Ultrafast Proton Transfer to Solvent:  Molecularity and Intermediates from Solvation- and Diffusion-Controlled Regimes

Abstract

Photoinduced proton transfer (PT) from cations 6-hydroxyquinolinium (6HQc) and 6-hydroxy-1-methylquinolinium (6MQc) to water and alcohols, and solvation of the zwitterionic conjugate base 1-methylquinolinium-6-olate (6MQz) were studied with stationary and transient absorption spectroscopy and by quantum chemical calculations. Transient emission spectra from 6MQz in acetonitrile and protic solvents shift dynamically to the red without changing their shape and intensity. The shift matches the solvation correlation function C(t) either measured with known solvatochromic probes coumarin 343 and coumarin 153 or derived from infrared/dielectric-loss data on neat solvents. This indicates that 6MQz monitors the solvation dynamics and that no intramolecular electron transfer occurs on a subpicosecond or longer time scale. The PT dynamics S(t) from 6HQc and 6MQc closely follows C(t), being initially 2-3 times slower. This allows for the conclusion that PT is controlled by solvation, with a barrier of 2 kJ/mol. In water, a pre-condition of this ultrafast reaction seems to be hydrogen-bonding between the negatively charged oxygen and two water molecules, resulting in a complex 6HQc:H2O:H2O. The complex is stable due to a high (47 kJ/mol) bonding energy between 6HQc and a water molecule. In acetonitrile, the reaction equilibrium is strongly shifted to the cation. There an intermediate PT state was detected, which may be ascribed to the cationic form 6HQc:H2O due to residual water impurities. In water-acetonitrile mixtures, the ultrafast solvent-controlled PT is followed by a diffusion-controlled reaction; the measured rate kD approximately 1010 s-1 M-1 is characteristic for simple bimolecular diffusion. The dependence of the short-time PT signal on water concentration can be fitted with a Poisson distribution of water molecules around the cation. Altogether, the short-time and long-time behaviors provide strong evidence that diffusion of only one water molecule is sufficient to detach the proton. Subsequent solvent stabilization of the products completes the PT reaction.

Ultrafast Photoinduced Charge Separation in Naphthalene Diimide Based Multichromophoric Systems in Liquid Solutions and in a Lipid Membrane

“…However, this negative band is 30 nm redshifted relative to the stimulated emission spectrum of R1, calculated by multiplying its steady-state fluorescence intensity by λ 4 . 57 Moreover, no negative signal due to the bleach of the S 0 -S 1 absorption can be observed around 530 nm. This indicates that the S 1 -state absorption spectrum of R1 most probably extends over the whole visible region and masks the groundstate bleach signal and the blue side of the stimulated emission spectrum.…”

Wagging motion of hydrogen-bonded wire in the excited-state multiple proton transfer process of 7-hydroxyquinoline·(NH3)3 cluster

“…However, some key molecular processes of proton transfer via hydrogen-bonded wire in biological protein have remained poorly understand so far due to the complex environment, such as the green fluorescent protein (GFP) [32]. Hence, studying on hydrogen-bonding dynamics in the excited-state multiple proton transfer (ESMPT) via hydrogen-bonded wire becomes one of the most popular topics in nowadays, which is very useful for biological investigations [33][34][35][36][37][38][39][40][41][42][43][44][45][46].…”

“…And the resulting ketonic form (8KQ) is supposed to be barely emissive [35]. On the other hand, the hydrogen-bonded wire can be hardly formed in 6-hydroxyquinoline (6HQ) [37][38][39], which requires an extreme long bridge to connect the functional groups. We believe that, the hydrogen-bonded wire with two or three components (H 2 O, NH 3 or alcohols, etc.)…”

“…They explained that the fluorescence is forbidden due to symmetry. However, with the same symmetry, the fluorescence from both the cationic and anionic forms of 6HQ have been observed in emission spectra [37,39]. Meanwhile, by using CASSCF and CASPT2 calculations without symmetry constraints, Serrano-Andrés and co-workers suggested that [43], ESMPT would be preferred over hydrogen-atom transfer in the excited-state tautomerization of 7HQÁ(NH 3 ) 3 .…”

Early events associated with the excited state proton transfer in 2-(2′-pyridyl)benzimidazole

“…17,18 Proton transfer in acetonitrile-water mixture from strong photoacids 6-hydroxyquinolinium and 6-hydroxy-1-methylquinolinium is associated with a rather long time constant of approximately 1 ns, the process being diffusion controlled. 19 Long time constants of approximately 100 ps have been observed in the perylene quinones, hypocrellin, and calphostin C as a result conformational change and chain dynamics associated with the proton transfer a͒ Author to whom correspondence should be addressed. Electronic mail: anindya@chem.iitb.ac.in.…”