Phantom excited state

In carrying out he calculations we use essentially the same procedure as in the case of benzene and naphthalene. As an additional simplification, however, we neglect entirely all the excited states of the molecule, since their contribution to the total energy is comparatively small, and since they would complicate the calculations tremendously if retained. Another slight modification of the procedure is necessitated by the fact that a free radical possesses an odd number of electrons, one of which must remain unpaired. This is taken care of formally by introducing a phantom orbit X with an accompanying phantom electron which is paired with the odd electron.4 In the subsequent... 

Scheme 1. Three-state kinetic scheme as minimal basis to explain the photophysics of donor-acceptor substituted stilbenes. E (primary excited n, n state, essentially planar geometry) and P ( Phantom Singlet state , twisted double bond) are the classical states discussed for stilbene, A (twisted single bonds, state(s) of TICT nature, up to 4 different possibilities in donor-acceptor substituted stilbenes like DNS) correspond to motions along different reaction coordinates than for P ...

Theoretical calculations have been made on stilbene which are relevant to photoisomerization dynamics. MNDO calculations of stilbene potential energy properties shows no evidence of a doubly excited "phantom" state but a singly excited state with adiabatic rotation around the central ethylene bond has only a small barrier on this path23T Calculations of dipole moments, optical spectra, and second order hyperpolarizability coefficients of some mono- and disubstituted stilbene molecules allows the design of useful nonlinear optical molecules 38. 

Time-resolved microwave conductivity technique has been employed to study the photoexcited state behavior of TPE and tetramethoxy TPE. The change in dipole moment upon excitation was found to be in the order of 7.5D for these compounds [65]. This has been dted as direct evidence for the twisted dipolar or zwitterionic nature of the singlet excited state (phantom state). 

A theoretical study of the excited states of stilbene and stilbenoid donor-acceptor dye systems as potential laser was performed [4]. Semiempirical calculations within the CNDO/S framework were used to characterize the nature of the phantom-singlet excited state P (double-bond twisted geometry) of stilbene and stilbenoid donor-acceptor dye systems including 4-(dimethylamino)styrylpyridylmethylium and DCM laser dyes. It was shown that for stilbene, a slight geometric symmetry reduction is... 

The photochemistry of a-methylstilbene (5) resembles stilbene photochemistry in many ways. However, as pointed out earlier, both the cis and trcms isomers are nonclassical acceptors of triplet excitation. This suggests that both the cis and trcms triplet states correspond to high-energy vibrational levels of the twisted or phantom triplet. Azulene does not alter the photo-... 

Steer et al. [8, 48] have proposed that the dark state is a new state, C, that is formed when two electrons are lifted from the n orbital and placed into the 7t orbital in a doubly excited configuration, infill )1. Multireference CASSCF calculations of Strickler and Gruebele [6] indicate, however, that this phantom state is very likely the C 1 B2( ,ct ) electronic state, formed from the one electron n —> a excitation. Their calculations show that the C state is planar and intersects the nonplanar B 1 1(71,71 ) state at increasing C—S bond distances ( 2.5 A). The barrier separating the B from the C state ranges from 300 cm-1 (0 = 0) to about 1650 cm-1 (0=45°). Calculations of the transition dipole moments along the C—S bond shows that the (C p X), element is about three orders of magnitude smaller than (B p X) and provides the rationalization of C state darkness. 

Using azulene as a triplet quencher it has been shown that triplet states are not involved in the cis-trans isomerization of stilbene on direct excitation. Triplet-sensitized cis-trans isomerization, however, is observed and proceeds in both directions through a minimum in the triplet potential energy surface at a twisted geometry, often referred to as the triplet phantom state P. ... 

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