Engineering Metal Halide Perovskite Nanocrystals with BODIPY Dyes for Photosensitization and Photocatalytic Applications

The sensitization of surface-anchored organic dyes on semiconductor nanocrystals through energy transfer mechanisms has received increasing attention owing to their potential applications in photodynamic therapy, photocatalysis, and photon upconversion. Here, we investigate the sensitization mechanisms through visible-light excitation of two nanohybrids based on CsPbBr3 perovskite nanocrystals (NC) functionalized with borondipyrromethene (BODIPY) dyes, specifically 8-(4-carboxyphenyl)-1,3,5,7-tetramethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BDP) and 8-(4-carboxyphenyl)-2,6-diiodo-1,3,5,7-tetramethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (I2-BDP), named as NC@BDP and NC@I2-BDP, respectively. The ability of I2-BDP dyes to extract hot hole carriers from the perovskite nanocrystals is comprehensively investigated by combining steady-state and time-resolved fluorescence as well as femtosecond transient absorption spectroscopy with spectroelectrochemistry and quantum chemical theoretical calculations, which together provide a complete overview of the phenomena that take place in the nanohybrid. Förster resonance energy transfer (FRET) dominates (82%) the photosensitization of the singlet excited state of BDP in the NC@BDP nanohybrid with a rate constant of 3.8 ± 0.2 × 1010 s-1, while charge transfer (64%) mediated by an ultrafast charge transfer rate constant of 1.00 ± 0.08 × 1012 s-1 from hot states and hole transfer from the band edge is found to be mainly responsible for the photosensitization of the triplet excited state of I2-BDP in the NC@I2-BDP nanohybrid. These findings suggest that the NC@I2-BDP nanohybrid is a unique energy transfer photocatalyst for oxidizing α-terpinene to ascaridole through singlet oxygen formation.

The role of the oxime group in the excited state deactivation processes of indirubin

The introduction of an oxime group into indirubin (INR) derivatives, including INROx, MINROx, and 6-BrINROx, and its impact on the spectral and photophysical properties of INR was investigated using a combination of fast-transient absorption (fs-TA/fs-UC) and steady-state fluorescence techniques. The oxime group introduces structural modifications that promote a rapid keto-enol tautomeric equilibrium and enhance the excited-state proton transfer (ESPT) process compared to its analogue, INR. In the oxime-indirubin derivatives investigated, the ESPT process is notably more efficient than what is observed in INR and indigo, occurring extremely fast (<1 ps) in all solvents, except for the viscous solvent glycerol. The more rapid deactivation mechanism precludes the formation of an intermediate species (syn-rotamer), as observed with INR. These findings are corroborated by time-dependent density functional theory (TDDFT) calculations. The work demonstrates that introducing an oxime group to INR, whether in nature or in the laboratory, results in an enhancement of its photostability.

Effect of Co Doping on the Physical Properties and Organic Pollutant Photodegradation Efficiency of ZnO Nanoparticles for Environmental Applications

This paper presents a comprehensive investigation of the synthesis and characterization of Zn1-xCoxO (0 ≤ x ≤ 0.05) nanopowders using a chemical co-precipitation approach. The structural, morphological, and vibrational properties of the resulting ZnO nanostructures were assessed through X-ray diffraction, scanning electronic microscopy, and Raman spectroscopy to examine the influence of cobalt doping. Remarkably, a notable congruence between the experimental results and the density functional theory (DFT) calculations for the Co-doped ZnO system was achieved. Structural analysis revealed well-crystallized hexagonal wurtzite structures across all samples. The SEM images demonstrated the formation of spherical nanoparticles in all the samples. The vibrational properties confirmed the formation of a hexagonal wurtzite structure, with an additional Raman peak corresponding to the F2g vibrational mode characteristic of the secondary phase of ZnCo2O4 observed at a 5% cobalt doping concentration. Furthermore, a theoretical examination of cobalt doping's impact on the elastic properties of ZnO demonstrated enhanced mechanical behavior, which improves stability, recyclability, and photocatalytic activity. The photocatalytic study of the synthesized compositions for methylene blue (MB) dye degradation over 100 min of UV light irradiation demonstrated that Co doping significantly improves photocatalytic degradation. The material's prolonged lifetime, reduced rate of photogenerated charge carrier recombination, and increased surface area were identified as pivotal factors accelerating the degradation process. Notably, the photocatalyst with a Zn0.99Co0.01O composition exhibited exceptional efficiency compared to that reported in the literature. It demonstrated high removal activity, achieving an efficiency of about 97% in a shorter degradation time. This study underscores the structural and photocatalytic advancements in the ZnO system, particularly at lower cobalt doping concentrations (1%). The developed photocatalyst exhibits promise for environmental applications owing to its superior photocatalytic performance.

Highly Selective Fluorescent Sensors: Polyethylenimine Derivatives of Triphenylamine and Coumarin for GTP and ATP Interaction via Fluorescence Lifetime Imaging Microscopy

Chemical derivatives of polyethylenimine (PEI) receptors with either triphenylamine (TPA) or 7-hydroxy-4-methyl-coumarin (Cou) form stable complexes with adenine and guanine nucleotides in water. The host-guest complex modulation is found to be based on noncovalent molecular interactions such as π-π stacking and hydrogen bonding, which are dependent on the aromatic moieties attached to the polyaminic (PEI) backbone. PEI-TPA acts as a chemosensor with a recognition driving force based on aggregation-induced emission (AIE), involving π-π interaction between the nucleic base and TPA. It detects GTP by a chelation enhancement quenching effect of fluorescence (CHEQ) with a measured logarithm stability constant, log β = 7.7. By varying the chemical characteristics of the fluorophore, as in the PEI-Cou system, the driving force for recognition changes from a π-π interaction to an electrostatic interaction. The coumarin derivative detects ATP with a log β value one order of magnitude higher than that for GTP, allowing for the selective recognition of the two nucleotides in a 100% aqueous solution. Furthermore, fluorescence lifetime imaging microscopy (FLIM) allows for a correlation between the selectivity of PEI-TPA toward nucleotides and the morphology of the structures formed upon ATP and GTP recognition. This study offers valuable insights into the design of receptors for the selective recognition of nucleotides in water.

