Photophysics of Acyl- and Ester-DBD Dyes: Quadrupole-Induced Solvent Relaxation Investigated by Transient Absorption Spectroscopy

Abstract

A new generation of wavelength-tunable, fluorescent dyes, so-called DBD ([1,3]dioxolo [4,5-f ][1,3]benzodioxole) dyes, were developed a few years ago, and they showed great potential as probes, for example, for fluorescence microscopy. However, their photophysics is not fully explored and leaves open questions regarding their large fluorescence Stokes shifts and sensitivity to solvent conditions of differently substituted DBD dyes. To improve the understanding of the influence of the substitution pattern of the DBD dyes on their respective photophysics, transient absorption spectroscopy (TAS) was used, that is, a pump−probe experiment on the femtosecond timescale. TAS allows measurements of excited states, ground state recovery, solvent relaxation, and fluorescence properties on time scales of up to several nanoseconds. Two different DBD dye samples were investigated: acyl-and estersubstituted DBD dyes. Experiments were carried out in solvents with different polarities using different excitation energies and at different viscosities. Based on the experimental data and theoretical calculations, we were able to determine the conformational changes of the molecule due to electronic excitation and were able to investigate solvent relaxation processes for both types of DBD dyes. By generalizing the theory for quadrupole-induced solvent relaxation developed by Togashi et al., we derived quadrupole moments of both molecules in the ground and excited state. Our data showed differences in the binding of polar solvent molecules to the dyes depending on the substituent on the DBD dye. In the case of water as the solvent, an additional efficient quenching process in the electronically excited state was revealed, which was indicated by the observation of solvated electrons in the TAS signals.

Investigating the Sulfur “Twist” on the Photophysics of DBD Dyes

“…As can be seen in Figure , the transient behavior of the oxygen ald-DBD dye is similar to the previously investigated acyl- and ester-DBD dyes . There are several transient absorption signals (TA, positive signals) at around 400 and 690 nm for the O 4 -DBD dye and 550 nm for the S 4 -DBD dye that are visible.…”

“…As we have shown before, 13 we can also find the extinction coefficient of the transient absorption ε TA from the ratio of ΔOD of the TA and GSB signal and the ground-state extinction coefficient ε GS at the corresponding wavelength:…”

“…As can be seen in Table 3, the transferred charge upon excitation in the ald-O 4 -DBD dye is about 0.02 e, which is less than half of the amount of the ester-and acyl-O 4 -DBD dyes with 0.07 ± 0.03 and 0.10 ± 0.04, respectively. 13 This behavior can be explained by the fact that a negative charge that is transferred onto the substituents oxygen is better stabilized on a ketone or ester than on an aldehyde group, due the additional side chain. However, due to the large error of Δq for the S 4 -DBD dye we can draw no conclusion on the magnitude of the larger push−pull effect due to sulfur substitution.…”

“…The most promising feature of the DBD dyes is their very large Stokes shift, due to a strong charge-transfer character and high quadrupole moment, which is beneficial for application where the emission needs to be well separated from the excitation wavelength, like in optical microscopy applications.…”

pH-Sensitive Fluorescence Switching of Pyridylanthracenes: The Effect of the Isomeric Pattern

“…In the transient absorption (TA) measurements, , a Ti:sapphire laser system (Spitfire Ace PA, Spectra Physics) operated at 800 nm was used. The transient absorption spectrometer (TAS, Newport) was combined with a HE-TOPAS (Light Conversion) as the pump source (a pump wavelength of 275 nm, unless stated otherwise) at a 1 kHz repetition rate and a 40 fs pulse width.…”