Luminescence of Free and Self-Trapped Excitons in α- and β-Perylene Crystals

Nishimura, Hitoshi; Yamaoka, Tohru; Mizuno, Ken-ichi; Iemura, Motoaki; Matsui, Atsuo

doi:10.1143/jpsj.53.3999

Cited by 75 publications

(81 citation statements)

References 16 publications

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“…The similarity between the exciton dynamics measured on and off the microstructures can be explained by the strong exciton-phonon interaction, by which the free exciton gains sufficient energy to surmount the barrier to the self-trapped state. As further indicated by the identical decay times, rate-limited occupation is sustained in this temperature range 17 . Below 30 K, a divergence of about 0.5 ns between the decay times of the two excitonic states can be detected.…”

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confidence: 52%

“…Important questions raised by the PL spectra are: how does the conversion of the excited free exciton into the self-trapped state at elevated temperatures occur and what is the related energy barrier ? This phenomenon, which has been described in detail for several polyaromatic crystalline materials including rubrene 12,[16][17][18] , will also be considered here with a special focus on the impact of the confined surface geometry. The experimental decay times shown in Fig.…”

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Enhanced photoluminescence from self-organized rubrene single crystal surface structures

Stöhr

Beirne

Michler

et al. 2010

Applied Physics Letters

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We report on crystalline pyramidal structures grown via self-organization on the rubrene (001) surface. The analysis of their spectral response by means of photoluminescence with micrometer lateral resolution reveals an intensity enhancement on-top of the surface structures. As we demonstrate this intensity increase can be related to the excitation processes at the molecular level in combination with exciton confinement within the pyramids.PACS numbers: 78.55. Kz, 78.47.-p Keywords: rubrene single crystals, self-organized surface structures, photoluminescence, exciton confinementThe optical properties of organic single crystals have been the subject of intense research for many decades. Their low symmetry and the correlation between their structural and optical properties result in complex phenomena such as Davydov-splitting, large hyperpolarizabilities and highly anisotropic optical parameters 1-3 . In contrast to their inorganic counterparts, the optical surface properties are not affected e.g. by dangling bonds. Furthermore, exciton binding energies of up to 1 eV favor the use of ordered molecular structures for optoelectronic applications at room temperature 4,5 . However, fundamental aspects of the exciton formation, excitonic states and lifetimes are still under discussion. The existence of a delocalized excitonic band versus the possible formation of a so-called "self-trapped" excitonic state, caused by the relaxation of the local molecular environment, highlights one of these key questions 6,7 . In this context, the material dependent diffusion constants for the various excitonic species can only be estimated by laborious methods such as transient optical gratings on molecular crystals of high quality 8,9 . In this paper we describe an alternative approach to access the optical properties of ordered organic materials by analyzing the exciton dynamics in self-organized pyramidal structures on the (001) rubrene crystal surface. Their regular arrangement and crystallinity prove to be beneficial for optical studies in contrast to approaches based on randomly oriented micro-and nanocrystals grown by precipitation 10 . The structural quality and the spatial extension enable the investigation of their optical characteristics in comparison with those of the planar (001) rubrene surface by photoluminescence with micrometer spatial resolution (µ-PL). High quality rubrene crystals were grown by sublimation under streaming H 2 . By Laue diffraction the crystal surface was identified as (001). Additional polarization studies corroborate that the microstructures are optically active indicating their crystallinity. The pyramidal baseline ratio of 2:1 is in agreement with the ratio of the unit cell vectors of the (ab)-plane. The angles between the facets and the pyramidal base plane indicate that the facets coincide with low index planes such as {103} and {013}. Details of the underlying growth mechanisms leading to these structures will be presented in a forthcoming article. µ-PL measurements were performed in a helium...

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confidence: 52%

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confidence: 99%

Enhanced photoluminescence from self-organized rubrene single crystal surface structures

Stöhr

Beirne

Michler

et al. 2010

Applied Physics Letters

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“…1, left hand side) interact via short ranging Dexter coupling which can, nevertheless, lead to significant triplet exciton diffusion lengths as in general the life time of the triplet states exceeds that of the respective singlet states by orders of magnitude. [25–29] States with charge transfer (CT) character (Fig. 1, two right hand panels) arise by excitation of an electron from one dye molecule to another one in close proximity.…”

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confidence: 99%

Comparison of the electronic structure of different perylene‐based dye‐aggregates

et al. 2012

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Aggregates of functionalized polycyclic aromatic molecules like perylene derivatives differ in important optoelectronic properties such as absorption and emission spectra or exciton diffusion lengths. Although those differences are well known, it is not fully understood if they are caused by variations in the geometrical orientation of the molecules within the aggregates, variations in the electronic structures of the dye aggregates or interplay of both. As this knowledge is of interest for the development of materials with optimized functionalities, we investigate this question by comparing the electronic structures of dimer systems of representative perylene-based chromophores. The study comprises dimers of perylene, 3,4,9,10-perylene tetracarboxylic acid bisimide (PBI), 3,4,9,10-perylene tetracarboxylic acid dianhydride (PTCDA), and diindeno perylene (DIP). Potential energy curves (PECs) and characters of those electronic states are investigated which determine the optoelectronic properties. The computations use the spin-component-scaled approximate coupled-cluster second-order method (SCS-CC2), which describes electronic states of predominately neutral excited (NE) and charge transfer (CT) character equally well. Our results show that the characters of the excited states change significantly with the intermolecular orientation and often represent significant mixtures of NE and CT characters. However, PECs and electronic structures of the investigated perylene derivatives are almost independent of the substitution patterns of the perylene core indicating that the observed differences in the optoelectronic properties mainly result from the geometrical structure of the dye aggregate. It also hints at the fact that optical properties can be computed from less-substituted model compounds if a proper aggregate geometry is chosen.

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“…Previous models for STE in molecular crystals have concentrated on the inter-molecular distance as a suitable configuration coordinate [12,13]. However, from the vibronic progressions observed in absorption and PL of dissolved Table 1 Shift DE F of the HOMO-LUMO transition in different molecular dimers in a-PTCDA with respect to a monomer in the non-rectangular geometry compatible with the crystalline phase, as determined by B3LYP/3-21G calculations performed with Gaussian98.…”

Section: Self-trapped Excitonsmentioning

confidence: 99%