TDDFT Calculations of Electronic Spectra of Benzoxazoles Undergoing Excited State Proton Transfer

Abstract

Energies and oscillator strengths of vertical transitions for various rotameric and tautomeric species of 2-(2'-hydroxyphenyl)benzoxazole (HBO), 2,5-bis(2-benzoxazolyl)phenol (DBP) and 2,5-bis(2-benzoxazolyl)hydroquinone (BBHQ) have been calculated in the ground and first excited states with the use of TDDFT methods. The TDDFT results demonstrate good correspondence to the frequencies of absorption and fluorescence bands of the benzoxazoles reported for measurements in supersonic jets and solution, but fail to predict relative energies of the enol and keto tautomers of DBP and BBHQ in the excited state. Low intensity of the fluorescence bands attributed to the conformations of HBO and DBP that do not undergo excited state proton transfer is shown to be caused by low concentrations of the conformations in the ground state. For the three compounds large-amplitude twisting of the keto tautomer is found to be one of radiationless processes resulting in decrease of the fluorescence with a large Stokes shift.

Optical absorption and luminescence spectra of 2,5-di-(2-benzoxazolyl)phenol in the solid state

“…Finally, the quantum chemical calculations [9] demonstrate that the rotation of benzoxazole and phenol parts of the keto structure decreases the gap between the ground and first excited states, which approaches a small value, 0.28 eV. The barrier for the rotation in excited state S 1 is low (the estimated value being 0.08 eV) and the molecule can be distorted by the crystal field in its excited state.…”

“…Additional single-point calculations are needed for inclusion of double excitations. It has been found [9] that the calculations with the B3LYP functional yield a qualitatively wrong result predicting the energy of the enol structure by ca. 0.14 eV lower than that for the keto structure.…”

Theoretical modeling of two-photon absorption of benzoxazoles exhibiting excited state proton transfer

“…The most stable conformations of the molecules are considered. The molecule of DBP can exist as two conformations which have close values of energy; they are formed by the rotation of the non-hydrogen-bonded moieties [7], the most stable structure is shown in Fig. 1c.…”

Arresting consecutive steps of a photochromic reaction: studies of β-thioxoketones combining laser photolysis with NMR detection

“…We hope that the complicated scheme of processes responsible for the photochromism in b-thioxoketones can provide a challenge for the rapidly growing area of theoretical description of photoinduced proton/hydrogen transfer phenomena. [55][56][57][58][59][60][61][62][63][64][65]…”