Absorption and emission spectra of ultraviolet B blocking methoxy substituted cinnamates investigated using the symmetry-adapted cluster configuration interaction method

Abstract

The absorption and emission spectra of ultraviolet B (UVB) blocking cinnamate derivatives with five different substituted positions were investigated using the symmetry-adapted cluster configuration interaction (SAC-CI) method. This series included cis- and trans-isomers of ortho-, meta-, and para-monomethoxy substituted compounds and 2,4,5-(ortho-, meta-, para-) and 2,4,6-(ortho-, para-) trimethoxy substituted compounds. The ground and excited state geometries were obtained at the B3LYP/6-311G(d) and CIS/D95(d) levels of theory. All the compounds were stable as cis- and trans-isomers in the planar structure in both the S(0) and S(1) states, except the 2,4,6-trimethoxy substituted compound. The SAC-CI/D95(d) calculations reproduced the recently observed absorption and emission spectra satisfactorily. Three low-lying excited states were found to be relevant for the absorption in the UV blocking energy region. The calculated oscillator strengths of the trans-isomers were larger than the respective cis-isomers, which is in good agreement with the experimental data. In the ortho- and meta-monomethoxy compounds, the most intense peak was assigned as the transition from next highest occupied molecular orbital (next HOMO) to lowest unoccupied molecular orbital (LUMO), whereas in the para-monomethoxy compound, it was assigned to the HOMO to LUMO transition. This feature was interpreted as being from the variation of the molecular orbitals (MOs) due to the different substituted positions, and was used to explain the behavior of the excited states of the trimethoxy compounds. The emission from the local minimum in the planar structure was calculated for the cis- and trans-isomers of the five compounds. The relaxation paths which lead to the nonradiative decay were also investigated briefly. Our SAC-CI calculations provide reliable results and a useful insight into the optical properties of these molecules, and therefore, provide a useful tool for developing UVB blocking compounds with regard to the tuning of the photoabsorption.

TDDFT and MS‐CASPT2 Study of the Excited States of Para‐Methoxymethylcinnamate

“…However, the long‐range corrected DFT functionals (CAM‐B3LYP and ωB97XD) give the correct energy order for the excited states. In previous studies, the SAC‐CI and SCS‐CC2 methods gave the same energy ordering as obtained using TD‐CAM‐B3LYP, TD‐ωB97XD, and MS‐CASPT2 in this work. According to these results, the conventional DFT exchange‐correlation functionals such as B3LYP and PBE0 are inadequate for calculating the excitation energies of p ‐MMC.…”

“…The T V of the S 1 (ππ*) state calculated using MS‐CASPT2 based on SA5‐CAS(12, 14) with aug‐cc‐pVDZ basis sets (4.20 eV) is 0.52 eV smaller than that previously obtained using MS‐CASPT2/6‐31G* (4.72 eV) . Although the UV–vis spectrum of gas phase p ‐MMC is not available, the UV–vis spectra in hexane and cyclohexane both show the λ max value of 290 nm (=4.28 eV) . The T V values of the S 1 (ππ*) state calculated in hexane and cyclohexane using TD‐CAM‐B3LYP/aug‐cc‐pVDZ with the integral equation formalism polarizable continuum model (IEFPCM) were 4.25 and 4.23 eV, respectively.…”

“…In addition, although a second ππ* excited state exists (ππ*′ state), it was not considered in the aforementioned study . In other studies, results calculated using TDDFT with conventional functionals such as B3LYP and PBE0 differed from those obtained using ab initio methods (SAC‐CI and the spin‐component scaled version of the approximate coupled‐cluster singles and doubles, SCS‐CC2), which indicated that the nπ* state lies at a higher energy than the second ππ* state (ππ*′) in the vertical excitation energy ( T V ) calculations . The energy ordering of the excited states of p ‐MMC is crucial for estimating the energy barrier for internal conversion, which plays a decisive role in the UV‐blocking mechanism.…”

“…The core structure of 2‐ethylhexyl‐4‐methoxycinnamate is that of para ‐methoxymethylcinnamate ( p ‐MMC), which plays a crucial role as a UV filter. Therefore, numerous experimental and theoretical investigations have been performed in order to elucidate the mechanism of the UV‐blocking activity of p ‐MMC …”

“…Inspired by these experimental evidences, theoretical studies of the excited states of p ‐MMC generally use single reference‐based methods such as time‐dependent density functional theory (TDDFT) and symmetry adapted cluster‐configuration interaction (SAC‐CI) . Recently, multireference ab initio methods adopted for studying the mechanism of photoisomerization have shown that it takes place via S 1 /S 0 CI .…”