Solvent Effects on 3-Keto-1H-pyrido[3,2,1-kl]phenothiazine Fluorescence in Polar and Protic Solvents

Abstract

The photophysical and photochemical properties of fluorophores are always affected by solvents. 1À26 These include solvent polarity, viscosity, temperature, and specific solvent and fluorophore interaction. There are two kinds of solvent effects: the general and the specific solvent effects.1,2 General solvent effects are due to the interactions of the fluorophore dipole with its environment. Solvent polarity is a major factor of general solvent effects. For example, an internal charge-transfer (ICT) state was formed for coumarin-151 (C151) in a polar solvent. Compared with the local excited (LE) state, the ICT state has a smaller nonradiative decay rate (k nr ). Thus, the fluorescence of C151 in high-polarity solvents enhanced with the decrease of k nr .3 In contrast to C151, the ICT state of neutral red has a smaller radiative decay rate (k r ) compared with its LE state. Therefore, the fluorescence quantum yield of neutral red decreased in highpolarity solvents. 4 Specific solvent effects, which are produced by interactions between the fluorophore and neighboring solvent molecules, are more complex than general solvent effects. The fluorescence properties of C151 and neutral red in protic solvents are also affected by specific solvent effects.3,4 Excitedstate proton transfer (ESPT) is considered a very important reaction in the excited-state dynamics of fluorophore.5À11 Electron transfer between the solute and the solvent also affects the photophysical and photochemical properties of fluorophores.14 Actually, the solvent effects on the excited-state dynamics of fluorophores are essential to its photophysical and photochemical properties. Investigating solvent effects on excited-state dynamics helps one to understand the fluorescence properties of fluorophores. Numerous theoretic efforts have been made to describe the excited-state dynamics of fluorophores in different circumstances.11À13,27À33 The energy gap law is well established.27 Lim et al. explored a "proximity effect" model to understand the photophysical properties of nitrogen-hytero cyclic or aromatic carbonyl compounds. 12,13 Zgierski et al. found a biradical radiationless decay channel in natural bases. 29À32Recently, Han et al. reported some environment-sensitive phenothiazine dyes.15 Among these (Figure1) are 3-keto-1H-pyrido[3,2,1-kl]phenothiazine (PTZ-5) and 2,3-dihydro-3-keto-1H-pyrido[3,2,1-kl]phenothiazine (PTZ-4), both of which show interesting photophysical properties. In protic solvents, PTZ-5 has shown significantly higher fluorescence quantum yields than in aprotic solvents, whereas PTZ-4 was quenched entirely. However, the effects of solvent properties on the fluorescence properties of these two dyes remain unclear.This paper focuses on the solvent effects on PTZ-5 fluorescence properties. PTZ-4 was also investigated as a comparison. The time-dependent density functional theory (TD-DFT) method was used to investigate the electronic states of PTZ-5 and PTZ-4. The general solvent effects on the fluorescence properties of PTZ-5 and PTZ-4...

Hydrogen Bonding Effects on the Photophysical Properties of 2,3-dihydro-3-keto-1H-pyrido[3,2,1-kl]phenothiazine

“…Figures 1 (d) and (e) show that the fluorescence spectra of PTZ5 in THF and methanol are independent of the excitation wavelength. Only one relaxed emission state exists, as results suggest, which is consistent with the findings in previous studies [24,29]. Larger Stoks shifts more in fluorescence for PTZ5 in methanol than in THF, which may be due to enhanced hydrogen bonds at the optimized S 1 .…”

“…The S 1 vertical excited energy of A-2MeOH calculated at the S 0 -optimized geometry was 2.6402 eV. As reported previously, the S 1 vertical excited energies of PTZ5 (in THF) and PTZ5-MeOH are 3.5081 and 3.4120 eV, respectively [24]. These results indicate the redshift in absorption spectra from THF to methanol.…”

“…However, the fluorescence of PTZ4 was quenched, whereas that of PTZ5 was enhanced in protic solvents. Theoretical investigations on the photophysical properties of PTZ5 in different solvents have been previously reported [24]. We found that the enhanced fluorescence of PTZ5 in polar protic solvents is due to the larger energy gap between the lowest two-singlet excited states and inhibition of the out-of-plane bending of C−H by the formation of hydrogen-bonded complexes [24].…”

“…The emission rate constants k f of PTZ5 showed no significant changes in various solvents [29]. In addition, the emission state of PTZ5 was always the bright state (π, π * ) [24]. Figures 1 (d) and (e) show that the fluorescence spectra of PTZ5 in THF and methanol are independent of the excitation wavelength.…”

“…According to previous reports, we know that the secondary hydrogen bond between C−H and oxygen is an important factor that influences the excited-state dynamics of fluorophores [24]. Considering that there are two C16−H bonds in PTZ4, two methanol molecules were added to form hydrogen-bonded complexes with PTZ4 (Fig.3, A2MeOH).…”