Synthesis of novel multifunctional carbazole-based molecules and their thermal, electrochemical and optical properties

Two novel carbazole-based compounds 7a and 7b were synthesised as potential candidates for application in organic electronics. The materials were fully characterised by NMR spectroscopy, mass spectrometry, FTIR, thermogravimetric analysis, differential scanning calorimetry, cyclic voltammetry, and absorption and emission spectroscopy. Compounds 7a and 7b, both of which were amorphous solids, were stable up to 291 °C and 307 °C, respectively. Compounds 7a and 7b show three distinctive absorption bands: high and mid energy bands due to locally excited (LE) transitions and low energy bands due to intramolecular charge transfer (ICT) transitions. In dichloromethane solutions compounds 7a and 7b gave emission maxima at 561 nm and 482 nm with quantum efficiencies of 5.4% and 97.4% ± 10%, respectively. At positive potential, compounds 7a and 7b gave two different oxidation peaks, respectively: quasi-reversible at 0.55 V and 0.71 V, and reversible at 0.84 V and 0.99 V. At negative potentials, compounds 7a and 7b only exhibited an irreversible reduction peak at −1.86 V and −1.93 V, respectively.

In this work, we designed two novel 2-(N-hexylcarbazol-3'-yl)-4/5-formylpyridine compounds (7a and 7b), where 4/5pyridinecarboxyaldehyde was attached to the 3-position of carbazole via the 2-position of the pyridine ring. These two compounds (7a and 7b) can be used in OLEDs, solar cells and sensor studies either directly or with small modifications. Here we report the synthesis, full characterisation and properties of these two novel compounds (7a and 7b).

Thermal properties
The thermal properties of compounds 7a and 7b were investigated by thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC). For TGA, compounds 7a and 7b were heated at 20 °C/min under nitrogen atmosphere. The decomposition temperatures (T d 5% ) corresponding to 5% weight losses for 7a and 7b were 291 °C and 307 °C, respectively. For DSC, compounds 7a and 7b were first heated to 450 °C and then cooled down to room temperature at 20 °C/min under a nitrogen atmosphere. Compounds 7a and 7b showed only clear melting transitions (T m ) at 95 °C and 86 °C, respectively. Upon first cooling and second heating, no phase transitions were observed at all. TGA and DSC curves of compounds 7a and 7b are depicted in Figure 1. Thermal properties of compounds 7a and 7b are also summarised in Table 1.  Electrochemical properties The redox behaviour of compounds 7a and 7b was investigated by cyclic voltammetry in dichloromethane solution under argon atmosphere using tetrabutylammonium hexafluorophosphate as the electrolyte (Figure 2). A platinum disk was used as a working electrode, silver wire as the reference electrode and platinum wire as the counter electrode. The ferrocene-ferrocenium redox couple was used as an internal reference. compound At positive potentials, compounds 7a and 7b exhibited two oxidation peaks; one is quasi-reversible at 0.55 V (7a) and 0.71 V (7b), and the other is reversible at 0.84 V (7a) and 0.99 V (7b). At negative potentials, compounds 7a and 7b only exhibited an irreversible reduction peak at −1.86 V and −1.93 V, respectively. The highest occupied molecular orbital and the lowest unoccupied molecular orbital energy levels (E HOMO , E LUMO ) of compounds 7a and 7b were also calculated from the half-way anodic oxidation and onset cathodic reduction peak potentials, with respect to the energy level of ferrocene (4.8 eV below vacuum level) [38] by using the following equations; [39] E HOMO = −(4.8 + E 1/2 ox ) and E LUMO = −(4.8 + E onset red ). The HOMO-LUMO energy gap was calculated both from electrochemical data using Equation 1 and from optical data using Equation 2 [25,40,41]. (1) The optical energy gap (E g o ) was higher than the electrochemical energy gap (E g e ) for compounds 7a and 7b. The oxidation and reduction potentials and the HOMO-LUMO energy levels of both compounds are summarised in Table 2 and the energy levels are depicted in Figure 3.

Optical properties
The absorption properties of compounds 7a and 7b were investigated in dichloromethane using a Duetta Fluorescence and Absorbance Spectrometer. Each compound (7a/7b) displayed three distinctive absorption bands in the UV-vis spectra: high energy bands and mid energy bands were assigned to π-π* and n-π* transitions, whereas the low energy bands were assigned to an intramolecular charge transfer (ICT) transition. The ICT band of 7b at 373 nm was more intense than the ICT band of 7a at 378 nm. This observation confirms that conjugation enhances ICT band intensity [42]. In 7b, the formyl group is at the para position to the carbazole ring, thus giving rise to conjugation. In 7a, however, the formyl group is at the meta position to the carbazole ring.
The photoluminescence (PL) properties of compounds 7a and 7b were investigated in dichloromethane using a Duetta Fluorescence and Absorbance Spectrometer. Compounds 7a and 7b gave emission maxima at 561 nm and 482 nm, respectively. The UV-vis and PL spectra of the compounds are given in Figure 4.

