Spectral Signature of 2-[4-(Dimethylamino)styryl]-1-methylquinolinium Iodide: A Case of Negative Solvatochromism in Water

Abstract

Photophysics of the 2-[4-(dimethylamino) styryl]-1-methylquinolinium iodide (DASQMI) molecule has been studied in different solvents by steady-state and time-resolved emission spectroscopy and also with quantum chemical calculations. The probe molecule exhibits a strong solvent-polarity-dependent characteristic. The low-energy fluorescence band of DASQMI shows an anomalous 40 nm blue shift in water from that in dimethyl sulfoxide (DMSO); though in deuterium oxide the normal trend of red shift was observed. A marked increase in intensity of this band at 77 K and an increase in lifetime in viscous solvent point clearly to the intramolecular charge-transfer (ICT) character of the low-energy band. From the temperature-dependent emission and emission spectra in mixed solvents, the negative solvatochromism of DASQMI has been established, which means that the ICT state moves toward ground state with polarity and hydrogen-bond ability and beyond a critical dielectric constant coupled with protic nature of the solvent ground state gets further stabilized to show anomalous blue shift. In ethanol, below a critical temperature, 253 K, a blue shift starts due to greater solvent molecular polarization. A third long-lifetime component with dominant 75% amplitude was observed only in aqueous solution and may be due to the cis-isomer of hydrophobic DASQMI, a stable form in the excited state predicted from polarizable continuum model (PCM) calculations in water with 6-31G+(d,p) as basis set.

Effect of the π Bridge and Acceptor on Intramolecular Charge Transfer in Push–Pull Cationic Chromophores: An Ultrafast Spectroscopic and TD‐DFT Computational Study

“…On going from the low‐polarity DCM to the highly polar water (W), the spectral blueshifts in the absorption band are 4240, 2130, and 3540 cm −1 for A , B , and C , respectively. An investigation of the absorption and fluorescence properties of B in a few solvents has been reported in a recent paper;28 the obtained results are quite different with respect to ours (results are oddly blueshifted with respect to those widely reported in the literature for R ) but because the NMR spectroscopy analysis of this compound leaves no doubt about its structure and the quantum mechanical calculations excellently reproduce the experimental spectra, we consider our data reliable. The important solvatochromism observed for the A – C compounds may be an indication of a certain charge‐transfer character of the S 0 –S 1 transition (further evidence is provided by QC calculations, see below), as also other experimental findings seem to indicate.…”

A Facile Molecular Machine: Optically Triggered Counterion Migration by Charge Transfer of Linear Donor‐π‐Acceptor Phosphonium Fluorophores

“…This observation reflects the higher electron‐acceptor ability of the ‐ + PPh 3 versus ‐PPh 2 O group. The values of λ abs for all the studied species 1 – 9 are only slightly dependent on the solvent polarity, as the solvation sphere remains virtually unrelaxed upon fast vertical excitation . Nevertheless, for all the salts, the lowest‐lying absorptions in water are systematically blue‐shifted with respect to DCM solutions (hypsochromic shifts of 90 cm −1 for 1 to 1183 cm −1 for 6 ); this can be termed as negative solvatochromism, as described for other ionic bipolar dyes…”

Selective Formation of Twisted Intramolecular Charge Transfer and Excimer Emissions on 2,7-bis(4-Diethylaminophenyl)-fluorenone by Choice of Solvent

“…However, to date, there has been little investigation on the effect of changing the solvent used, which is a very simple environment change [ 19 ]. Previous studies have reported only shifts of the emission peaks when the solvent was changed [ 22 , 23 , 24 ].…”