Highly Coupled Heterobicycle‐Fused Porphyrin Dimers: Excitonic Coupling and Charge Separation with Coordinated Fullerene, C₆₀

Abstract

Porphyrin dimers have been widely explored and studied owing to their importance in photosynthetic systems. A vast variety of dimers linked by different groups and at different angles have been synthesized and studied; however, the means by which to synthesize rigidly fused porphyrins with direct conjugation of the chromophores remains limited. Such a class of porphyrins may possess interesting properties that unconjugated or stacked dimers may not exhibit. In this study, bisbenzimidazolefused porphyrin dimers and their mono-and bisÀ zinc derivatives are synthesized and characterized. As a consequence of excitonic coupling, these dimers exhibit a split Soret band irrespective of the metal ion in the porphyrin cavity. Steadystate fluorescence and excitation spectra followed by femtosecond transient absorption spectral studies of the heterometallated dimer, (free-base and zinc porphyrin) reveals the occur-rence of efficient singlet-singlet energy transfer (> 95 % efficiency and rate constant > 10 12 s À 1 ) within the dyad. Further, donor-acceptor conjugates were formed by metal-ligand axial coordination of phenyl imidazole functionalized C 60 and were characterized by a variety of physicochemical techniques. Excited state charge separation from both singlet and triplet excited states of ZnP in the conjugates has been established. The lifetime of the final charge-separated state was in the 30-40 μs range revealing charge stabilization. Interestingly, no charge separation in the conjugate derived from the heterometallated dimer was observed wherein excitation transfer dominated the process. The present study brings out the importance of the rigid π-spacer connecting porphyrin dimers in governing the energy and electron transfer events when coupled with an electron acceptor.

Excited Charge Separation in a π-Interacting Phenothiazine–Zinc Porphyrin–Fullerene Donor–Acceptor Conjugate

“…10,33−39 For instance, Tian and coworkers have developed zinc porphyrin−viologen (ZnP−V) nanoparticles in an aqueous solution that showed a charge separation (CS) state lifetime of up to 4.3 ms. 30 This can be recognized as charge hopping induced by aggregation or distance modification between the donor and acceptor induced by electronic interaction. 31 D'Souza and coworkers 32 while transient absorption studies suggested that the lifetime of the final charge-separated state was in the 30−40 μs range. 32 Poddutoori and coworkers have reported a triad system in which a secondary donor bis-triphenylamino-boron-dipyrromethane (TPA 2 -BDP) was covalently connected to the axial position of Al(III) porphyrin (AlPorF 3 ), a primary donor and an acceptor pyridine appended naphthalenediimide (NDI) at another axial position, and observed a charge-separated state of (TAP 2 −BDP) •+ -AlP or F 3 −NDI •− .…”

“…31 D'Souza and coworkers 32 while transient absorption studies suggested that the lifetime of the final charge-separated state was in the 30−40 μs range. 32 Poddutoori and coworkers have reported a triad system in which a secondary donor bis-triphenylamino-boron-dipyrromethane (TPA 2 -BDP) was covalently connected to the axial position of Al(III) porphyrin (AlPorF 3 ), a primary donor and an acceptor pyridine appended naphthalenediimide (NDI) at another axial position, and observed a charge-separated state of (TAP 2 −BDP) •+ -AlP or F 3 −NDI •− . 10 Zhu et al developed two triads featuring fulleropyrrolidine−perylene tetracarboxylicdiimide−freebase porphyrin (FPP) and its zinc derivative (ZnFPP) for organic solar cells with a device efficiency of 0.35%.…”

“…The photodynamics of D–A systems revealing charge-separated species of appreciable lifetimes is not only useful for understanding the initial events of photosynthesis but also useful in the design of efficient materials for optoelectronic applications. , Quite a few molecular or supramolecular systems, such as dyads, triads, tetrads, etc., have been elegantly designed and studied to witness long-lived charge separation mimicking “antenna-reaction center” events of photosynthesis. − Of the various scaffolds used for building D–A systems, porphyrins, structurally close to the natural photosynthetic chlorophyll pigment and due to well-established synthetic protocols, have conquered this area of research. Many groups, including ours, have reported multimodular systems by introducing secondary donors/acceptors on the porphyrin macrocycle to achieve sequential electron and hole transfer, resulting in the generation of long-lived charge-separated species. ,− For instance, Tian and coworkers have developed zinc porphyrin–viologen (ZnP–V) nanoparticles in an aqueous solution that showed a charge separation (CS) state lifetime of up to 4.3 ms .…”

“…This can be recognized as charge hopping induced by aggregation or distance modification between the donor and acceptor induced by electronic interaction . D’Souza and coworkers have developed bis-benzimidazole-fused porphyrin hetero and homo dimers in which the axial position of zinc porphyrin is coordinated by phenyl imidazole-functionalized C 60 . The split in the Soret band suggested the presence of excitonic coupling, while transient absorption studies suggested that the lifetime of the final charge-separated state was in the 30–40 μs range .…”

“…D’Souza and coworkers have developed bis-benzimidazole-fused porphyrin hetero and homo dimers in which the axial position of zinc porphyrin is coordinated by phenyl imidazole-functionalized C 60 . The split in the Soret band suggested the presence of excitonic coupling, while transient absorption studies suggested that the lifetime of the final charge-separated state was in the 30–40 μs range . Poddutoori and coworkers have reported a triad system in which a secondary donor bis-triphenylamino-boron-dipyrromethane (TPA 2 -BDP) was covalently connected to the axial position of Al­(III) porphyrin (AlPorF 3 ), a primary donor and an acceptor pyridine appended naphthalenediimide (NDI) at another axial position, and observed a charge-separated state of (TAP 2 –BDP) • + -AlP or F 3 –NDI • – .…”