Isoporphyrins are porphyrin tautomers with a saturated meso carbon and thus an interrupted pi 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 porphyrin 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 (similar to 600 cm(-1)) and an unusually short singlet excited-state lifetime at room temperature (140 +/- 15 ps), photophysical properties distinctly different from those of the canonical porphyrin tautomers. The only porphyrins to exhibit marginally similar perturbations of their photophysical properties are those with severely nonplanar macrocycles whose pi systems are significantly destabilized by the conformational distortions and thus approach the interrupted pi 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 pi systems in vitro and, by extrapolation, in vivo as well.