Mechanisms of Excited-State Energy-Transfer Gating in Linear versus Branched Multiporphyrin Arrays

Abstract

We have investigated electrochemical switching of excited-state electronic energy migration in two optoelectronic gates with different architectures. Each gate consists of diarylethyne-linked subunits: a borondipyrrin (BDPY) input unit, a Zn-porphyrin transmission unit, a free-base-porphyrin (Fb-porphyrin) output unit, and a Mg-porphyrin redox-switched site connected either to the Fb porphyrin (linear gate) or to the Zn porphyrin (branched, T gate). Both the linear and branched architectures show Fb-porphyrin emission when the Mg porphyrin is neutral and nearly complete quenching when the Mg porphyrin is oxidized to the π-cation radical. To determine the mechanism of gating, we undertook a systematic photophysical study of the gates and their dyad and triad components in neutral and oxidized forms, using static and time-resolved optical spectroscopy. Two types of photoinduced energy-transfer (and/or charge-transfer) processes are involved in gate operation: transfer between adjacent subunits and transfer between nonadjacent subunits. All of the individual energy-transfer steps that funnel input light energy to the fluorescent output element in the neutral systems are highly efficient, occurring primarily by a through-bond mechanism. Similarly efficient energytransfer processes occur between the BDPY and the Zn and Fb porphyrins in the oxidized systems, but are followed by rapid and efficient energy/charge transfer to the redox-switched site and consequent nonradiative deactivation. Energy/charge transfer between nonadjacent porphyrins, which occurs principally by superexchange, is crucial to the operation of the T gate. Collectively, our studies elucidate the photophysics of gating and afford great flexibility and control in the design of more elaborate arrays for molecular photonics applications.

Light Harvesting and Directional Energy Transfer in Long‐Lived Homo‐ and Heterotrimetallic Complexes of FeII, RuII, and OsII

“…[1][2][3][4][5][6][7] Visible-light-harvesting ensembles have been prepared with a variety of chromophores including porphyrins, metalloporphyrins, and transition metal complexes having metal-to-ligand charge-transfer (MLCT) excited states. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] Among transition metal complexes, low-spin d 6 metal complexes derived from polypyridine ligands have been extensively investigated because of their excellent photoredox properties. [27,28] In particular, the unique combination of electrochemical and spectroscopic properties of Ru II and Os II complexes, which can be modulated in a systematic manner by synthetic manipulation, led to their widespread application for various light-induced functions such as molecular machines, dye-sensitized solar cells, optical sensors, and molecular electronics.…”

Resonance energy transfer: The unified theory revisited

“…[17][18][19] Notable amongst more complex, multichromophore systems are cyclodextrins 20,21 and other multiporphyrin arrays. [22][23][24] Dendrimers also feature prominently in recent research; these highly branched macromolecules comprise chromophores linked in fractal or other highly symmetric geometries 25 and much interest has focused upon energy transfer from their dendritic constituents to photoactive cores. 26 Dendrimers display a variety of photophysical effects resulting from intramolecular resonance energy transfer, including photoisomerization, 27,28 light-harvesting, 29,30 and directed energy transfer or funneling.…”

Energy transfer in macromolecular arrays

“…The bi-exciton initial state is described by (9) and, due to the extra excitation present in each state, we use fourth-order time-dependent perturbation theory to describe the photophysics. Each contribution corresponds to a quantum amplitude for three-centre energy transfer (3CET).…”

“…20 Taking the first term in (9) as an example, we can see that A and B are the two excited species (C is essentially dormant throughout this channel) -the three transfer pathways, in the context of the idealized dendrimer, are illustrated in figure 2. Adopting the language of interaction-pairs 21 we can correctly describe the three 3CET pathways.…”

“…6 These have been successfully designed to mimic behaviour observed in actual photosynthetic systems, 7,8 and also as the basis for optical molecular switches. 9 In this work we highlight theoretical studies into the formation and subsequent transfer of both excitons and bi-excitons within the above systems. Using the tools of molecular quantum electrodynamics (QED), 10 a recently developed statesequence technique 11 allows the correct formulation of the behaviour of both types of exciton transfer within an artificial three-fold symmetric edifice.…”