New styryl based organic chromophores including free amino and azomethine groups: Syntheses, photophysical, NLO and thermal properties

In this manuscript, we have successfully synthesized and characterized a new series of styryl based push-pull dyes containing free amino group and their Schiff bases derivatives in which dicyanomethylene was used as the acceptor group and different para-substituted alkylamines as the donor groups and 2-pyridyl as proton sensitive group. All compounds showed absorption in the visible region and green-red emission with low quantum yields. The photophysical properties were examined in various solvents with different polarities. The absorption and emission maxima were shifted


Introduction
Push-pull organic molecules are a class of organic compound comprising of electron donating group in one end and electron withdrawing group in other end. This compound exhibits strong photophysical modulation when exposed to a passing wave of photons. This phenomenon caused by the structural modification that is triggered by significant electron migration and inevitable change in electronic composition during excited states. This change is followed by solvatochromic behavior, strong color modulation and/or fluorescence increase. The changes observed generally explain how the molecule behave in a solution for its ground and excited state [1,2,3]. The important feature of this molecule class is exceptional polarizability which is crucial criterion for NLO materials. Nowadays extended π conjugation containing NLO materials have shown extensive usages particularly in signal processing, optical storage and telecommunication devices [4,5,6,7]. On the structural basis NLO chromophores are classified in two major groups: Inorganic and Organic based NLO chromophore [8,9]. Inorganic based NLO chromophores have to be isolated in single crystal form before their implementation that is not efficient and rewarding for commercial purposes nowadays. Fortunately the latter class NLO chromophores does not require such a thing to be implemented in advance, along with the flexibility to derivate their structures make them affordable candidates for implementation in various NLO devices [10]. The flexibility to swap electron donors and acceptors groups within the molecules gives us an ability to fine tune intramolecular charge transfer (ICT) intensity that comprehends chromophores NLO behaviors [11]. To understand the concept of NLO, first we may need to discuss about the first hyperpolarizability (β) value of a chromophore. Hyperpolarizability value indicates how convenient electron transfers occur within a molecule between the two ends, in this case electron donor and acceptor groups. Good NLO chromophores have usually a high value of both first and second hyperpolarizability, which explains intrinsically why push-pull molecules are good NLO chromophores. In order to inflate the total hyperpolarizability (βtop) value there is a necessity to pump electron density in the conjugation that is done by interchanging or adding stronger of either electron donor or acceptor groups.
Furthermore, the electron density found within heterocycles in the push-pull system has a contribution in affecting βtop value of the system.

4
Hydroxide plays significance influences in environmental chemistry. Its presence in aqueous medium directly determining pH level that affects organisms living in the corresponding area [12]. For that, pH sensitive compounds are critical in various sensor applications. These compounds have a tendency to show different spectral properties upon protonation/deprotonation process [13].
Fluorescent dyes chemosensors posses unique merits such as low energy consumption, ease of handling and remarkable selectivity and notable sensitivity [14].
These traits makes them outstanding detecting capability in wide range of condition.
Up until now, there are many investigations regarding pH sensitive fluorescent dyes with different approaches that led to an better understanding of hydroxide detection in organic or aquaeous environment however some failed to offer cheaper option due to synthesis complexities and/or expensive initial reagents. In the other hand, ICT in some fluorescent dyes is a notable characteristic that if optimized enough will trigger the appearence of NLO behavior. Among dyestuffs classes push-pull fluorescent dyes are renowned to own such special behaviors. These push pull dyes generate more charge delocalization upon excitation thus enhance both polarizability and fluorescence emission [15]. The charge delocalization upon excitation leads to redshifted emmision which is viable for various substrate detection in biological tissues and samples [16].
To address the aformentioned economical issues we introduced Schiff base which is easy and cheap to synthesis and pronounced for their for their antimicrobial, antifungal, antiviral and anticancer activity to the main structure of molecules investigated in the study [17,18]. Recently, there were massively growing interests in study regarding push pull organic molecules that makes use of dicyanomethylene group act as strong electron withdrawing group coupled with various donor conjugate through connecting π conjugation bridge [19,20,21,22]. Here, heterocyclic amines would be a great electron donating group thanks to their high electron density around nitrogen atom 5 [23,24]. Apart from that, heterocyclic amines have also makes their presences in various field from pharmaceutical to engineering and chemical sensing department [25,26,27,28,29,30,31,32]. Furthermore, a study regarding fluorescent probes [33] and NLO application for this compound has also been reported [34,35]. We have also conducted an NLO study for new series of styril based push-pull compounds in the last five years. Styryl compound contains unsaturated double bond, a structure similar to azomethine group, that connect electron donating and withdrawing group at its two ends. Based from the result alone, these compounds offers good NLO characteristic when compared to Disperse Red 1 as well as noteworthy thermal stability with dissociation temperatures for up to 300 o C [19,21].
Many have conducted study of plausible deprotonation process of bonded hydroxile group nearby ortho positioned azomethine bridge. While these approach can target certain anions however they show strong affinity towards cyanide and fluoride [20,36,37,38,39,40].
The azomethine bridge inside Schiff base is weak electron donating group. However, it still contributes to the total ICT thus increases molecular polarizability. This coupled with extended π conjugation will definitely makes a promising candidate for various NLO studies and applications [19,41,42,43].
Here, we report on syntheses and full photophysical characterization of a new series of styryl based organic compounds containing free amino group and their Schiff bases derivatives. The both series have various electron donating groups in the order of donor strength julolidine, pyrolidine, piperidine and morpholine. And also, it was synthesized additional two compounds bearing low pH sensitive 2-pyridinyl group. The pH sensitive properties of newly prepared Schiff bases against TBAOH were examined inside DMSO, furthermore their reverse protonation were also investigated using TFA.
The obtained all photophysical changes were saved under UV-lamp and ambient light.
The structural and electronic properties of all newly synthesized compounds were studied using DFT calculation. To determine thermal properties for all compounds TGA analyses were done under inert atmosphere.

