Does diphenylacetylene (tolan) fluoresce from its second excited singlet state? Semiempirical MO calculations and fluorescence quantum yield measurements

Abstract

It is confirmed by measurements of fluorescence spectra and quantum yields that the fluorescence in tolan originates from the same state that causes the absorption band at lowest energy. The temperature dependence of the fluorescence quantum yield shows that this state is thermally deactivated with an activation energy of Ea = 14.0 kJ/mol. Geometry optimizations of the states SO, SI, and TI of tolan with the semiempirical AM1 method lead to planar structures with Dzh symmetry. Potential energy curves along the triple-bond stretching coordinate have been calculated for several low-lying excited states with a combination of the AM1 and the INDO/S methods. It is found that for large triple-bond lengths, the llA,-state with UT* character becomes thelowest excited singlet state. It is proposed that thermal deactivation of SI( llBlu) leads to this state. Nonvertical excitation of 11A, could explain the weak lines found in supersonic jet experiments below the onset of the llA, -l1B1,, transition.

Recent Developments in Ultrafast Time-Resolved Vibrational Spectroscopy of Electronically Excited States

“…Thus, the CC stretch frequency of S1 DPA has been obtained as 1567 cm -1 , which suggests a double bond character of the central CC bond. This very low CC frequency is in good harmony with a quantum chemical calculation that predicted an extraordinary long CC bond distance for the S1 state [18].…”

“…Diphenylacetylene (DPA) is known for its extraordinary photophysics, which is thought to be caused by strong interactions of closely lying excited singlet states with the environments [15][16][17][18]. In rare gas matrices, a clear mirror-image relationship between the ultraviolet absorption and fluorescence spectra was found, indicating that the emission originates from the excited singlet state to which the absorption terminates [16].…”

“…Thus, in solution, the emitting state is not the S i state but the S2 state. In order to fill the gap between the photophysics in matrices and solution, Ferrante et al proposed a scheme which was based on empirical molecular orbital calculations [18]. According to their scheme, the ground-state absorption is assigned to the So(A1g)-S2(B1u) transition.…”

Photophysical Properties of Diphenylacetylene and Diphenylbutadiyne Derivatives in Solution Phase

“…The absorption band of DPA observed at 290 mn was assigned to the 1B1u state, while the lowest excited singlet state was predicted to be 2 1Αg state [8]. Recent INDO/S calculation predicted that the S1 state has Au symmetry [6]. Under the supersonic beam conditions, the energy gap between the 1Β1u and 2 1Αg states is less *The results of this paper were initially presented at The Jabłoński Centennial Conference on Luminescence and Photοphysics, July 28-27, 1998, Toruń, Poland. than 300 cm -1 [9].…”

“…The level inversion of the lowest excited singlet states of dicyano-diphenylbutadiyne was observed. The thermally activated S2 <-S1 internal conversion was not negligible and the sum of the rate constants of 52 → S1 and S2 ←S1 internal conversions was estimated to be about Recently, characters of the low lying excited singlet states of diphenylacetylene (DPA) and its derivatives have been studied in the solution phase [1][2][3][4][5][6][7]. The absorption band of DPA observed at 290 mn was assigned to the 1B1u state, while the lowest excited singlet state was predicted to be 2 1Αg state [8].…”