Ethylthio-substituted sandwich phthalocyaninato europium (III) semiconductors for sensing NO2 and NH3: Effect of the extended π-conjugate systems on tuning the conductivity and sensing behavior

Abstract

By using the π-conjugated phthalocyanine macrocycle as the versatile building block, a series of five sandwich-type ethylthio substituted phthalocyaninato europium complexes, namely double-decker Eu[Pc(SC 2 H 5 ) 8 ] 2 ( Pc-1 ), triple-decker Eu 2 [Pc(SC 2 H 5 ) 8 ] 3 ( Pc-2 ), and their corresponding dimers, [Pc(SC 2 H 5 ) 8 ] 2 Eu 2 [BiPc(SC 2 H 5 ) 12 ] ( Pc-1@Pc-1 ), [Pc(SC 2 H 5 ) 8 ] 3 Eu 3 [BiPc(SC 2 H 5 ) 12 ] ( Pc-1@Pc-2 ) and [Pc(SC 2 H 5 ) 8 ] 4 Eu 4 [BiPc(SC 2 H 5 ) 12 ] ( Pc-2@Pc-2 ), are synthesized and prepared into the solution-processed films by a simple quasi-Langmuir-Shäfer (QLS) method. Combination between the extending π-conjugated system in the longitudinal and transverse directions of Pc macrocycles and/or radical nature of Pc-1 unit among different semiconducting molecules result in unusually small energy gaps (0.345–0.91 eV). Consequently, all of the semiconductors exhibit excellent conductivities. Among these materials, the conductivity for the radical species Pc-1@Pc-1 , Pc-1@Pc-2 and Pc-1 is about 3–4 times larger than that for the non-radical compounds Pc-2@Pc-2 and Pc-2 . Moreover, the QLS films of five semiconductors take excellent linear responses for both oxidizing NO 2 (100–300 ppb) and reducing NH 3 (4–8.6 ppm). Respectively, the sensitivity (in % ppm −1 ) gets increased in the order of Pc-1 < Pc-2 < Pc-1@Pc-1 < Pc-1@Pc-2 < Pc-2@Pc-2 for NO 2 , and Pc-1@Pc-2 < Pc-1 < Pc-1@Pc-1 < Pc-2@Pc-2 < Pc-2 for NH 3 . Depending on the highly extended π-conjugated systems, Pc-2@Pc-2 and Pc-2 films achieve the highest sensitivity of 208.2% ppm −1 and 0.97% ppm −1 to NO 2 and NH 3 , respectively. In addition, with a less than 2 min response time within a limit of detection at 10 ppb for NO 2 and 0.48 ppm for NH 3 , good reproducibility and selectivity have been revealed for the Pc-2@Pc-2 and Pc-2 films among the best gas sensors obtained so far for all the solution-processed films based on organic semiconductors in dry air at room temperature. More importantly, it is firstly demonstrated that the high NO 2 sensing is resulted from low Oxd 1 , and high NH 3 sensing is resulted from high Red 1 among the sandwich Pc-based semiconductors. With the highly extended π-conjugated systems of Pc, Pc-2@Pc-2 and Pc-2 films achieve the highest sensitivity to NO 2 and NH 3 . • Five sandwich phthalocyanines with extending π-conjugate systems are synthesized. • Best sensing performances are achieved to NO 2 in ppb and NH 3 in sub-ppm levels. • High NO 2 sensing is determined by low Oxd 1 , and high NH 3 sensing by high Red 1 . • Increasing in conductivity matches well with decreased trend of their energy gaps. • A new opportunity is offered for development of low-cost highly sensitive sensors.