The structural and dynamic non-rigidity of two types of hydrogen-bonded complexes was investigated: cyclic dimers A∙∙∙A of 4-n-alkyloxy and 4-n-alkyl-substituted aromatic acids (A) and complexes A∙∙∙B∙∙∙A with hydrogen bonds O-H∙∙∙N, where B1 – 4,4′-bipyridine and B2 – 4,4′-azopyridine. Dynamic non-rigidity depends on temperature and is determined by the values of the intramolecular vibrational amplitudes (VibAmp) of various structural groups of the complexes. VibAmp were calculated using the force field obtained by the method DFT/CAM-B3LYP/6-311++G**. It was shown that the core of acid dimers, including intermolecular hydrogen bonds, can be considered rigid over a wide temperature range. The dynamic non-rigidity of the A∙∙∙A dimers is mainly determined by the vibrations of the alkyl substituents. A correlation has been established between the VibAmp of the terminal methyl groups l((H3)C···C(H3)) of acid dimers and the temperatures of phase transitions TLC→I in homologous series of 4-n-alkyloxycinnamic, 4-n-alkyloxybenzoic and 4-n-alkylbenzoic acids, which exhibit an odd–even effect. The higher the vibrational amplitude l((H3)C∙∙∙C(H3)), the lower the temperature of the phase transition TLC→I, since the increased vibrational amplitudes prevent ordered intermolecular interactions that stabilize the structure of the liquid crystal. The structural and dynamic non-rigidity of A∙∙∙A dimers and A∙∙∙B∙∙∙A complexes was compared. The non-rigid core of the A∙∙∙B∙∙∙A complex contributes to a significant increase in both the VibAmp of the core and the terminal groups of substituents and a decrease in the TC→LC temperature of the phase transition “crystal-liquid crystal” in comparison with that for acid dimers. The concept of the structural and dynamic non-rigidity of possible hydrogen-bonded complexes can serve as a basis for understanding the influence of various factors and types of hydrogen bonds on the stability of complexes and the manifestation of mesomorphic properties of systems consisting of supramolecules.

С помощью квантово-химических расчетов на теоретическом уровне DFT/B3LYP/cc-pvTZ изучены процессы транс—цис-изомеризации азобензола (AZB), 4,4'-азопиридина (AZP) и азоксибензола (AZOB). Выбранные объекты могут послужить реперными точками при исследовании внутримолекулярных перегруппировок для множества соединений, содержащих азо- и азоксимостиковую группу. Определены геометрические и электронные характеристики транс- и цис-изомеров AZB и AZP и четырех изомеров AZOB, а также переходных состояний (TS) между ними. Установлено, что два изомера AZOB, содержащие семиполярную связь N→O, обладают значительно более низкой энергией, чем два изомера с мостиковой группой NON циклического строения. Переход между низкоэнергетическими транс—цис-изомерами происходит через TS с сохранением связи N→O без миграции атома кислорода от одного атома азота к другому. Выполнен NBO-анализ электронного строения и отмечены факторы, стабилизирующие полученные геометрические конфигурации всех изомеров и переходных состояний.

Processes of trans–cis isomerization of azobenzene (AZB), 4,4′-azopyridine (AZP), and azoxybenzene (AZOB) are studied using quantum chemical calculations at the DFT/B3LYP/cc-pvTZ level of theory. The chosen objects can serve as reference points when studying intramolecular rearrangements of numerous compounds containing an azo group and an azoxy bridging group. Geometric and electronic characteristics of trans and cis isomers AZB and AZP and four AZOB isomers, as well as transition states (TSs) between them, are determined. It is established that two AZOB isomers containing a semi-polar bond N→O have a significantly lower energy than two isomers with a cyclic NON bridging group. The transition between the low-energy trans–cis isomers proceeds via a TS while preserving the N→O bond so that the oxygen atom does not migrate between the nitrogen atoms. The NBO analysis of the electronic structure is performed, the factors stabilizing the obtained geometric configurations of all isomers and transition states are determined.

The TD-DFT/B97-D/6-311G** method is used to calculate the electronic absorption spectra of А⋯Х⋯А supramolecules with intermolecular hydrogen bonds (HBs), where А is 4-n-propoxycinnamic acid, Х is a nonmesogen molecule with bifunctional HB acceptors. Nonmesogens Х (B, C, D, E) consist of two pyridyl fragments connected by bridging groups of different natures: 4,4′-bipyridine (В), 1,2-bis(4-pyridyl)ethane (С), 1,2-bis(4-pyridyl)ethylene (D), 4,4′-azopyridine (Е). It is shown that the introduction of bridging groups (–CН2–CН2–, –CН=СН–, –N=N–) significantly affects the energy of frontier orbitals and electronic absorption spectra (EAS) of nonmesogens Х in the near-UV region. The EAS of two structural units A⋯В⋯A, A⋯A, which may be formed due to the self-assembly in two-component mesogen А–nonmesogen В systems, are compared. Due to hydrogen bonds (O–H⋯N and O–H⋯O), having different types and strengths in these complexes, the high-intensity band shifts to shorter wavelengths in complex А⋯В⋯А and to longer wavelengths in complex А⋯А with respect to the λ = 333 nm band of monomer А. It is shown that EAS of hydrogen-bonded complexes А⋯Х⋯А are not superpositions of EAS of individual components. The spectra show electronic transitions related to charge transfer between the components of the complexes. Using nonmesogens Х of different nature in supramolecules А⋯Х⋯А will allow one to change purposefully their electro-optical properties and to achieve intense absorption in particular regions of UV spectra.

