Energy singlet state

In 95% ethanol, 4,4 -BPDC shows a strong fluorescence and relatively weaker phosphorescence (relative intensity 10-2 times the fluorescence). This indicates, though perhaps unexpected, the lowest energy singlet state is a (ir,ir ) state... 

FIGURE 2. Some configurations describing the low-energy singlet states of linear polyenes... 

Parasuk and Cramer s calculations indicate that triplet vinylnitrene is 15 kcal/mol more stable than the lowest energy singlet state. The lowest energy singlet state is open shell and resembles a 1,3-biradical. 

The only head-to-head polymer which has been examined for excimer fluorescence is polystyrene 25). Unfortunately, the synthetic route to this polymer leaves a number of stilbene-based structures in the sample, which have a lower-energy singlet state than either PS monomer (285 nm) or excimer (330 nm). Thus, fluorescence from these intrinsic stilbene traps was seen in the spectra of head-to-head PS in pure films and, to a lesser extent, in fluid solution. In the latter, the fluorescence of PS monomer was predominant, and the small amount of stilbene fluorescence was increased when a nonsolvent (methanol or cyclohexane) was added to the 2-methyl-tetrahydrofuran solution. In films of the polymer, stilbene fluorescence was the major spectral band, although some PS excimer fluorescence was also present in the spectrum. No monomer fluorescence at 285 nm was detected from films. Given the impure nature of the head-to-head PS sample, no conclusions on excimer formation in these systems could be drawn. 

The proposed mechanism for the chlorinative cleavage involves the initial excitation of the acceptor to its lowest-energy singlet state. Electron transfer to the permethyloli-gogermane produces initially the radical anion of the acceptor and the radical cation of the... 

Cr +) have been probed by absorption and luminescence spectroscopy. The strongest interaction occurs for the NF complex, where the lowest energy singlet state is very close in energy to a spin allowed crystal field band, giving rise to intense vibronic patterns. ... 

Fig. 2. The ordering of low-energy singlet states in polyenes and carotenoids. Labels refer to molecules belonging to the Cji, point group. Arrows with x s refer to symmetry-forbidden electronic transitions.

---