Synthesis, Spectral and Theoretical Studies of Phenothiazine Embedded Pentaphyrins(2.1.1.1.1)

Condensation of phenothiazine-based tripyrrane and dipyrroethene diol in CH₂Cl₂ under mild acid catalyzed conditions followed by DDQ oxidation afforded the first examples of phenothiazine-embedded pentaphyrins(2.1.1.1.1) instead of the expected conjugated phenothiazine-embedded pentaphyrins(2.1.1.1.1). The formation of phenothiazine-embedded pentaphyrins(2.1.1.1.1) containing one phenothiazine and four pyrrole rings connected via four meso-sp² carbons and two meso-sp³ carbons in porphyrinoids framework was confirmed by HR-MS, ¹H, ¹³C, ¹H-¹H COSY, NOESY and HMBC NMR spectra. DFT optimized structures of macrocycles showed that the phenothiazine moiety was deviated from the mean plane defined by six meso carbons and macrocycle adopted a puckered-like structure. The absorption spectra of phenothiazine-embedded pentaphyrins(2.1.1.1.1) showed one relatively sharp absorption band in the region of 350-500 nm and one broad absorption band in the low energy region of 500-750 nm which were bathochromically shifted in their protonated form with distinct colour change of the solution. The macrocycles were weakly fluorescent with broad fluorescence band in the range of 500-850 nm with peak maxima in the range of 708-715 nm and quantum yields of 8-11 %. The electrochemical studies revealed that macrocycles were stable under redox conditions and indicated their electron rich nature. TD-DFT studies agreed with the experimental observations.