Phantom state

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. 

Ma, J. and Zimmt M.B., Equilibration between the fluorescent and zwitterionic phantom states in alkyl-substituted tetraphenylethylenes. J. Am. Chem.SOc. (1992) 114 9723-9724. 

The presence of an underlying weak singlet transition below the main band of these molecules was also supported by Shpolskii spectra (98) and by two-photon spectroscopy (99-101). Thus in spite of the failure to detect the "phantom" state in a study of environmental effects on the fluorescence of diphenylhexatriene (102), the direct observations of the phantom state in the a, U) diphenyl derivatives seem to be numerous and probative. Since a Ag J-A- transition is symmetry-allowed for two photon absorption, the latter experiments supported the Ag identification for the underlying state. 

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. 

Equations (2.1.68) show that the long-range expansion limit ag(k -+ oo) is finite so that the mean-square size of a large chain is still proportional to N. This confirms that we are indeed in the 0 state, although the chain is somewhat expanded with respect to the phantom state. Considering that the characteristic ratio is given by <(r N) o/Nl [see (2.1.6)] and that r(iV) = T(q = 0) [see (2.1.28)], we have... 

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. ... 

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). 

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