Optical Spectra of Isolated s-cis- and s-trans-Bithiophene: Torsional Potential in the Ground and Excited States

Abstract

Fluorescence excitation and fluorescence spectra of 2,2'-dithenyl (commonly known as bithiophene) seeded into a supersonic helium expansion show complete vibronic resolution. The fluorescence excitation spectrum has two components whose relative intensity depends on expansion conditions. Measurement and analysis of these spectra for systematic variation of expansion conditions leads to their assignment to the s-trans and s-cis conformers and provides information on the potential for s-trans to s-cis interconversion in the ground and first excited singlet states.

Density-functional study of the evolution of the electronic structure of oligomers of thiophene: Towards a model Hamiltonian

“…The coupling with the molecular vibration modes can in principle also be calculated in this way, but remains a subject for future studies. However reasonable estimates of these can also be obtained from the optical 15 and photoelectron spectra presented here. Before presenting the results, we look at some of the basic information concerning the monomers, which will turn out to be important in describing what happens as we increase N in the oligomers.…”

“…In order to model the effects of rotational disorder and structural defects on the electronic properties of the oligothiophenes, we studied the ground-and excited-state properties of ␣-2T as a function of the dihedral angle between both monomer units. Chadwick and Kohler 15 found experimental evidence of the coexistence of cisoid and transoid bithiophenes in a supersonic expansion. The ratio is dependent on the temperature of the expansion, and the enthalpy difference between the two structures was found to be 1.16 Ϯ0.13 kcal/mol.…”

“…The ratio is dependent on the temperature of the expansion, and the enthalpy difference between the two structures was found to be 1.16 Ϯ0.13 kcal/mol. One should, of course, realize that the dihedral angles in the gas phase are 72°for the cisoid form and 64°for the transoid form, 15 while those molecular structures are flat in the solid phase. 39 Because interaction between the molecules in the crystal are only weak, we expect a rather shallow potential well for torsion.…”

“…Various forms of optical spectroscopy together with theoretical interpretations have provided us with the location of the lowest-energy singlet excitations, [13][14][15][16][17][18][19][20][21][22][23][24][25][26] as well as in some cases the triplets. 27 As we will discuss below, the lowestenergy singlet excitations show a 1/N-like dependence ͑with N the number of rings͒ on the oligomer length, whereas the lowest triplets are considerably lower in energy.…”

On-the-fly ab initio semiclassical dynamics: Identifying degrees of freedom essential for emission spectra of oligothiophenes

“…However, we disregard this effect since the resulting shift is expected to be small for a broad class of solvents. 2,37,38 Also, the exact prediction of the spectrum position is partly beyond the level of the ab initio setup employed here (see Sec. III).…”