Optical characteristics of molecular aggregates with inhomogeneous monomers’ energies

The dynamical characteristics of one-dimensional molecular aggregate with the randomness in exciton hopping is considered. We show that the randomness of the distribution of signs of the exciton hopping of the aggregate dramatically changes the dynamical characteristics. In the homogeneous case, the dynamical characteristics of the molecular aggregate manifest narrow lines at edges of the band of Frenkel excitons of the aggregate. On the other hand, we show that the considered randomness yields nonzero contributions to the dynamical characteristics of all excitons of the band.

Within the linear response theory, we have calculated the resonance absorption of the light and resonance luminescence for the one-dimensional molecular aggregate. We have shown that the randomness of the distribution of signs of the interaction between dipoles of the aggregate dramatically changes the parameters of the absorption and luminescence. In the homogeneous case, the resonance response of the exciton band of the molecular aggregate produces narrow lines at edges of the band. On the other hand, inhomogeneities yield contributions to optical characteristics of all excitons of the band.

The problem of optical absorption by aggregates of molecules whose wavefunctions do not overlap is formulated and it is shown that there are five exact relationships (sum rules) between the absorption spectrum of the aggregate and the spectra of the constituent molecules. An approximation satisfying these sum rules is developed in which aggregate absorption is given explicitly as a function of the spectra of the constituent molecules. This approximation, which makes use of a complete set of vibrational wavefunctions for the aggregate, is a generalization of the weak and strong coupling models in the sense that it contains each as a limiting case. Moreover, it is far superior to either in both the intermediate and strong coupling cases as shown by comparing it to an exact calculation on a model dimer.

A brute‐force numerical investigation has been carried out on the hopping of excitons in a three‐dimensional molecular aggregate. Possibilities of vibronic decay, rapid chemical reactions of saturated species, radiative decay of overpopulated molecules, and cooperative chemical reactions involving saturated exciton populations on traps of two different types have been considered. Investigation have been performed with two types of initial distribution of excitons—facial and random—and for 10,000 or, sometimes, for 20,000 time steps each of duration 1ps. Several interesting observations have been made from this computer experiment: (1) The total number of occurrences of fast reactions depends upon the initial distribution of excitons. (2) It decreases if other exciton depleting processes are at work. (3) It also depends on the pattern of placement of traps. (4) The location of impurities also affects the rate of occurrence of these reactions. Thus, more reactions occur when the excitons are initially concentrated on one face and traps are suitably located on the path of flow of these excitons. A random initial distribution tends to equilibrate the excitons quickly over all the lattice points, thus giving rise to fewer reactions. (5) The number of reactions need not necessarily increase with the number of reaction centers; in fact, it decreases as more centers are added when the supply of excitons is severely limited. (6) A Complicated dynamics results when different types of additional processes, viz., enhanced fluorescence, radiative emissions, and cooperative chemical reactions are simultaneously allowed. The cooperative process has been clearly found to dominate. A first‐order rate constant of about 108 s‐1 has been calculated for the occurrence of the cooperative process. This rate is affected when other nonconserving processes are switched on. Observations (1), (4), and (5) are the most important conclusions of our work. They lie outside the scope of traditional models such as the random walk model, the diffusion model, and the lattice model for the migration of excitons in a molecular aggregate. © 1993 John Wiley & Sons, Inc.

