Ultrafast Excited-State Proton Transfer to the Solvent Occurs on a Hundred-Femtosecond Time-Scale

Abstract

Steady-state and ultrafast time-resolved techniques were used to study a newly synthesized photoacid, phenol-carboxyether dipicolinium cyanine dye, QCy9. We found that the excited-state proton transfer (ESPT) to water occurs at the remarkably short time of about 100 fs, k(PT) ≈ 1 × 10(13) s(-1), the fastest rate reported up to now. On the basis of the Förster-cycle, the pK(a)* value is estimated to be -8.5 ± 0.4. In previous studies, we reported the photoacidity of another superphotoacid, the QCy7 for which we found an ESPT rate constant of ~1.25 × 10(12) s(-1), one-eighth that of the QCy9 compound. We found a kinetic isotope effect of the ESPT of about two.

Photoacid behaviour in a fluorinated green fluorescent protein chromophore: ultrafast formation of anion and zwitterion states

“…The calculation leads to a of –4, which, although less negative than those reported by Simkovich et al ,30 places DF p -HBDI firmly in the class of super-photacids ( < –2). However, the rate of ESPT seems to be not only a function of the solute (donor), but also of the solvent (acceptor).…”

“…We can investigate this more quantitatively by using a Forster cycle to calculate the p K a of the excited electronic state of DF p -HBDI, 30,31. This is obtained spectroscopically from: …”

Superior Photoprotective Motifs and Mechanisms in Eumelanins Uncovered

“…Such proton transfer often occurs along a pre-existing hydrogen bond and therefore becomes very fast, frequently on the ∼100 fs time scale or even faster. 36 ESPT to solvent has been studied for more than 60 years, 52,57,58 and many theoretical models have been proposed to describe various aspects of the transfer process, e.g. the dependence of transfer kinetics on solvent, temperature, pressure, deuterium substitution, complex formation, etc.…”

The critical size of hydrogen-bonded alcohol clusters as effective Brønsted bases in solutions

“…Compared to water, the H-bond network of ROH is different in that the presence of the alkyl group (R) allows for only one H-bond donating site and two H-bond accepting sites in the hydroxyl group (OH), yielding the (quasi) one-dimensional linear H-bonded chain. 14 Here, we study the intermolecular proton-transfer reaction from a photoacid, [15][16][17][18][19][20][21] which is as strong as sulfuric acid (pK a B À2) when photoexcited, 22 to various ROHs of different basicity in the condensed phase. 10 The H-bond accepting (and donating) ability of bulk ROH, like that of bulk water, is known to be superior to that of ROH monomers.…”