Electronic Structure and Excited-State Dynamics of Rylene-Tetrapyrrole Panchromatic Absorbers

Panchromatic absorbers have potential applications in molecular-based energy-conversion schemes. A prior porphyrin-perylene dyad (P-PMI, where "MI" denotes monoimide) coupled via an ethyne linker exhibits panchromatic absorption (350-700 nm) and a tetrapyrrole-like lowest singlet excited state with a relatively long singlet excited-state lifetime (tau(S)) and increased fluorescence quantum yield (Phi(f)) versus the parent porphyrin. To explore the extension of panchromaticity to longer wavelengths, three arrays have been synthesized: a chlorin-terrylene dyad (C-TMI), a bacteriochlorin-terrylene dyad (B-TMI), and a perylene-porphyrin-terrylene triad (PMI-P-TMI), where the terrylene, a pi-extended homologue of perylene, is attached via an ethyne linker. Characterization of the spectra (absorption and fluorescence), excited-state properties (lifetime, yields, and rate constants of decay pathways), and molecular-orbital characteristics reveals unexpected subtleties. The wavelength of the red-region absorption band increases in the order C-TMI (705 nm) < PMI-P-TMI (749 nm) < B-TMI (774 nm), yet each array exhibits diminished Phi(f) and shortened tau(s) values. The PMI-P-TMI triad in toluene exhibits Phi(f) = 0.038 and tau(s) = 139 ps versus the all-perylene triad (PMI-P-PMI) for which Phi(f) = 0.26 and tau(s) = 2000 ps. The results highlight design constraints for auxiliary pigments with tetrapyrroles to achieve panchromatic absorption with retention of viable excited-state properties.