Excited-State Proton Transfer:  From Constrained Systems to “Super” Photoacids to Superfast Proton Transfer

Abstract

We have used knowledge of the electronic structure of excited states of acids to design molecules that exhibit enhanced excited-state acidity. Such "super" photoacids are the strongest reversible photoacids known and allow the time evolution of proton transfer to be examined in a wide array of organic solvents. This includes breaking/formation of the hydrogen bonds in hundreds of femtoseconds, solvent reorientation and relaxation in picoseconds, proton dissociation, and, finally, diffusion and geminate recombination of the dissociated proton, observed in nanoseconds.

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.…”

“…We suggest a picture similar to that proposed to explain the solvent dependence of the COOH-NH ESIPT process of the DHICA monomer 20 and indicated by quantum chemistry calculations of ICA−water complexes. 28 Water molecules are envisaged to take an active part in the transfer as in the model of Tolbert et al, 52,67 suggesting a proton accepting network formed by solvent molecules and OH substituents, or the solvent wire model of Leutwyler et al 68,69 Intermolecular ESPT involving proton transfer chains has also been extensively studied and discussed for various biological systems such as GFP. 64−66 Here, in the case of DHICA dimers and oligomers we suggest that the several OH-groups of the DHICA oligomers and surrounding solvent molecules define a pre-existing network of hydrogen bond acceptors along which the expelled proton can be transferred and solvated.…”

Solvent‐Dependent Photoacidity State of Pyranine Monitored by Transient Mid‐Infrared Spectroscopy

“…[1][2][3][4][5][6][7][8][9][10] Photoacids are known to donate a proton in aqueous solutions, or even in less polar solvents in the case of enhanced photoacids, where electron-withdrawing groups, such as cyano, sulfonate or perfluoroalkylsulfonyl groups, have been introduced. [7] The mechanism of the excited-state proton transfer (ESPT) to water, which typically occurs with relatively slow pace on time scales of tens to hundreds of picoseconds, [6] remains unclear.…”

Long‐Range Proton Conduction across Free‐Standing Serum Albumin Mats

“…The proton can then diffuse to bulk water or recombine with the excited anion to form the ROH* form in what is known as geminate recombination 33. Since the ROH* and RO −* forms have different emission wavelengths (for HPTS, 440 and 535 nm, respectively), it is relatively easy to follow their time‐resolved and steady‐state fluorescence 34, 35, 36, 37. In pure water, the proton diffuses rapidly from the photoanion, which results in a predominant RO −* species, as can be seen in the steady‐state emission spectra (Figure 2a).…”