Ultrafast Through-Space Electronic Energy Transfer in Molecular Dyads Built around Dynamic Spacer Units

Abstract

A pair of complementary molecular dyads have been synthesized around a 1,2-diaminocyclohexyl spacer that itself undergoes ring inversion. Despite these conformational exchange processes, the donor and acceptor occupy quite restricted spatial regions, and they are not interchangeable. The donor and acceptor pair comprise disparate boron dipyrromethene dyes selected to display favorable electronic energy transfer (EET). Steady-state emission spectroscopy confirms that through-space EET from donor to acceptor is almost quantitative, aided by the relatively short separations. Ultrafast time-resolved fluorescence spectroscopy has allowed determination of the rates of EET for both dyads. Surprisingly, in view of the close proximity of donor and acceptor (center-to-center separations less than 20 Å), the EET dynamics are well-accounted for in terms of the computed molecular conformations and conventional Förster theory. One dyad appears as a single family of conformations, but EET for the second dyad corresponds to dual-exponential kinetics. In this latter case, an intramolecular hydrogen bond helps stabilize an open geometry, wherein EET is relatively slow.

Molecular Porous Photosystems Tailored for Long‐Term Photocatalytic CO2 Reduction

“…Those findings are in good agreement with the calculated electronic structure of the lowest excited state of Cp*Rh@PyBpyCMP and Cp*Rh@PyBpyCMP (Figure ) exhibiting a strong LMCT character that results from the localization of the electron on the bipyridine moiety close to the metal center, whereas for the non‐complexed bipyridine, the excitation is localized on the chromophore moiety (pyrene or perylene, respectively). Therefore, our time‐resolved emission measurements firmly establish the efficient quenching of the excited states of the chromophores to form a LMCT‐like state with the electron localized close to the metal center . Indeed, the high quenching efficiencies for the LE states of up to 90 % are particularly remarkable because the Rh/chromophore ratio of only 1:20 in the solids necessarily implies long‐range interactions.…”

“…Therefore,o ur time-resolved emission measurements firmly establish the efficient quenching of the excited states of the chromophores to form aL MCT-like state with the electron localized close to the metal center. [24,50,51,53,54] Indeed, the high quenching efficiencies for the LE states of up to 90 %a re particularly remarkable because the Rh/chromophore ratio of only 1:20 in the solids necessarily implies long-range interactions.I nc omparison to the LE state,t he ICT states observed in both photosystems are less efficiently quenched (Figure 4e,f;S upporting Information, Table S6). These results suggest that the formation of ICT states lowers the efficiency of the photoinduced electron transfer (quenching), and thus reduces the yield of activation of the LMCT state especially in the pyrene-based materials (for further discussion, see the Supporting Information, Section 3.8).…”

Ultrafast Resonance Energy Transfer in Ethylene-Bridged BODIPY Heterooligomers: From Frenkel to Förster Coupling Limit

“…Some of us have demonstrated that the choice of solvent is able to force squaraine polymers to switch between coiled and zigzag superstructures, triggering ultrafast intrachain energy transfer events between these coexistent morphologies on a time scale of ∼70 fs . Several dye scaffolds were involved in similar studies on EET rates with either flexible tethers between the donor–acceptor pair, − a more rigid connectivity (alkynyl, − sp 2 –sp 2 , − stiff aliphatic linker, metal center), or even orthogonally aligned units. − In many cases, especially in the short-range regime, experimental data did not meet theoretical predictions, which led to further improvements of the theory, such as refined calculations of electronic couplings, , the introduction of electron–nuclear coupling effects (e.g., shared vibrational modes), the involvement of through-bond/exchange interactions, and the consideration of quantum coherence. − Another important question however is whether the rate of intramolecular EET processes is dependent on the aggregation modes, coupling strengths, and the extent of energetic disorder within the interacting donor and acceptor units, respectively.…”