Optical and Electron Paramagnetic Resonance Studies of the Excited States of 4-tert-Butyl-4′-Methoxydibenzoylmethane and 4-tert-Butyl-4′-Methoxydibenzoylpropane

Abstract

The excited states of 4-tert-butyl-4'-methoxydibenzoylmethane (BM-DBM) and 4-tert-butyl-4'-methoxydibenzoylpropane (BM-DBP), the model of the keto form of BM-DBM, have been studied through measurements of UV absorption, fluorescence, phosphorescence, and electron paramagnetic resonance spectra in ethanol at 77 K. The energy levels and lifetimes of the lowest excited triplet (T(1)) states of BM-DBP, dibenzoylpropane (DBP), the model of the keto form of dibenzoylmethane (DBM), and the keto and enol forms of BM-DBM and DBM were determined. The energy level of the T(1) state of the keto form is much higher than that of the enol form in BM-DBM. The effect of tautomerization on the T(1) lifetime is small in DBM but large in BM-DBM. The methoxy and tert-butyl groups play an important role in lengthening the T(1) lifetimes of BM-DBP and the keto form of BM-DBM. The nature of the T(1) states of the keto and enol forms of BM-DBM can be explained in terms of the mixing of the (3)npi* and (3)pipi* states. The observed energy levels and the lifetimes of the T(1) states of the UV absorbers provide a useful suggestion for designing more photostable UV absorbers.

Photoexcited States of UV Absorbers, Benzophenone Derivatives

“…As is clearly seen in Table that the T 1 energies of the BP derivatives studied are ~24 500 cm −1 . The T 1 energies of the BP derivatives studied are higher than those of the most widely used representative of UV‐A absorber, 4‐ tert ‐butyl‐4′‐methoxydibenzoylmethane (BMDBM, Avobenzone, Parsol 1789) and the most widely used representative of UV‐B absorber, octyl methoxycinnamate (OMC, 2‐ethylhexyl 4‐methoxycinnamate, Octinoxate, Parsol MCX) . These BP derivatives may act as triplet energy donors for BMDBM and OMC in the mixtures of UV absorbers.…”

Photoexcited Singlet and Triplet States of a UV Absorber Ethylhexyl Methoxycrylene

“…As the observed D ** value of EHMCR is fairly smaller than those of methoxyphenyl, tolyl, ethylenic and carbonyl fragments, the two unpaired electrons localize on neither the methoxyphenyl, nor the tolyl, nor the ethylenic, nor the carbonyl fragment. The delocalized character of the unpaired electrons has been reported for some UV absorbers, such as OMC, 2‐methylphenyl 4‐methoxycinnamate, dioctyl 4‐methoxybenzylidenemalonate and BMDBM . On the other hand, the EPR B min signal of the T 1 state of OCR is too weak to be observed.…”

“…The delocalized character of the unpaired electrons has been reported for some UV absorbers, such as OMC, 2-methylphenyl 4-methoxycinnamate, dioctyl 4-methoxybenzylidenemalonate and BMDBM (22)(23)(24)(25). On the other hand, the EPR B min signal of the T 1 state of OCR is too weak to be observed.…”

“…(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21) It is important to study how each UV absorber in the mixture behaves under UV irradiation. Therefore, it is especially important to determine the energy levels of the lowest excited singlet (S1) and triplet (T1) states of individual UV absorbers (22)(23)(24)(25) and discuss the nature of the photoexcited states of UV absorbers and their deactivation processes.…”

Energy-donor phosphorescence quenching study of triplet–triplet energy transfer between UV absorbers

“…1, BMDBM is excited mainly through the excitation at 355 nm (28,200 cm À 1 ) and at 365 nm (27,400 cm À 1 ). The energy levels of the lowest excited singlet (S 1 ) states of BMDBM (enol form), OMC and DOMBM have been reported to be 25,600 cm À 1 , 29,900 cm À 1 and 29,100 cm À 1 , respectively [30,34,35]. The energy levels of the S 1 states of these UV absorbers were estimated from the intersection point of the UV absorption and fluorescence spectra [36].…”