Azusa Kikuchi

Ultrafast Photodissociation Dynamics of a Hexaarylbiimidazole Derivative with Pyrenyl Groups: Dispersive Reaction from Femtosecond to 10 ns Time Regions

The photodissociation dynamics of a hexaarylbiimidazole (HABI) derivative with two pyrenyl groups was investigated by time-resolved transient absorption spectroscopy and fluorescence measurements. Transient absorption spectroscopy revealed that photodissociation took place in the wide time region of <100 fs to 10 ns. On the other hand, fluorescence time profiles showed the dynamic red shift in the time region <100 ps. The apparent dispersive photodissociation process was attributed to the increase in the interaction between the pyrenyl moiety in the excited state and the other moiety in the ground state, resulting in the gradual increase of the activation energy for the crossing between the attractive potential surface of an excited pyrenyl unit and the repulsive potential surface.

Photoexcited Singlet and Triplet States of a UV Absorber Ethylhexyl Methoxycrylene

The excited states of UV absorber, ethylhexyl methoxycrylene (EHMCR) have been studied through measurements of UV absorption, fluorescence, phosphorescence and electron paramagnetic resonance (EPR) spectra in ethanol. The energy levels of the lowest excited singlet (S1) and triplet (T1) states of EHMCR were determined. The energy levels of the S1 and T1 states of EHMCR are much lower than those of photolabile 4‐tert‐butyl‐4′‐methoxydibenzoylmethane. The energy levels of the S1 and T1 states of EHMCR are lower than those of octyl methoxycinnamate. The weak phosphorescence and EPR Bmin signals were observed and the lifetime was estimated to be 93 ms. These facts suggest that the significant proportion of the S1 molecules undergoes intersystem crossing to the T1 state, and the deactivation process from the T1 state is predominantly radiationless. The photostability of EHMCR arises from the 3ππ* character in the T1 state. The zero‐field splitting (ZFS) parameter in the T1 state is D** = 0.113 cm−1.

Triplet–Triplet Energy Transfer from a UV-A Absorber Butylmethoxydibenzoylmethane to UV-B Absorbers

The phosphorescence decay of a UV‐A absorber, 4‐tert‐butyl‐4′‐methoxydibenzolymethane (BMDBM) has been observed following a 355 nm laser excitation in the absence and presence of UV‐B absorbers, 2‐ethylhexyl 4‐methoxycinnamate (octyl methoxycinnamate, OMC) and octocrylene (OCR) in ethanol at 77 K. The lifetime of the lowest excited triplet (T1) state of BMDBM is significantly reduced in the presence of OMC and OCR. The observed quenching of BMDBM triplet by OMC and OCR suggests that the intermolecular triplet–triplet energy transfer occurs from BMDBM to OMC and OCR. The T1 state of OCR is nonphosphorescent or very weakly phosphorescent. However, we have shown that the energy level of the T1 state of OCR is lower than that of the enol form of BMDBM. Our methodology of energy‐donor phosphorescence decay measurements can be applied to the study of the triplet–triplet energy transfer between UV absorbers even if the energy acceptor is nonphosphorescent. In addition, the delayed fluorescence of BMDBM due to triplet–triplet annihilation was observed in the BMDBM–OMC and BMDBM–OCR mixtures in ethanol at 77 K. Delayed fluorescence is one of the deactivation processes of the excited states of BMDBM under our experimental conditions.

Photoexcited States of UV Absorbers, Benzophenone Derivatives

The UV absorption, phosphorescence and phosphorescence‐excitation spectra of benzophenone (BP) derivatives used as organic UV absorbers have been observed in rigid solutions at 77 K. The triplet–triplet absorption spectra have been observed in acetonitrile at room temperature. The BP derivatives studied are 2,2′,4,4′‐tetrahydroxybenzophenone (BP‐2), 2‐hydroxy‐4‐methoxybenzophenone (BP‐3), 2,2′‐dihydroxy‐4,4′‐dimethoxybenzophenone (BP‐6), 5‐chloro‐2‐hydroxybenzophenone (BP‐7) and 2‐hydroxy‐4‐n‐octyloxybenzophenone (BP‐12). The energy levels and lifetimes of the lowest excited triplet (T1) states of these BP derivatives were determined from the first peak of phosphorescence. The time‐resolved near‐IR emission spectrum of singlet oxygen generated by photosensitization with BP‐7 was observed in acetonitrile at room temperature. BP‐2, BP‐3, BP‐6 and BP‐12 show photoinduced phosphorescence enhancement in ethanol at 77 K. The possible mechanism of the observed phosphorescence enhancement is discussed. The T1 states of 2‐hydroxy‐5‐methylbenzophenone, 4‐methoxybenzophenone and 2,4′‐dimethoxybenzophenone have been studied for comparison.

Electronic structure of light-induced lophyl radical derived from a novel hexaarylbiimidazole with π-conjugated chromophore

A novel photochromic hexaarylbiimidazole with a bithienyl group as an extended pi-conjugation unit was synthesized and the light-induced lophyl radical was found to be stabilized due to the delocalization of an unpaired electron, and to strongly absorb near-infrared light.

