Elucidating Inner Workings of Naturally Sourced Organic Optoelectronic Materials with Ultrafast Spectroscopy

Recent advances in sustainable optoelectronics including photovoltaics, light-emitting diodes, transistors, and semiconductors have been enabled by π-conjugated organic molecules. A fundamental understanding of light-matter interactions involving these materials can be realized by time-resolved electronic and vibrational spectroscopies. In this Minireview, the photoinduced mechanisms including charge/energy transfer, electronic (de)localization, and excited-state proton transfer are correlated with functional properties encompassing optical absorption, fluorescence quantum yield, conductivity, and photostability. Four naturally derived molecules (xylindein, dimethylxylindein, alizarin, indigo) with ultrafast spectral insights showcase efficient energy dissipation involving H-bonding networks and proton motions, which yield high photostability. Rational design principles derived from such investigations could increase the efficiency for light harvesting, triplet formation, and photosensitivity for improved and versatile optoelectronic performance.

A solvent-assisted ESIPT fluorescent dye for F-/Ag+ sensing and high-resolution imaging of the cilia in live cells

A solvent-assisted ESIPT fluorescent dye was synthesized and used as a probe (2-PPN) for the detection of F^-/Ag⁺ and high-resolution imaging of the cilia in live cells. The developed ESIPT fluorophore exhibited strong tautomeric fluorescence in protic solvents and normal emission in aprotic solvents, which is a significant departure from that of conventional intramolecular ESIPT compounds. The H-binding interaction of F^- and the chelation of Ag⁺ with the ESIPT module of 2-PPN resulted in significant tautomeric emission quenching. From this basis, the 2-PPN-based assays for the detection of F^- and Ag⁺ were established. The detection limit for F^- and Ag⁺ sensing is 2.4 nM and 1.5 nM, respectively. The selective experimental results showed that no tautomeric fluorescence change of 2-PPN could be observed in the presence of the other inorganic ions in the same medium, revealing high selectivity of 2-PPN to F^- and Ag⁺. Furthermore, MTT assay experiments proved that the probe 2-PPN exhibited low cytotoxicity and good cell membrane permeability. The probe was also further successfully utilized to image the cilia in vitro MCF7 cells, displaying its high-resolution imaging performance.Graphical abstract.

Supramolecular and suprabiomolecular photochemistry: a perspective overview

In this perspective review article, we have attempted to bring out the important current trends of research in the areas of supramolecular and suprabiomolecular photochemistry. Since the spans of the subject areas are very vast, it is impossible to cover all the aspects within the limited space of this review article. Nevertheless, efforts have been made to assimilate the basic understanding of how supramolecular interactions can significantly change the photophysical and other related physiochemical properties of chromophoric dyes and drugs, which have enormous academic and practical implications. We have discussed with reference to relevant chemical systems where supramolecularly assisted modulations in the properties of chromophoric dyes and drugs can be used or have already been used in different areas like sensing, dye/drug stabilization, drug delivery, functional materials, and aqueous dye laser systems. In supramolecular assemblies, along with their conventional photophysical properties, the acid-base properties of prototropic dyes, as well as the excited state prototautomerization and related proton transfer behavior of proton donor/acceptor dye molecules, are also largely modulated due to supramolecular interactions, which are often reflected very explicitly through changes in their absorption and fluorescence characteristics, providing us many useful insights into these chemical systems and bringing out intriguing applications of such changes in different applied areas. Another interesting research area in supramolecular photochemistry is the excitation energy transfer from the donor to acceptor moieties in self-assembled systems which have immense importance in light harvesting applications, mimicking natural photosynthetic systems. In this review article, we have discussed varieties of these aspects, highlighting their academic and applied implications. We have tried to emphasize the progress made so far and thus to bring out future research perspectives in the subject areas concerned, which are anticipated to find many useful applications in areas like sensors, catalysis, electronic devices, pharmaceuticals, drug formulations, nanomedicine, light harvesting, and smart materials.

Inhibition of the prototropic tautomerism in chrysazine by p-sulfonatocalixarene hosts

This study reveals the unusual inhibition of excited-state prototropic tautomerism of Chrysazine by p-sulfonatocalix[4,6]arene hosts.

Novel 6-(1H-benzo[d]imidazol-2-yl) benzo[a]phenazin-5-ol Derivatives with Dual Emission and Large Stokes Shift Synthesis, Photophysical Properties and Computational Studies

Novel phenazine containing dyes were obtained by the condensation of 5-hydroxybenzo[a]phenazine-6-carbaldehyde and 5-chloro-benzo[a]phenazine-6-carbaldehyde with 1,2-diaminobenzene. The dyes were characterized by FT-IR, 1H NMR, elemental analysis and mass spectra. The UV-vis absorption and fluorescence emission spectra of the dyes were studied in solvents of differing polarity; the dyes exhibited excited state intra molecular proton transfer. The structural changes due to excited state intramolecular proton transfer (ESIPT) phenomenon in terms of bond angle, bond distances and geometry were investigated with the help of DFT computations. The computed absorption and emission were in agreement with the experimental absorption and emission.

