Photoinduced electron transfer (ET) of a series of aromatic electron donors (D) to the singlet or triplet excited state of a flavin analogue (10-methylisoalloxazine: MeFl) and intermolecular back electron transfer (BET) from MeFl(*-) to D(*+) in benzonitrile (PhCN) has been investigated in light of the Marcus theory of ET. The rate constants of intermolecular photoinduced ET (k(et)) from D to the singlet excited state ((1)MeFl*) and the triplet excited state ((3)MeFl*) were determined by fluorescence quenching and enhanced decay rates of triplet-triplet (T-T) absorption by the presence of D, respectively. The k(et) values increase with an increase in the ET driving force to reach the diffusion-limit value that remains constant with a further increase in the ET driving force. Nanosecond laser flash photolysis was performed to determine the rate constants of intermolecular BET (k(bet)) from MeFl(*-) to D(*+) in PhCN. In contrast to the case of k(et), the driving force dependence of k(bet) shows a pronounced decrease towards the highly exothermic region. The reorganization energy (lambda) of intermolecular BET is determined to be 0.68 eV by applying the Marcus equation in the inverted region, where the k(bet) value decreases with increasing the BET driving force. The slowest BET was observed for BET from MeFl(*-) to N,N-dimethylaniline radical cation (DMA(*+)) with the k(bet) value of 3.5 x 10(6) M(-1) s(-1), which is 1600 times smaller than the diffusion rate constant in PhCN (5.6 x 10(9) M(-1) s(-1)). Then, DMA was linked to the 10-position of isoalloxazine to synthesize a DMA-flavin linked dyad (10-[4'-(N,N-dimethylamino)phenyl]-isoalloxazine: DMA-Fl). Photoexcitation of DMA-Fl results in photoinduced ET from the DMA moiety to the singlet excited state of Fl moiety to form the charge-separated (CS) state (DMA(*+)-Fl(*-)) that has an extremely long lifetime (2.1 ms) in PhCN at 298 K.

Inverse opal TiO2 may offer a novel and promising solution for enhancing the light harvesting efficiency of dye-sensitized solar cells (DSSCs). Its large interconnected pores enable a better penetration of the dye sensitizers via the matrix pores, making this material surpasses the efficiency of conventional TiO2 electrodes. Moreover, it also exhibits a photonic band gap that may enable a significant change in its dye absorbance by the adjustment of the photon localization near the red edge of the photonic band gap to the position of dye absorption. In this study, we report a simple method of fabrication of inverse opal TiO2, wherein the voids in artificial opal latex are filled with nanosized TiO2 particles by adding a drop of TiCl4 into the latex matrix, hydrolyzing, and heating. In this process, we investigate the effect of different heat treatment times on the properties of inverse opal TiO2. Photoacoustic (PA) characterization shows that longer heat treatment times could produce more defect sites. The presence of defects causes the inhibition of electron transfer and results in a decrease in incident photon-to-current conversion efficiency (IPCE). CdSe quantum dots were adsorbed onto inverse opal TiO2 by chemical deposition. The blue shift of PA spectra relative to the bulk CdSe and the gain in IPCE were clearly observed. This result indicates the quantum confinement effect and photosensitization of CdSe quantum dots.

Electron donor (D) substituted 3-ethoxycarbonylcoumarin (CM) derivatives [D-CM: D = 4-diphenylaminophenyl (DPA), 4-diethylaminophenyl (DEA), 4-dimethylaminophenyl (DMA), and 2-methyl-4-dimethylaminophenyl (MeDMA)] are synthesized and characterized. Photoinduced electron transfer (ET) from the D moiety to the acceptor (CM) and back electron transfer (BET) are investigated by femtosecond and nanosecond laser flash photolysis measurements. Femtosecond laser excitation at 355 nm of a deaerated acetonitrile (MeCN) solution of D-CM shows generation of the singlet charge-separated (CS) state [(1)(D(.+)-CM(.-))] by ET from D to the singlet excited state of the CM moiety ((1)CM*), and this is followed by rapid decay within 3 ns to afford the triplet excited state (D-(3)CM*). Nanosecond laser excitation of a deaerated MeCN solution of D-CM results in formation of the triplet CS state by ET from D to (3)CM*. The quantum yield of formation of the triplet CS state [(3)(DPA(.+)-CM(.-))] in the presence of iodobenzene (PhI) in deaerated MeCN increases with increasing concentration of PhI to reach 27 % at 0.5 M PhI. The triplet CS state decays by bimolecular BET because of the long CS lifetime by unimolecular BET. Formation of the long-lived triplet CS state was confirmed by electron spin resonance (ESR) measurements. The photorobust nature of DPA-CM is demonstrated by multiple laser pulse excitation (>1000 times) at 355 nm. The photoinduced ET and BET rate constants of a series of D-CM are thoroughly analyzed in light of the Marcus theory of electron transfer.

The investigations were made on photoinduced electron transfer (ET) from the singlet excited state of rubrene (1RU*) to p-benzoquinone derivatives (duroquinone, 2,5-dimethyl-p-benzoquinone, p-benzoquinone, 2,5-dichloro-p-benzoquinone, and p-chloranil) in benzonitrile (PhCN) by using the steady state and time-resolved spectroscopies. The photoinduced ET produces solvent-separated type charge-separated (CS) species and the charge-recombination (CR) process between RU radical cation and semiquinone radical anions obeys second-order kinetics. Not only the CS species but also the triplet excited state of RU (3RU*) is seen in the transient absorption spectra upon laser excitation of a PhCN solution of RU and p-benzoquinone derivatives. The comparison of their time profiles clearly suggests that the CR process between RU radical cation and semiquinone radical anions to the ground state is independent from the deactivation of 3RU*. This indicates that the CR in a highly exergonic ET occurs at a longer distance with a large solvent reorganization energy, which results in faster ET to the ground state than to the triplet excited state that is lower in energy than the CS state. Photoinduced ET from 3RU* in addition from 1RU* also occurs when p-benzoquinone derivatives with electron-withdrawing substituents were employed as electron acceptors.

We demonstrated a new approach to fabricate an ultraviolet (UV) photodetector with a pentacene transistor structure where photoactive molecules of 6-[4′-(N,N-diphenylamino)phenyl]-3-ethoxycarbonylcoumarin (DPA-CM) were introduced into a poly(methyl methacrylate) (PMMA) gate dielectric. DPA-CM molecules strongly absorb UV light and form stable charge-separation states. When a negative gate voltage was scanned to a gate electrode of the transistor, the charge-separation states of DPA-CM molecules were converted into free electrons and holes. The free electrons traversed and subsequently reached an interface of the PMMA:DPA-CM layer and a polystyrene buffer layer, inducing accumulation of additional holes in a pentacene channel. Therefore, under 2.54 mW/cm2 of 365 nm UV irradiation, a marked increase in drain current by 6.1 × 102 times were obtained from the transistor. Moreover, the phototransistor exhibited a high light responsivity of 0.12 A/W which is about one order of magnitude larger than that of a conventional pentacene phototransistor [Lucas et al., Thin Solid Films 517, 280 (2009)]. This result will be useful for manufacturing of a high-performance UV photodetector.