Solvatochromism
In general, ICT-based absorption and emission bands show solvent dependency. This is better known as solvatochromism. The ICT behaviour of compounds 7a and 7b was further investigated in different solvents. Normalised UV-vis spectra of compounds 7a and 7b in different solvents are depicted in Figure 5. The spectral profiles remained almost unchanged in different solvents, but there is greater variance in the spectra of compound 7b.
The PL spectra of compounds 7a and 7b displayed either dual emission bands or a single emission band. This was dependent on the excitation wavelength chosen and the solvent used. It is believed that the dual emission was due to mixed locally excited (LE) and intramolecular charge transfer (ICT) states and the single emission was due to the ICT state. Photoluminescence (PL) spectra of compounds 7a and 7b in different solvents are shown in Figure 6.
Upon excitation at π-π*/n-π* bands (λ exc = 245-349 nm for 7a and 248-356 nm for 7b), the PL spectra of compound 7a in most solvents depicted dual emission bands, one from the    locally excited state and one from ICT. On the other hand, the PL spectra of compound 7b in most solvents interestingly depicted only a single emission band from ICT. Upon excitation at the ICT band (λ exc = 370-377 nm for 7a and 367-378 nm for 7b), only a single emission band was observed for both compounds (7a and 7b). As seen in Figure 6, a redshift was observed in the emission maxima as the microscopic solvent polarity [43,44], E T (30), increased from toluene to dimethyl sulfoxide (see also Table 3). A 141 nm red-shift was observed for 7a (from 465 nm to 606 nm) and an 86 nm redshift was observed for 7b (from 423 nm to 509 nm). In comparison for 7b, this red-shift was more pronounced for 7a. This indicates that the excited state dipole moment is much greater than the ground state dipole moment.

Quantum yields
The relative fluorescent quantum yields (ϕ FL ) of compounds 7a and 7b were determined in dichloromethane by using rhodamine B (ϕ FL = 49% at λ exc =355 nm) in ethanol as reference [45]. ϕ FL of compounds 7a and 7b was 5.4% and 97.4%, respectively. An estimated error in quantum yield calculations is ca. 10%. The details of the calculations are given in Supporting Information File 1. Surprisingly, compound 7b, in which the formyl group is at the para position to the carbazole ring, was much more emissive than compound 7a, in which the formyl group is at the meta position to the carbazole ring.

Conclusion
In this work, two novel compounds 7a and 7b were successfully synthesised in good yields and demonstrated good thermal stability. Compounds 7a and 7b showed intramolecular charge transport properties with positive solvatochromism. Whilst 7a showed very low emission intensity, 7b showed very high emission intensity. It is noted that the conjugation in compound 7b encompasses the N atom of the carbazole ring and the formyl functionality (viz. the donor/acceptor units of the ICT component), whereas the link by conjugation between the same functionalities in 7a is missing. The resulting stronger ICT compo-nent in 7b explains the big difference in photophysical properties.

Experimental
All reagents were standard reagent grade and purchased from Sigma-Aldrich, Merck and Alfa Aesar. Inert reactions were performed under an argon atmosphere. Nuclear magnetic resonance (NMR) spectra were obtained on an Agilent Premium Compact NMR spectrometer (600 MHz for 1 H NMR, 150 MHz for 13 C NMR) with tetramethylsilane as internal standard. Elemental analysis was performed on a Costech Elemental system. The IR spectra were obtained (4000-400 cm −1 ) using a Shimadzu IRAffinity-1S Fourier transform infrared spectrophotometer. The mass spectra were obtained by Bruker microTOFq mass spectrometers to obtain low-and high-resolution spectra using electron ionisation (EI) or electrospray ionisation (ESI) techniques. UV, PL and photoluminescence quantum yields were measured on a Duetta two-in-one fluorescence and absorbance spectrometer from Horiba Scientific. Both absorption and emission solutions for reference and samples had a concentration of 10 −6 M. CV measurements were obtained using a CH Instruments 602E electrochemical workstation with iR compensation using dry dichloromethane. Thermogravimetric analysis was conducted using a Netzsch TG 209 F3 Tarsus Thermogravimetric Analyser under a constant flow of nitrogen. Differential scanning calorimetry was determined on a Netzsch DSC 214 Polyma instrument.

Supporting Information
The Supporting Information features the followings: 1) 1 H NMR and 13 C NMR spectra; 2) FTIR spectra; 3) mass and HRMS spectra; 4) calculations of relative fluorescence quantum yields.