3,6,13,16-Tetraalkylporphycenes: synthesis and exploration of the effect of alkyl groups on the structure, photophysical properties, and basicity

Abstract

Two new 3,6,13,16-tetraalkylporphycenes were synthesized following a rational approach. The reason behind the lower yield of the desired α,α′-bipyrroles was unraveled. The σ-donating effect of alkyl-substituents was more profound than that of the reported positional isomeric tetraalkylporphycenes. The tetramethyl derivative (1a) exhibited a square-type core unlike the tetraethyl (1b) and tetrapropyl (1c) counterparts. The latter two derivatives (1b–1c) unusually displayed a rectangular core as noticed in metalloporphycenes owing to coordination with the guest. This could be correlated to the enhanced basicity of the macrocycles upon increasing the alkyl chain at the inner β-positions. Both the tetramethyl and tetraethyl freebase porphycenes exhibited enhanced fluorescence (Φ_f: 11–12%) and singlet oxygen generation ability (Φ_Δ: 18–22%) compared to 3,6,13,16-tetrapropylporphycene (Φ_f: 6%; Φ_Δ: 18%).