Indoles photophysical processes

A substantial literature exists describing experimental and theoretical studies of the photophysical processes of indoles. In part, this reflects the well-established use of the magnitude of the Stokes shift seen in fluorescence spectrum of the indole chromophore to probe the local environment of tryptophan residues in biological molecules [4]. However, it also reflects the complexity of indole photophysics and the fact that some aspects remain controversial. Only an overview is presented here, because a proper discussion of the photophysics of the indole ring would easily fill this chapter. Unfortunately, no recent, comprehensive review of the topic is available however, early publications have been summarized [4,5], and recent papers in this area provide leading references and excellent brief reviews of subsequent work [6]. 

Table 1 lists some of the photophysical properties of indole of relevance for planning photochemical reactions. The numbers should be taken as guidelines only, given the perturbations that can result from changing structure and medium. The data in Table 1 indicate that fluorescence and intersystem crossing of the singlet excited state are both fast, efficient processes. Consequently, both triplet photochemistry and fast singlet photochemistry can be expected. 

Moreover, in aqueous solution there is always a solvated-electron energy level sitting 3.5 eV below vacuum level, and the Sq Sj excitation of an aqueous chromophore may be enough energy to access such a state. Common aromatic chromophores have photoionization thresholds corresponding to ultraviolet wavelengths (e.g., 4.35 eV for indole ), so that these solvated-electron states may be very much in play in solution-phase photochemistry and photophysics. The molecular-level details of the electron ejection process, and the structure of the initially formed solvated electron, remain poorly understood. 

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