Intramolecular excimers of open forms of 2H-benzopyran, 2H- and 3H-naphthopyrans in solution: TD-DFT/DFT analysis.

We hypothesized that in the open forms of diphenyl-substituted photochromic compounds, immediately after the photoinduced cleavage of the C-O bond, one of the phenyl rings forms a stack with an aromatic system at the other end of the alkyl linker. The formation of these intramolecular stacking excimers is made possible by a known increase in the rotational mobility of the linker double bonds in the excited state. The flexibilities of linkers are analyzed in terms of changes in their dihedral angles. After internal conversion, stacking is partially broken but it still prevents the recovery of the C-O bond. This explains the significantly greater stability of the open forms of diphenyl-substituted benzopyran and naphthopyran compared to dimethyl-substituted ones. Open structures with residual stacking are intermediate and by thermal motion, they transform into stable trans-trans and cis-trans forms. However, in our opinion, they significantly affect the equilibrium of closed and open isomers in solution. In particular, the calculated stacking energy of 2H-naphthopyran turned out to be higher than that of 3H-naphthopyran, which partly explains the significantly greater stability of the open form of the former. For the DFT/TD-DFT calculations performed in this work, the following functionals were selected to provide reliable stacking for the open forms of all three studied compounds: APFD, M052X, M06HF, as well as B3LYP with D3(BJ) Grimme dispersion correction. The 6-311++G(d,p) basis set and cyclohexane solvent modeled using IEFPCM were also used.

4a,4b-Dihydrophenanthrene → cis-stilbene photoconversion: TD-DFT/DFT study.

CONTEXT: DHP → CS photoconversion was analyzed in terms of electron density redistribution for the first time. The following explanation for the non-recovery of the C4a-C4b bond upon CS relaxation is proposed: during this process, the Coulomb repulsion energy between these pairs of atoms increases by almost one and a half times, and their bonding by an electron at LUMO is insufficient to recover the C4a-C4b bond. According to calculations, upon CS relaxation, the linker connecting the benzene rings undergoes significant structural changes. In this case, the distance between the C4a and C4b atoms increases from 3.00 Å to 3.28 Å. Calculations showed that the C4a-C4b vibration of the DHP bond has a very low intensity. Therefore, thermal motion does not contribute to the rupture of this bond. METHODS: All calculations were performed using the Gaussian16 software package at the B3LYP/6-311 + + G(d,p)/IEFPCM theory level. B3LYP was the only hybrid functional supported by Gaussian16, which ensured the cleavage of the C4a-C4b bond of DHP while optimizing its S1 excited state. A quantitative description of the redistribution of electron density in the studied conformers was carried out using the analysis of the NPA of atomic charges. Cyclohexane was used as an implicitly specified non-polar solvent. Visualization of molecular orbitals, and electron densities, as well as plotting of calculated IR spectra, were performed using the Gaussview6 software package.

TD-DFT analysis of excitation of the closed-form spironaphthopyran in methanol solution: The contribution of vibronic transitions and intermolecular hydrogen bonds.

A set of forty hybrid functionals, combined with a 6-31++G(d,p) basis set and an IEFPCM solvent description, was applied to calculate the S0→S1 excitation of the spironaphthopyran (SNP) photochromic compound. None of the hybrid functionals resulted in Cspiro-O bond cleavage upon optimization of the S1 excited state of the SNP, in contrast to the spiropyran BIPS which we studied earlier. At the same time, the deformations of the SNP during excitation turned out to be very small (in contrast to those for BIPS) which made it possible to calculate its vibronic absorption spectrum. The best agreement between the calculated and experimental absorption bands was demonstrated by the SOGGA11X functional. The main result of this work is confirmation of the vibration participation in the breaking of the Cspiro-O bond in the S1 excited state of the SNP. This assumption based on experimental data was previously put forward by other authors. Strong H-bonds between the SNP and two methanol molecules enhance the photoinduced distortions of the latter and are slightly weakened upon excitation.

TD-DFT study of BIPS spiropyran: Effects of functionals and high-polarity solvent on Cspiro O bond dissociation and recovery.

