2-nitro-7,12,17-tri-tert-butylporphycene: Spectroscopy, photophysics, and tautomerism

Nitration of 2,7,12,17-tert-butylporphycene, leading mainly to meso-substituted 9-nitro- and 9,20-nitro- derivatives, also yielded a small amount of porphycene bearing three tert-butyl moieties and the nitro group at the [Formula: see text] position. The electronic absorption spectra of 2-nitro and 9-nitroporphycenes are similar, but the photophysical characteristics strongly differ. Unlike the meso-substituted derivative, 2-nitroporphycene emits intense, long-lived (ns) fluorescence. Strong enhancement of fluorescence intensity is due to the absence of steric interactions between tert-butyl and nitro moieties. The two porphycenes also differ in their tautomeric properties. 2-nitroporphycene exists in one trans-tautomeric form, whereas two trans-tautomers of similar energies coexist in the meso derivative. These results are well reproduced by quantum-chemical calculations. The possibility of changing photophysics and tautomerism by shifting the position of the substituent may be exploited while designing porphycenes targeted for specific applications, such as photodynamic therapy or building molecular switches.