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

Abstract

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

Panchromatic Absorbers Tethered for Bioconjugation or Surface Attachment

“…The chosen perylene ( 11 ) bears two aryloxy groups (one in each bay region) and a 2,6-diisopropylphenyl group at the N -imide site, which together impart structural features that enable good solubility of the perylene in hydrocarbon solvents [ 46 ]. The ethyne is located at the perylene 9-position, a site of considerable electron density in the frontier molecular orbitals and a site known to afford substantial electronic communication upon covalent attachment to the porphyrin [ 42 , 45 , 46 , 47 , 48 , 49 , 50 ]. The ethynyl-perylene-monoimide 11 is an advanced functional dye that has emerged from extensive studies over several decades beginning with the pioneering work of Langhals [ 59 , 60 , 61 , 62 , 63 , 64 , 65 ].…”

“…For comparison purposes, the absorption and fluorescence spectra of a benchmark perylene ( 33 ) [ 46 ] and the trans -AB zinc porphyrin 31 are shown in panels c and d, respectively. Perylene 33 includes a phenyl group at the terminus of the ethyne and a 2,6-diisopropylphenyl substituent at the N -imide position ( Chart 3 ) and is displayed correctly in the original report of synthesis and characterization [ 46 ], but is shown incorrectly with the 2,5-di- tert -butylphenyl substituent at the N -imide position in a subsequent report [ 50 ]. Molar absorption coefficient values are reported for triads 13 , 23 , 27 , and 28 , as well as for benchmark ethynyl-perylene-monoimide 11 .…”

“…This unexpected finding in 2013 [ 41 ] did not close the experimental program but instead prompted an additional decade of research [ 45 , 46 , 47 , 48 , 49 , 50 ] to establish the physical basis for the origin of panchromaticity. An ideal panchromatic absorber was found to be a porphyrin bearing two perylene-monoimides each joined via a single ethyne unit bridging the perylene 9-position and a porphyrin meso-position ( 4 , Chart 1 ).…”

Trimethylsilanolate-Promoted Activation of Alkynyl Trimethylsilanes: Hiyama-Type Sonogashira Cross-Coupling for the Synthesis of Arylene–Ethynylene-Linked Porphyrin Arrays

“…It has been shown that the dimer formation of the copper acetylide mediates the Glaser homocoupling (Scheme A). − The conditions free from a copper cocatalyst should then be rational to mitigate the oxidative homocoupling . Therien and coworkers performed the extensive modular synthesis of arylene–ethynylene-linked porphyrins under the Heck-type copper-free Sonogashira conditions, using a palladium precatalyst with triphenylarsine (AsPh 3 ) as the ligand, providing arylene–ethynylene-linked porphyrin dimers in moderate yields (Route A2 ). − For this approach, tri­( o -tolyl)­phosphine (P­( o -Tol) 3 ) is also applicable instead of AsPh 3 . , The Heck-type copper-free conditions are also tolerant of the in situ protodesilylation of alkynyl TMSs by methanolic potassium carbonate (Route A4 ) . The copper-free modular synthetic methodology is effective in avoiding the unexpected copper insertion to free-base porphyrins, as originally developed by Lindsey and coworkers. − Additionally, the Heck-type Sonogashira cross-coupling reaction with AsPh 3 achieves the exclusive polycondensation of arylene–ethynylene-linked porphyrin arrays, as recently optimized by Harvey and coworkers (Route A2 ). , The Sonogashira cross-coupling approaches including the Heck-type copper-free conditions hold a substantial advantage in the synthesis of arylene–ethynylene-linked porphyrin arrays, as long as the desired reactions kinetically prevail in the competitive side reactions.…”

“…60−63 For this approach, tri(otolyl)phosphine (P(o-Tol) 3 ) is also applicable instead of AsPh 3 . 64,65 The Heck-type copper-free conditions are also tolerant of the in situ protodesilylation of alkynyl TMSs by methanolic potassium carbonate (Route A4). 61 The copper- free modular synthetic methodology is effective in avoiding the unexpected copper insertion to free-base porphyrins, as originally developed by Lindsey and coworkers.…”