Solvent-Driven Self-Organization of Meso-Substituted Porphyrins: Morphological Analysis from Fluorescence Lifetime Imaging Microscopy

A morphological analysis of different thin films of meso-tetra-p-(di-p-phenylamino)phenylporphyrin, H₂T(TPA)₄P, was made by fluorescence lifetime imaging microscopy (FLIM) and scanning electron microscopy (SEM). A comprehensive study of H₂T(TPA)₄P was undertaken through UV/vis absorption and fluorescence techniques in different solvents, solvent mixtures and in thin films. In solution, occurrence of intramolecular energy transfer from the triphenylamine (TPA) moieties to the porphyrin core, with quenching efficiencies in the order of 94-97%, is observed. The energy transfer rate constants are determined assuming Förster's dipole-dipole and Dexter's electron exchange mechanisms. In drop-cast-prepared thin films, from samples with different solvent mixtures, the photoluminescence (PL) quantum yield (Φ_PL) decreases ∼1 order of magnitude compared to the solution behavior. FLIM and SEM experiments showed the self-organization and morphology of H₂T(TPA)₄P in thin films to be highly dependent on the solvent mixture used to prepare the film. In chloroform, the solvent's evaporation results in the formation of elongated and overlapped microrod structures. Introduction of a cosolvent, namely, a polar cosolvent, promotes changes in the morphology of the self-assembled structures, with the formation of three-dimensional spherical structures and hollow spheres. H₂T(TPA)₄P dispersed in a polymer matrix shows enhanced Φ_PL values when compared to the drop-cast films. FLIM images showed coexistence of three different states or domains: aggregated, interface, and nonaggregated or less-aggregated states. This work highlights the importance of FLIM in the morphological characterization of heterogeneous films, together with the photophysical characterization of nano- and microdomains.

The role of solvents and concentrations in the properties of oxime bearing A2B corroles

A comprehensive study on the electronic spectral, photophysical and acid-base properties of phenyl- and methyl-oxime corrole derivatives and of triphenylcorrole (model corrole) has been performed, aiming to shed light on the existing species in the ground and excited states. Solvents and corrole concentration are found to govern the properties of the studied compounds and are determinants of their applicability in in vivo studies. In THF, the neutral corrole has two tautomeric forms (T1 and T2). In DMSO, the deprotonated form shows a characteristic long-wavelength Q band slightly shifted to blue when compared with the T1 tautomer and a higher fluorescence quantum yield. In ACN, with the increase of the corrole concentration formation of an aggregate due to homoconjugation (with dimer characteristics) is observed, and pioneeringly reported using UV-Vis and fluorescence studies and confirmed by carrying out titrations with TFA. The effect of the oxime group on the pK values of a corrole is found to influence the formation of a homoconjugate, namely by precluding its formation (at higher concentrations) when compared with the model corrole. TDDFT electronic quantum calculations support the experimental observations, namely the existence of tautomers and deprotonated species, with their respective electronic spectral features, further allowed proposing a structure for the homoconjugate complex in ACN. The characteristics of the oxime-corroles, namely a pK of ∼ 5, absorption and emission at ca. 650 nm and solvent dependent properties, make them good candidates for their use in biological systems either as probes, sensors, or as new sensitizers for photodynamic therapy.

Enhanced thermal and photo-stability of a para-substituted dicumyl ketone intercalated in a layered double hydroxide

A ketodiacid, 4,4′-dicarboxylate-dicumyl ketone (3), has been intercalated into a Zn, Al layered double hydroxide (LDH) by a coprecipitation synthesis strategy. The structure and chemical composition of the resultant hybrid material (LDH-KDA3) were characterized by powder X-ray diffraction (PXRD), FT-IR, FT-Raman and solid-state 13C{1H} NMR spectroscopies, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), thermogravimetric analysis (TGA), and elemental analysis (CHN). PXRD showed that the dicarboxylate guest molecules assembled into a monolayer to give a basal spacing of 18.0 Å. TGA revealed that the organic guest starts to decompose at a significantly higher temperature (ca. 330°C) than that determined for the free ketodiacid (ca. 230°C). Photochemical experiments were performed to probe the photoreactivity of the ketoacid in the crystalline state, in solution, and as a guest embedded within the photochemically-inert LDH host. Irradiation of the bulk crystalline ketoacid results in photodecarbonylation and the exclusive formation of the radical-radical combination product. Solution studies employing the standard myoglobin (Mb) assay for quantification of released CO showed that the ketoacid behaved as a photoactivatable CO-releasing molecule for transfer of CO to heme proteins, although the photoreactivity was low. No photoinduced release of CO was found for the LDH system, indicating that molecular confinement enhanced the photo-stability of the hexasubstituted ketone. To better understand the behavior of 3 under irradiation, a more comprehensive study, involving excitation of this compound in DMSO-d6 followed by 1H NMR, UV-Vis and fluorescence spectroscopy, was undertaken and further rationalized with the help of time-dependent density functional theory (TDDFT) electronic quantum calculations. The photophysical study showed the formation of a less emissive compound (or compounds). New signals in the 1H NMR spectra were attributed to photoproducts obtained via Norrish type I α-cleavage decarbonylation and Norrish type II (followed by CH3 migration) pathways. TDDFT calculations predicted that the formation of a keto-enol system (via a CH3 migration step in the type II pathway) was highly favorable and consistent with the observed spectral data.

