Photoinduced Proton Transfer and Isomerization in a Hydrogen-Bonded Aromatic Azo Compound: A CASPT2//CASSCF Study

Abstract

Intramolecularly hydrogen-bonded aromatic azo compound 1-cyclopropyldiazo-2-naphthol (CPDNO) exhibits complicated excited-state behaviors, e.g., wavelength-dependent photoinduced proton transfer and photoproducts. Its photochemistry differs from that of common aromatic azo compounds in which cis-trans photoisomerization is dominant. To rationalize the intriguing photochemistry of CPDNO at the atomic level, we have in this work employed the complete active space self-consistent field (CASSCF) and its second-order perturbation (CASPT2) methods to explore the S0, S1, and S2 potential-energy profiles relevant to enol-keto proton transfer and isomerization reactions. It is found that the proton transfer along the bright diabatic (1)ππ* potential-energy profile is almost barrierless, quickly forming the fluorescent (1)ππ* keto minimum. In this process, the dark (1)nπ* state is populated via a (1)ππ*/(1)nπ* crossing point, but the proton transfer on this dark state is suppressed heavily as a result of a large barrier. In addition, two deactivation paths that decay the S1 enol and keto minima to the S0 state, respectively, were uncovered. For the former, it is exoenergetic and thereby thermodynamically favorable; for the latter, it is a little endothermic (ca. 5 kcal/mol). Both are energetically allowable concerning the available total energy. Finally, on the basis of the present results, the experimentally observed wavelength-dependent photoproducts were explained very well.

Computational Photochemistry of the Azobenzene Scaffold of Sudan I and Orange II Dyes: Excited‐State Proton Transfer and Deactivation via Conical Intersections

“…It can be found that Wiberg bond index of the N2⋅⋅⋅H7 hydrogen bond is more than twice as large in the 1 ππ* state than in the S 0 state; whereas the counterpart in the 1 n π* state is rather smaller. This hydrogen‐bond feature of augmenting in the 1 ππ* state but weakening in the 1 n π* state could hold true for similar systems 34. 74–76…”

“…17 These applications are heavily interrelated to thermal and photoinduced isomerizations of the azo group 18–20. To design and synthesize new azobenzene compounds with better performance, much experimental and theoretical effort has been dedicated in previous years to understanding the isomerization mechanisms of azobenzene and its variants at atomistic level 21–34…”

Using density based indexes to characterize excited states evolution

“…More in detail, starting from the structures reported in Ref. , we have focused on the description of the relaxed minimum energy proton‐transfer profile occurring along the 1 ππ* PES. Using the optimized geometries reported in Ref.…”

“…Using the optimized geometries reported in Ref. , single point calculations were performed at the CASPT2 level of theory. Analogously to what has been done in Ref.…”

“…was employed to avoid the presence of intruder states, as described in Ref. . All these post‐Hartee–Fock calculations (CASSCF and CASPT2) were performed using the MOLCAS 8.0 quantum package.…”