Spectrochimica Acta Part A 58 (2002) 2245 – 2255 Electronic states of 1,6,6a 4 -trithiapentalene and its 2,5-dimethyl and 2,5-diphenyl derivatives. Ultraviolet-visible linear dichroism spectroscopy and time-dependent density functional theory calculations Jens Spanget-Larsen * Department of Life Sciences and Chemistry, Roskilde Uniersity, Building 18.1, P.O. Box 260, DK-4000 Roskilde, Denmark Received 6 November 2001; accepted 3 December 2001 Dedicated to Professor Rolf Gleiter on the occasion of his 65th anniversary. Abstract The electronic transitions of 1,6,6a 4 -trithiapentalene (TTP) and its 2,5-dimethyl and 2,5-diphenyl derivatives (DMTTP and DPTTP) are investigated by ultraviolet-visible linear dichroism spectroscopy on molecular samples partially aligned in stretched polyethylene, and by time-dependent density functional theory calculations (TD-B3LYP/ 6-31G*). The theoretical predictions are in good agreement with the observed polarization spectra, allowing a detailed assignment of observed transitions to calculated electronic states. According to the theoretical results,  – * excitations involving orbitals associated with the unique SSS three-center bonding in the TTP ring system play a fundamental role in the description of low-lying electronic states. The visible absorption band close to 20 000 cm -1 and the dominant peak in the near-ultraviolet region around 39 000 cm -1 can thus be assigned to transitions with a high degree of  – * character. The situation is slightly more complicated in the case of DPTTP because sterically induced twisting of the phenyl groups leads to mixing of  and  orbitals. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Trithiapentalenes; Electronic transitions; Linear dichroism spectroscopy; Polarization; Time-dependent density functional theory www.elsevier.com/locate/saa 1. Introduction 1,6,6a 4 -Trithiapentalene (TTP) is the most well known and most studied example of a cyclic -system with a formally hypervalent heteroatom, and its molecular and electronic structure has been a subject of investigation for decades [1–5]. TTP can be considered as a 10 -electron bicyclic ring system with C 2v symmetrical equilibrium ge- ometry, incorporating a linear arrangement of three sulfur atoms with unusually long and weak SS bonds [6–13]: * Tel.: +45-4674-2710; fax: +45-4674-3011 E-mail address: spanget@ruc.dk (J. Spanget-Larsen). 1386-1425/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S1386-1425(01)00726-0