A minor substituent modification leads to a large change in photophysics: 9,20-dialkyl-substituted porphycenes

Porphycene - a constitutional isomer of porphyrin - exhibits spectral and photophysical properties that surpass those of the parent macrocycle in the context of potential applications. Three novel derivatives of porphycene bearing two cyclohexyl, isopropyl or tert-butyl functionalities at the meso positions (9,20) have been obtained and characterized using structural and spectroscopic techniques combined with quantum chemical calculations. Substitution with isopropyl or cyclohexyl does not significantly affect fluorescence quantum yields and lifetimes with respect to the parent, unsubstituted porphycene. In contrast, introducing two tert-butyl moieties at the meso positions results in extremely weak emission, which, however, can be recovered by placing the chromophore in a viscous environment. This is explained by large amplitude geometry relaxation in the lowest excited singlet state, leading to enhanced S₀ ← S₁ internal conversion. Since nonradiative S₁ deactivation occurs directly to S₀, the triplet state is not populated, and no singlet oxygen is produced. Porphycenes that exhibit such photophysical properties emerge as prospective functional materials in areas that exploit light-matter interactions. Factors that make these compounds so attractive include (a) strong absorption in the long wavelength range of the visible spectrum; (b) extreme sensitivity of emission intensity to viscosity of the environment; (c) very good photostability, (d) non-toxicity, and (e) possibility of use as fluorophores in the single molecule regime.