title: Presence of two emissive minima in the lowest excited state of a push-pull cationic dye unequivocally proved by femtosecond up-conversion spectroscopy and vibronic quantum-mechanical computations.
abstract: The long-standing controversy about the presence of two different emissive minima in the lowest excited state of the cationic push-pull dye o-(p-dimethylamino-styryl)-methylpyridinium (DASPMI) was definitively proved through the observation of dual emission, evidenced by both experimental (femtosecond up-conversion measurements) and theoretical (density functional theory calculations) approaches. From the fluorescence up-conversion data of DASPMI in water, the time resolved area normalized spectra (TRANES) were calculated, showing one isoemissive point and therefore revealing the presence of two distinct emissive minima of the excited state potential energy hypersurface with lifetimes of 0.51 and 4.8 ps. These spectroscopic techniques combined with proper data analysis allowed us to discriminate the sub-picosecond emitting state from the occurrence of ultrafast solvation dynamics and to disentangle the overlapping fluorescence (very close in energy) of the two components. Vibronic computations based on TD-DFT potential energy surfaces fully confirm those results and provide deeper insights about the key factors playing a role in determining the overall result. The two emissive minima have different structural and electronic characteristics: on one hand, the locally excited (LE) minimum has a flat geometry and an electric dipole moment smaller than the ground state; on the other hand, the twisted-intramolecular-charge-transfer (TICT) minimum shows a rotation of the methylpyridinium moiety with respect to the rest of the structure, and has an electric dipole moment significantly larger than the ground state.
authors.affiliation: †Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy.
authors.forename: Enrico
authors.initials: E
authors.lastname: Benassi
authors.affiliation: ‡Department of Chemistry, Biology and Biotechnology and Centro di Eccellenza sui Materiali Innovativi Nanostrutturati (CEMIN), University of Perugia, via Elce di Sotto 8, 06123 Perugia, Italy.
authors.forename: Benedetta
authors.initials: B
authors.lastname: Carlotti
authors.affiliation: †Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy.
authors.forename: Mireia
authors.initials: M
authors.lastname: Segado
authors.affiliation: ‡Department of Chemistry, Biology and Biotechnology and Centro di Eccellenza sui Materiali Innovativi Nanostrutturati (CEMIN), University of Perugia, via Elce di Sotto 8, 06123 Perugia, Italy.
authors.forename: Alessio
authors.initials: A
authors.lastname: Cesaretti
authors.affiliation: ‡Department of Chemistry, Biology and Biotechnology and Centro di Eccellenza sui Materiali Innovativi Nanostrutturati (CEMIN), University of Perugia, via Elce di Sotto 8, 06123 Perugia, Italy.
authors.forename: Anna
authors.initials: A
authors.lastname: Spalletti
authors.affiliation: ‡Department of Chemistry, Biology and Biotechnology and Centro di Eccellenza sui Materiali Innovativi Nanostrutturati (CEMIN), University of Perugia, via Elce di Sotto 8, 06123 Perugia, Italy.
authors.forename: Fausto
authors.initials: F
authors.lastname: Elisei
authors.affiliation: †Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy.
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Barone