Photo-deactivation pathways of a double H-bonded photochromic Schiff base investigated by combined theoretical calculations and experimental time-resolved studies

Interrogating the nature of aggregates formed in a model azine based ESIPT coupled AIE active probe: stark differences in photodynamics in the solid state and aggregates in water

A novel Schiff base 4-bromo-2-((E)-((E)-(1-(naphthalen-2-yl)ethylidene)hydrazono)methyl)phenol (BNHMP) was synthesized and characterized by NMR, ESI-MS, FTIR and single crystal X-ray diffraction studies. In the solution phase, BNHMP shows prominent emission from the keto-form, a consequence of excited state intramolecular proton transfer (ESIPT). The quantum yield and excited state lifetime decrease in polar solvent THF compared to relatively non-polar solvent DCM. Interestingly, in aqueous solution (pH 7.0), the quantum yield along with the excited state lifetime undergoes tremendous increment. Dynamic light scattering experiments and FESEM reveal the formation of aggregates in water as reflected by the increased hydrodynamic radius of BNHMP in water. Hence, aqueous phase studies revealed BNHMP to be an AIE active probe. On the other hand, BNHMP shows huge emission intensity in the solid state. Interestingly, the emission decay behavior of BNHMP changes upon excitation, as BNHMP shows very broad absorption in the solid state. Upon excitation at 360 nm, a triexponential decay pattern is found, which changes to a biexponential one upon excitation at 450 nm. Meticulous analysis of the fluorescence lifetimes led to the assignment of J and H aggregates coexisting in the solid state with the former dominating the photodynamics. A judicious comparison of the lifetime behavior in the solid state to that in water leads to the conclusion that BNHMP undergoes AIE by the formation of J and H aggregates to an equal extent, a phenomenon starkly different from the solid-state scenario. The current results hold significance as this is among a few reports where such comprehensive spectrodynamic dissection has been performed for an ESIPT-AIE active Schiff base in solution as well as in the solid phase, thereby giving a holistic vision of the nature and fate of aggregation occurring in such azine based systems and subsequently advancing the understanding of such systems in terms of their photo behavior.

Theoretical Investigation of Excited-State Intramolecular Double-Proton Transfer Mechanism of Substituent Modified 1, 3-Bis (2-Pyridylimino)-4,7-Dihydroxyisoindole in Dichloromethane Solution

In this paper, density functional theory (DFT) and time-dependent DFT (TDDFT) methods were used to investigate substituent effects and excited-state intramolecular double-proton transfer (ESIDPT) in 1, 3-bis (2-pyridylimino)-4, 7-dihydroxyisoindole (BPI–OH) and its derivatives. The results of a systematic study of the substituent effects of electron-withdrawing groups (F, Cl and Br) on the adjacent sites of the benzene ring were used to regulate the photophysical properties of the molecules and the dynamics of the proton-transfer process. Geometric structure comparisons and infrared (IR) spectroscopic analysis confirmed that strengthening of the intramolecular hydrogen bond in the first excited state (S1) facilitated proton transfer. Functional analysis of the reduced density gradient confirmed these conclusions. Double-proton transfer in BPI–OH is considered to occur in two steps, i.e., BPI–OH (N) [Formula: see text] BPI–OH (T1) [Formula: see text] BPI–OH (T2), in the ground state (S0) and the S1 state. The potential-energy curves (PECs) for two-step proton transfer were scanned for both the S0 and S1 states to clarify the mechanisms and pathways of proton transfer. The stepwise path in which two protons are consecutively transferred has a low energy barrier and is more rational and favorable. This study shows that the presence or absence of coordinating groups, and the type of coordinating group, affect the hydrogen-bond strength. A coordinating group enhances hydrogen-bond formation, i.e., it promotes excited-state intramolecular proton transfer (ESIPT).

