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The properties of the excited perylenediimide dianion (PDI 2-∗ ) and its intramolecular electron transfer (ET) behavior were examined using femtosecond laser flash photolysis on PDI 2- and PDI 2- -acceptor (A) dyads. Upon laser excitation, the dianion of PDI first generated a singlet (S 1 ) state, followed by a triplet (T 1 ) state. In all of the dyads, rate constants of the intramolecular ET from PDI 2-∗ (S 1 ) varied with the driving forces. Comparison of the PDI 2-∗ -A and PDI •-∗ -A dyad systems revealed higher rate constants for the ET from PDI 2-∗ than from PDI •-∗ , which can be attributed to the difference in electronic coupling for the ET. These findings elucidate the previously unexplored photoinduced ET characteristics of PDI 2- , thereby advancing the groundwork for photochemical studies of excited multi-ions and their applications in related materials.

For the first time, the dynamics of excited fullerene dianions and associated intramolecular electron transfer (ET) were directly investigated by using femtosecond pump-probe laser flash photolysis on selectively reduced C 60 , pyrrolidino[60]fullerene (C 60 H), and dyads including C 60 -naphthalenediimide (NDI) and C 60 -pyromellitimide (PI). Upon near-infrared laser excitation, the excited dianion of C 60 or C 60 H displayed two states with lifetimes of less than one and several tens of ps, attributed to prompt internal conversion from the theoretically predicted S n state. Furthermore, the ET processes from the excited C 60 2- in dyad molecules, including C 60 2- -NDI •- and C 60 2- -PI •- , were confirmed with varied ET rate constants due to the difference in the driving force for ET. The current findings provide a clear description of the hitherto uncharted excited-state and photoinduced ET characteristics of fullerene dianions, paving the way for photochemical studies of excited multi-ions (excited multi-polarons) and their application in organic semiconducting materials.

[ n]Cycloparaphenylenes ([ n]CPPs; n, number of phenyl rings) have gained considerable attention because they exhibit interesting properties owing to their highly strained structure and radially oriented p orbitals. Recently, [ n]CPPs with n ≥ 5 have been synthesized, but the ring-size dependence of the deactivation processes of the excited states has not been explained particularly for smaller [ n]CPPs ( n ≤ 7). In the present study, we characterized the deactivation processes of [ n]CPPs (5 ≤ n ≤ 12) using transient absorption spectroscopy at sub-pico-, sub-nano-, nano-, and microsecond time scales. Although the fluorescence quantum yield increased with the ring size, the longest S 1 -state lifetime was observed with [8]CPP, and both the decrease and increase of the ring size resulted in the decrease of the lifetime. Characterization of the intersystem crossing and internal conversion processes explained unique ring-size dependence of the deactivation processes of [ n]CPPs, i.e., the enhanced radiation rate of the larger CPP and the fast internal conversion rate of smaller CPP dominate their S 1 -state lifetimes.

Structural changes of aromatic imides upon one-electron reduction are investigated by time-resolved resonance Raman spectroscopy during pulse radiolysis. Significant downshifts are observed for both the aromatic ring stretching and carbonyl stretching modes, which are related to a reduction of the bond order and increase of the charge density on these moieties. For three aromatic imides, i.e., 1,8-naphthalene imide (1,8-NI), 2,3-naphthalene imide (2,3-NI), and naphthalene diimide (NDI), the extent of structural changes is found to follow the order: 2,3-NI > 1,8-NI > NDI, reflecting the influence of charge distribution on molecular structure. To further investigate this phenomenon, a series of homologous NDI derivatives with a substituted phenyl group at the imide position are studied. The Raman peaks between 1550 and 1600 cm -1 , which are assigned to aromatic stretching vibrations of the NDI moieties, are found to be sensitive to the charge distribution: stronger electron-withdrawing substituents result in these peaks shifting to slightly higher wavenumbers. As supported by a spin density analysis, despite the fact that the added charge is mostly localized on the NDI moiety, in the presence of an electron-withdrawing group, the subtle charge is likely to delocalize on the phenyl fragment, alleviating the effect of one-electron reduction on the molecular structure.

Strained polyaromatic carbon molecules exhibit interesting properties owing to enhanced interactions between p orbitals of sp 2 carbons; sumanene (SUM) is one of these bowl-shaped π-conjugated molecules (π-bowls). In the present study, the radical cation and radical anion of SUM were characterized by radiation chemical methods, that is, pulse radiolysis and γ-ray radiolysis. Absorption spectra of the SUM radical cation and radical anion in a wide spectral region ranging from the visible to near-IR were successfully obtained. Absorption spectra of the SUM radical cation and radical anion are similar in shape to those of the corresponding species of triphenylene (TP), which possesses a planar structure and the same π-electron system as SUM. However, the SUM radical anion showed lower peak shifts than the TP radical anion. Theoretical calculations revealed that the MOs responsible for the electronic transitions of the SUM radical anion are different from those of the TP radical anion, in contrast to the radical cations. These results demonstrate that the strains in the molecular geometries affect the electronic transitions of radical ions.