We performed a theoretical analysis of the BIPS photochemical cycle using an extensive set of forty hybrid functionals and taking into account a highly polar solvent (methanol). The functionals with a small fraction of the exact Hartree-Fock exchange (%HF) showed the predominant S0 → S2 transition with the strengthening of the Cspiro O bond. At the same time, functionals with medium and high %HF (including those with long-range correction) gave a dominant S0 → S1 transition with weakening or breaking of the Cspiro O bond, which corresponds to the experimental results. The influence of a highly polar solvent on the photochemical electrocyclic transformations of BIPS turned out to be significant. The number of functionals causing dissociation of the Cspiro O bond decreased from 10 to 7 compared to the gas phase. The magnitude of the oscillator strength has increased by approximately one and a half times. Structural distortions of the BIPS molecule during excitation (both with and without Cspiro O bond cleavage) significantly decreased in methanol compared to the gas phase. The two strong hydrogen bonds of methanol molecules with the oxygen and nitrogen atoms of spiropyran also have a significant effect on its excitation. They lead to a change in the predominant transition from S0 → S2 to S0 → S1 for five functionals. The number of functionals giving dissociation of the Cspiro O bond decreased from seven to four (M08HX, M052X, CAM-B3LYP, and M11). After the opening of the excited BIPS molecule, both of its strong H-bonds with methanol are preserved. Of this set of four functionals, only M052X and CAM-B3LYP exhibited the dominant HOMO-1 → LUMO configuration observed in high-level computations by other authors. Therefore, both of these functionals are recommended for modeling the photochemical cycle of this spiropyran. The photochemical cycle of BIPS was theoretically analyzed. The redistribution of the electron density in this cycle was quantitatively described using the differences in NPA of the atomic charges. The most important result of this analysis was the electrostatic mechanism of the approach of Cspiro and oxygen atoms at the fourth stage, which causes further reduction of the Cspiro O bond.

Theoretical study of the excitation of proflavine H-dimers in an aqueous solution: the effect of functionals and dispersion corrections.

Analysis of the ground and excited states of the H-dimer of the proflavine dye cation (PF) in an aqueous solution was performed using the DFT/TD-DFT method with an implicit specification of the aqueous environment and using various hybrid functionals (APFD, B2PLYP, B3LYP, B3PW91, BMK, CAM-B3LYP, M05, M052X, M06, M062X, M06HF, mPW2PLYP, PBE0, PW6B95, and ωB97XD), Grimme dispersion corrections, and Becke-Johnson damping. To our knowledge, this is the first theoretical study on the dimerization of charged monomers. The use of the B2PLYP, B3LYP, B3PW91, BMK, CAM-B3LYP, PBE0, M05, mPW2LYP, and PW6B95 functionals without additional dispersion corrections led to dimer dissociation due to Coulombic repulsion between PF cations. At the same time, the M052X, M06, M062X, and M06HF functionals without dispersion corrections showed reliable stabilization of PF2cations. APFD and ωB97XD with built-in dispersion corrections also performed well. This photoinduced attraction is explained by the different overlaps of the respective molecular orbitals. In this case, there is no significant intermolecular charge transfer in PF2. The electron density shifts in dye molecules during their dimerization significantly exceeded those upon excitation of both the free monomer and the dimer. The transition moments M showed that the CAM-B3LYP, M052X, M062X, M06HF, and ωB97XD functionals gave M(monomer) < M(H-dimer), while all other functionals used in this work showed M(monomer) > M(H-dimer). The former pattern was described using the strong coupling theory for H-aggregates. The interaction between PF molecules in the H-dimer is indeed strong and, therefore, is correctly described precisely using the functionals with the long-range correction or large exact Hartree-Fock exchange. Only these five functionals were found to satisfy the condition λmax(H-dimer) < λmax(monomer), which is observed experimentally for PF and is explained using the Kasha exciton theory. They alone give rise to very strong intramolecular vibrations in the spectrum of the excited dimer. The vibronic absorption spectra could only be calculated using the M062X, M06HF, and ωB97XD functionals, which had the lowest RMSD between the ground and excited states. Based on the results obtained, we recommend the CAM-B3LYP (with dispersion correction), M052X, M062X, M06HF, and ωB97XD functionals for the theoretical description of aromatic cation dimers in both the ground and excited states.

Acriflavine in aqueous solution: excitation and hydration.

Using TD-DFT/DFT, the ground and excited states of the acriflavine dye were studied in an aqueous medium. The mutual influence of photoexcitation and strong hydrogen bonds with the solvent was studied by comparing the purely implicit and combined modeling of the aqueous environment of the dye. The excitation of acriflavine was calculated considering the vibronic coupling. The effect of photoexcitation on dye vibrations was analyzed. The spatial structure of the acriflavine H-dimer was obtained and its absorption was estimated.

