Soret emission from water-soluble porphyrin thin films: effect on the electroluminescence response Marta P erez-Morales, * a Gustavo de Miguel, a Henk J. Bolink, b Marı ´a T. Martı ´n-Romero a and Luis Camacho a Received 30th January 2009, Accepted 9th April 2009 First published as an Advance Article on the web 15th May 2009 DOI: 10.1039/b902066g The emission properties of TSPP [tetrakis(4-sulfonatophenyl) porphyrin] and TMPyP [tetrakis(4-methylpyridyl) porphyrin] in thin films, prepared both by the Langmuir–Blodgett technique and the spin-coating method, have been investigated. Surprisingly, in most of the samples analyzed, the emission spectra do not show the usual two bands typical of porphyrins (in the region 650–750 nm), and depend strongly on the excitation energy. The origin of a new emission band detected at 480–500 nm is discussed in terms of the nature of the porphyrin aggregates. Moreover, we demonstrate that the presence or absence of this band in the fluorescence spectrum is directly related to the generation of electroluminescence in organic light-emitting diodes (OLEDs), built from such porphyrin films as emitting layers. Introduction The synthesis of organic emitting materials has been significantly developed in the last few decades, partly due to the great progress made in electroluminescence device technology, since the first report from researchers at Kodak in 1987. 1 Although emitting materials at high energies are very versatile in terms of their potential use as host-emitters (emitter itself) or dopants, the use of red-emitters to be used as dopants in OLEDs is practically limited. 2 Red fluorophores acting as host-emitters, which are usually polar or non-polar molecules but p-conjugated systems, tend to aggregate in the solid state, due to either attractive dipole–dipole interactions or effective intermolecular p-stacking. As a result of such possible phenomena, concentration quench- ing takes place and leads to, in most cases, weakly or non- emissive thin layers. To solve this problem one may either adopt a guest–host doped emitter system or avoid the formation of red- emitter aggregates in the films. The second option can be obtained by controlling the molecular organization and orien- tation in thin films which can be achieved by the Langmuir– Blodgett (LB) technique. The LB technique thus provides a promising and versatile method for the construction of molecular devices. 3 Chowdhury et al. 4 reported an electrolumi- nescent device exclusively built from LB multilayers containing an amphiphilic TMPyP derivative as a red-emitter, and studied the influence of the transfer surface pressure, i.e., the possibility of molecular aggregation, on the characteristics of the device. Luminescence properties of metallic and free-base porphyrins have been widely described. 5 Commonly, typical emission spectra of porphyrins consist of two emission peaks, Q(0,0) and Q(0,1), which are mirror images of their respective absorption bands at Q(0,0) and Q(1,0). Porphyrins generally have a rela- tively large energy gap between the first excited singlet state S 1 (Q band) and the second one S 2 (Soret band). Furthermore, in contrast to S 1 , the Soret band corresponds to a strongly allowed transition, so that the S 2 / S 0 radiative transition is very fast. For these reasons, porphyrins are expected to fluoresce not only from the first excited state, but also from the S 2 band. Soret emission in porphyrins was firstly observed by Bajema et al. for Zn tetrabenzoporphyrin (ZnTBP) in octane. 6 After this pio- neering work, many investigations concerning the S 2 fluorescence of porphyrins have been reported, although all of them were related to metallic derivatives in solution. 7–9 In these reports, the origin of the S 2 emission of diverse porphyrins is discussed, paying special attention to the factors determining the rate of S 2 – S 1 internal conversion. It was found that this rate is determined not only by the (S 2 –S 1 ) energy gap, but also by the frequency mode. 10 Kurabayashi et al. 8 attributed the Soret emission observed in several metalloporphyrins, and the absence of this band in the metal-free analogues, to the lower vibration frequencies of the N–metal bonds, compared to the central N–H bonds in metal-free rings, as it leads to longer lifetimes. Soret emission of non-metallic porphyrins has been rarely observed. 9,11 Akimoto et al. found B-emission in a free-base porphyrin, but it was much weaker than the Q fluorescence (Ø F Q / Ø F B 400), 9 and as a consequence the lifetime of the S 2 state could not be estimated due to the low intensity of the band. More recently, the S 2 emission of other free-base porphyrins has been demonstrated. 11 In the case of a tetraoxaporphyrin derivative, B- emission is considerably stronger than Q, but not for the unsubstituted porphyrin H 2 P and its corresponding diproto- nated species H 4 P 2+ . Also, Soret emission from a non-metallic porphyrin linked quinoxaline in basic solution has been reported by Kim and Jaung. 12 Schick observed, for the first time, S 2 emission from Zn and Cu porphine derivatives in monolayers assembled by the LB technique, which was drastically enhanced over the S 2 fluores- cence measured in solution, which possibly originated from a Departamento de Qu´ ımica F´ ısica y Termodin amica Aplicada, Universidad de C ordoba, Campus de Rabanales, Ed. Marie Curie, C ordoba, Spain E-14071. E-mail: qf2pemom@uco.es; Fax: +34 957 218618; Tel: +34 957 218618 b Instituto de Ciencia Molecular, Universidad de Valencia, Pol´ ıgono La Coma s/n, Paterna, Valencia, Spain E-46980 This journal is ª The Royal Society of Chemistry 2009 J. Mater. Chem., 2009, 19, 4255–4260 | 4255 PAPER www.rsc.org/materials | Journal of Materials Chemistry