Exciton delocalization plays a prominent role in the photophysics of molecular aggregates, ultimately governing their particular function or application. Deoxyribonucleic acid (DNA) is a compelling scaffold in which to template molecular aggregates and promote exciton delocalization. As individual dye molecules are the basis of exciton delocalization in molecular aggregates, their judicious selection is important. Motivated by their excellent photostability and spectral properties, here, we examine the ability of squaraine dyes to undergo exciton delocalization when aggregated via a DNA Holliday junction (HJ) template. A commercially available indolenine squaraine dye was chosen for the study given its strong structural resemblance to Cy5, a commercially available cyanine dye previously shown to undergo exciton delocalization in DNA HJs. Three types of DNA-dye aggregate configurations-transverse dimer, adjacent dimer, and tetramer-were investigated. Signatures of exciton delocalization were observed in all squaraine-DNA aggregates. Specifically, strong blue shift and Davydov splitting were observed in steady-state absorption spectroscopy and exciton-induced features were evident in circular dichroism (CD) spectroscopy. Strongly suppressed fluorescence emission provided additional, indirect evidence for exciton delocalization in the DNA-templated squaraine dye aggregates. To quantitatively evaluate and directly compare the excitonic Coulombic coupling responsible for exciton delocalization, the strength of excitonic hopping interactions between the dyes was obtained by simultaneously fitting the experimental steady-state absorption and CD spectra via a Holstein-like Hamiltonian, in which, following the theoretical approach of Kühn, Renger, and May, the dominant vibrational mode is explicitly considered. The excitonic hopping strength within indolenine squaraines was found to be comparable to that of the analogous Cy5 DNA-templated aggregate. The squaraine aggregates adopted primarily an H-type (dyes oriented parallel to each other) spatial arrangement. Extracted geometric details of the dye mutual orientation in the aggregates enabled a close comparison of aggregate configurations and the elucidation of the influence of dye angular relationship on excitonic hopping interactions in squaraine aggregates. These results encourage the application of squaraine-based aggregates in next-generation systems driven by molecular excitons.

Thin films of molecular aggregates of pseudoisocyanine were obtained. Delocalization length of exciton wave function was measured with an assumption that an absorption line width is defined by disorder induced mixing of the lowest states of the one-exciton band. Predicted effect of size enhancement of the nonlinear optical properties for molecular aggregates was observed. The assumption used to determine the delocalization length is shown to coincide with the observed optical nonlinearity size enhancement phenomenon.

Low-temperature spectral dynamics of excitons in molecular aggregates

The model of spherical molecular aggregate of nonionic surfactant is proposed. This model allows for the maximal (in accordance with packing rules) penetration of water molecules into an aggregate and is an alternative to the droplet model of molecular aggregate. Necessary conditions for the applicability of a model named quasi-droplet model are formulated. Based on this model, the dependence of the work of molecular aggregate formation on the aggregation number and surfactant monomer concentration in solution that plays the key role for the theory of micellization is studied. The equation is derived for the coordinates of maximum and minimum of aggregate formation work on the aggregation number axis arising with an increase in the concentration of micellar solution. Model calculations of the thermodynamic characteristics of the kinetics of micellization are performed. The approximation of the work of molecular aggregate formation allowing for the analytical study is constructed.

It is well known that Frenkel excitons are not true bosons or fermions. I show that the fundamental electronic excitations in a linear (noncyclic) one-dimensional molecular aggregate consisting of N coupled two-level absorbers with site disorder are in fact fermions, allowing one to calculate IallR of the ${2}^{\mathit{N}}$ excited-state eigenfunctions and eigenenergies with a single N\ifmmode\times\else\texttimes\fi{}N matrix diagonalization. As an application, the third-order nonlinear absorption coefficient for a molecular aggregate with site disorder is calculated.

Molecular Materials with Short Radiative Lifetime for High-Speed Light-Emitting Devices