Paracyclophanes (PCPs), which exhibit interesting properties due to their transannular interactions, have been employed as a spacer in various electron transfer (ET) systems. In the present work, we investigated ET processes in dyads and triads containing [2.2]PCP or [3.3]PCP as donors to study their properties in multistep ET processes. The dyad molecules of PCP and 1,8-naphthalimide (NI) as a photosensitizing electron acceptor exhibited charge separation (CS) upon excitation of NI. In addition, triads of NI, PCP, and carbazole showed charge shift after an initial CS, thus confirming multistep ET. In this study, we demonstrated that use of [3.3]PCP in place of [2.2]PCP enhanced the initial CS rate. Lower oxidation potentials and a smaller reorganization energy for [3.3]PCP are shown to be key factors for this enhanced CS rate. Both of these properties are closely related to the strained structure of PCP; hence, the present results demonstrate the importance of strain in ET chemistry.

Stilbene (St) derivatives have been investigated for many years because of their interesting photochemical reactions such as cis-trans isomerization in the excited states and charged states and their relation to poly(p-phenylenevinylene)s. To clarify their charged state properties, structural information is indispensable. In the present study, radical cations and radical anions of St derivatives were investigated by radiation chemical methods. Absorption spectra of radical ion states were obtained by transient absorption measurements during pulse radiolysis; theoretical calculations that included the solvent effect afforded reasonable assignments. The variation in the peak position was explained by using HOMO and LUMO energy levels. Structural changes upon one-electron oxidation and reduction were detected by time-resolved resonance Raman measurements during pulse radiolysis. Significant downshifts were observed with the CC stretching mode of the ethylenic groups, indicative of the decrease in the bonding order. It was confirmed that the downshifts observed with reduction were larger than those with oxidation. On the other hand, the downshift caused by oxidation depends significantly on the electron-donating or electron-withdrawing nature of the substituents.

Hoop-shaped π-conjugated molecules have attracted much attention. In this study, the radical ions of [4]cyclo-2,7-pyrenylene ([4]CPY), a cyclic tetramer of pyrene, and [4]cyclo-4,5,9,10-tetrahydro-2,7-pyrenylene ([4]CHPY) were investigated using radiation chemical methods, namely, γ-ray radiolysis and pulse radiolysis. The absorption spectra of the radical ions of [4]CPY and [4]CHPY showed clear peaks in the near-IR and UV-vis regions similar to those of [8]cycloparaphenylene ([8]CPP). Theoretical calculations using time-dependent density functional theory provided reasonable assignments of the observed absorption bands. It was indicated that the C4-C5 and C9-C10 ethylene bonds of [4]CHPY do not contribute to the electronic transitions, resulting in absorption spectra similar to those of [8]CPP. On the other hand, it was confirmed that the allowed electronic transitions of the radical ions of [4]CPY are different from those of the radical ions of [4]CHPY and [8]CPP.

The structures of various para-substituted biphenyls (Bp-X; X = -OH, -OCH3, -CH3, -H, -CONH2, -COOH, and -CN) and their radical anions (Bp-X(•-)) were investigated by time-resolved resonance Raman spectroscopy combined with pulse radiolysis. The inter-ring C1-C1' stretching modes (ν6) of Bp-X were observed at ∼1285 cm(-1), whereas the ν6 modes of Bp-X(•-) with an electron-donating or -withdrawing substituent were significantly up-shifted. The difference (Δf) between the ν6 frequencies of Bp-X and Bp-X(•-) showed a significant dependence on the electron affinity of the substituent and exhibited a correlation with the Hammett substituent constants (σp). In contrast to Bp-H(•-) with a planar geometry, the theoretical and experimental results reveal that all Bp-X(•-) with an electron-donating or -withdrawing substituent have a slightly twisted structure. The twisted structure of Bp-X(•-) is due to the localization of the unpaired electron and negative charge density on one phenyl moiety in Bp-X(•-).

Cycloparaphenylenes (CPPs), a class of hoop-shaped conjugated macrocycles, have attracted the attention of researchers in various fields because of their interesting properties. Although their properties in the singlet-excited state have been reported, there is no systematic information on the triplet-excited state. In the present study, the properties of triplet-excited [n]CPP (n = 8-12, where n denotes the number of phenyl rings) were comprehensively investigated. The phosphorescence peak shifted to the shorter-wavelength side with increasing ring size of the CPPs, indicating smaller triplet energy for smaller CPPs. It was found that the triplet energy of a smaller CPP is even smaller than those of poly(p-phenylene)s, indicating that small hoop-shaped conjugated macrocycles are effective in realizing low-band gap materials. By applying laser flash photolysis, the Tn-T1 absorption spectra of CPPs were obtained, from which the size-dependence of energy levels of higher triplet states were also determined. Generation of singlet oxygen by the energy transfer from a triplet-excited CPP was confirmed. The generation yield became smaller with increasing size of the CPP in accordance with the triplet yield. From these observations, the size-dependence of the deactivation pathways is explained.

Electron and energy transfer processes from higher excited states are attractive for efficient photoenergy utilization because energy dissipating internal conversion processes can be avoided. In the present study, charge separation processes in meso-substituted Sb- and Ge tetraphenylporphyrins (TPPs) were investigated by means of laser spectroscopy. For both Sb- and GeTPPs, S2- and S1-fluorescence emissions were confirmed. Sb- and GeTPPs containing a meso-substituent with a higher electron donor-ability exhibited charge separation from the S2 state, and this was confirmed by transient absorption spectroscopy and a fluorescence up-conversion method. Charge separation was faster for SbTPP with a donor at the meso position than for SbTPP bearing a donor as an axial ligand. The faster charge separation of the present TPPs was attributed to a larger electronic coupling due to a larger HOMO electron density at the meso-carbon of the porphyrin ring. In addition, the charge separation processes in the present meso-substituted TPPs occurred under the adiabatic condition. Contribution of the intramolecular exciplex to the relaxation process was also indicated for the S1 state of GeTPPs.

