Subphthalocyanines for Visible-Light-Driven Hydrogen Evolution: Tuning Photocatalytic Performance with Molecular Design

Abstract

In this work, a series of subphthalocyanines (SubPcs) with a carboxylic acid anchoring group at the axial position were used as photosensitizers of TiO2 for photocatalytic hydrogen evolution from water under visible light irradiation. SubPc derivatives with various peripheral substituents were successfully prepared to systematically investigate the dependence of photocatalytic performance on electron-donating units (i.e., bisthiophene or thioether) at the peripheral position of the SubPcs. SubPc 2/TiO2/Pt shows the best photocatalytic activity among the three dye-sensitized photocatalysts, with a hydrogen evolution rate of 1.104 mmol·g–1·h–1. After 24 h irradiation, SubPc 2/TiO2/Pt achieved a remarkable catalytic activity for the production of H2 (19.96 mmol·g–1) with a TON value of 40 734 and a high STH efficiency of 2.1%. Density functional theory (DFT) and time-dependent DFT (TD-DFT) approaches were used to elucidate further structural and electrical properties, including the interaction patterns of tailored SubPcs. It is worth noting that the theoretical computations exhibit good conformity with the empirical data. The predicted fluctuations in photocatalytic activity detected in SubPc systems were shown to be closely associated with frontier molecular orbital (FMO) characteristics, noncovalent interaction (NCI) patterns, and the electron-donating nature of the fragments located at the peripheral positions.

Solar light-driven hydrogen evolution by co-catalyst-free subphthalocyanine-sensitized photocatalysts

“…The highest occupied molecular orbital energy levels ( E HOMO ) of SubPc were calculated by subtracting the optical bandgap from the E LUMO value according to our previous reports. 22 The LUMO levels of all SubPcs were determined to be around −0.74 V and exhibited negligible variation among the different peripheral substituents. As expected, electron-donating SC 3 H 7 and amine units increased the HOMO energy level of SubPc 1 and 3 compared to the levels of SubPc 2 and 4 (Fig.…”

“…19 In our previous works, many SubPc derivatives were employed as sensitizers in photocatalytic hydrogen production systems, yielding highly promising results thanks to their suitable HOMO–LUMO levels and strong light absorption properties. 20–23 In particular, higher photocatalytic activities were obtained with SubPc derivatives having electron-donating substituents (Scheme 1). 22…”

“…20–23 In particular, higher photocatalytic activities were obtained with SubPc derivatives having electron-donating substituents (Scheme 1). 22…”

“…The SubPc derivatives were synthesized following our previous procedure 20–22 and are illustrated in Scheme 2. The axial functionalization of SubPcs with carboxy groups was carried out in two steps.…”

“…The peripheral amination of this SubPc was accomplished by attaching two bis(4-methoxyphenyl)amine groups to the SubPc ring, according to the literature procedure. 20–22 In the final step, the Sonagashira coupling reaction between the 4-ethynyl benzoic acid group and the monoiodo SubPc derivative was conducted in toluene in the presence of Pd 2 (dba) 3 , BINAP, and Cs 2 CO 3 to obtain the target product. Further details about the synthesis of SubPcs can be found in the ESI †…”