Elaborating the excited-state double proton transfer mechanism and multiple fluorescent characteristics of 3,5-bis(2-hydroxypheny)-1H-1,2,4-triazole

Recently, Krishnamoorthy and coworkers reported a new type of proton transfer, which was labeled as 'proton transfer triggered proton transfer', in 3,5-bis(2-hydroxypheny)-1H-1,2,4-triazole (bis-HPTA). In this work, the excited-state double proton transfer (ESDPT) mechanism and multiple fluorescent characteristics of bis-HPTA were investigated. Upon photo-excitation, the intramolecular hydrogen bonding strength changed and the electron density of bis-HPTA redistributed. These changes will affect the proton transfer process. In S₀ state, the proton transfer processes of bis-HPTA were prohibited on the stepwise and concerted pathways. After vertical excitation to the S₁ state, the ESIPT-II process was more likely to occur than the ESIPT-I process, which was contrary to the conclusion that the ESIPT-II process is blocked and the ESIPT-II process takes place after the ESIPT-I process proposed by Krishnamoorthy and coworkers. When the K2 tautomer was formed through the ESIPT-II process, the second proton transfer process on the stepwise pathway was prohibited. On another stepwise pathway, after the ESIPT-I process (form the K1 tautomer), the second proton transfer process should overcome a higher potential barrier than the ESIPT-I process to form ESDPT tautomer. On the concerted pathway, the bis-HPTA can synchronous transfer double protons to form the ESDPT tautomer. The ESDPT tautomer was unstable and immediately converted to the K2 tautomer via a barrierless reverse proton transfer process. Thus, the fluorescent maximum at 465 nm from the ESDPT tautomer reported by Krishnamoorthy and coworkers was ascribed to the K2 tautomer. Most of the fluorophores show dual fluorescent properties, while the bis-HPTA undergoing ESDPT process exhibited three well-separated fluorescent peaks, corresponding to its normal form (438 nm), K1 tautomer (462 nm) and K2 tautomer (450 nm), respectively.

Synthesis and Photobehavior of a NewDehydrobenzoannulene-Based HOF with Fluorine Atoms: From Solution to Single Crystals Observation

Hydrogen-bonded organic frameworks (HOFs) are the focus of intense scientific research due their potential applications in science and technology. Here, we report on the synthesis, characterization, and photobehavior of a new HOF (T12F-1(124TCB)) based on a dehydrobenzoannulene derivative containing fluorine atoms (T12F-COOH). This HOF exhibits a 2D porous sheet, which is hexagonally networked via H-bonds between the carboxylic groups, and has an interlayers distance (4.3 Å) that is longer than that of a typical π–π interaction. The presence of the fluorine atoms in the DBA molecular units largely increases the emission quantum yield in DMF (0.33, T12F-COOH) when compared to the parent compound (0.02, T12-COOH). The time-resolved dynamics of T12F-COOH in DMF is governed by the emission from a locally excited state (S1, ~0.4 ns), a charge-transfer state (S1(CT), ~2 ns), and a room temperature emissive triplet state (T1, ~20 ns), in addition to a non-emissive triplet structure with a charge-transfer character (T1(CT), τ = 0.75 µs). We also report on the results using T12F-ester. Interestingly, FLIM experiments on single crystals unravel that the emission lifetimes of the crystalline HOF are almost twice those of the amorphous ones or the solid T12F-ester sample. This shows the relevance of the H-bonds in the photodynamics of the HOF and provides a strong basis for further development and study of HOFs based on DBAs for potential applications in photonics.

Substituent Effect on the Photophysics and ESIPT Mechanism of N,N'-Bis(salicylidene)-p-phenylenediamine: A DFT/TD-DFT Analysis

Excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) processes are widely exploited in the designing of organic materials for multifarious applications. This work explores the aftereffects of combining both ESIPT and ICT events in a single molecule, namely, N,N'-bis(salicylidene)-p-phenylenediamine (BSP) exploiting DFT and TD-DFT formalisms. The PBE0 functional employed in the present study is found to yield results with better accuracy for excited-state calculations. The results reveal that introduction of electron donor (-NH₂) and electron acceptor (-NO₂) substituents on BSP produces a strikingly red-shifted emission with respect to the corresponding emission from the unsubstituted analogue in polar solvents. This red-shifted emission originated due to the coupled effect of ESIPT and planar-ICT (PICT) processes from the coplanar geometry adopted by the substituted molecule (s-BSP). Based on the computed potential energy curves, the ground-state intramolecular proton transfer (GSIPT) was found to take place more favorably in s-BSP than in BSP under all solvent conditions. In the case of ESIPT, the barrier and relative energies of the phototautomers of s-BSP were slightly higher than BSP, which shows that simultaneous substitution of -NH₂ and -NO₂ groups causes slight perturbation to the ESIPT process. Overall, the computed results show that simultaneous substitution of suitable electron donor and acceptor substituents provides profitable changes in the photophysical properties of ESIPT molecules like BSP. These molecular-level insights will pave way for designing better materials for diverse applications.

