Akira Sugimoto

Intramolecular excimer formation and photoinduced electron-transfer process in bis-1,8-naphthalimide dyads depending on the linker length.

The photophysical properties of bis-1,8-naphthalimide (NI-L-NI) dyads with different linkers ( L = -C 3H 6-, -C 4H 8-, -C 6H 12-, -C 8H 16-, and -C 9H 18-) as well as the reference NI derivative (NI-C 7H 15) were investigated in CH 3CN and H 2O/CH 3CN (v/v = 1:9). The normal fluorescence peak of (1)NI*-L-NI was observed at 379 nm together with a broad emission at longer wavelength both in aprotic CH 3CN and in H 2O/CH 3CN, which is assigned to an excimer, (1)(NI-L-NI)*. The excimer emission maximum was blue-shifted with increasing length of the linker. The photoinduced electron-transfer process of NI-L-NI was also investigated in both solvents by using nanosecond-laser flash photolysis. The T 1-T n absorption band for (3)NI*-L-NI was observed around 470 nm in both solvents. In H 2O/CH 3CN, NI-L-NI is solvated with H 2O in the ground state to exist as solvated NI-L-NI. In the excited triplet state, the NI radical anion (NI (*-)) was generated via the intramolecular quenching of (3)NI*-L-NI by another NI moiety. The solvated NI (*-)-L-NI may undergo the proton abstraction process to give NI(H) (*)-L-NI, which can be confirmed by the transient absorption band at 410 nm. This band was not observed in pure aprotic CH 3CN.

Excitation Energy Dependence of Photoinduced Processes in Pentathiophene−Perylene Bisimide Dyads with a Flexible Linker

In the present paper, photoinduced processes in the dyad molecules of pentathiophene (5T) and perylene-3,4:9,10-bis(dicarboximide) (PDI) with a flexible alkyl linker (propyl or hexyl) were investigated by using femtosecond laser flash spectroscopy in various solvents. Since absorption of 5T covers the wavelength region where absorption of PDI has minima and fluorescence of 5T overlaps with absorption of PDI, combination of 5T and PDI is favorable to achieve light energy harvesting as well as efficient electron transfer. When the sample was excited at the PDI moiety of the dyad, charge separation occurred almost quantitatively even in nonpolar solvent as well as in polar solvents. When the 5T moiety of the dyad was excited, efficient energy transfer to the PDI moiety from which charge separation occurred was confirmed, indicating that 5T acts as an antenna of the charge separation system, like a photosynthesis system of a plant. On the basis of Forster and Marcus theories and the estimated energy and electron-transfer rates, it was indicated that these dyads tend to take a folded structure in all solvents investigated.

Benzophenones in the higher triplet excited states

Transient phenomena of benzophenone (BP) in the higher triplet excited state (Tn) have been investigated by the two-colour two-laser excitation method. Triplet energy transfer from BP(Tn) to quenchers (Q) occurred within the duration of a laser pulse (5 ns) to give Q(T1) with higher triplet energy than that of BP(T1). The quantum yield of the triplet energy-transfer quenching of BP(Tn) by CCl4 was found to be 0.0023 +/- 0.0002 from the bleaching of the transient absorption of BP(T1) and the absorbed photon number. It appears that internal conversion from BP(Tn) to BP(T1) is the predominant process. The lifetimes (tauTn) of BP(Tn) and several substituted benzophenones (BPs) in the higher triplet excited state [BPs(Tn)] were estimated from the dependence of the Q concentration on the efficiency of the triplet energy-transfer quenching of BP(Tn) by Q, and found to be 110-450 ps, depending on the nature of the substituents on the BPs. The effect of the substituents on tauTn may be explained by the energy gap between the Tn and T1 states, because the main deactivation pathway for BPs(Tn) is the internal conversion process. In contrast, the substituent effect on the lifetimes of BPs(T1) cannot be explained by the energy gap law. The transient behaviour of Q(T1) depends on the properties of the quencher. Sequential triplet energy transfer from Q(T1) to BP occurred for p-dichlorobenzene and tert-butylbenzene as quenchers, while Q(T1) reacted partly with Q to form triplet excimers (3Q2*) for benzene, chlorobenzene, and o-dichlorobenzene as quenchers. When CCl4 was used as the quencher, the homolytic cleavage of a C-Cl bond of CCl4(T1) occurred to give Cl* and Cl3C* radicals.

Electron transfer from oligothiophenes in the higher triplet excited states.