Excited state deactivation mechanisms in Shikonin rationalized from its naphthoquinone parent structures

Shikonin, a naphthoquinone dye, is a molecule of colour of natural origin, whose peculiar properties have not yet been fully rationalized. Its core structure consists of a di-hydroxy-naphthoquinone with an additional non-aromatic hydroxy group. From a comprehensive study involving fast spectroscopic techniques (fs-TA and fs-UC) and TDDFT electronic structure calculations on shikonin (Shk) and its derivatives 5-hydroxy-1,4-naphthoquinone (5HNQ), 5,8-diacetoxy-1,4-naphthoquinone (DiAc), 5,8-dihidroxy-1,4-naphthoquinone (DHNQ) and acetylshikonin, AcShk, it is shown that intramolecular excited state proton transfer (ESIPT) is present and is determinant in the deactivation of the hydroxy containing molecules. This is mirrored by the dominance of the internal conversion deactivation channel. In Shk, the non-aromatic hydroxy group determines the preferred conformer in both the ground- and excited-state, as reflected in the doubling of the fluorescence quantum yield value of this molecule relative to DHNQ. From fs-UC, a kinetic isotopic effect of 1.7 was obtained for DHNQ.

Aggregation-Induced Emission with Alkynylcoumarin Dinuclear Gold(I) Complexes: Photophysical, Dynamic Light Scattering, and Time-Dependent Density Functional Theory Studies

Aggregation-induced emission (AIE) has gained a remarkable amount of interest in the past 20 years, but the majority of the studies are based on organic structures. Herein, three dinuclear gold(I) complexes, with the general formula [PPh₂XPPh₂-Au₂-Coum₂], where the Au(I) atom is linked to three different diphosphanes [PPh₂XPPh₂; DPPM for X = CH₂ (1.1), DPPP for X = (CH₂)₃ (1.2), and DPPA for X = C≡C (1.3)] and the propynyloxycoumarin precursor (1, 4-methyl-substituted coumarin), have been synthesized. The compounds present AIE characteristics, AIEgens, with high luminescence quantum yields in the solid state when they are compared to dilute solutions. Photophysical studies (steady-state and time-resolved fluorescence) were obtained, with AIE being observed with the three gold(I) complexes in acetonitrile/water mixtures. This was further corroborated with dynamic light scattering measurements. Time-dependent density functional theory (TDDFT) electronic calculations show that the compounds have different syn and anti conformations (relative to the coumarin core) with 1.1 syn and 1.2 and 1.3 both anti. From time-resolved fluorescence experiments, the augment in the contribution of the longer decay component is found to be associated with the emission of the aggregate (AIE effect) and its nature (involving a dimer) rationalized from TDDFT electronic calculations.

Ring-Fused meso-Tetraarylchlorins as Auspicious PDT Sensitizers: Synthesis, Structural Characterization, Photophysics, and Biological Evaluation

Novel 4,5,6,7-tetrahydropyrazolo[1,5-a]pyridine-fused meso-tetraarylchlorins, with different degrees of hydrophilicity (with methyl ester, hydroxymethyl, and carboxylic acid moieties), have been synthesized and their photophysical characterization as well as in vitro photocytotoxicity assessment against human melanoma and esophageal and bladder carcinomas was carried out. An integrated analysis of the photosensitizers’ performance, considering the singlet oxygen generation data, cell internalization, and intracellular localization, allowed to establish relevant structure-photoactivity relationships and the rationalization of the observed photocytotoxicity. In the diacid and monoalcohol series, chlorins derived from meso-tetraphenylporphyrin proved to be the most efficient photodynamic therapy agents, showing IC50 values of 68 and 344 nM against A375 cells, respectively. These compounds were less active against OE19 and HT1376 cells, the diacid chlorin with IC50 values still in the nano-molar range, whereas the monohydroxymethyl-chlorin showed significantly higher IC50 values. The lead di(hydroxymethyl)-substituted meso-tetraphenylchlorin confirmed its remarkable photoactivity with IC50 values below 75 nM against the studied cancer cell lines. Subcellular accumulation of this chlorin in the mitochondria, endoplasmic reticulum, and plasma membrane was demonstrated.

Cucurbituril hosts as promoters of aggregation induced emission of triphenylamine derivatives

Three ligands bearing triphenylamine as a core and one, two or three acyclic polyamine chains, TPA1p, TPA2p and TPA3p, respectively, have been studied by potentiometric and photophysical (UV-Vis, steady-state and time-resolved fluorescence) techniques. The host-guest interaction with cucurbit[7]uril, CB7, has been investigated in aqueous solution showing aggregation induced emission behaviour when encapsulated into a CB7 cavity. From fluorescence emission it is revealed that the charged polyamine chains are the unit entering into CB7 and from the Job plots the stoichiometries are found to vary from 1 : 1 to 1 : 3 L : CB7 ratios. Interactions of the charged amines with the portals of CB7 restrict rotation of the benzene units in the triphenylamine backbone (free rotor effect), decreasing the radiationless internal conversion channel at the expense of the enhancement of fluorescence. Dynamic light scattering and resonance Rayleigh scattering experiments show that TPA3p-CB7 complexes involve formation of aggregates with a mean size of 126 ± 5 nm and a dispersity factor of 0.117, indicating a monodisperse distribution and supporting the important conclusions of this work: formation of emissive aggregates through the AIE effect.

