Conductive Directly Fused Metalloporphyrin Coatings By Chemical Vapour Deposition – from Singly to Triply Fused
Authors: Boscher Nicolas D. , Bengasi Giuseppe , Baba Kamal , Heinze Katja , Frache Gilles 
Published: February 2020

Porphyrins, whose central structure is composed of four pyrrole units joined by four methine bridges, have been selected by nature to fulfil the two main catalytic phenomena allowing life, i.e. photosynthesis and respiration. Their remarkable functional properties led to the development of numerous porphyrinic compounds for various technological applications, including catalysis and photocatalysis. Porphyrin’s properties can be adjusted by tuning the central metal ion or the pendant groups located at the meso or beta positions. The properties of porphyrin-based devices are also greatly dependent on their integration mode since porphyrins may be directly used dissolved in a liquid medium, covalently grafted onto a surface, embedded in a matrix or polymerised. Conjugated porphyrin polymers,[1] including directly fused porphyrins,[2] display a cooperative effect promoted by conjugated covalent links between porphyrins that yields superior catalytic performances.[1,2] However, the two main synthetic routes towards conjugated poly(porphyrins), (i) oxidative polymerization using a suitable oxidant or (ii) electrodeposition, are a major drawback to their integration into devices. Indeed, the weak solubility of porphyrin monomers, oligomers and polymers requires the introduction of bulky pendant groups to form conjugated poly(porphyrins) in which the tetrapyrrole moieties responsible for the targeted properties end-up diluted.

In this work, we report a straightforward chemical vapour deposition approach for the simultaneous synthesis and deposition of conjugated poly(porphyrin) coatings from a simple and commercially available metalloporphyrin, i.e. nickel(II) 5,15-(diphenyl)porphyrin (NiDPP).[3] Our approach relies on the vapour-phase delivery of NiDPP and an oxidant towards a substrate on which they adsorb and react to form conjugated polymer coatings. The direct fusion of NiDPP is notably demonstrated by ultraviolet-visible-near infrared spectrophotometry (UV-Vis-NIR) and high-resolution mass spectrometry (HRMS). Among the investigated oxidants, FeCl3 appears as the candidate of choice for the oxidative fusion of NiDPP in CVD and control of the reactants ratio alternatively allowed the formation of singly-fused P(NiDPP) coatings or multiply-fused (doubly and triply) P(NiDPP) coatings such as evidenced by HRMS. UV-Vis-NIR and conductivity atomic force microscopy (C-AFM) highlight the strong dependence of the optical absorption and electrical conductivity of the fused NiDPP films on the degree of conjugation, opening up the possibility to tune the optoelectronic properties of the P(NiDPP) coatings. Noticeably, the multiply fused P(NiDPP) tapes in thin film form exhibit an electrical conductivity up to 7·10–1 S·cm–1 coupled an absorption deep in the NIR.
Our gas-phase reaction approach considerably simplified the integration of fused metalloporphyrins into devices.[3] Following to the very recent report of the superior catalytic activity of fused copper porphyrin dimers,[2] the oCVD reaction of metalloporphyrins will allow the straightforward preparation of potent heterogeneous single‐site catalysts.
[1] J. Chen, P. Wagner, L. Tong, G. G. Wallace, D. L. Officer, G. F. Swiegers – A Porphyrin-Doped Polymer Catalyzes Selective, Light-Assisted Water Oxidation in Seawater – Angew. Chem. Int. Ed.
2012, 51, 1907.
[2] D. Khusnutdinova, B. L. Wadsworth, M. Flores, A. M. Beiler, E. A. Reyes Cruz, Y. Zenkov, G. F. Moore – Electrocatalytic Properties of Binuclear Cu(II) Fused Porphyrins for Hydrogen Evolution – ACS Catal.
2018, 8, 9888.
[3] G. Bengasi, K. Baba, G. Frache, P. Gratia, K. Heinze, N. D. Boscher – Conductive Fused Porphyrin Tapes on Sensitive Substrates by a Chemical Vapor Deposition Approach – Angew. Chem. Int. Ed.
2018, DOI: 10.1002/anie.201814034.

Figure 1