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The polarized electronic spectra and electric field spectra of benzo-diazoles. II. 2,1,3-benzothiadiazole
Authors:Tien-Sung Lin  James R. Braun
Affiliation:Department of Chemistry, Washington University, St. Louis, Missouri 63130, USA
Abstract:
The S1 ← S0 absorption spectra of 2,1,3-benzothiadiazole (BTD) have been measured at 4.2 K in four different host crystals: naphthalene, durene, p-dichlorobenzene (DCB) and p-dibromobenzene. Detailed vibrational analyses are given for BTD imbedded in napthalene and DCB. The polarization measurements show that the S1 state has B2 symmetry, like its selenium analogue (BSD). The transition is dominated by a single totally symmetric mode - 484 cm?1. The Herzberg-Teller coupling contributes only a very small fraction of the total intensity. The Stark measurements of a DCB sample containing both BTD and BSD enabled us to compare the charge distribution of BTD and BSD in the state S1. The Stark splittings of BTD are 17% greater than the splittings of BSD. Reorganization of the σ-core during the excitation is used to explain the difference. The drastic change in dipole moment upon excitation implies that the S1←S0 transitions of BTD and BSD are not localized in the six-membered ring as suggested by previous workers. Weak phosphorescence of BTD in napthalene and DCB and singlet-triplet absorption spectrum of neat BTD have been observed. The heavy atom effect of spin-orbit coupling is to explain the ST absorption intensity of BTD and BSD.
Keywords:
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