Subphthalocyanine-type dye with enhanced electron affinity: Effect of combined azasubstitution and peripheral chlorination

https://doi.org/10.1016/j.dyepig.2020.108944Get rights and content

Highlights

  • Effect of azasubstitution in hexachlorinated subphthalocyanine has been studied.

  • Novel hexaazaanalogue of peripherally chlorinated subphthalocyanine has been prepared.

  • Azasubstitution strongly enhances the acceptor properties of the macrocycle and shifting the 1st reduction potential to −0.20 V.

  • Presence of nitrogen atoms in meso-positions or/and in fused rings determines solvatochromism in acid medium.

Abstract

Novel subphthalocyanine-type dye with enhanced electron-affinity was prepared by trimerization of 5,6-dichloropyrazine-2,3-dicarbonitrile in the presence of BCl3 in p-xylene. The obtained perchlorinated pyrazine fused subporphyrazine [Cl6Pyz3sPA] was characterized by mass-spectrometry, 13C and 11B NMR, IR and UV-VIS spectroscopy, cyclovoltammetry. Its molecular structure was confirmed by single crystal X-ray diffraction. To elucidate the combined effect of hexaazasubstitution and peripheral chlorination in subphthalocyanines on their electronic structure and spectral features, DFT and TD DFT calculations (B3LYP/pcseg-2 basis set) were used. Fusion of the pyrazine fragments instead of benzene rings strongly enhances the electron affinity of the macrocycle due to stabilization of the LUMO. Thus, the first reduction potential is observed at −0.20 V vs Ag/AgCl, that is macrocycle in [Cl6Pyz3sPA] is 0.5 V more easily reduced than in the peripherally hexachlorinated subphthalocyanine [Cl6sPc], the most popular subphthalocyanine-type acceptor for organic photovoltaics. The HOMO-LUMO gap is increased, so that the maximum of the Q band corresponding to the lowest ππ* transition 2a2→1e* shifts hypsochromically by 36 nm and appears at 535 nm. A distinct feature of the peripherally chlorinated macrocycles is appearance of two bands in the UV-region (B1 at ~300 nm and B2 at ~340 nm) corresponding to the electronic transitions 1a1→1e* and 2a1→1e*, respectively. Azasubstitution decreases fluorescence quantum yield, ΦF = 0.20 for [Cl6Pyz3sPA] and 0.37 for [Cl6sPc] in CH2Cl2. Spectrophotometric titrations indicate that unlike [Cl6sPc], that undergoes consecutive protonation of three meso-nitrogens in CH2Cl2-CF3COOH-H2SO4 medium, the presence of pyrazine rings in [Cl6Pyz3sPA] and their involvement in the acid-base interaction decrease the basicity of nitrogens in the meso-positions. The obtained data along with the photoelectric tests on the sublimed thin films point out at the prospects of [Cl6Pyz3sPA] for the photovoltaic applications.

Introduction

Organic molecules combining extended π-chromophore system with enhanced electron donor or electron acceptor properties can be used as functional materials in various fields of organic electronics. Thus, the search for an optimal donor-acceptor pair is very important for achieving high conversion efficiency in organic photovoltaic cells (OPVC). While conjugated semiconducting polymers with donor and acceptor moieties are widely utilized in the solution-based OPVC processing, the vacuum processed OPVC require low molecular weight semiconductors. Today, a large number of small-molecule semiconductors possessing p-type conductivity are known. These are porphyrins, phthalocyanines, polyarenes or squaranine dyes, which are effective as electron donors, but among n-type semiconductors (acceptors) fullerenes still prevail [1,2]. Novel stable non-fullerene acceptors can widen the selection of an effective D/A pair for the molecular heterojunction in organic photovoltaics [2] and can find applications as n-type materials in other fields.

