Highly selective room-temperature NO2 sensors based on a fluoroalkoxy-substituted phthalocyanine

Abstract

A new n-type 2(3),9(10),16(17),23(24)-tetrakis(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyloxy)phthalocyanine H₂[Pc(OCH₂(CF₂)₆CF₃)₄] (1) is designed and synthesized. The introduction of electron-withdrawing pentadecafluorooctyloxy chains onto the peripheral positions of the phthalocyanine macrocycle successfully realizes the n-type organic semiconducting nature of the compound. Two kinds of films are fabricated by simple drop-casting (DC) and vacuum deposited (VD) methods, respectively. Examination by spectroscopic methods and morphology observation revealed that H₂[Pc(OCH₂(CF₂)₆CF₃)₄] molecules in the DC film adopt the J aggregation mode, whereas those in the VD film adopt the H aggregation mode, with the increasing molecular stacking order of DC < VD films. Moreover, these two types of films are used for low concentrations of NO₂ detection. The highly sensitive n-type responses to NO₂ gas in the range of 100 ppb to 1 ppm are obtained within a dynamic exposure period of 1 min at room temperature. Depending on the favorable hydrogen bonding between the analytes and the fluoroalkoxy groups or phthalocyanine core of compound 1, excellent selectivity for electrophilic NO₂ against electrophobic NH₃ and H₂S is achieved successfully. The responses of the compound in the DC and VD films are all linearly correlated to the NO₂ concentration from 100 ppb to 1 ppm with a significant increase in sensitivity from 7.54% ppm⁻¹ for the former to 12.05% ppm⁻¹ for the latter. The present work not only implies the great potential of the fluoroalkoxy-substituted phthalocyanine derivatives in the field of chemical sensors, but more importantly provides a new strategy through the elaborate molecular design combined with a suitable film fabrication technique to obtain ultrasensitive and selective sensors for the detection of trace amounts of gas.