Emission and Color Tuning of Cyanostilbenes and White Light Emission.

White-light-emitting diodes are energy efficiency replacement of conventional lighting sources. Herein, we report the luminescent behavior of three simple cyanostilbenes with triphenylamine-donating groups bearing different electron-withdrawing groups (phenyl, pyridyl, and p-nitrophenyl) in a common donor (D)-π-acceptor (A) α-cyanostilbene construct along with their thermal and electrochemical properties. The density functional theory (DFT) studies reveal that aggregation-induced emission characteristic feature of the D-π-A dyes is inversely proportional to the intramolecular charge transfer (ICT) effect, that is, phenyl-and pyridyl-substituted compounds show characteristic aggregation-induced emission in water, whereas the ICT effect is dominant for the nitro derivative. The extent of ICT and the solvatochromic emission shifts, from blue to red, depend on the strength of the electron-withdrawing group. White luminescence and tunable emission colors are obtained by careful admixtures of these cyanostilbenes bearing triphenylamines. The results rationalized through DFT and time-dependent DFT calculations follow a consistent trend with the energy levels measured from the electrochemical and optical studies. Thermogravimetric analysis and differential scanning calorimetry studies showed excellent thermal stability of the compounds. The scanning electron microscopy and dynamic light scattering measurements were performed to reveal the formation of aggregates. This strategy involving synthetically simple and structurally similar molecules with different emission properties has potential applications in the fabrication of multicolor and white-light-emitting materials.

Introduction

Organic materials emitting white light have attracted significant attention for their potential applicability in lighting devices and for efficient energy utilization.[1,2] In general, the white light emission is obtained by blending of two or more complementary colors that cover the full spectrum of visible wavelength ranging from 400 to 700 nm.[3−7] Traditionally, the white light emission has been achieved with the use of single chromophoric systems,[8] self-assembly,[9,10] complexation with macrocyclic hosts,[11] supramolecular gelation[12,13] or through the use of natural pigments from vegetable extracts,[14,15] and related processes.[16−18] A scaffold with suitable electronic push–pull substituents separated by a π-spacer is prevalent in the organic dyes exhibiting strong emission or color characteristics. Such molecular materials with electron-donating and electron-accepting substituents connected through conjugated π-electrons (D−π–A) have been extensively investigated because of their abundant applications for organic electronic  and biological applications.[17,19−25] Among many known derivatives, there is an active interest toward materials exhibiting unique aggregation-induced emission (AIE or AIEE) for their excellent emissive properties in the solid or aggregated state due to restricted molecular rotations with subsequent reduction of radiationless decay processes.[26−29] α-Cyanostilbenes belongs to one such class of π-conjugated substrates that exhibit this unique emission behavior and were widely utilized for optical and biological applications.[30−35] Cyanostilbenes are considered to be an attractive platform  as different chemical entities can be synthetically incorporated in the core structure in the form of the donor (D) or acceptor (A) groups  and other functional moieties via routine synthetic procedures.[36] Self-assemblies, nanostructures, supramolecular complexes, fluorescent probes, sensors, supramolecular materials, and several other functional materials with cyanostilbene core have been investigated by taking advantage of this synthetic tunability.[37−40] Previously, derivatives based on tetraphenylethylene, a well-known AIE substrate, is shown to emit white light in the presence of a suitable polymer host or dopant.[41−43] Although cyanostilbenes have also been explored as excellent optoelectronic materials, the multicolor tunability and achieving of white light emission are not reported to the best of our knowledge. Here, in this work, we explored α-cyanostilbene derivatives bearing triphenylamine donor and variable electron-acceptor groups (nitro, pyridyl, and phenyl) separated by cyanovinyl π-spacer [Scheme ]. This design ensures a facile but variable charge transfer between two terminals. All the compounds exhibited solvatochromic emission with a change in solvent polarity but emitted white light when these compounds were mixed at specific proportions. Fine tuning the mixing ratios of these three compounds allowed a broad emission range spanning from blue to red regions. This strategy can pave the way for developing varieties of organic dye admixtures to create a molecular palette for tuning emissions and colors. The results involving the photophysical, electrochemical, and density functional theory (DFT) and time-dependent DFT (TDDFT) studies are detailed below.