Tryptanthrin derivatives as efficient singlet oxygen sensitizers

Halogenated tryptanthrin and aminotryptanthrin were synthesized from indigo or isatin precursors. Dibromo- and tetrabromo-tryptanthrin were obtained from indigo dyes following green chemistry procedures, through microwave-assisted synthesis in mild oxidation conditions. Spectral and photophysical properties of the compounds, including quantitative determination of all the different deactivation pathways of S₁ and T₁, were obtained in different solvents and temperatures. The triplet state (T₁) has a dominant role on the photophysical properties of these compounds, which is further enhanced by the halogens at the fused-phenyl rings. Substitution with an amino group, 2-aminotryptanthrin (TRYP-NH₂), leads a dominance of the radiative decay channel. Moreover, with the sole exception of TRYP-NH₂, S₁ ~  ~  > T₁ intersystem crossing constitutes the dominant route, with internal conversion playing a minor role in the deactivation of S₁ in all the studied derivatives. In agreement with tryptanthrin, emission of the triplet state of tryptanthrin derivatives (with exception of TRYP-NH₂), was observed together with an enhancement of the singlet oxygen sensitization quantum yield: from 70% in tryptanthrin to 92% in the iodine derivative. This strongly contrasts with indigo and its derivatives, where singlet oxygen sensitization is found inefficient.

Comprehensive Investigation of the Photophysical Properties of Alkynylcoumarin Gold(I) Complexes

Six gold(I) complexes (R₃P-Au-Coum) containing three different alkynylcoumarin chromophores (Coum) with different electron-donating and electron-withdrawing characteristics and two different water-soluble phosphanes (PR₃ = PTA (a) and DAPTA (b)) have been synthesized (1a,b, unsubstituted coumarin; 2a,b, 4-methyl substituted coumarin; 3a,b, 3-chloro and 4-methyl substituted coumarin). A comprehensive study of the photophysical properties of the R₃P-Au-Coum, together with their propynyloxycoumarin precursors 1-3, was performed in solution at room and low temperatures. Spectral and photophysical characteristics of the R₃P-Au-Coum essentially depend on the electronic characteristics of the propynyloxycoumarin ligand. The presence of the Au(I) atom was found to be responsible for an increase of the intersystem crossing, with triplet state quantum yield values, ϕ_T, ranging from ∼0.05 to 0.35 and high coumarin phosphorescence quantum yield values for derivatives 1 and 2; fluorescence dominates the deactivation in derivatives 3. Efficient singlet oxygen photosensitization was observed for the new compounds 3a,b. From TDDFT calculations, the relevant HOMO and LUMO of the compounds, i.e., those involved in the transitions, are dominated by the frontier orbitals associated with the coumarin core. The Au(I)-phosphane structure introduces a new transition assigned to an intraligand transition involving the phosphane ligand, and π(C≡C) system, to the p orbitals of phosphorus and gold atoms.

Cationic Diazapentacenium Polymers Made in a Sequence of CN Cross Coupling Polymerization and Acid-Mediated Postpolymerization Cyclization

Polycationic stepladder polymers containing 5,12-diazapentacenium bistriflate repeat units are made in a two-step sequence of a carbon-nitrogen cross coupling polymerization and subsequent postpolymerization cyclization. The deeply colored products show a rather weak conjugative interaction between the dicationic diazapentacenium repeat units along the polymer chains.

Sulfonated tryptanthrin anolyte increases performance in pH neutral aqueous redox flow batteries

Aqueous organic redox flow batteries (AORFBs) hold great promise as low-cost, environmentally friendly and safe alternative energy storage media. Here we present aqueous organometallic and all-organic active materials for RFBs with a water-soluble active material, sulfonated tryptanthrin (TRYP-SO₃H), working at a neutral pH and showing long-term stability. Electrochemical measurements show that TRYP-SO₃H displays reversible peaks at neutral pH values, allowing its use as an anolyte combined with potassium ferrocyanide or 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt monohydrate as catholytes. Single cell tests show reproducible charge-discharge cycles for both catholytes, with significantly improved results for the aqueous all-organic RFB reaching high cell voltage (0.94 V) and high energy efficiencies, stabilized during at least 50 working cycles.

Tuning J-aggregate Formation and Emission Efficiency in Cationic Diazapentacenium Dyes

Enhancement of the luminescence efficiency of two new diazapentacenium salts (D1 and D2) of more than 55 for D1 and 22 times for D2) in poor solvents, acetonitrile and/or dichloromethane, was observed and rationalized as formation of emissive J-aggregates. Both compounds displaying 4-n-decylphenyl substituents at the 7,14-carbons and phenyl (D1) or 2,6-difluorophenyl (D2) substituents at the quaternary nitrogen atoms in 5,12-positions have been synthetized in a two-step procedure involving a two-fold Buchwald-Hartwig-type CN cross-coupling and an electrophilic Friedel-Crafts-type cyclization. The optical properties of the dicationic diazapentacenium salts in various solvents and in thin films have been investigated by steady-state and time-resolved absorption and photoluminescence spectroscopies. In thin films and in good solvents, isolated molecules coexist with aggregates. Nonetheless, D1 is seven times more emissive than D2, reflecting a higher J-aggregate contribution in the former.

Red-Purple Photochromic Indigos from Green Chemistry: Mono-tBOC or Di-tBOC N-Substituted Indigos Displaying Excited State Proton Transfer or Photoisomerization