Excited-state intramolecular hydrogen transfer (ESIHT) of 1,8-dihydroxy-9,10-anthraquinone (DHAQ) characterized by ultrafast electronic and vibrational spectroscopy and computational modeling

We combine ultrafast electronic and vibrational spectroscopy and computational modeling to investigate the photoinduced excited-state intramolecular hydrogen-transfer dynamics in 1,8-dihydroxy-9,10-anthraquinone (DHAQ) in tetrachloroethene, acetonitrile, dimethyl sulfoxide, and methanol. We analyze the electronic excited states of DHAQ with various possible hydrogen-bonding schemes and provide a general description of the electronic excited-state dynamics based on a systematic analysis of femtosecond UV/vis and UV/IR pump-probe spectroscopic data. Upon photoabsorption at 400 nm, the S2 electronic excited state is initially populated, followed by a rapid equilibration within 150 fs through population transfer to the S1 state where DHAQ exhibits ESIHT dynamics. In this equilibration process, the excited-state population is distributed between the 9,10-quinone (S2) and 1,10-quinone (S1) states while undergoing vibrational energy redistribution, vibrational cooling, and solvation dynamics on the 0.1-50 ps time scale. Transient UV/vis pump-probe data in methanol also suggest additional relaxation dynamics on the subnanosecond time scale, which we tentatively ascribe to hydrogen bond dynamics of DHAQ with the protic solvent, affecting the equilibrium population dynamics within the S2 and S1 electronic excited states. Ultimately, the two excited singlet states decay with a solvent-dependent time constant ranging from 139 to 210 ps. The concomitant electronic ground-state recovery is, however, only partial because a large fraction of the population relaxes to the first triplet state. From the similarity of the time scales involved, we conjecture that the solvent plays a crucial role in breaking the intramolecular hydrogen bond of DHAQ during the S2/S1 relaxation to either the ground or triplet state.

Ultrafast resonance energy transfer in the umbelliferone–alizarin bichromophore

Fast and efficient intramolecular energy transfer takes place in the umbelliferone–alizarin bichromophore; the process is well described by the Förster mechanism.

A time-resolved infrared vibrational spectroscopic study of the photo-dynamics of crystalline materials

Time-resolved infrared vibrational spectroscopy is a structurally sensitive probe of the excited-state properties of matter. The technique has found many applications in the study of molecules in dilute solution phase but has rarely been applied to crystalline samples. We report on the use of a sensitive pump-probe time-resolved infrared spectrometer and sample handling techniques for studies of the ultrafast excited-state dynamics of crystalline materials. The charge transfer excited states of crystalline metal carbonyls and the proton transfer of dihydroxyquinones are presented and compared with the solution phase.

Solvatochromic study of 1,2-dihydroxyanthraquinone in neat and binary solvent mixtures

Preferential solvation of a solvatochromic probe has been studied in binary mixtures comprising of a non-protic and a protic solvent. The non-protic solvents employed are carbon tetrachloride (CCl(4)), acetonitrile (AcN) and N,N-dimethyl formamide (DMF) and the protic solvents are methanol (MeOH) and ethanol (EtOH). The probe molecule exhibits different spectroscopic characteristics depending upon the properties of the solubilizing media. The observed spectral features provide an indication of the microenvironment immediately surrounding the probe. Solvatochromic shifts of the ground and excited states of the probe were analysed by monitoring the charge transfer absorption band and the fluorescence emission spectra in terms of the solute-solvent and solvent-solvent interactions. Fluorescence emission spectra show the dual emission due to excited state proton transfer nature of the probe molecule. The effect of solvent and the excitation energy on dual emission are also studied. The observed magnitude of the Stokes shift in the above solvents has been used to deduce experimentally the dipole moment ratio of the probe molecule for the excited state to the ground state. The dipole moment of excited state is higher than the ground state.

Reactivity of aromatic amines with triplet 1,8-dihydroxyanthraquinone: a laser flash photolysis study

The property of the lowest excited triplet states of 1,8-dihydroxyanthraquinone (DHAQ) was investigated by using time-resolved laser flash photolysis at 355nm in organic solvents, i.e. acetonitrile and cyclohexane. The transient absorption spectra of the excited triplet DHAQ were obtained in acetonitrile, which have an absorption maximum at 480nm and two broad absorption bands around 350 and 650nm. 3DHAQ(*) is efficiently quenched by triphenylamine (TPA) via photoinduced electron transfer pathway, which was testified by the finding of TPA radical cation. In addition, aniline derivatives such as N,N-dimethylaniline (DMA), 3,5,N,N-tetramethylaniline (TMA), 4-dimethylaminobenzoic acid (DMABA) and dimethyl-p-toluidine (DMT) could also quench 3DHAQ(*) rapidly. Evidence for electron transfer interaction with anilines in acetonitrile was obtained from transient spectral characterization of formed radicals. Experimental k(q) values approach the diffusion-controlled rate limit, and decrease significantly from DMT (1.85x10(10)M-1s-1) to DMABA (1.95x10(9)M-1s-1). These k(q) values depend on the charge density on the "N" atom of anilines, which could be quantified by Hammett sigma constant.

Following the Solvent Directly during Ultrafast Excited State Proton Transfer

Excited state intramolecular proton transfer in 1-chloroacetylaminoanthraquinone is investigated from the perspective of the solvent. Using a new two-dimensional nonlinear optical spectroscopy the solvent response is probed directly as the proton transfer takes place. The measurements indicate that solvent reorganization controls the proton transfer in acetonitrile by dynamically shifting the position of equilibrium in the excited state, even on subpicosecond time scales.