Transient Absorption and Resonance Raman Investigations on the Axial Ligand Photodissociation of Halochromium(III) Tetraphenylporphyrin

The axial ligand photodissociation processes of halochromium(III) tetraphenylporphyrin (XCrIIITPP, X = Cl, Br) have been investigated in noncoordinating and coordinating solvents by transient Raman and absorption spectroscopic techniques. In noncoordinating solvents such as benzene, the upshift of the ν2 and ν4 bands and the disappearance of CrX stretching mode in the transient Raman spectra demonstrate the core size reduction of the porphyrin macrocycle accompanied by the photodissociation of axial halogen ligand atoms in the excited state. In coordinating solvents such as tetrahydrofuran (THF), where the solvent molecule is already attached to XCrIIITPP as an axial ligand to form XCrIIITPP(THF), the transient spectroscopic data indicate that the axial halogen ligand atoms photodissociate to form the five-coordinate CrIIITPP(THF) on photoexcitation. The temporal evolutions of photoinduced absorption and bleaching signals of XCrIIITPP in benzene exhibit biphasic decay profiles with time constants of 1 and 20 ms. The shorter decay is likely due to the four-coordinate photoexcited CrIIITPP* species, and the relatively slow decay component seems to be the recombination process returning to the original five-coordinate XCrIIITPP species. On the other hand, a significant reduction in the lifetime of photoexcited ClCrIIITPP in THF was observed as compared with that in benzene. This behavior seems to be caused by the excited five-coordinate CrIIITPP(THF)* species, which decays rapidly with a time constant of 632 ps due to the participation of low-energy states in the deactivation process below the normally emissive tripmultiplet (π,π*) states. The electronic nature of the lowest excited state of the five-coordinate CrIIITPP(THF)* species is suggested to possess (π,dπ) charge transfer character based on the comparison of transient Raman and absorption spectral features with those of other paramagnetic metalloporphyrins. We explain the axial ligand photodissociation processes in terms of the electron density change in metal d orbitals, which is particularly sensitive to the interaction with σ-donor axial ligands

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 = −C3H6−, −C4H8−, −C6H12−, −C8H16−, and −C9H18−) as well as the reference NI derivative (NI−C7H15) were investigated in CH3CN and H2O/CH3CN (v/v = 1:9). The normal fluorescence peak of 1NI*−L−NI was observed at 379 nm together with a broad emission at longer wavelength both in aprotic CH3CN and in H2O/CH3CN, 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 T1−Tn absorption band for 3NI*−L−NI was observed around 470 nm in both solvents. In H2O/CH3CN, NI−L−NI is solvated with H2O 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 3NI*−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 CH3CN

Fluorescence Properties of Si-Linked Oligothiophenes

The fluorescence properties of Si-linked oligothiophenes were investigated by using various spectroscopic methods. From the molecular orbital calculation, it was indicated that Si-linked oligothiophenes tend to take structures with a smaller repulsion between oligothiophene entities and methyl groups on the Si-atom. Steady state absorption spectra showed red-shifted absorption peaks due to the σ−π conjugation when compared with the corresponding oligothiophenes. Peak shift to the longer wavelength side was also confirmed in the fluorescence spectra. Si-linked terthiophenes showed excimer formation upon excitation with a pulsed laser, although its formation yield was as small as ∼0.1−0.2. On the other hand, Si-linked pentathiophenes do not form the excimer probably because of the lower mobility of larger oligothiophenes. From the anisotropy decay, excitation energy migration among the oligothiophenes was confirmed. The excitation energy migration was discussed on the basis of the incoherent energy hopping mechanism. The effect of σ−π conjugation on the present energy transfer was indicated

Singlet Energy Migration along an Alternating Block Copolymer of Oligothiophene and Oligosilylene in Solution

