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.

Structure–Photochemical Function Relationships in the Photobasicity of Aromatic Heterocycles Containing Multiple Ring Nitrogen Atoms

“…We used the same computational approach to calculate pK a * as in our previous study. 29 −E S 0 B is the difference in the electronic energy of the photobasic bright state, S PBS , evaluated at the S PBS optimized geometry and the electronic energy of the ground state, S 0 , evaluated at the S 0 optimized geometry. For all but the azaindole derivatives, we considered the excited-state protonation of both ring nitrogen atoms.…”

“…To facilitate comparison with the results from our previous study, the strongest mono-and disubstituted quinoline derivatives (open black symbols) with amine or methoxy substituents are also included in Figure 3. 29 Figure 3 shows that only one of the 1,8-naphthyridine compounds, 1,8-naphthyridine-3,4-diamine, was found to be a strong photobase with pK a * = 16.46. We attribute the relative scarcity of 1,8-naphthyridine strong photobases to the high symmetry of the parent compound which impedes the development of significant charge-transfer character in the S 0 → S PBS transition.…”

“…The overall substitution trends observed in our previous study of the quinoline derivatives are also seen in the 1,8- naphthyridine data. 29 For the monosubstituted 1,8-naphthyridines, the ring nitrogen on the same ring as the substituent is much less photobasic than the ring nitrogen on the opposite ring. In particular, for 1,8-naphthyridine-4-amine, pK a * = 13.01 for the nitrogen on the fused pyridine ring (N8) but pK a * = 2.84 for the nitrogen directly opposite from the amine group (N1).…”

“…This trend suggests that the thermodynamic driving force for photobasicity is only partially captured by Δq* and the degree of charge-transfer character in the S 0 → S PBS transition; we reached a similar conclusion in our previous study of quinoline photobases. 29 As shown in Figure S15 in the Supporting Information, adding a third benzene ring to cinnoline and quinazoline to create benzo[g]cinnoline and benzo[g]quinazoline, respectively, lowers the excitation energy, with all of the compounds we considered predicted to absorb in the visible region of the spectrum. Six of the benzo[g]cinnolines are strong photobases, all of which are disubstituted with both electron-donating substituents either on the terminal fused benzene ring or in positions 5 and 6.…”

“…Building off of this experimental work, we recently reported our computational study of how the thermodynamic driving force for excited-state protonation is tuned by changes to the molecular structure of photobases derived from quinoline. 29 We systematically explored the effect of substituent position on the photobasicity of the monosubstituted quinolines and found that the strongest photobases had electron-donating groups on the fused benzene ring in the 5, 6, or 8 position, although the presence of intramolecular hydrogen bonding between the substituent in the 8 position and the ring nitrogen atom greatly decreased the photobasicity strength. We showed that electron-donating substituents in these positions generate a buildup of electron density on the fused benzene ring in the ground state that shifts onto the fused pyridine ring in the excited state, resulting in an increase in electron density at the ring nitrogen atom.…”