Krishan Kumar

Cryptochromes are blue-light-sensitive flavoproteins that play central roles in biological function. In European robin (Erithacus rubecula) ErCry4a proteins, optical excitation of their flavin chromophore forms a long-lived radical pair through a sequence of electron-transfer steps across a tetradic chain of tryptophan residues, making them primary candidates for magnetoreception in night-migratory songbirds. Recent quantum chemical calculations indicate that nonadiabatic couplings play a central role in the energy- and charge-transfer processes initiated by optical excitation. Here, we study these dynamics in ErCry4a using ultrafast transient absorption spectroscopy with 10 fs time resolution in the 450 nm spectral range. We uncover a rapid, sub-50 fs red shift in stimulated emission, quenched within 360 fs by electron transfer from a nearby tryptophan moiety. While high-frequency excited-state vibrations are rapidly damped, coherent motion involving several low-frequency vibrations persists during both the initial energy relaxation and the subsequent electron transfer. This is evidenced by probing the coherent vibrational motion of the formed FAD•– radical anion and is independently validated by blocking the electron transfer through site-selective tryptophan mutation. Our results provide insight into the role of nonadiabatic couplings for the initial steps of cryptochrome photoactivation and suggest a general strategy for redox-state-specific monitoring of charge transfer dynamics by probing coherent vibrational motion.

Photoactivated intermolecular electron transfer (ET) in cryptochromes proceeds along chains of aromatic residues and creates a spatially separated pair of radical electrons. Ultrafast time-dependent spectroscopy can provide experimental insight into this process and theoretical estimates of charge transfer rates are commonly obtained via Marcus theory. Here, we present a new perspective on the ET in European robin cryptochrome 4a (ErCry4a) that synthesizes insights from real-time ET calculations, ultrafast spectroscopic measurements and analytical derivations. The simulations exemplify that molecular vibrations play an essential role in enabling the ET dynamics, which was further rationalized through analytical derivations. Ultrafast pump-probe spectroscopy provided experimental access to the first 1.5 ns of the ET cascade, where multiple radical pair recombination rates arise due to the dynamic equilibrium along the ET chain. We show that the motions of the protein environment and the ET dynamics are inseparably coupled, violating the timescale separation required for Marcus theory. The presented results highlight that non-equilibrium coupling between electronic and nuclear motion dominates ET kinetics in ErCry4a during the first nanosecond after photo-excitation. The findings exemplify the limits of Marcus theory and refine the interpretation of ultrafast spectroscopic signatures in cryptochromes.

Flavins are the chromophores in several blue-light-sensitive photoreceptor proteins and act as redox cofactors in many enzymes that are relevant for biological processes. Despite their biological relevance and numerous detailed optical investigations of their photophysical properties, the ultrafast nonequilibrium dynamics of their elementary optical excitations are not yet fully known. Here, we use ultrafast coherent vibrational spectroscopy with a 10-fs time resolution in the 450-nm spectral range to study their excited state coherent vibrational dynamics. We observe that coherent wavepacket motion along high-frequency C-C stretching modes with a ∼20-fs period is rapidly damped on a 20-fs time scale. In contrast, coherent motion along several low-frequency modes persists for much longer. We attribute this to a mode-selective intramolecular vibrational energy redistribution driven by nonadiabatic couplings between the optical bright state and a close-lying dark electronic state, in accordance with model calculations. Our results may be of relevance for the formation of long-lived radical pair states in magnetic-field sensitive proteins.

Absorption cross sections of multiexcitons are determined using pump-repump-probe transient absorption spectroscopy.

Applying an MCMC sampling approach for global target analysis enables us to determine the contributions of multiple exciton decay via Auger recombination and the contribution of surface exciton states to the observed decay dynamics.

Excitonic properties of colloidal quantum dots are strongly dependent on size and surface properties. The same is predicted to be valid for the properties of multiexcitons. To realize applications exploiting the generation of multiexcitons in colloidal nanocrystals, it requires a comprehensive understanding of size and surface influence e.g., on the lifetimes of multiexciton species. In this study, we employ intensity-dependent transient absorption spectroscopy to probe multiexcitons in long-chained organic (phosphonic acids and trioctylphosphine oxide), and short inorganic ligand (sulfide, S2−) capped colloidal CdSe quantum dots of four different sizes. To analyse the intensity-dependent transient absorption data a global fit method based on Markov Chain Monte Carlo sampling was employed. Applying a simple Auger recombination model lifetimes and spectra of multiexciton species were analysed. The spectra obtained for different multiexciton species exhibit both size and surface functionalization dependent features, which allow us to distinguish between different species. Independent of the surface modification, we find that the multiexciton lifetimes follow the volume scaling laws established earlier. Owing to the strong surface hole trapping induced by the S2− ligands, S2−-capped QDs show prolonged multiexciton lifetimes compared to the QDs capped with the native organic ligands with long alkyl chains. Deconvoluting contributions of bleach, stimulated emission and photoinduced absorption in the species spectra enables us to determine multiexciton binding energies. We observe a decrease of binding energies with increasing size and a clear reduction in multiexciton binding energies for the S2− -capped QDs. The observed trends can be explained by changes in the overlap of electron and hole wave functions depending on the QDs diameter and the charge carrier localization which can be induced by trapping in surface defect sites.