Optical Properties Of Organic Materials Research Articles

Regioisomerism in Thienothiophene-Based Covalent Organic Frameworks─A Tool for Band-Gap Engineering.

The craft of tuning optical properties is well-established for crystalline inorganic and hybrid solids. However, a far greater challenge is to tune the optical properties of organic materials systematically by design. We now introduce a synthesis concept that enables us to alter the optical properties of crystalline covalent organic frameworks (COFs) systematically using isomeric structures of thienothiophene-based building blocks (T23/32T) combined with a variety of tetratopic aromatic amines, e.g., the Wurster moiety (W-NH2). This concept is demonstrated for the synthesis of COFs in bulk and film forms and provides highly crystalline and porous isomeric COFs featuring predesigned photophysical properties. The band gap of the framework can be tuned continuously and precisely by chemically doping the pristine W23TT COF with its related constitutional isomer building block. Density-functional theory investigations of COF model compounds indicate that the extent of π-conjugation is among the key characteristics enabling the band-gap engineering.

Significant Improvement of Solid‐State One‐/Two‐Photon Excited Luminescence in HOFs via Constructing Hydrogen‐Bonded Solvate Model

AbstractA hydrogen‐bonded solvate (HBS) model is herein proposed to construct hydrogen‐bonded organic frameworks (HOFs), for enhancing the solid‐state one‐/two‐photon excited fluorescence (1/2PEF) of organic fluorophores. Two photonic HOFs (ZJU‐HOF‐25 and ZJU‐HOF‐26) are synthesized by using the same organic fluorophore and the fluorophore in them features similar arrangement. The most obvious structural distinction between the two HOFs is the existence of solvent molecules in the crystal cell of ZJU‐HOF‐25. ZJU‐HOF‐25 is regarded as an ideal HBS material where solvent molecules separate fluorophores apart just like dissolving them. But ZJU‐HOF‐26 contains no solvent molecules and the fluorophores are connected directly via hydrogen bonds among them. A significant quantum yield (η) increase is realized from amorphous fluorophores (η ≈0%) to HBS‐based ZJU‐HOF‐25 (η ≈21.75%), while ZJU‐HOF‐26 displays poor 1PEF (η ≈0%). ZJU‐HOF‐25 also exhibits much stronger 2PEF with the two‐photon action cross section larger than ZJU‐HOF‐26 by almost two orders of magnitude. These phenomena are mainly attributed to that the solvent molecules in ZJU‐HOF‐25 benefit the homogeneous separation and conformation fixation of fluorophores, suppressing the non‐radiative transition. The HBS model thus shows the fantastic validity to improve solid‐state optical properties of organic materials, providing guidance for exploring advanced optical materials.

Optoelectronic properties of one-dimensional molecular chains simulated by a tight-binding model

Studying optical properties of organic materials is important due to the rapid development of organic light-emitting diodes, solar cells, and photon detectors. Here, for the first time, we have performed tight-binding calculations for singlet excitons, in combination with first-principles calculations of the excited states in molecular dimers, to describe the optical properties of a zinc-phthalocyanine one-dimensional molecular chain. We have included the intra-molecule and charge-transfer excitations and the coupling between them. Our calculations have successfully interpreted a body of experimental UV–visible optical spectra of transition-metal phthalocyanines. Compared with the previous ab initio calculations for a molecular dimer, the optical absorptions at the split peaks of the Q-bands can be comparable, which indicates the importance of the coupling between the intra-molecular and charge-transfer excitons.

Refined standards for simulating UV–vis absorption spectra of acceptors in organic solar cells by TD-DFT

Photoexcitation property of acceptor plays a vital role to understand fundamental physical process of acceptor excitation in organic solar cells (OSCs), which should be described at a reliable level and characterized by ultraviolet-visible (UV–vis) absorption spectra. Generally, adiabatic linear-response time-dependent density functional theory (LR-TD-DFT) as a suitable framework, which requires to select exchange correlation functionals (XCFs) and basis sets to concentrate the approximation of models, is widely recognized. However, selecting reliable theoretical methods tends to consume computational cost and time, and optimal selections have not yet reached general standards. Herein, a systematic investigation including XCF, basis set, calculation density, solvation effect, and full width at half maximum (FWHM) was undertaken to quantify the photoexcitation properties of typical fullerene acceptors (FAs) and non-fullerene acceptors (NFAs). The results reveal that maximum absorption wavelength (λmax) of NFAs is strongly correlated with the fraction of “exact” Hartree-Fock exchange (EEX), thus establishing selection standards. Moreover, calculation density is the primary factor to ensure the integrity of spectra. This work provides reasonable schemes for characterization conditions, which further lays foundation for the reliability of theoretical calculations that will help to advance the theoretical characterization of optical properties of organic materials.

