Photophysical Properties of Diphenylacetylene and Diphenylbutadiyne Derivatives in Solution Phase

Abstract

By using the femtosecond and picosecond transient absorption measurement techniques, we have investigated diphenylatetylene, diphenylbutadiyne, and their methoxy and cyano derivatives in the solution phase.Both the rapid S2 → S1 internal conversion in the subpicosecond time scale and the triplet formation in the tens of picosecond region were observed for diphenylbutadiyne and its methoxy derivatives, while the dynamic behavior of dimethoxy-diphenylacetylene was quite similar to that of diphenylacetylene. 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]. 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

Unusual Solvatofluorochromism of the Asymmetrically Substituted Aromatic Acetylene Derivative CNAacDMA**

“…The transition from fluorescent ππ* state to weakly fluorescent πσ* state required crossing a small energy barrier [15] . The prediction arising from the calculation were confirmed experimentaly [26,31,32,33] . Thus, in the case of diaryl derivatives of acetylene apart from the emission from LE (locally excited) or CT (charge transfer) state, fluorescence of photoisomer can be simultaneously recorded [15] …”

“…However, no stable photochemical reaction product was found during irradiation of the compound. Thus, most likely a dual fluorescence band is the result of emission from two different excited states, for instance – locally excited (LE) and charge transfer (CT) states just like for 4‐ N,N ‐dimethylamino diphenylacetylene [27,31,32,33] . To confirm this, the fluorescence excitation spectra at different wavelengths of observation were measured.…”

Experimental and theoretical studies of spectroscopic properties of simple symmetrically substituted diphenylbuta-1,3-diyne derivatives

“…Their calculations also predict that the attachment of an electron-withdrawing group to diphenylacetylene increases the energy of the πσ* state and the ππ* → πσ* state switch is not expected to occur, while the attachment of an electron-donating group to diphenylacetylene enhances the state switch from the initially excited ππ* state to the πσ* state. The experimental confirmation of the presence of the πσ* state was the strong πσ* ← πσ* absorption at about 700 nm in the picosecond transient spectra for different DPA derivatives containing electron-donating substituents and the lack of such absorption for electron-withdrawing substituents 17,18,[20][21][22][24][25][26] as predicted by theoretical calculations. The recent work of Saltiel and Kumar 23 on the photophysics of tolane fully supports theoretical calculations.…”

“…The shorter average fluorescence lifetime is observed in acetonitrile solution (574 ps) and MeTHF (610 ps) whereas the longer in MCH (1.271 ns) and hexadecane (1.069 ns). Bi-exponential fluorescence intensity decay could be a result of the equilibrium between S 1 and S 2 states as suggested by Hirata et al 25 based on picosecond transient absorption spectroscopy or the presence of two non-equilibrating conformers. However, the lack of dependence of fluorescence spectra and fluorescence quantum yield on the excitation wavelength and fluorescence excitation spectra on monitoring the wavelength 50 indicates the equilibrium between S 1 and S 2 states in MCH, MeTHF as well as in MeCN (exemplary data for ester are given in Fig.…”

“…28 This can be explained by assuming the presence of the close location of the optically forbidden 1 1 A 1u state above the 1 1 B 1u state. 24,25,27,28 Moreover, Thulstrup et al 29 based on linear dichroism in the IR and UV range and DFT theoretical calculations concluded that even at cryogenic temperatures diphenylbuta-1,3-diyne is a highly dynamic molecule. Due to the lack of systematic studies of symmetrically substituted diphenylbuta-1,3-diyne derivatives containing an electronwithdrawing or an electron-donating group, and at the same time considerable interest of systems containing the diyne motif, 30 we decided to study the spectral and photophysical properties of such derivatives (Fig.…”