Ultrafast Excited-State Proton-Transfer Reaction of 1-Naphthol-3,6-Disulfonate and Several 5-Substituted 1-Naphthol Derivatives

Exploring Autoionization and Photoinduced Proton-Coupled Electron Transfer Pathways of Phenol in Aqueous Solution

The excited state dynamics of phenol in water have been investigated using transient absorption spectroscopy. Solvated electrons and vibrationally cold phenoxyl radicals are observed upon 200 and 267 nm excitation, but with formation time scales that differ by more than 4 orders of magnitude. The impact of these findings is assessed in terms of the relative importance of autoionization versus proton-coupled electron transfer mechanisms in this computationally tractable model system.

Solvent dependence of excited-state proton transfer from pyranine-derived photoacids

Steady-state and time-resolved techniques were employed to study the excited-state proton-transfer (ESPT) rate of two newly synthesized 8-hydroxy-1,3,6-pyrenetrisulfonate (pyranine, HPTS) derived photoacids in three protic solvents, water, methanol and ethanol. The ESPT rate constant k(PT) of tris(1,1,1,3,3,3-hexafluoropropan-2-yl)-8-hydroxypyrene-1,3,6-trisulfonate, 1a, whose pK(a)* ~ -4, in water, methanol and ethanol is 3 × 10(11) s(-1), 8 × 10(9) s(-1) and 5 × 10(9) s(-1) respectively. (8-Hydroxy-N1,N3,N6-tris(2-hydroxyethyl)-N1,N3,N6-trimethylpyrene-1,3,6 trisulfonamide, 1b) is a weaker acid than 1a but still a strong photoacid with pK(a)* ~ -1 and the ESPT rate in water, methanol and ethanol is 7 × 10(10) s(-1), 4 × 10(8) s(-1) and 2 × 10(8) s(-1). We qualitatively explain our kinetic results by a Marcus-like free-energy correlation which was found to have a general form suitable for describing proton transfer reactions in both the proton-adiabatic and the proton-non-adiabatic limits.

Unraveling ultrafast photoinduced proton transfer dynamics in a fluorescent protein biosensor for Ca(2+) imaging

Imaging Ca(2+) dynamics in living systems holds great potential to advance neuroscience and cellular biology. G-GECO1.1 is an intensiometric fluorescent protein Ca(2+) biosensor with a Thr-Tyr-Gly chromophore. The protonated chromophore emits green upon photoexcitation via excited-state proton transfer (ESPT). Upon Ca(2+) binding, a significant population of the chromophores becomes deprotonated. It remains elusive how the chromophore structurally evolves prior to and during ESPT, and how it is affected by Ca(2+) . We use femtosecond stimulated Raman spectroscopy to dissect ESPT in both the Ca(2+) -free and bound states. The protein chromophores exhibit a sub-200 fs vibrational frequency shift due to coherent small-scale proton motions. After wavepackets move out of the Franck-Condon region, ESPT gets faster in the Ca(2+) -bound protein, indicative of the formation of a more hydrophilic environment. These results reveal the governing structure-function relationship of Ca(2+) -sensing protein biosensors.

Vibrational excitation induced proton transfer in hydrated Nafion membranes

We study the energy relaxation and structural relaxation dynamics of hydrated protons in Nafion membranes at different hydration levels using femtosecond infrared transient absorption spectroscopy. At low hydration levels we observe that the excitation of the proton vibration of an Eigen-like proton hydration structure leads to a structural relaxation process in which the Eigen-like structure evolves to a Zundel-like proton hydration structure. This reorganization leads to a transfer of the proton charge and closely follows the mechanism of infrared-induced adiabatic proton transfer that has been proposed by S. Hammes-Schiffer, J. T. Hynes, and others. At high hydration levels, the spectral dynamics are dominated by vibrational energy relaxation and subsequent cooling of the proton hydration structures and the surrounding water molecules. Using a kinetic analysis of the transient spectral data, we determine the rates of proton transfer, vibrational energy relaxation, and cooling as a function of hydration level. We find that infrared-induced proton transfer occurs at all hydration levels but becomes less observable at high hydration levels due to the increasingly dominant influence of the vibrational energy relaxation.

Effective targeting of proton transfer at ground and excited states of ortho-(2′-imidazolyl)naphthol constitutional isomers

Besides electronic effects at the excited state, ground and excited state acidities are largely affected by proximity and structural alignment.

