Structure-Photochemical Function Relationships in Nitrogen-Containing Heterocyclic Aromatic Photobases Derived from Quinoline

Abstract

Photobases are compounds which become strong bases after electronic excitation.Recent experimental studies have highlighted the photobasicity of the 5-R quinoline compounds, demonstrating a strong substituent dependence to the # * . In this paper we describe our systematic study of how the photobasicity of four families of nitrogen-containing heterocyclic aromatics are tuned through substituents. We show that substituent position and identity both significantly impact the # * . We demonstrate that the substituent effects are additive and identify many disubstituted compounds with substantially greater photobasicity than the most photobasic 5-R quinoline compound identified previously. We show that the addition of a second fused benzene ring to quinoline, along with two electron-donating substituents, lowers the S0®SCT vertical excitation energy into the visible while still maintaining a # * > 14.Overall, the structure-function relationships developed in this study provide new insights to guide the development of new photocatalysts that employ photobasicity.

Computationally Probing the Mechanism of the Blue-Light-Driven O–H Functionalization of Alcohols by Aryldiazoacetates: Photobasicity or Carbene Chemistry

“…As illustrated in Table S3 in the Supporting Information, the strong photobase 5-aminoquinoline has ΔΔG = −14.3 kcal/mol, the weak photobase 5-cyanoquinoline has ΔΔG = −4.4 kcal/ mol, and 4-aminoquinoline, which is a stronger base in the ground state, has ΔΔG = 2.2 kcal/mol. 22 For the cis-and trans-conformers of MPDA, the calculated ΔΔG values of 17.3 and 17.9 kcal/mol demonstrate that electronic excitation causes a large decrease in the basicity of the diazo carbon of MDPA. This thermodynamic analysis, in conjunction with the results presented in Figure 2 and Tables S1 and S2, demonstrates that the blue-light-induced O−H functionalization of alcohols by MDPA does not occur through photobasicity.…”

“…The Forster cycle has been used by us and others to relate the thermodynamic driving force for photobasicity, ΔG*, to ground-state energetics and spectroscopic observables 22,24,38…”

“…The adiabatic energy gaps required for calculating ΔΔG were obtained using the same computational procedure as used in our previous studies of N-heterocycle photobases. 22,23 Specifically, the geometry optimizations used the ωB97X-D exchange-correlation functional with a (99,590) integration grid, a def2-SVPD basis set, and a CPCM solvation model. The excited-state geometry optimizations were performed within the Tamm Dancoff approximation (TDA).…”

Can Brønsted Photobases Act as Lewis Photobases?

“…We used electronic structure calculations to further characterize the Lewis photobasicity of 5-R-quinolines. Our approach is similar to the one used by the Petit group studying proton transfer energetics of a series of photobases 37,38 with some small modifications made to suit the study of dative bond complexes. Their computational results were shown to correlate quite well with experimental values for the excitedstate drive for proton transfer in quinoline photobases.…”

“…Their computational results were shown to correlate quite well with experimental values for the excitedstate drive for proton transfer in quinoline photobases. 37 We will outline our procedure here and refer the reader to Petit's prior work for a more detailed discussion. Paralleling the experimental work, a computational Forster cycle requires the calculation of three energies: the 0−0 gap between the ground state and the photobasic electronic ), the 0−0 gap between the ground state and the photobasic electronic state of the adducted quinoline (ΔE 00 adduct ), and the ground-state energy for adduct dissociation (ΔE GS ).…”