Ground- and Excited-State Double Proton Transfer in Lumichrome/Acetic Acid System:  Theoretical and Experimental Approach

Abstract

Experimental time-resolved spectral and photon counting kinetic results confirm formation of an isoalloxazinic excited state via excited-state double proton transfer (ESDPT) catalyzed by a carboxylic acid molecule that forms a hydrogen-bond complex with the parent alloxazine molecule. This isoalloxazinic tautomer manifests itself as a distinct long-lived emissive species formed only in such alloxazine derivatives that were not substituted at the N1 nitrogen atom, being a product of the excited-state reaction occurring from the alloxazinic excited state. Theoretical calculations support the idea that the ESDPT occurs by the concerted mechanism. The calculated activation barrier in the excited state is much lower than the same barrier in the ground state and even disappears for the HOMO-1 to LUMO excitation, which explains the fact that the reaction takes place in the excited-state only. The reaction rate estimated from the emission kinetics is ca. 1.4 x 10(8) dm3 mol(-1) s(-1) in ethanolic solutions of lumichrome with added acetic acid.

Biologically inspired pteridine redox centres for rechargeable batteries

“…2e). After reduction (that is, discharge), the appearance of isoalloxazinic fluorescence at 520 nm and the disappearance of alloxazinic fluorescence at 450 nm at the discharged state were clearly observed, which agreed with previous reports on phototautomerism 26,27 . After recharging, the LC showed fluorescence at 450 nm again, suggesting that the tautomerism of the alloxazinic to the isoalloxazinic structure was completely reversible during the lithium-coupled redox reaction.…”

“…On the basis of these photophysical differences, previous studies on LC observed an interesting phenomenon called excited-state tautomerism, which is a structural isomerism from the alloxazinic to the isoalloxazinic type (Fig. 1c) 26,27 . However, few studies have explored the redox chemistry of LC.…”

“…After recharging, the LC showed fluorescence at 450 nm again, suggesting that the tautomerism of the alloxazinic to the isoalloxazinic structure was completely reversible during the lithium-coupled redox reaction. The photo-induced tautomerism of LC has been well characterized 26,27 , but the similar tautomerism to the isoalloxazinic structure that occured electrochemically during lithium-coupled electron reduction was unexpected.…”

Steady‐State Spectroscopy of the 2‐(N‐methylacetimidoyl)‐1‐naphthol Molecule

“…The TD/CIS/DFT methodology with constraints has given good results when studying the excited state intramolecular proton transfer in 2‐(2′‐hydroxyphenyl)imindazoles . The calculations leading to relative energies and geometrical parameters for the normal and tautomeric forms of the lumichrome–formic acid complex are also reported to be successful for the first excited state when “calculating at the TD‐B3LYP/6‐31G(d,p) for geometries along the reaction path in the first excited state calculated at CIS/6‐31G(d,p) level.” Also, the ESIPT process for the first excited state of HAN calculated at the TD/CIS/DFT level is reasonably good— from a qualitative point of view —when compared with the results obtained at the TD‐DFT/B3LYP//TZVP level with excited state full geometry optimization (see Figure S1, Supplementary Material). Excited state calculations for the MAN molecule at the TD/DFT‐B3LYP level with a basis set 6‐31G** and full geometry optimization are in progress.…”

Spectroscopic study on the intermolecular double proton transfer in 4-(naphthalen-1-yl)-6-octyl-1,3,5-triazin-2-amine with acid

“…The excited-state double proton transfer (ESDPT) refers to intramolecular or intermolecular PT reactions where two protons change their location in one molecule [15][16][17] or an intermolecular process (dimer [18][19][20] or solvent [21,22]/acid [23,24] assistance). However, few intermolecular proton transfer reactions of 1,3,5-triazine derivatives have been reported to the best of our knowledge.…”