Fused polycyclic compounds: Synthesize, enhancing TPA cross-section by modifying torsional multiple ring

Abstract Four fused polycyclic compounds (DNBT, DNTB, DNPY, and DNNM) with different substituents were designed and synthesized to achieve excellent nonlinear optical materials. Broadband reverse saturation absorption performance based on TPA was carried out by Z-scan. Transient absorption spectrum showed that the different substituents can significantly regulate the ultrafast kinetic processes of their excited states. The relationship between excited states charge structure and nonlinear optical properties were studied by density functional theory (DFT) and sum over states (SOS) model. TPA of these compounds is modulated by local excitation in DBPy and TA fragments. Intra-molecular charge transfer weakens the TPA cross-section. These compounds are potential optical limiting (OL) materials in visible light range. The clear mechanism show that the nonlinear optical properties of organic materials can be improved purposefully through theoretical and experimental methods.

Cocrystallisation enhanced optical properties of organic materials – quantum crystallography meets crystal engineering

Cocrystallisation enhanced optical properties of organic materials – quantum crystallography meets crystal engineering

Controllable Molecular Packing Motif and Overlap Type in Organic Nanomaterials for Advanced Optical Properties

The optical properties of organic materials are very sensitive to subtle structural modification, and a proper understanding of the structure-property relationship is essential to improve the performance of organic electronic devices. The phase transitions of the η-CuPc to the α-CuPc, then to the β-CuPc were investigated using In situ X-ray diffraction and the differential scanning calorimetry (DSC). The five stages in the phase-transition process from low to high-temperature were observed, which consisted of (1) the η-CuPc; (2) a mixture of the η- and α-CuPc; (3) a mixture of the η-, α- and β-CuPc; (4) a mixture of the α- and β-CuPc; and (5) the β-CuPc. The vibrational and optical properties at different phase-transition stages were correlated to molecular packing motif and molecule overlap type through systematic analyses of the Fourier–transform infrared, Raman and UV-VIS spectra. Moreover, the mechanism for the morphology evolution was also discussed in detail.

Standing wave spectrometer with semi-transparent organic detector

Optical properties of organic materials such as polymer–fullerene mixtures enable realization of semi-transparent photodetectors with superior design and features for standing wave spectrometer applications.

Re-arrangements of 4-[(4H-1,2,4-triazol-4-ylimino)methyl]phenol with different inorganic anions: Crystal structure and fluorescence properties

Fluorescent materials have many interesting applications, but it remains difficult to control the luminescence of organic materials and in particular to retain the same luminescence in solution and in the solid state, a property of interest for various imaging applications. In this work, the influence of inorganic-anions on the crystal structure and physical–chemical properties of a series organic salts L+Cl− (1), L+ClO4− (2), L+NO3−·H2O (3), 2L+SO42−·2H2O (4) (L=4-[(4H-1,2,4-triazol-4-ylimino)methyl]phenol) has been investigated. Single crystal X-ray analyzes revealed that salt 1 formed a linear structure connected by H⋯O interactions, while salts 2–4 all formed 2D network structures through O⋯O interactions. The protonated L+s adopt face-to-face slipped π-stacked arrangement in salts 1 and 3, while display monomer arrangements in 2 and 4, the latter can be attribute to the entrapment of heavy anions in the crystal lattices. Optical-properties measurements of these four salts have shown L+ arrangements dependent on solid-state properties: salt 1 and 3 show broad emission bands (λ1=478nm, λ2=487nm) with a red shift of about 60nm relative to its fluorescence in solvents, while 2 and 4 are similar to each other, exhibiting sharp bands (λ3=453nm, λ2=433nm) with the higher ϕPL values (ϕPL1=0.08, ϕPL1=0.15, ϕPL1=0.06, ϕPL1=0.14). These results demonstrated the significant influence of inorganic-anions on the organic salts and the optical properties of organic materials could be modulated by entrapping different anions in lattice.

