Research ArticleSpectroscopic behavior of alloxazine-based dyes with extended aromaticity: Theory vs Experiment
Graphical abstract
Introduction
Organic compounds derived from the tricyclic isoalloxazine and alloxazine have been subjects of several studies in photophysics and electrochemistry [[1], [2], [3], [4]], revealing them as efficient luminescent chromophores. Substituted flavins, i.e., isoalloxazines, are widely spread in animals and plants as a part of flavoproteins and coenzymes. For example, riboflavin, also known as vitamin B2, supports the integrity of mucous membranes, skin, eyes, and the nervous system. Various molecules containing benzo[g]pteridine moiety were, therefore, proved to be suitable candidates for future application in bioorganic molecular sensors and semiconductor technology [5,6].
Although the alloxazine (AL) and its tautomer, isoalloxazine (iAL), are closely related compounds (see Fig. 1) differing only by the shift of hydrogen atom from N(1) to N(10) position, their spectroscopic and photophysical properties are quite dissimilar. In particular, the absorption spectra of alloxazine sample in aqueous solution differ from those of isoalloxazine by a hypsochromic shift of both long-wavelength maxima from about 440 nm to 340 nm to about 380 nm and 330 nm, respectively [7]. The shape of absorption spectrum is determined by the equilibrium concentration of both tautomers. In aqueous solutions, the equilibrium can be changed by solvent acidity and temperature. At pH = 4 and room temperature, alloxazine in aqueous solution is expected to be free of a tautomeric isoalloxazine contribution, while at pH = 10, a partial presence of isoalloxazine (9%) is estimated [8].
The isoalloxazine exhibits one order of magnitude larger fluorescence quantum yields and correspondingly longer fluorescence lifetimes than alloxazine [9]. The fluorescence wavelength maximum of 456 nm was detected at pH = 4 for excitation wavelength λexc = 330 nm. At pH = 10, the emission maximum at 530 nm is observed following the excitation wavelength of 440 nm. The solvent acidity in aqueous solutions or in different solvents combined with the photoexcitation also supports the formation of the isoalloxazine tautomer [[10], [11], [12]]. This proton transfer from N(1) to N(10) atom was experimentally also observed for other flavin derivatives in water or pyridine solutions, e.g. lumichrome [13] or 6,7-dicyano-lumazine [14]. The possible role of tautomerization process and deprotonated specie for different pH in water and BDHC micelles was discussed by Choudhury et al. and Prukała et al. [15,16].
Recently, Richtar et al. [17,18] suggested and applied two efficient approaches to the synthesis of various novel alloxazine derivatives. Most prepared compounds exhibit high thermal stability and versatile chemical use. The fluorescence spectra of certain compounds in aqueous and aprotic solvents (DMSO, CHCl3) consist of more than one emission band with different intensities. The authors assumed that this multichromophoric behavior results dominantly from an intramolecular proton transfer [17].
Although this study has demonstrated that the optical properties are changed by the modification of initial alloxazine core, the variation effect of the terminal part in molecular structure on the chemical and electronic structure was not systematically analyzed. In this context, we decided to perform a quantum chemical analysis of (iso)alloxazine and their eight derivatives where the aromatic chromophore is fused to the smaller benzo[g]pteridine moiety (see Fig. 1). The partial aims of this study are: (1) to calculate the optimal geometries of molecules in the electronic ground-state and lowest excited-state in the gas-phase and in alkaline dimethylsulfoxide environment (DMSO); (2) to evaluate the gas-phase energies of frontier molecular orbitals (MOs) and (3) to calculate the TD-B3LYP and TD-CAM-B3LYP electron transitions contributing to absorption and fluorescence spectra. The obtained theoretical results are compared with the experimental absorption and fluorescence spectra measured for available samples in alkaline DMSO. In this context, the possible contribution of deprotonated anionic species or iso-tautomers to the experimental spectra in DMSO is also discussed.
Section snippets
Computational details
The Gaussian 16 program package was used for performing the quantum chemical calculations [19]. Optimal geometries of the studied molecules were calculated by the Density Functional Theory (DFT) with the B3LYP functional [20,21] without any constraints (energy cut-off of 10−5 kJ × mol−1, final RMS energy gradient under 0.01 kJ × mol−1 × A−1). The 6-31 + G** basis set of atomic orbitals was applied [22,23]. In pursuance of optimized B3LYP geometries, the vertical singlet transition energies and
Chemical structure
Compounds in this study are based on the N(1)H-alloxazine core and its N(10)H iso-tautomeric form with various aromatic rings condensed to one end. Despite their differences, the local aromaticity of three rings common for all the derivatives (denoted A, B, and C in Fig. 1) can be compared utilizing the HOMED index (see Supplementary material). The effect of chemical structure modification on the aromaticity can be deduced from this comparison [[31], [32], [33], [34]]. In the parent alloxazine (
Conclusions
The optimal geometries and electronic structure of (iso)alloxazine and eight condensed alloxazine derivatives were investigated using the Density Functional Theory. The local aromaticity of benzo[g]pteridine core was described using the HOMED aromaticity indices and the influence of chemical structure on the static dipole moment was discussed. We have shown that the consecutive addition of aromatic rings could lead to an increase of static dipole moments by about 2 D with respect to the parent
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The work has been supported by Slovak Research and Development Agency (APVV-19-0024) and VEGA 1/0461/21. V.L. thanks to Ministry of Education, Science, Research and Sport of the Slovak Republic for funding within the scheme “Excellent research teams”. We are grateful to the HPC center at the Slovak University of Technology in Bratislava, which is a part of the Slovak Infrastructure of High Performance Computing (SIVVP project, ITMS code 26230120002, funded by the European region development
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