Isoporphyrins are porphyrin tautomers with a saturated meso carbon and thus an interrupted π system. We report here steady-state optical absorption, fluorescence, and fluorescence polarization data as well as time-resolved results that detail the significant effects of porphyrine tautomerization on the photophysical properties of a metallo-isoporphyrin, zinc 2,3,5,5′,7,8,12,18-octamethyl-13,17-bis(3-methoxy-3-oxopropyl) isoporphyrin perchlorate (2). Besides the red-shifted, low-energy absorption bands diagnostic of metallo-isoporphyrins, 2 exhibits a large Stokes shift of its fluorescence emission (approximately 600 cm-1) and an unusually short singlet excited-state lifetime at room temperature (130 ± 15 ps), photophysical properties distinctly different from those of the canonical prophyrin tautomers. The only porphyrins to exhibit marginally similar perturbations of their photophysical properties are those with severely nonplanar macrocyles whose π systems are significantly destabilized by the conformational distortions and thus approach the interrupted π systems of isoporphyrins (Gentemann et al: J. Am. Chem. Soc. 1994, 116, 7363). In addition to providing the first insights into the photophysical consequences of porphyrin tautomerization, the results for the isoporphyrin further document the sensitivity of the fundamental electronic and excited-state properties of porphyrinic chromophores to modulation of their π systems in vitro and, by extrapolation, in vivo as well.
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Gentemann et al. (1995) studied this question.