Coumarin 343 in aqueous solution: theoretical analysis of absorption.

Vibronic coupling and hydration were taken into account when describing the absorption of coumarin C343 (both neutral and anionic forms) in an aqueous media. It was shown that the B3LYP functional with the 6-31 + + G(d,p) basis set and the IEFPCM solvent continuum model give theoretical vibronic absorption spectra, which are coincide with the experimental ones. Of the structural differences between C3430 and C343-, there is a different twisting of the carboxyl group additionally changing due to excitation. Upon excitation, a significant shift in the electron density occurs from the C10 atom to the C4 atom only. Thus, a charge transfer on the scale of the entire molecule does not occur. Different hydration complexes with strongly bound water molecules have been analyzed.

Excitation of neutral red dye in aqueous media: comparative theoretical analysis of neutral and cationic forms.

The main goal of this work was the theoretical interpretation of the absorption spectra of neutral red in an aqueous solution (both neutral NR0 and protonated NR+ forms). To achieve this problem, TD-DFT/DFT calculations with different hybrid functionals, the IEFPCM solvent model, and the 6-31 + + G(d,p) basis set were used. MN12SX functional provided the best agreement with the experiment for both dye forms. While the absorption band of the cationic form of the dye in the visible region of the spectrum is due to one transition S0 → S1 (HOMO-LUMO), for its neutral form, there are two transitions S0 → S1 (HOMO → LUMO) and S0 → S2 (HOMO-1 → LUMO), with the latter having a higher intensity. The protonation of the dye chromophore introduces significant changes in HOMO shape. At the same time, LUMOs are almost the same for the protonated and neutral forms of the NR. During the transition from NR0 to the S1 state, its dipole moment increases more significantly than during the transition to the S2 state. Calculations confirmed the assumption of Singh et al. about the existence of two closely spaced excited states of NR0, the first of which has a larger dipole moment. However, the hypothesis of these authors about the corresponding intramolecular charge transfer, as well as the huge value of the dipole moment of this excited state (~ 20 D) declared by them, was not confirmed by present calculations. It was shown that the photoinduced charge redistribution in both the neutral and cationic forms of the dye is local, and the corresponding dipole moment is ~ 10 D. This agrees with other early theoretical work by Aaron et al. The influence on the NR+ absorption spectrum of hydrating water molecules was also analyzed. It was found that the interplay of electrostatic and site-specific contributions leads to the fact that NR solvatochromism does not have a pronounced dependence on the polarity of the solvent.

Excitation of rhodamine 800 in aqueous media: a theoretical investigation.

The main goal of this work was to obtain a calculated absorption spectrum of rhodamine 800 in an aqueous solution, which most accurately reproduces the experimental one. To achieve this result, I used the hybrid functionals supported by Gaussian 16 software package. In this case, the basis set (6-31++G(d,p)) and the solvent model (IEFPCM) were not varied. The B3PW91 functional gave the best agreement with the experimental absorption spectrum of the dye in an aqueous medium. B3P86, B971, B972, B98, X3LYP, APF, HSE06, and N12SX functionals also give good absorption energy coincidence. The B3PW91/6-31++G(d,p)/IEFPCM theory level chosen in this way made it possible to calculate the various characteristics of rhodamine 800 in the ground and excited states. An important result of this work was the establishment of the vibronic nature of the short-wavelength smaller maximum of the absorption spectrum. The influence of the strong H-bond of the exocyclic nitrogen atom with the water molecule on the dye excitation was analyzed.

Photoexcitation of oxazine 170 dye in aqueous solution: TD-DFT study.

The vibronic absorption spectra of OX170 dye in an aqueous solution using 40 hybrid functionals, the 6-31 + + G(d,p) basis set, and the SMD solvent model were calculated. It turned out that the long-range corrected ωB97XD functional provided the best agreement with the experiment in the positions of the main maximum and the short-wavelength shoulder. Calculations showed that this shoulder is vibronic and is not caused by a separate electronic transition. At the same time, the shoulder intensity in the calculated spectrum turned out to be lower than in the experimental one. Various parameters of the OX170 cation in the ground and excited states (IR spectra, atomic charges, dipole moments, and transition moment) were calculated. Maps of the distribution of electron density and electrostatic potential have been built. The influence of four strong hydrogen bonds of the dye with water molecules on the absorption spectrum was analyzed. It was shown that these bonds are strengthened upon OX170 excitation. It was found that explicit assignment of water molecules strongly bound to the dye leads to a redshift of the calculated spectrum by ≈15 nm as a whole, and worsened its shape. Photoexcitation of the dye leads to a noticeable polarization of only one of the four considered water molecules (associated with the endocyclic nitrogen atom in the central ring of the chromophore, the electron density on which increases the most).

