Efficient Excited-state Proton Transfer Research Articles

Ultrafast excited-state proton transfer dynamics in dihalogenated non-fluorescent and fluorescent GFP chromophores.

Green fluorescent protein (GFP) has enabled a myriad of bioimaging advances due to its photophysical and photochemical properties. To deepen the mechanistic understanding of such light-induced processes, novel derivatives of GFP chromophore p-HBDI were engineered by fluorination or bromination of the phenolic moiety into superphotoacids, which efficiently undergo excited-state proton transfer (ESPT) in aqueous solution within the short lifetime of the excited state, as opposed to p-HBDI where efficient ESPT is not observed. In addition, we tuned the excited-state lifetime from picoseconds to nanoseconds by conformational locking of the p-HBDI backbone, essentially transforming the nonfluorescent chromophores into highly fluorescent ones. The unlocked superphotoacids undergo a barrierless ESPT without much solvent activity, whereas the locked counterparts exhibit two distinct solvent-involved ESPT pathways. Comparative analysis of femtosecond transient absorption spectra of these unlocked and locked superphotoacids reveals that the ESPT rates adopt an "inverted" kinetic behavior as the thermodynamic driving force increases upon locking the backbone. Further experimental and theoretical investigations are expected to shed more light on the interplay between the modified electronic structure (mainly by dihalogenation) and nuclear motions (by conformational locking) of the functionalized GFP derivatives (e.g., fluorescence on and off).

Unraveling the Mechanism of the Photodeprotection Reaction of 8‐Bromo‐ and 8‐Chloro‐7‐hydroxyquinoline Caged Acetates

Photoremovable protecting groups (PPGs) when conjugated to biological effectors forming “caged compounds” are a powerful means to regulate the action of physiologically active messengers in vivo through 1-photon excitation (1PE) and 2-photon excitation (2PE). Understanding the photodeprotection mechanism is important for their physiological use. We compared the quantum efficiencies and product outcomes in different solvent and pH conditions for the photolysis reactions of (8-chloro-7-hydroxyquinolin-2-yl)methyl acetate (CHQ-OAc) and (8-bromo-7-hydroxyquinolin-2-yl)methyl acetate (BHQ-OAc), representatives of the quinoline class of phototriggers for biological use, and conducted nanosecond time-resolved spectroscopic studies using transient emission (ns-EM), transient absorption (ns-TA), transient resonance Raman (ns-TR2), and time-resolved resonance Raman (ns-TR3) spectroscopies. The results indicate differences in the photochemical mechanisms and product outcomes, and reveal that the triplet excited state is most likely on the pathway to the product and that dehalogenation competes with release of acetate from BHQ-OAc, but not CHQ-OAc. A high fluorescence quantum yield and a more efficient excited-state proton transfer (ESPT) in CHQ-OAc compared to BHQ-OAc explain the lower quantum efficiency of CHQ-OAc relative to BHQ-OAc.

Water/humidity and ammonia sensor, based on a polymer hydrogel matrix containing a fluorescent flavylium compound

Flavylium compounds can conveniently be encapsulated in water permeable cross-linked poly(2-hydroxyethyl methacrylate) (PHEMA) polymer hydrogel matrices. The polymer is also permeable to gases, for example ammonia and hydrogen chloride. The ability of the polymer to concentrate water from the surrounding atmosphere was demonstrated. The absorption and emission spectra of the encapsulated flavylium compounds are very dependent on the pH, and by consequence ammonia (or hydrogen chloride) can be easily detected. The intensity of the fluorescence emission of the flavylium cation (AH+ species obtained in acidic media) is very dependent on the water content due to the efficient excited state proton transfer involving the AH+* excited state and water. The combination of flavylium emission sensitivity to water and ability of the polymer to concentrate water from the surrounding atmosphere, confers this material potential utility as a humidity sensor with high sensitivity.