Because of their excellent optical and electronic properties, oligofluorenes and polyfluorenes have been investigated for years. Recently developed star-shaped oligomers bearing a truxene or isotruxene core are interesting two-dimensional oligomers. Since employment of a condensed ring system will be effective in further extension of π-conjugation system, we studied electronic and vibrational properties of radical cation of CITFn, star-shaped condensed oligomer with isotruxene core and fluorene unit, by means of the radiation chemical methods. Absorption spectra of radical cation of CITFn were measured in the wide spectral range, which revealed extended π-conjugation of CITFn. Furthermore, time-resolved resonance Raman spectra during pulse radiolysis revealed that the oxidation of CITFn induced structural change to enhance quinoidal character. The Raman data and theoretical calculation indicated that the rigid framework of the present star-shaped oligomer which makes the oligomer a planar structure is quite important in extension of the conjugation pathway.

In the present study, delocalization of a positive charge in π-stacked multi-benzene rings in multilayered para- and meta-cyclophanes, in which benzene rings are connected by propyl chains to form a chromophore array with the face-to-face structure, was investigated by means of transient absorption spectroscopy during the pulse radiolysis using dichloroethane as a solvent. The local excitation and charge resonance (CR) bands were successfully observed. It was revealed that the CR band shifted to the longer wavelength side with the number of the benzene rings. The stabilization energy estimated from the peak position of the CR band showed the efficient charge delocalization over the cyclophanes. Furthermore, the CR bands showed the slight spectral change attributable to the change in distribution of the conformers. The substantially long lifetime of the CR band can be explained on the basis of the smaller charge distribution on the outer layers of the multilayered cyclophanes.

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.

In the present study, photoinduced electron transfer (ET) processes of supramolecular donor-acceptor dyads of Zn 2,3,7,8,11,12,17,18-octaethylhemiporphycene (ZnHPc) and axial ligands were investigated by using various spectroscopic methods. The formation of 1:1 dyads was confirmed by absorption spectral change during titration of axial ligand. The association constants were determined from the spectral change. Quenching of the fluorescence intensity was observed when electron acceptor ability of the axial ligand increased. The driving forces for ET were estimated based on the estimated redox potentials and structural parameters. It became clear that various ZnHPc dyads showed ET because of slightly higher donor-ability and larger excitation energy of ZnHPc when compared to the corresponding dyads of Zn porphycene (ZnPcn). The transient absorption spectra during the sub-picosecond laser flash photolysis showed the formation of charge separated state, that is, radical cation of ZnHPc and radical anion of axial ligand, from the singlet excited ZnHPc. The observed ET rates were compared with the previously reported values for Zn porphyrins and ZnPcn. The ET rates of ZnHPc were located between those observed with porphyrins and ZnPcn supramolecular dyads, even when the -DeltaG values were similar to each other. This observation was explained on the basis of the variation in reorganization energy and electronic coupling (V) values. Furthermore, distribution of HOMO electron density gave a plausible explanation for the variation in V values of these dyads.

The electron transfer processes of Zn octaethylporphycene (ZnPcn), a structural isomer of Zn octaethylporphyrin, have been investigated mainly using transient absorption spectroscopy. To form a supramolecular donor-acceptor dyad, imide compounds bearing a pyridine group at the N position of the imides have been used as an acceptor. The N atom of the pyridine ring can coordinate to the central Zn ion of ZnPcn. Formation of a supramolecular donor-acceptor dyad, that is, pentacoordinated ZnPcn, was confirmed by steady-state absorption spectroscopy using toluene as a solvent. Charge separation upon excitation of ZnPcn was indicated by efficient fluorescence quenching, especially when pyromellitic diimide was used as the acceptor. Electron transfer processes were confirmed by subpicosecond transient absorption spectroscopy, in which generation of a radical anion of the acceptor and a radical cation of ZnPcn, which was identified by means of gamma-ray radiolysis, was confirmed. It became clear that the charge separation rate was smaller than that of the corresponding supramolecular dyads of Zn tetraphenylporphyrin and Zn octaethylporphyrin despite a similar driving force. This observation indicates a larger internal reorganization energy and a smaller coupling element of the ZnPcn dyad.

Photochemical properties in the lowest singlet and triplet excited states (S(1) and T(1)) and in the higher triplet excited states (T(n)) of p-phenylbenzoyl derivatives (PB-X) having C-O and C-S bonds were investigated in solution using a stepwise two-color two-laser flash photolysis technique. PB-Xs (X = OPh and SPh as a leaving group) undergo alpha-bond dissociation in the S(1) state, while the C-O bonds in PB-OH and -OMe were stable upon the 266-nm laser flash photolysis. The T(1)(pi,pi*) states of PB-X were efficiently produced during the 355-nm laser flash photolysis of PB-X in the presence of benzophenone as a triplet sensitizer. The T(1)(pi,pi*) states of PB-Xs deactivate to the ground-state without producing any intermediates. However, when PB-Xs (X = OPh and SPh) in the T(1) states (PB-X(T(1))) were excited upon the 430-nm laser flash photolysis, their disappearance was observed. These observations indicate that PB-X(T(1)) (X = OPh and SPh) is excited to PB-Xs in the T(n) states which decompose through the C-X bond cleavage. From the transient absorption measurements, quantum yields (Phi(dec)) of the disappearance of PB-X(T(1)) were determined while bond dissociation energies (BDE) of the C-X bonds of PB-X were calculated by computations. On the basis of the Phi(dec) and the BDE values, it was shown that the rates of the decomposition process of PB-X in the T(n) states were expressed in an energy gap low form as a function of BDE. Features of the C-X bond cleavage of PB-X in the T(n) states were discussed.

