Tetrapyrazinoindoloporphyrazine Langmuir-Blodgett Films

Abstract

Phthalocyanines are attractive due to their advantageous properties such as chemical and thermal stability, optical and electronic properties, and ordered crystal film growth. They have found potential applications in the field of organic light emitting diodes, organic field effect transistors, organic solar cells and sensors.[1,2] Introduction of two nitrogen atoms into the equivalence of the 1,4-positions of the phthalocyanine benzene units leads to tetrapyrazinoporphyrazines. Owing to the additional nitrogen atoms, tetrapyrazinoporphyrazines are less electron-rich than the corresponding phthalocyanines. Tetrapyrazino[2,3-b]indoloporphyrazines have been synthesized by substituting an indole group at the 2,3 position of the pyrazine. (Figure 1) These molecules behave as push-pull intramolecular charge-transfer chromophoric systems in which the indole ring works as a donor group and the pyrazine ring as an acceptor group.[3] In this work, we have fabricated tetrapyrazino[2,3-b]indoloporphyrazine films using Langmuir-Blodgett (LB) technique and analyzed the films using X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS), electron loss spectroscopy (ELS), UV-vis spectroscopy, atomic force microscopy (AFM), and contact angle measurement. Figure 2 shows UV-vis spectra of copper and metal-free tetrapyrazino[2,3-b]indoloporphyrazine LB multilayer films on quartz. The intense Soret bands appeared at near 360 nm with a slight shoulder at 440 nm can be assigned to the fused pyrolopyrazine ring structure. Broad Q bands are observed at visible region. XPS has been used to verify the formation of tetrapyrazino[2,3-b]indoloporphyrazine LB films on Au/Si(111) substrates. XPS survey spectra of the LB films (not shown here) detected C 1s and N 1s peaks for a metal-free LB film and C 1s, N 1s, and Cu 2p peaks for copper-coordinated LB films. Deconvolution of N 1s peaks reveal the changes in the electronic structures of the film caused by addition of copper into the center of the tetrapyrazinoindoloporphyrazine. UPS has been used to compare the valence electronic states of the copper and metal-free tetrapyrazino[2,3-b]indoloporphyrazine LB films. In the case of multilayer LB films, HOMO peaks are appeared at 1.4 eV, however, in the case of monolayer LB films, the HOMO peaks are shifted to lower binding energy due to the existence of the band bending near the interface. ELS, combined with UV-visible spectroscopy, has been performed to calculate the electronic bandgaps of the films. Contact angle measurement for the LB films on Au/Si(111) were performed using water as the contacting liquid. The measured contact angles for UV/Ozone cleaned substrate, copper tetrapyrazino[2,3-b]indoloporphyrazine monolayer , and multilayer LB films are 18, 78, and 83°, respectively. AFM has been used to reveal the surface roughness of LB monolayer on Au/Si(111). The measured surface roughness of UV/Ozone cleaned substrate and copper tetrapyrazino[2,3-b]indoloporphyrazine monolayer on Au/Si(111) are 6.8 and 10.5 ?, respectively.

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