Aromatic Crystals Research Articles

Direct Population of Triplet States for Efficient Organic Afterglow through the Intra/Intermolecular Heavy-Atom Effect.

Organic afterglow is a fascinating phenomenon with exceptional applications. However, it encounters challenges such as low intensity and efficiency, and typically requires UV-light excitation and facile intersystem crossing (ISC) due to its spin-forbidden nature. Here, we develop a novel strategy that bypasses the conventional ISC pathway by promoting singlet-triplet transition through the synergistic effects of the intra/intermolecular heavy-atom effect in aromatic crystals, enabling the direct population of triplet excited states from the ground state. The resulting materials exhibit a bright organic afterglow with a remarkably enhanced quantum efficiency of up to 5.81%, and a significantly increased organic afterglow lifetime of up to 157 microseconds under visible light. Moreover, given the high-efficiency visible-light excitable organic afterglow emission, the potential application is demonstrated in lifetime-resolved, color-encoded, and excitation wavelength-dependent pattern encryption. This work demonstrates the importance of the direct population method in enhancing the organic afterglow performance and red-shifting the excitation wavelength, and provides crucial insights for advancing organic optoelectronic technologies that involve triplet states.

Influence of different linkages on the mesomorphic properties of aromatic ring system liquid crystals

In order to study the effect of the central linkage on the mesomorphic behaviour of aromatic ring liquid crystals we have synthesized two homologues groups, group A includes compounds 1a-d and group B includes compounds 2a-e. Group A has azo, azomethine, azo-ester, and azomethine-ester central linkages whereas the group B has azomethine, ether-azomethine, heterocyclic, and methylene-azomethine central linkages. The synthesized compounds were identified and characterized by elemental analyses and spectroscopic methods (proton nuclear magnetic resonance and Fourier transform infrared). The mesomorphic properties were investigated by polarizing optical microscopy (POM) with a heating stage and differential scanning calorimetry (DSC) thermograms with an aim to investigate the influence of different types of linkages on the mesomorphic properties in such aromatic ring systems. The results revealed that the change of the central linkages has a crucial effect on the mesomorphic properties when compared to the influence of different types of linkages. We found that the synthesised compounds (1a-d) showed smectic phase with different thermal ranges, while the synthesised compounds (2a-e) showed nematic phase except the compounds 2c-e which did not exhibit any liquid crystalline phase.

Synthesis of aromatic liquid crystal π-conjugated copolymers bearing three-ring pyrimidine-based mesogens, and optical texture observations

Poly(thiophene-co-phenylene) type π-conjugated polymers having two mesogens in monomer repeat unit were synthesized using the Migita–Kosugi–Stille coupling reaction. Thermotropic liquid crystal behavior of the series of polymers was examined with polarizing optical microscopy observations and differential scanning calorimetry. The produced polymers showed unique optical textures of smectic liquid crystal.

Synthesis and properties of wholly aromatic phosphorus-containing thermotropic liquid crystal copolyesters with excellent fibre formation ability

ABSTRACT In this study, wholly aromatic phosphorus-containing thermotropic liquid crystal polymers (TLCP) derived from 6-hydroxy-2-naphthenic acid (HNA), terephthalic acid (TA) and 10-(2,5-Dihydroxypheny)-10 h-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO-HQ) are successfully synthesised via one pot melt polymerisation method. Although, the resulted copolyesters are amorphous, high glass transition temperature (182–196 °C) is observed due to the existence of kinked DOPO-HQ unit and crankshaft HNA monomer. The nematic liquid crystal behaviour is found for all of the copolyesters under POM at relatively low temperature (190–207 °C). The copolyesters show good thermal stability and high char yield under both nitrogen and air atmosphere, which of the initial degradation temperature (T5%) and char yield are above 450 °C and 50%, 439 °C and 29%, respectively. Dynamic rheology results show that viscous modulus of the obtained copolyesters are much more prominent under both frequency and temperature sweeping mode, indicating have excellent processing ability. The fibres can be easily drawn from the melt, and exhibit well oriented fibrillated structure. The convenient polymerisation method, excellent thermal stability and fibre formation ability of the phosphorus-containing TLCP endow them much potential use in the safety protection field.

Charge transfer interaction and vibrational spectral investigation of 2-amino-5-nitropyridinium sulfamate

A hetero aromatic nonlinear optical crystal, 2-amino-5-nitropyridinium sulfamate was synthesized by slow evaporation method. An optimized molecular structure and theoretical vibrational spectra were obtained and analyzed by use of density functional theory. The observed vibrational wavenumbers in the experimental spectra were compared with the calculated results. Scaled quantum mechanical force field methodology with normal coordinate analysis was used for the assignment of experimental infrared and Raman spectra. The second harmonic generation and photoluminescence properties of the crystal were studied. The charge transfer interaction between the donor acceptor moieties were obtained by means of natural bond orbital analysis.