Photochemical properties of p-phenylphenacyl derivatives (PP-X) having C-halide, C-S, and C-O bonds in the lowest (T 1) and higher (T n ) triplet excited states were investigated in solution by using single-color and stepwise two-color two-laser flash photolysis techniques. PP-Xs (X = Br, SH, and SPh) undergo beta-bond dissociation in the lowest singlet excited states (S 1) while the C-X bonds of other PP-Xs are stable upon 266-nm laser photolysis. The T 1(pi,pi*) states of PP-X were efficiently produced during 355-nm laser photolysis of benzophenone as a triplet sensitizer. Triplet PP-Xs deactivate to the ground state without photochemical reactions. Upon 430-nm laser photolysis of the T 1 states of PP-X (X = Br, Cl, SH, SPh, OH, OMe, and OPh), decomposition of PP-X in the T n states was found. On the basis of the changes in the transient absorption, quantum yields (Phi dec) of the decomposition of PP-X in the T n states were determined, while bond dissociation energies (BDE) of the C-X bonds were calculated by computations. According to the relationship between the Phi dec and BDE values, it was shown that the decomposition of PP-X in the T n state is due to beta-cleavage of the corresponding C-X bond, and that the state energy of the reactive T n for the C-O bond cleavage differs from that for the C-halide and C-S bond cleavage. The reaction profiles of the C-X bond cleavage of PP-X in the T n states were discussed.

Excited-state properties of radical cations of substituted oligothiophenes ( nT (*+), n denotes the number of thiophene rings, n = 3, 4, 5) in solution were investigated by using various laser flash photolysis techniques including two-color two-laser flash photolysis. nT (*+) generated by photoinduced electron transfer to p-chloranil or resonant two-photon ionization (RTPI) by using the first 355-nm ns laser irradiation was selectively excited with the second picosecond laser (532 nm). Bleaching of the absorption of nT (*+) together with growth of a new absorption was observed during the second laser irradiation, indicating the generation of nT (*+) in the excited state ( nT (*+)*). The D 1 state lifetime was estimated to be 34 +/- 4, 24 +/- 2, and 18 +/- 1 ps for 3T (*+), 4T (*+), and 5T (*+), respectively. In the presence of hole acceptor (Q), bleaching of nT (*+) and growth of Q (*+) were observed upon selective excitation of nT (*+) during the nanosecond-nanosecond two-color two-laser flash photolysis, indicating the hole transfer from nT (*+)(D 1) to Q. Recovery of nT (*+) was also observed together with decay of Q (*+) because of regeneration of nT (*+) by hole transfer from Q (*+) to nT at the diffusion-limiting rate. It was suggested that the hole transfer rate ( k HT) from nT (*+)(D 1) to Q depended on the free-energy change for hole transfer (-Delta G = 1.41-0.46 eV). The estimated k HT faster than the diffusion-limiting rate can be explained by the contribution of the static quenching for the excited species in the presence of high concentration of Q (0.1-1.0 M).

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.

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.

In the present paper, we synthesized a series of benzophenone (BP)-N-methylphthalimide (MePI) dyads (Cn, n = 3, 6, and 9, where n denotes the number of methylene in the linker) and investigated the photochemical properties and intramolecular triplet-triplet energy transfer from BP(T1) to MePI. Formation of two different intramolecular complexes was found, that is, a ground-state complex and a singlet exciplex. The formation of the triplet-equilibrium between MeBP and MePI was observed. The triplet-equilibrium constant (1.0 and 1.1 for C6 and C9, respectively) and forward ((3.8 +/- 1.3) x 107 and (3.9 +/- 1.2) x 107 s-1 for C6 and C9, respectively) and back ((3.8 +/- 1.3) x 107 and (3.6 +/- 1.2) x 107 s-1 for C6 and C9, respectively) energy transfer rates were estimated from the result of transient absorption measurements. From the van't Hoff plots, enthalpy and entropy change for the equilibrium formation were estimated.

Electron transfer (ET) reactions from the S(1) and S(2) states of some porphyrins and phthalocyanines to the axial ligand have been investigated by means of femtosecond laser flash photolysis. As the axial ligand, which acts as an acceptor, we synthesized an asymmetric pyromellitic diimide (PI) compound that has an alkyl chain and a pyridine ring on N and N' atoms, respectively. The pyridine ring of PI can coordinate to Zn of tetrapyrrole macrocycles. The coordination was confirmed by UV-vis and (1)H NMR spectra. ET from the S(1) state of Zn tetraphenylporphyrin (ZnTPP), Zn octaethylporphyrin (ZnOEP), Zn phthalocyanine (ZnPc), and Zn naphthalocyanine (ZnNc) to PI was confirmed with transient absorption spectroscopy by observing PI*-. ET from the S1 state occurred at the rate constant of (8.6 ps)(-1) - (78 ps)(-1), and the yield was almost unity. Furthermore, ET from the S(2) state of ZnTPP and ZnPc to PI was confirmed. ET from the S(2) state of ZnPc was observed for the first time. The ET rate from the S(2) state was faster than that from the S(1) state. In the case of ZnOEP-PI and ZnNc-PI complexes, ET from the S(2) state was not observed.

