Twisted Intramolecular Charge Transfer State of a "Push-Pull" Emitter

Abstract

The excited state Raman spectra of 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) in the locally-excited (LE) and the intramolecular charge transfer (ICT) states have been separately measured by time-resolved stimulated Raman spectroscopy. In a polar dimethylsulfoxide solution, the ultrafast ICT of DCM with a time constant of 1.0 ps was observed in addition to the vibrational relaxation in the ICT state of 4-7 ps. On the other hand, the energy of the ICT state of DCM becomes higher than that of the LE state in a less polar chloroform solution, where the initially-photoexcited ICT state with the LE state shows the ultrafast internal conversion to the LE state with a time constant of 300 fs. The excited-state Raman spectra of the LE and ICT state of DCM showed several major vibrational modes of DCM in the LE and ICT conformer states coexisting in the excited state. Comparing to the time-dependent density functional theory simulations and the experimental results of similar push-pull type molecules, a twisted geometry of the dimethylamino group is suggested for the structure of DCM in the S₁/ICT state.

Keywords: excited-state dynamics; femtosecond stimulated raman spectroscopy; intramolecular charge transfer; push-pull emitter; twisted intramolecular charge transfer.

Scheme 1

(a) Molecular structures of 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) with plausible structural changes upon the intramolecular charge transfer, (b) summary of the photophysics of DCM in polar solvents. Planar and twisted dimethylamino structures were optimized in the time-dependent density functional theory (TDDFT) simulations at the B3LYP/6-311G(d,p) level.

Figure 1

(a) Absorption (solid lines) and emission (dotted lines) spectra of DCM in n-hexane, CHCl₃, and dimethylsulfoxide (DMSO) solutions. The excitations at 405 (n-hexane) and 485 nm (CHCl₃ and DMSO) were used for the emission measurements; the evolution associated difference spectra (EADS) of DCM in (b) DMSO and (c) CHCl₃ solution for the transient absorption results obtained with 403 nm excitation.

Figure 2

(a) Femtosecond stimulated Raman spectra of DCM in DMSO solution with 403 nm excitation and (b) the population and structural dynamics of major vibrational modes: ν₁₅ (ph) and ν₁₄ (ph) at 1057 and 1288 cm⁻¹; ν_9a (ph) at 1200–1212 cm⁻¹; ν_19b (py) modes at 1471–1492 cm⁻¹ and δ_CH3 (ph) or ν_19a (ph) mode at 1436–1428 cm⁻¹; and ν_as,C≡N at 2170–2175 cm⁻¹. All the kinetic traces were fit with a number of exponential functions convoluted with a Gaussian function for the instrument response function of the experiment. Solid and dotted lines represent the fit results for the population and structural dynamics, respectively. The “ph” and “py” denote the phenyl and pyran ring, respectively.

Figure 3

(a) Femtosecond stimulated Raman spectra of DCM in CHCl₃ solution with 403 nm excitation and (b) the population and structural dynamics of ν₁₅ (ph) at 1057 cm⁻¹ and ν_8b (ph) at 1495 cm⁻¹; δ_CH3 (ph) at 1415–1428, δ_CH3 (ph) + ν_19b (py) at 1448–1462 cm⁻¹; ν_as,C≡N at 2171–2174 cm⁻¹. All the kinetic traces were fit with a number of exponential functions convoluted with a Gaussian function for the instrument response function of the experiment. Solid and dotted lines represent the fit results for the population and structural dynamics, respectively. The “ph” and “py” denote the phenyl and pyran ring, respectively.

Figure 4

Evolution associated difference spectra (EADS) for (a) the intramolecular charge transfer (ICT) and (b) locally-excited (LE) state of DCM in CHCl₃ and DMSO solution with the specific time constants. The kinetic information obtained from the global analysis over all the vibrational modes can be slightly different from the kinetics of a specific vibrational mode.

Figure 5

The Raman spectra of DCM in the ground state and the S₁/LE and S₁/ICT excited states; (a) the experimental results, (b) the TDDFT simulation results at B3LYP/6-311G(d,p) level for the optimized structures in the ground and S₁ excited states. The spectral changes expected for ν_19a, ν_19b, or δ_CH3 modes were displayed with color-codes.