Excitons In Organic Crystals Research Articles

Light–Matter Coupling Strength Controlled by the Orientation of Organic Crystals in Plasmonic Cavities

Strong light–matter coupling is a powerful mechanism to engineer materials properties and a platform to study polariton physics. Excitons in organic crystals are interesting candidates for the investigation of light–matter coupling due to the large magnitude and well-defined orientation of their transition dipole moments. We demonstrate the coupling of excitons in tetracene crystals to optical modes in open cavities formed by anisotropic arrays of plasmonic nanoparticles and investigate the coupling strength as a function of the alignment of the exciton dipole moment to the cavity field. The anisotropy of the cavity and the crystal provides a practical method to tune the light–matter coupling strength from weak to the onset of the strong coupling regime, by rotating the crystal with respect to the plasmonic array. The possibility to control the coupling within a single excitonic material, paves the way to study the effects of the coupling strength on polariton physics, such as exciton-polariton dynamics, transport, or condensation.

Opposite Anisotropy Effects of Singlet and Triplet Exciton Diffusion in Tetracene Crystal.

We observe the diffusion anisotropy difference between singlet and triplet excitons in organic crystals; that is, singlet and triplet excitons may have completely different spatial direction preference for diffusion. This phenomenon can be ascribed to the distinct dependence of different excitonic couplings (Coulomb Förster vs. exchange Dexter) existing in singlet and triplet excitons on their intermolecular distance and intermolecular orientation. Such a discovery provides insights for understanding the fundamental photophysical process in a vast range of organic condensed‐phase systems and optimizing the efficiency of organic optoelectronic materials.

Dynamical Localization Limiting the Coherent Transport Range of Excitons in Organic Crystals.

Exciton or energy transport in organic crystals is commonly described by a series of incoherent hoppings. This picture is no longer valid if the transport range is on the order of the exciton coherent (or delocalization) size. However, coherent effects are often neglected because the exciton wave function generally localizes to a few molecules within an ultrafast time scale (<1 ps) after photoexcitation. Here, by using time-resolved photoemission spectroscopy and nanometer-thick zinc phthalocyanine crystals, we are able to observe a transition from the coherent to incoherent transport regime while the exciton coherent size is decreasing as a function of time. During the transition, a distinct phonon mode is excited, which suggests that the electron-vibrational interaction localizes the exciton and reduces its coherent size. It is anticipated that the coherent transport range can be increased by controlling the electron-vibrational coupling. An enhanced coherent transport range can be advantageous in applications such as organic photovoltaics.

A density matrix based approach for studying excitons in organic crystals

A theoretical analysis of molecular excitons in crystalline p-Nitro-Aniline (PNA) is presented. The approach is general and can be straightforwardly extended to the calculation of the exciton structure of other molecular crystals or aggregates. Based on the evaluation of transition density matrices (TDM), the method allows to easily classify the excited states of interacting molecules on the basis of those of the constituent molecules. Exciton couplings of selected dimers within the PNA crystal have been evaluated and the exciton dispersion has been determined. The experimental absorption spectrum of PNA powders can be analysed in details based on the exciton density of states.

Transient absorption microscopic study of triplet excitons in organic crystals

We have constructed a transient absorption microscope to study triplet excitons in organic crystals. Using the microscope, transient absorption of small crystals can be measured and the spatial distribution of triplet excitons can be obtained. Additionally, it was found that the contrast of the image becomes much better than that of a conventional bright-field microscope. These characteristic features of the transient absorption microscope will be discussed. We also report on the absorption spectra in benzophenone, anthracene and α- and β-perylene crystals. Absorption spectra due to triplet excitons in the crystals were similar to the corresponding solution spectra, although there were some minor differences.

Charged Frenkel excitons in organic crystals

In molecular crystals a charge carrier and a molecular (Frenkel) exciton are attracted to each other. This attraction arises from the increase of the molecular static polarizability upon electronic excitation, and may be responsible for the formation of a two-particle bound state of the exciton and the free charge. Such charged Frenkel excitons are analogous to trions (bound states of a Wannier–Mott exciton and a charge carrier) in inorganic semiconductors. In this paper we develop a theory of charged excitons in molecular crystals, and apply this theory to study the spectra of charged Frenkel excitons in two-dimensional structures, which serve as a rather good approximation for layered crystals like anthracene, tetracene. We show that the binding energy of charged Frenkel excitons can be of the order of several hundred cm −1. Thus, they may be stable at room temperatures, in contrast to trions in inorganic semiconductors. Particularly interesting is the possibility of forming charged triplet excitons in the crystal of tetracene where the energy of the lowest triplet exciton is much smaller than the energy of the lowest excitation in ions. This circumstance prevents the transformation of charged triplet excitons into an ion excitation with short lifetime and promotes the formation of rather stable charged triplet excitation. We briefly discuss the optical properties of charged Frenkel excitons and the effects of a static electric field.