Is the origin of green fluorescence in unsymmetrical four-ring bent-core liquid crystals single or double proton transfer?

The origin of green fluorescence in unsymmetrical four-ring bent-core liquid crystals (BCLCs) is not understood clearly.

Unravelling Why and to What Extent the Topology of Similar Ce-Based MOFs Conditions their Photodynamic: Relevance to Photocatalysis and Photonics

Metal-organic frameworks (MOFs) are emerging materials for luminescent and photochemical applications. Armed with femto to millisecond spectroscopies, and fluorescence microscopy, the photobehaviors of two Ce-based MOFs are unravelled: Ce-NU-1000 and Ce-CAU-24-TBAPy. It is observed that both MOFs show ligand-to-cluster charge transfer reactions in ≈100 and ≈70 fs for Ce-NU-1000 and Ce-CAU-24-TBAPy, respectively. The formed charge separated states, resulting in electron and hole generation, recombine in different times for each MOF, being longer in Ce-CAU-24-TBAPy: 1.59 and 13.43 µs than in Ce-NU-1000: 0.64 and 4.91 µs. The linkers in both MOFs also undergo a very fast intramolecular charge transfer reaction in ≈160 fs. Furthermore, the Ce-NU-1000 MOF reveals excimer formation in 50 ps, and lifetime of ≈14 ns. The lack of this interlinkers event in Ce-CAU-24-TBAPy arises from topological restriction and demonstrates the structural differences between the two frameworks. Single-crystal fluorescence microscopy of Ce-CAU-24-TBAPy shows the presence of a random distribution of defects along the whole crystal, and their impact on the observed photobehavior. These findings reflect the effect of linkers topology and metal clusters orientations on the outcome of electronic excitation of reticular structure, key to their applicability in different fields of science and technology, such as photocatalysis and photonics.

A theoretical study of the photodynamics of salicylidene-2-anthrylamine in acetonitrile solution

The ultrafast photoinduced processes of salicylidene-2-anthrylamine (2-AntSA) in acetonitrile solution have been investigated using DFT/TD-DFT static electronic structure calculations and excited state ab initio molecular dynamics simulations. Two different isoenergetic enol ground-state structures with suitable geometry for excited state intramolecular proton transfer (ESIPT) where chosen for the excited-state dynamics. The S1 relaxed potential energy profiles for the excited state intramolecular proton transfer and the N[double bond, length as m-dash]C double bond isomerization reactions predict that both reactions occur over an energy barrier and that they are competitive processes in the deactivation of the Franck-Condon state. The photodynamic simulations show that the ESIPT occurs in the femtosecond time scale for both conformers (77 and 213 fs for IA and IIA, respectively) and that the speed is modulated by the ability of the conformers to evolve toward a planar conformation in the S1 state. The trajectories predict two conical intersections which provide nonradiative relaxation pathways to the S0 state. The first one is located in the twisted enol region, where the proton transfer process is unlikely, and only occurs for the conformer IIA in a time scale ≥600 fs. The second conical intersection is located in the cis-keto region, and represents an effective depopulation channel toward the trans-keto form. All our results are in remarkably good agreement with experiments.

The photoinduced isomerization mechanism of the 2-(1-(methylimino)methyl)-6-chlorophenol (SMAC): Nonadiabatic surface hopping dynamics simulations

The photophysical properties of the Schiff base family are crucial for their applications such as molecular switches and molecular memories. However, it was found that the photophysical behavior is not uniform for all Schiff base molecules, which shows a significant substituent dependent property. In this article, we studied the photoisomerization mechanism of one Schiff base chlorosubstituted derivative 2-(1-(methylimino)methyl)-6-chlorophenol by employing geometrical optimization, energy profiles scanning, and on-the-fly dynamical simulations. Three types of minimum energy conical intersections were located on the S₁/S₀ crossing seam, with two characterized by twisting motion of the C=N bond and one featured with the excited state intramolecular proton transfer process and then twisting motion around the C=C bond [excited-state intramolecular proton transfer process (ESIPT)-then-twisting]. By a combination of the dynamics simulation results with the energy profiles scanned along with the ESIPT coordinate, it was found that the photophysical property of the targeted molecule is different from that of most Schiff base members, which prefer to decay by a twisting motion around the C=N bridge bond rather than the ESIPT-then-twisting channel. The minor ESIPT channel is probably governed by a tunneling mechanism. The proposed deactivation mechanism can provide a reasonable explanation for the observations in the experiment and would provide fundamental indications for further design of new and efficient photochromic products.