Scheme 1

Synthesis of the cyanostilbene derivatives

Results and Discussion

Absorption and Emission of Cyanostilbenes

A series of cyanostilbene derivatives were synthesized [Scheme ] and investigated for their absorption and emission behavior in solution. The compounds have a triphenylamine donor linked to an aromatic ring [phenyl 1, pyridyl 2, and p-nitrophenyl 3] separated by a π-spacer (double bond). All the compounds also have an electron-withdrawing cyano substituent on the double bond. The absorption and emission maxima of the compounds are given in Table . Compound 1 with no electron-withdrawing group shows the absorption maxima at ∼394 nm in dioxane [Figure S1A]. Compound 2 bearing a pyridyl substituent absorbs at ∼418 nm [Figure A] and compound 3 substituted with a stronger electron-withdrawing nitro group absorbs at ∼437 nm [Figure B]. In effect, increasing the electron-withdrawing strength shifts the absorption maxima to a longer wavelength in the order, 3 > 2 > 1, owing to greater electron delocalization. A shorter wavelength band observed at 300 nm is attributed to the transitions of triphenylamine chromophore.[44] The absorption at longer wavelength region is due to the charge-transfer transition of the donor (triphenylamine) to acceptor (cyano, pyridine, or nitro) groups. Changing the solvent polarity does not result in any significant absorption changes, but in water, all the compounds show broad absorption with bathochromic shifts [Figures and S1A] attributed to the formation of aggregates.

Table 1

Absorption and Emission Properties of (1–3) in Solvents of Varying Polarities

	1			2			3
solvents	λabs (nm)	λem (nm)	Stokes shift (cm–1)	λabs (nm)	λem (nm)	Stokes shift (cm–1)	λabs (nm)	λem (nm)	Stokes shift (cm–1)
heptane	392	446/470	4233	419	465/493	6582	433	496	2933
THF	396	510	5644	424	546	5269	440	596	5948
dioxane	394	496	5219	418	517	4581	437	548	4635
acetonitrile	392	543	7093	419	588	6859	433	quenched
methanol	395	536	6659	425	587	6493	434	quenched
water	408	517	5167	435	561	5163	463	626	5623

Figure 1

Absorption spectra of (A) stilbene 2 and (B) stilbene 3 in different solvents. The concentration of molecules of 10 μM was taken in 1 mL of the solvent of different polarities.

With increasing solvent polarity, cyanostilbene 1 shows bathochromic emission shifts from 447 nm in nonpolar heptane to 543 nm in polar acetonitrile with overall emission shifts of +95 nm [Figure S1B]. Similar solvatochromic emission shifts were noted for compound (2) [∼+123 nm shift from heptane (465 nm) to acetonitrile (588 nm)]. Compound 3 emits at 496 nm in heptane and similar to the other compounds (1 and 2), bathochromic emission shifts are observed with emission at 548 nm in dioxane [Figure B]. In polar solvents, the emission of nitro-substituted 3 is severely quenched because of the strong intersystem crossing and subsequent nonradiative transitions.[45] In water, interesting emission changes are noted. Compound 1 shows emission at ∼517 nm, 2 at 561 nm, and 3, despite quenched emission in polar solvents, emits at 626 nm. These emission maxima are also associated with intensity enhancement [∼5 fold in 1, ∼4.3 fold in 2, and ∼56 folds intensity enhancement in 3 in comparison with the observed emission in acetonitrile, a polar solvent]. These emission changes in water are attributed to aggregation-induced emission.[46,47]

Figure 2

Emission spectra of (A) stilbene 2 and (B) stilbene 3 in solvents. The excitation wavelength is 410 nm (2) and 430 nm (3), respectively. Concentration [10 μM].

To achieve a better insight, the emission of these compounds was measured in binary solvent mixtures of dioxane and water. For this, 10 μL aliquots of the stock solution of the compound in dioxane were added to 1 mL of water to measure the emission spectrum. Stilbene 1 emits at ∼497 nm in dioxane and increase in water fractions yields solvatochromic emission shifts with maximum emission wavelength (545 nm) at 60% water fraction [Figure S2]. Further increase in water fraction results in enhancement of the emission intensity (∼31 fold rise with respect to emission intensity observed in dioxane) with a final emission maximum at ∼517 nm. Likewise, compound 2 emits at 517 nm in dioxane, and with increased water percent, the maximum emission wavelength is noted at 592 nm at 60% water (∼8.8-fold emission intensity enhancement) with the final emission maximum at ∼561 nm [Figure A]. Compound 3 emits at ∼550 nm in dioxane. Upon increasing the water fraction, gradual emission shifts with quenched emission are observed [Figure B] with a final emission maximum at ∼620 nm. However, in this case, emission shifts were noted with no emission intensity enhancement. As noted earlier [Figure B], the emission of compound 3 in water is due to the formation of aggregates.[47]

Figure 3

Emission in the dioxane-water binary mixture (A) cyanostilbene 2 and (B) cyanostilbene 3. The visual emission colors seen under UV illumination (365 nm) are shown in Figure S3.

