Absorption Spectra Of Organic Molecules Research Articles

Computer aided molecular design coupled to deep learning techniques as a less-expensive approach to design organic photoredox catalysts

To date, the prediction of electronic absorption spectra of organic molecules by computational chemistry methods is still limited, with differences of even 100 nm between the theoretical and experimental values, often at relatively high computational cost. In this work, we present a computationally cheaper approach for the design of new molecules that may be candidates for organic photoredox catalysts based on the prediction of their absorbance in the visible region of the electromagnetic spectrum. This problem is important to promote the development and use of solar energy in chemical processes, as an exemplary application. The design of new chromophoric molecules is approached as a two-step procedure. In the first step, a Mixed-Integer Nonlinear Programming (MINLP) model was formulated for the In-silico design of organic chromophores. A quantitative structure–activity relationship model was developed for the prediction of the difference in energy from the HOMO to the LUMO orbitals of organic dyes. This molecular design approach produced 4 molecules, where 3 have not been previously reported. In the second step, a large chemical compound database was leveraged to develop a deep learning model aimed at forecasting the absorption of visible light by such and related compounds. We have taken advantage of the deep learning model to verify that the light absorption between this model and the results of the MINLP formulation match with reasonable accuracy. For 3 of the 4 molecules, both results appear similar, allowing to verify that the molecules could in fact offer the desired absorbance at the target wavelength.

Electronic Descriptors for Supervised Spectroscopic Predictions.

Spectroscopic properties of molecules hold great importance for the description of the molecular response under the effect of UV/vis electromagnetic radiation. Computationally expensive ab initio (e.g., MultiConfigurational SCF, Coupled Cluster) or TDDFT methods are commonly used by the quantum chemistry community to compute these properties. In this work, we propose a (supervised) Machine Learning approach to model the absorption spectra of organic molecules. Several supervised ML methods have been tested such as Kernel Ridge Regression (KRR), Multiperceptron Neural Networs (MLP), and Convolutional Neural Networks. [Ramakrishnan et al. J. Chem. Phys. 2015, 143, 084111. Ghosh et al. Adv. Sci. 2019, 6, 1801367.] The use of only geometrical-atomic number descriptors (e.g., Coulomb Matrix) proved to be insufficient for an accurate training. [Ramakrishnan et al. J. Chem. Phys. 2015, 143, 084111.] Inspired by the TDDFT theory, we propose to use a set of electronic descriptors obtained from low-cost DFT methods: orbital energy differences (Δϵia = ϵa - ϵi), transition dipole moment between occupied and unoccupied Kohn-Sham orbitals (⟨ϕi|r|ϕa⟩), and when relevant, charge-transfer character of monoexcitations (Ria). We demonstrate that with these electronic descriptors and the use of Neural Networks we can predict not only a density of excited states but also get a very good estimation of the absorption spectrum and charge-transfer character of the electronic excited states, reaching results close to chemical accuracy (∼2 kcal/mol or ∼0.1 eV).

Comparison of optical absorption spectra of organic molecules and aggregates computed from real frequency dependent polarizability to TD-DFT and the dipole approximation

We compare the performance of an approach using real frequency dependent polarizability to compute optical absorption spectra to linear-response time-dependent density functional theory (TD-DFT) for small organic dyes, oligomers of different lengths (oligothiophenes), and molecular clusters representing a molecular crystal (pentacene). For pentacene, the spectra computed with the two methods are also compared to the spectrum computed for clusters and the periodic solid using the dipole approximation. The approach based on real polarizability produces spectra in good agreement with TD-DFT for small molecules. The (artificial) redshift for longer oligomers is slightly more significant with the polarizability-based method than with TD-DFT. For pentacene clusters, TD-DFT produces reasonable spectra with a hybrid functional, but a significant redshift is introduced with a generalized gradient approximation functional due to the presence of charge transfer transitions. This problem is slightly attenuated with the polarizability-based method. The dipole approximation results in spectra much redshifted vs both TD-DFT and the polarizability-based method and in a different trend with cluster size.

A REVIEW: TERAHERTZ SPECTROSCOPY AS A VIABLE DYNAMIC TOOL FOR PROTEIN AND ORGANIC MOLECULES CHARACTERIZATION

THz spectroscopy is a relatively new spectroscopic method in which the THz useful range offrequencies is from 300 cm-1 down to 3 cm-1, a spectral region unachievable by FTIR spectroscopy.Multiple resonances in the absorption spectra of organic molecules exist in the THz range around 0.1–10 THz(3 to 300 cm-1) where these resonances arise from electromagnetic wave interaction of the THz sourcewith the low-frequency, and hence large- scale, vibrational modes within the organic macromolecules. These majorvibrational modes correspond to the quaternary or overall structure of macromolecules. The concerted motion ofprotein structure can also be investigated by THz spectroscopy, which enables both energy and momentum transferat the picosecond time scale. The short bursts of far-IR radiation in an ultra-fast time scale in THz spectroscopy allowresearchers to study the dynamic processes of various materials for a large variety of applications. This reviewpaper discusses the probability of utilizing THz spectroscopy as a probe to determine the intermolecular structures of macromolecules as well as basic technical knowledge in THz spectroscopy.  Keywords:Terahertz, spectroscopy, vibrational, proteins, organic molecules

Effects of Coexisting Isomers on Two-Photon Absorption of Organic Molecules in Solutions

Isomer-dependent two-photon absorption spectra of organic molecules in solutions have been identified by time-dependent density functional theory calculations. Experimentally observed enhancements in two-photon absorptions of V-shaped hydroxypyrimidine derivatives are not due to the formation of molecular dimers but to mainly the co-occurrence of geometrical isomers. Solvent effects on both one- and two-photon absorption spectra have been carefully examined by using a supermolecular approach in combination with a polarizable continuum model. Special attention has been paid to understand the effects of hydrogen bonds on electronic structure and absorption spectra of these chromophores. Our calculated results are very helpful for understanding the corresponding experimental observations.

