Unveiling the photophysical and morphological properties of an acidochromic thiophene flanked dipyrrolopyrazine-based chromophore for optoelectronic application

A series of dipyrrolopyrazine (DPP) based chromophores featuring thiophene and varied donor (N,N-dimethylamine, NH2, OMe) and acceptor (CF3, CN, NO2) appendages have been synthesized. The structures and properties of the chromophores were investigated by absorption spectroscopy, electrochemistry, differential scanning calorimetry, and thermogravimetric analysis. X-ray crystallographic analysis indicates a planar geometry for the molecule 7g. Surface morphological studies reveal the formation of microrods and nanorods. The acidochromic behavior of the chromophore which shows a prominent red-shift in the absorption spectra owing to the protonation of the pyrazine segment provides a valuable opportunity to assess the sensory response. Acid dependent spectral changes could be successfully applied to detect pH in biological fluids and acid impurities in solvents.


Introduction
In recent years, a large number of p-conjugated organic molecules, mainly push-pull chromophores, have received a lot of interest owing to their applications in a wide range of electronic and optoelectronic devices. One commonly used strategy to design p-electron chromophores is to end-terminate suitable conjugated bridges with strong electron donor and acceptor substituents. [1][2][3][4][5][6][7] This D-p-A arrangement 8 ensures efficient intramolecular charge transfer (ICT) between donor and acceptor and generates a dipolar push-pull system. The electronic and structural properties of chromophores with donoracceptor (D-p-A) substituted organic compounds are of considerable interest because of their potential applications in nonlinear optical materials (NLO), [9][10][11] light emitting diodes (OLED), 12,13 and sensors 14 as well as organic eld effect transistors (OFET). [15][16][17] Beside these widely used applications, pushpull chromophores are also used in organic photovoltaic cells (PVCs), 18 dye sensitised solar cells (DSSC), [19][20][21][22] bulkheterojunction solar cells (BHJ), 23 and biological imaging. 24,25 Direct interaction in push-pull chromophores provides the pconjugated molecule with additional properties such as dipolar character, intense colour, chemical and thermal robustness. 26 Conversely, many organic materials like polyenes suffer from low thermal stability. Therefore extensive research efforts have been directed towards modication of donor, acceptor, and pconjugated moieties.
Synthetic studies have shown that replacement of benzene ring with easily delocalizable ve membered heteroaromatic ring (furan, pyrrole or thiophene) [27][28][29][30][31][32] play vital role in determining the overall electron donating ability of the substituent. However, the pyrrole-based bridge, the analogue of thiophene moieties, has seldom been noticed. Pyrrole 33,34 containing chromophores as the p-conjugated bridge were found to display good optical properties in comparison with their analogues with furan or thiophene. This was attributed to the higher electron density in the pyrrole moiety compared to thiophene and furan.
Thiophene have been used as efficient electron donor as it imparts an enhance ICT in the chromophore, thiophene is undoubtedly among most explored heterocyclic moiety and has already found many applications 35 in organic electronic and photonics because of its high chemical and photophysical stability compared to the other heteroaromatics. Thiophene as a part of push-pull molecules has been investigated by Filip Bureš et al. 36 and other groups. The pyrazine rings have also been extensively used as electron withdrawing part of push-pull systems. [37][38][39][40][41][42] Linear chromophores incorporating pyrazine moiety exhibits interesting emission properties. 43 Only a few examples of incorporating pyrazine rings as p-linker in pushpull system have been reported. However, to the best of our knowledge, thiophene anked DPP chromophores have not been studied as a part of push-pull chromophores.
We have previously introduced p-conjugated DPP skeletal backbone [44][45][46] and their photophysical properties. In this work, we decided to investigate the effect of presence and position of electron-rich and electron-decient moieties on the photophysical properties. The most efficient charge-transfer between these two functionalities is achieved when they are attached to the C-2 and C-6 positions of the dipyrrolopyrazine. DPP was used as p-conjugated bridge functionalized with most powerful electron donor 47 (N,N-dimethylamine, NH 2 , OMe) and acceptor 48,49 (CF 3 , CN, NO 2 ). CF 3 and CN groups are introduced as they are expected to affect the molecular packing mode by hydrogen bonding interaction. Acid dependent uorescent materials [50][51][52] have been intensively studied in terms of practical applications in the eld of sensors, memories, and display device. Thus we investigated the sensitivity of these chromophores toward acids to explore their feasibility towards sensor industry.

