Excited-State Proton Transfer from Embedded Photoacids to Silanols on Mesostructured Surfaces: Role of Structural Heterogeneity

Abstract

As a common process in nature, excited-state proton transfer (ESPT) is becoming increasingly attractive due to its wide applications in smart sensors and materials. Compared with well-known ESPT in bulk solution, knowledge of ESPT between excited-state photoacids and silanols on mesostructured surfaces is still limited. Herein, we present our work on ESPT behavior in photoacid-embedded mesostructured CTAB−silica films by using steady and picosecond time-resolved fluorescence spectroscopy. Compared with ESPT in solutions, two unique characteristics of photoacid−silanol ESPT were revealed: (1) deprotonated silanols act as ESPT acceptors, while protonated silanols affect ESPT by changing the basicity of the silica surface; (2) largely distinguished properties of Q 3 /Q 2 silanols and their non-uniform distribution on the surface lead to heterogeneous properties of the surface such as basicity that greatly affect ESPT dynamics. Our work provides new insight into the ESPT mechanism in mesostructured systems and inspiration for designing smart sensors and materials that utilize the sensitivity to proton concentration.

Fluorescence quenching of deprotonated phenylurea through twisting motion induced by an electron-donating substituent group

“…[1][2][3][4][5][6][7][8] Excited-state proton transfer reactions can be classified as excited-state intramolecular proton transfer (ESIPT) [9][10][11][12][13][14][15][16] and excited-state intermolecular proton transfer (ESPT). [17][18][19][20][21][22][23] The dynamic mechanism of ESIPT reaction is relatively perspicuous and can be usually described as a four-step cyclic phototautomerization process: excitation of the compound, proton transfer (PT) in the excited state, deexcitation to the ground state, and reversed PT in the ground state. Meanwhile, solute-solvent ESPT is often described via the well-known Eigen-Weller model 24,25 as a two-step reaction: formation of contact ion pairs (CIPs*) through a short-range PT from proton donor species (photoacid for example) to proton acceptor species (solvent or basic molecules) in the excited state, followed by separation of CIPs* into free ions dominated by diffusion motion.…”