In indigo, excited state proton transfer (ESPT) is known to be associated with the molecular mechanism responsible for highly efficient radiationless deactivation. When this route is blocked (partially or totally), new deactivation routes become available. Using new green chemistry procedures, with favorable green chemistry metrics, monosubstitution and disubstitution of N group(s) in indigo, by tert-butoxy carbonyl groups, N-(tert-butoxycarbonyl)indigo (NtBOCInd) and N,N'-(tert-butoxycarbonyl)indigo (N,N'tBOCInd), respectively, were synthetically accomplished. The compounds display red to purple colors depending on the solvent and substitution. Different excited-state deactivation pathways were observed and found to be structure- and solvent-dependent. Trans-cis photoisomerization was found to be absent with NtBOCInd and present with N,N'tBOCInd in nonpolar solvents. Time-resolved fluorescence experiments revealed single-exponential decays for the two compounds which, linked to time-dependent density functional theory (TDDFT) studies, show that with NtBOCInd ESPT is extremely fast and barrierless-predicted to be 1 kJ mol^-1 in methylcyclohexane and 5 kJ mol^-1 in dimethylsulfoxide-, which contrasts with ∼11 kJ mol^-1 experimentally obtained for indigo. An alternative ESPT, competitive with the N-H···O═C intramolecular pathway, involving dimer units is also probed by TDDFT and found to be consistent with the experimentally observed time-resolved data. N,N'tBOCInd, where ESPT is precluded, shows solvent-dependent trans-cis/cis-trans photoisomerization and is surprisingly found to be more stable in the nonemissive cis conformation, whose deactivation to S₀ is found to be solvent-dependent.

Novel fluorinated ring-fused chlorins as promising PDT agents against melanoma and esophagus cancer

Investigation of novel 4,5,6,7-tetrahydropyrazolo[1,5-a]pyridine-fused chlorins, derived from 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin, as PDT agents against melanoma and esophagus cancer is disclosed. Diol and diester fluorinated ring-fused chlorins, including derivatives with 2-(2-hydroxyethoxy)ethanamino groups at the phenyl rings, were obtained via a two-step methodology, combining SNAr and [8π + 2π] cycloaddition reactions. The short-chain PEG groups at the para-position of the phenyl rings together with the diol moiety at the fused pyrazole ring promote a red-shift of the Soret band, a decrease of the fluorescence quantum yield and an increase of the singlet oxygen formation quantum yield, improving the photophysical characteristics required to act as a photosensitizer. Introduction of these hydrophilic groups also improves the incorporation of the sensitizers by the cells reaching cellular uptake values of nearly 50% of the initial dose. The rational design led to a photosensitizer with impressive IC50 values, 13 and 27 nM against human melanoma and esophageal carcinoma cell lines, respectively.

Aggregation-Induced Emission: From Small Molecules to Polymers-Historical Background, Mechanisms and Photophysics

The enhancement of photoluminescence through formation of molecular aggregates in organic oligomers and conjugated organic polymers is reviewed. A historical contextualization of aggregation-induced emission (AIE) phenomena is presented. This includes the loose bolt or free rotor effect and J-aggregation phenomena, and discusses their characteristic features, including structures and mechanisms. The basis of both effects is examined in key molecules, with a particular emphasis on the AIE effect occurring in conjugated organic polymers with a polythiophene (PT) skeleton with triphenylethylene (TPE) units. Rigidification of the excited state structure is one of the defining conditions required to obtain AIE, and thus, by changing from a flexible ground state to rigid (quinoidal-like) structures, oligo and PTs are among the most promising emerging molecules alongside with the more extensively used TPE derivatives. Molecular structures moving away from the domination of aggregation-caused quenching to AIE are presented. Future perspectives for the rational design of AIEgen structures are discussed.

Deep in blue with green chemistry: influence of solvent and chain length on the behaviour of N- and N,N′- alkyl indigo derivatives

Using green chemistry procedures the synthesis of N-alkyl (NC_nInd) and N,N′-dialkyl (N,N′C_nInd) indigo derivatives, with n = 1–3, 6, 8, 12 and 18, was undertaken, leading to compounds with blueish to greenish colors in solution. The effect of the alkyl chain length on the spectral (including color) and photophysical properties of the compounds was explored. This was done with solvents of different viscosities and polarities (dielectric constants). From time-resolved fluorescence and femtosecond-transient absorption (fs-TA) for the NC_nInd derivatives with n = 1 and 2, the decays are, in methylcyclohexane (MCH) and n-dodecane, single-exponential, while in 2-methyltetrahydrofuran (2MeTHF) they are bi-exponential. The excited state proton transfer (ESPT) is ultrafast (<1 ps) for NC_1,2Ind in MCH and n-dodecane, supported by time-dependent density functional theory (TDDFT) calculations, thus showing that both the chain length and solvent influence the ESPT process. For N,N′C_nInd, from time-resolved experiments, and with the exception of the shortest member of the series, N,N′C₁Ind, two conformers are found to be present in the excited state.

Using green chemistry procedures the synthesis of N- and N,N′-alkyl indigo derivatives was undertaken and the effect of the alkyl chain length on the spectral (including color) and photophysical properties of the compounds explored.

Correction: Deep in blue with green chemistry: influence of solvent and chain length on the behaviour of N- and N,N′-alkyl indigo derivatives

Correction for ‘Deep in blue with green chemistry: influence of solvent and chain length on the behaviour of N- and N,N′-alkyl indigo derivatives’ by Daniela Pinheiro et al., Chem. Sci., 2020, DOI: 10.1039/d0sc04958a.

A water-soluble bithiophene with increased photoluminescence efficiency and metal recognition ability

A new water-soluble tri-tert-butyl-bithiophenesulfonamide (α2-tbS) was synthesized and a comprehensive spectroscopic and photophysical study was undertaken in organic solvents and water at different pH values. In contrast to the behaviour found for the parent (and un-substituted) α,α'-bithiophene (α2), in which radiationless decay processes are the main excited-state deactivation channels, the tert-butylsulfonamide derivative presents a significant fluorescence quantum yield (φF) (ca. one order of magnitude higher than that of α2). The high φF allowed further exploring α2-tbS as a selective fluorimetric sensor for metal ions. A strong selectivity towards Cu(ii) is observed at neutral pH values, whereas at pH = 9.5 a strong quenching upon the addition of Hg(ii) is observed. An additional high sensitivity of 0.64 ± 0.02 ppm towards Cu(ii) was observed, well below 1.25 ppm (∼20 μM), the maximum value allowed in drinking water by the EPA.