Photoinduced processes of a series of phosphorus tetraphenylporphyrin (PTPP) derivatives ([PTPP-(NHC6H4X)2]+Cl-, X = OCH3, CH3, H, Cl, CF3, and CN) have been investigated by using femtosecond laser flash photolysis mainly. PTPP with OH as an axial ligand showed S2 fluorescence upon excitation of the Soret band. The S2 fluorescence lifetime was estimated to be 1.5 ps. On the other hand, both S2 and S1 fluorescence bands of PTPP-(NHC6H4X)2 were difficult to observe, indicating the existence of an additional deactivation process such as charge separation (CS). From MO calculation and cyclic voltammetry, PTPP and the axial ligand are expected to act as an acceptor and a donor, respectively, upon excitation of PTPP. CS via the S2 state was confirmed during the femtosecond laser flash photolysis by observing the transient absorption of radical anion of PTPP. Furthermore, CS via the S1 state of PTPP was also observed. The CS rate via the S1 state was faster than that from the S2 state. The free energy dependence of the electron-transfer rates was discussed on the basis of Marcus theory.

We studied the energy transfer processes in the molecular array consisting of pyrene (Py), biphenyl (Ph2), and bisphthalimidethiophene (ImT), (Py-Ph2)2-ImT, during two-color two-laser flash photolysis (2-LFP). The first laser irradiation predominantly generates ImT in the lowest triplet excited state (ImT(T1)) because of the efficient singlet energy transfer from Py in the lowest singlet excited state to ImT and, then, intersystem crossing of ImT. ImT(T1) was excited to the higher triplet excited state (Tn) with the second laser irradiation. Then, the triplet energy was rapidly transferred to Py via a two-step triplet energy transfer (TET) process through Ph2. The efficient generation of Py(T1) was suggested from the nanosecond-picosecond 2-LFP. The back-TET from Py(T1) to ImT was observed for several tens of microseconds after the second laser irradiation. The estimated intramolecular TET rate from Py(T1) to ImT was as slow as 3.1 x 104 s-1. Hence, long-lived Py(T1) was selectively and efficiently produced during the 2-LFP.

Bimolecular hole transfer quenching of the 1,3,5-trimethoxybenzene radical cation (TMB*+) in the excited state (TMB*+*) by hole quenchers (Q) such as biphenyl (Bp), naphthalene (Np), anisole (An), and benzene (Bz) with higher oxidation potentials than that of TMB was directly observed during the two-color two-laser flash photolysis at room temperature. From the linear relationships between the inverse of the transient absorption changes of TMB*+ during the second 532-nm laser excitation versus the inverse of the concentration of Q, the rate constant of the hole transfer from TMB*+* to Q was estimated to be (8.5 +/- 0.4) x 10(10), (1.4 +/- 0.7) x 10(11), (1.3 +/- 0.6) x 10(11), and (6.4 +/- 0.3) x 10(10) M(-1)s(-1) for Bp, An, Np, and Bz, respectively, in acetonitrile based on the lifetime of TMB*+*. The estimated rate constants are larger than the diffusion-controlled rate constant in acetonitrile. Short lifetime, high energy, and high oxidation potential of TMB*+* cause the lifetime-dependent quenching process or static quenching process as the major process during the quenching of TMB*+* by Q as indicated by the Ware's theoretical model. The subsequent hole transfer from Q*+ to TMB, giving TMB*+, was found to occur at the diffusion-controlled rate for Bp and An as Q. For Q such as Np and Bz, the dimerization of Q*+ with Q to give dimer radical cation (Q2*+) occurred competitively with the hole transfer from Q*+ to TMB.

One-electron oxidation of alcohols such as methanol, ethanol, and 2-propanol by 1,3,5-trimethoxybenzene radical cation (TMB*+) in the excited state (TMB*+*) was observed during the two-color two-laser flash photolysis. TMB*+ was formed by the photoinduced bimolecular electron-transfer reaction from TMB to 2,3,5,6-tetrachlorobenzoquinone (TCQ) in the triplet excited-state during the first 355-nm laser flash photolysis. Then, TMB*+* was generated from the selective excitation of TMB*+ during the second 532 nm laser flash photolysis. Hole transfer rate constants from TMB*+* to methanol, ethanol, and 2-propanol were calculated to be (5.2 +/- 0.5) x 10(10), (1.4 +/- 0.3) x 10(11), and (3.2 +/- 0.6) x 10(11) M-1 s-1, respectively. The order of the hole transfer rate constants is consistent with oxidation potentials of alcohol. Formation of TCQH radical (TCQH*) with a characteristic absorption peak at 435 nm was observed in the microsecond time scale, suggesting that deprotonation of the alcohol radical cation occurs after the hole transfer and that TCQ radical anion (TCQ*-), generated together with TMB*+ by the photoinduced electron-transfer reaction, reacts with H+ to give TCQH*.

