Engineering opposite electronic polarization of singlet and triplet states increases the yield of high-energy photoproducts

Abstract

Efficient photosynthetic energy conversion requires quantitative, light-driven formation of high-energy, charge-separated states. However, energies of high-lying excited states are rarely extracted, in part because the congested density of states in the excited-state manifold leads to rapid deactivation. Conventional photosystem designs promote electron transfer (ET) by polarizing excited donor electron density toward the acceptor (“one-way” ET), a form of positive design. Curiously, negative design strategies that explicitly avoid unwanted side reactions have been underexplored. We report here that electronic polarization of a molecular chromophore can be used as both a positive and negative design element in a light-driven reaction. Intriguingly, prudent engineering of polarized excited states can steer a “U-turn” ET—where the excited electron density of the donor is initially pushed away from the acceptor—to outcompete a conventional one-way ET scheme. We directly compare one-way vs. U-turn ET strategies via a linked donor–acceptor (DA) assembly in which selective optical excitation produces donor excited states polarized either toward or away from the acceptor. Ultrafast spectroscopy of DA pinpoints the importance of realizing donor singlet and triplet excited states that have opposite electronic polarizations to shut down intersystem crossing. These results demonstrate that oppositely polarized electronically excited states can be employed to steer photoexcited states toward useful, high-energy products by routing these excited states away from states that are photosynthetic dead ends.

Electronic structure and photophysics of a supermolecular iron complex having a long MLCT-state lifetime and panchromatic absorption

“…4D), in close agreement with the 160-ps 3 MLCT lifetime acquired from ultrafast pump-probe spectroscopic data. The magnitudes of these matched lifetimes, coupled with the energy separation between the absorption and emission band maxima (Stokes shift = 3,076 cm −1 ) are consistent with an assignment of the FeNHCPZn photoluminescence as phosphorescence resulting from a 3 MLCT → S 0 radiative transition; note that the magnitude of this Stokes shift resembles that evinced for RuPZn phosphorescence (35). Despite the weak nature of this room-temperature phosphorescence, these data 1) provide an important measure of the 3 MLCT-state energy (E 0,0 = 810 nm = 1.53 eV) and 2) demonstrate direct 3 MLCT → S 0 photoluminescence from an Fe(II) complex using a conventional excitation source that does not employ fluorescence upconversion experimental methods (44,45).…”

“…Note that FeNHCPZn relaxation dynamics stand in marked juxtaposition to those described for FePZn, as FeNHCPZn's 3 MLCT NIR absorption decays simultaneously with the ground-state recovery (τ 3MLCT = 160 ps; Figs. 3C and 4), without the observance of any other new excited-state absorption signals: this excited-state dynamical behavior is identical to that manifest by RuPZn (26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36). These data thus indicate that 3,5 MC states play a negligible role in excited-state relaxation processes associated with the 3 MLCT state of FeNHCPZn.…”

“…However, at longer delay times (t > 1 ps), while the excited-state absorption features of FePZn resemble those of conventional Fe(II) complexes such as Fe(tpy) 2 (17), FeNHCPZn spectral evolution shows remarkable correspondence to that manifested by the RuPZn benchmark, congruent with the fact that this ligand design realizes an Fe(II) complex that features a low-lying excited state that recapitulates the MLCT photophysics elucidated for (polypyridyl)metal(II)-ethyne-(porphinato)zinc(II) supermolecular chromophores that exploit the heavy metals ruthenium and osmium (26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36). In sharp contrast to FeNHCPZn, the FePZn 3 MLCT state decays on an ultrafast timescale, accompanied by the rise of a new transient absorption signal having substantial oscillator strength over the ∼450-500-nm spectral region, which lies to the red of the PZn-derived B-band bleaching signal.…”

“…Ultrafast pump-probe transient absorption spectroscopic data acquired at early delay times (<1 ps) demonstrate that both FePZn and FeNHCPZn exhibit excited-state absorption features similar to those characteristic of bis-(terpyridyl)metal(II)ethyne-(porphinato)zinc(II) supermolecules such as RuPZn (Fig. 3) (26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36). Immediately following S 0 → S 1 photoexcitation, near-infrared (NIR) transient absorption signals centered at ∼780 nm are evident for both Fe(II) supermolecules ( Fig.…”

Supermolecules steer electrons down a wrong-way street

“…The problem of ET directionality has long fascinated scientists and engineers (1), and researchers in the field of electron donor (D)-acceptor (A) interactions are now deeply immersed in developing methods to exert control over electron motion in molecules, metamaterials, and bulk materials on micrometer and nanometer scales. The PNAS paper by Polizzi et al (2) reports results on directional ET reactions in so-called "supermolecules," inspired by the ET cascade found in the photosynthetic reaction center of plants and certain bacteria. This fundamental work is certain to enhance and stimulate new research in the field.…”