Specific solvent effects on the charge separation efficiency in photoinduced electron transfer processes

Bodipy Dimer for Enhancing Triplet-Triplet Annihilation Upconversion Performance

Triplet-triplet annihilation upconversion (TTA-UC) has considerable potential for emerging applications in bioimaging, optogenetics, photoredox catalysis, solar energy harvesting, etc. Fluoroboron dipyrrole (Bodipy) dyes are an essential type of annihilator in TTA-UC. However, conventional Bodipy dyes generally have large molar extinction coefficients and small Stokes shifts (<20 nm), subjecting them to severe internal filtration effects at high concentrations, and resulting in low upconversion quantum efficiency of TTA-UC systems using Bodipy dyes as annihilators. In this study, a Bodipy dimer (B-2) with large Stokes shifts was synthesized using the strategy of dimerization of an already reported Bodipy annihilator (B-1). Photophysical characterization and theoretical chemical analysis showed that both B-1 and B-2 can couple with the red light-activated photosensitizer PdTPBP to fulfill TTA-UC; however, the higher fluorescence quantum yield of B-2 resulted in a higher upconversion efficiency (η_UC) for PdTPBP/B-2 (10.7%) than for PdTPBP/B-1 (4.0%). This study proposes a new strategy to expand Bodipy Stokes shifts and improve TTA-UC performance, which can facilitate the application of TTA-UC in photonics and biophotonics.

Enantioselective Total Synthesis of Dysiherbols A, C, and D

Herein, we report the enantioselective total synthesis of dysiherbols A, C, and D, a unique group of 6/6/5/6/6 pentacyclic quinone/hydroquinone sesquiterpenes, featuring a photo-induced quinone-alkene [2 + 2] cycloaddition and a tandem [1,2]-anionic rearrangement/cyclopropane fragmentation as key elements. Based on our total synthesis, the originally proposed structures of dysiherbols C and D have been revised. Detailed computational studies were carried out to gain deep insight into the unprecedented [1,2]-anionic rearrangement, which revealed that the transformation, albeit a symmetry-forbidden process, proceeded through a concerted manner owing to the release of high ring-strain energy and the evolution of local aromaticity in the transition state. Taking all, the present work offers a mechanistically interesting and synthetically useful approach to accessing dysiherbols and related congeners.

The Rehm-Weller experiment in view of distant electron transfer

The driving-force dependence of bimolecular fluorescence quenching by electron transfer in solution, the Rehm-Weller experiment, is revisited. One of the three long-standing unsolved questions about the features of this experiment is carefully analysed here, that is, is there a diffusional plateau? New experimental quenching rates are compiled for a single electron donor, 2,5-bis(dimethylamino)-1,3-benzenedicarbonitrile, and eighteen electron acceptors in acetonitrile. The data are analysed in the framework of differential encounter theory by using an extended version of the Marcus theory to model the intrinsic electron-transfer step. Only by including the hydrodynamic effect and the solvent structure can the experimental findings be well modelled. The diffusional control region, the "plateau", reveals the inherent distance dependence of the reaction, which is shown to be a general feature of electron transfer in solution.

Primary electron-transfer dynamics in 2-phenylindole-9-cyanoanthracene system. A comparative study with 2-methylindole

Electrochemical measurements by cyclic voltammetry predict the possibility of occurrence of photoinduced electron-transfer (PET) reactions between the ground state of 2-phenylindole (2PI) (electron donor) and the excited singlet of 9-cyanoanthracene (9CNA) molecule acting as an electron acceptor. However, 2PI should be expected to behave as a relatively weaker electron donating agent than the structurally related donor 2-methylindole (2MI) as it possesses higher oxidation potential value. Both steady-state and time-resolved spectroscopic measurements in the polar acetonitrile (ACN) and ethanol (EtOH) solvents show that the fluorescence quenching phenomenon of 9CNA in presence of 2PI is primarily due to the involvement of dynamic process which in high probability should be PET. Nevertheless, in less polar tetrahydrofuran (THF) medium, the quenching of 9CNA results from the combined effect of dynamic and static modes. The transient absorption spectra, measured by using nanosecond laser flash photolysis, of 9CNA in presence of 2PI exhibit the signature of the bands of the anionic species of 9CNA, cation of the donor 2PI and the contact neutral radical. Observations of the transient absorption at the different delays infer that ion-recombination mechanism is responsible for production of the monomeric triplets of both 9CNA and 2PI. From the transient absorption decays in ACN medium, it has been demonstrated that the diffusional separation of ions from geminate ion-pair is facilitated in the case of 2MI-9CNA pair whereas for 2PI-9CNA system the energy wasting charge recombination dominates over the process of charge dissociation. From the above observations, the possibility of developing much potential photosynthetic model compounds with the donor 2MI, rather than with the other donor 2PI molecule has been hinted.

Synthesis and studies on spectroscopic as well as electron donating properties of the two alkoxy benzo[b]thiophenes

Synthesis, characterization, steady state and time resolved, using time correlated single photon counting as well as laser flash photolysis techniques, spectroscopic investigations were made for two alkoxy benzo[b]thiophene molecules: 5-methoxy benzo[b]thiophene (5MBT) and 5-methoxymethyl benzo[b]thiophene (5MMBT). In both non-polar n-heptane (NH) and polar acetonitrile (ACN) solvents and at ambient temperature the electronic absorption spectra of these thiophenes exhibit different band systems whose assignments were made from the measurements of the steady state excitation polarization spectra. Steady state fluorescence spectra of these molecules in the different polarity solvents show the presence of non-specific interactions. From the redox properties of the benzothiophenes, measured by cyclic voltammetry, their electron donating properties were observed in the presence of the well-known electron acceptor 9cyanoanthracene (9CNA). Further, detailed studies by laser flash photolysis techniques show that ion-recombination mechanism predominates after the initial excitation of the acceptor moiety using the third harmonic of Nd:YAG laser. This recombination together with the external heavy atom effect (the donor containing 'sulphur' atom) appears to be responsible for the formation of the triplet of the monomeric acceptor 9CNA. From the steady state experiments it is shown that both in non-polar NH and highly polar ACN the quenching in the fluorescence emission of 9CNA in the presence of the benzothiophene donors is brought about primarily by the external heavy atom effect and in ACN, although the presence of the photoinduced ET reaction is confirmed, this process seems, from the observed bimolecular dynamic quenching rate, kq to be significantly masked by the external heavy atom effect.