Room-Temperature Phosphorescence and Efficient Singlet Oxygen Production by Cyclometalated Pt(II) Complexes with Aromatic Alkynyl Ligands

The synthesis of five novel cyclometalated platinum(II) compounds containing five different alkynyl-chromophores was achieved by the reaction of the previously synthesized Pt-Cl cyclometalated compound (1) with the corresponding RC≡CH by a Sonogashira reaction. It was observed that the spectral and photophysical characteristics of the cyclometalated platinum(II) complexes (Pt-Ar) are essentially associated with the platinum-cyclometalated unit. Room-temperature emission of the Pt-Ar complexes was attributed to phosphorescence in agreement with DFT calculations. Broad nanosecond (ns)-transient absorption spectra were observed with decays approximately identical to those obtained from the emission of the triplet state. From the femtosecond-transient absorption (fs-TA) data, two main excited-state decay components were identified: one in the order of a few picoseconds was assigned to fast intersystem crossing to populate the triplet excited-state and the second (hundreds of ns) was associated with the decay of the transient triplet state. In general, efficient singlet oxygen photosensitization quantum yields were observed from the triplet state of these complexes.

Evaluation of the supramolecular interaction of Congo red with cucurbiturils using mass spectrometry and spectroscopic methods

Cucurbit[n]urils decolourise aqueous solutions of Congo red by forming outer-surface adducts, which are also detected in gas-phase ESI-MS studies.

Luminescent halogen-substituted 2-(N-arylimino)pyrrolyl boron complexes: the internal heavy-atom effect

New luminescent halogen-substituted 2-iminopyrrolyl boron complexes exhibited an internal-heavy atom effect depending on the position of the halogen atom, and activity in OLEDs.

Deactivation Routes in Gold(I) Polypyridyl Complexes: Internal Conversion Vs Fast Intersystem Crossing

An electronic spectral and photophysical characterization of three gold(I) complexes containing heterocyclic chromophores differing in the number and arrangement of pyridine rings (pyridine, bipyridine, and terpyridine, with the acronyms pD, bD, and tD respectively) was performed. Quantum yields of fluorescence, internal conversion and triplet state formation, together with the rate constants for singlet to triplet intersystem crossing, S₁ ∼ ∼ ∼ S₀ internal conversion and fluorescence were measured in order to equate the impact of fast triplet state formation on the amount of triplets formed. The results showed a correlation between the increase on the measured decay values of S₁ (leading to the main formation of T₁) and the increase in the charge transfer (CT) character of the lowest energy transition, as evaluated from the orthogonality of the frontier orbitals. The measured triplet state quantum yields range from ∼50-60% to 70%, whereas the intersystem crossing rate constants differ by almost 2 orders of magnitude, from 9.4 × 10⁹ s^-1 for tD to 8.1 × 10¹¹ s^-1 for bD. This constitutes an evidence for the existence of a correlation between the intersystem crossing and the internal conversion mechanisms.

Interactions and Supramolecular Organization of Sulfonated Indigo and Thioindigo Dyes in Layered Hydroxide Hosts

Supramolecularly organized host-guest systems have been synthesized by intercalating water-soluble forms of indigo (indigo carmine, IC) and thioindigo (thioindigo-5,5'-disulfonate, TIS) in zinc-aluminum-layered double hydroxides (LDHs) and zinc-layered hydroxide salts (LHSs) by coprecipitation routes. The colors of the isolated powders were dark blue for hybrids containing only IC, purplish blue or dark lilac for cointercalated samples containing both dyes, and ruby/wine for hybrids containing only TIS. The as-synthesized and thermally treated materials were characterized by Fourier transform infrared, Fourier transform Raman, and nuclear magnetic resonance spectroscopies, powder X-ray diffraction, scanning electron microscopy, and elemental and thermogravimetric analyses. The basal spacings found for IC-LDH, TIS-LDH, IC-LHS, and TIS-LHS materials were 21.9, 21.05, 18.95, and 21.00 Å, respectively, with intermediate spacings being observed for the cointercalated samples that either decreased (LDHs) or increased (LHSs) with increasing TIS content. UV-visible and fluorescence spectroscopies (steady-state and time-resolved) were used to probe the molecular distribution of the immobilized dyes. The presence of aggregates together with the monomer units is suggested for IC-LDH, whereas for TIS-LDH, IC-LHS, and TIS-LHS, the dyes are closer to the isolated situation. Accordingly, while emission from the powder H₂TIS is strongly quenched, an increment in the emission of about 1 order of magnitude was observed for the TIS-LDH/LHS hybrids. Double-exponential fluorescence decays were obtained and associated with two monomer species interacting differently with cointercalated water molecules. The incorporation of both TIS and IC in the LDH and LHS hosts leads to an almost complete quenching of the fluorescence, pointing to a very efficient energy transfer process from (fluorescent) TIS to (nonfluorescent) IC.

Advances on photodynamic therapy of melanoma through novel ring-fused 5,15-diphenylchlorins

The synthesis, photophysical behaviour and photosensitization ability of novel 4,5,6,7-tetrahydropyrazolo[1,5-a]pyridine-fused 5,15-diphenylchlorins against melanoma cells are described. All studied chlorins were found to be extremely active against melanoma cell lines A375 showing IC₅₀ values below 20 nM. Furthermore, a dihydroxymethyl diphenylchlorin was identified as an excellent candidate to allow modulating of different types of cell death, apoptosis vs. necrosis, by varying its concentration. This can be explored as a tool to improve the effectiveness of PDT since inflammatory response resulting from necrotic cell death after PDT can activate the antitumor immune response with implications also regarding the vascular damage. This feature combined with very low cytotoxicity against human melanoma cells in the absence of light activation and against human fibroblast HFF-1 cells makes this chlorin a candidate of choice as a photosensitizer for PDT. A comprehensive photophysical investigation including the determination of quantum yields for fluorescence, singlet oxygen sensitization and internal conversion, lifetimes and rate constants of all the excited state deactivation processes has been undertaken.

