Characterization, Second-Harmonic Generation, and Gas-Sensitive Properties of Langmuir-Blodgett Films of 1,8-Naphthalimide-tri-tert-butylphthalocyanine

Langmuir-Blodgett (LB) films of a new metal-free asymmetrically substituted phthalocyanine (Pc), 1,8-naphthalimide-tri-tert-butylphthalocyanine (NaBuPc), were fabricated and characterized by UV-vis spectrometry, atomic force microscopy, and small-angle X-ray diffraction (SAXRD). The results proved that the asymmetrically substituted Pc not only possesses good solubility in common organic solvents but also possesses an ideal LB film forming ability. From its surface pressure-area isotherm and SAXRD pattern, the configuration of the phthalocyanine molecules both at the air-water interface and in the LB films was demonstrated at a tilted angle (72°) to the water surface or solid substrate. The second-harmonic generation (SHG) of a threelayer LB film of NaBuPc was measured. Its SH intensity relative to a Y-cut quartz wedge  is approximately 1.76. The values of second-order susceptibility (x⁽²⁾) and the molecular hyperpolarizability (β) from the multilayers are 8.32 x 10^-9 and 5.97 x 10^-30 esu, respectively. Gas sensors based on the LB films of NaBuPc demonstrated good gas sensitivity to nitrogen dioxide at room temperature. A linear relationship between the logarithm of the concentration of NO₂ and the resistance of the sensor was observed, providing a basis for the application as gas sensors in the future.