Photophysical Properties and Electronic Structure of Hydroporphyrin Dyads Exhibiting Strong Through-Space and Through-Bond Electronic Interactions

Abstract

Electronic interactions between tetrapyrroles are utilized in natural photosynthetic systems to tune the light-harvesting and energy-/charge-transfer processes in these assemblies. Such interactions also can be employed to tailor the electronic properties of tetrapyrrolic dyads and larger arrays for use in materials science and biomedical research. Here, we have utilized static and time-resolved optical spectroscopy to characterize the optical absorption and emission properties of a set of chlorin and bacteriochlorin dyads with varying degrees of through-bond (TB) and through-space (TS) interactions between the constituent macrocycles. The dyads consist of two chlorins or two bacteriochlorins joined by a linker that utilizes a triple–double–triple-bond (enediyne) motif in which the double-bond portion is an ester-substituted ethylene or o-phenylene unit. The photophysical studies are coupled with density functional theory (DFT) calculations to probe the ground-state molecular orbital (MO) characteristics of the dyads and time-dependent DFT calculations (TDDFT) to elucidate excited-state properties. The latter include electronic characteristics of the singlet excited-state manifold and the absorption transitions to these states from the electronic ground state. A comparison of the MO and calculated spectral properties of each dyad with the linker present versus disrupted (by eliminating the double-bond portion) gives insight into the relative contributions of TB versus TS interactions to the electronic properties of the dyads. The results show that the TB and TS contributions are additive (constructively interfere), which is not always the case for molecular dyads. Most of the dyads have shorter lifetimes of the lowest singlet excited state compared to the parent monomer, which derives from increased S1 → S0 internal conversion. The enhancement is greater for the dyads in benzonitrile than in toluene. The studies provide insights into the nature of the electronic interactions between the constituents in the tetrapyrrole arrays and how these interactions dictate the spectral properties and excited-state decay characteristics.

Engineering giant excitonic coupling in bioinspired, covalently bridged BODIPY dyads

“…The energy difference within the corresponding MOs (ΔHOMO and ΔLUMO, Table 5) reflect the strength of the ground-state electronic interactions; lack of ground-state interaction would be manifested as two degenerate HOMOs and LUMOs each localized on the individual BODIPY units. 46,53,54 We note that ΔHOMO and ΔLUMO are similar when calculated using either the RSH-PCM or the SRSH-PCM model.…”

“…Moreover, the short-range excitonic coupling significantly modifies the ground-state electronic structure of individual chromophores. 46,53,54 In synthetic arrays where chromophores are connected by molecular linkers, “through-bond” coupling is also possible, due to electronic π-conjugation through conjugated linking moieties, which further modifies the ground-state electronic structure of chromophores. 46,54…”

“…A variety of chromophores have been studied for such synthetic arrays, including porphyrins, 22,23,37,38,46,54 acenes, 32,35,36,39,41 rylenediimides, 25,30,40 squaraines, 34,42 and (mero)cyanines. 43,44 Coupling strengths (calculated as half of the energy between the maxima in the absorption band) as large as 1488 cm −1 have been obtained using cyanines.…”

“…Moreover, the short-range excitonic coupling significantly modifies the ground-state electronic structure of individual chromophores. 46,53,54 In synthetic arrays where chromophores are connected by molecular linkers, ''through-bond'' coupling is also possible, due to electronic p-conjugation through conjugated linking moieties, which further modifies the ground-state electronic structure of chromophores. 46,54 A variety of chromophores have been studied for such synthetic arrays, including porphyrins, 22,23,37,38,46,54 acenes, 32,35,36,39,41 rylenediimides, 25,30,40 squaraines, 34,42 and (mero)cyanines.…”

Dyads with tunable near-infrared donor–acceptor excited-state energy gaps: molecular design and Förster analysis for ultrafast energy transfer

“…The TB and short-range p-p TS interactions are ground-state in nature and give rise to linear-combination MOs, MO splitting, split redox waves and excited states with characteristics of the two constituents. [78][79][80] The longer-range dipole-dipole coupling is excited-state in nature and does not afford the linear-combination MOs or split redox waves characteristic of ground-state interactions. In the following, the linker-mediated interactions are referred to as TB, the long-range dipole-dipole coupling as TS, and the short-range (through-space) interactions as direct p-p overlap.…”