Planar conformation ground electronic state

A number of fluorescent dyes with internal charge transfer mechanism allow the molecule to twist (rotate) between the electron donor and electron acceptor moieties of the fluorescent dipole. In most cases, the twisted conformation is energetically preferred in the excited Si state, whereas the molecule prefers a planar or near-planar conformation in the ground state. For this reason, photoexcitation induces a twisting motion, whereas relaxation to the ground state returns the molecule to the planar conformation. Moreover, the Si — So energy gap is generally smaller in the twisted conformation, and relaxation from the twisted state causes either a... 

Electron spin resonance (ESR) studies of the urazole-bridged 1,3 diradicals 64 derived from the azoalkanes 63 confirm a triplet ground state for these species. The nearly zero symmetry parameter, that is, Elhc= 0.0004 0.0001 cm-1, for the triplet diradical 64 of the diphenyl azoalkane 63 establishes a planar conformation <1995JOC308, 1997JA10673>. 

Wiberg and Schreiber have examined the rotational barriers in acrolein and vinylamine. If resonance structure 13 participates in the description of acrolein, then the population of C3 should be greater in the rotated conformer relative to the planar form. Similarly, if resonance structure 14 participates in the description of vinylamine, then the population on C2 should be smaller in the rotated conformer relative to the ground state planar conformer. The calculated topological charges (Table 28) indicate an electronic distribution exactly opposite to that expected. The authors conclude that dipolar resonance structures are unimportant in these molecules. 

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