Solvatochromism of pyranine-derived photoacids

Abstract

Photoacidity is frequently found in aromatic alcohols where the equilibrium dissociation constant increases by some orders of magnitude upon electronic excitation. In this study we investigated the solvatochromism of a family of recently synthesized super-photoacids and their methylated counterparts based on pyrene. The chemical similarity of these molecules on the one hand and their differing photoacidity with pKa* values between -0.8 and -3.9 on the other allow for gaining insights into the mechanisms contributing to excited-state proton transfer. Three different solvent scales, namely Lippert-Mataga, Kamlet-Taft and Catalán, were independently employed in this study and gave consistent results. We found the strongest correlation of the excited-state acidity with the dipolarity of the excited state, pem ranging from -1775 cm(-1) to -2500 cm(-1), and a concomitant change in the permanent dipole moment of roughly 14 Debye. Spectral changes due to varying basicity of the solvent, which probes the conjugated property of the solute, are found to be less indicative of the graduation of excited-state acidity, i.e. bem values between -700 and -1200 cm(-1). The solvent acidity is the only parameter with a distinct influence on the electronic spectra of the deprotonated species. The low values of aem ~ 400 cm(-1), which are 3-4× smaller than aabs and aexc, indicate the low basicity of these species in the excited state. Triggered by semiempirical theoretical calculations, the energetic splitting between the two lowest excited states could be related to the excited-state acidity and points to alterations in the electronic mixing of locally excited and charge-transfer states, caused by the substituents. Differences between the threefold negatively charged pyranine and the new neutral photoacids are also discussed.

Multiresponsive Photo-, Solvato-, Acido-, and Ionochromic Schiff Base Probe

“…This is in agreement with the fact that hydrogen donor−acceptor characteristics of some molecules change in the excited state, 45,46 due to intramolecular charge transfer properties (ICT), see the Theoretical Calculations section.…”

Complexes of a naphthalimide photoacid with organic bases, and their excited-state dynamics in polar aprotic organic solvents

“…19,26 As a result, the optimization of the S 1 state of the dHONIÁ Á ÁIM complex directly leads to the deprotonated form indicating a nearly barrierless proton transfer i.e. The proton affinities were calculated as a negative enthalpy change (ÀDH 298 ), i.e.…”

“…[7][8][9][10][11][12][13][14][15][16][17][18] While the authors generally agree that the fastest and slowest time components correspond to solvation dynamics and the formation of the anionic species after the ESPT reaction, respectively, the intermediate step has had several interpretations such as relaxation to an intermediate electronic state, 9 a slow charge redistribution, 12,13 solvation dynamics or hydrogen-bond rearrangement, 14 or an additional step in the original two-step ESPT mechanism. 19 The overall picture is not very clear and a detailed description of the ESPT process in organic solvents is missing. 12,13 In their studies, the authors attributed the ambiguous fast process to a slow charge redistribution producing an intramolecular charge-transfer (ICT) state preceding the proton-transfer step.…”

A DFT and TDDFT investigation of interactions between 1-hydroxypyrene and aromatic amino acids

“…Therefore, it is very important to know their interactions with biologically important molecules. They can form intra-and intermolecular charge transfer complexes in solution [20]. Ground state interactions between pyrene and some amine systems have also been observed formerly [21].…”