Suppressed Triplet Exciton Diffusion Due to Small Orbital Overlap as a Key Design Factor for Ultralong-Lived Room-Temperature Phosphorescence in Molecular Crystals.

Persistent room-temperature phosphorescence (RTP) under ambient conditions is attracting attention due to its strong potential for applications in bioimaging, sensing, or optical recording. Molecular packing leading to a rigid crystalline structure that minimizes nonradiative pathways from triplet state is often investigated for efficient RTP. However, for complex conjugated systems a key strategy to suppress the nonradiative deactivation is not found yet. Here, the origin of small rates of a nonradiative decay process from triplet states of conjugated molecular crystals showing RTP is reported. Optical microscopy analysis showed that, despite a favorable molecular stacking, an aromatic crystal with strong RTP is characterized by small diffusion length and small values of the diffusion coefficient of triplet excitons. Quantum chemical calculations reveal a large overlap between the lowest unoccupied molecular orbitals but very small overlap between the highest occupied molecular orbitals (HOMOs). Inefficient electron exchange caused by the small overlap of HOMOs prevents triplet excitons from diffusing over long distances and consequently from quenching at defect sites inside the crystal or at the crystal surface. These results will allow design of comprehensive molecular structures to obtain molecular solids with more efficient RTP.

Intrinsic Analysis of Radiative and Room-Temperature Nonradiative Processes Based on Triplet State Intramolecular Vibrations of Heavy Atom-Free Conjugated Molecules toward Efficient Persistent Room-Temperature Phosphorescence.

The radiative rate ( kp) of the lowest triplet excited state (T1) and the nonradiative rate based on intramolecular vibrations at room temperature [ knr(RT)] from T1 for heavy atom-free conjugated structures are determined by considering the triplet yield and quenching rate from T1. Donor substitution did not strongly influence knr(RT) but greatly enhanced kp. The knr(RT) values were comparable between donor-substituted molecules and nonsubstituted molecules, which we explain by similar vibrational spin-orbit coupling (SOC) related to the transition from T1 to the ground state (S0). We attribute the enhancement of kp induced by donor substitution to the appearance of a large SOC between high-order singlet excited states (Sm) and T1 together with the large transition dipole moments of the Sm-S0 transitions. Knowledge of this mechanism is important for developing future efficient persistent room-temperature phosphorescence from doped aromatic materials and aromatic crystals.

Density Functional Theory Modeling of Solid-State Nuclear Magnetic Resonances for Polycyclic Aromatic Hydrocarbons

We acknowledge funding from C. Madrid (Grant S2013/MIT-2753), MINECO (Grants MAT2014-54231, MAT2016-78625-C2-2P, MAT2016-78362-C4-2-R, and FIS2015-6422-C2-1-P), EU (Grant ERC-2013-SYG-610256 NANOCOSMOS), Generalitat Valenciana (Grant PROMETEO/2017/139); the University of Alicante and computing resources from CTI-CSIC.

Sensitizer-Free Photon Upconversion in Single-Component Brominated Aromatic Crystals

A new methodology of single-component photon upconversion (UC) based on S0-to-T1 (S−T) absorption is demonstrated. Although the S−T absorption is spin-forbidden, modification of anthracene chromophores with heavy bromine atoms allows the direct population of the excited triplets in crystals upon near-infrared (NIR) excitation with incident light intensities of ∼W cm−2. The consequent triplet energy migration (TEM) and triplet-triplet annihilation (TTA) events in crystals lead to blue UC emission with a large anti-Stokes shift over 0.9 eV. The generality of the S−T absorption-based UC is further confirmed with crystals of brominated perylene-derivatives that show a green UC emission under a longer-wavelength NIR excitation (λex=856 nm). The single-component UC with large anti-Stokes shifts observed for brominated aromatic crystals compensates for the shortcomings of conventional two-component solid UC system, i. e., energy losses intrinsic to the intersystem crossing of the sensitizers and the triplet energy transfer from sensitizer to emitter. It offers a simple and complementary UC methodology to the conventional two-photon absorption (TPA) and TTA-UC.