The electron transfer from the benzophenone ketyl radical in the excited state (BPH(.-)(D(1))) to several quenchers (Qs) was investigated using nanosecond/picosecond two-color two-laser flash photolysis and nanosecond/nanosecond two-color two-laser flash photolysis. The electron transfer from BPH(.-)(D(1)) to Qs was confirmed by the transient absorption and fluorescence quenching measurements. The intermolecular electron-transfer rate constants were determined using the Stern-Volmer analysis. The driving force dependence of the electron-transfer rate was revealed.

A dissociation mechanism of the S-S bond in the alpha,alpha'-dinaphthyl disulfide radical anion (NpSSNp*-) in organic solution was investigated on the basis of transient absorption measurements and DFT calculations. NpSSNp*- generated during gamma-radiolysis of NpSSNp in MTHF at 77 K showed the absorption band at 430 nm, which shifted to 560 nm with an increase of the ambient temperature up to room temperature. With the aid of DFT calculations at the B3LYP/6-31G(d) level, the shift of the absorption band was interpreted in terms of molecular conformational changes of NpSSNp*- due to the elongation of the S-S bond. It was observed that NpSSNp*- dissociates into naphthylthiyl radical and thionaphtholate anion in organic solution with a first-order rate constant in the magnitude of 10(6) s-1. From Arrhenius plots of the decay rate constants of NpSSNp*- in a temperature range of 160-293 K, an activation energy for the S-S bond cleavage in NpSSNp*- in solution was determined along with a frequency factor. Based on the state energies of NpSSNp*- calculated at the B3LYP/6-31G(d) level, a Morse-like energy potential for the S-S bond cleavage of NpSSNp*- is depicted as a function of the S-S bond distance.

The solvent effects on ketyl radicals of benzophenone derivatives (BPD) in the excited state (BPDH*(D1)) were investigated. Absorption and fluorescence spectra of BPDH*(D1) in various solvents were measured using nanosecond-picosecond two-color two-laser flash photolysis. The fluorescence peaks from BPDH*(D1) showed a shift due to the dipole-dipole interaction with the solvent molecules. The dipole moments (mu(e)) of BPDH*(D1) were estimated to be 7-10 D, indicating that BPDH*(D1) are highly polarized. It was revealed that the fluorescence lifetime (tau(f)) depends on mu(e) in acetonitrile because the stabilization by solvent molecules affects the tau(f) value in polar solvents, predominantly. On the contrary, the conformation of BPDH*(D1) plays an important role in cyclohexane because the efficiency of the unimolecular reaction from BPDH*(D1) depends on the conformation. The substituent effect on the electron transfer from BPDH*(D1) to their parent molecules was also discussed.

The properties and reactivity of the 9-xanthyl radical (X(*)) in the doublet excited state (X(*)(D(1))) were investigated using nanosecond-picosecond two-color two-laser flash photolysis. The absorption and fluorescence spectra of X(*)(D(1)) were observed for the first time. The reactivity of X(*)(D(1)) toward a series of halogen donors and electron acceptors in acetonitrile and 1,2-dichloroethane (DCE) was investigated. It is confirmed that X(*)(D(1)) has a halogen abstraction ability from a series of halogen donors. On the basis of the solvent effect on the quenching rate constants of X(*)(D(1)), an electron transfer from X(*)(D(1)) to CCl(4) was indicated.

Photodissociation of naphthalene (Np) dimer radical cation (Np2*+) to give naphthalene radical cation (Np*+) and Np and the subsequent regeneration of Np2*+ by the dimerization of Np*+ and Np were directly observed during the two-color two-laser flash photolysis in solution at room temperature. When Np2*+ was excited at the charge-resonance (CR) band with the 1064-nm laser, the bleaching and recovery of the transient absorption at 570 and 1000 nm, assigned to the local excitation (LE) and CR bands of Np2*+, respectively, were observed together with the growth and decay of the transient absorption at 685 nm, assigned to Np*+. The dissociation of Np2*+ proceeds via a one-photon process within the 5-ns laser flash to give Np*+ and Np in the quantum yield of 3.2 x 10(-3) and in the chemical yield of 100%. The recovery time profiles of Np2*+ at 570 and 1000 nm were equivalent to the decay time profile of Np*+ at 685 nm, suggesting that the dimerization of Np*+ and Np occurs to regenerate Np2*+ in 100% yield. Similar experimental results of the photodissociation and regeneration of Np2*+ were observed during the pulse radiolysis-laser flash photolysis of Np in 1,2-dichloroethane. The photodissociation mechanism can be explained based on the crossing between two potential surfaces of the excited-state Np2*+ and ground-state Np*+.

The intramolecular dimer radical cation and charge-transfer complex of various cyclophanes were investigated by using pulse radiolysis measurements. The charge resonance band due to the dimer radical cation of cyclophanes appeared in the near-IR region, which showed a blue-shift as the distance between the two benzene rings of cyclophane decreased. The stabilization energy of the dimer radical cation, which was estimated from the peak position of the charge resonance band, was explained by the exchange interaction, while the substituent effect was small. The absorption peak of the charge-transfer complex with chlorine atom also showed the shift in accordance with the oxidation potential of cyclophanes.

