Development of a coherent picture of enhanced fluorescence in the aggregated/solid state of molecular materials requires an exploration of the concomitant inhibition of intra and intermolecular non-radiative energy loss pathways. This necessitates a fluorophore that exhibits a systematic variation of the emission enhancement (solid over solution) upon subtle structural tuning at the molecular and supramolecular levels. Diaminodicyanoquinodimethanes with an imidazolidine moiety (1a), reported in 1962 but never structurally characterized, is shown to be ideally suited for this. 1a and its N-ethyl (1b) and N,N 0 -dimethyl (1c) derivatives are synthesized by a modified route and structurally characterized. Systematic change in the molecular structure (a crucial torsion angle varying from B31 to 501) and hence assembly in crystals, increases the fluorescence enhancement from B30 (1a) to B900 (1c). A methodology based on ab initio and lattice energy calculations and analysis of the organization of molecules and their transition dipoles in crystals is developed, to quantitatively assess the inhibition of excited state relaxation and relative energy transfer rates in solids. This approach provides insight into the contribution of intra and intermolecular pathways to the structural tuning of the emission enhancement in 1a-c, and a rational basis to tailor highly emissive molecular solids.
It is now well established that water-like anomalies can be reproduced by a spherically symmetric potential with two length scales, popularly known as core-softened potential. In the present study we aim to investigate the effect of attractive interactions among the particles in a model fluid interacting with core-softened potential on the existence and location of various water-like anomalies in the temperature-pressure plane. We employ extensive molecular dynamic simulations to study anomalous nature of various order parameters and properties under isothermal compression. Order map analyses have also been done for all the potentials. We observe that all the systems with varying depth of attractive wells show structural, dynamic, and thermodynamic anomalies. As many of the previous studies involving model water and a class of core softened potentials have concluded that the structural anomaly region encloses the diffusion anomaly region, which in turn, encloses the density anomaly region, the same pattern has also been observed in the present study for the systems with less depth of attractive well. For the systems with deeper attractive well, we observe that the diffusion anomaly region shifts toward higher densities and is not always enclosed by the structural anomaly region. Also, density anomaly region is not completely enclosed by diffusion anomaly region in this case.
We consider molecules confined to a microcavity of dimensions such that an excitation of the molecule is nearly resonant with a cavity mode. The molecular excitation energies are assumed to be Gaussianly distributed with mean ϵ a and variance σ. We find an asymptotically exact solution for large number density [Formula: see text]. Conditions for the existence of the polaritonic states and expressions for their energies are obtained. Polaritonic states are found to be quite stable against disorder. Our results are verified by comparison with simulations. When ϵ a is equal to energy of the cavity state ϵ c, the Rabi splitting is found to increase by [Formula: see text], where [Formula: see text] is the coupling of a molecular excitation to the cavity state. An analytic expression is found for the disorder-induced width of the polaritonic peak. Results for various densities of states and the absorption spectrum are presented. The dark states turn “gray” in the presence of disorder with their contribution to the absorption increasing with σ. Lifetimes of the cavity and molecular states are found to be important, and for sufficiently large Rabi splitting, the width of the polaritonic peaks is dominated by them. We also give analytical results for the case where the molecular levels follow a uniform distribution. We conclude that the study of the width of the polaritonic peaks as a function of the Rabi splitting can give information on the distribution of molecular energy levels. Finally, the effects of (a) orientational disorder and (b) spatial variation on the cavity field are presented.
We consider the escape of a particle trapped in a metastable potential well and acted upon by two noises. One of the noises is thermal and the other is Poisson white noise, which is non-Gaussian. Using path integral techniques, we find an analytic solution to this generalization of the classic Kramers barrier crossing problem. Using the “barrier climbing” path, we calculate the activation exponent. We also derive an approximate expression for the prefactor. The calculated results are compared with the simulations, and a good agreement between the two is found. Our results show that, unlike in the case of thermal noise, the rate depends not just on the barrier height but also on the shape of the whole barrier. A comparison between the simulations and the theory also shows that a better approximation for the prefactor is needed for agreement for all values of the parameters.
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