Scanning Electron Microscopy Measurements

The unique emission noted in the aqueous media for cyanostilbenes is attributed to the formation of aggregates. To assess the aggregate morphology, scanning electron microscopy (SEM) images were recorded. The SEM images reveal spherical aggregates with diameters of 270 nm for 1 and 365 nm for 2, and for 3, particles of smaller diameter (123–193 nm) were observed [Figure ]. The morphology of these aggregates is distinct to that seen in their powdered form [Figure S4]. The dynamic light scattering (DLS) experiments of the compounds in water and their binary mixtures show particles in a size range of 195–550 nm [Table S1]. The size distribution obtained in DLS measurements is slightly greater than that measured through SEM, indicating the influence of added solvent on the size of the particles.

Figure 4

SEM images of 1, 2, 3, and 1 + 2 + 3 taken as a drop-cast image (10 μM) in water over a silicon wafer.

Emission Tuning: Emission of Binary Mixtures in Solution and Solid State

Compounds 1–3 emit at different wavelengths in water, that is, 517 nm for 1, 560 nm for 2, and 626 nm for 3. These wavelengths approximately correspond to green, yellow, and red emission regions. Because the designed molecules have strong structural similarity, excepting for the end groups, we assumed that these molecules could perfectly blend and show unique emission characteristics. For this, we performed absorption and emission measurements in different combinations of 1 + 2, 1 + 3, 2 + 3, and 1 + 2 + 3 in equimolar concentrations (10 μM in 1 mL of water). The absorption and emission of these blends are shown in Figure A. Interestingly, the emission of all of these blends is different from what is observed for the corresponding individual counterparts. Equimolar proportions of 1 + 2 show unique emission at 541 nm. Similarly, a combination of 1 and 3 or 2 and 3 yields emission at 607 and 612 nm, respectively. The emission of 1 and 3 also shows a shoulder band at ∼570 nm. Finally, a combination of cyanostilbenes 1 + 2 + 3 results in an emission maxima at 588 nm (Figure A). Thus, by simple mixing of the structurally similar fluorophores, tunable emission bands are observed. These blends show visually distinct colors under UV illuminator (Figure B). The corresponding emission spectra obtained from the binary mixture titration  are shown in the Supporting Information [Figure S5] and also yields visually distinct colors. SEM image of 1 + 2 + 3 show formation of closely associated spherical aggregates [Figure ].

Figure 5

Normalized spectra of compounds 1, 2, and 3 and their combinations (10 μM) in water (A) emission and (B) color changes under UV illumination (365 nm).

Emission Tuning in Solid State

To determine the solid-state tunable emission, we measured the emission of individual fluorophores and their combinations in the solid state [Figure A]. For this, 10 mg of each fluorophore was taken, and their spectra were measured individually. Later, desired combinations were mixed and annealed together with dioxane as the annealing solvent. Similar to the solution state observations, distinct emission signatures were also obtained in the solid state [Figure A] indicating a perfect blend. For better visualization, we represented the emission in the CIE color-space diagram.[48] The CIE (Commission Internationale de l’Eclairage proceedings) diagram given below shows emissions of molecules  1, 2, and 3 in green, yellow, and reddish-yellow regions, respectively. The combinations (2 + 3), (1 + 3), and (1 + 2) appear in orange, yellow, and yellowish green regions, respectively. The CIE chromaticity coordinate for molecules (1–3 and mixtures) appears in the green and yellow-red region. The CIE color coordinates (Figure B) obtained for the compounds are given in the parenthesis: 1 (0.21, 0.60), 2 (0.53, 0.44); 3 (0.67, 0.23), and for the mixtures are as follows: 1 + 2 (0.41, 0.53), 1 + 3 (0.53, 0.38), 2 + 3 (0.63, 0.32), and 1 + 2 + 3 (0.43, 0.47).