David Parker Craig 1919–2015

David Craig was an outstanding Australian theoretical chemist whose academic life oscillated between Australia (University of Sydney and Australian National University (ANU)) and the UK (University College London). The Craig Building of the Research School of Chemistry of the ANU was named in his honour in 1995. He was President of the Australian Academy of Science from 1990 to 1994, and the Academy's David Craig Medal, which recognizes outstanding contributions to chemistry research, was inaugurated in his honour. His best-known research is in the fields of quantum theory and spectroscopy of aromatic molecules, molecular crystals, quantum electrodynamics and chirality.

Synthesis of aromatic liquid crystals with asymmetric diester based on rod-like multi-ring system by two-step esterification method

ABSTRACTA novel method of two-step esterifications was developed to synthesize compounds with asymmetric double ester groups. By using this method, six rod-like double ester compounds were prepared with p-hydroxy benzaldehyde, p-hydroxy benzoic acid bicyclohexyl carboxylic acid, cyclohexyl benzoic acid and biphenyl carboxylic acid substituted by n-propyl and n-pentyl as main reactants. The structures and properties of target compounds were confirmed by IR, MS, 1H NMR, elemental analysis, differential scanning calorimetry (DSC) and hot stage polarizing optical microscope (HS-POM). Typical yields of the target molecules were more than 70%. All the molecules have mesophases with the textures of nematic type, indicating a rod-like molecule with a longer rigid skeleton can keep its mesophases. There was no clearing point observed for any of the derivatives before they decomposed so that the temperature ranges of the mesophases could not be determined. The energy differences between frontier molecular orbitals (HOMO-LUMO) (Eg) of the compounds were calculated by cyclic voltammetry (CV). The terminal ring system has an obvious influence on the energy levels and the energy gaps (Eg).

Electrochemical Characterization of Synthetic Hybrid DNA Molecular Wires

Over the past several years, considerable amount of research have been conducted on DNA conductivity studies and charge transfer mechanisms for potential applications in micro and nano-scale electronic devices. In general, there is a consensus that DNA molecule behaves as a one-dimensional aromatic crystal with π-electron conductivity. Along these lines, our recent work has shown that DNA molecular wire’s electrical properties are strongly influenced by the ionization potential and percentage content of its base-pairs that form these π-orbital stacking. Our previous work has also shown the effect of external effects such as humidity, electrical fields, and UV illumination on the long-term stability of DNA molecular wires. In general, with time, there is measurable significant decrease in the conductivity in DNA wires exposed to ambient conditions. Such external effects could also result in the physical change of DNA molecular wires that in turn compromise their suitability as nanoelectronic component. As a viable solution to long-term stability problems with DNA molecular wires and also tuning their conductivity, we are currently working on synthesizing hybrid DNA molecular wires made of natural base-pairs and synthetic base-pairs with lower ionization potentials and more π-orbital overlapping. One strong candidate is tricyclic cytosine analogue (such as (8-MeO) tC0), a synthetic unnatural base, which possesses (i) a lower ionization potential (~ 0.65 V) compared to natural bases, G (1.29 V), C (1.6 V), A (1.42 V), T (1.7 V) and (ii) more π-orbital stacking than the natural bases due to its tricyclic structure. This unnatural base can be a good choice as it minimally perturbs the structure of B-form DNA. Subsequently, we investigate the charge transfer mechanism of short hybrid doubled stranded DNA (16 bp) consisting of both natural and unnatural bases and compared it to the natural DNA strand of the same length. The whole hybrid DNA sequence is NH2-5’-AtC0TACAGTCATCGCtC0tC0-3’ where some bases were substitute with tricyclic cytosine analogues (tC0). Two methods are used to study the charge transport through hybrid DNA wires. One is based on the charge transport between donor and acceptor (oxidant and hole trap). The oxidant (donor) is the metallointercalator which has positive charge with either Ru or Rh complex. This intercalator does not perturb the structure of the DNA π stacking as it binds covalently with DNA. The hole trap is two (8-MeO) tC0 molecules in a row, as it can potentially have the lowest ionization potential in the DNA sequence. The donor is then photo-excited and the fluorescence of the donor quenched through the DNA strand during charge transfer to the acceptor. The Ru or Rh complex are positioned at the 5’ of the DNA sequence, apart from the hole trap. A DNA 5 ‘terminus has to be terminated with amine group in order to interact with carboxyl group of Ru or Rh complex through EDC-NHS reaction. The other method is based on electrochemical detection. The sequence to be used is the same as the first method, however, it is terminated with the thiol group SH-5’-AtC0TACAGTCATCGCtC0tC0-3’ in order to be attached easily to gold electrodes. The radox active material (Methylen Blue) acts as an intercalator and its electrocatalytic signal is measured while the electrons pass through the DNA strands attached to the surface. The DNA strands are in the Ferrocyanide solution [Fe(CN)6]-3 which can be reduced by the reduction of methyl blue. The chronocoulometry techniques is then used to measure the electron transport in DNA strand. Between the two methods, we demonstrate that synthetic molecular wires consisting of both natural and strategically placed synthetic bases with lower ionization potential and π stacking will offer a stable and electrically tunable molecular wire.

