Picosecond excited-state lifetimes of protonated indazole and benzimidazole: The role of the N-N bond

Certain chemical groups give rise to characteristic excited-state deactivation mechanisms. Here, we target the role of a protonated N-N chemical group in the excited-state deactivation of protonated indazole by comparison to its isomer that lacks this group, protonated benzimidazole. Gas-phase protonated indazole and protonated benzimidazole ions are investigated at room temperature using picosecond laser pump-probe photodissociation experiments in a linear ion-trap. Excited state lifetimes are measured across a range of pump energies (4.0-5.4 eV). The (1)pi pi* lifetimes of protonated indazole range from 390 & PLUSMN; 70 ps using 4.0 eV pump energy to & LE;18 ps using 4.6 eV pump energy. The (1)pi pi* lifetimes of protonated benzimidazole are systematically longer, ranging from 3700 & PLUSMN; 1100 ps at 4.6 eV pump energy to 400 & PLUSMN; 200 ps at 5.4 eV. Based on these experimental results and accompanying quantum chemical calculations and potential energy surfaces, the shorter lifetimes of protonated indazole are attributed to pi sigma* state mediated elongation of the protonated N-N bond.

Automated summary

This study investigates the role of a protonated N-N chemical group in the excited-state deactivation of protonated indazole compared to its isomer protonated benzimidazole. Protonated indazole exhibits shorter lifetimes attributed to pi sigma* state mediated elongation of the protonated N-N bond, as determined by experimental results, quantum chemical calculations, and potential energy surfaces.