4-Dimethylaminochalcone as fluorescent probe: effect of the medium polarity on relaxation processes in the excited state

Abstract

Relaxation processes in a 4 dimethylaminochalcone molecule after excitation with a light pulse of duration 70 fs were studied. During 0.4-1 ps after excitation, an absorbance of an excited state S 1 with a maximum at ~460 nm is formed in both polar and nonpolar media. Subsequent relaxation processes depend on the polarity of the medium. In nonpolar hexane, the 4 dimethylaminochalcone molecule transits to the triplet state having an absorption maxi mum at 570 nm (lifetime longer than 600 ps) for 20 ps. In polar aprotic acetonitrile, the absorbance at 460 nm decreases slowly (during hundreds of picoseconds), indicating that the molecules return to the ground state. The induced emission from the level S 1 in a region of 520-550 nm and fluorescence from the same level with a maximum at 537 nm are also observed in acetonitrile. Thus, a reason for a sharp decrease in the fluorescence yield on going from polar to nonpolar media was found. The mechanism of fluorescence quenching of 4 dimethylaminochalcone in nonpolar media is confirmed by the data on phosphorescence. The phosphorescence of 4 dimethylaminochalcone is observed at -196 °C in nonpolar sol vents, indicating a triplet excited state, while no phosphorescence is revealed in polar solvents.Key words: fluorescent probe, 4 dimethylaminochalcone, femtosecond dynamics, polarity of medium, triplet state.4 Dimethylaminochalcone (DMAC) as a fluorescent probe is used for studying biological membranes and lipo proteins. 1-6 It is poorly soluble in water and, hence, binds to lipids in the presence of mem branes or lipoproteins. 1 In polar solvents, the quantum yield of fluores cence can reach 0.26; however, the fluorescence of DMAC in water is almost completely quenched due to the forma tion of a hydrogen bond (other protic solvents quench DMAC flluorescence in the same manner). 3,4 When bind ing to proteins or lipids, a DMAC molecule is shielded from water, and the fluorescence yield can increase to 0.1 and higher. However, if the molecule has "drown" in the lipid depth and got into a nonpolar environment, the yield decreases to 0.01 and lower. Thus, it seems clearly why polar protic solvents quench fluorescence, while rea sons for fluorescence quenching in nonpolar media are unknown. In this work, we attempted to elucidate this problem. Experimental4 Dimethylaminochalcone was synthesized and kindly pre sented by B. M. Krasovitskii (Institute for Single Crystals, Na tional Academy of Sciences, Kharkov, Ukraine). 7 Acetonitrile, 95% EtOH, methylcyclohexane, and hexane (Aldrich) were used.The femtosecond dynamics of transient absorption spectra of DMAC was studied on an experimental system based on a femtosecond CPM dye laser designed at the N. N. Semenov Institute of Chemical Physics of the Russian Academy of Sci ences. 8 The "pump-probe" spectroscopy technique was used. The second harmonic from the main sequence of pulses gener ated by the system was used as pump pulses. The bearing wave length of a pump pulse was 308 nm, and the duration was 70 fs. Probe pulses were carr...

Solvent Polarity Dependent Excited State Dynamics of 2′-Hydroxychalcone Derivatives

“…The well-fitted evolution associated difference spectra (EADSs) and concentrations of transient species are obtained (see Figure ), and all the obtained time constants in different solvents are listed in Table . In CHX, after photoexcitation, the molecule has a high probability of ISC into triplet state (τ 1 ) according to the nanosecond TA results and previous calculations, , and the second lifetime component (τ 2 ) for DEAMC in CHX is assigned to the relaxation of the T 1 state back to the ground state. In THF, we attribute the first component (τ 1 ) to vibrational cooling or average solvation time of the initially Franck–Condon (FC) state, which is reached to an equilibrium situation through intramolecular vibrational energy redistribution and energy dissipation to the solvent molecules.…”

“…Earlier studies have shown that DEAHC has no or weak green fluorescence in solution but is strongly emissive around 700 nm when in bulk crystals; DEAMC is dark in both crystalline form and nonpolar solution but emits brightly in polar solution. , To explore their crucial microscopic dynamic mechanism, in this work, solvent-dependent excited state dynamics of both DEAMC and DEAHC are extensively investigated by using steady state spectroscopy, femtosecond and nanosecond transient absorption spectroscopy. We found that dramatic solvation effect induces stabilization of ICT state, restricting efficient formation of triplet states, enhancing the fluorescence yield of DEAMC . ,, Moreover, the stronger solvation process in polar solvents can further facilitate efficient ESIPT reaction in DEAHC .…”

“…We found that dramatic solvation effect induces stabilization of ICT state, restricting efficient formation of triplet states, enhancing the fluorescence yield of DEAMC. 41,43,44 Moreover, the stronger solvation process in polar solvents can further facilitate efficient ESIPT reaction in DEAHC.…”

Electronically excited states of membrane fluorescent probe 4-dimethylaminochalcone. Results of quantum chemical calculations

“…So, the calculations show that after the initial excitation of a DMAC molecule to the 1 (p, p*) state, an effective cascade of relaxation 1 (p, p*) -1 (n, p*) -3 (p, p*) can occur. This results in rapid (several picoseconds, according to our experimental estimates) 11 population of the triplet 3 (p, p*) state.…”

“…10 Basic processes that define the probe's fluorescence occur in its excited states. 11 This means that in order to assess specific contributions of radiative and nonradiative processes to the molecule's relaxation path, we had to determine the nature of its lowest excited states. This task is traditionally solved with quantum chemistry methods.…”