Molecularly Dispersed Donors in Acceptor Molecular Crystals for Photon Upconversion under Low Excitation Intensity

For real-world applications of photon upconversion based on the triplet-triplet annihilation (TTA-UC), it is imperative to develop solid-state TTA-UC systems that work effectively under low excitation power comparable to solar irradiance. As an approach in this direction, aromatic crystals showing high triplet diffusivity are expected to serve as a useful platform. However, donor molecules inevitably tend to segregate from the host acceptor crystals, and this inhomogeneity results in the disappointing performance of crystalline state TTA-UC. In this work, a series of cast-film-forming acceptors was developed, which provide both regular acceptor alignment and soft domains of alkyl chains that accommodate donor molecules without segregation. A typical triplet sensitizer, PtII octaethylporphyrin (PtOEP), was dispersed in these acceptor crystals without aggregation. As a result, efficient triplet energy transfer from the donor to the acceptor and diffusion of triplet excitons among regularly aligned anthracene chromophores occurred. It resulted in TTA-UC emission at low excitation intensities, comparable to solar irradiance.

Perspectives of Applicability and Validity of Tait Equation for Liquids

Recently, a number of state equations for liquids have been proposed but none of them has a universal character (the possibility of application to all classes of liquids) as well as a sufficient theoretical basis and necessary accuracy. One of the most well-known empirical state equations of liquids is the Tait equation and its modified versions. A theoretical justification of the Tait equation does not exist despite numerous attempts to solve this problem. The authors have verified the Tait equation for aromatic hydrocarbons and nematic liquid crystals on the basis of experimental density data. The above mentioned studies allow concluding that the Tait equation can be used for prediction of the density over a wide range of pressures for different classes of liquids if the data taken from a narrow range of pressures are known. Theoretical justification of the empirical Tait equation is possible within the cluster model proposed by the authors.

Pressure-Induced Fluorescence of Pyridine

Two-photon excitation profiles and fluorescence spectra have been measured as a function of pressure in a diamond anvil cell up to 15.5 GPa in crystal phases I and II and in the glassy form of pyridine. The fluorescence emission intensity increases by about 6 orders of magnitude in going from the liquid to the crystalline phases at 3 GPa and further increases with pressure. This is explained by an energy inversion of the lowest (1)B(1) (nπ*) and (1)B(2) (ππ*) excited states likely due to the involvement of the lone pair of the N atom in intermolecular CH···N bonds. These interactions characterize the crystal phases and are stabilized by pressure. The glassy form, accordingly, is characterized by a much weaker fluorescence. Excimer emission is also observed. Comparison of the emission of several samples with different compression and annealing histories, the lack of reversibility in the excimer emission with decompression, and the larger relative intensity of the excimer band in the glassy form suggest that excimer formation occurs at crystal defects. This results support the conclusions of a previous investigation proposing that pressure-induced reactivity of pyridine is limited to crystal defects and agrees with the present knowledge of the solid-state chemistry of aromatic crystals.

Ageing of spherulites of an industrial relevant aromatic amine derivative

AbstractPolycrystalline spherulites of an aromatic amine derivative have been precipitated in a batch process by pH‐shift with hydrochloric acid from stirred aqueous solutions. The time dependent behaviour of the spherulites has been studied during crystallization in the temperature range from 5 °C to 60 °C. Cake resistance values have been obtained from batch filtration tests performed at 2 bar pressure difference at different stages of the crystallization process. The FBRM mean chord length of the crystals decreases with time as a result of crystal ageing into plate‐like crystals. The rate of the ageing process increases with temperature. XRD‐studies show no significant differences in the crystal structure during the ageing process, and the mechanism of the transformation was not established. The filterability of aromatic amine crystals deteriorates as the crystallization progresses. The decrease in the filterability is attributed to the appearance of small plate‐like crystals and a change in the interaction between the crystal surface and the solution, during the ageing of the particles. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Combination bands in vibronic spectra of molecular crystals

