White Luminescent Silica Layers: The Molecular Design Beneath

The development of white-light-emitting materials with improved colour stability holds great promise for the production of highly efficient large-area light sources with potential use in displays and lighting applications.[1-5] The replacement of conventional light sources with solid-state lamps based on lightemitting diodes (LEDs) has recently become a research area of general interest and is expected to have positive effects in terms of energy consumption.[6] White-light-emitting devices are usually obtained by the appropriate mixing of red, green and blue emitters, generally organized in multilayered structures. Another common approach involves the use of polymer/ polymer or polymer/small-molecule blends as emissive layers,[2] host materials doped with several fluorescent dyes or composite nanostructures.[5, 7-12] In recent years we have explored the possibility to prepare highly luminescent materials with simple architecture and colour tunability through incorporation of innovative lanthanide complexes in optically transparent single layers of silica glass.[13-16] Photoluminescent lanthanide antenna complexes provide sharp and intense emission lines upon ultraviolet (UV) light irradiation. The sensitization process involves the effective intramolecular energy transfer from the light-harvesting unit (antenna) to the Ln3+ ion from which the radiative deactivation of the excited state takes place (antenna effect).[14, 17, 18] Lanthanide antenna complexes have been employed for a wide range of photonic applications and as luminescent probes in biomedicine.[14, 17-21] The soft sol-gel solution process paves the way for the simultaneous doping of inorganic or hybrid matrices (host) with a variety of optically active species (guest),[9, 18, 20, 22, 23] and discloses the possibility to develop new materials and shaped nanostructures with tunable light-emitting properties. The material colour output can be designed a priori as a function of the chemical and photophysical behaviour (absorption and emission profiles, emission quantum yield) of the guest luminophores, their relative amount, the nature of the host matrix, and the excitation wavelength. We report herein on the preparation of white luminescent silica single layers by coupling the peculiar emissive properties of Ln antenna complexes (Ln3+_1, Figure 1a) [Ln= Eu (red), Tb (green), 1=10-[(4-methoxybenzoyl)methyl]- 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid] with efficient blue-emitting carbazole derivatives. Various carbazole derivatives have been widely used as hole transporting,[24] luminescent,[25, 26] and photoconductive[27] materials for optical applications.[24, 28] Among them, 9H-carbazole-9-ethanol (Cz, Figure 1 a) was identified as more suited to our purpose. The ab- sorption spectrum of Cz in dichloromethane (Figure 1b) shows the typical bands associated with the π-π* transitions.[15, 29]. © 2010 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.