Subphthalocyanines (sPc) (Chart 1) are non-planar macroheterocyclic compounds consisting of three isoindole fragments assembled around tetrahedral boron (III) center, which bears halogen atom or group (usually aryloxy) in the axial position [3,4]. While phthalocyanines (Pc) containing four isoindole units are mostly planar and strongly absorb visible light in the red region, subphthalocyanines have a bowl-shaped structure of the macrocycle and are distinguished by intense absorbance in the green region. Unsubstituted sPc are easily available and basically used as electron donors in OPVC [5,6], though there are examples of their accepting (n-type) behavior [7,8]. Partial or full halogenation of benzene rings stabilizes the frontier π-molecular orbitals (both HOMO and LUMO) and enhances the electron affinity of the subphthalocyanine molecule, thus making halogenated sPc an attractive alternative to fullerenes as electron-acceptors for photovoltaic applications [[8], [9], [10]]. By varying the nature, number and position of the halogen atoms in the macrocycle, one can not only shift the HOMO and LUMO downwards, but also change the HOMO-LUMO gap of the acceptor. In the OPVC design, this is a convenient approach to tune the interface gap energy and thereby to achieve the high open-circuit voltage [11]. Although the use of the perhalogenated subphthalocyanines ([F12sPc] [12]) or [Cl12sPc] [13]) increases the D/A interface gap in the junction, the most impressive results are currently obtained for peripherally hexachlorinated derivative, [Cl6sPc] [[14], [15], [16], [17]], see also [18] for review].

Direct substitution of carbons in the fused benzene ring with electron-accepting heteroatoms is another way to strongly influence the frontier MO levels, as demonstrated in our recent works on heterocyclic subphthalocyanine analogues containing instead of benzene rings fused π-electron deficient heterocycles - pyrazine [[19](b), [19], [19](a)] or 1,2,5-thia (selena)diazole [[20], [20](a), [20](b), [20](c), [21], [22]]). Halogenation of the fused pyrazine rings might further increase the electron-deficient nature and acceptor properties of the macrocycle. Recently, this trend was revealed for perchlorinated tetrapyrazinoporphyrazine and its metal complexes with AlIII GaIII and InIII [23]. The FeII [24] and SnIV [25] complexes were also reported and structurally characterized. In the present study, we report on the synthesis and characterization of hexachlorinated tripyrazinoporphyrazinatoboron (III) chloride, [Cl6Pyz3sPA] (Scheme 1), and compare its optical and chemical properties with those of the well-known subphthalocyanine analogue [Cl6sPc] (Chart 2).

Section snippets

General

Mass-spectrometric measurements were carried out on a MALDI TOF Shimadzu Biotech Axima Confidence and high-resolution ESI maXis Bruker mass-spectrometers in negative and positive modes. 11B and 13C spectra were measured with Bruker Avance 500 spectrometer. Electronic absorption spectra were recorded using a Cary 60 spectrophotometer. The IR spectra were obtained on a Cary 630 FT-IR spectrometer. Commercially available solvents were dried and distilled prior to use.

Synthesis

Synthesis and characterization

Subphthalocyanines and subporphyrazines are usually prepared as boron (III) complexes by template cyclotrimerization of the corresponding dicarbonitrile in the presence of BCl3 in p-xylene [3,4,14,19,20]. We have applied a similar approach for the synthesis of hexachlorotripyrazinosubporphyrazinatoboron (III) chloride, [Cl6Pyz3sPA] (see Chart 2 and Scheme 1) from 5,6-dichloropyrazine-2,3-dicarbonitrile. The dinitrile and BCl3 were taken in 1:1 ratio. A 3-fold excess of BCl3 was necessary since

Conclusions

A novel subphthalocyanine-type dye - perchlorinated pyrazine fused subporphyrazine [Cl6Pyz3sPA] was synthesized and fully characterized. Combined effect of hexaazasubstitution and peripheral chlorination in benzene rings strongly enhances the electron affinity of the subphthalocyanine macrocycle due to stabilization of the LUMO. The first reduction occurs at −0.20 V vs Ag/AgCl, that is by 0.50 V more easily than for peripherally hexachlorinated subphthalocyanine [Cl6sPc]. Azasubstitution

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by Russian Science Foundation (grant 17-13-01522). The authors are very thankful to Dr. Valentina Ilyushenkova (Institute of Organic Chemistry RAS) for her assistance in the measurements of high-resolution mass-spectra.

