Ilhan Yavuz

Theoretical Study of the Molecular Ordering, Paracrystallinity, And Charge Mobilities of Oligomers in Different Crystalline Phases

Molecular ordering and charge transport have been studied computationally for 22 conjugated oligomers fabricated as crystal or thin-film semiconductors. Molecular dynamics (MD) simulations are employed to equilibrate crystal morphologies at 300 K. The paracrystalline order parameter, g, is calculated to characterize structural order in the materials. Charge-transport dynamics are predicted using kinetic Monte Carlo methods based on a charge-hopping mechanism described by the Marcus theory of electron transfer to calculate charge-transfer rates using the VOTCA package. We introduce an error function to assess the reliability of our computed values to reproduce experimental hole mobilities in both crystalline and thin-film morphologies of the 22 conjugated oligomers. For each of the oligomers, we compute hole mobility with three different theoretical models incorporating increasing measures of disorder: (1) a perfect crystal, based on the experimentally derived crystal structure, with no disorder, (2) an MD-equilibrated structure incorporating thermal disorder into the crystal structure, and (3) model 2 above but also incorporating energetic disorder arising from variations in site energies. For the series of known crystals with long-range order, we find that the perfect crystal model produces hole mobilities giving the best fit to experimental data. For the series of thin-film morphologies with short-range order, we observe that the presence of both thermal and energetic disorder is essential for accurate calculation. We also discuss the interplay between hole mobility and other charge-transport parameters in these morphologies, such as reorganization energy and energetic disorder.

Structure, Binding Energies, and IR-Spectral Fingerprinting of Formic Acid Dimers.

We describe equilibrium structures for a variety of species likely to be formed as intermediate species in the dimerization of formic acid to produce the stable C2h-symmetric doubly H-bonded dimer and perhaps produced as the vapor is irradiated. For several low-lying species the rearrangement pathways to the stable form are characterized at the MP2/6-311+G(d,p) level of theory, with optimized structures and vibrations computed with full counterpoise corrections for basis set superposition error. Estimates of vibrational frequencies with corrections for anharmonicity suggest that infrared transitions (CO stretches and OH out-of-plane motions) could signal the presence of species less stable than the C2h dimer, observable in irradiation studies of formic acid vapor.

A Solid‐State Effect Responsible for an Organic Quintet State at Room Temperature and Ambient Pressure

A stable organic diradicaloid with an intermolecular quintet at room temperature as a polycrystalline solid is studied. The conclusion is supported by the observation of the ΔMs = ±2 forbidden transition, electron spin resonance (ESR) simulations, and density functional theory (DFT) calculations. In addition, the molecule, as the active component of a device, is an outstanding near-infrared photodetector with detectivity over 10(11) cm Hz(1/2) W(-1) at 1200 nm.

The effect of hexyl side chains on molecular conformations, crystal packing, and charge transport of oligothiophenes

We report substituent effects on conformational preferences and hole mobilities of 2,5-bis-(thiophen-2-yl)thieno[3,2-b]thiophenes (BTTT) monomer and dimer, and hexyl derivatives. We employ single-crystal X-ray diffraction, quantum mechanical calculations, and thin-film transistors to explore the difference between monomer, dimer, and effect of hexyl substitution. The hexyl-substituted molecules show marked differences in solid-state packing compared to the unsubstituted analogs. Most notably, the alkylated monomer crystal structure exhibits terminal thiophenes in the syn conformation. In contrast, the unsubstituted monomer adopts the more common anti conformation. The hexyl-substituted dimer, however, features a mixture of syn and anti thiophenes. Gas phase conformations of oligomers rationalize the intrinsic conformational preferences. We use a multimode simulation to compute hole mobilities and find excellent agreement with experiment. Theoretical results support our hypothesis that alkyl side chains cause these small molecules to adopt orientations that enhance hole mobilities by an order of magnitude upon hexyl substitution of the monomer.

Quantitative prediction of morphology and electron transport in crystal and disordered organic semiconductors

The morphologies and electron mobilities for 20 single-crystal and 21 thin-film organic n-type semiconductors are predicted using a multi-mode methodology previously applied by our group for p-type materials [I. Yavuz, et al., J. Am. Chem. Soc., 2015, 137, 2856–2866]. The calculations simulate Marcus charge-hopping with a kinetic Monte Carlo method using the VOTCA package of Andrienko et al. The calculations assume perfect order for single crystal morphologies, but structural disorder is incorporated into thin-film calculations using molecular dynamics to simulate the energetic disorder of thin-film morphologies. Predicted electron mobilities for both morphologies are typically within one order of magnitude.

Enhancement of high-order harmonic generation in the presence of noise

We report on our simulations of the generation of high-order harmonics from atoms driven by an intense femtosecond laser field in the presence of noise. We numerically solve the non-perturbative stochastic time-dependent Schrodinger equation and observe how varying noise levels affect the frequency components of the high harmonic spectrum. Our calculations show that when an optimum amount of noise is present in the driving laser field, roughly a factor of 45 net enhancement can be achieved in high-order harmonic yield, especially, around the cut-off region. We observe that, for a relatively weak noise, the enhancement mechanism is sensitive to the carrier-envelope phase. We also investigate the possibility of generating ultra-short intense attosecond pulses by combining the laser field and noise and observe that a roughly four orders of magnitude enhanced isolated attosecond burst can be generated.

Focal point analysis of torsional isomers of acrylic acid

The thermochemistry of acrylic acid has presented challenges owing to its high reactivity, tendency to dimerize in the gas phase, and the existence of two very nearly equal energy conformational isomers. Well-tested thermochemical schemes including G2, G3, G4, and CBS-QB3 agree in the prediction that the s-cis syn structure is the most stable of the torsional isomers, with the s-cis anti form lying 3 kJ mol−1 or less higher in energy. Microwave spectra suggest a value of 0.63 kJ mol−1. The energy barrier between these forms is in the neighbourhood of 25 kJ mol−1 according to a MP2/cc-pVDZ calculation. We present estimates of the relative energies of all four torsional isomers and the rotational barrier based on a variant of the Focal Point Analysis developed by Császár and co-workers. These calculations, extending to the CCSD(T)/cc-pV5Z level, predict that the s-cis anti torsional isomer is the most stable form, in contrast to prior estimates. The s-cis syn form lies about 2.9 kJ mol−1 higher, while the s-trans syn and anti forms lie at about 21.7 and 23.3 kJ mol−1, respectively. We estimate the rotational barrier between the s-cis trans and s-cis anti structures to be about 23.9 kJ mol−1. Error ranges derived from the fit to extrapolation forms suggest that our estimates have an uncertainty of about 0.1 kJ mol−1.

High-order-harmonic generation from Rydberg states at fixed Keldysh parameter

Because the commonly adopted viewpoint that the Keldysh parameter

Dielectronic Recombination in He-like, Li-like and Be-like Iodine and KLM Resonances

\gamma

Level-resolved distorted-wave cross-sections of electron impact ionization of Ar5+

determines the dynamical regime in strong field physics has long been demonstrated to be misleading, one can ask what happens as relevant physical parameters, such as laser intensity and frequency, are varied while