Excited state structural events of a dual-emission fluorescent protein biosensor for Ca²⁺ imaging studied by femtosecond stimulated Raman spectroscopy

Fluorescent proteins (FPs) are luminescent biomolecules that emit characteristic hues upon irradiation. A group of calmodulin (CaM)-green FP (GFP) chimeras have been previously engineered to enable the optical detection of calcium ions (Ca(2+)). We investigate one of these genetically encoded Ca(2+) biosensors for optical imaging (GECOs), GEM-GECO1, which fluoresces green without Ca(2+) but blue with Ca(2+), using femtosecond stimulated Raman spectroscopy (FSRS). The time-resolved FSRS data (<800 cm(-1)) reveal that initial structural evolution following 400 nm photoexcitation involves small-scale coherent proton motions on both ends of the chromophore two-ring system with a <250 fs time constant. Upon Ca(2+) binding, the chromophore adopts a more twisted conformation in the protein pocket with increased hydrophobicity, which inhibits excited-state proton transfer (ESPT) by effectively trapping the protonated chromophore in S1. Both the chromophore photoacidity and local environment form the ultrafast structural dynamics basis for the dual-emission properties of GEM-GECO1. Its photochemical transformations along multidimensional reaction coordinates are evinced by distinct stages of FSRS spectral evolution, particularly related to the ∼460 and 504 cm(-1) modes. The direct observation of lower frequency modes provides crucial information about the nuclear motions preceding ESPT, which enriches our understanding of photochemistry and enables the rational design of new biosensors.

Synthesis and spectroscopic characterization of 1,8-naphthalimide derived "super" photoacids

The ground- and excited-state acid-base properties of three novel naphthalimide-based “super” photoacids were studied using steady-state and time-resolved spectroscopy. The compounds exhibit pKa = 8.8-8.0 and pKa* = -1.2 to -1.9. The decrease in both ground- and excited-state pKa is achieved by attachment of an electron withdrawing group (sulfonate) on the aromatic system. All compounds are deprotonated upon excitation in alcohols and DMSO. Good correlation is established between the pKa* and the ratio of the neutral and anion emission intensities in a certain solvent. The excited-state intermolecular proton transfer to solvent (H2O and DMSO) is explained by a two-step model. In the first step, short-range proton transfer takes place, resulting in the formation of a contact ion pair. Free ion pairs are formed in the diffusion controlled second step.

Highly photostable "super"-photoacids for ultrasensitive fluorescence spectroscopy

The photoacid 8-hydroxypyren-1,3,6-trisulfonic acid (HPTS, pyranine) is a widely used model compound for the examination of excited state proton transfer (ESPT). We synthesized five "super"-photoacids with varying hydrophilicity and acidity on the basis of HPTS. By chemical modification of the three sulfonic acid substituents, the photoacidity is enhanced by up to more than five logarithmic units from pK*≈ 1.4 to ∼-3.9 for the most acidic compound. As a result, nearly quantitative ESPT in DMSO can be observed. The novel photoacids were characterized by steady-state and time-resolved fluorescence techniques showing distinctively red shifted spectra compared to HPTS while maintaining a high quantum yield near 90%. Photostability of the compounds was checked by fluorescence correlation spectroscopy (FCS) and was found to be adequately high for ultrasensitive fluorescence spectroscopy. The described photoacids present a valuable palette for a wide range of applications, especially when the properties of HPTS, i.e. highly charged, low photostability and only moderate excited state acidity, are limiting.

Early time excited-state structural evolution of pyranine in methanol revealed by femtosecond stimulated Raman spectroscopy

To understand chemical reactivity of molecules in condensed phase in real time, a structural dynamics technique capable of monitoring molecular conformational motions on their intrinsic time scales, typically on femtoseconds to picoseconds, is needed. We have studied a strong photoacid pyranine (8-hydroxypyrene-1,3,6-trisulfonic acid, HPTS, pK(a)* ≈ 0) in pure methanol and observed that excited-state proton transfer (ESPT) is absent, in sharp contrast with our previous work on HPTS in aqueous solutions wherein ESPT prevails following photoexcitation. Two transient vibrational marker bands at ~1477 (1454) and 1532 (1528) cm(-1) appear in CH3OH (CD3OD), respectively, rising within the instrument response time of ~140 fs and decaying with 390-470 (490-1400) fs and ~200 ps time constants in CH3OH (CD3OD). We attribute the mode onset to small-scale coherent proton motion along the pre-existing H-bonding chain between HPTS and methanol, and the two decay stages to the low-frequency skeletal motion-modulated Franck-Condon relaxation within ~1 ps and subsequent rotational diffusion of H-bonding partners in solution before fluorescence. The early time kinetic isotope effect (KIE) of ~3 upon methanol deuteration argues active proton motions particularly within the first few picoseconds when coherent skeletal motions are underdamped. Pronounced quantum beats are observed for high-frequency modes consisting of strong phenolic COH rocking (1532 cm(-1)) or H-out-of-plane wagging motions (952 cm(-1)) due to anharmonic coupling to coherent low-frequency modes impulsively excited at ca. 96, 120, and 168 cm(-1). The vivid illustration of atomic motions of HPTS in varying H-bonding geometry with neighboring methanol molecules unravels the multidimensional energy relaxation pathways immediately following photoexcitation, and provides compelling evidence that, in lieu of ESPT, the photoacidity of HPTS promptly activates characteristic low-frequency skeletal motions to search phase space mainly concerning the phenolic end and to efficiently dissipate vibrational energy via skeletal deformation and proton shuttling motions within the intermediate, relatively confined excited-state HPTS-methanol complex on a solvent-dependent dynamic potential energy surface.