Spectroscopy and dynamics of dehydrobenzo[12]annulene derivatives possessing peripheral carboxyphenyl groups: theory and experiment

In this work, we report on the results of theoretical and experimental studies of a series of dehydrobenzoannulene (DBA) derivatives (Nu-T12 [5,6,11,12,17,18-hexadehydrotribenzo[a,e,i]cyclododecene], T12-COOMe [5,6,11,12,17,18-hexadehydro-2,3,8,9,14,15-hexakis(4-methoxycarbonylphenyl)tribenzo[a,e,i]cyclododecene] and T12-COOH [5,6,11,12,17,18-hexadehydro-2,3,8,9,14,15-hexakis(4-carboxyphenyl)tribenzo[a,e,i]cyclododecene]) in N,N'-dimethylformamide (DMF) solutions. The theoretical and experimental findings show that the S₀ → S₁ transition of these molecules is forbidden. Time-resolved spectroscopy measurements determined a lifetime of ∼100 ps of the transition from the first electronical excited (S₁) state. These molecules also emit through charge transfer (CT) species, with lifetimes of ∼1 and ∼4.5 ns. In addition to this, Nu-T12 and T12-COOMe in DMF solutions exhibit an emission from their triplet state in 35 and 24.5 ns, respectively. However, T12-COOH strongly interacts through H-bonds with DMF molecules, leading to the formation of new species having a proton-transferred character, whose emission spectrum is red-shifted and its lifetime from the S₁ state is ∼25 ns. Using nanosecond (ns) flash photolysis, we also observed the presence of non-emissive triplet states, in addition to the emissive ones. The theoretical calculations suggest that this non-radiative triplet state originates from a CT structure of the emissive triplet one. The new findings presented here elucidate the photobehaviour of three DBA derivatives of relevance to crystalline Hydrogen-Bonded Organic Framework (HOF) materials. The photophysical data provide a strong basis to explore and to better understand the photodynamics of HOF crystals.

Photochemistry of Zr-based MOFs: ligand-to-cluster charge transfer, energy transfer and excimer formation, what else is there?

Understanding of the photocatalytic behaviour of Zr-based MOFs is fundamental for the improved design of new and more efficient photocatalysts. The present work describes steady-state and photodynamical studies on the behavior of two MOFs: Zr-2,6-naphthalene dicarboxylate (Zr-NDC) and Zr-4-amino-2,6-naphthalene dicarboxylate (Zr-NADC, 65% NDC/35% NADC) in dichloromethane (DCM) and N,N-dimethylformamide (DMF) suspensions. In the DMF suspension, the Zr-NDC MOF exhibits excimer formation in 280 ps due to the interactions between neighboring linkers. Using the femtosecond transient absorption (fs-TA) technique we have found that the Zr-NADC MOF exhibits a ligand-to-cluster charge transfer (LCCT) event in ∼170 and <100 fs in DMF and DCM, respectively. The Zr-NDC MOF shows similar LCCT in <100 fs in both solvents. The Zr-NADC MOF in DMF suspension shows an energy transfer (ET) from the excited NDC linkers to the NADC ones in 1.1 ps. Flash photolysis experiments demonstrate dominant radiative electron-hole (e^--h⁺) recombination from two different trap states in 310 ns and 1.3 μs, and 32 ns and 1.0 μs in the photoexcited Zr-NADC and Zr-NDC MOFs in DMF, respectively. Excitation of the NDC linkers in the Zr-NADC and Zr-NDC MOFs in DCM allows for the characterization of the dynamics associated with the charge-separated state and the non-radiative recombination from the trap states (45 ns and 0.3 μs). Direct excitation of the NADC linkers (410 nm) in the Zr-NADC MOF in DCM suspension produces a radiative (e^--h⁺) recombination in 3.5 μs. Further experiments in the presence of electron donor (N,N,N',N'-tetramethyl-p-phenylenediamine, TMPD) and acceptor (methyl viologen, MV²⁺) molecules corroborate the formation of charge separated states in these MOFs. These findings are relevant for understanding the photocatalytic and photonic behaviours of these MOFs.