The molecular crystals’ vibronic spectra with combination bands have been studied using the complete vibronic approach. Those vibronic spectra consist of one electronic excitation (Frenkel exciton) plus vibrational quanta of two different intramolecular vibrational modes. The linear absorption spectra of one-dimensional models of some aromatic crystals have been calculated applying the methods of canonical transformations and Green functions (at T = 0) in the following cases: (i) vibronic spectra of naphthalene and benzene which consist of Frenkel exciton plus one phonon of non-totally symmetrical vibration plus one/two phonons of totally symmetrical vibrations; (ii) vibronic spectra of an anthracene-like model and other two models with combination bands with two different totally symmetrical vibrations (one of them is weakly coupled with Frenkel exciton). In this paper, we show that the linear absorption demonstrates considerable changes in the one-phonon and two-phonon vibronic spectra caused by the indirect coupling between vibrational modes (appearing because of their coupling with a Frenkel exciton). Our simulations of absorption spectra prove the opportunity to observe some peculiarities like the manifestation of various molecular configurations and edge anomalies in the shape of the absorption bands.

Investigation of emission properties of doped aromatic derivative organic semiconductor crystals

Abstract Fluorescence measurements have been made on pure and doped bulk, mechanically polished wafers of crystalline m-DNB and benzil obtained by cutting ingots grown by the Bridgman–Stockbarger method modified for organic compounds crystallization. By comparison with pure matrices, we have investigated the effect of an inorganic dopant (iodine, silver, sodium) and of an organic dopant (m-DNB, naphthalene) on the emission characteristics (position and shape) of these molecular crystals. A slight shift of the emission peaks through high energy and an intense emission peak situated around 2.35 eV correlated with the local trapping level attributed to structural defects, which are involved in radiative processes, have been evidenced in iodine-doped m-DNB. The emission peak of m-DNB-doped benzil situated in the high-energy range (2.97 eV) is associated with direct emission activity of m-DNB, suggesting that this is an active impurity in benzil molecular matrix. We have not observed in benzil any evidence of indirect action of the impurity molecules (atoms) associated with the traps represented by the structural defects that generate changes in the energy levels of the neighbouring molecules and are correlated with different growth conditions. We have not remarked any involvement of the studied inorganic metallic impurities and of some organic impurities, such as naphthalene, in the radiative recombination processes in benzil matrix.

Vibronic approach in the theory of the excitonic spectra of molecular crystals

The excitonic and vibronic spectra of a molecular chain have been studied on using a complete vibronic approach (without separation in Hilbert subspaces) which has been developed in this paper. The methods of canonical transformations and Green's functions are applied to find out analytical expressions as continued fractions for the linear optical susceptibility near the frequencies of the Frenkel exciton and its first and/or subsequent three vibronic replicas. The approximation of the dynamical theory, i.e., the case of a narrow exciton band compared with the energy of the vibrational quantum, is modified to treat vibronic spectra with totally symmetric phonons by considering both the linear and quadratic exciton-phonon coupling. The excitonic and vibrational parameters for anthracene, naphthalene, and benzene crystals as well as for three other models of molecular chains have been used in calculating the linear absorption spectra of the one-dimensional models of those crystals. The main results concern the shape of the absorption maxima and the manifestations of joint and unbound exciton-phonon configurations in the one-, two-, and three-phonon vibronic spectra depending on the dominant impact of the linear or (and) quadratic exciton-phonon coupling. The picture of the calculated spectra has been compared with the data of previous theoretical studies. It agrees with the available experimental data on the linear absorption in the three aromatic crystals.

Molecular orientation of 4-hydroxy-4′-biphenylcarbonitrile on the polyimide surfaces with long alkyl chains

It is important to have control over the nucleation, growth and orientation of organic thin films. The present study focuses on the orientation the dipolar linear organic molecule, 4-hydroxy-4′-biphenylcarbonitrile (HBC), on aligned polyamide films. The polymer surfaces were oriented by using rubbing method and photo-alignment method. UV–vis spectroscopic study illustrated that the photo-reaction of cinnamate containing polyamide proceed by photo-dimerization reaction. The photo-aligned polyimide can make perpendicular orientation of nematic liquid crystals. The IR absorption intensity ratios of alkyl group versus nitrile group measured with transmission mode and reflection mode are about 2–3 and 1.3, respectively. The low intensity ratio for reflection measurement was due to the CN vibrational mode can be better matched with inclined light and accordingly the absorbance increases. Since the molecules are perpendicular to the surface, the light from inclined angle to the substrate activates the vibration of CN group more effectively. These results clearly demonstrate that the orientation influences the nucleation and oriented growth of aromatic crystals. The crystallinity, mesoscopic and side chain orientation and hydrophobicity of polymer surface are important factors that induce the orientation of guest molecules.