Figure 6

(A) Emission in the solid state of 1–3 and the mixtures; (B) CIE plot based on the solid-state emission.

White Light Emission

To obtain white light, there is a need for broad emission covering the visible light wavelength region. To accomplish this, typically an additive mixture of complementary colors (blue and green) or (red, green and blue: RGB) are needed. The cyanostilbenes are characterized by moderate to strong solvatochromism with a change in solvent polarity. In water, aggregation-induced emission features are obtained, but the emission maxima of these compounds lie outside of the RGB requirements. The binary or the ternary mixtures, although yielded unique emission signatures and colors, failed to yield broad emission spectra required for the white light emission. The unique emission signatures could be a result of intermolecular energy transfer.[49−51] We examined our data closely and observed that the emission maxima of the compounds in tetrahydrofuran (THF) closely corresponds to the RGB region. Mixing of these compounds thus should result in the white light emission. Accordingly, the spectra measured yields broad emission from 380 to 700 nm for the binary mixtures of 1 and 3 [10 + 5 μM] as well as ternary mixtures of 1, 2, and 3 (20 + 5 + 20 μM) [Figure A]. The CIE coordinates [Figure B] observed for the binary mixture (1 + 3) and the ternary mixture combination [1 + 2 + 3] are (0.39, 0.37) and (0.33, 0.31), respectively, that correspond to the white light emission [inset Figure A].

Figure 7

Emission spectra of (A) 1, 2, and 3 and the combinations of (1–3) and (1 + 2 + 3) in THF at an excitation wavelength of 410 nm. (B) CIE plot of binary mixtures of 1 + 3 and 1 + 2 + 3 and the corresponding colors obtained by mixture of 1 + 2 + 3.

Molecular Geometry and Transitions of the Molecules

Density functional studies of the molecular geometries have proven to be an invaluable method to gain insights about the structural and optoelectronic properties. DFT-optimized geometries of the molecules exhibit a significant variation of torsional deviations in the gas phase using DFT/B3LYP/6-311G(d,p) level of theory. The mean value of the dihedral angle between the planes of C–N–C–C atoms from diphenylamine unit is predicted to be ∼53° (Figure ). The phenyl unit deviates from the plane with a torsional twist of ∼25° upon its attachment to the diphenylamine part, and the vinylene unit maintains coplanarity (2°–5°) with the acceptor units. On the other hand, torsion of the α-cyano group is considerably decreased with respect to increasing acceptor strength from the terminal phenyl (29.4°) to pyridyl (22.1°) and 4-nitrophenyl units (21.8°). The ground-state dipole moment (μg) of the compounds 1, 2, and 3 is computed to be 6.25, 9.28, and 12.26 D, respectively. It is noted that increasing the acceptor strength gradually augmented the ground-state dipole moment of the molecules. The greater dipole moment (μe) obtained from transient state evidences the positive solvatochromism of the molecules.

Figure 8

Optimized geometry of the molecules with selected torsional deviation predicted using DFT/B3LYP/6-311G(d,p) level of theory and comparison of TDDFT simulation vs experimental results along with their oscillator strength obtained from various functionals (B3LYP, CAM-B3LYP, and M062X)/6-311G(d,p)/CPCM(DMF) level of theory.

Electron density distribution analysis has been carried out to understand how the terminal acceptor units influence the charge distribution on α-cyanostilbenes. In highest occupied molecular orbital (HOMO), the electron density is distributed over the entire molecular system irrespective of the acceptor substitution. Nevertheless, the electron density of lowest unoccupied molecular orbital (LUMO) inclined toward the acceptor side upon photoexcitation, but it is difficult to predict the charge distribution over the α-cyano, and the terminal acceptor units as they seem identical (Figure ). However, remarkable intramolecular charge transfer (ICT) effect is found in the molecules. It is of crucial importance to understand the role of α-cyano and terminal acceptor units on the optoelectronic properties such as AIE, absorption, and emission.[52] Hence, to examine the exact contribution from the individual molecular segments toward the electron density population of the molecules in terms of their molecular orbital density, we performed partial density of states (PDOS) calculations by partitioning the molecular system into several segments such as diphenylamine (D), phenylvinylene (π), α-cyano (A1), and terminal acceptor units (A2), respectively.

