Tuning Wavefunction Mixing in Push-Pull Molecules: From Neutral to Zwitterionic Compounds

Fluorescence emission mechanism for the π-conjugated zwitterion 2,4-Bisimidazolylphenol base on ESIPT: A TDDFT theoretical reconsideration

Molecules with zwitterionic characteristics exhibit significant potential for utilization in nonlinear optics, optoelectronics, and organic lasers owing to their large dipole moments. Recently, the synthesized compound 2,4-bis (4,5-diphenyl-1H-imidazol-2-yl) phenol (2,4-bImP) by Sakai et al. has been noticed for its unique photochromic properties in solvents [J. Phys. Chem. A, 125 (2021), 4784-4792]. The observed fluorescence in chloroform was attributed to the keto tautomer. Based on the excited state intramolecular proton transfer, the photochromism of 2,4-bImP in chloroform was interpreted as zwitterion production. However, the zwitterion with a specific electronic structure can be in resonance with the conventional neutral structure. The impact of the resonance contribution from the zwitterion and the conventional neutral structure on fluorescence attribution was not taken into account in the previous studies. In this investigation, the ESIPT mechanism of the 2,4-bImP in chloroform has been explored using both the density functional theory and the time-dependent density functional theory. The optimized geometric configuration parameters illustrate the molecular resonant properties. The calculated fluorescence spectra on the basis of the optimization results further corroborate that the fluorescence peaks after proton transfer originates from the resonance of the zwitterionic and the neutral configuration. The zwitterionic nature of the molecule was demonstrated by electrostatic potential and atomic dipole modified Hesfeld atomic charge (ADCH) analysis. Furthermore, the characterization of potential energy curves and IR spectrum further verified the resonance of both the zwitterionic and neutral structures. The results reveal that the 2,4-bImP molecule generates the neutral o-quinoid structure and the zwitterionic structure resonance phenomenon following ESIPT. The aforementioned resonance structure offers novel insights into the ascription of fluorescence. These discoveries establish the theoretical foundation for the exploration and development of zwitterions.

Indane-1,3-Dione: From Synthetic Strategies to Applications

Indane-1,3-dione is a versatile building block used in numerous applications ranging from biosensing, bioactivity, bioimaging to electronics or photopolymerization. In this review, an overview of the different chemical reactions enabling access to this scaffold but also to the most common derivatives of indane-1,3-dione are presented. Parallel to this, the different applications in which indane-1,3-dione-based structures have been used are also presented, evidencing the versatility of this structure.

Conjugated donor–acceptor substituted systems involving the 1,3-indandione-derived electron accepting moieties

Conjugated donor–acceptor molecules are the focus of research owing to their unusual photo- and electro-physical properties. At the same time, several unusual features of these compounds are difficult to explain or predict. Here we present our results on the synthesis, X-ray structures and D-NMR spectra providing a deeper insight into the conjugation within the derivatives involving the 1,3-indandione-derived series of compounds with varying electron acceptor strength and conjugating bridge length. The X-ray structures show the presence of several intermolecular short contacts strongly affecting the molecular geometries. In solution, the coalescence temperatures corresponding to the rotation of the phenylamino moiety of all derivatives do not exceed 246 K indicating the unhindered rotation at room temperature. Using B3LYP/aug-cc-pVDZ, the calculated model chemistry barriers to rotation, dipole moments and first hyperpolarizabilities are within experimental error. We conclude that neglecting the electron donating properties of bridges themselves and internal rotation about the single bonds taking part in conjugation can result, for instance, in misinterpretation of their room temperature NMR spectra and overestimation of the computed molecular dipole moments by more than 5 D.

Determination of barriers to rotation using the D-NMR technique within a series of push–pull derivatives sheds light on the inability of stronger acceptors to considerably increase the dipole moments and nonlinear optical response.

