The Gas-Phase Photophysics of Eosin Y and its Maleimide Conjugate

Abstract

The use of the xanthene family of dyes as fluorescent probes in a wide range of applications has provided impetus for the studying of their photophysical properties. In particular, recent advances in gas-phase techniques such as FRET that utilize such chromophores have placed a greater importance on the characterization of these properties in the gas phase. Additionally, the use of synthetic linker chains to graft the chromophores in a site-specific manner to their target system is ubiquitous. There is, however, often limited information on how the addition of such a linker chain may affect the photophysical properties of the chromophores, which is of fundamental importance for interpretation of experimental data reliant on grafted chromophores. Here, we present data on the optical spectroscopy of different protonation states of Eosin Y, a fluorescein derivative. We compare the photophysics of Eosin Y to its maleimide conjugate, and to the thioether product of the reaction of this conjugate with cysteamine. Comparison of the mass spectra following laser irradiation shows that very different relaxation takes place upon addition of the maleimide moiety but that the photophysics of the bare chromophore are restored upon addition of cysteamine. This radical change in the photophysics is interpreted in terms of charge-transfer states, whose energy relative to the S1 ← S0 transition of the chromophore is dependent on the conjugation of the maleimide moiety. We also show that the shape of the absorption band is unchanged in the gas-phase as compared to the solution-phase, showing a maximum with a shoulder toward the blue, and examination of isotope distributions of the isolated ions show that this shoulder cannot be due to the presence of dimers. Consideration of the fluorescence emission spectrum allows a tentative assignment of the shoulder to be due to a vibrational progression with a high Franck-Condon factor.

Excited States of Xanthene Analogues: Photofragmentation and Calculations by CC2 and Time‐Dependent Density Functional Theory

“…In fact, vibronic coupling is made responsible for the observed shoulder in the absorption and emission of rhodamines in solution. Recently, we also evidenced such a shoulder in the gas‐phase photodetachment spectra of eosine‐containing anions, for which a broad, almost 0.5 eV band was observed. However, for each of those species only a single shoulder was observed.…”

“…For systems with a fragmentation efficiency of 1.0, the total fragmentation yield will be dominantly driven by the absorption, as is the case for 7 . On the contrary, rhodamine derivatives 1 – 6 possess high fluorescence quantum yields, and thus, the relaxation processes that lead to direct one‐photon fragmentation in the gas phase are suppressed . At the same time, within the lifetimes of their S 1 states, absorption of a second photon (termed resonant 1+1 two‐photon absorption or excited‐state absorption) can lead to population of higher lying singlet states from which fragmentation more likely occurs.…”

“…Indeed, in accord with this consideration, 7 displays a linear laser power dependence of the photofragmentation, whereas the fragmentations of 1 – 6 show a quadratic power dependence. It may be shown by the self‐quenching properties of rhodamines, by two‐color 1+1′ absorption schemes, by comparison of nonfluorescent analogues, or by fluorescence excitation measurements that the absorption maxima lie on the very red edges of the observed photofragmentation bands. We therefore fitted each of the photofragmentation spectra with one to three Gaussians, depending upon the number of bands visible (one for 7 ; two for 2 , 4 , and 5 ; three for 1 , 3 , and 6 ).…”

Molecular Fluorescence Imaging Spectroscopy for Mapping Low Concentrations of Red Lake Pigments: Van Gogh's Painting The Olive Orchard

“…The maroon‐colored derivative endmember mapped to fluorescent areas, including the maroon outline of the woman and ladder and a tree trunk on the right (Figure B). The corresponding reflectance spectrum of this endmember (Figure A, left) shows absorption sub‐bands near 520 and 557 nm, characteristic of insect‐based red lakes and due to the S 0 (π)→S 1 (π*) electronic transition . The associated molecular fluorescence spectrum (Figure A, middle) has an emission maximum near 600 nm, which is red‐shifted relative to that seen for the eosin lake used in the breast of the woman (580 nm) but not quite as shifted as in a prior study where the corrected emission maxima of insect‐based red lakes in solid paint samples occurred between 615–630 nm .…”

“…Once corrected for self‐absorption, the molecular fluorescence emission spectra collected with either laser illumination (Supporting Information, Figure S1) or broadband illumination (Figures D and 3 A inset) all exhibit spectral features intrinsic to eosin lake: an emission maximum near 550 nm, with shoulders of decreasing intensity near 590 and 665 nm. The bands centered near 550 and 590 nm have been assigned to the S 1 (π*)→S 0 (π) emission with the lower energy 590 nm shoulder resulting from a transition to a vibrational level within the electronic ground state . The band centered near 665 nm possibly results from an interaction between the metal substrate and the ligand.…”