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

Abstract

Quantum-chemical calculations of ground and excited states for membrane fluorescent probe 4-dimethylaminochalcone (DMAC) in vacuum were performed. Optimized geometries and dipole moments for lowest-lying singlet and triplet states were obtained. The nature of these electronic transitions and the relaxation path in the excited states were determined; changes in geometry and charge distribution were assessed. It was shown that in vacuum the lowest existed level is of (n, π*) nature, and the closest to it is the level of (π, π*) nature; the energy gap between them is narrow. This led to an effective (1)(π, π*) →(1)(n, π*) relaxation. After photoexcitation the molecule undergoes significant transformations, including changes in bond orders, pyramidalization angle of the dimethylamino group, and planarity of the molecule. Its dipole moment rises from 5.5 Debye in the ground state to 17.1 Debye in the (1)(π, π*) state, and then falls to 2 Debye in the (1)(n, π*) state. The excited (1)(n, π*) state is a short living state; it has a high probability of intersystem crossing into the (3)(π, π*) triplet state. This relaxation path explains the low quantum yield of DMAC fluorescence in non-polar media. It is possible that (3)(π, π*) is responsible for observed DMAC phosphorescence.

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

“…Previous quantum chemical calculation by Romanov et al has shown that after photoexcitation 4-dimethylaminochalcone undergoes efficient 1 (π, π)* → 1 (n, π)* → 3 (π, π)* relaxation process in nonpolar media. With the increase in solvent polarity, the charge transfer state is quickly stabilized to ICT′ (solvent-stabilized ICT state) state by dipolar interaction with solvent molecules, which will suppress the efficient ISC process …”

“…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 .…”

Luminescent properties of chalcone and its aminoderivatives

“…The largest shift (0.8 eV) corresponds to the 4-(N, N dimethylamino ′ − )-chalcone, which is shown to have also high fluorescence quantum yield (0.6 in dimethylformamide) when compared to the non-fluorescent chalcone [12,14]. In paper [8] the energies of singlet and triplet ππ ⁎ and nπ ⁎ excited states were simulated using quantum chemical approach for 4-(N, N dimethylamino ′ − )-chalcone and its complex with water molecules in a gas phase. However, dipoledipole interactions were not taken into account, so the obtained results can be compared only to spectral properties in binary mixture of nonpolar solvent with small amount of water.…”

“…However, it hardly be said about the properties of chalcone and its amino-derivatives. Due to the flexible -CQC-C(-)QO chain and non-planar structure [8,9] they have distinguishing features, such as fluorescence quenching in protic solvents [4,[10][11][12][13][14].…”

Effect of solvent polarity on the photophysical properties of chalcone derivatives

“…13 In addition, chalcones are well known for biological activities such as anti-inammatory, 14 antibacterial 15,16 anti-parasitic, 17 antitumor 18 and antifungal. 19,20 This is due to a, b unsaturated keto group (-C]C-C(-)]O) and nonplanar structure 21,22 present in chalcones. A compound having donor-acceptor functionality with conjugation called a pull-push system, displays interesting photophysical properties.…”