Cryogenic Ion Fluorescence Spectroscopy: FRET in Rhodamine Homodimers and Heterodimers

Abstract

The internal electronic communication between two or more light‐absorbers is fundamental for energy‐transport processes, a field of large current interest. Here we have studied the intrinsic photophysics of homo‐ and heterodimers of rhodamine cations where just two methylene units bridge the dyes. Gas‐phase experiments were done on frozen molecular ions at cryogenic temperatures using the newly built LUNA2 mass spectroscopy setup in Aarhus. Both absorption (from fluorescence excitation) and dispersed‐fluorescence spectra were measured. In the gas phase, there is no dielectric screening from solvent molecules, and the effect of charges on transition energies is maximum. Indeed, bands are redshifted compared to those of monomer dyes due to the electric field that each dye senses from the other in a dimer. Importantly, also, as two chemically identical dyes in a homodimer do not experience the same field along the long axis, each dye has separate absorption. At low temperatures, it is therefore possible to selectively excite one dye. We find that fluorescence is dominantly from the dye with the lowest transition energy no matter which dye is photoexcited. Hence our work unequivocally demonstrates Förster Resonance Energy Transfer even in homodimers where one dye acts as donor and the other as acceptor.

Cryogenic fluorescence spectroscopy of oxazine ions isolated in vacuo

“…This gives rise to more than a doubling of the Stokes shift from 14 cm −1 to 37 cm −1 . The Stokes shifts measured for the oxazines are similar to those for rhodamine cations (17–25 cm −1 ) 26 but smaller than those for fluorone anions and that for the dimethyl-oxyluciferin anion; in the latter case the shift is an order of magnitude larger (360 cm −1 ) 14 Both absorption and fluorescence spectra of rhodamine cations are, however, much less well-resolved than those of oxazines, which is likely associated with differences in the importance of low frequency modes.…”

“…The fluorescence spectrum (blue) exhibits maximum intensity at 565.5 ± 0.5 nm, and the spectrum is significantly broader but still shows appreciable vibrational structure. In our previous work 9,14,25,26 on a range of similar fused three-ring systems (pyronin Y, rhodamines, fluorones) and dimethyl-oxyluciferin, the fluorescence spectra were also broader and more congested than the fluorescence-excitation spectra. We showed that it was caused by an increased effective temperature of the ions due to excess energy deposited by the absorbed laser photon.…”

Empirical Calibration of a Cylindrical Ion Trap for Mass-Selected Gas-Phase Fluorescence Spectroscopy

“…The CIT is made of copper, and the entrance and exit electrodes are in physical contact with a container filled with liquid nitrogen. As a result, ions are cooled to about 100 K (not relevant here but in other experiments to reduce spectral congestion − ). The CIT accumulates ions for several repetitions (typically 200), to operate at maximum occupation, rendering the setup insensitive to ion-current fluctuations.…”