Aminorhodamine (ARh): A Bichromophore with Three Emission Bands in Low Temperature Glasses

Abstract

At first glance, aminorhodamine (ARh) is a typical pH responsive fluorescent, rhodamine-type dye. However, hidden under the typical rhodamine absorption band, ARh has another electronic transition of similar energy, but polarized orthogonal to that of the rhodamine chromophore. This transition-assigned to an arylpyrylium type chromophore contained in the system-is responsible for the sensor action of the dye. ARh is non-fluorescent, while protonation of a donor amino group turn on a strong rhodamine-type emission. At low temperature in frozen solution emission from both electronic subsystems of ARh are observed. In order to achieve more complete understanding of the photophysical mechanisms in this type of fluorescent probes, ARh and its protonated counterpart HARh were studied by absorption and fluorescence spectroscopy, computational chemistry, and at low temperatures in solid solution. Results from fluorescence anisotropy and time-resolved fluorescence spectra establish a bichromophore model and suggest that a remarkable weak coupling between the two nearly isoenergetic excited states in ARh enables the dual emission. All the complicated properties observed for ARh was accounted for by a bichromophore model describing the electronic system of ARh as a bichromophore constituted by a rhodamine and an arylpyrylium subsystem.

Tetramethoxy‐Aminorhodamine (TMARh): A Bichromophore, an Improved Fluorophore, and a pH Switch

“…Radically new fluorophores are rarely found, and most useful dyes are based on known molecular units such as rhodamine, fluorescein, borondipyrrole, cyanine, etc . We recently reanalyzed the electronic structure and pH switching of aminorhodamine ( ARh , Figure ) based on the discovery of a hidden electronic transition, corresponding to an optically induced charge shift in an arylpyrylium chromophore integrated with the rhodamine chromophore . ARh can best be described as an arylpyrylium/rhodamine bichromophore system.…”

“…We set out to synthesize this compound as we expected that beneficial properties of ARh would be obtained, but with a response in a more relevant pH‐range owing to the increased electron density of the peripheral phenyl substituent. Tetramethoxy‐aminorhodamine ( TMARh , Figure ) was synthesized in a manner different from the usual condensation between phthalic anhydride and methoxy aniline used to make rhodamines, including ARh . Despite the orthogonal orientation of the aminoaryl arm, TMARh proved to be a true bichromophore.…”

“…The protonated on state resembles rhodamine B and has the same photophysical properties, that is, similar fluorescence quantum yield, absorption, and emission maxima. We recently found that the off state was not quenched in an excited‐state process (ICT or TICT), but was due to the bichromophoric nature of the system . Two fundamentally different chromophores are present in the native form of ARh : an arylpyrylium chromophore and a rhodamine chromophore, with an energy that make them the first (S 1 ) and second (S 2 ) excited state in the same fully conjugated molecular structure.…”

“…The first excited state is efficiently quenched by internal conversion, resulting in a dark off state. Protonation results in increased transition energy in the arylpyrylium chromophore, resulting in an efficient fluorescent dye dominated by the rhodamine chromophore that now constitutes the first excited state (S 1 , HARh) …”

“…The unusual feature in the absorption spectrum of TMARh is the broad low‐intensity absorption on the red side of the main absorption band, which is evident by spectral comparison between TMARh , HTMARh (protonated TMARh ), and rhodamine 101 in Figure . Similar to the analysis of ARh the absorption of TMARh can be resolved by deconvolution using Gaussian functions. In the resolved spectrum, the individual bands can be assigned to the two chromophores in the system.…”