Synthesis
A simple and efficient protocol was employed as outlined in Scheme 1. Extended 2-thiophene and triuoromethanesubstituted p-linkers 2 and 5, which are required for the synthesis of chromophores 4(a-e) and 7(f-g) were prepared in a modular manner. 45,46 Synthesis of target chromophores was accomplished by two-step process. First C-C coupling via Sonogashira coupling reaction followed by intramolecular cyclization. PdCl 2 (PPh 3 ) 2 catalysed Sonogashira cross-coupling reaction between free amine (2,5) and acetylene containing electron withdrawing groups (CF 3 , CN, NO 2 ) and electron donating groups (thiophen-2-yl, thiophen-3-yl, N,N-dimethylaniline, NH 2 , OMe) under microwave (MW) afforded C-C coupled product 3(a-e) and 6(f-g) in modest yield, along with small amount of cyclic product, separation of the mixture was tedious in some case so the mixture was directly used for next step. The reaction with 1-ethynyl-4-nitrobenzene and 4-ethynylbenzonitrile was sluggish and low-yielding, although the reaction conditions were optimised, increased temperature or reaction time led to decomposition.
Next, base induced intramolecular cyclization of 3(a-e) and 6(f-g) under MW condition furnished corresponding cyclic compounds 4(a-e) and 7(f-g) with moderate yield. Intramolecular cyclization of 3e afforded only traces of desired chromophores 4e. Purication of products was carried out by means of standard column chromatography. All the prepared chromophores are easily soluble in regular organic solvents such as chloroform, dichloromethane, methanol to give bluishgreen solution and the compounds can be stored for long time without decomposition. The structures of prepared chromophores were unambiguously conrmed by their spectral and analytical data (Table 1).

Crystallographic analysis
The X-ray crystallography was performed to investigate solidstate packing and the interactions in the chromophores. In our previous work, 44 we have shown the non planar arrangement in 1,7-dimethyl-2-p-tolyl-1,7-dihydrodipyrrolo[2,3-b:3 0 ,2 0 -e] pyrazine which is barrier for effective D-A conjugation. Thus we prepared chromophores possessing D and A connected to plinker (DPP) through C-2 and C-6 position. This arrangement assured planarization of the entire p-conjugated system. Single crystal suitable for structural analysis was obtained by recrystallization from DCM. The illustration of crystal packing for 7g is shown in Fig. 1. X-ray crystallographic analysis indicates planar geometry of the molecules with p-stacking, which facilitates ICT process through the molecule which in turn nely tunes the absorption. Compound 7g crystallised in the monoclinic system with space group P2(1)/c, and the unit cell dimensions of (a) 15.07 (2)Å, (b) 8.80 (10)Å, (c) 15.41 (2)Å, a ¼ 90 , b ¼ 115.2(10) , and g ¼ 90 .

Optical properties
Absorption spectra of the chromophores were recorded in DCM solution. All of the chromophores exhibited a broad intense absorption band ( Fig. 2(a)). The position of the absorption Scheme 1 Synthetic scheme employed for the preparation of DPP-based chromophore.
bands is inuenced by the functional groups of the compound and assumes a trend in the order (7g < 7f < 4c < 4d < 4b < 4a) reecting the impact of donor moiety. Chromophores with electron donating group exhibit bathochromic shi due to involvement of conjugative delocalization. Extended conjugation present in these derivatives lead to red shi prole. On the a Trace amount of cyclization product was also observed.  Table 2. Solvatochromism for the D-A molecules originate due to the difference in the dipole moment. Chromophore 4a studied in this work exhibited slight red shi when recorded in polar solvents indicating negligible intramolecular interactions in the ground state. A representative variation of absorption spectra with different solvents is illustrated in Fig. 3(a). Fluorescence spectra of the compounds displayed moderately intense emission spectra in DCM solution when excited at their absorption maxima. The most red-shied emission prole was observed for amine derivative (4a) while the thiophen-3-yl (7g) displayed the shorter wavelength emission.
Representative illustrations showing the emission prole are displayed in Fig. 2(b). Emission spectra of the chromophores were also examined in a series of solvents with varying polarity index to identify the impact of the polarity of the solvent on the excited state of the chromophores. For the chromophore 4a representative illustrations showing the inuence of the solvent polarity on the emission prole are displayed in Fig. 1 (ESI data †). The emission prole of the chromophores exhibited a positive solvatochromism with the bathochromically shied emission maxima in the polar solvents such as DMF, DMSO, and MeOH. Chromophores showed different types of interactions with the nonpolar and polar solvents. This suggests that less polar solvents, solvation effect is present while for the polar solvents additional specic interactions such as dipole-dipole relaxation plays a major role.

