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

Abstract

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.

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

“…Details of emission measurements at 77 K have been described previously [23]. The excitation laser power, attenuated through a variable neutral density filter (Sigma Koki, VND-50U), was monitored by a power meter (Ophir, Nova) equipped with a thermopile sensor (Ophir, 3A) which was positioned at the front of the Dewar vessel.…”

“…The observed deviation can be explained by the triplet-triplet annihilation and/or the triplet-triplet energy transfer from BMDBM to the UV-B absorbers. The observed nonexponential decay curves were analyzed by the method reported previously [23].…”

“…The electron paramagnetic resonance and timeresolved phosphorescence spectra of the UV-B absorbers have been measured following the excitation of only BMDBM in ethanol at 77 K [29,30]. Very recently, the triplet-triplet energy transfer from BMDBM to a nonphosphorescent UV-B absorber, OCR, has been observed by measuring the phosphorescence decay of the energy donor, BMDBM [23]. We reported that the phosphorescence decay measurement of the triplet-energy donor by use of a nanosecond excitation is an effective method for studying the intermolecular triplet-triplet energy transfer even if the T 1 state of the energy acceptor is nonphosphorescent [23].…”

“…Very recently, the triplet-triplet energy transfer from BMDBM to a nonphosphorescent UV-B absorber, OCR, has been observed by measuring the phosphorescence decay of the energy donor, BMDBM [23]. We reported that the phosphorescence decay measurement of the triplet-energy donor by use of a nanosecond excitation is an effective method for studying the intermolecular triplet-triplet energy transfer even if the T 1 state of the energy acceptor is nonphosphorescent [23]. We have also reported that the effect of the addition of a new UV-B absorber, dioctyl 4-methoxybenzylidenemalonate (DOMBM, Scheme 1), on the photostability of BMDBM [30].…”

“…The strong absorption of the enol form in the UV-A range is greatly reduced after exposure to UV-A radiation and a new absorption band of the keto form appears in the UV-C range resulting in a reduction in UV-A-protective power of BMDBM [17,18]. The photoinstability of BMDBM can be reduced by the addition of UV-B absorbers, such as 2-ethylhexyl 2-cyano-3,3-diphenylacrylate (octocrylene, OCR, Scheme 1), acting as a quencher of BMDBM excited states [8,14,[19][20][21][22][23][24]. However, this combination does not cover a wide range of wavelengths sufficiently because the molar absorption coefficient of OCR is moderate in the UV-B range [25,26].…”