Journal of Photochemistry and Photobiology (Sep 2020)
Optical spectroscopy of cryogenic metalated flavins: The O2(+) isomers of M+lumiflavin (M=Li–Cs)
Abstract
Flavin complexes are used by nature in many photobiological processes, because their photochemical response in the visible range can be strongly modulated by their environment. Herein, we report optical spectra of mass-selected metal complexes of lumiflavin (LF) with alkali metal ions (M=Li–Cs) recorded in the visible range by photodissociation (VISPD) at cryogenic temperatures (T<20 K). VISPD spectra are measured in a tandem mass spectrometer coupled to a cryogenic ion trap and an electrospray ionization source. The vibrationally-resolved VISPD spectra obtained in the 420–450 nm range are assigned to the S0←S1 (ππ*) transition of the O2(+) isomers of M+LF by comparison to time-dependent density functional theory calculations (PBE0/cc-pVDZ) coupled to multidimensional Franck–Condon (FC) simulations. The preferred binding motif and interaction strength strongly depend on the size of M+. The spectra of M+LF with the smaller M+ ions (M=Li and Na) are attributed to the most stable O2+ isomers characterized by a N1–M–O2 chelate binding motif in which M+ can interact with the lone pairs of both N1 and O2 of LF. Because of steric interaction with the CH3 group at N10, the tricyclic aromatic ring is slightly bent and the VISPD spectra feature low-frequency out-of-plane bending modes of LF. Such an O2+ structure is sterically repulsive for the larger M+ ions (M=Rb and Cs), which instead form planar O2 minima with nearly linear C2–O2–M bonds. Their VISPD spectra are characterized by low-frequency in-plane bending modes (β) describing the M+…LF interaction. The VISPD spectrum of K+LF with the intermediate-size K+ ion is more complex and features very low-frequency modes resulting from a very shallow potential along the O2↔O2+ isomerization coordinate, which cannot be described well by harmonic FC simulations. Electronic S1 excitation slightly weakens the M+…LF interaction (6–9%), as deduced from the small ΔS1 blueshifts upon metalation at the O2(+) binding site, consistent with the charge reorganization deduced from the molecular orbitals involved in ππ* excitation. Overall, the effects of O2(+) complexation of LF are drastically different from those of the previously studied O4+ isomers, which are characterized by large ΔS1 redshifts based on the strong increase in the M+…LF interaction (up to 20%) upon the same ππ* excitation of the LF chromophor. The drastic site-specific variation in the photophysical response of the O2(+) and O4+ isomers arises mainly from the different effect of electronic excitation on the M+…LF bond strength rather than from the metal-induced change of the electronic structure and molecular orbitals of LF responsible for ππ* excitation.