Exploring the ESIPT dynamical processes of two novel chromophores: symmetrical structure CHC and asymmetric structure CHN

The detailed ESIPT dynamical processes of CHC (symmetrical structure) and CHN (asymmetric structure) chromophores were revealed and compared using the TDDFT method at the B3LYP/6-31+G(d,p) level.

Photodynamics of Zr-based MOFs: effect of explosive nitroaromatics

The present work describes the fluorescence spectroscopy and photodynamics of two different Zr mixed-linkers MOFs (Zr-NDC/Tz and Zr-NDC/CN) and their interactions with nitroaromatics.

Tuning ESIPT fluorophores into dual emitters

Using first-principle approaches, we show how ESIPT can be controlled by fine-tuning of substituents, hence leading to new dual emitters.

Dyes undergoing excited-state intramolecular proton transfer (ESIPT) are known to present large Stokes shifts as a result of the important geometrical reorganisation following photon absorption. When the ESIPT process is not quantitative, one can obtain dual emitters characterised by two distinct fluorescence bands, observed due to emissions from both the canonical and ESIPT isomers. However, dual emission generally requires to maintain a very specific balance, as the relative excited-state free energies of the two tautomers have to be within a narrow window to observe the phenomenon. Consequently, simple chemical intuition is insufficient to optimise dual emission. In the present contribution, we investigate, with the help of quantum-mechanical tools and more precisely, time-dependent density functional theory (TD-DFT) and algebraic diagrammatic construction (ADC), a wide panel of possible ESIPT/dual emitters with various substituents. The selected protocol is first shown to be very robust on a series of structures with known experimental behaviour, and next is applied to novel derivatives with various substituents located at different positions. This work encompasses the largest chemical library of potential ESIPT compounds studied to date. We pinpoint the most promising combinations for building dual emitters, highlight unexpected combination effects and rationalise the impact of the different auxochromes.

Dye chemistry with time-dependent density functional theory

In this perspective, we present an overview of the determination of excited-state properties of "real-life" dyes, and notably of their optical absorption and emission spectra, performed during the last decade with time-dependent density functional theory (TD-DFT). We discuss the results obtained with both vertical and adiabatic (vibronic) approximations, choosing relevant examples for several series of dyes. These examples include reproducing absorption wavelengths of numerous families of coloured molecules, understanding the specific band shape of amino-anthraquinones, optimising the properties of dyes used in solar cells, mimicking the fluorescence wavelengths of fluorescent brighteners and BODIPY dyes, studying optically active biomolecules and photo-induced proton transfer, as well as improving the properties of photochromes.

Stepwise versus concerted mechanism of photoinduced proton transfer in sec-1,2-dihydroquinolines: effect of excitation wavelength and solvent composition

Excited-state proton transfer in 2,2,4,6-tetramethyl-1,2-dihydroquinoline was studied in dependence on solvent composition in binary mixtures of MeOH with nonpolar pentane and polar acetonitrile by an ultrafast pump-probe technique with λ(pump) = 350 nm and spectroscopic detection. The concerted mechanism was observed earlier at λ(pump) = 308 nm, whereas the stepwise mechanism is clearly observed at λ(pump) = 350 nm. The "fast" (200 fs) component is attributed to PT from MeOH molecule to the C(3) of the heterocycle to give the carbocationic structure in the highest vibrational ground state. This process is succeeded by the "slow" PT from the NH group to the MeO(-) anion in the cluster of MeOH molecules to give corresponding 2,3-dihydroquinoline. The former process is independent of the solvent composition, whereas the latter depends on the solvent composition in the MeOH-C5H12 mixtures with a characteristic time increasing from 15 ps in bulk MeOH to 150 ps in the mixture containing 2 vol % MeOH. This result is explained in terms of stabilization of the ionic species in the less polar microenvironment.

