To assess the performance of the R‐AlPc materials as a cannabinoid sensing layer, they were integrated into OTFT‐based sensors. The R‐AlPc OTFTs were characterized before and after exposure to 300 ppm THC vapor, 300 ppm CBD vapor (Figures 4 and S11, Supporting Information), 20 × 10⁻⁶ m THC solution and 20 × 10⁻⁶ m CBD solution (Figure S12, Supporting Information) to determine how the device performance changed in response to cannabinoid exposure. A similar sensor response, in terms of trends, between the R‐AlPcs and THC and CBD solution and vapor was observed, thus for simplicity this section will focus on the discussion of THC and CBD vapor. Overall, compared to Cl‐AlPc, devices made with 345F‐AlPc, mCr‐AlPc and pCr‐AlPc experienced a greater change in µ _h, g _m and I _On/Off, while devices made with all phenoxy‐AlPcs experienced larger shifts in V _T compared Cl‐AlPc (Figure 4). The performance of these materials surpasses those previously reported using Cl‐AlPc,^{[ 12 ]} but does not quite reach the sensitivity achieved using ZnPc as the sensing layer.^{[ 13 ]} Other previously reported MPc cannabinoid sensors have reached similar sensitivities, but have required the addition of a sensitizing probe to the surface to achieve these results.^{[ 9} , ¹⁰ , ^{11 ]}

Additionally, some phenoxy‐AlPcs experienced selective responses to one cannabinoid over the other. However, the reason for this response is ultimately unknown. A selective response to THC and CBD has previously been reported for Cl‐AlPc and other MPcs in OTFTs,^{[ 9} , ¹⁰ , ¹¹ , ^{12 ]} and their interactions in solution have been investigated, suggesting strong coordination between the nitrogen ring and central metal of the Pc with the conjugated ring of the cannabinoids, causing changes in their electrical properties.^{[ 10 ]} It is unclear if these interactions in solution are reproduced in a thin‐film. The selectivity observed with the phenoxy‐AlPcs could be due to a combination of similar electrochemical interactions and the interactions of THC and CBD with the different crystal structures and morphologies of the phenoxy‐AlPcs. The absence of single crystal data for each of the phenoxy‐AlPcs makes it difficult to draw conclusions, but these results indicate that the choice of phenoxy group on the AlPc could be used to provide a more sensitive cannabinoid sensing layer in OTFT based sensors.

As previously observed, THC and CBD can coordinate strongly with R‐AlPc molecules, changing the Pcs electronic properties, and therefore the substituted phenoxy group could improve these interactions.^{[ 10 ]} Additionally, THC and CBD can induce structural changes in thin films, also leading to reductions in µ _h, I _On/Off, V _T, and changes in subthreshold slope and defect density.^{[ 11} , ¹² , ^{13 ]} Figure  5 represents the AFM, GIWAXS and OTFT transfer curves for 345F‐AlPc and Cl‐AlPc before and after exposure to THC or CBD vapor to investigate the effect of phenoxy functionalization of AlPc. The AFM images of 345F‐AlPc films show significant reorganization or recrystallization after exposure to THC or CBD, leading to larger crystals with wider grain boundaries and increased surface roughness from 3.9 to 14.8 nm and 18.9 nm, respectively. In comparison, the same exposure led to a reduction in grain size for Cl‐AlPc films and a drop in surface roughness from 10.3 to 5.6 nm and 9.06 nm for THC and CBD, respectively (Figure 5). Changes in grain size and film morphology can affect the number and quality of charge transport pathways available, altering the defect density and causing a reduction in performance as a result of structural interactions between THC or CBD with Pc thin films.^{[ 11 ]} An increase in grain size, as seen with 345F‐AlPc, can lead to a decrease in defect density due to the larger crystal structures (Figure S11, Supporting Information), but a simultaneous decrease in the quality of charge transport pathways due to the larger grain boundaries and more irregular grains, leading to the observed overall reduction in µ_h and V_T .^{[ 38} , ⁶¹ , ^{63 ]} A similar trend was observed with the other phenoxy‐AlPc films (Figures S15 and S16, Supporting Information). The orientation of the molecules within these grains may also help explain the reduced performance.