Acidochromism
Another interesting phenomenon of the present chromophores was its uorescent behavior stimulated by acid (acidochromism). Addition of CF 3 COOH (TFA) to the chromophores elicits a red-shi in both absorption and emission and the corresponding spectra are shown in Fig. 4. Similarly a sharp decrease in the emission intensity was noticed on the addition of TFA to DCM solutions with bathochromic shi, due to dipolar relaxation of D-A interactions from the excited state. 53 In the absorption spectra of selected chromophores 4a (D-plinker-D) and 7f (A-p-linker-D) the higher wavelength band was progressively red-shied giving two main uorescent peaks, with a color change from green (blue) to yellow. Similar effects were  observed in the emission spectra. These changes could be ascribed to the protonation effect of TFA. The proton induced shis in all chromophores are reminiscent. 54 The pyrazine segment is protonated to generate pyrazinium ion, which trigger ICT between D-A (Scheme 2). Consequently, chromophores 4a and 7f are consanguine, pyrazine and thiophene core together dictates their absorption behavior. A representative illustration of changes for 4a and 7f in the absorption prole on incremental addition of TFA is shown in Fig. 5. In DCM solution, color of chromophores changes from green to yellowish as shown in Fig. 4. Photographic image of chromophores under normal light are shown in ESI data † (Fig. 2). Interestingly, adding triethylamine (TEA) to this system could restore its initial green (blue) state which neutralizes the effect arising due to the addition of TFA. The observation of isobestic points suggests the presence of neutral and protonated forms in equilibrium. These chromophores exhibit acid-base equilibrium as illustrated in Scheme 2.

Electrochemical properties
The redox behavior of the chromophores 4(a-d) and 7(f-g) were scrutinized by cyclic voltammetry and their redox potentials and energy levels are shown in Table 2. The highest occupied molecular orbital (HOMO) energy levels, estimated from the oxidation onset. The LUMO levels are estimated from the HOMO value and the optical bandgap. The electronic interaction of donor and acceptor moieties affects the oxidation potentials. The donors interact effectively with acceptor moiety and lead to a reduction in the band gap. Thus lower oxidation potentials are accepted for the strong donor containing chromophores 4a and 4b. While larger oxidation potentials observed in 7g (Fig. 6). In agreement with these generalizations, the oxidation potentials of the chromophores assumed the order: 4a < 4b < 4c < 4d < 7f < 7g.

Thermal properties
Thermal behaviour of chromophores 4(a-d) and 7(f-g) were studied by thermogravimetric analysis (TGA) and differential  scanning calorimetry (DSC). All the chromophores exhibited good thermal stabilities with the decomposition temperature (T d ) higher than 300 C under nitrogen atmosphere. The highest thermal stability observed in 4a, while 4d displayed the lowest in the series. This may be due to the different properties of modied groups (NMe 2 , NH 2 , OMe, CF 3 ). The data indicate that the modied groups can inuence the thermal stabilities of chromophores ( Table 3). The phase transitions have been studied by DSC under nitrogen with a heating rate of 10 C min À1 in the temperature range of 30-700 C, neither phase transition nor thermal decomposition was observed upto 180 C (Fig. 7). Sharp endothermic melting peak observed in the range 180-264 C, indicates highly crystalline nature of the compounds. As determined by DSC and TGA experiments, compounds 4(a-d) and 7(f-g) has remarkable thermal stability for an organic material.