Single and double intramolecular proton transfers in the electronically excited state of flavone derivatives

3,7-Dihydroxyflavone derivatives containing carbonyl fragments were synthesized. Results of the fluorescent spectroscopy investigations indicate that one of them undergoes Excited State Intramolecular Double Proton Transfer (ESIDPT).

Photochromism and fluorescence modulation of pyrazolone derivatives in the solid state

Two novel photochromic pyrazolones were synthesized, and their photochromic performance was investigated in the solid state.

Ultrafast excited state hydrogen atom transfer in salicylideneaniline driven by changes in aromaticity

Quantum chemical topology shows that (i) the ultrafast excited state intramolecular proton transfer in salicylideneaniline occurs after considerable loss in aromaticity upon photoexcitation and (ii) the transferred species has a charge intermediate between that in a bare proton and a neutral hydrogen atom.

A pair of Schiff base enantiomers studied by absorption, fluorescence, electronic and vibrational circular dichroism spectroscopies and density functional theory calculation

A pair of enantiomeric Schiff bases were synthesized and characterized, in particular their absolute configurations were determined by vibrational circular dichroism spectroscopy.

A theoretical exploration of the nonradiative deactivation of hydrogen-bond complexes: isoindole–pyridine and quinoline–pyrrole

CC2 potential energy profiles of the ground and excited states of the isoindole–pyridine complex along the proton transfer reaction coordinate are studied.

Color- and morphology-controlled self-assembly of new electron-donor-substituted aggregation-induced emission compounds

Four electron-donor-substituted aggregation-induced emission (AIE) compounds, N,N'-bis(4-methoxylsalicylidene)-p-phenylenediamine (BSPD-OMe), N,N'-bis(4-methylsalicylidene)-p-phenylenediamine (BSPD-Me), N,N'-bis(salicylidene)-p-phenylenediamine (BSPD), and N,N'-bis(4-hydroxylsalicylidene)-p-phenylenediamine (BSPD-OH), are designed and synthesized. They are all found to exhibit controlled self-assembly behaviors and good thermal properties. By changing the terminal electron-donor groups, they are controlled to self-assemble into three emission colors (green, yellow, and orange) and four morphologies (microblocks, microparticles, microrods, and nanowires) in THF/water mixtures. Their self-assembled structures were investigated with scanning electron microscopy (SEM), fluorescent microscopy images, transmission electron microscopy (TEM), and powder X-ray diffraction (PXRD) techniques. In addition, the emission colors of BSPD-OH can be successfully controlled to three colors (green → yellow → orange) through simply changing the water fraction (fw). Their thermal gravimetric analysis (TGA) results indicate that their thermal decomposition temperatures (Td, corresponding to 5% weight loss) range from 282 to 319 °C. Their differential scanning calorimetry (DSC) data show that BSPD-OH bears a glass-transition temperature (Tg) of 118 °C. The good Td and Tg values will ensure them to be luminogens for organic light-emitting diodes (OLEDs). The theoretical calculations and single-crystal X-ray diffraction (XRD) analysis of BSPD-OMe and BSPD suggest that the stronger electron donor substituent can twist the molecular conformation, decrease the degree of π conjugation, increase the energy gap, and then induce the emission colors' blue shift and morphology variation. The results are meaningful in controlling the emission colors and self-assembly shapes of these derivatives, and they also provide a novel but facile way to get color-tunable AIE luminogens for OLEDs.

Femto- to micro-second photobehavior of photosensitizer drug trapped within a cyclodextrin dimer