Uncovering the Intramolecular Emission and Tuning the Nonlinear Optical Properties of Organic Materials by Cocrystallization

AbstractThe spectroscopic and photophysical properties of organic materials in the solid‐state are widely accepted as a result of their molecular packing structure and intermolecular interactions, such as J‐ and H‐aggregation, charge‐transfer (CT), excimer and exciplex. However, in this work, we show that Spe‐F4DIB cocrystals (SFCs) surprisingly retain the energy levels of photoluminescence (PL) states of Spe crystals, despite a significantly altered molecular packing structure after cocrystallization. In comparison, Npe‐F4DIB cocrystals (NFCs) with new spectroscopic states display different spectra and photophysical behaviors as compared with those of individual component crystals. These may be related to the molecular configuration in crystals, and we propose Spe as an “intramolecular emissive” material, thus providing a new viewpoint on light‐emitting species of organic chromophores. Moreover, the nonlinear optical (NLO) properties of Npe and Spe are firstly demonstrated and modulated by cocrystallization. The established “molecule‐packing‐property” relationship helps to rationally control the optical properties of organic materials through cocrystallization.

Uncovering the Intramolecular Emission and Tuning the Nonlinear Optical Properties of Organic Materials by Cocrystallization.

The spectroscopic and photophysical properties of organic materials in the solid-state are widely accepted as a result of their molecular packing structure and intermolecular interactions, such as J- and H-aggregation, charge-transfer (CT), excimer and exciplex. However, in this work, we show that Spe-F4 DIB cocrystals (SFCs) surprisingly retain the energy levels of photoluminescence (PL) states of Spe crystals, despite a significantly altered molecular packing structure after cocrystallization. In comparison, Npe-F4 DIB cocrystals (NFCs) with new spectroscopic states display different spectra and photophysical behaviors as compared with those of individual component crystals. These may be related to the molecular configuration in crystals, and we propose Spe as an "intramolecular emissive" material, thus providing a new viewpoint on light-emitting species of organic chromophores. Moreover, the nonlinear optical (NLO) properties of Npe and Spe are firstly demonstrated and modulated by cocrystallization. The established "molecule-packing-property" relationship helps to rationally control the optical properties of organic materials through cocrystallization.

Investigations on the Low Temperature Solution growth, etching, laser damage threshold, Photoluminescence, electrical characterization and nonlinear optical properties of organic material: Ammonium hydrogen l-malate

Optically good quality bulk organic nonlinear optical crystal ammonium hydrogen l-malate (AHM) of crystal size 25mm×26mm×26mm has been grown by slow cooling (Low Temperature Solution) method. The grown crystals were subjected to various characterization studies such as chemical etching, UV–vis spectrum, birefringence, laser damage threshold, Photoluminescence studies and second harmonic generation studies. The quality was ascertained by birefringence studies. Piezoelectric charge coefficients of the grown crystal have been determined. The dielectric constant, dielectric loss and ac conductivity of the compound were calculated at different temperatures and frequencies to analyze the electrical properties.

Assessment of long-range corrected functionals for the prediction of non-linear optical properties of organic materials

We calculate the hyperpolarizability of the prototypical chromophore p-nitroaniline in gas phase using different long-range corrected Density Functional Theory (LC-DFT) methods and compare it with experimental data. While traditional DFT methods are inadequate for predicting hyperpolarizabilities, LC-DFT functionals provide very good agreement with experiment (less than 10% error) for this archetypal organic molecule with large nonlinear optical (NLO) susceptibility. Additionally, we use the LC-ωPBE functional to calculate the structures and predict the hyperpolarizabilities of two recently synthesized potential organic NLO materials. The structures are compared with experimental measurements and found to be in excellent agreement.

Synthesis and Two-Photon Absorption Enhancement of Porphyrin Macrocycles

Improving the nonlinear optical properties of organic materials through transition to macromolecular architectures of highly conjugated chromophores has been shown to be a viable strategy for generating materials suitable for TPA applications. In this study we display a simple and elegant method by which to synthesize macrocycles of porphyrin dimers. Two-photon absorption studies show 2 orders of magnitude enhancement of cross-section of the material, giving a maximum delta(2) of 10(6) GM.

