TY - JOUR

T1 - Absorption and fluorescence spectroscopy of cold proflavine ions isolated in the gas phase

AU - Lindkvist, Thomas Toft

AU - Kjær, Christina

AU - Langeland, Jeppe

AU - Vogt, Emil

AU - Kjaergaard, Henrik G.

AU - Nielsen, Steen Brøndsted

N1 - Funding Information: We acknowledge support from the Independent Research Fund Denmark - Natural Sciences (Grant No. 3103-00072B), the NOVO Nordisk Foundation (Grant Nos. NNF20OC0064958 and NNF22OC0080193), and the High Performance Computer Centre at the University of Copenhagen. Also, we would like to thank Dr. Iden Djavani-Tabrizi and Professor Rebecca Jockusch for enlightening discussions. Funding Information: We acknowledge support from the Independent Research Fund Denmark \u2013 Natural Sciences (Grant No. 3103-00072B), the NOVO Nordisk Foundation (Grant Nos. NNF20OC0064958 and NNF22OC0080193), and the High Performance Computer Centre at the University of Copenhagen. Also, we would like to thank Dr. Iden Djavani-Tabrizi and Professor Rebecca Jockusch for enlightening discussions. Publisher Copyright: © 2024 Author(s).

PY - 2024/5/14

Y1 - 2024/5/14

N2 - Proflavine, a fluorescent cationic dye with strong absorption in the visible, has been proposed as a potential contributor to diffuse interstellar bands (DIBs). To investigate this hypothesis, it is essential to examine the spectra of cold and isolated ions for comparison. Here, we report absorption spectra of proflavine ions, trapped in a liquid-nitrogen-cooled ion trap filled with helium-buffer gas, as well as fluorescence spectra to provide further information on the intrinsic photophysics. We find absorption- and fluorescence-band maxima at 434.2 ± 0.1 and 434.7 ± 0.3 nm, corresponding to a Stokes shift of maximum 48 cm−1, which indicates minor differences between ground-state and excited-state geometries. Based on time-dependent density functional theory, we assign the emitting state to S2 as its geometry closely resembles that of S0, whereas the S1 geometry differs from that of S0. As a result, simulated spectra involving S1 exhibit long Franck-Condon progressions, absent in the experimental spectra. The latter displays well-resolved vibrational features, assigned to transitions involving in-plane vibrational modes where the vibrational quantum number changes by one. Dominant transitions are associated with vibrations localized on the NH2 moieties. Experiments repeated at room temperature yield broader spectra with maxima at 435.5 ± 1 nm (absorption) and 438.0 ± 1 nm (fluorescence). We again conclude that prevalent fluorescence arises from S2, i.e., anti-Kasha behavior, in agreement with previous work. Excited-state lifetimes are 5 ± 1 ns, independent of temperature. Importantly, we exclude the possibility that a narrow DIB at 436.4 nm originates from cold proflavine cations as the band is redshifted compared to our absorption spectra.

AB - Proflavine, a fluorescent cationic dye with strong absorption in the visible, has been proposed as a potential contributor to diffuse interstellar bands (DIBs). To investigate this hypothesis, it is essential to examine the spectra of cold and isolated ions for comparison. Here, we report absorption spectra of proflavine ions, trapped in a liquid-nitrogen-cooled ion trap filled with helium-buffer gas, as well as fluorescence spectra to provide further information on the intrinsic photophysics. We find absorption- and fluorescence-band maxima at 434.2 ± 0.1 and 434.7 ± 0.3 nm, corresponding to a Stokes shift of maximum 48 cm−1, which indicates minor differences between ground-state and excited-state geometries. Based on time-dependent density functional theory, we assign the emitting state to S2 as its geometry closely resembles that of S0, whereas the S1 geometry differs from that of S0. As a result, simulated spectra involving S1 exhibit long Franck-Condon progressions, absent in the experimental spectra. The latter displays well-resolved vibrational features, assigned to transitions involving in-plane vibrational modes where the vibrational quantum number changes by one. Dominant transitions are associated with vibrations localized on the NH2 moieties. Experiments repeated at room temperature yield broader spectra with maxima at 435.5 ± 1 nm (absorption) and 438.0 ± 1 nm (fluorescence). We again conclude that prevalent fluorescence arises from S2, i.e., anti-Kasha behavior, in agreement with previous work. Excited-state lifetimes are 5 ± 1 ns, independent of temperature. Importantly, we exclude the possibility that a narrow DIB at 436.4 nm originates from cold proflavine cations as the band is redshifted compared to our absorption spectra.

U2 - 10.1063/5.0204331

DO - 10.1063/5.0204331

M3 - Journal article

C2 - 38726936

AN - SCOPUS:85192758089

VL - 160

JO - The Journal of Chemical Physics

JF - The Journal of Chemical Physics

SN - 0021-9606

IS - 18

M1 - 184306

ER -