Towards a Theoretical Description of UV‐VIS Absorption Bands of Organic Molecules

AbstractThe UV‐Vis absorption spectra of organic molecules are calculated with a new ansatz based on the simultaneous coupling of several vibrations during the electronic transition. Even with the simplest assumption of a Boltzmann weighted statistical mixing of the vibrational modes it is possible to reproduce the experimental absorption spectra, e.g. of naphthalene, surprisingly well. Using this method the homogeneous line width can be determined with an error of less than 30% from the common absorption spectra.

Extinction Coefficients of Triplet–Triplet Absorption Spectra of Organic Molecules in Condensed Phases: A Least-Squares Analysis

A global least-squares technique is developed to assist in the critical evaluation of data consisting of large sets of measurements. The technique is particularly designed to handle sets of data where many of the measurements are relative measurements. A linearization procedure is used to reduce the inherently nonlinear problem to a traditional multivariate linear regression. The technique developed here is used to evaluate extinction coefficients, ε’s, of triplet–triplet absorption (TTA) spectra of organic molecules in condensed phases. A previous assumption, that there are no solvent effects on the TTA spectra, is investigated and modified so that a group of compounds measured in benzene is treated separately. The set of 445 ε’s obtained from the global least-squares fit, including these solvent effects, is presented in the accompanying tables. How these least-squares results can be used in a hierarchy of TTA ε standards is discussed. Further solvent effects such as the separation of polar and nonpolar media and the influence of temperature are probed.

Triplet–Triplet Absorption Spectra of Organic Molecules in Condensed Phases

We present a compilation of spectral parameters associated with triplet–triplet absorption of organic molecules in condensed media. The wavelengths of maximum absorbance and the corresponding extinction coefficients, where known, have been critically evaluated. Other data, for example, lifetimes, energies and energy transfer rates, relevant to the triplet states of these molecules are included by way of comments but have not been subjected to a similar scrutiny. Work in the gas phase has been omitted, as have theoretical studies. We provide an introduction to triplet state processes in solution and solids, developing the conceptual background and offering an historical perspective on the detection and measurement of triplet state absorption. Techniques employed to populate the triplet state are reviewed and the various approaches to the estimation of the extinction coefficient of triplet–triplet absorption are critically discussed. A statistical analysis of the available data is presented and recommendations for a hierarchical choice of extinction coefficients are made. Data collection is expected to be complete through the end of 1984. Compound name, molecular formula and author indexes are appended.

Triplet—triplet absorption and polarization spectra of trans-stilbene and some of its derivatives

The triplet—triplet absorption spectra of organic molecules having short triplet state lifetimes, such as trans-stilbene, 4,4′-dihydroxy stilbene, 4,4′-diamino-2,2′-stilbene disodium disulfate and 4,4′-diphenyl stilbene, were measured. Ultraviolet cw lasers were used as excitation sources. Employing photoselection spectroscopy, the polarizations of the triplet—triplet absorption spectra were also recorded.

A new method of exhibiting the line structure in nonuniformly broadened absorption spectra of organic molecules

A new method of exhibiting the line structure in nonuniformly broadened absorption spectra of organic molecules

“Hole-burning spectra”. A new method for obtaining fine structure in absorption spectra of organic molecules

A new method is proposed for obtaining fine structure in broad-band absorption spectra of organic molecules in solid solutions at low temperature. This method is based on the laser burning of a series of narrow gaps in inhomogeneously broadened absorption spectra. A special spectral set-up has been developed for the measurement of the differential spectrum (a “hole-burning spectrum”) of “burnt” and “unburnt” samples. As an example the hole-burning spectrum of Zn-tetraphenylporphine in ethanol has been considered.

Use of 95 percent ethyl alcohol as a solvent for ultra-violet spectroscopy.

IN connexion with a problem on the determination of the ultra-violet absorption spectra of organic molecules dissolved in 95 per cent ethyl alcohol1, it was necessary to determine the suitability of the alcohol as a solvent in the region of 2000 A. There is a report2 that carefully purified 95 per cent ethyl alcohol3 is suitable for the region of 2000 A., although no experimental results were given. Observations in this region have been reported by MacLean, Jencks and Acree4 for ordinary commercial 95 per cent ethanol, and they find that unpurified alcohol is completely opaque in this region.

Ultraviolet Absorption Spectra of Organic Molecules. III. Mechanical Interference of Substituent Groups with Resonance Configurations

Ultraviolet Absorption Spectra of Organic Molecules. III. Mechanical Interference of Substituent Groups with Resonance Configurations

Ultraviolet Absorption Spectra of Organic Molecules: The Dependence upon Restricted Rotation and Resonance

Ultraviolet Absorption Spectra of Organic Molecules: The Dependence upon Restricted Rotation and Resonance

A Partial Analysis of Some Infra-Red Absorption Spectra of Organic Molecules in Dilute Solution

An attempt is made to resolve into component parts a number of relatively complex infra-red absorption coefficient curves for several OH-containing organic molecules in the region of the first overtone absorption of the OH group. Such analyses suggest certain relations between the structure of the molecule and its effect upon the character of the absorption of the absorbing group.