The molecules of color in Portuguese postage stamps (1857–1909)

The nature of the pigments or dyes used to dye the first Portuguese postage stamps has remained unknown until now. In this work, a study has been made of the inks used for red, rose, purple and orange colors in a selected number of Portuguese postage stamps from the period 1857 to 1909. This is based on analysis involving a variety of techniques (X-ray fluorescence, UV-Vis spectroscopy, HPLC-MS/HPLC-DAD, steady and time resolved fluorescence). It was found that the inks included, among others, the inorganic pigments cinnabar (HgS), lead oxide (Pb3O4) and chromate (PbCrO4), lead sulphide (PbS), and the organic compounds carminic acid and Eosin Y. The study demonstrated a non-destructive analysis method for identification of two molecules of color involving the UV-Vis (for carminic acid and Eosin Y) and fluorescence spectra, together with quantum yields and lifetimes (for Eosin Y).

Platinum(II) Ring-Fused Chlorins as Near-Infrared Emitting Oxygen Sensors and Photodynamic Agents

Novel near-infrared luminescent compounds based on platinum(II) 4,5,6,7-tetrahydropyrazolo[1,5-a]pyridine-fused chlorins are described. These compounds have high photostability and display light emission, in particular simultaneous fluorescence and phosphorescence emission in solution at room temperature, in the biologically relevant 700-850 nm red and near-infrared (NIR) spectral region, making them excellent materials for biological imaging. The simultaneous presence of fluorescence and phosphorescence emission at room temperature, with the phosphorescence strongly quenched by oxygen whereas fluorescence remains unaffected, allows these compounds to be used as ratiometric oxygen sensors in chemical and biological media. Both steady-state (fluorescence vs phosphorescence intensities) and dynamic (dependence of phosphorescence lifetimes upon oxygen concentration) luminescence approaches can be used. Photocytotoxicity studies against human melanocytic melanoma cells (A375) indicate that these compounds display potential as photosensitizers in photodynamic therapy.

An Indigo Carmine-Based Hybrid Nanocomposite with Supramolecular Control of Dye Aggregation and Photobehavior

Zn-Al layered double hydroxides (LDHs) containing solely indigo carmine (IC) or 1-hexanesulfonate (HS) anions, or a mixture of the two with different HS/IC molar ratios, were prepared by the direct synthesis method and characterized by various techniques. Hydrotalcite-type phases were obtained with basal spacings of 17.6 Å for the LDH intercalated by IC (IC-LDH) and 18.2-18.3 Å for the other materials containing HS. From the basal spacing for IC-LDH and UV/Vis spectroscopic data, it is proposed that the dye molecules assemble within the interlayer galleries to form a J-type stacking arrangement. A comprehensive electronic spectral and photophysical study was undertaken for IC in solution and all materials, aiming to obtain a detailed characterization of the host-guest and guest-guest interactions. In solution (the solvent surrounded "isolated" molecule), IC presents a fast excited state proton transfer with rate constants of ∼1.2-1.4×10(11)  s(-1) , which is linked to the very efficient radiationless deactivation channel. In the solid state it is shown that incorporation of IC within the LDH decreases the level of aggregation, and that further addition of HS induces the appearance of isolated IC units within the LDH galleries. The indigo carmine-based nanocomposites reported constitute a step forward in the design of hybrid materials with tunable properties.

Controlling the fluorescence behavior of 1-pyrenesulfonate by cointercalation with a surfactant in a layered double hydroxide

Zn-Al layered double hydroxides (LDHs) containing solely 1-pyrenesulfonate (PS) or 1-heptanesulfonate (HS) anions, or a mixture of the two with HS/PS molar ratios ranging between ca. 7.5 and 82, were prepared by the direct synthesis method and characterized by powder X-ray diffraction, thermal and elemental analyses, scanning electron microscopy, and FT-IR, FT-Raman, and (13)C{(1)H} CP MAS NMR spectroscopies. Well-ordered intercalates were obtained with basal spacings of 18.8 Å for the LDH intercalated by PS and 19.2-19.4 Å for the other materials containing HS. The photophysics of the solids, as well as the PS probe dissolved in water and common organic solvents (aiming to compare the behavior of the "isolated" molecule with that in the solid), were investigated by steady-state and time-resolved fluorescence techniques. The fluorescence spectra of the solid samples display two bands with maxima at 376 and 495 nm. Depending on the HS/PS ratios, the band intensity ratio (obtained at 375 and 520 nm) changes, reflecting different contributions from monomer and dimer species. The decays collected at 375 nm are biexponentials with a major component (∼97% of the total fluorescence) of 105 ns for the highest HS/PS ratio, which further loses importance with an increase in the PS content. When the decays are collected at 480 and 520 nm, the fits are triexponentials with a major component varying from 108 to 124 ns, attributed to an excimer. Steady-state and time-resolved measurements with PS in solution (ethanol, methanol, DMF, DMSO, and water) were also measured, and a comparison of the vibronic I1/I3 ratio and lifetimes in water (65 ns) with those in the LDHs indicates that the PS probe in the cointercalated LDHs is surrounded by the HS surfactant.

