Correlation among Hydrogen Bond, Excited-State Intramolecular Proton-Transfer Kinetics and Thermodynamics for −OH Type Proton-Donor Molecules

Abstract

A series of new molecules bearing alkyl-substitutes on the parent molecule 1-hydroxy-11H-benzo­[b]­fluoren-11-one (HBF) has been designed and synthesized, which possesses an intramolecular hydrogen bond (H-bond) between −OH proton donor and carbonyl proton acceptor. All studied molecules present an equilibrium type of excited-state intramolecular proton transfer (ESIPT) at 298 K. The alkyl-substitutions at various positions subtly alter the intramolecular H-bond strength, which then fine-tune the excited-state equilibrium and hence thermodynamics between normal and tautomer species. These, in combination with finite rates of ESIPT resolved by femtosecond fluorescence upconversion techniques, lead to the establishment of an empirical relationship among H-bond strengths, ESIPT kinetics, and thermodynamics for HBF series of molecules, in which the stronger H-bond is, the faster and more exergonic ESIPT is. The intensity ratio for normal versus proton-transfer tautomer emission can be systematically tuned, demonstrating the harness of −OH type ESIPT reaction via facile alkyl-substituent perturbation.

Chapter Open for the Excited-State Intramolecular Thiol Proton Transfer in the Room-Temperature Solution

“…3NTF exhibits shorter a SO distance (2.925 Å) than 3TF (2.996 Å) and 3FTF (2.956 Å), which, empirically, indicates that 3NTF possesses the strongest H-bond among the three molecules. 33 Indirectly, this is verified by the computational simulation in which 3NTF indeed has the shortest HO bond length (2.049 Å) than that of 3TF (2.064 Å) and 3FTF Scheme 1. Two Synthetic Routes Used to Synthesize 3TF and the Synthesis of 3FTF, 3NTF, and 3MeTF a (2.057 Å) in the ground-state geometry (see Figure S41).…”

“…the non-proton transfer, normal emission). Such phototautomerism offers ample fundamental insights and can be specified with a barrierless process, a barrier induced, or a reversible process. − On the other hand, the diversification in the ESIPT mechanism renders different emission properties in terms of single, dual, and even multiple emissions that can be fine-tuned by varying the functionality, offering the versatility in applications such as in sensing, − bioimaging, − organic lasing, − and light-emitting diodes. ,,, …”

Counterion Migration Driven by Light-Induced Intramolecular Charge Transfer

“…Thus, it decreases the energy gap between the S 1 and S 0 states (see Figures S60, 61 and Tables S5, S6 for the computational approaches). 56 The relatively small absorption coefficient of the lowest-lying absorption peak ( Figure 1 and Table 1 ) and lack of vibronic progression imply that the lowest-lying absorption bands, in part, possess a charge-transfer (CT) character. Time-dependent density functional theory (TDDFT) computation also shows that the lowest lying transition is attributed to the charge transfer from the diphenyl amino (DPA) moiety to the pyridinium, which supports the above-mentioned hypothesis (see Figure S61 ).…”

Adiabatic deprotonation as an important competing pathway to ESIPT in photoacidic 2-phenylphenols

“…These results strongly support the previous theoretical approach and represent the first example of ESIPT to C-atom enhancement by using steric hindrance as a tool for controlling ground-state conformer distribution, and it also constitutes one of the few examples of ESIPT tuning via alkylsubstituent perturbation. [28]…”