Radical cations of trans-stilbene and substituted trans-stilbenes (stilbenes and the radical cations denote Sand S(*+), respectively) were generated from the resonant two-photon ionization (TPI) in acetonitrile with irradiation of one-laser (266- or 355-nm laser) and with simultaneous irradiation of two-color two-lasers (266- and 532-nm or 355- and 532-nm lasers) with the pulse width of 5 ns each. The formation yields of S(*+), the TPI efficiency, depended on the properties of S in the lowest and higher singlet excited state (S(S(1)) and S(S(n))), generated from one-photon excitation with 266- or 355-nm laser and from two-photon excitation with simultaneous irradiation of 266- and 532-nm or 355- and 532-nm lasers, respectively. The TPI efficiency using two-color two-lasers increased compared with that using one-laser. It is confirmed that the TPI proceeds through two-step two-photon excitation with the S(0) --> S(1) --> S(n)() transition. In addition to the electronic character of S(S(0)) which depends on the substituent of S, oxidation potential, and molar absorption coefficient of the S(0) --> S(1) absorption as well-known important factors for the TPI efficiency, it is shown that properties of S(S(1)) and S(S(n)) such as lifetimes, electronic characters of S(S(1)) and S(S(n)), molar absorption coefficient of the S(1) --> S(n) absorption, and ionization rate from S(S(n)) are also important.

Dual intermolecular electron transfer (ELT) pathways from 4,4'-dimethoxybenzophenone (1) ketyl radical (1H*) in the excited state [1H*(D1)] to the ground-state 4,4'-dimethoxybenzophenone [1(S0)] were found in 2-methyltetrahydrofuran (MTHF) by observing bis(4-methoxyphenyl)methanol cation (1H+) and 4,4'-dimethoxybenzophenone radical anion (1*-) during nanosecond-picosecond two-color two-laser flash photolysis. ELT pathway I involved the two-photon ionization of 1H* following the injection of electron to the solvent. The solvated electron was quickly trapped by 1(S0) to produce 1*-. ELT pathway II was a self-quenching-like ELT from 1H*(D1) to 1(S0) to give 1H+ and 1*-. From the fluorescence quenching of 1H*(D1), the ELT rate constant was determined to be 1.0 x 10(10) M(-1) s(-1), which is close to the diffusion-controlled rate constant of MTHF. The self-quenching-like ELT mechanism was discussed on the basis of Marcus' ELT theory.

The C-O bond cleavage from benzophenone substituted with 4-CH2OR (p-BPCH2OR, 1-3), such as p-phenoxymethylbenzophenone (1, R= C6H5) and p-methoxymethylbenzophenone (2, R= CH3), occurred by a stepwise two-photon excitation during two-color, two-laser flash photolysis. On the other hand, no C-O bond cleavage occurred from p-hydroxymethylbenzophenone (3, R = H). The first 355-nm laser excitation of 1-3 generates p-BPCH2OR in the lowest triplet excited state (T1) which has an absorption at 532 nm. When p-BPCH2OR(T1) is excited with the second 532-nm laser to p-BPCH2OR in the higher triplet excited state (T(n)), the C-O bond cleavage occurred within the laser flash duration of 5 ns. The quantum yields of the C-O bond cleavage during the second 532-nm laser irradiation were found to be 0.015 +/- 0.007 and 0.007 +/- 0.003 for 1 and 2, respectively. Although these values are low, the diminishing 1(T1) or 2(T1) was found to convert, in almost 100% yield, to phenoxyl (C6H5O*) and p-benzoylbenzyl (BPCH2*) radicals or methoxyl (CH3O*) and BPCH2* radicals, respectively. The T(n) excitation energy, the energy barrier along the potential surface between the T(n) states and product radicals, and delocalization of the T(n) state molecular orbital including BP and CH2OR (R = C6H5, CH3, H) moieties are important factors for the occurrence of the C-O bond cleavage. It is found that the C-O bond cleavage and production of free radicals, such as BPCH2*, C6H5O*, and CH3O*, can be performed by a stepwise two-photon excitation. The present study is an example in which the chemical reactions can be selectively initiated from the T(n) state but not from the S1 and T1 states.

We have investigated the excited-state properties and singlet oxygen ((1)Delta(g)) generation mechanism in phthalocyanines (4M; M = H(2), Mg, or Zn) and in low-symmetry metal-free, magnesium, and zinc tetraazaporphyrins (TAPs), that is, monobenzo-substituted (1M), adjacently dibenzo-substituted (2AdM), oppositely dibenzo-substituted (2OpM), and tribenzo-substituted (3M) TAP derivatives, whose pi conjugated systems were altered by fusing benzo rings. The S(1)(x) and S(1)(y) states (these lowest excited singlet states are degenerate in D(4)(h) symmetry) split in the low-symmetry TAP derivatives. The excited-state energies were quantitatively determined from the electronic absorption spectra. The lowest excited triplet (T(1)(x)) energies were also determined from phosphorescence spectra, while the second lowest excited triplet (T(1)(y)) states were evaluated by using the energy splitting between the T(1)(x) and T(1)(y) states previously reported (Miwa, H.; Ishii, K.; Kobayashi, N. Chem. Eur. J. 2004, 10, 4422-4435). The singlet oxygen quantum yields (Phi(Delta)) are strongly dependent on the pi conjugated system. In particular, while the Phi(Delta) value of 2AdH(2) is smallest in our system, that of 2OpH(2), an isomer of 2AdH(2), is larger than that of 4Zn, in contrast to the heavy atom effect. The relationship between the molecular structure and Phi(Delta) values can be transformed into a relationship between the S(1)(x) --> T(1)(y) intersystem crossing rate constant (k(ISC)) and the energy difference between the S(1)(x) and T(1)(y) states (DeltaE(S)(x)(T)(y)). In each of the Zn, Mg, and metal-free compounds, the Phi(Delta)/tau(F) values (tau(F): fluorescence lifetime), which are related to the k(ISC) values, are proportional to exp(-DeltaE(S)(x)(T)(y)), indicating that singlet oxygen ((1)Delta(g)) is produced via the T(1)(y) state and that the S(1)(x) --> T(1)(y) ISC process follows the energy-gap law. From the viewpoint of photodynamic therapy, our methodology, where the Phi(Delta) value can be controlled by changing the symmetry of pi conjugated systems without heavy elements, appears useful for preparing novel photosensitizers.

