: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

How do molecules aggregate in solution, and how do these aggregates consolidate themselves in crystals? What is the relationship between the structure of a molecule and the structure of the crystal it forms? Why do some molecules adopt more than one crystal structure? Why do some crystal structures contain solvent? How does one design a crystal structure with a specified topology of molecules, or a specified coordination of molecules and/or ions, or with a specified property? What are the relationships between crystal structures and properties for molecular crystals? These are some of the questions that are being addressed today by the crystal engineering community, a group that draws from the larger communities of organic, inorganic, and physical chemists, crystallographers, and solid state scientists. This Perspective provides a brief historical introduction to crystal engineering itself and an assessment of the importance and utility of the supramolecular synthon, which is one of the most important concepts in the practical use and implementation of crystal design. It also provides a look to the future from the viewpoint of the author, and indicates some directions in which this field might be moving.

Crystal engineering: from molecule to crystal.

Organophosphorus π-Conjugated Materials

Singlet fission is a photophysical reaction in which a singlet excited electronic state splits into two spin-triplet states. Singlet fission was discovered more than 50 years ago, but the interest in this process has gained a lot of momentum in the past decade due to its potential as a way to boost solar cell efficiencies. This review presents and discusses the most recent advances with respect to the theoretical and computational studies on the singlet fission phenomenon. The work revisits important aspects regarding electronic states involved in the process, the evaluation of fission rates and interstate couplings, the study of the excited state dynamics in singlet fission, and the advances in the design and characterization of singlet fission compounds and materials such as molecular dimers, polymers, or extended structures. Finally, the review tries to pinpoint some aspects that need further improvement and proposes future lines of research for theoretical and computational chemists and physicists in ...

Theoretical Modeling of Singlet Fission

We present here the systematic synthesis and comparative physicochemical characterization of a series of regiochemically varied and core size extension-modulated arene(perfluoroarene)-thiophene oligomers. The molecules investigated are: 5,5''-diphenyl-2,2':5',2'':5'',2'''-quaterthiophene (1), 5,5'-bis[1-[4-(thien-2-yl)phenyl]]-2,2'-dithiophene (2), 4,4'-bis[5-(2,2'-dithiophenyl)]-biphenyl (3), 5,5''-diperfluorophenyl-2,2':5',2'':5'',2'''-quaterthiophene (4), 5,5'-bis[1-[4-(thien-2-yl)perfluorophenyl]]-2,2'-dithiophene (5), 4,4'-bis[5-(2,2'-dithiophenyl)]-perfluorobiphenyl (6), 5,5''-diperfluorophenyl-2,2':5',2''-tertthiophene (7), 5,5'-diperfluorophenyl-2,2'-dihiophene (8), and 5,5-diperfluorophenylthiophene (9). Trends in optical absorption and emission parameters, molecular structures as defined by single-crystal X-ray diffraction, as well as electrochemical redox processes are described. The morphologies and microstructures of the vapor-deposited films grown over a range of growth temperatures have also been characterized. Field-effect transistor (FET) measurements demonstrate that all of these materials are FET-active and, depending on the molecular architecture, exhibit comparably good p- or n-type mobility when optimum film microstructural order is achieved. A very large n-channel mobility of approximately 0.5 cm2/Vs with I(on)/I(off) ratios > 10(8) is achieved for films of 4.

Fluorocarbon-modified organic semiconductors: molecular architecture, electronic, and crystal structure tuning of arene- versus fluoroarene-thiophene oligomer thin-film properties

α,α′‐dimethyl substituted oligothiophenes with increasing chain length (from the dimer up to the hexamer) were recently synthesized by chemical methods. In this paper we have investigated the vibrational Fourier transform‐IR and Fourier transform‐Raman spectra of solid α,α′‐dimethyl substituted oligothiophenes in the neutral state. The data are consistent with the existence of a chain‐length dependent π electron delocalization: a large frequency dispersion with conjugation length is observed for some Raman and infrared active vibrational modes, particularly at the high‐energy side of the aromatic C=C stretching region. Vibrational assignments are proposed for the main vibrational features in the whole spectral range. This vibrational spectroscopic analysis of the solid samples thus becomes a tool for deriving information on the structure of these neutral materials in solution and in the doped state.

Efficient π electrons delocalization in α,α′‐dimethyl end‐capped oligothiophenes: A vibrational spectroscopic study

We present the results of a detailed study carried out on a few α,α′-dimethyl end-capped oligothiophenes which have been in situ doped with dry iodine vapors, at different concentrations. The doping process of these well sized oligomers (from dimer to hexamer) has been monitored by using Fourier transform infrared absorption and Fourier transform raman scattering spectroscopies. These data, when combined with the vibrational full assignment of the oligomers in the neutral state and the electronic absorption bands recorded for both neutral and doped compounds, provide a complete spectroscopic characterization of a full series of positive polaron-type model defects of doped polythiophene. We have also performed density functional theory quantum-chemical calculations in order to analyze the effects of ionization on the geometries and vibrational spectra of these systems.

Vibrational spectra of charged defects in a series of α,α ' -dimethyl end-capped oligothiophenes induced by chemical doping with iodine

The degree of π conjugation in a novel series of molecular materials containing a central oligothiophene moiety of variable length, with its end α positions capped by phenyl groups (PnTP; n=1–4) is closely inspected by means of Fourier-transform infrared and Fourier-transform Raman spectroscopies in the neutral state as solids. Density functional theory quantum chemical calculations were performed for each co-oligomer, at the B3LYP/6-31G** level, to obtain the optimized molecular geometry and force field. The thermal stability of these oligothiophenes has been also investigated, at the molecular level, by recording infrared and Raman spectra at different temperatures between −170 °C and 160 °C.

Efficiency of the π conjugation in a novel family of α, α'-bisphenyl end-capped oligothiophenes by means of Raman spectroscopy

http://www.biblioteca.uma.es/bbldoc/tesisuma/1664282x.pdf

Vibrational study of push–pull chromophores for second-order non-linear optics derived from rigidified thiophene π-conjugating spacers

In this paper we report on the vibrational Fourier transform infrared (FT–IR) and Fourier transform Raman (FT–Raman) spectra of a few donor–acceptor–substituted bi- ter-, and quaterthiophenes, as solids. The relevant vibrational information has been interpreted with the help of spectroscopic data previously collected on a model series of α,α′-dimethyl end-capped oligothiophenes, in their neutral, radical cationic, and dicationic forms. We have also performed density functional theory (DFT) quantum-mechanical calculations to analyze the effects of the intramolecular charge transfer on the geometries and vibrational spectra of these systems.

J. Casado

Papers

Efficient π electrons delocalization in α,α′‐dimethyl end‐capped oligothiophenes: A vibrational spectroscopic study

Vibrational spectra of charged defects in a series of α,α ' -dimethyl end-capped oligothiophenes induced by chemical doping with iodine

Efficiency of the π conjugation in a novel family of α, α'-bisphenyl end-capped oligothiophenes by means of Raman spectroscopy

Vibrational study of push–pull chromophores for second-order non-linear optics derived from rigidified thiophene π-conjugating spacers

Analysis of the intramolecular charge transfer in donor–acceptor α,α′-substituted oligothiophenes from their vibrational spectra