Optical and Time-Resolved Electron Paramagnetic Resonance Studies of the Excited States of a UV-B Absorber (4-Methylbenzylidene)camphor

Abstract

The excited states of UV-B absorber (4-methylbenzylidene)camphor (MBC) have been studied through measurements of UV absorption, phosphorescence, triplet-triplet (T-T) absorption, and steady-state and time-resolved electron paramagnetic resonance spectra in ethanol. The energy level and lifetime of the lowest excited triplet (T(1)) state of MBC were determined. The energy level of the T(1) state of MBC is much lower than that of photolabile 4-tert-butyl-4'-methoxydibenzoylmethane. The weak phosphorescence and strong time-resolved EPR signals, and T-T absorption band of MBC were observed. These facts suggest that the significant proportion of the lowest excited singlet (S(1)) molecules undergoes intersystem crossing to the T(1) state and the deactivation process from the T(1) state is predominantly radiationless. The quantum yields of singlet oxygen production by MBC determined by time-resolved near-IR luminescence measurements are 0.05 ± 0.01 and 0.06 ± 0.01 in ethanol and in acetonitrile, respectively. The photostability of MBC arises from the (3)ππ* character in the T(1) state. The zero-field splitting parameters in the T(1) state are D = 0.0901 cm(-1) and E = -0.0498 cm(-1). The sublevel preferentially populated by intersystem crossing is T(y) (y close to in-plane short axis and to the C═O direction).

A theoretical study of the UV absorption of 4-methylbenzylidene camphor: from the UVB to the UVA region

“…The experimental absorption spectrum of 4-MBC, in ethanol, is characterized by an intense and broad band in the UVB region. 26 This feature of the spectrum is indeed captured by our calculated vibrationally resolved spectrum of 4-MBC, thus supporting the use of the current methodology. Interestingly, this feature is conserved in both DSBC1 and DSBC2.…”

“…33 The reported spectra were simulated by using a convoluting Gaussian function presenting a half width at half maximum of 0.25 eV, in order to better compare with experimental results. 26 A maximal number of 50 overtones for each mode and 40 combinations on each pair of modes were included. The spectra were computed in implicit ethanol.…”

Photophysics of a UV‐B Filter 4‐Methylbenzylidene Camphor: Intersystem Crossing Plays an Important Role

“…To fill this gap, in this work, we have employed combined CASPT2 and CASSCF methods to systematically explore the spectroscopic properties, excited‐ and ground‐state geometric and electronic structures, relevant conical intersections and crossing points, and excited‐state deactivation pathways of 4MBC. Finally, we have proposed a corresponding photophysical mechanism to rationalize recent experimental observations …”

“…Recently, Kikuchi et al. have employed a series of experimental techniques to study UV absorption, phosphorescence, triplet–triplet (T–T) absorption, and steady‐state and time‐resolved electron paramagnetic resonance (EPR) spectra of 4MBC in ethanol and acetonitrile solutions . The observed strong EPR signal and T–T absorption band of 4MBC show that 4MBC undergoes efficient intersystem crossings to triplet states from initially populated excited singlet states.…”

Photoexcited States of UV Absorbers, Benzophenone Derivatives

“…The ideal UV absorbers should ensure that the absorbed UV energy is transformed into vibrational energy without harmful reactions. Therefore, the nature of the photoexcited states of UV absorbers and their deactivation processes have been under active investigation (5)(6)(7)(8)(9).…”