Organic electroluminescence from singlet and triplet exciplexes: Exciplex electrophosphorescent diode M. Cocchi a , D. Virgili a , C. Sabatini a , J. Kalinowski b, * a Institute of Organic Synthesis and Photoreactivity, National Research Council of Italy (ISOF-CNR), I-40129 Bologna, Italy b Department of Molecular Physics, Gdan ´ sk University of Technology, ul. G. Narutowicza 11/12, 80-952 Gdan ´ sk, Poland Received 24 December 2005 Available online 21 February 2006 Abstract We have observed emission from singlet and triplet exciplexes in electron donor (D)–acceptor (A) organic electroluminescent systems. The exciplex singlet-to-triplet fraction appears to be a function of electronic properties of the (D–A) complex molecular components and applied electric field. The fluorescence from exciplex singlets could be eliminated using a heavy atom (Gd)-substituted acceptor molecule allowing the efficient intersystem crossing of singlet into triplet exciplex states. By this a pure exciplex electrophosphorescent diode has been realized. These findings are of importance for the understanding of the exciplex formation mechanism and, thus, for material choice in device applications. Ó 2006 Elsevier B.V. All rights reserved. If an electroluminescent (EL) emitter consists of two- or more component materials, particularly electron donors (D) and electron acceptors (A), two-molecule underlain excited states are formed when a D-located hole (D + ) approaches an A-located electron (A  ), we deal with exci- plexes [1], jDAæ * . They can be detected through the struc- tureless emission band shifted towards the red as compared with the spectrum of light emitted by each of interaction-free molecular (monomer) components. Exci- plexes are considered as the molecular base of the commu- nication network of the living systems [2]. They are found to be of importance for studying fundamental quantum physics and chemistry [3,4] as well as for many practical applications like exciplex fluorescence imaging technique [5], fluorescent molecular thermometry [6] or white light emitting displays [7]. Understanding the nature and prop- erties of exciplexes is therefore of great general interest and crucial to the design of exciplex-affected organic devices for use in displays, lasers and other illumination applications. Efficient exciplex emission has recently been observed with organic light-emitting-diodes (LEDs) [8]. The question arises whether it is possible to distinguish sin- glet and triplet exciplex emission and what would be the effect of their spin character on the EL quantum yield. Can the exciplex eletrophosphorescent (EPH) organic LEDs be realized? Here we report exciplex EL from a diamine derivative (TPD):bathocuproine (BCP):polycarbonate (PC) blend used as an emitter in single-layer LEDs. Our devices gener- ate either blue light characteristic of the emission of singlet 1 jTPD:BCPæ * and greenish-yellow light characteristic of the emission of triplet 3 jTPD:BCPæ * exciplexes. Using an organic complex of gadolinium as an electron acceptor a pure exciplex EPH organic structure has been realized. Generally, the exciplex is described by a function jDAi  ¼ c 1 jDAi  loc þ c 2 jDAi  CT ð1Þ expressing the quantum mechanical mixing of the locally (LOC) excited exciplex configuration jDAi  loc ¼ a 1 jD  Aiþ a 2 jDA  i, and charge-transfer (CT) exciplex configuration jDAi  CT ¼ b 1 jD þ A  iþ b 2 jD  A þ i. Whereas, the coefficients c 1 , c 2 determine the contributions of the LOC and CT con- figurations to the exciplex, the coefficients a 1 and a 2 are the amplitudes of the two component state of the LOC config- uration emerging as a result of the excitonic resonance 0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.01.082 * Corresponding author. Fax: +48 58 664 6535. E-mail address: kalino@polnet.cc (J. Kalinowski). www.elsevier.com/locate/cplett Chemical Physics Letters 421 (2006) 351–355