Photobasicity in Quinolines: Origin and Tunability via the Substituents’ Hammett Parameters

Abstract

Coupling between electronic excitation and proton transfer is relevant to the kinetics of redox reactions, in particular those involved in solar-tofuel light harvesting. A prime example of such coupling occurs in photoacids, where electronic excitation leads to proton release in the excited state. Here, we systematically study the inverse of this effect, photobasicity, in which a molecule becomes more basic in the excited state compared to the ground state. This endows the molecule with light induced proton removal capability which is anticipated to be of use in driving reactions where proton transfer is kinetically challenging. To investigate the origins and tunability of photobasicity, a set of 5-R-quinoline derivatives (R = {NH 2 , CH 3 O, H, Br, Cl, CN}) were selected and their changes in pK a upon electronic excitation in aqueous solutions were determined. The Hammett parameters σ p of these substituents, indicative of their electron withdrawing capability, span a range of −0.7 to +0.7. Using Forster cycle analysis, the acid dissociation equilibria in the ground and first excited state were determined. The ground state pK a obeys an expected linear relationship with respect to the Hammet parameter σ p . An important finding of our work is that the excited state pK a * also obeys a linear relationship with respect to σ p . Interestingly, the excited state pK a * is ∼5 times more sensitive to the electron-withdrawing power of the substituent than the ground state pK a . We attribute this difference to the larger polarizability of the excited state charge density. Increase in pK a due to optical excitation ranging between 2.2 (R = CN) and 10.6 (R = NH 2 ) units were observed within the set. This substantial range of ΔpK a values may find use in applications such as oxidation catalysis, in which optically induced removal of protons could speed up reaction kinetics. Finally, we comment on the correlation between photobasicity and enhancement of electronic charge density on the heterocyclic nitrogen upon optical excitation.

Shedding new light on an old molecule: quinophthalone displays uncommon N-to-O excited state intramolecular proton transfer (ESIPT) between photobases

“…Excited state tautomerization is usually accompanied by electron redistribution after vertical excitation 34 , and the resulting change in the relative acidity and basicity is explained by the change in electron density at ‘heavy atoms’ (nitrogen or oxygen) 35 . Calculations of electrostatic charge at atoms related to photoacidity have been presented as supporting evidence in several studies of established systems 33, 36 . Results of our NBO calculation may be interpreted in similar way.…”

“…More cases are known if dimerized molecules are considered 32 . However, what makes our case unique is that the proton donor of QPH is quinoline, which is better known for its photobasicity, meaning that it is more likely to accept a proton in the excited state 33 .…”

Quinone 1 e– and 2 e–/2 H+ Reduction Potentials: Identification and Analysis of Deviations from Systematic Scaling Relationships

“…Linear correlations between thermodynamic properties have provided insights into many catalytic systems, 15d,23 and the pKa values and reduction potentials of quinone derivatives have been shown previously to correlate with Hammett parameters. 7d,24 Therefore, the pKa and E° data acquired for the 134 quinones were plotted as a function of an effective Hammett constant, defined as the sum of the substituents’ Hammett constants (Σσ).…”

Synthesis, characterization, and cellular localization of a fluorescent probe of the antimalarial 8-aminoquinoline primaquine

“…The 1 L B state is a bond centered state with excess charge density on the quino-lone ring while the 1 L B state is an atom centered excess charge density state with the excess charge density localized on the heterocyclic nitrogen. 9 The spectrum of coumarin contains a single transition centered at 320 nm that arises from a strongly allowed π-π* transition overlapping with a weaker 𝑛-π* transition. 10 The absorption spectra of the PQCP complex are essentially the sum of the PQand coumarin spectra, indicating there are no intramolecular interactions between the ring systems of the two chro- mophores within the complexes.…”