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 (τ_S) and increased fluorescence quantum yield (Φ_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 π-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 orderC-TMI(705 nm) <PMI-P-TMI(749 nm) <B-TMI(774 nm), yet each array exhibits diminished Φ_fand shortened τ_Svalues. ThePMI-P-TMItriad in toluene exhibits Φ_f= 0.038 and τ_S= 139 ps versus the all-perylene triad (PMI-P-PMI) for which Φ_f= 0.26 and τ_S= 2000 ps. The results highlight design constraints for auxiliary pigments with tetrapyrroles to achieve panchromatic absorption with retention of viable excited-state properties.