Substituted stilbenes fluorescence lifetime

The objective of this article is to review critically the bimolecular photochemical reactions of the stilbenes. While the emphasis will be placed upon the unsubstituted stilbenes t-1 and and c-1, comparisons with substituted stilbenes and stilbene analogues will be made in some cases. In most of the reactions that will be described, the reactive excited state is the lowest singlet of t-1, - -t. Because of the short lifetime of It, unimolecular processes (fluorescence and trans-cis isomerization) can compete with bimolecular reactions under normal reaction conditions. While competing unimolecular processes are... 

The fluorescence Intensity of substituted stilbenes and stilbene analogues provides a useful indicator of photochemical reactivity. Virtually all of the reported bimolecular photochemical reactions of electronically excited stilbenes involve stilbenes which are fluorescent at room temperature in solution. The absence of fluorescence is indicative of a singlet lifetime too short (< 100 ps) to allow for efficient bimolecular quenching. 

A series of aminostilbenes (lA-C, 2A-C) have been synthesized to test the effect of substitution of the amino group upon the photophysics and photochemistry of stilbenes [45]. This study indicated that the photophysics properties of trans-2-aminostilbene, lA, and trans-3-aminostilbene, IB, were similar. as-2-Aminostilbene, 2A, and ds-3-aminostilbene, 2B, showed similar fluorescent lifetimes. Anomalous behavior of emission anisotropy for short-living derivatives of stilbenes was described [46]. The absorption, fluorescence and polarization excitation, and emission spectra... 

The lifetimes of the first excited state of substituted stilbenes in solvents of different polarities were detected. An example of the time-resolved fluorescence decay profile is shown in Figure 11.6. 

Adams and Cherry (78) have investigated the effects of stilbene substitution on the behavior of their excited complexes with fumaronitrile and find that the rate constants for fluorescence and nonradiative decay are insensitive to substitution, but that the rate constant for intersystem crossing is increased by electron-donating substituents (lower stilbene oxidation potential). This trend is attributed to a decrease in the energy gap between the excited complex and locally excited 3t (Fig. 4). The observed energy gap dependence of the exciplex lifetime could also account for the absence of fluorescence (or cycloadduct formation, see Section IV-B) from the excited charge-transfer complexes of t-1 with stronger electron acceptors such as maleic anhydride (76) or tetracyanoethylene (85). 

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