Comprehensive investigation of absolute optical properties of organic materials

The dispersive absorption coefficient and refractive index are the key optical functions in optimizing the performance of organic optoelectronic devices. The optical functions of organic materials have been generally determined and reported by, for example, using ellipsometry and photoluminescence excitation. However, these methods cannot provide much physical understanding of the optical functions. In addition, conclusive studies on the origins of the optical functions of the organic materials are limited. In this paper, we first determine the absolute optical functions by using ellipsometry against different oscillator models. We can then find out the better oscillator model to be used with ellipsometry together for determining the experimental optical functions. The next step is to determine and explain the origins of the experimentally determined optical functions by investigating the molecular orbitals and electronic transitions of organic molecules through time-dependent density functional theory. This comprehensive study is conducted on two popular materials of tris(8-hydroxyquinoline) aluminium (Alq3) and 4,4′ bis[N-(1-napthyl)-N-phenyl-amino]-biphenyl (NPD).

Organic Nanoclusters on Inorganic Nanostructures for Tailoring the Emission Properties of Organic Materials

AbstractInorganic nanostructures have been intensively studied because of their size‐dependent emission color as a result of quantum confinement effects. In contrast, no significant size dependence of the optical properties is expected for organic nanostructures owing to the weak nearest‐neighbor interactions in molecular crystals; however, the optical properties of organic materials can be significantly affected by aggregation and molecular packing, which can result in large differences between the emission spectra of solutions and thin films. Thus, the emission spectra of organic nanostructures may differ considerably from the bulk material despite the absence of quantum confinement effects. We demonstrate that 3,4,9,10‐perylenetetracarboxylicdiimide (PTCDI) nanoclusters grown on ZnO nanorods exhibit an intense blue‐white photoluminescence, different from the weak red emission of PTCDI films. Light emitting diodes consisting of ZnO nanorods and PTCDI nanoclusters also exhibited white emission.

Three-dimensional nonlinear optical chromophores based on through-space delocalization.

Six permutations of 4-fold donor and/or acceptor substitution of paracyclophane at the 4, 7, 12, and 15 positions were synthesized to probe the phenomenon of three-dimensional delocalization on the nonlinear optical properties of organic materials. The interplay between through-bond intramolecular charge transfer (ICT) as well as three-dimensional, or through-space, ICT processes gives rise to large quadratic hyperpolarizability values. The determination of dipolar (beta(J)(=1)) and octupolar (beta(J)(=3)) irreducible tensor contributions to the overall beta tensor value is made possible by the polarized harmonic light scattering technique at 1.32 microm. The electric field-induced second-harmonic generation technique was also used at 1.91 microm for comparison. Significant experimental beta values for members of the series made of two centrosymmetric benzene-like units are a clear signature of a purely through-space ICT between two aryl subunits. The two configurational isomers that pair two dipolar donor-acceptor chromophores also exhibit octupolar character. Analysis of these two with an additive model for beta(J)(=1) and beta(J)(=3) reveals a strong three-dimensional inter-ring charge transfer.

Magnetic polymer chains: a Hubbard Study

Molecular or polymeric-based magnetic materials have been the subject of increasing interest in recent years. Organic magnetic structures have been theoretically predicted by Ovchinnikov. One of the most attractive features of these new materials is the possibility of combining mechanical, electrical and/or optical properties of organic materials with magnetic properties. In the present work, we study a class of one-dimensional chains with special topologies that could lead to magnetic order. Using the Hubbard model, we find that these materials should exhibit ferrimagnetic ordering at half-filled band, in agreement with a theorem by Lieb. Away from half filling these systems display a rich magnetic phase diagram.

Lead phthalocyanine reverse saturable absorption optical limiters

Systematic studies of the nonlinear optical properties of organic materials have led to the development of promising new materials for optical limiting. Several heavy metal substituted phthalocyanines are effective materials for optical limiters in the visible and near IR. Nonlinear absorption is the principal mechanism near the limiting threshold. In fast optical systems, refractive contributions also contribute at higher energies. These refractive contributions are predominantly thermal. The spectral window over which the limiter operates can be engineered by altering both the main ring and peripheral substitution of the molecules.

Nonlinear Optical Effects in Polar Dielectric Materials

The electro-optical and nonlinear optical properties of organic materials are reviewed. Molecular crystals, Langmuir-Blodgett films and poled polymers are shown to have interesting properties for applications as high-speed electro-optic modulators, optical frequency converters or all optical signal processors.