Synthesis
As depicted from Scheme 1 the reaction pathway starts from compound 1 (2-(1-(4aminophenyl)ethylidene)malononitrile), its reaction with some benzaldehydes in mildly basic environment through aldol condensation gives compound 3-7 in moderate to good yields, generally without the need for chromatographic purification.

Absorption and emission properties
To assess the effect of the solvent on the absorption and emission spectra of 3-7 and 8-12 (c=10 μM for absorption, and 1 μM for emission), various solvents with different polarity were employed at room temperature ( Figure 2, Table 1 and Table S1, see  Table 1 and Table S1 (see SI).   (Table S1, see SI). In the calculations, it was obtained that the absorption wavelengths for the studied compounds changed in order 3>4>5>6>7 and also 8>9>10>11>12 in DMSO, as consistent with experimental data. The solvent effects on the absorption spectra were seen for 4 and 5 with a bathochromic shift about 16-17 nm from DMSO to toluene while the shifted was only 7 nm for 3 and 6. In case of 8-11, it was obtained about 13-15 nm. As again, there was no solvatochromic behavior observed for 7 and 12 in calculations, as seen experimentally. As given in Table 1, the calculated absorption maxima correlated with experimental ones for all compounds. In calculations, the main peaks were found to correspond to the H→L transitions from highest occupied molecular orbital (HOMO, H) to lowest unoccupied molecular orbital (LUMO, L) with the highest contributions for 3, 4, 5 and 7. For 6, the highest contributions were from the transitions H-1→L.
The solvatochromic studies in emission were done in an effort to gain an insight into the photophysical behaviour of these new push-pull dyes bearing free amino and azomethine groups. Therefore, the fluorescence spectra of all dyes 3-7 and 8-12 were recorded in same solvents with various polarities which was used in UV-Vis studies.
The results on emission behavior with solvents used are summarized in Table 1 and   Table S1 (see SI). The emission properties of all compounds are also essentially regular dependent of the polarity parameter of solvents. However, the largest bathochromic shifts of emission maxima for both serie of compounds were observed  [44,45,46,47,48,49,50,51,52]. As examples, the emission spectra of compounds 3 and

Substituent effects
From the investigation, Schiff bases exhibit greater red shift than its styryl counterpart in both absorption and emission spectra, for instance compound 8 reaches its emission maximum at 636 nm whereas compound 3 at 617 nm in DCM. Both styryl and Schiff bases show far greater red shift compared to compared to their previous counterparts [21]. In addition, these results are slightly overshade by triphenylamine-based NLO compounds that were synthetized by our group [19], this explain that triphenylamine is better electron donating group than heterocyclic amines substituent studied in this study. It can be seen from Figure

Photophysical changes in high pH values
Like others push-pull dyes, all synthetized compounds show significant red shift for both emission and absorption maxima with increasing solvent polarity. However, pyridine bearing compounds (7 and 12) doesn't show same behavior at all, which was explained by a similar delocalization of the charge in the ground and excited state [60].
This means that these compounds undergo weaker or no ICT phenomenon.
Regardless, all compounds that exhibit low intensity fluorescence under excitation at absorption maxima indicate non-radioactive deactivation channel that occurs readily with light source [58]. Neither absorption and emission maxima correlate well with other solvent parameter such as hydrogen-bonding ability, dipole moment, acidity or basicity [54,55]. With this, investigated photophysical changes produce no correlation with solvent polarity as quantified by ET 30 which indicates solvent properties other than polarity contribute the molecules stabilization or destabilization in their ground/excited states [59].
Unfortunately, the solubility limitation of all synthetized compounds doesn't allow us to conduct wide range of solvent observation that is required for multiparameter analysis.
In conjunction with solvent interaction study hydroxide interaction study was conducted for compound 8-12. Based on the observations alone compounds 8-12 have the ability to detect hydroxide ion inside DMSO solution. Here they underwent prototopic equilibrium that led to changes of UV-Vis absorption spectra when exposed to basic environment [61]. The interaction between the compound and hydroxide was understood from the bathochromic-shifted maximum of compound 12, as presented in due to the phenoxy group is more pronounce electron donor than the phenol group [62]. Furthermore, reversibility test using TFA (Trifluoroacetic acid) shows that all compounds except compound 8 can be brought to initial state hence marks the sensor repeatability usage for every single analysis process.

