Shedding new light on an old molecule: quinophthalone displays uncommon N-to-O excited state intramolecular proton transfer (ESIPT) between photobases

Abstract

Excited state dynamics of common yellow dye quinophthalone (QPH) was probed by femtosecond transient absorption spectroscopy. Multi-exponential decay of the excited state and significant change of rate constants upon deuterium substitution indicate that uncommon nitrogen-to-oxygen excited state intramolecular proton transfer (ESIPT) occurs. By performing density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations, we found that adiabatic surface crossing between the S1 and S2 states takes place in the photoreaction. Unlike most cases of ESIPT, QPH does not exhibit tautomer emission, possibly due to internal conversion or back-proton transfer. The ESIPT of QPH presents a highly interesting case also because the moieties participating in ESIPT, quinoline and aromatic carbonyl, are both traditionally considered as photobases.

Time-resolved photoelectron spectroscopy of organic molecules in aqueous solutions

“…This fast change of the photoelectron spectrum is attributed to the solvent cage rearrangement, which is followed by electronic relaxation on one or several timescales specific to the solute. For the case of Quinoline Yellow the decay is reminiscent of the recently proposed excited state intramolecular proton transfer [10]. The relaxation of Methyl Orange and Metanil Yellow is consistent with ultrafast trans-cis isomerization typical for azo-benzene derivatives.…”

Spectral Signatures of Ultrafast Excited-State Intramolecular Proton Transfer from Computational Multi-edge Transient X-ray Absorption Spectroscopy

“…Many technologies and biological functions such as stable white light generation, enzymatic reactions, and several cellular processes rely on efficient energy transfer mediated by excited-state intramolecular proton transfer (ESIPT). − Ultrafast ESIPT processes occurring on the sub-100 fs time scale often involve coupled motions of electrons and atoms. The efficiency and speed of an ESIPT is dependent on a number of different factors, including the coupling between atomic and electronic degrees of freedom and the extent of electronic delocalization on the excited state.…”

Femtosecond Extreme Ultraviolet Photoelectron Spectroscopy of Organic Molecules in Aqueous Solution

“…31 Recently, relaxation dynamics of quinophthalone attracted interest, possibly featuring an uncommon N-to-O excited-state intramolecular proton transfer (ESIPT). 33 We first conduct XUV-only experiments to access the ground-state photoelectron spectrum of the QY molecule in solution. Figure 1b shows photoelectron spectra of a 10 mM solution of QY in water (orange line) and photoelectron spectra of water without QY (blue line), both resulting from ionization by 26.5 eV XUV photons.…”

“…We now turn to investigation of the relaxation dynamics of QY upon photoexcitation. QY is a yellow dye with absorption peaking at 412 nm in water solutions and low fluorescence yield, 33 which indicates fast relaxation pathways for the bright electronic state. We follow these relaxation dynamics in a timedependent pump−probe experiment carried out by introducing a UV pump pulse at a 400 nm wavelength and scanning the relative delay between the UV pump pulse and the XUV probe pulse while recording photoelectron spectra for each delay step.…”

“…To assess the origin for the two slower time scales, 1.3 and 90 ps, we refer to the recent publication of Han and coworkers, 33 who investigated relaxation of quinophthalone and deuterated quinophthalone in cyclohexane and concluded that the first relaxation process in the excited state is the intramolecular proton transfer (ESIPT) taking place between the N atom of quinoline and the O atom of 1,3-indandione. The fast decay component that they observed is 3.3 ps and depends on deuteration of the molecule at the NH site.…”