Indian Journal of Chemistry Vol. 40A. September 2001, pp. 933-938 Tuning of wavelength of exciplex emission of pyrene and perylene in fluorene matrix S R Pujari, S A Jadhavt & S R Patil* Departme nt of Chemistry, Fluorescence Spectroscopy Laboratory, Shivaji University. Kolhapur 416 004, Maharashtra, India Recei ved 30 August 2000; revised 5 Jun e 2001 The exciplex e mi ssion of pyrene and perylene has been studied systematically in fluorene matri x. The fluoresce nt flu orene act as energy dono r. The fluorescence spectra of mixed stack crystals of fluorene are recorded on spectrofluorometer by monitoring the excitation wavelength at 363 nm. Addition of 10- 1 mol pyrene and 10- 1 mol perylene quenches th e structured monome ri c emission of fluorene and new structureless emission band appear at a wavelength 510 nm _ The broad band is attributed to the emission arising from the exciplex formed between pyrene and perylene. The observed emission does not correspond to the pyrene-perylene exciplex emission without fluorene matrix. It is believed th at th e varying degree of mol ec ular overlapping changes the wavelength of exciplex emission of pyrene and perylene. Systematic va ri a ti on of concentration of pyrene and perylene in fluorene matrix has given mixed stack crystals of flu o re ne having molecular ove rl apping of diff erent degrees. The fluorescence spectra of these mixed crystals of fluorene show exciplex emissions at diff ere nt wavelengths in the range 485-560 nm. The tuning of exciplex e mi ssion wavelength is poss ibl e by controlling co ncentration of pyrene and perylene in fluorene matrix. Photophysical and electrical processes of molecular organic semiconductors need to be understood for potential applications of these materials in the field of electroluminescence and 10"1 cost light emitting material si.2. Tuning of the colours of phosphor material is an attractive topic in fabrication of organic light emitting diodes. Creation of the materials emitting with desired colours can be achieved by mixing homogeneously red, yellow or green emitting material s in a material emitting with a blue or violet colour. Fluorene is a planar molecule which crystaliizes in orthorhombic form with four molecules per unit cell. ft is a wide band gap organic semiconducting material and fluoresce in violet sector of the visible spectrum. We found that fluorene solubalise polynuclear aromatic hydrocarbons in molten state from which its mixed crystals can be grown. Use of planner and highly conjugated aromatic hydrocarbons as an impurity in fluorene can give mixed stack luminophors emitting in the green, yellow and orange region of the spectrum. The highly fluorescent pyrene and perylelle whose lattice contains four molecules per unit cell favouring formation of excimers, dimers Dept. of Chemistry, Shahajiraje Mahavidya1aya, Khatav. India etc. were chosen as guest molecules 3 .4. The prese nt work aims at the preparation of mixed crystals of fluorene containing pyrene and perylene impurities. The fluorescence spectra of these polycrystalline luminophors were recorded and discussed with a view to understand the fluorescent behaviour of mi xed stack crystals. Materials and Methods Fluorene obtained from Merck-Schuchardt (Hohen- byunn) Germany was recrystallized from ethanol wi th charcoal, and then from ethanol alone. The recrys- tallized fluorene is fluorescent and exhibits violet fluorescence. The fluorescent grade pyrene and perylene procured from Fluka AG, Buchs SG, (Switzerland) were used without further purification. The purity of all the components were confirmed by comparing the fluorescence spectra with th ose reported in the literature. The mixed stack crystals of fluorene conta1l111lg pyrene and perylene were prepared by conventional solid state reaction techniques. Appropriate amounts of pyrene, perylene and fluorene were intimately mixed, placed in a silica crucible and heated in close furnace at temperature 217°C for about 5 h. The melt was then cooled and the mixed crystals were crushed to a fine powder.