In the present paper, we have investigated the inter- and intramolecular electron transfer processes from the higher triplet excited state (T(n)) of oligothiophenes (3T and 4T). In the case of the intermolecular systems, two-color two-laser flash photolysis using nanosecond lasers was applied to the solution including benzophenone, oligothiophene, and halogenated benzene as a photosensitizer, an electron donor, and an electron acceptor, respectively. The first laser light irradiation generated the lowest triplet excited state (T(1)) of oligothiophene via energy transfer from benzophenone. Upon the second laser light irradiation, the absorption band of the radical cation of oligothiophene appeared with the simultaneous bleaching of the absorption band of the T(1) state, indicating the electron transfer from the T(2) state of the oligothiophene to the electron acceptor. The observed electron transfer rate dependent on the free energy change was explained on the basis of the Marcus theory. The intramolecular electron transfer in the dyad molecule of oligothiophene and acceptor was investigated using the two-color two-laser flash photolysis employing femtosecond laser. Upon the second laser light irradiation, which generates the T(n) state, the kinetic trace of the absorption band of T(1) state showed the bleaching and recovery, the rate of which depends on the driving force for the charge separation from the T(2) state of the oligothiophene. This observation suggests the existence of charge separation process from the T(2) state, and the observation of the charge-separated state was difficult probably due to the low charge separation yield and fast charge recombination.

Mesolysis Mechanisms of Aromatic Thioether Radical Anions Studied by Pulse Radiolysis and DFT Calculations

The mesolysis mechanisms for eight aromatic thioether radical anions (ArCH2SAr(•-)) generated during radiolysis in 2-methyltetrahydrofuran were studied by spectroscopic measurements and DFT calculation. Seven of ArCH2SAr(•-) underwent mesolysis via dissociation of the σ-bond between the benzylic carbon and sulfur atoms, forming the corresponding radical and anion with the stepwise mechanism or concerted mechanism. Conversely, no mesolysis in the benzyl β-naphthyl sulfide radical anion was found. From the Arrhenius analysis of the mesolysis with the stepwise mechanism, apparent activation energies (ΔEexp) were determined and compared with those (ΔEcal) estimated by the DFT calculations. Two types of C-S bond dissociation are possible to give the C radical and S anion (ArCH2(•)/ArS(-)) and the C anion and S radical (ArCH2(-)/ArS(•)). The dissociation energies (BDE(ArCH2(•)/ArS(-)) and BDE(ArCH2(-)/ArS(•))) were estimated by the DFT calculations, and BDE(ArCH2(•)/ArS(-)) were found to be smaller than BDE(ArCH2(-)/ArS(•)). The formation of ArCH2(•)/ArS(-) was observed on the mesolysis of five ArCH2SAr(•-), while one ArCH2SAr(•-) provided ArCH2(-)/ArS(•). Chemical properties governing the mesolysis mechanisms of ArCH2SAr(•-) are discussed.

Integration of grating couplers in two-story waveguides for rotary displacement sensing

An integrated-optic device, constructed by stacking two types of grating coupler in a two-story structure of waveguides, is proposed for sensing angular displacement of spindle rotation. In the first story a guided wave is diffracted by a grating coupler and becomes a sensing beam. The sensing beam is reflected by a mirror with a quarter-wave plate attached to a spindle head and is coupled back into the second story by another grating coupler. We measured the rotary displacement of the spindle by detecting variation of polarization direction of the reflected beam. A prototype device has been designed and fabricated, and the operation principle is experimentally confirmed.

Excited-State Properties of Radical Anions of C70 and Its Derivatives: Significant Differences from the Case of C60

Excited-state dynamics of the C70 radical anion (C70•–*) were investigated for the first time by applying femtosecond laser flash photolysis to reduced C70 and its derivatives, including C70•–, C70H•–, C70•–-pyromellitimide (PI), and C70•–-naphthalenediimide (NDI)•–. The transient absorption spectra of C70•– and C70H•– upon near-infrared laser excitation revealed the relaxation processes of the D1 and vibrational hot ground (D0hot) states. Chemical modification of C70 was found to decrease the excited-state lifetimes. In the cases of dyad molecules (C70•–*-PI and C70•–*-NDI•–), the deactivation occurring through dual intramolecular electron transfer (ET) pathways from both C70•– (D1) and C70•– (D0hot) was revealed by spectroscopic analysis. Furthermore, the ET pathways for C70•–*-NDI•– generating NDI2– were proposed based on the energetically preferred products. The differences observed between the dyad systems of C70•–* and C60•–* can be explained by the lower geometric symmetry of C70. The current findi...