The vibronic absorption spectrum and electronic properties of Azure B in aqueous solution: TD-DFT/DFT study.

The vibronic absorption spectrum of Azure B (AB) in an aqueous solution is calculated using the time-dependent density functional theory (TD-DFT). The results of calculations are analyzed using all hybrid functionals supported by Gaussian16, the 6-31++G(d,p) basis set, and the IEFPCM and SMD solvent models. The solvent model IEFPCM gave significantly underestimated values of λmax in comparison with the experiment. This is a manifestation of the TD-DFT "cyanine failure". However, the SMD model made it possible to obtain good agreement between the calculation results and experimental data. The best fit was achieved using the X3LYP functional. According to our calculations, the shoulder in the visible absorption spectrum of AB has a vibronic origin. However, the calculated shoulder is weaker than the experimental one. Explicit assignment of two water molecules, which form strong hydrogen bonds with a dye molecule, leads to a shift of the calculated absorption spectrum to longer wavelengths by approximately 17 nm but does not lead to an improvement in its shape. Comparative analysis of the calculated vibronic absorption spectra of Azure B with those obtained earlier for Azure A and methylene blue showed that the presence and intensity of the short-wavelength shoulder are determined by the location of the bands of higher vibronic transitions relative to the band of the 00 â†’ 00 main transitions. Photoexcitation leads to an increase in the dipole moment of the dye molecule. An insignificant photoinduced electron transfer was found in the central ring of the chromophore of the dye molecule.

The vibronic absorption spectra and electronic states of acridine orange in aqueous solution.

The time-dependent density functional theory (TD-DFT) was used to obtain vibronic absorption spectra of acridine orange dye (AO) in an aqueous solution that were in good agreement with the experiment. The protonated and neutral forms of the dye have been investigated. The results of calculations using various functionals and basis sets have been analyzed. The best agreement with experiment was given by the level of theory X3LYP/6-31G(d,p). AO molecular orbitals involved in electronic transitions due light absorption in the visible region of the spectrum have been obtained. The dipole moments and atomic charges of the ground and excited states of the AO molecule have been calculated. Maps of the electrostatic potential have been drawn. An insignificant photoinduced electron transfer was found in the central ring of the chromophore of the dye molecule. According to our calculations, the vibronic coupling and the Boltzmann distribution play a significant role in the absorption spectra of the AO.

Interaction of pseudoephedrine and azithromycin with losartan: Spectroscopic, dissolution and permeation studies.

This study aims at investigating the potential effect of selected cationic drugs (azithromycin (AZN) and pseudoephedrine sulfate (PSD) on the dissolution profile and intestinal permeation of losartan potassium (LOS) that might occur due to ion pair salt formation. DSC, FT-IR and 1H NMR indicated the formation of ion pair salts between LOS and each of AZN and PSD. Based on NMR chemical shifts calculations, utilizing specialized software, the most likely structures of the salt were proposed and revealed interesting structural features. The obtained ion pair products were shown to have lower aqueous solubilities (water and phosphate buffer pH 6.8) and higher apparent partition coefficient values compared to the parent compound. Neither of the cations affected the dissolution of LOS tablet (Cozaar® 100 mg) in the studied media (HCl pH 1.2 and phosphate buffer pH 6.8). Interestingly, AZN significantly increased the dissolution of LOS in phosphate buffer pH 4.5 (f2 = 33), and an explanation based on distinguished association pattern between AZN and LOS (CH/π) was offered. Employing permeation test across Caco-2 cells monolayer, the apparent permeability coefficient (Papp) of LOS increased significantly (from 0.9 × 10-5 cm/s to 1.8 × 10-5 cm/s) in the presence of the selected cations. Therefore, while the employed cationic drugs were not shown to form ion pair salts under the in-vitro dissolution conditions, they may still participate in significant in-vivo interaction with LOS.