Unusual response to environmental polarity in a nonlinear-optical benzylidene-type chromophore containing a 1,3-bis(dicyanomethylidene)indane acceptor fragment

One of the strongest known electron-accepting fragments used in the synthesis of organic dyes for applications in nonlinear optics (NLO) is 1,3-bis(dicyanomethylidene)indane (BDMI). By studying a benzylidene-type push-pull chromophore bearing a 5-carboxy-BDMI electron-acceptor and 4-(dimethylamino)aniline donor fragment, we demonstrate that this class of compounds can show unusual response to the polarity of the surrounding medium. The combined results of UV-Vis absorption spectrometry, NMR experiments and computational modeling indicate that the studied compound undergoes a geometrical transformation that involves an increase in the torsion angle ω between the aniline and indane ring systems with the rise of the polarity of the surrounding medium. This process is partly facilitated by an increased rotational freedom around ω in more polar solvents, as detected experimentally by NMR and predicted by calculations. Regarding the practical application aspects, computations predict that the solvent-polarity-induced increase of torsion ω would lead to a notable decrease in the first hyperpolarizability (β) value. This was detected experimentally, as hyper-Rayleigh scattering (HRS) data showed a drop in the compound's NLO activity from β_HRS(532)  = 513 × 10^-30 esu in toluene to β_HRS(532)  =  249 × 10^-30 esu in acetonitrile. This places limitations on the NLO applications of the studied compound and its structural analogues, as the surrounding medium (solvent of polymer matrix) with the lowest possible polarity needs to be used to maximize their NLO efficiency.

Long-living optical gain induced by solvent viscosity in a push–pull molecule

The combination of continuum and ultrafast pump–probe spectroscopy, in viscous and non-viscous environments, with DFT and TDDFT calculations, is effective in unraveling important features of the twisted intramolecular charge transfer mechanism in a new push–pull molecule that possesses aggregation induced emission properties.

Franck–Condon factors—Computational approaches and recent developments

Algorithms and methodologies for the calculation of Franck–Condon integrals are reviewed. Starting from the standard approach based on the Cartesian representation of the normal modes and the use of Duschinsky's transformation and recursion formulas, methods for treating large displacements of the equilibrium positions and anharmonic and non-Condon effects are considered. Finally, focusing attention on problems arising from the use of recurrence relations, some of the proposed solutions are critically reviewed along with new alternative approaches.

2-(4-Methyl-phen-yl)-6-nitro-1,3-benzoxazole

The title compound, C14H10N2O3, is a π-conjugated mol-ecule containing a benzoxazole aromatic fused heterobicycle. The benzoxazole ring system is planar within 0.01 Å. The mol-ecule assumes an approximately flat conformation, the benzoxazole ring system forming dihedral angles of 6.52 (12) and 7.4 (3)° with the benzene ring and the nitro group, respectively. In the crystal, mol-ecules are connected by very weak C-H⋯O hydrogen inter-actions, forming chains running parallel to the a or c axes. The methyl H atoms are disordered over two sets of sites of equal occupancy rotated by 60°.

3,3'-({4-[(4,5-Di-cyano-1H-imidazol-2-yl)diazen-yl]phen-yl}imino)-dipropionic acid

The title compound, C₁₇H₁₅N₇O₄, is a push–pull non-linear optical chromophore containing a di­alkyl­amino donor group and the di­cyano­imidazolyl acceptor separated by a π-conjugated path. The benzene and imidazole rings are not coplanar, making a dihedral angle of 10.0 (2)°. In the crystal, mol­ecules are linked by an extended set of hydrogen bonds and several motifs are recognized. Pairs of mol­ecules are held together by hydrogen bonding between carb­oxy O—H donor groups and diazenyl N-atom acceptors, forming R₂²(24) ring patterns across inversion centres. Four-mol­ecule R₄⁴(28) ring motifs are formed, again across inversion centres, through hydrogen bonding involving carb­oxy O—H donor groups and diazenyl and imidazole N-atom acceptors. Four-mol­ecule R₄⁴(42) patterns are formed among mol­ecules related by translation and involve carb­oxy O—H and imidazole N—H donor groups with carbonyl O-atom and imidazole N-atom acceptors.

Generating function approach to the calculation of spectral band shapes of free-base chlorin including Duschinsky and Herzberg-Teller effects

A generating function approach to the calculation of spectral band shapes including Duschinsky and Herzberg-Teller effects is proposed and applied to the computation of the free-base chlorin Q absorption bands, using molecular geometries and normal vibrations obtained by density functional theory computations. The results clearly show that non-Condon effects can significantly affect the relative intensities of the weakest Q(y) and, to a lesser extent, Q(x) bands. The proposed approach is extremely powerful and can be used in the cases where the molecular size makes the direct calculation of Franck-Condon integrals by recurrence formulas prohibitive.