Abstract
Aim. To perform a combined experimental and computational study on the luminescent properties of practically important class of organic dyes – 4-aminophthalimides.Results and discussion. The absorption and fluorescence spectra of 4-aminophthalimide derivatives in polar protic and aprotic solvents were computed and matched vs. the experimental data. The changes in emission spectra are mainly related to the NH2-group derivatization. The methyl substitution of amide hydrogen causes a batochromic shift of about 7 nm in the absorption peak and a negligible hypsochromic shift in the fluorescence peak, while introducing alkyl substituents to the amine moiety causes bathochromic shifts in absorption and emission peaks of 30 – 40 nm and 10 – 60 nm, respectively.Experimental part. Absorption and emission wavelengths were computed by the standard algorithm based on the ground state geometry optimization (equilibrium solvation), vertical excitation with nonequilibrium solvation, and the TD-DFT geometry optimization of the excited state structures. A reliable hybrid B3LYP functional was used in combination with DZ and TZ-quality basis sets.Conclusions. The computed absorption wavelengths are in excellent agreement with the experimental data and are only slightly solvent-dependent. At the same time, the discrepancy with the experiment for Stokes shifts reaches about 20 % at IEF-PCM-TD-B3LYP/6-31G(d). However, the general tendency for both absorption and fluorescence wavelengths is identical for all solvents within one molecule. Received: 24.12.2019Revised: 31.01.2020Accepted: 27.02.2020
Highlights
The general tendency for both absorption and fluorescence wavelengths is identical for all solvents within one molecule
Since the experimental fluorescence spectra of 4-aminophthalimide derivatives are determined by the allowed transitions between the S0/S1 electronic states [15], the frontier orbitals of 1 and its methyl derivative 2 were first analyzed through the optimization of their S1-state geometries (Fig. 2)
The absorption wavelengths obtained from the vertical excitation of 1 with the linear response solvation are in good agreement with the experiment, while the computed emission wavelengths exhibit considerably larger deviation and result in ca 40 nm deviations in the theoretical Stokes shifts from those observed experimentally for 1
Summary
We studied the luminescent properties of 4-aminophthalimide 1 derivatives, namely 4-aminoN-methylphthalimide 2, 4-amino-N-ethylphthalimide 3, 4-amino-N-(n-propyl)phthalimide 4, 4-amino-N-(isopropyl)phthalimide 5, 4-amino-N-cyclohexylphthalimide 6, 4-amino-N-(1-adamantyl)phthalimide 7, 4(N-ethyl)amino-N-methylphthalimide 8, and 4-(N,Ndiethyl)amino-N-methylphthalimide 9 (Fig. 1) prepared as described earlier [2, 5]. The computations of the vertical excitation step of substituted 1 with even relatively small basis sets provided absorption wavelengths in good agreement with the experiment. The absorption wavelengths obtained from the vertical excitation of 1 with the linear response solvation are in good agreement with the experiment, while the computed emission wavelengths exhibit considerably larger deviation and result in ca 40 nm deviations in the theoretical Stokes shifts from those observed experimentally for 1. In order to compute the fluorescence wavelengths for all molecules studied a small basis set 6-31G(d) was chosen as it allowed us to optimize the most time-consuming stage of the frequencies calculation of excited states for the whole set of compounds. Theoretical and experimental absorption and fluorescence wavelengths, as well as Stokes shifts, computed for a set of 4-aminophthalimide derivatives using IEF-PCM-TD-B3LYP/6-31G(d) approach with bulk solvation effects modeled in methanol. The Stokes shifts were computed by subtracting vertical absorption and emission energies and converting to the wavelength scale
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