A new approach to the synthesis of the sterically crowded photostable and fluorescent triphenodioxazines

Highlights

• A new expedient method is suggested for the preparation of photostable triphenodioxazines.

• The compounds absorb in the 500–600 nm spectral region and exhibit intense fluorescence.

• CV determined and DFT calculated HOMO/LUMO energy levels meet requirements of DSSCs.

Abstract

A new method for the synthesis of the pentacyclic triphenodioxazines (TPDOs) involving condensation of 6,8-di-(tert-butyl)-3H-phenoxazin-3-one with o-aminophenols has been suggested and a series of sterically crowded 2,4-di-(tert-butyl)-benzo[5,6][1,4]oxazino[2,3-b]phenoxazines (TPDOs) 4 has been prepared. The compounds are well soluble in nonpolar and polar solvents, absorb in the strongest emissive part of the solar spectrum, display intense fluorescence and are characterized by excellent photostability at air conditions. X-ray diffraction study reveals planar structures of heteropentacene cores TPDOs and π-π stacking of the molecules of some of them in crystal. The energy levels of the frontier MOs of the studied TPDOs assessed using the electrochemical methodology and calculated by the DFT B3LYP/6–311++G(d,p) method lie outside the energy domain confined by TiO₂ conductivity zone and redox-mediator I⁻/I₃⁻, thus meeting the requirements of dyes suitable for DSSCs. ESR and electron absorption and emission spectra of radical anion and dianion of 4 appeared under reduction with a potassium mirror in THF were reported and interpreted based of the DFT calculations.

Introduction

Synthesis [1] of the first benzo[5,6][1,4]oxazino[2,3-b]phenoxazine (triphenodioxazine, TPDO) 1 (R = R₁ = H) and confirmation of its structure [2] dated back to 19th century rapidly initiated industrial application of these compounds as commercial dyes and pigments which found widespread use in dyeing various textiles and leathers [3,4]. In the recent years, significant attention has been regained to TPDOs as promising materials for the development of organic electronics, particularly air stable n-type organic field-effect transistors (OFET) [[5], [6], [7]], luminescent devices [8] and dyes possessing spectral and redox properties suitable for using in dye-sensitized solar cells (DSSCs) [7,[9], [10], [11], [12]]. Originally, TPDOs were obtained by oxidation of o-aminophenol at very rigorous conditions (see Ref. [3] and refs. therein), whereas the later developed more convenient synthetic strategies are generally based on the condensation of tetrahalogeno-p-benzoquinone (R = Cl, Br) with arylamines [[13], [14], [15]] or nucleophilic substitution of dichloro-p-benzoquinone with the substituents (R = t-Bu, CCSi(i-Pr)₃) chosen to ensure better solubility and appropriate spectral parameters of the target compounds by potassium 2-nitrophenoxide followed by the subsequent oxidation and cyclization of the formed intermediate products [5,9]. A recently suggested modification of this approach involves reduction of 2-nitrophenoxides to the corresponding o-aminophenol derivatives condensed then with 2,5-dihydroxy-p-benzoquinone [11]. Scheme 1 provides a shortcut representation of the methods [6,7,[10], [11], [12], [13], [14], [15]] currently employed for the preparation of TPDOs 1.

Large aromatic molecules, such as porhyrines, perylenes, carbazoles and other, behave as excellent electron donors or acceptors and their structures and, consequently, optical, photophysical and electrochemical properties can be rationally tailored to meet the requirements of dye-sensitizers implemented in the DSSCs devices [[16], [17], [18], [19], [20]]. TPDOs demonstrating the photovoltaic conversion efficiencies (PCE) currently ranged from 4.30% to 6.30% [10,12] were shown to belong to this group of stable π-extended systems [6,7,[10], [11], [12]]. To improve the efficiency of TPDOs a larger number of this class compounds should be synthesized and tested by the properties critical for the dye-sensitizers of DSSCs. The crucial role of the photosensitizer consists in sunlight harvesting and electron injection from the LUMO level of excited dye into the conduction band of the photo active anode (usually mesoporous titanium dioxide). Therefore, the principal characteristics of a TPDO as a potential efficient sensitizer must include the intense absorption in the strongest emissive part of the solar spectrum (500–600 nm) and good solubility in nonpolar solvents. Electrochemical properties of the compound must be compatible with a LUMO energy level being lower than the low potential of the conduction band of the anode (approximately −0.5 V vs NHE in the case of TiO₂), and a reduction potential of the oxidized chromophore being higher than the potential for oxidation of I⁻ (in the case of the I⁻/I₃⁻ redox system) by approximately 0.5 V vs NHE to be able to regenerate the dye [16,17]. Of special importance are the air and photostability of a TPDO [18,21] determined by the persistent character of its radical cation formed in the course of the oxidation process.

The search for new TPDO compounds possessing the aforementioned properties should be based on the development of novel synthetic routes which would allow further improving the important characteristics of TPDOs and providing for increase in PCE. With this goal, we report in this study on a new straightforward methods for the synthesis of TPDOs which are based on condensation of preparatively accessible 3H-phenoxazin-3-ones [22,23] with various o-aminophenols 3 (Scheme 2) and the direct reaction of o-quinone with o-aminophenols with the formation of an intermediate 3-imino-3H-phenoxazin-2-ol (Scheme 3) [24].