Figure 9

Schematic diagram shows the energy level comparison of the molecules estimated from electrochemical method vs DFT predictions and the isodensity surface plots of the frontier molecular orbitals (FMOs).

As displayed in Figure , the electron density is uniformly distributed in HOMO that is revealed from the molecular orbital density contribution of the individual segments. On the other hand, in LUMO,  distinct variation was observed with respect to terminal acceptor units. It can be explained as follows:  Compound 1 measured in dioxane showed the emission peak at 496 nm. At 100% water, versatile intramolecular rotation behavior of vertically positioned α-cyano unit to the principal axis of the molecular system is restricted upon aggregation and yields a red-shifted emission peak.[47] Explaining this from LUMO-PDOS point of view, the contribution from the molecular orbital density of α-cyano segment is found to be 25% for the compound 1 which is the highest in the series. As a result, AIE of compound 1 is obtained at 517 nm with a ∼31-fold enhancement in the emission intensity. For compound 2, AIE peak appears at  a longer wavelength (561 nm) from its monomer peak at 517 nm in dioxane, but its emission intensity enhancement is significantly lowered to eight-fold. The moderate electron-accepting pyridyl unit abstracts the electron density from the α-cyano unit which decreases the intensity of AIE peak but increases the ICT character of the system.[36] In the case of compound 3, the nitro group retains 63% of the electron density population on it by eliminating the electron density from α-cyano group upon photoexcitation. As a result, the ICT character of compound 3 is shifted to longer wavelength (109 nm), but the AIEE effect is almost quenched. To say in general words, “acceptor strength of the peripheral units in a D−π–A system is inversely proportional to AIE effect”;  Compound 1: dominant AIE character > compound 2: moderate (AIE + ICT) effect > compound 3: dominant ICT character.

Figure 10

Percent contribution of molecular orbital electron density of the individual segments and its correlation with their AIE behavior of the molecules.

To simulate the experimental absorption of the molecules measured in dimethylformamide (DMF) and to get a deeper understanding of the excited-state transitions, TDDFT calculations were performed using different hybrid functionals such as B3LYP, CAM-B3LYP, and M06-2X with the 6-311G(d,p) basis set in DMF solvent medium. B3LYP functional reproduced an overestimated wavelength to the experimental results, whereas CAM-B3LYP yielded an underestimation for all of the molecules. The excitation energy predicted by M06-2X functional provides excellent agreement with the experimental results, and the corresponding values predicted using three functionals along with the oscillator strength are summarized in Table . The maximum absorption shifts to longer wavelength upon increasing the acceptor strength.[53] We also calculated the Mulliken population analysis for the molecules using the common 6-311G(d,p) basis set to get qualitative information on the partial atomic charge density of the atoms. All the heteroatoms bear a negative charge, thereby inducing a positive charge on the proximal carbon atoms, as shown in Figure S6.

Table 2

Calculated Eigenvalues of the FMOs of the Molecules  along with the HOMO–LUMO Gap, Excitation Energy along with Oscillator Strength, Major Transitions, and Ground and Transient Dipole Moment of the Molecules.

	HOMOa (eV)	LUMOa (eV)	Eoptb (eV)	HOMOc (eV)	LUMOc (eV)	EHLgapc (eV)	λB3LYPmaxd (nm)	λCAM-B3LYPmaxe (nm)	λM062Xmaxf (nm)	major compositionf	μgg (D)	μeh (D)
1	–5.45	–2.38	3.07	–5.44	–2.27	3.17	437.2 (1.11)	376 (1.32)	391 (1.28)	HOMO → LUMO (94%), H – 1 → LUMO (2%)	6.25	6.67
2	–5.50	–2.58	2.92	–5.55	–2.47	3.08	449.8 (1.13)	387.1 (1.34)	421.3 (1.31)	HOMO → LUMO (94%), H – 1 → LUMO (3%)	9.28	9.95
3	–5.50	–2.72	2.78	–5.56	–2.58	2.98	539.1 (0.89)	412.6 (1.52)	439.9 (1.39)	HOMO → LUMO (96%), H – 1 → LUMO (4%)	12.26	13.09

Deduced from the formulas HOMO = −[4.4 + Eox] and LUMO = −[Eopt – HOMO] from the ca. corresponding oxidation potentials of the molecules measured in DMF with 0.1 M tetrabutylammonium perchlorate (Ta BAPC) as a supporting electrolyte with a scan rate of 50 mV s–1.