References (58)

  • C. Zhan et al.

    New advances in non-fullerene acceptor based organic solar cells

    RSC Adv

    (2015)
  • A. Meller et al.

    Phthalocyaninartige bor-komplexe

    Monatshefte Fur Chemie

    (1972)
  • Christian G. Claessens et al.

    Subphthalocyanines: singular nonplanar aromatic CompoundsSynthesis

    Reactivity, and Physical Properties

    (2002)
  • C.A. Dearden et al.

    High voltage hybrid organic photovoltaics using a zinc oxide acceptor and a subphthalocyanine donor

    Phys Chem Chem Phys

    (2014)
  • G.E. Morse et al.

    Boron subphthalocyanines as organic electronic materials

    ACS Appl Mater Interfaces

    (2012)
  • N. Beaumont et al.

    Boron subphthalocyanine chloride as an electron acceptor for high-voltage fullerene-free organic photovoltaics

    Adv Funct Mater

    (2012)
  • N. Beaumont et al.

    Acceptor properties of boron subphthalocyanines in fullerene free photovoltaics

    J Phys Chem C

    (2014)
  • H. Gommons et al.

    Perfluorinated subphthalocyanine as a new acceptor material in a small-molecule bilayer organic solar cell

    Adv Funct Mater

    (2009)
  • A. Mizrahi et al.

    Axial/peripheral chloride/fluoride-substituted boron subphthalocyanines as electron acceptors

    Inorg Chem

    (2020)
  • K. Cnops et al.

    Energy level tuning of non-fullerene acceptors in organic solar cells

    J Am Chem Soc

    (2015)
  • G.E. Morse et al.

    Fluorinated phenoxy boron subphthalocyanines in organic light-emitting diodes

    ACS Appl Mater Interfaces

    (2010)
  • J.S. Castrucci et al.

    Boron subphthalocyanines as triplet harvesting materials within organic photovoltaics

    J Phys Chem Lett

    (2015)
  • P. Sullivan et al.

    Halogenated boron subphthalocyanines as light harvesting electron acceptors in organic photovoltaics

    Adv Energy Mater

    (2011)
  • H. Lee et al.

    Interfacial electronic structure of Cl6SubPc non-fullerene acceptors in organic photovoltaics using soft X-ray spectroscopies

    Phys Chem Chem Phys

    (2017)
  • B. Verreet et al.

    Decreased recombination through the use of a non-fullerene acceptor in a 6.4% efficient organic planar heterojunction solar cell

    Adv Energy Mater

    (2014)
  • C. Duan et al.

    The role of the axial substituent in subphthalocyanine acceptors for bulk-heterojunction solar cells

    Angew Chem

    (2017)
  • G.L. Pakhomov et al.

    Hexachlorinated boron(III) subphthalocyanine as acceptor for organic photovoltaics: a brief overview

    Recent Adv. Boron-Containing Materials. IntechOpen

    (2020)
  • M. Hamdoush et al.

    Heterocyclic subphthalocyanine analogue – boron(III) subporphyrazine with fused 1,2,5-thiadiazole rings

    Macroheterocycles

    (2016)
    M. Hamdoush et al.

    Perfluorinated subphthalocynine analogues containing fused 1,2,5-thiadiazole fragments

    J Fluor Chem

    (2017)
    M. Hamdoush et al.

    Molecular sructure of 1,2,5-selenadiazolo- dibenzosubporphyrazinatoboron(III) chloride and influence of perfluorination and perchlorination on its spectral properties

    Macroheterocycles

    (2020)
  • G.L. Pakhomov et al.

    Thiadiazole fused subporphyrazines as acceptors in organic photovoltaic cells

    Macroheterocycles

    (2017)
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