**4.1. Elimination of exciplex emission by introducing an intermediate layer between the hole-transporting and the emitting layers**

To prove the exciplex origin of the long-wavelength EL, we have fabricated several control devices in which the long-wave EL bands are eliminated.

One of the methods for preventing exciplex emission is the insertion of an additional layer between the hole-transporting and electron-transporting materials [22,25-27]. CBP is considered as one of the materials appropriate for such layers [27,54,55]. We have fabricated two control devices with Zn(PSA-BTZ)2 as emitting layer and CBP as the intermediate layer: ITO/PTA/NPD/CBP/Zn(PSA-BTZ)2/Al:Ca (device D3) and ITO/PTA/CBP/Zn(PSA-

BTZ)2/Al:Ca (device D4). Figure 2 shows the EL spectra of devices D3 and D4 (Figure 2a, curve 3 and Figure 2b, curve 2, respectively). In both cases, the EL spectra contain no wide band around 560 nm and exhibit only one band in the blue region with the maximum at 471 nm (device D3) and 469 nm (device D4), which may be attributed mainly to the intrinsic emission of the Zn(PSA-BTZ)2 complex.

Similar to the devices based on Zn(PSA-BTZ)2, the exciplex band can be eliminated by introducing the intermediate layer of CBP between NPD and Zn(POPS-BTZ)2 or PTA and Zn(TSA-BTZ)2. The EL spectrum of the device ITO/PTA/NPD/CBP/Zn(POPS-BTZ)2/Al:Ca (device D9) is shown in Figure 4 (curve 2). The exciplex band in the region of 540 nm is absent, and only the intrinsic emission of Zn(POPS-BTZ)2 at *λ*max = 460 nm is observed. The EL spectrum of the device ITO/PTA/CBP/Zn(TSA-BTZ)2/Al:Ca (device D12) is shown in Figure 3b (curve 2). The exciplex band in the region of 580 nm is absent, and only the intrinsic emission of Zn(TSA-BTZ)2 at *λ*max = 465 nm is observed.
