Stationary External Electric Field-Mimicking the Solvent Effect on the Ground-State Tautomerism and Excited-State Proton Transfer in 8-(Benzo[d]thiazol-2-yl)quinolin-7-ol

Abstract

The effect of the external electric field on the ground-state tautomerism in 8-(benzo[d]thiazol-2-yl)quinolin-7-ol has been studied by using density functional theory. The compound exists as an enol tautomer (off state) and under the influence of the external electric field a long-range intramolecular proton transfer can occur, placing the tautomeric proton at the quinolyl nitrogen atom (on state). This is a result of the much higher dipole moment of the end keto tautomer and indicates that the external electric field can be used to mimic the implicit solvent effect in tautomeric systems. In the excited state, the further stabilization of the most polar on state leads to a situation when the excited-state intramolecular proton transfer becomes impossible, limiting the intramolecular rotation to the conical intersection region.

Keywords: DFT; external electric field; proton transfer; solvent effect; switching; tautomerism.

Scheme 1

Compounds for which tautomerism has been studied under EEF up to now.

Scheme 2

Ground-state tautomeric forms of HQBT. The angle of rotation of the rotor (benzothiazole) against the stator (7-hygroxy quinoline) is indicated in pink as a twisting angle α.

Figure 1

Potential energy surface (PES) of HQBT in toluene in ground and first singlet excited state. The stationary forms are indicated in blue, the transition states—in red and the Frank–Condon state—in green. The numerical values present relative energy DE/relative Gibbs free energy DG and are given in kcal/mol units. The ground-state PES in acetonitrile is presented with the same colors but in transparency. In the excited state, the oscillator strength and the angle of rotation of the rotor against the stator are given in brackets, the latter is underlined.

Figure 2

Theoretically predicted spectra of the ground-state tautomers of HQBT in toluene given as simulated absorption curves (left) and single transitions with corresponding oscillator strengths (right).

Figure 3

Direction of the dipole moment vectors (in light blue) of the ground-state tautomeric forms and transition states of HQBT in respect to the local coordinate system (in green). The atoms are given as follows: O (red), N (blue), S (yellow), C (grey), H (white). The values of the dipole moments and twisting angle (see Scheme 2) are given for information.

Figure 4

Orientation of E in respect to the local coordinate system when the rotation around the Z-axis is allowed and EEF is applied in the direction of the dipole moment (left) and against it (right). In the middle, the result with a molecule, fixed in respect to the Z-axis, is shown when the EEF is applied against the direction of the dipole moment. In this particular case the applied EEF is of 0.005 au.

Figure 5

Contour map of the ground-state PESs of HQBT in toluene presented as a function of the EEF strength.

Figure 6

Potential energy surface (PES) of HQBT in toluene in ground and first singlet excited state at EEF of 0.005 (A) and 0.01 (B) au. The stationary forms are indicated in blue, the transition states—in red, the Frank–Condon state—in green. The numerical values present relative energy DE/relative Gibbs free energy DG and are given in kcal/mol units. In the excited state, the oscillator strength and the angle of rotation of the rotor against the stator are given in brackets, the latter is underlined.

Figure 6

Potential energy surface (PES) of HQBT in toluene in ground and first singlet excited state at EEF of 0.005 (A) and 0.01 (B) au. The stationary forms are indicated in blue, the transition states—in red, the Frank–Condon state—in green. The numerical values present relative energy DE/relative Gibbs free energy DG and are given in kcal/mol units. In the excited state, the oscillator strength and the angle of rotation of the rotor against the stator are given in brackets, the latter is underlined.