Luminescence of anthracene molecules in single crystal matrices

Abstract

Optical properties of single fluorene/phenanthrene crystals doped with various mole fraction of anthracene, 10 -5 to a few %, were investigated at 10 K. In low concentration samples, sharp structures attributed to isolated anthracene molecules were observed. Inhomogeneous width in their 0-0 energy were found to be ~20 cm -1 . When concentration is higher, spectral species, which are distinctly different from the isolated molecule spectrum, were observed and their origins are discussed. . For microcrystallites consisting of small number of molecules, appreciable energy gaps in the lowest electronic excitation level (0-0) are expected between crystallites consisting of n and n+1 molecules. However, gaps are considered to be difficult to observe with PMMA as a matrix, since 0-0 energies of crystallites of the same size would have distribution width due to inevitable local inhomogeneity. In this study, to achieve uniform environment, single crystals of fluorene and phenanthrene were chosen as matrices. Optical properties were investigated for various anthracene mole fraction samples (10 -5 to a few %) to estimate inhomogeneous width in low concentration samples and to search spectral species different from the one from isolated molecules in high concentration samples.

Difference in optical properties of anthracene molecule aggregates in fluorene and phenanthrene crystals

“…It means that in real system, not only the splitting and low energy shift of the lowest electronic state but also electron-lattice interaction has to be taken into account. On the other hand, in the previous study on phenanthrene crystals doped with a few % of anthracene, no dimer luminescence with lattice relaxation was observed [2]. It suggests a possibility that electron-lattice relaxation in dimer state can be controlled by changing host crystals.…”

Optical study of anthracene molecules doped in fluorene single crystals

“…Type A luminescence, which is dominant in samples with anthracene concentration less than 0.1%, is attributed to isolated anthracene molecules in the fluorene lattice [2]. Luminescence of types B and C, which appears as concentration increases, is expected to be due to small anthracene clusters.…”

“…However, inhomogeneity of polymer environment prevented one from observing energy gaps expected between small nanocrystallites of different sizes, which gives rise to ambiguity in assignment of obtained spectra to a certain size of nanocrystallites. To gain insight from knowledge of small clusters of aromatic molecules in homogeneous environment, we studied anthracene molecules in single crystals of fluorene, and found that luminescence spectra of three types, A, B and C, appear depending on anthracene concentration [2].…”

Optical properties of anthracene‐fluorene mixed crystals fabricated by cell method

“…Bulk crystals of anthracene (A) molecules (C 14 H 10 ), having small exciton-phonon coupling constant g of 0.85 [1], show free exciton luminescence composed of many sharp lines due to intra molecular vibronic transitions similar to that of isolated molecules. On the other hand, in fluorene (F) (C 13 H 10 ) crystals doped with a few% of anthracene, broad luminescence bands with non-zero Stokes-shift energy are observed [2]. This luminescence originates from adjacent A molecule pair in F lattice [3].…”

“…Absorption spectrum for a sublimation sample (bottom gray curve) consists of peaks originating from isolated A molecules, M [2], and those originating from two A molecules situated at adjacent sites in F lattice, hereafter denoted as D. Assignment of D has been deduced from the concentration dependence of intensity ratio of D band to M band for sublimation samples [3]. Peaks at higher energy side of the M band are vibronic structures.…”