Band gap obtained from the onset absorption of the molecules measured in DMF.

Computed energy levels of the molecules obtained from the DFT/B3LYP/6-311G(d,p) level of theory.

Excitation energy of the molecules (oscillator strength values in parenthesis) predicted from TDDFT/B3LYP level of theory.

Excitation energy of the molecules (oscillator strength values in parenthesis) predicted from CAM-B3LYP level of theory.

Excitation energy of the molecules (oscillator strength values in parenthesis) predicted from M06-2X/6-311G(d,p)/CPCM(DMF) level of theory.

Ground state dipole moment of the molecules obtained from the DFT/B3LYP/6-311G (d,p) level of theory.

Transient dipole moment obtained from the M06-2X/6-311G(d,p) level of theory.

The ESP surface analysis reveals the most negative valued potential localization sites on the cyano and terminal electron acceptor units, respectively, that can efficiently undergo intermolecular interactions upon photoexcitation (Figure S7). The geometrical coordinates of the molecules are given in Figure S8.

Electrochemical Studies

The redox activity of the complexes was studied by cyclic voltammetry (CV) using a standard three-electrode configuration. In DMF, the scan started from the cathodic (reduction) direction toward the anodic side (oxidation). Irreversible oxidation features were observed for all of the three complexes ∼0.4–0.8 V versus ferrocene. Compound 1 exhibits a small shoulder on oxidation side around 0.45 V versus ferrocene and a subsequent clear oxidation feature at 0.65 V versus ferrocene. Nasar and co-workers have attributed the first peak as oxidation of triphenylamine group for analogous compounds.[54] The second feature is possibly because of the two successive oxidations of the conjugated double bond present in the stilbene framework, which subsequently produces the radical cation. Compound 2 also showed analogous anodic features; however, the position of those peaks were altered. The triphenylamine oxidation peak shifted slightly toward cathodic direction (negative side) at 0.43 V versus ferrocene, where the olefin oxidation peak moved further anodic (positive direction), and it was observed at 0.70 V versus ferrocene. Compound 3 exhibited two strong oxidation features around 0.55 and 0.70 V versus ferrocene (Figure S9). Similar oxidation features for the molecules and the molecular mixtures were also noted in water (30 μL of a 10 μM stock solution of dioxane is added to 3 mL of H2O). Here, the potentials reported are with respect to a standard hydrogen electrode (SHE). Compounds 1 and 2 have shown oxidation features at 1.14 and 1.16 V versus SHE [Figure and Table S2]. Interestingly, when these two compounds were mixed, an average oxidation potential (0.15 V vs SHE) was observed for the same oxidation process rather than retaining the original features. Analogously, mixing compounds 2 and 3, or 1 and 3, results in similar averaging of oxidation processes (Figure B). The electrochemical data suggests that the compounds were interacting as a mixture through π–π stacking that altered their electronic distribution.

Figure 11

CV data of (A) compounds 1, 2, and 3 and (B) their two component mixtures in 3% dioxane–water solvent. The data were recorded with a glassy carbon disc electrode at 0.1 V/s scan rate. Each scan was started from 0.4 V to a positive direction.

Thermal Stability

For potential applications in organic electronics, thermal stability is an important parameter. The temperature stability of the molecules was measured using thermogravimetric analysis (TGA) and differential scanning calorimetric (DSC) analysis. The molecular systems bearing triphenylamine units are expected to have appreciable thermal stability because of extensive delocalization and the strong possibility of π–π interactions. The TGA and DSC measurements reveal that the synthesized molecules are stable in the range of 312–350 °C with insignificant (∼5%) weight loss.

Thermal stability of the molecules gradually increased in the following order: 1 < 2 < 3 (Figure S10). The high thermal stability of triphenylamine derivatives is attributed to the presence of a greater number of phenyl units. The melting point of molecules obtained from DSC was different from TGA because of tight molecular packing, and the values are found in the range of 146–194 °C. The excellent thermal stability of the compounds reveals their utilization in optoelectronic device applications.