The interactions of 5,10,15,20-tetrakis(4-sulfonatophenyl)-porphyrin (TSPP, a singlet oxygen photosensitiser molecule) with a hexa-2,4-diynediyl bridged β-cyclodextrin dimer (CD-CD) in aqueous solutions of pH 7 were studied using steady-state UV-visible absorption/emission and femto- to millisecond time-resolved spectroscopy. TSPP forms 1 : 1 complexes with CD-CD (K(e) = 1.9 × 10(8) M(-1) at 293 K). The value of K(e) indicates a high affinity of TSPP to form complexes with CD-CD. The chemical nano-cavity has a notable effect on the fluorescence lifetimes of the Q(x) state (9.3 ns in water and 10.8 ns in CD-CD). The rotational times (410 ps for TSPP in water and 0.03 ns (12%) and 1.1 ns (88%) for the TSPP:CD-CD complexes) indicate the robustness of the formed entities, and fast depolarization of emission, most probably involving the porphyrin skeleton and phenyl ring motions. The ultrafast femtosecond component (60-100 fs) of TSPP is moderately affected by the confining environment, which instead strongly influences the ps component (1-2 ps in water and 5 ps within CD-CD) assigned to the vibrational relaxation of the Q(x) state. Moreover, a 50 ps component emerges in the emission transients in the 640-720 nm range, and which is assigned to a thermalization of the hot Q(x) state. The effect of O2 on the triplet state of the encapsulated TSPP was also studied and discussed in light of the shielding effect of the CD-CD cavity. We observed comparable quantum yield (0.62 and 0.69) of the generated singlet molecular oxygen of TSPP without and with CD-CD. We believe that our results on the molecular interaction between TSPP and CD-CD from femtosecond to millisecond regime at both ground and electronically first excited states give relevant information for improving our understanding of this kind of caged drugs, and thus for a better design of drug:nanocarrier complexes. A particular implication for the use of CD-CD as a drug carrier is the high affinity of this host for complex formation with TSPP, while the yield of singlet oxygen generation is still high.

Photochromic cycle of 2'-hydroxyacetophenone azine studied by absorption and emission spectroscopy in different solvents

This paper reports on the investigations of the synthesized di-(o-hydroxyaryl ketoimine) compound by the steady state absorption and emission techniques as well as picosecond time resolved emission and femtosecond transient absorption methods in different solvents. The results of the experimental observation have been supported by the theoretical DFT and TD-DFT calculations. The theoretical data have revealed the completed influence of the environmental polarity on particular conformers of studied compound. Dependencies between the activation rate constant and polarizability function as well as Kamlet-Abbond-Taft hydrogen-bonding parameter have been obtained in different solvent. The mechanism of photodynamic changes of di-(o-hydroxyaryl ketoimine) is presented.

Femtosecond to second studies of a water-soluble porphyrin derivative in chemical and biological nanocavities

The interactions of 5,10,15,20-tetrakis(4-sulfonatophenyl)-porphyrin (TSPP) with a quaternary ammonium modified β-cyclodextrin (QA-β-CD) and human serum albumin (HSA) protein in aqueous solutions at pH 7 were studied using steady-state, stopped-flow, and femtosecond to millisecond spectroscopy. TSPP forms 1:1 and 1:2 complexes with QA-β-CD (K(1) = 1.9 × 10(5) M(-1) and K(2) = 7 × 10(3) M(-1)) at 293 K, whereas with the HSA protein only 1:1 complex (K(1) = 1.7 × 10(6) M(-1)) has been found. The chemical and biological nanocavities have notable effects on the fluorescence lifetimes of the Q(x) state (from 9.3 to 11.1 ns in QA-β-CD and 11.6 ns in HSA). Furthermore, the rotational times (400 ps for the free TSPP, 1.6 and 19 ns for QA-β-CD and HSA protein complexes, respectively) clearly indicate the robustness of the formed entities. The confined environment does not affect much the fs dynamics (0.1-0.2 ps) of the encapsulated molecule. However, it clearly affect the ps one (1-2 ps (H(2)O) and 5-10 ps (QA-β-CD and HSA)). The effect of O(2) on the relaxation of the triplet state of the free and encapsulated TSPP is also studied and the obtained results are discussed in light of the shielding effect provided by the chemical and biological cavities. The observed difference, longer triplet lifetime upon encapsulation, might be relevant to the efficiency of this porphyrin in photodynamic therapy. The presteady-state kinetics of the TSPP:HSA has been studied by the stopped-flow spectrometer, and a two-step model was proposed for the complexation processes. The results show the importance of the initial association step for the overall ligand recognition process. This first step occurs with rate constant of ~4 × 10(5) M(-1) s(-1), which is about 5 orders of magnitude larger than the rate constant of the consecutive relaxation processes. We believe that our observations of molecular interaction between TSPP, QA-β-CD, and HSA protein from femtosecond to second at both ground and electronically first excited state give detailed information to improve our understanding of this kind of system and thus for a better design of drug delivery nanocarriers.