Though inhomogeneous particles are common for atmospheric aerosol, inhomogeneity is not taken into account in present-day remote sensing retrieval algorithms. Effects of inhomogeneity on radiation scattering can however have an impact on the quality of aerosol retrievals, as shown in Mishenko et al., 2016. The current study aims to address this gap by an attempt to introduce aerosol inhomogeneity parameterization into the aerosol remote sensing retrieval algorithm – GRASP (Dubovik et al., 2021). First, we focus on AERONET measurements as a global network with an efficient retrieval algorithm with the aim to identify situations where currently employed homogeneous aerosol model does not reproduce correctly the radiation field or gets to the limits of the field of solutions. We examine AERONET retrievals using results of operational AERONET algorithm, but also those obtained using an independent recently developed version of GRASP/Component algorithm (Li et al., 2019) applied to AERONET. A notable part of these retrievals, under atmospheric conditions suspected to cause particles inhomogeneity, present questionable values for retrieved refractive index, i.e. values of its real part reach the algorithmic limit. This situation unveils potential mismatch of employed aerosol microphysical model. At the second step we model the aerosol inhomogeneity by Mie calculations for layered spheres (core/shell) particle structure with an ammonium nitrate/sulfate liquid shell and various composition of core, relying on some field results reported in (Unga et al. 2018). We then examine the response of the obtained optical characteristics to variation in core/shell model. Namely, the phase function and degree of linear polarization are compared for several core radii, refractive indexes and particles size distributions. We present comparative analysis for the effects of structure changes over size changes and study their differences relative to homogeneous particle model. This analysis reveals potential sensitivity of remote sensing to particles inhomogeneity and serves for parameterization of core/shell model in the remote sensing algorithm GRASP. The updated GRASP/Component algorithm will then be applied to previously identified cases of questionable AERONET retrievals. ReferencesDubovik O., Fuertes D., Litvinov P., et al.: A Comprehensive Description of Multi-Term LSM for Applying Multiple a Priori Constraints in Problems of Atmospheric Remote Sensing: GRASP Algorithm, Concept, and Applications. Front. Remote Sens. 2:706851, 2021. doi:10.3389/frsen.2021.706851Unga F., Choël M., Derimian Y., et al. : Microscopic Observationsof Core-Shell Particle Structure and Implications for Atmospheric Aerosol Remote Sensing. Journal of Geophysical research 123:24, 2018. doi:10.1029/2018JD028602Michael I. Mishchenko, Janna M. Dlugach, and Li Liu, "Linear depolarization of lidar returns by aged smoke particles," Appl. Opt. 55, 9968-9973, https://doi.org/10.1364/AO.55.009968, (2016).Li, L., Dubovik, O., Derimian, Y., et al.: Retrieval of aerosol components directly from satellite and ground-based measurements, Atmos. Chem. Phys., 19, 13409–13443, https://doi.org/10.5194/acp-19-13409-2019, 2019.

In molecular and cellular biology, dissolved ions and molecules have decisive effects on chemical and biological reactions, conformational stabilities, and functions of small to large biomolecules. Despite major efforts, the current state of understanding of the effects of specific ions, osmolytes, and bioprotecting sugars on the structure and dynamics of water H-bonding networks and proteins is not yet satisfactory. Recently, to gain deeper insight into this subject, we studied various aggregation processes of ions and molecules in high-concentration salt, osmolyte, and sugar solutions with time-resolved vibrational spectroscopy and molecular dynamics simulation methods. It turns out that ions (or solute molecules) have a strong propensity to self-assemble into large and polydisperse aggregates that affect both local and long-range water H-bonding structures. In particular, we have shown that graph-theoretical approaches can be used to elucidate morphological characteristics of large aggregates in various aqueous salt, osmolyte, and sugar solutions. When ion and molecular aggregates in such aqueous solutions are treated as graphs, a variety of graph-theoretical properties, such as graph spectrum, degree distribution, clustering coefficient, minimum path length, and graph entropy, can be directly calculated by considering an ensemble of configurations taken from molecular dynamics trajectories. Here we show percolating behavior exhibited by ion and molecular aggregates upon increase in solute concentration in high solute concentrations and discuss compelling evidence of the isomorphic relation between percolation transitions of ion and molecular aggregates and water H-bonding networks. We anticipate that the combination of graph theory and molecular dynamics simulation methods will be of exceptional use in achieving a deeper understanding of the fundamental physical chemistry of dissolution and in describing the interplay between the self-aggregation of solute molecules and the structure and dynamics of water.

We study the Frenkel exciton dynamics in a one-dimensional molecular aggregate with disorder in both the on-site energies and transition dipole moments. Using a tensorial generalization of the coherent-potential approximation, we calculate the linear absorption spectra, the exciton density of states, and the coherence length responsible for the linear optics. In particular, we consider the purely orientational disorder in the transition dipoles, and show that our theory agrees well with the numerical simulations. In addition to features shared by other disordered aggregate models, we show that the strength of orientational disorder affects the anisotropy degree of the optical response with the main components of the optical susceptibility tensor being characterized by nonequal coherence lengths. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Generalized Kasha’s Model: T-Dependent Spectroscopy Reveals Short-Range Structures of 2D Excitonic Systems

Interfacial Behavior of Mixed Systems of Glycerylether-Modified Silicone and Polyoxyethylene-Modified Silicone