On the ordering of the first two excited electronic states in all-trans linear polyenes

Reported experimental evidence of the relative position of the first two excited electronic states in linear polyenes was carefully examined and compared with that derived from time dependent density functional theory (TDDFT) theoretical calculations performed at the B3LYP level on optimized geometries. The energy values for the first two triplet states B-3(u) and (3)A(g), obtained from TDDFT calculations, were found to be highly strongly correlated with the experimental values. Also, the theoretical calculations for the electronic transition 1 (1)A(g)-->1 B-1(u) were also extremely well correlated with their experimental counterparts; even more important, the three reported experimental data for 1 (1)A(g)-->2 (1)A(g) transitions in these systems conformed to the correlation for the TDDFT 1 (1)A(g)-->1 B-1(u) transition. The first excited electronic state in the linear polyenes studied (from ethene to the compound consisting of 40 ethene units, P-40) was found to be B-1(u). The energy gap between the excited states 2 (1)A(g) and 1 B-1(u) decreased with increasing length of the polyene chain, but not to the extent required to cause inversion, at least up to P-40. In the all-trans linear polyenes studied, the widely analyzed energy gap from the ground electronic state to the first excited singlet state for infinitely long chains may be meaningless as, even in P-40, it is uncertain whether the ground electronic state continues to be a singlet. (C) 2004 American Institute of Physics.