Unveiling the Eigen-Weller ion pair from the excited state proton transfer kinetics of 3-chloro-4-methyl-7-hydroxycoumarin

The prototropic reactions of the first excited singlet state of 3-chloro-4-methylumbelliferone (3Cl4MU), in dioxane:water mixtures (Dx:H2O), were revisited using ps-time-resolved fluorescence techniques. The data response to the dielectric constant of the mixtures revealed the presence of an additional fourth kinetic species, kinetically coupled to the neutral (N*), the tautomeric (T*), and anionic (A(-)*) forms of 3Cl4MU, which is assigned to the elusive geminate (A(-)*···H(+)) ion pair. From the data analysis, all rate constants of the prototropic and diffusion processes involved were separately evaluated. The results showed that, whenever the geminate ionic pair is not kinetically detected, the evaluated values for deprotonation and protonation rate constants can substantially deviate from the real ones, depending on the efficiencies of pair recombination and dissociation. Finally, the results provide convincing kinetic evidence for the Eigen-Weller mechanism (intermediacy of the geminate ionic pair) in a quasi-aqueous medium, which to our knowledge had not yet been given.

The effect of polyaromatic hydrocarbons on the spectral and photophysical properties of diaryl-pyrrole derivatives: an experimental and theoretical study

A new class of diaryl-pyrrole derivatives of the polyaromatic hydrocarbons (PAH) benzene, naphthalene, anthracene and pyrene were synthesized in a multicomponent reaction under microwave irradiation and studied in solution at room (293 K) and low (77 K) temperature. The study includes a complete spectroscopic evaluation (singlet-singlet and triplet-triplet absorption, fluorescence and phosphorescence spectra) as well as photophysical evaluation (fluorescence, phosphorescence and triplet lifetimes together with fluorescence and triplet occupation and singlet oxygen sensitization quantum yields). From the above evaluation, a complete set of deactivation rate constants (kF, kIC and kISC) could be obtained. The study was further complemented with TDDFT calculations. It is shown that, with the exception of the anthracene derivative, the diaryl-pyrrole moiety strongly influences the spectral and photophysical properties of the PAH and that with the exception of the benzene derivative, the excited state internal conversion deactivation channel of the diaryl-pyrrole derivatives is higher than that of the PAH counterparts.

Perkin's and Caro's mauveine in Queen Victoria's lilac postage stamps: a chemical analysis

Mauveine, a chemical icon, is no longer commercially available. If nowadays one wanted to have a sample of the original Perkin, or Caro, mauveine, and see its colour, where would one find it? The answer is on UK Victorian 6d postage stamps from 1867-1880. This was found from a comparison with historical samples of mauveine, from both William Perkin and a Heinrich Caro sample (here analysed for the first time). These have distinctly different compositions and this was used to identify the origin of mauveine in the postage stamps, with evidence found for mauveine made by both Perkin's and Caro's synthesis.

Self-assembly of poly{1,4-phenylene-[9,9-bis(4-phenoxy-butylsulfonate)]fluorene-2,7-diyl} with oppositely charged phenylenevinylene oligoelectrolytes

The interaction of the water-soluble conjugated polyelectrolyte (CPE) poly{1,4-phenylene-[9,9-bis(4-phenoxy-butylsulfonate)]fluorene-2,7-diyl} (PBS-PFP) (degree of polymerization, DP, ∼3-6) with various concentrations of a homologous series of oppositely charged amphiphilic phenylenevinylene oligomers was investigated in water:dioxane mixtures and in aqueous micellar solutions of the non-ionic surfactant n-dodecylpentaoxyethylene glycol ether. The excellent spectral overlap between the CPE fluorescence and the conjugated oligoelectrolyte (COE) absorption indicates that energy transfer between these is a highly favored process, and can be tuned by changing the COE chain length. This is supported by time-resolved fluorescence data. The overall results provide support for different types of self-assembly, which are sensitive to the solvent environment and to the size of the phenylenevinylene oligoelectrolyte chain. It is suggested that large aggregates are formed in water:dioxane mixtures, while decorated core-shell structures are present in the surfactant solutions.

Understanding the interaction between trivalent lanthanide ions and stereoregular polymethacrylates through luminescence, binding isotherms, NMR, and interaction with cetylpyridinium chloride

Complexation of isotactic, syndiotactic, and atactic poly(methacrylic acid), PMA, with trivalent lanthanide ions has been studied in water at a degree of neutralization 0.5. Metal ion binding is shown by quenching of cerium(III) fluorescence, enhancement of Tb(III) luminescence, and lanthanide-induced line broadening in the PMA (1)H NMR spectra. Comparison with lanthanide-acetate complexation suggests carboxylate binds in a bidentate fashion, while Ce(III) luminescence quenching suggests an ≈3:1 carboxylate:metal ion stoichiometry, corresponding to charge neutralization. The presence of both free and bound Ce(III) cations in PMA solutions is confirmed from luminescence decays. Studies of Tb(3+) luminescence lifetime in H2O and D2O solutions show complexation is accompanied by loss of 5-6 water molecules, indicating that each bidentate carboxylate replaces two coordinated water molecules. The behavior depends on pH and polyelectrolyte stereoregularity, and stronger binding is observed with isotactic polyelectrolyte. Binding of cetylpyridinium chloride, CPC, in these systems is studied by luminescence, NMR, and potentiometry. NMR and Tb(3+) luminescence lifetime studies show the strongest binding with the isotactic polymer. Binding of surfactant to poly(methacrylate) in the presence of lanthanides is noncooperative, i.e., it binds to the free sites; binding isotherms in the presence of lanthanides are shifted to higher free surfactant concentrations, compared with sodium ions, have lower slopes and show a clear two-step binding mechanism. While CPC readily replaces the Na(+) ions of poly(methacrylate) and binds very strongly (low critical association concentrations), exchange is much more difficult with the strongly bound trivalent lanthanide ions. Effects of tacticity are seen, with surfactant interacting most strongly with isotactic chains in the initial stages of binding, while in the final stages of binding the interaction is strongest with atactic poly(methacrylate).