Для супрамолекул с межмолекулярными водородными связями (ВС) типа А⋯Х⋯А, где А — 4-н-пропилоксикоричная кислота, Х — молекула немезогена с бифункциональными акцепторами ВС, методом TD-DFT/B97-D/6-311G** рассчитаны электронные спектры поглощения. Немезогены Х (B, C, D, E) состоят из двух пиридильных фрагментов с разной природой мостиковой группы между ними: В — 4,4'-бипиридин, С — 1,2-бис(4-пири­дил)этан, D — 1,2-бис(4-пиридил)этилен, Е — 4,4'-азопиридин. Показано, что введение мостиковых групп (—CН2—CН2—, —CН=СН—, —N=N—) существенно изменяет энергии граничных орбиталей и электронные спектры поглощения (ЭСП) немезогенов Х в области ближнего УФ. Выполнено сравнение ЭСП двух структурных единиц A⋯В⋯A, A⋯A, которые могут быть образованы в результате самосборки в двухкомпонентных системах мезоген А—немезоген В. Отмечено, что наличие разных по типу (O—H⋯N и O—H⋯O) и силе водородных связей в этих комплексах приводит к смещению высокоинтенсивной полосы в область меньших длин волн в комплексе А⋯В⋯А и в область больших длин волн — в комплексе А⋯А по отношению к полосе λ = 333 нм мономера А. Показано, что ЭСП водородосвязанных комплексов А⋯Х⋯А не являются суперпозицией ЭСП отдельных компонентов. В них наблюдаются электронные переходы, связанные с переносом заряда между составляющими комплексов. Использование немезогенов Х разной природы в супрамолекулах А⋯Х⋯А позволит направленно изменять их электрооптические свойства и обеспечивать интенсивное поглощение в заданных областях УФ спектра.

IR spectra of individual 4-n-dodecyloxybenzoic acid (A) and 4-pyridyl 4′-n-dodecyloxybenzoate (B) compounds as well as IR spectra of 2A:1B and 1A:1B systems were recorded. For the assignment of the experimental vibrational spectra a series of quantum chemical calculations of DFT(B97-D)/6-311++G** level was carried out. Hydrogen-bonded complexes of types A⋯A, A⋯B, as well as various trimers that can be formed at different component ratios in system A-B were simulated. The geometric structure of these complexes was optimized and the vibrational frequencies were calculated. The conclusions on the molecular organization of system A-B for different ratios of the components A and B were based on the interpretation of IR spectra and the analysis of calculated thermodynamic characteristics of self-assembly processes. Thus, it is determined that the system A consists of cyclic A⋯Acycl dimers; in 1A:1B system the H-complexes of A⋯B type are formed. In the 2A:1B system in the process of self-assembly, instead of the complexes A⋯A⋯B and A⋯B⋯A of stoichiometric composition, the complexes A⋯B and A⋯Acycl are formed in the ratio 2:1. These results are confirmed by the dilatometric method data.

The variants of structural organization in the systems “mesogen – nonmesogen” are considered. The systems contain p-n-propyloxycinnamic acid (A), as a mesogenic component, and nonmesogenic Ph–X–Ph compounds: phenyl benzoate (B, where X = –COO–), azobenzene (C, where X = –N=N–) and N-benzylideneaniline (D, where X = –CH=N–). Quantum-chemical modeling of possible structural units in such systems has been performed. It was shown that all assumed A∙∙∙X(Ph)2 H-complexes do not have electronic and geometric anisotropy and have a lower intermolecular interaction energy than the cyclic dimer of acid A∙∙∙A. The calculated values of the Gibbs free energy of complexation reactions also indicate a low probability of the formation of A∙∙∙X(Ph)2 type H-complexes. It is noted that the “length” of the A∙∙∙A dimer is comparable with the doubled “length” of Ph–X–Ph molecules, which, like the acid dimer, have a rod-like structure favorable for the formation of nematic and smectic LC phases. Based on the analysis of the quantum chemical calculations, it was assumed that Ph–X–Ph can be embedded between acid cyclic dimers A∙∙∙A and can facilitate reduce intermolecular interactions in the system, which reduces the temperature of Cr–LC transitions. The proposed structural organization of systems A: Ph–X–Ph is confirmed by an experimental IR spectrum for a similar system, in which the bands corresponding to the vibrational frequencies of the acid dimer and to individual molecules of alkyloxy substituted phenyl benzoate B are recorded.