Synthesis of Four- and Five-Membered Heterocycles Derived from an Iminophosphorane

Reactions of an iminophosphorane bearing the Martin ligand with a ketone, an isothiocyanate, and an alkyne gave the corresponding cycloadducts, 1,3,2 u 5 -oxazaphosphetidine, 1,3,2 u 5 -diazaphosphetidine-4-thione, and 1,2 u 5 -azaphosphetine, respectively, while that with dimethyl acetylenedicarboxylate (DMAD) and successive hydrolysis afforded the 1,2 u 5 -oxaphosphole-(2 H )-one. Thermal reactions of the cycloadducts were also studied.

Excited states of menthyl anthranilate

The photophysical properties of menthyl anthranilate (MA), a UV-A absorber, have been studied through measurements of UV absorption, fluorescence, triplet-triplet absorption and time-resolved thermal lens in ethanol at room temperature and/or 77 K. The phosphorescence and time-resolved electron paramagnetic resonance spectra have also been observed in ethanol at 77 K. The energy levels of the lowest excited singlet (S(1)) state and triplet (T(1)) state were determined. The quantum yields of fluorescence, phosphorescence and S(1) → T(1) intersystem crossing (ISC) were also determined. From the data on the lifetime and quantum yield, the rate constants of the radiative decay, internal conversion and ISC of the excited states of MA were determined. The exceptionally high ISC quantum yield of MA shows that the deactivation processes of the S(1) state of MA are different from those of most UV absorbers such as cinnamate, salicylate, 2-hydroxybenzophenone, benzotriazole and dibenzoylmethane derivatives, where the internal conversion rates of the S(1) states are much faster than the ISC rates. The observed T(1) lifetime and zero-field splitting parameters suggest that the T(1) state of MA possesses almost pure (3)ππ* character.

Photoexcited triplet states of UV-B absorbers

The excited states of UV-B absorbers, ethylhexyl triazone (EHT) and diethylhexylbutamido triazone (DBT), have been studied through measurements of UV absorption, fluorescence, phosphorescence, triplet-triplet absorption and electron paramagnetic resonance spectra in ethanol. The energy levels of the lowest excited singlet (S1) and triplet (T1) states and quantum yields of fluorescence and phosphorescence of EHT and DBT were determined. In ethanol at 77 K, the deactivation process of EHT and DBT is predominantly fluorescence, however, a significant portion of the S1 molecules undergoes intersystem crossing to the T1 state. The observed phosphorescence spectra, T1 lifetimes and zero-field splitting parameters suggest that the T1 state of EHT can be assigned to a locally excited (3)ππ* state within p-(N-methylamino)benzoic acid, while the T1 state of DBT can be assigned to a locally excited (3)ππ* state within p-(N-methylamino)benzoic acid or p-amino-N-methylbenzamide. The quantum yields of singlet oxygen generation by EHT and DBT were determined by time-resolved near-IR phosphorescence measurements in ethanol at room temperature. EHT and DBT did not exhibit significantly antioxidative properties by quenching singlet oxygen, in contrast to the study by Lhiaubet-Vallet et al.

Optical and electron paramagnetic resonance studies of the excited triplet states of UV-B absorbers

The energy levels and lifetimes of the lowest excited triplet (T1) states of UV-B absorbers, 2-ethylhexyl salicylate (EHS) and homomenthyl salicylate (HMS), and their deprotonated anions (EHS(-) and HMS(-)) were determined through measurements of phosphorescence and electron paramagnetic resonance (EPR) spectra in rigid solutions at 77 K. The observed T1 energies of EHS and HMS are higher than those of butylmethoxydibenzoylmethane, the most widely used UV-A absorber, and octyl methoxycinnamate, the most widely used UV-B absorber. The T1 states of EHS, HMS, EHS(-) and HMS(-) were assigned to almost pure (3)ππ* state from the observed T1 lifetimes and zero-field splitting parameters. EHS and HMS with an intramolecular hydrogen bond show a photoinduced phosphorescence enhancement in ethanol at 77 K. The EPR signals of the T1 states of EHS and HMS also increase in intensity with UV-irradiation time (photoinduced EPR enhancement). The T1 lifetimes of EHS and HMS at room temperature were determined through triplet-triplet absorption measurements in ethanol. The quantum yields of singlet oxygen production by EHS and HMS were determined by using time-resolved near-IR phosphorescence.

Vibrational coherence of bis-imidazole derivative, BDPI-2Y, observed by ultrafast spectroscopy

ABSTRACT Ultrafast pump-probe measurement was applied to investigate the excited state deactivation dynamics of bisimidazolyl radical, 1,4-bis-(4,5-diphenylimidazol-2-ylidene)cyclohexa-2,5-diene (BDPI-2Y). BDPI-2Y is non-fluorescent and the recovery of the ground state took place in the time scale of femtoseconds to picoseconds, indicating a direct ultrafast nonradiative decay from the excited state. Coherent intermolecular vibrations were also observed and the low-frequency mode at 150 cm−1 and 128 cm−1 had a phase shift of ∼ π compared to the mode at 290 cm−1.