Two-Photon Absorption and Multiphoton Excited Fluorescence of Acetamide-Chalcone Derivatives: The Role of Dimethylamine Group on the Nonlinear Optical and Photophysical Properties

Abstract

This work studied the effect of different electron-withdrawing and electron-donating groups on the linear and nonlinear optical properties of acetamide-chalcone derivatives. The results showed that the addition of the dimethylamine group led to a large fluorescence emission (71% of fluorescence quantum yield in DMSO solution) that can be triggered by two and three-photon excitations, which is essential for biological applications. Furthermore, dimethylamine also red-shifts the lower energy state by approximately 90 nm, increasing the two-photon absorption cross-section of the lower energy band by more than 100% compared with the other studied compounds. All compounds presented two-electronic states observed through one and two-photon absorption spectroscopy and confirmed by Quantum Chemistry Calculations (QCCs). QCC results were also used to model the experimental two-photon absorption cross-sectional spectrum by the Sum-Over-States (SOS) approach, revealing a dependence between the coupling of the ground state with the first excited state and the transition dipole moment between these states.

Keywords: SOS model; acetamide-chalcones; dimethylamine group; two and three-photon excited fluorescence emission; two-photon cross-section.

Figure 1

Molecular structure of the compounds studied. R’ represents the position of different substituents.

Figure 2

One-photon absorption spectrum (continuous black lines) and 2PA spectrum measured through the tunable Z-Scan technique (circles) of (a) ChH, (b) ChCH3, (c) ChCH2CH3, (d) ChOCH3, (e) ChOCH2CH3, (f) ChN(CH3)2, (g) ChBr, and (h) ChNO2. The experimental error associated with 2PA measurements is 20%. The red lines represent the SOS model used to model the experimental 2PA results.

Figure 3

Frontier Molecular Orbitals of all studied compounds obtained through the PCM-CAM-B3LY/6-311++G(d,p) approach for DMSO solvent.

Figure 4

Fluorescence anisotropy (red circles) for compound ChN(CH3)2. The inset displays the fluorescence spectrum for different excitation wavelengths.

Figure 5

Fluorescence quantum yields of ChN(CH3)2 for different solvents (in ascending order of ET(30): toluene, tetrahydrofuran, dichloromethane, dimethylsulfoxide, dimethylformamide, ethanol, and methanol) determined through the Brouwer method.

Figure 6

(a) linear absorption and (b) fluorescence emission of ChN(CH3)2 for different surrounding media (solvents). (c) represents the Stoke Shift (in wavenumber units) for different values of solvent polarity (in ascending order of $\mathsf{\Delta }F:$ toluene, chloroform, dichloromethane, dimethyl sulfoxide, dimethylformamide, and methanol), and (d) is the experimental (circles) fluorescence decay. The red line represents the adjustment considering the fluorescence experimental measurements and the instrument response function were convoluted.

Figure 7

Quadratic (black circles) and cubic (red circles) dependence of the pulse energy with the fluorescence intensity confirming that the fluorescence of ChN(CH3)2 is triggered by two (900 nm) and three (1190 nm) photon excitations. The continuous lines represent the linear fitting.