X-ray diffraction
X-ray diffraction pattern of prepared chromophores are displayed in Fig. 8. All of the chromophores exhibit strong diffraction intensities attributed to p-p interactions.
Similarly the X-ray diffraction pattern of chromophores 7f and 7g exhibits a primary diffraction peak at 2q ¼ 5.5 (d-spacing   16.0Å) and 9.3 (d-spacing 9.4Å). This indicates that chromophores easily form well resolved molecular structure and well dened diffraction peaks indicating a high degree of crystallinity.

Surface morphology
The morphology of thin lm was also studied by scanning electron microscopy (SEM) in order to obtain more information about the thin-lm forming properties of these chromophores. Thin lms were prepared from drop casting THF and DCM solution of 4(a-d) and 7(f-g) on bare silica substrate followed by drying in air. SEM analysis reveals that compound 4(a-b) forms microrods and nanorods with an average length of 76 mm with thickness of 3 mm and 72 Â 18 mm, respectively. Chromophores 4(c-d) also form rod like microstructures as observed in Fig. 9. SEM image of 7f reveals "chrysanthemum" like morphology, these ower-like supramolecular architectures is 60-70 mm in diameter, constructed from nanobelts with a thickness of 300-500 nm and width of 3-5 mm. Compound 7g form square facet with typical length-to-width ratio of the facts was about 10 mm (Fig. 10). The major driving force for formation of their supramolecular assemblies is the dipole-dipole interaction. The morphologies of the microstructures can be tuned by changing solvents. SEM image reveals well-ordered particles with 1-D and 3-D microstructure indicating highly crystalline nature with diverse particle size.

General methods
Solvents and reagents were purchased of reagent grade and used without further purication. 1 H NMR and 13 C NMR spectra were recorded on a Bruker 500 MHz NMR instrument. The chemical shis were reported as d (ppm) relative to a deuterated solvent as an internal reference and coupling constants (J) are reported in hertz (Hz). High-resolution mass spectrometry was performed using a 6550 iFunnel Q-TOF LC/ MS system. Melting points were obtained from DSC  thermograms. UV-vis and uorescence spectra were recorded using standard 1 cm quartz cells on Varian cary-50 spectrophotometer and Cary Eclipse Fluorescence Spectrophotometer (excitation slit 5 nm). The spectra were recorded by using freshly prepared dilute solution. Compounds were excited at their absorption maxima. TGA and DSC were performed with TGA 3 plus (mettlertoledo) and DSC 2 STAR system (mettlertoledo) respectively, under nitrogen with heating rate of 10 C min À1 . Cyclic voltammetry experiments were performed with ZIVE-SP2 LAB. All measurements were carried out at room temperature with a conventional three-electrode conguration consisting of a platinum working electrode, a platinum wire auxiliary, and a Ag/AgCl reference electrode. Ferrocene was used as an internal standard at a scan rate of 50 mV s À1 . The solvent in all experiments was DCM, and the supporting electrolyte was 0.1 M tetrabutylammonium hexauorophosphate. Single-crystal X-ray diffraction data were collected on Bruker SMART APEX II X-ray crystallography with CCD area detector SHELXL structural analysis program. Structure solution was done by direct method and rened by a full-matrix least-square method on F 2 . Powder XRD were analysed with Smartlab using Cu Ka radiation (l ¼ 1.5406Å). SEM images were taken from FEI Quanta 3D FEG scanning electron microscope.

Conclusions
We have demonstrated the design, synthesis, and characterization of DPP-based chromophores with donor-acceptor molecular architecture with well established and straightforward methodologies. The structural, optoelectronic, and thermal properties were investigated in detail. X-ray crystallographic analysis of 7g, reveals p-conjugated framework with a planar geometry. The abilities of these chromophores to function as pH sensors were demonstrated with dramatic color changes upon the introduction of acid. These ndings suggest that suitable design of the molecules and a sound understanding of their spectroscopic properties could enable to develop promising pH sensors. Surface morphological studies reveal the formation of microrods with diverse particle size. Undoubtedly, our results provide important guidelines for designing DPP-based molecular semiconductors, indicating that through rational design and synthesis, DPP can be a highly favorable building block for efficient electron charge-transport in optoelectronics.

Conflicts of interest
There are no conicts to declare.