Conclusions

In conclusion, we successfully obtained white light emission by using different CIE color coordinates (0.39, 0.37) and (0.33, 0.31) for binary and ternary combinations, respectively, of structurally similar D−π–A based on α-cyanostilbene constructs with varying electron-withdrawing groups. The synthesized molecules show excellent solvatochromic emission because of ICT as well as strong emission in the solid state. Varying proportions of the compounds yield tunable emission signatures and distinct colors. DFT studies illustrated a fine-tuning of AIE and ICT effects with respect to the terminal acceptor units present in the compounds. The electrochemical results also suggested intermolecular interactions in mixture conditions. Thus, these results have highlighted the potential of using tunable photoactive organic compounds for the fabrication of tunable optical materials through simple structural changes.

Experimental Section

Reagents required for the synthesis of the phenylacrylonitrile derivatives were obtained from Aldrich, Alfa Aesar, Sigma-Aldrich, and S. D. FineChem. The absorption spectral data were recorded using Analytic Jena, Specord 210 spectrophotometer. Fluorescence emission spectra were obtained using Horiba Fluorolog-3 spectrofluorimeter. 1H and 13C NMR spectra were obtained with a Bruker Avance 500 MHz NMR spectrometer. The accurate mass analysis was performed using a Waters-Synapt G2S (ESI-QToF) mass spectrometer. For the absorption or fluorescence studies, 10 μL of the desired fluorophore stock solution in dioxane (∼10–3 M) was added to 1 mL (effective concentration ≈ 10 μM) of the desired solvent media. Typically, the excitation wavelengths were set at the absorption maxima (λabs) of the compounds under investigation, and the emission (λem) reported is uncorrected. All the measurements were performed at room temperature. SEM analysis was carried out using a field-emission SEM (JSM 7600F JEOL). For this purpose, one drop of the sample [∼10 μM solutions] was deposited on a Si wafer mounted on an aluminum stub with the help of a double-sided adhesive carbon tape. The samples were heat-dried at 35 °C for 12 h and vacuum dried for 30 min to ensure complete removal of any residual water and coated with platinum before being analyzed. TGA was performed with a TGA/SDTA 851e (Mettler Toledo) thermal analyzer using a heating rate of 10 °C min–1 under a nitrogen atmosphere in the temperature range of 33–550 °C. CV measurements were performed on Metrohm Autolab Potentiostat (PGSTAT101) with the dyes at variable scan rates in an organic solvent (DMF) or organic solvent–water mixture. A 1 mm diameter glassy carbon disc electrode was used as a working electrode, whereas a Pt wire and Ag/AgCl in saturated KCl solution were used as a counter and a reference electrode, respectively. Ferrocene and K4Fe(CN)6 were used as internal standards for organic (0 V vs ferrocene couple) and aqueous solutions (or +0.36 V vs SHE), respectively.

Synthetic Methods

The synthetic strategy for the synthesis of desired derivatives is given in Scheme .

Synthesis of Triphenylamine-1-yl Acetonitrile Derivatives (1–3)

Substituted phenylacetonitrile derivatives (3.6 mmol) and formyl triphenylamine (3.6 mmol) was taken in a round-bottomed flask. To this, 120 μL of piperidine in 10 mL of ethanol was added, and the contents of the flask were heated for 24 h at 80 °C. The reaction was cooled to room temperature and washed with ethanol. Light yellow 1, dark yellow 2, and red 3 colored products obtained were purified using column chromatography. The characterization data is given in the Supporting Information Figures S11–S18.

(Z)-3-(4-(Diphenylamino)phenyl)-2-phenylacetonitrile (1)

UV (dioxane, λabs: 395 nm, ε = 20 282 M–1 cm–1), mp 174–176 °C. Yield 94%. FT-IR (cm–1): 2215.3 (−C≡N), 1520.62 (−C=C aliphatic), 1584.28 (C=C aromatic), 3020.5 (=C–H aromatic), 3010 (=C–H aliphatic). 1H NMR (500 MHz, CDCl3, δ ppm): 7.783–7.767 (d, J = 8.0 Hz, 2H), 7.652–7.636 (d, J = 8 Hz, 2H), 7.437–7.408 (t, J = 8 Hz, 3H), 7.365–7.350 (d, J = 7.5 Hz, 1H), 7.330–7.299 (t, J = 7.5 Hz, 4H), 7.168–7.153 (d, J = 7.5 Hz, 4H), 7.135–7.106 (t, J = 7.5 Hz, 2H), 7.058–7.042 (d, J = 8 Hz, 2H); 13C NMR (125 MHz, CDCl3, δ ppm): 151.06, 150.46, 146.23, 144.29, 142.56, 131.48, 129.69, 126.09, 124.93, 123.37, 120.14, 119.90, 119.58, 117.78, 104.51; (ESI–MS) m/z: ([M + H]+) calcd mass, 372.1626; found, 373.1612.

