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, FT-IR, 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 291C and 307C, respectively. Compounds 7a and 7b show three distinctive absorption bands: high and mid energy bands due to locally 2 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.


Introduction
Carbazole derivatives have found many different applications in a variety of technologically important areas, such as organic light emitting diodes (OLEDs), organic photovoltaics (OPVs), dye synthesised solar cells (DSSCs) and sensors [1,2]. In OLEDs, carbazole derivatives are frequently used as host materials [3][4][5]. In this respect the most frequently used host materials are 1,3-bis(Ncarbazolyl)benzene (mCP) [6] and polyvinylcarbazole (PVK) [7]. Carbazole derivatives, either just by themselves or in combination with iridium, are also used as emissive materials in OLEDs [8]. In this respect TCTzC, which bears a dithienylbenzothiadiazole unit and four alkyl-linked peripheral carbazole groups, is used in the construction of saturated red emissive OLEDs [9]. Carbazole-based homoleptic or heteroleptic iridium(III) complexes were also reported in the construction of different OLEDs [10][11][12][13]. In OPVs, carbazole derivatives are frequently used as small molecule p-type (electron donating) materials or electron 3 accepting (n-type) materials with a variety of donor-acceptor combinations [14,15]. In sensor studies, carbazole derivatives are used as fluorophores. In this regard many different carbazole-based fluorophores are reported in the literature [16][17][18][19]. Some of the carbazole derivatives were used as colourimetric anion sensors [20], and others as biothiol sensors [21][22][23]. Research continues on carbazole derivatives to find new materials with novel properties. It is therefore essential that one should design a molecule that has multifunctional usage in many different areas of technology [24].
Since carbazole is a relatively inexpensive material with unique properties such as high hole-transporting mobility [25,26], pronounced thermal stability [2] and high fluorescent quantum yields [27,28] our attention was focused on carbazole derivatives [29]. In addition to that, carbazole is a rigid aromatic molecule [30] with many different modification sites for multifunctionalisation [31,32].
In this work, we designed two novel 2-(N-hexylcarbazol-3'-yl)-4/5-formylpyridine compounds (7a and 7b), where 4/5-pyridinecarboxyaldehyde was attached to the 3position 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). Compounds 7a and 7b were heated at 20 °C/min under nitrogen atmosphere. The decomposition temperatures (Td 5% ) corresponding to 5% weight losses for 7a and 7b were 291 o C and 307 o C, respectively.
The morphological properties of compounds 7a and 7b were investigated by differential scanning calorimetry (DSC). Compounds 7a and 7b were first heated to

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 7 platinum wire as the counter electrode. The ferrocene-ferrocenium redox couple was used as an internal reference. 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 (EHOMO, ELUMO) 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] EHOMO= -(4.8 + E1/2 ox ) and ELUMO= -(4.8 + Eonset red ). The HOMO-LUMO energy gap was calculated both from electrochemical data using eq. 1 and from optical data using eq. 2.

Eg elec = ELUMO-EHOMO (1)
Eg abs = 1240/onset abs (2) [40][41][42] The optical energy gap (Eg o ) was higher than the electrochemical energy gap (Eg 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 using a Duetta Fluorescence and Absorbance Spectrometer in dichloromethane. Each compound (7a/7b) displayed three distinctive absorption bands in the UV-vis spectra: high energy bands and mid energy bands were assigned to -* and n-* transitions, whereas the low energy bands were assigned to an intramolecular charge transfer (ICT) transition.
The photoluminescence (PL) properties of compounds 7a and 7b were investigated using a Duetta Fluorescence and Absorbance Spectrometer in dichloromethane.
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 10 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 for compound 7b.  As seen in Figure 6, a red-shift was observed in the emission maxima as the microscopic solvent polarity [43,44], ET(30), increased from toluene to dimethyl sulfoxide (see also

Quantum Yields
The relative fluorescent quantum yields (FL) of compounds 7a and 7b were determined in dichloromethane by using Rhodamine B (FL=49% at exc=355 nm) in ethanol as reference [45]. 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 the Supporting Information. Surprisingly compound 7b, in which the formyl group is at the 5-position of the pyridine ring (para to carbazole), was much more emissive than compound 7a, in which the formyl group is at the 4-position of the pyridine ring (meta to carbazole).

Conclusions
In

Experimental
All reagents were standard reagent grade and purchased from Sigma-Aldrich, Merck

Supporting Information
The Supporting Information features the followings: 1) 1 H NMR and 13 C NMR spectra; 2) FT-IR Spectra