pH sensitive study of 8 in aqueous solution
As how hydroxide ion play critical role in our ecosystem, pH value in water body also does its importance to maintain physiological stability both inside and outside living organism. The pH value remains the most important condition for a natural cycle inside our body in order to run flawlessly, it also judges whether a plant or animal can continue to live in a specific environment. For these reasons, the necessity to develop new kinds of pH sensors has been quite vital in order to control pH condition in the world we lived in. It has been thoroughly studied that optical pH sensor boast for their not having 18 electrical interference and remarkable sensitivity. Due to their low cost manufacturing and maintenance this class of pH sensor has been met their usages in both clinical and environmental analysis, especially in the form of optrodes [63,64,65]. At the same time, fluorescent pH indicators offer better selectivity and sensitivity than other classes [16]. Aside from that, they provide wide range of measurement techniques such as polarization and energy transfer [14]. Based on this fact we had investigated compound   To determine pKa value of organic compound, wide range of methodologies such as solubility measurement [66,67], potentiometry [68,69] and UV-Vis absorption spectroscopy [70,71] can be applied. In our case, determination of pKa of compound linearity at defined range of calibration graph, thus contradicts aggregation [15].

NLO properties
In order to obtain an insight into the NLO property of the studied compounds, first-order hyperpolarizability (β) and other related measurements such as polarizability (α) and dipole moment (µ) were calculated at B3LYP/6-31+G (d,p) level in gas phase. In

Thermal Properties
We conducted thermogravimetric analysis to understand whether the molecule is sufficient for NLO study. Great NLO molecule generally has high thermal dissociation figures for around 250 o C. In NLO study energetic photons tend to emit heat the longer it takes place that's why this thermal condition is crucial for any candidate for NLO molecules [19,72]. As depicted from Figure

Materials, Methods and Instrumentation
All commercially available chemicals were reagent grade and used without further purification. The compounds 1 and 2 which were successfully synthesized from published literatures [42,43].

Photophysical studies
Compounds 3-7 (10 µM for absorption and 1 µM for emission) were studied in six various solvents with different polarity (Toluene, THF, DCM, DMSO, ACN, MeOH), whereas compound 8-12 were studied with the same manner while excluding MeOH due to their low solubilities. All absorption spectra were registered using Shimadzu 1800 spectrophotometer while emission spectra were recorded using Hitachi F-7000 Fluorimeter. All samples were measured inside 1 cm x 1 cm quartz cuvvetes with approximately 2 mL in volume. The deprotonation and reverse protonation studies of compounds 8-12 were conducted inside DMSO. To each DMSO solution 5 equiv of TBAOH were introduced and immediately UV-Vis absorption spectra were taken with the same concentration mentioned above. This titration was repeated until there was no observable change in sample spectra. The titrant used in this titration is 10 mM TBAOH in DMSO. Reverse protonation processes for each compound 8-12 were conducted after TBAOH titration using 10 mM TFA in DMSO, 5 equiv of TFA were introduced to fully deprotonated solutions incrementally until there was no observable change in spectra. All of the procedures above were done at room temperature (25 o C). Emission changes during deprotonation and reverse protonation processes for compound 8-12 were also been registered using fluorimeter in the same manner with the same concentration stated earlier for emission study at room temperature (25 o C).
All photographs were taken using SONY RX100 pocket camera with ISO values of 200 and variable aperture at "Program Auto" mode.

pKa determination for Compound 8
Solubility of compound 8 in deionized water were conducted by studying its calibration graph at 505 nm. The calibration graph was obtained using UV-Vis absorption spectrophotometry by the following procedure, 5 µM of 8 solution were prepared and immediately its absorbance at 505 nm were registered, into the same solution 10 µL from 1 mM compound 8's stock solution were added incrementally followed by recording its absorbance value after each addition. From the results absorbance vs Concentration graph was drawn in order to obtain the calibration graph and its linearity were investigated by calculating R 2 value in Microsoft Excel.
The spectra of compound 8 were recorded using Shimadzu 1800 spectrophotometer in Britton-Robinson buffer solutions [79] with pH values ranging from 5.5 to 11 at room temperature (25 o C). Determination of compound 8 pKa has been done by dividing new band wavelength intensity with its own maxima and compared them with corresponding pH values which led to a crude sigmoid function. Obtained function was then fitted and rebuilt from scratch using "Curve Fitting" method and DoseResp approach in order to obtain a clean sigmoid function.

Computational methods
The geometry of all compounds in their ground states have been calculated by Density Functional Theory at B3LYP/6-31+G(d,p) level in gas phase [80,81]. This method were also applied to study non-linear optic (NLO) property of each compounds on their ground states. Theoretical absorption spectra in different solvents were calculated with time dependent DFT (TD-DFT) method at the same level. For calculations in solvents, Polarizable Continuum Model (PCM) was used [82,83]. All the calculations were done using Gaussian 09 program [84].

Supporting Information
Supporting information text