The 2D scattering patterns and the azimuthally integrated GIWAXS patterns of Cl‐AlPc thin‐films displayed in Figure 5 exhibit no large structural changes after THC or CBD exposure. However, exposure to THC and CBD vapor resulted in a change in molecular order of the Cl‐AlPc film with a slight increase in film crystallinity evident by the rounded reflection at q = 0.56 Å⁻¹, compared to the more arched shaped of the same reflection pre‐exposure to cannabinoids. These results are consistent with previous reports for Cl‐AlPc films upon exposure to THC and CBD (liquid solution or vapor).^{[ 12 ]} Conversely, 345F‐AlPc films experience significant microstructural changes while maintaining approximately the same degree of molecular order and crystallinity. The initial thin films display high intensity reflections at q = 0.61 Å⁻¹ and q = 0.65 Å⁻¹ along the q_z axis. When exposed to THC or CBD, a change in the ratio of these two peaks suggests a reorganization in the thin‐film. A change in the average molecular orientation was also detected from primarily ≈52° to a more mixed orientation with ≈25°, determined by linecuts with respect to χ (Figure 5). This means there could be a mix of molecules oriented more face‐on than edge‐on to the substrate upon exposure to cannabinoids, which is associated with reduced mobility and lower I _On.^{[ 13} , ^{38 ]} This can help explain the observed reduced µ _h with the larger crystallite size seen by AFM, which is generally associated with improved µ _h. Similar relative changes in peak intensities from the (100) plane at q = ≈0.50 Å⁻¹ and (10‐2) plane at q = 0.68 Å⁻¹ of β‐ZnPc films have been reported after exposure to THC vapor in conjunction with a shift in the average molecular orientation from ≈45° to ≈74° for β‐ZnPc molecules relative to the substrate.^{[ 13 ]} The more face‐on oriented films had a lower I _On/Off, and lower µ _h.^{[ 13 ]} While no single crystal data are available for the phenoxy‐AlPc derivatives, a similar shift in 345F‐AlPc molecular orientation is likely taking place with exposure to THC and CBD due to the similar changes detected by GIWAXS. In fact, all other phenoxy‐AlPc films experienced similar shifts in their film morphology (Figures S13 and S15–S17, Supporting Information), with pronounced grain restructuring and crystal reorganization similar to that of 345F‐AlPc. The integrated GIWAXS patterns demonstrate that the phenoxy substitutions promote more reorganization of R‐AlPc films upon THC vapor exposure that is not observed with Cl‐AlPc. By GIWAXS (Figure S10, Supporting Information) and XRD (Figure S14, Supporting Information), the d‐spacing of the phenoxylated‐AlPc films are smaller than Cl‐AlPc, suggesting smaller intermolecular distances. However, as evidenced by lower sublimation temperature, the phenoxy groups are likely disrupting the strength of the intermolecular interactions, corroborating the higher tendency for the phenoxy‐AlPc films to reorganize upon exposure to cannabinoids.

The R‐AlPc films were also exposed to a 20 × 10⁻⁶ m liquid THC solution, and 20 × 10⁻⁶ m CBD solution, where similar results were observed (Figures S18–S23, Supporting Information). Significant changes in the thin‐film morphology were observed for the phenoxy‐AlPc films, resulting in the larger changes observed in their device performance. Again, 345F‐AlPc experienced significant surface morphology changes in response to THC and CBD vapor resulting in an increase in grain size and surface roughness. This surface reorganization was less significant upon exposure to THC and CBD solutions, suggesting the solvent interferes with the surface interactions. These results demonstrate selective effects of THC relative to CBD which can facilitate ratiometric cannabinoid analysis,^{[ 9 ]} and the improvement of OTFT sensitivity to both cannabinoids with the implementation of phenoxylated AlPc.

Finally, Raman spectroscopy was used to further probe the interactions between the R‐AlPc molecules and cannabinoids in a thin film (Figure S24, Supporting Information). Upon exposure, there appears to be shifting of the peaks in both location and intensity around the isoindole region of the spectra, at ≈1530–1540 cm⁻¹.^{[ 64} , ^{65 ]} Spectra of THC and CBD alone were collected to confirm that these shifts were not caused merely by the presence of the cannabinoids (Figure S25, Supporting Information). These results indicate a change in the environment around the R‐AlPc molecules as a result of interaction with THC/CBD in the thin film, suggesting the interactions between the MPc and the analyte are present in the solid state.

Phenoxylation of R‐AlPc alters the crystal packing of R‐AlPc molecules, reducing the strength of the intermolecular interactions compared to Cl‐AlPc. As THC has been shown to coordinate with the nitrogen ring of AlPcs in previous work,^{[ 10 ]} the tight crystal structure is likely interrupted by the introduction of THC, causing more drastic shifts in film structure. Consequently, the addition of the phenoxy groups changed the response of AlPc to analytes and generated a more sensitive cannabinoid sensor through the introduction of new crystal structures. This demonstrates the utility of molecular engineering as a strategy to fine tune the thin‐film properties of Pcs for sensor optimization through the enhancement of Pc‐analyte interactions.