Micelle effect on ground and excited state proton transfer reactions involving the 4-methyl-7-hydroxyflavylium cation

In aqueous solution, the 4-methyl-7-hydroxyflavylium ion (AH+) undergoes a ground state dissociation reaction to form the quinoidal base A and, when excited, an efficient excited state proton transfer (ESPT) leading to *A. The effect of micelles on the ground and excited state proton transfer reactions has been investigated. As far as the acid–base properties of the ground state system are concerned, negatively charged SDS micelles stabilize the acidic form AH+, while the positively charged CTAB and, to a lesser extent, the neutral Triton X 100 micelles destabilize AH+. Moreover, the neutral form A exhibits a strong decrease in molar absorption coefficient that cannot be explained by electrostatic reasons only, suggesting that specific interactions of A with the hydrophobic part of the surfactants may play some role. Even the excited state dissociation constant (Ka*) is affected by the micelles, and the pKa* is positively or negatively shifted with respect to that in water by negatively or positively charged micelles, respectively. It was also observed that the excited states can be intrinsically affected by the micelles. In particular, the excited AH+ is not only stabilized by the negative SDS micelle, but also exhibits a luminescence lifetime longer than that in water, most probably because of the electrostatic interaction exerted on it by the negative charge of the micelles.

Proton transfer in neutral gas-phase clusters: α-Naphthol⋅(NH3)n

Efficient excited-state proton transfer in neutral acid–base clusters α-naphthol⋅Bn has been detected and studied by a combination of laser spectroscopic techniques (resonant two-photon ionization, fluorescence excitation, and emission spectroscopy). S1 state proton transfer was observed for B=NH3 and n≥4, as evidenced by several criteria: (a) large red shift and substantial broadening of the R2PI spectra of the n≥4 clusters relative to those of the bare α-naphthol and smaller clusters; (b) very large Stokes shift (∼8000 cm−1) of the emission spectra of the n≥4 clusters; (c) complete broadening of the fluorescence emission band for the n≥4 clusters; and (d) a striking similarity of the emission band position and width of the latter spectra to the emission spectrum of the α-naphtholate anion in basic aqueous solution. No proton-transfer reaction was observed for small solvent clusters with B=NH3 and n≤3, nor for any of the pair complexes studied, which involve a single base partner [B=triethylamine, 3-dimethylamino-1-aminopropane, 1,4-bis(dimethylamino)butane] which we have studied so far. This behavior illustrates the difficulty of achieving charge separation in neutral gas-phase complexes or clusters. A critical gas-phase proton affinity PAcrit =248±3 kcal/mol was determined for proton transfer to take place in the α-naphthol⋅Bn (or base B) system.

Excited-state proton transfer in neutral microsolvent clusters: α-naphthol·(NH 3) n

Efficient excited-state proton transfer was detected by a combination of laser spectroscopic techniques (laser-induced fluorescence, resonant two-photon ionization) in neutral microsolvent clusters α-naphthol·(NH 3) n , which were synthesized and cooled in supersonic expansions. Excited-state zwitterions α-naphtholate −·(NH 3) n ·H + are formed for n ⩾ 4, but not for n = 1–3.

Intramolecular single and double proton transfer in excited benzoxazoles. Distinction between concerted and stepwise reaction mechanisms

Two investigated“double” hydroxy-benzoxazoles, I and III, reveal the efficient excited state proton transfer (PT). The comparison between the spectroscopy of I and III and their monomethoxy-derivatives, II and IV, allows to distinguish between the processes of single and double PT. For bis-2,5-(2-benzoxazolyl)hydrochinine (I) the excited state equilibrium between primary (H—bonded) and tautometric (single PT) species is established, as is seen from the equality of both fluorescence lifetimes. In bis-3,6-(2-benzoxazolyl) pyrocatechol (II) two important facts are stated: excited states double PT takes place; the motions of two protons are most probably concerted. The concept of cooperativity is discussed basing on two examples of double intramolecular excited state PT reactions.