# Octachlorinated Metal Phthalocyanines (M = Co, Zn, VO): Crystal Structures, Thin-Film Properties, and Chemiresistive Sensing of Ammonia and Hydrogen Sulfide

## TL;DR

This study explores octachlorinated metal phthalocyanines as gas sensors, showing they can detect ammonia and hydrogen sulfide with high sensitivity and selectivity.

## Contribution

The first structural analysis of CoPcCl8 and demonstration of spin-coated films' superior sensing performance for ammonia and hydrogen sulfide.

## Key findings

- Spin-coated films of octachlorinated phthalocyanines show n-type semiconducting behavior with high conductivity.
- ZnPcCl8 has the highest ammonia response (45.3% at 10 ppm), while CoPcCl8 is most sensitive to hydrogen sulfide (LOD = 0.3 ppm).
- DFT calculations confirm the n-type character and band gaps of the materials.

## Abstract

Octachlorinated metal phthalocyanines (MPcCl8, M = Co, Zn, VO) represent an underexplored class of functional materials with promising potential for chemiresistive sensing applications. This work is the first to determine the structure of single crystals of CoPcCl8, revealing a triclinic (P-1) packing motif with cofacial molecular stacks and an interplanar distance of 3.381 Å. Powder XRD, vibrational spectroscopy, and elemental analysis confirm phase purity and isostructurality between CoPcCl8 and ZnPcCl8, while VOPcCl8 adopts a tetragonal arrangement similar to its tetrachlorinated analogue. Thin films were fabricated via physical vapor deposition (PVD) and spin-coating (SC), with SC yielding highly crystalline films and PVD resulting in poorly crystalline or amorphous layers. Electrical measurements demonstrate that SC films exhibit n-type semiconducting behavior with conductivities 2–3 orders of magnitude higher than PVD films. Density functional theory (DFT) calculations corroborate the experimental findings, predicting band gaps of 1.19 eV (Co), 1.11 eV (Zn), and 0.78 eV (VO), with Fermi levels positioned near the conduction band, which is consistent with n-type character. Chemiresistive sensing tests reveal that SC-deposited MPcCl8 films respond reversibly and selectively to ammonia (NH3) and hydrogen sulfide (H2S) at room temperature. ZnPcCl8 shows the highest NH3 response (45.3% to 10 ppm), while CoPcCl8 exhibits superior sensitivity to H2S (LOD = 0.3 ppm). These results suggest that the films of octachlorinated phthalocyanines produced by the SC method are highly sensitive materials for gas sensors designed to detect toxic and corrosive gases.