Spectroscopic evidence of the presence of an activation barrier in the otherwise barrierless excited state potential energy surface of auramine-O: A femtosecond fluorescence up-conversion study

Shahnawaz Rafiq, Pratik Sen

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11 Scopus citations

Abstract

A widely acclaimed model for the excited state relaxation dynamics of auramine-O involves orientational relaxation of dimethylanilino moieties along the barrierless excited state potential energy surface (PES). Such a model would necessitate similar excited state dynamics in media offering similar viscous drag. However, we have noticed an interesting experimental observation showing auramine-O to have ∼8 times larger fluorescence quantum yield in chloroform than in methanol, though both the solvents have the same viscosity. The femtosecond fluorescence transients of auramine-O in chloroform surprisingly depict a rise in fluorescence transients, which has not been observed before. This, along with the simultaneous observation of unexpectedly large fluorescence lifetime and multi-exponential transients (in chloroform) questions the thoroughly accepted barrierless model of auramine-O relaxation dynamics, as the barrierless model would demand a short lifetime and single-exponential decay. Temperature dependent quantum yield measurements along with solvent dependent excited state multi-coordinate time-dependent density functional theory calculations further unveil the exact nature of PES. All these results concomitantly conclude that, at-least in chloroform, upon photo-excitation auramine-O must pass over an activation barrier before damping the excited state population into ground state via a sink function through adiabatic coupling of the electronic states.

Original languageEnglish
Article number124302
JournalJournal of Chemical Physics
Volume139
Issue number12
DOIs
StatePublished - Sep 28 2013

ASJC Scopus subject areas

  • General Physics and Astronomy
  • Physical and Theoretical Chemistry

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