The singlet excited-state properties of the block copolymers of oligothiophene and oligosilylene in solution were investigated with several fast spectroscopic methods. Time-resolved fluorescence measurements at room temperature and in a glassy matrix revealed that the singlet excited states of the block copolymers are deactivated accompanying structural changes of the polymer. It became clear from the transient absorption spectroscopy that the absorption peak of the singlet excited state shifted to the longer wavelength side compared to that of the corresponding oligothiophenes because of the σ−π conjugation of the oligothiophene and oligosilylene. The intersystem crossing process generating the triplet excited state was also revealed by the transient absorption spectroscopy. Energy migration along the polymer chain was revealed by the fluorescence anisotropy measurements. The time constant for the energy migration became faster as the size of the oligothiophene in the polymer repeating unit became shorter. From comparison with the Förster theory, the energy migration process was attributed to an incoherent hopping mechanism

Intramolecular Electron Transfer from Axial Ligand to S₂-Excited Sb-Tetraphenylporphyrin

The S2 state properties of Sb-tetraphenylporphyrin (SbTPP) derivatives were investigated using subpicosecond spectroscopic methods. The S2 fluorescence of various SbTPP derivatives was observed for the first time. It was revealed that the S2 fluorescence lifetime changed depending on the donor-ability of the ligand because of the contribution of the charge separation to the S2 excited SbTPP, which was confirmed by transient absorption spectroscopy

Detection of Structural Changes upon One-Electron Oxidation and Reduction of Stilbene Derivatives by Time-Resolved Resonance Raman Spectroscopy during Pulse Radiolysis and Theoretical Calculations

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(<i>p</i>-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

Reaction-Environment-Dependent Photoaddition Reactions of <i>N</i>‑Phenyl Amino Acid Esters Possessing a Silyl Group with Fullerene C₆₀: Selective Formation of Aminomethyl-1,2-dihydrofullerenes vs Fulleropyrrolidines

The current study investigates SET-promoted photoaddition reactions of the silyl-group-containing N-phenylglycinates and N-phenylalaninates, N-((trimethylsilyl)­methyl)-N-phenyl-substituted glycinates and alaninates, respectively, with fullerene C60 to explore how the types of amino acid esters (AAEs) and molecular oxygen affect the photoaddition reaction efficiencies and chemoselectivity of in situ formed radical cations of AAEs. The results showed that under deoxygenated (N2-purged) conditions, photoreactions of N-phenylglycinates with C60 produced aminomethyl-1,2-dihydrofullerenes through the addition of α-amino radicals arising by sequential SET and desilylation processes from initially formed secondary anilines to C60. In oxygenated conditions, photoreactions of N-phenylglycinates with C60, albeit less efficient, took place to form fulleropyrrolidines through a pathway involving 1,3-dipolar cycloaddition of azomethine ylides to C60 assisted by in situ formed 1O2. The same types of photoproducts were observed with N-phenylalaninates, though the reactions were less efficient. The use of methylene blue (MB) as a photosensitizer in the photoreactions under oxygenated conditions was especially effective in enhancing the efficiency of fulleropyrrolidine formation. These results demonstrate that photoaddition reactions of silyl-tether-containing N-phenyl AAEs with C60 can be governed by the reaction conditions and the presence or absence of a photosensitizer employed

Structural Study of Various Substituted Biphenyls and Their Radical Anions Based on Time-Resolved Resonance Raman Spectroscopy Combined with Pulse Radiolysis

The structures of various <i>para</i>-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 (Δ<i>f</i>) 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 (σ_p). 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^•–

Transannular Distance Dependence of Stabilization Energy of the Intramolecular Dimer Radical Cation of Cyclophanes

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

Structural Relaxation in the Singlet Excited State of Star-Shaped Oligofluorenes Having a Truxene or Isotruxene as a Core

Oligofluorenes attract wide attention due to their excellent fluorescent properties. For the detailed understanding of the excited state properties, ultrafast processes have to be clarified. Here, we have investigated the structural relaxation in the singlet excited state of star-shaped oligofluorenes with a truxene or isotruxene core, to which oligofluorenes (n = 1–4) were attached. The transient absorption peak showed red-shift with time upon excitation. The fluorescence decay profiles in the picosecond domain showed the fast component in addition to the component corresponding to the singlet excited state lifetime. These ultrafast phenomena can be attributed to the structural relaxation, i.e., planarization, in the singlet excited state. The planarization process was supported by the theoretical calculation based on the time-dependent density functional theory. Furthermore, dependence of two-photon absorption cross section on the core of the star-shaped oligofluorene has been elucidated