Substituent effect on the lifetimes of a series of substituted naphthalenes (Np) in the higher triplet excited state (Tn) was studied with transient absorption measurements using the two-color two-laser flash photolysis technique. Lifetimes of Np(Tn) in cyclohexane solution were determined from the triplet energy transfer quenching by carbon tetrachloride to be 0.98-63 ps. The different lifetimes of Np(Tn) were explained by the energy gap law for the internal conversion from Np(Tn) to Np(T1), indicating that Np(Tn) quenched by carbon tetrachloride is assigned to Np(T2) with the longest lifetime among Np(Tn). The lifetime of Np(Tn) was correlative with the Hammett sigmap constant. Electronic characters of substituents showed a more significant influence on the energy of the T2 state than that of the T1 state.

Stepwise photocleavage of naphthylmethyl-oxygen (C-O) bonds of mono(substituted-methyl)naphthalenes [1- and 2-ROCH2Np, R = 4-benzoylphenyl (BP), phenyl (Ph), and methyl (CH3)] and bis(substituted-methyl)naphthalenes [1,8-(ROCH2)2Np and 1,4-(ROCH2)2Np, R = BP and Ph] was observed to give the naphthylmethyl radicals (NpCH2* or ROCH2NpCH2*) in almost 100% yield with two-step or three-step excitation by the two-color two-laser or three-color three-laser irradiation, respectively, at room temperature. The C-O bond cleavage quantum yields of 1-PhOCH2Np, 2-PhOCH2Np, 1,8-(PhOCH2)2Np, and 1,4-(PhOCH2)2Np were higher than those of 1-BPOCH2Np, 2-BPOCH2Np, 1,8-(BPOCH2)2Np, and 1,4-(BPOCH2)2Np. No C-O bond cleavage occurred from 1,8-(HOCH2)2Np and 2-CH3OCH2Np in the higher triplet excited state (T(n)). The experimental results show that the C-O bond cleavage was determined not only by the position of the substituents on Np but also by the type of the substituents. The C-O bond cleavage of 1-ROCH2Np was more efficient than that of 2-ROCH2Np. In the case of 1,8-(ROCH2)2Np and 1,4-(ROCH2)2Np (R = BP and Ph), the first C-O bond cleavage from the T(n) states occurred to give ROCH2-substituted naphthylmethyl radicals (1,8- and 1,4-ROCH2NpCH2*) when the T1 states, generated with the 308-nm first laser irradiation, were excited using the 430-nm second laser. The second C-O bond cleavage occurred when 1,8- and 1,4-ROCH2NpCH2* in the ground state [1,8- and 1,4-ROCH2NpCH2*(D0)] were excited to the excited states [1,8- and 1,4-ROCH2NpCH2*(D(n))] using the third 355-nm laser during the three-color three-laser flash photolysis at room temperature. It was revealed that acenaphthene was produced as the final product during the stepwise C-O bond cleavages of 1,8-(BPOCH2)2Np and 1,8-(PhOCH2)2Np. This is a successful example of stepwise cleavage of two equivalent C-O bonds in a molecule using the three-color three-laser photolysis method.

Formation of radical cation and charge-transfer complex of [3n]cyclophanes (n = 3, 5, 6) was investigated by transient absorption spectroscopy during pulse radiolysis. Radical cations of [3n]cyclophanes showed the charge resonance band around 700 nm which exhibited a blue-shift as the number of trimethylene bridges increased, indicating formation of highly stabilized intramolecular dimer radical cation of [3n]cyclophanes. The absorption peak of the charge-transfer complex with chlorine atom also showed the shift in accord with the oxidation potential of [3n]cyclophanes.

The properties and reactivities of the xanthone (Xn) ketyl radical (XnH*) in the doublet excited state (XnH*(D1)) were examined by using two-color two-laser flash photolysis. The absorption and fluorescence of XnH*(D1) were observed for the first time. Several factors governing the deactivation processes of XnH*(D1) such as interaction and reaction with solvent molecules were discussed. The remarkable change of reactivity of XnH*(D1) compared with that in the ground state (XnH*(D0)) was indicated from the experimental results. The rapid halogen abstraction of XnH*(D1) from some halogen donors such as carbon tetrachloride (CCl4) was found to occur. The halogen abstraction occurred more efficiently in the polar solvents than in the nonpolar solvents. It is suggested that the polar solvents promote the spin distribution of XnH*(D1) of the phenyl ring favorable to the halogen abstraction.