We have successfully observed the ESR signals of radical cations in the thin films of N,N¤-di(1-naphthyl)-N,N¤-diphenylbenzidine (NPB), typical hole-transport material, for the first time to our best knowledge. In order to characterize the radical cation state, ESR spectra obtained upon chemical doping with iodine are analyzed combined with density functional theory (DFT) calculation. The ESR line width is inversely correlated with doping concentration.Organic light-emitting diodes (OLEDs) have been actively studied because of their excellent characteristics such as high efficiency, low cost productivity, and flexibility.1,2 Aromatic amines are widely used as hole-transport materials in the devices. Among them, N,N¤-di(1-naphthyl)-N,N¤-diphenylbenzidine (NPB) (Scheme 1) is a typical one with high thermal stability.3 Blue-color luminescence was also reported in an OLED containing NPB layer.4 In order to understand the holetransport properties, it is quite important to elucidate the nature of cationic state. Since cations are usually accompanied by radical spins, electron spin resonance (ESR) spectroscopy is a useful technique for this purpose. Although the thin films of NPB have been characterized by various methods like photoemission, 5 time-of-flight, 6 infrared, 7 and Raman spectroscopies, 8 the ESR study of NPB film has not been reported to our best knowledge. We have utilized this technique for other organic materials in thin films 9 and in organic devices. 1012 In the present letter, we carried out chemical doping by exposing the NPB films to iodine vapor and successfully observed ESR signals arising from the induced radical cations. We also performed density functional theory (DFT) calculations to obtain insights to the radical cation state.By thermal vacuum evaporation under the pressure of 4 © 10 ¹4 Pa, NPB was deposited on a quartz substrate cleaned with isopropyl alcohol and acetone. The NPB film is 80nm in thickness and 0.84 cm 2 in area. Subsequently, the film was doped using iodine vapor under vacuum of 1 © 10 ¹1 Pa for 1 h, then it was sealed in an ESR sample tube. The doping concentration was reduced by exhausting the sample tube using a diffusion pump and was controlled by the length of the exhaustion time. ESR measurements for the sample were performed with a JEOL JES-FA200 ESR spectrometer.13 DFT calculations of the B3LYP/6-31G(d) level were carried out for an isolated NPB cation.14 The molecular geometry was fully optimized. The principal values of g tensor were computed using the gauge-independent atomic orbital method. Figure 1 shows observed ESR spectra: (a) spectrum observed right after the iodine doping and (b) that measured after an exhausting time of five hours. As seen in this figure, clear ESR signals were observed in both cases. In Figure 1a, g value, peak-to-peak ESR line width ¦H pp , and spin concentration of the ESR spectrum are 2.0040, 0.676 mT, and 2.65 © 10 20 cm
¹3, respectively. Meanwhile, in Figure 1b, they are 2.0036, 1.06 mT, and 2.12 © 10 19 cm
¹3, respectively. The spi...
