Competitive Energy and Electron Transfer in β‐Functionalized Free‐Base Porphyrin–Zinc Porphyrin Dimer Axially Coordinated to C₆₀: Synthesis, Supramolecular Formation and Excited‐State Processes

Abstract

Simultaneous occurrence of energy and electron transfer events involving different acceptor sites in a newly assembled supramolecular triad comprised of covalently linked free-base porphyrin-zinc porphyrin dyad, H P-ZnP axially coordinated to electron acceptor fullerene, has been successfully demonstrated. The dyad was connected through the β-pyrrole positions of the porphyrin macrocycle instead of the traditionally used meso-positions for better electronic communication. Interestingly, the β-pyrrole functionalization modulated the optical properties to such an extent that it was possible to almost exclusively excite the zinc porphyrin entity in the supramolecular triad. The measured binding constant for the complex with 1:1 molecular stoichiometry was in the order of 10 m revealing moderately stable complex formation. An energy level diagram constructed using optical, electrochemical and computational results suggested that both the anticipated energy and electron events are thermodynamically feasible in the triad. Consequently, it was possible to demonstrate occurrence of excited state energy transfer to the covalently linked H P, and electron transfer to the coordinated ImC from studies involving steady-state and time-resolved emission, and femto- and nanosecond transient absorption studies. The estimated energy transfer was around 67 % in the dyad with a rate constant of 1.1×10 s . In the supramolecular triad, the charge separated state was rather long-lived although it was difficult to arrive the exact lifetime of charge separated state from nanosecond transient spectral studies due to overlap of strong triplet excited signals of porphyrin in the monitoring wavelength window. Nevertheless, simultaneous occurrence of energy and electron transfer in the appropriately positioned energy and electron acceptor entities in a supramolecular triad was possible to demonstrate in the present study, a step forward to unraveling the complex photochemical events occurring in natural photosynthesis and its implications in building light energy harvesting devices.

Rational Construction and Efficient Regulation of Stable and Long-Lived Charge-Separation State in Fullerene Materials

“…7 CS can occur at an early stage, while electron−hole recombination to the 3 LEs is a more thermodynamically favorable process. 27 Despite the availability of CSs, the final nonradiative deactivation of the triplet state causes considerable energy loss. (III) The CSs energy levels are lower than those of 3 LEs of C 60 .…”

“…(II) CSs energy levels lower than 1 LEs of C 60 but higher than triplet LEs ( 3 LEs) of C 60 (∼1.5 eV) . CS can occur at an early stage, while electron–hole recombination to the 3 LEs is a more thermodynamically favorable process . Despite the availability of CSs, the final nonradiative deactivation of the triplet state causes considerable energy loss.…”

Push–Pull Porphyrins via β‐Pyrrole Functionalization: Evidence of Excited State Events Leading to High‐Potential Charge‐Separated States

“…By comparison with the earlier discussed absorptiona nd fluorescence spectra,t he first three negative peaks were attributed to ground state bleaching andt he latter two peaks were attributed to stimulated emission.T he near-IR peak at 1280 nm was attributed to singlet-singlet transition as such at rend waso bservede arlier for pristine porphyrins. [35] With time, decay of the positives ignals were noticed, however, within the 3nst ime window of our instrument most of the signal persisted, which is in agreement with longerl ifetimeo f 1 (8.73 ns). In the case of 2,t he instantaneously form 1 2*h ad positive peaks at 480, 586, 630, 732, 836 and 1320 nm.…”

Singlet and triplet energy transfer dynamics in self-assembled axial porphyrin–anthracene complexes: towards supra-molecular structures for photon upconversion

“…As part of the ongoing studies of energy and electron transfer reactions in porphyrin donor-acceptor system for various photochemical and electrochemical applications, 4,5,7,24,25,[31][32][33][34][35][36][37][38][39][40][41][42][43][44] we present a detailed photophysical study of a series of new, well-defined, self-assembled, axially coordinated ruthenium porphyrin-anthracene donor-acceptor systems. These systems consist of 2,3,7,8,12,13,17,18-octaethylporpyrin ruthenium(II) carbonyl (RuOEP(CO)) coordinated axially by five different pyridine functionalized anthracene based ligands with various bridge lengths (8-25 Å) or two different dendrimer ligands with 3-7 monomer units.…”