Dinuclear rhenium complexes highly luminescent in solid state

The interest for luminescent materials able to efficiently emit in solid state is continuously growing, because in most applications the dyes are used as solid film. Concentration-quenching effects, affecting both organic and organometallic emitters [1-2] usually decrease PLQY in solid state. We present here a series of dinuclear Re(I) complexes whose emission is higher in solid state than in solution, and strongly depends on the nature of the halide ancillary ligand. These complexes belong to the family of neutral tricarbonyl Re(I) complexes with general formula [Re2(CO)6(-1,2-diazine)(-X)2], where X could be halogen or hydride [3]. Some of them have recently gained interest for their intense yellow/green emission, occurring from triplet metal-to-ligand charge transfer (3MLCT) states [4], showing a modulation effect of the diazine substituents on wavelengths, lifetimes and quantum yields of the emission. Photoluminescent quantum yield (PLQY) up to 0.53 have been measured for the di-chloro complexes containing diazines bearing alkyl groups in both the  positions [5]. These properties allowed their successful use as phosphorescent dopants in organic light emitting devices (OLEDs) [5]. The influence of the halide ancillary ligand on the photophysical properties has been previously discussed in solution [2, 4]. Differently from the di-chloro derivative 1, the analogous di-bromo and di-iodo derivatives 2 and 3 are almost no emissive. We have now found an opposite behaviour in solid state. In fact all the complexes are very brilliant, showing intense yellow-orange emission whose maximum shifts to higher energy on going from 1 to 3 (from 560 nm for 1 and 2 to 548 nm for 3) with PLQY relatively low for 1 and higher for 2 and 3 (up to 0.50 for complex 2). This very high Aggregation Induced Emission (AIE) [6] will be discussed in terms of the restriction of the intramolecular roto-vibrational motions of the “Re2(CO)6(-X)2” scaffold imposed by the rigid environment, as evidenced by the strong decrease of the knr. Moreover a parallel increase of kr is noticed, which clearly shows the influence of the halides on determining not only the energy of the excited state, but also the nature of the lowest (emitting) state [7].