Two Roads Converged in a Yellow Dye: Unusual Spectral Broadening in the Emission of Auramine-O Possibly Caused by Low-Friction Intramolecular Rotation

Abstract

Steady-state and time-resolved optical techniques were employed to study the rather complex relaxation of excited states of Auramine-O (AuO) in several liquids at room temperature. We found three relaxation times in the decay of the pump−probe signals of the excited states of AuO. We focused our study on the short time decay, with a duration of within about 150−300 fs. We found that the temporal changes of the emission band of AuO could be divided into three behaviors, depending on the solvent characteristics. In dimethyl sulfoxide (DMSO), a hydrogen-bond-accepting solvent, AuO, shows, at short times, a relatively broad emission band with small changes in its peak position and width as a function of time. In acetonitrile and in acetic acid, both hydrogen-bond-donating solvents, we found large changes in the band peak and width as a function of time. Dichloromethane is a solvent lacking strong solvent interactions, it is apolar and is neither a strong hydrogen-bond-donator nor a strong hydrogen-bond-acceptor. For AuO in dichloromethane we found an oscillation with a time constant of 200 fs in the time-resolved emission signal. We attribute the rather large changes of the emission band with time in the short time window to the twist of the dimethylamino groups of the aniline groups of AuO.

Photophysics of Auramine-O: electronic structure calculations and nonadiabatic dynamics simulations

“…Table 1 collects the computed vertical excitation energies to S 1 at the S 0 minimum S0. Compared with the experimental value measured in methanol solution, 49 TD-CAM-B3LYP and MS-CASPT2 computations overestimate and underestimate the S 0 -S 1 vertical excitation energy, respectively; but, TD-B3LYP computations fortuitously give very good agreement. Our current OM2/MRCI computations give a reasonably accurate number: the vertical excitation energy to the S 1 state is 0.4 eV lower than the experimental value.…”

“…More recently, Erez et al employed steady-state and time-resolved optical techniques to study the short-time excited-state decay dynamics of Auramine-O with a duration of about 150-300 fs in several solutions at room temperature. 48,49 They found that its excited-state behavior could be divided into three time steps. The shortest decay time constant is associated with a strong emission with a band maximum at about 500 nm, which is attributed to the twisted motion of the dimethylamino group of the two anilines of Auramine-O.…”

Color in Bridge-Substituted Cyanines

“…For substituents with σ p + ≤ −0.7, the excited state is still dominated by the out-of-phase “twin” state |211⟩ – |112⟩, but the ground state becomes dominated by |202⟩, so a two-state resonance model will no longer describe well the ground and first excited states. The model therefore anticipates that the ground state of auramine-O (R = NH 2 ) is not normally written using the resonance notation in Scheme but rather as in Scheme . ,,− …”

Altered relaxation dynamics of excited state reactions by confinement in reverse micelles probed by ultrafast fluorescence up-conversion

“…For example, it is proposed that in some circumstances the ring torsion may be accompanied by changes in the orientation of the dimethylamino group (although recent calculations favour ring torsion as the major pathway). [135][136][137][138] Because of these complications it is best to compare AO dynamics measured in similar media. For consistency and to avoid effects associated with the specific probe we will focus on AO in this section.…”

“…A plausible alternative to AO as a probe of confinement effects on reaction dynamics would be Thioflavin T. Its ultrafast fluorescence was characterised by Huppert in detail, in a range of solvents and analysed with the GSE approach (Section 3). 56,137,148,149 This dye also has a radiationless decay pathway involving phenyl torsion, which gives rise to a degree of internal charge transfer. 56,149 It is thus quite similar to AO and has was accurately modelled using the GSE approach.…”