(Z)-3-(4-(Diphenylamino)phenyl)-2-(pyridine-4-yl)acrylonitrile (2)

UV (dioxane, λabs:421 nm, ε = 10 730 M–1 cm–1), mp 185–188 °C. Yield 94%. FT-IR (cm–1): 2245.52 (−C≡N), 1533.14 (−C=C aliphatic), 1577.5 (C=C aromatic), 3027.5 (=C–H aromatic), 2922.5 (=C–H aliphatic). 1H NMR (500 MHz, CDCl3, δ ppm): 8.652–8.642 (d, J = 5.0 Hz, 2H), 7.827–7.809 (d, J = 9.0 Hz, 2H), 7.592 (s, 1H), 8.531–7.521 (d, J = 5.0 Hz, 2H), 7.355–7.324 (t, J = 7.5 Hz, 4H), 7.54 (t, J = 7.5 Hz, 1H), 7.48 (t, J = 6.5 Hz, 1H); 13C NMR (125 MHz, CDCl3, δ ppm): 140.38, 134.59, 132.84, 131.30, 130.71, 129.86, 129.43, 129.20, 128.75, 127.98, 127.44, 126.35, 126.24, 126.14, 126.01, 124.99, 124.75, 124.58, 122.58, 118.16, 114.91; (ESI–MS) m/z: ([M + H]+) calcd mass, 373.1657; found, 374.1642.

(Z)-3-(4-(Diphenylamino)phenyl)-2-(4-nitrophenyl acrylonitrile) (3)

UV (dioxane, λabs:438 nm, ε = 9275 M–1 cm–1), mp 192–194 °C. Yield 94%. FT-IR (cm–1): 2215.3 (−C≡N), 1563.84 & 1342.35 (−NO2), 1507.53 (−C=C aliphatic), 1582.82 (C=C aromatic), 3030 (=C–H aromatic), 3020 (=C–H aliphatic). 1H NMR (500 MHz, CDCl3, δ ppm): 8.66 (d, J = 8.0 Hz, 1H), 8.60 (s, 1H), 8.31–8.24 (m, 2H), 8.22–8.03 (m, 3H), 7.86 (d, J = 7.5 Hz, 1H), 7.54 (t, J = 7.5 Hz, 1H), 7.48 (t, J = 6.5 Hz, 1H); 13C NMR (125 MHz, CDCl3, δ ppm): 140.38, 134.59, 132.84, 131.30, 130.71, 129.86, 129.43, 129.20, 128.75, 127.98, 127.44, 126.35, 126.24, 126.14, 126.01, 124.99, 124.75, 124.58, 122.58, 118.16, 114.91; (ESI–MS) m/z: ([M + H]+) calcd mass, 417.1477; found, 418.1409.

Computational Details

All the theoretical calculations have been performed within the framework of DFT using Gaussian 09 quantum ab initio software package. Geometry optimization of the molecules was carried out using DFT/B3LYP/6-311G(d,p) level of theory. The optimized geometries are subjected to vibrational frequency analysis to ensure the real minima on the potential energy surface. All the simulations were carried out in the gas phase, and no symmetry constraints were imposed during the optimization. These molecular structural coordinates were used as input for further single-point calculations including electron density distribution analysis of FMOs, TDDFT simulations, and PDOS analysis. TDDFT calculations (first 15 vertical singlet–singlet transitions) have been employed using various hybrid functionals such as B3LYP, CAM-B3LYP, and M06-2X with a standard 6-311G(d,p) basis set.[55] The integral equation formalism conductor-like the polarizable continuum model (C-PCM) within the self-consistent reaction field theory was used to describe the solvation of the molecules.[56] To interpret the major portion of the absorption spectrum and to analyze the nature of transitions, GaussSum 2.2.5 software has been utilized.[57] Mulliken population analysis and electrostatic potential surface (ESP) studies have been employed to examine the partial atomic charge and charge population of the molecules, respectively.