Progress on Hybrids and Mixed Alloys of Boron Subphthalocyanines (BsubPc) and Boron Subnaphthalocyanines (BsubNc); Tuning the Electrochemical Properties Towards Organic Electronic Applications
Authors: Bender Timothy P , Francesca Farac Nina , Bukuroshi Esmeralda 
Published: September 2023

Our research and development focus has been on the molecular design, synthetic methodology and in-depth characterization of boron subphthalocyanines (BsubPcs) and subnaphthalocyanines (BsubNcs) to apply them to organic electronic applications. BsubPcs, BsubNcs and their hybrids are macrocycles with a chelated central boron atom via nitrogen bonding, and is a p-conjugated macrocyclic material. Our focal point balances between the basic and applied chemistry, chemical engineering, their in-depth physical properties (electrochemistry included). We also do computational modelling. Once materials are developed, we merge them into organic electronics to function as light emitting, light absorbing and electronic conducting materials within organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs)/organic solar cells (OSCs). The basic electrochemical and photophysical properties, developing and evaluating the properties is critical to move onto such applications.

For this presentation, I will focus on our progress on the development of hybrids and mixed alloys of BsubPcs and BsubNcs.

Regarding hybrids, they are a mixture of BsubPcs and BsubNcs, and are formed via a reaction of a statistical distribution of phthalonitrile and 2,3-dicyanonaphthalene intermediates on reaction with BCl3; we have named them X-RnBsub(Pcx-Ncy); X the axial substituent, Rn the number of peripheral substitutions, Pcx being the number of 6 p-conjugated bonds, Ncy being the number of 10 p-conjugated bonds (Figure). On first study, we found that X-RnBsub(Pc1-Nc2) have low photostability. Therefore, we moved onto developing a ‘statistical’ synthetic methodology to target highest yields of X-RnBsub(Pc2-Nc1)s, and we did achieve relatively high yields. X-RnBsub(Pc2-Nc1)s therefore could be purified to enable physical characterization. Within a couple past publications, the unique absorption spectra was noted and we have confirmed its uniqueness. The X-RnBsub(Pc2-Nc1)s absorption spectra is as wide as if BsubPc and BsubNc were mixed together. This therefore has good potential to harvest a larger scope of photons from solar or to achieved FRET transfer from a host to emit via an OLED. We then did in-depth first electrochemical characterization, confirmed their electrochemical oxidation and reduction stability and also did in-depth photoluminescence characterization. Therefore, merging X-RnBsub(Pc2-Nc1)s into organic electronic devices as well as other chemistry to synthesis the hybrids is ongoing.

Regarding mixed alloys, in the past as we have shown that BsubNcs ended up being a mixed alloyed composition with random bay-position halogenation, that is formed during the reaction of BCl3 with 2,3-dicyanonaphthalene at temperature, which is the process to form the BsubNcs. The random bay-position halogenation has been shown to be impactful in a positive way within OPV devices, negative within OLED devices and also has electrochemical variations and good reversibility. Ongoing, given it is random halogenation, and some of the positive outcomes, this justifies considering additional mixed alloyed compositions.

We have also recently shown that mixed alloyed BsubPcs can be formed with three peripheral halogens in the alpha (a) or beta (b) positions, named as Cl-αCl3BsubPc, Cl-βCl3BsubPc, Cl-αF3BsubPc, and Cl-βF3BsubPc. The alloyed mixtures consisted of C1 and C3 isomers of trihalogenated BsubPcs. The physical properties characterization including photophysics, electrochemistry, and thermal characterization, were studied and were also positive and then were engineered into OSCs as electron donors and electrons acceptors to clarify their impact on device performance which had positivity aspects. Therefore, we moved onto developing a set of additional BsubPc mixtures. To be outlined for this presentation, we considered sets of phthalonitrile mixtures to form peripherally halogenated BsubPc mixtures during the formation of BsubPcs from these intermediates. For example, 4,5-dichlorophthalonitrile (Cl2-pn), 4,5-difluorophthalonitrile (F2-pn), tetrachlorophthalonitrile (Cl4-pn), and tetrafluorophthalonitrile (F4-pn) are commercially available compounds that were mixed together and yields a mixture of BsubPc which we named: X-Cl2nF2mBsubPc and X-Cl4nF4mBsubPc (X = Cl or F being a variant, Figure). We then did physical characterization and found positive aspects including electrochemical oxidation and reduction stability. Merging them into organic electronic devices is ongoing and might be present during this presentation time.

We did also recently develop additional chemistry methodology to develop additional mixed alloyed BsubPcs. The methodology was to develop phthalonitrile intermediates that have numbers of hydrogen, chlorine and fluorine substitutions and to take them onto the formation of the BsubPc mixed alloys. This is ongoing and may have some aspects to outline during this presentation.

We also do computational modelling to look into other relative impacts of the materials being developed, for example the HOMO and LUMO orbital distributions, the local ionization etc. This will also be outlined during this presentation.

Figure 1