Luminescence and ODMR studies of X-traps for triplet excitons in organic crystals

The excited triplet states of shallow X-traps in neat single crystals of 4,4-dichlorobenzophenone were studied at liquid helium temperature by luminescence spectroscopy and magnetic resonance with optical detection. The vibrational phosphorescence bands, the polarization of phosphorescence spectra, and the triplet zero-field splitting parameters of these traps have been analyzed. A specific model of the structure defect (a molecule is turned by 180° on its long axis) has been considered in organic crystals of benzophenone type which consist of molecules whose dipole moments compensate each other in a unit cell. The calculated depth of such a dipole X-trap for triplet excitons in 4,4-dichlorobenzophenone crystals at the account of a defect molecule relaxation upon its slewing was found to be comparable with the experimental one.

Enhancement of ODMR Effect in a System of Triplet Excitons at Threshold of Annihilational Bistability in Organic Crystals Containing Traps

Abstract It is shown that the sensitivity of a system of triplet excitons in organic crystals to microwave radiation under conditions that give rise to the development of a annihilational bistability significantly increases. A possibility to change intensity of crystal phosphorescence by an order of magnitude by applying microwave field resonant to the transitions between spin sublevels of trapped excitons has been demonstrated.

Electroabsorption due to excitons in crystalline molecular thin films grown by organic molecular beam deposition

We measure the electroabsorption due to the lowest absorption peak in ordered organic molecular crystalline films of 3,4,9,10-perylenetetracarboxylic dianhydride grown by organic molecular beam deposition. The application of electric fields induces both a Stark shift and a broadening of the exciton absorption line. We find a nearly spherical exciton of radius 14 ± 2 A ̊ , an effective hole mass of (0.13 ± 0.02) m 0, where m 0 is the free electron mass, and a binding energy of −150 ± 15 meV, suggesting that excitons in organic crystals with very small intermolecular stacking distances can be described by a combined Wannier and charge-transfer model.

B896 and B870 components of the Rhodobacter sphaeroides antenna: a hole burning study

Novel nonphotochemical hole burning action spectra are presented that yield the low-temperature absorption profiles of B896 and B870 and their underlying structures (linear electron-phonon coupling and site inhomogeneous broadening). The results establish that B896 and B870 are associated with the far more intense B875 and B850 bacteriochlorophyll {alpha} absorption bands, respectively, of the light harvesting I and II complexes. The homogeneous widths of the B896 and B870 zero-phonon holes are the same within experimental uncertainty, 3.2 cm{sup {minus}1} at 4.2 K, which corresponds to a total optical dephasing time of 6.6 ps. A number of interpretations for B870 and B896 are considered. Favored is one in which they are due to the lowest energy levels of the B850* and B875* exciton bands (asterisk denoting the S{sub 1}(Q{sub y}) state). Based on studies of the dephasing of excitons in organic crystals, the 6.6-ps dephasing of B896* is attributed to exciton scattering with energetic inequivalent neighboring unit cells. Such scattering and B870 to B875 energy transfer are suggested to be contributors to the dephasing of B870*. The effect of glasslike structural heterogeneity on the optical selection rules for unit cells of cyclic symmetry is also considered. 71 refs., 5 figs.

Relaxation of excitons in organic crystals with strong exciton-phonon coupling

Time-integrated and time-resolved luminescence spectra of pyrene and poly(phenylenevinylene) have been investigated and the direct and indirect self-trapping processes of hot free excitons are discussed. The existence of a self-trapping barrier between the free exciton state and the self-trapped state plays an essential role in exciton relaxation processes, in particular in the indirect relaxation process. Reported theories on branching of exciton relaxation paths are briefly compared to experimental data.

Evaluation of the triplet fine structure tensor in the DA-complex anthracene-PMDA by triplet-doublet resonance (TDR) recorded in the photocurrent

We have observed sharp triplet-doublet resonances in the photocurrent of single crystals of the organic donor-acceptor (DA) complex anthracene-pyromellitic dianhydride (A-PMDA). These resonances appear in a magnetic field and depend strongly on the orientation and the magnitude of this field. Resonances can be resolved at magnetic fields even well below 200 G. The evaluation of the resonance positions and their Field dependence provides a new method for the investigation of mobile triplet excitons in organic crystals. The triplet fine structure tensor orientation relative to the crystallographic axes is obtained with an accuracy of about one degree and coincides with the orientation of the anthracene molecule in the triclinic lattice. At room temperature the following fine structure parameters are determined: D/hc = ±(0.0665 ± 0.0005) cm −1 and E/hc = ∓(O.00765 ± 0.0001) cm −1. These values are about 4% lower than ESR results reported for 4 K.

Diffusion and mutual annihilation of triplet excitons in organic crystals

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTDiffusion and mutual annihilation of triplet excitons in organic crystalsR. E. MerrifieldCite this: Acc. Chem. Res. 1968, 1, 5, 129–135Publication Date (Print):May 1, 1968Publication History Published online1 May 2002Published inissue 1 May 1968https://doi.org/10.1021/ar50005a001RIGHTS & PERMISSIONSArticle Views307Altmetric-Citations30LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (830 KB) Get e-Alerts Get e-Alerts