Aggregation-Induced Emission and White Luminescence from a

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Article Cite This: ACS Omega 2018, 3, 13757−13771

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Aggregation-Induced Emission and White Luminescence from a Combination of π‑Conjugated Donor−Acceptor Organic Luminogens Paramita Das,†,‡,§ Atul Kumar,†,§ Aniket Chowdhury,† and Partha Sarathi Mukherjee*,† †

Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India Department of Chemistry, Asutosh College, 92, S. P. Mukherjee Road, Kolkata 700026, India

‡

ACS Omega 2018.3:13757-13771. Downloaded from pubs.acs.org by 5.101.219.51 on 10/22/18. For personal use only.

S Supporting Information *

ABSTRACT: Two new star-shaped phenyl- and triazine-core based donor−acceptor (D−A) type conjugated molecules bearing triphenylamine end-capped arms were synthesized and characterized as imminent organic optoelectronic materials. Photophysical properties of the compounds were explored systematically via spectroscopic and theoretical methods. Because of the presence of donor−acceptor interactions, these luminogens display multifunctional properties, for instance, high extinction coefficient, large stokes shift, and pronounced solvatochromic effect. The compounds also exhibited phenomenon known as aggregation-induced emission on formation of nano-aggregates in the tetrahydrofuran−water mixture. The aggregate formation was confirmed by transmission electron microscopy, scanning electron microscopy, and dynamic light scattering analyses. Moreover, by controlling the electron withdrawing ability of the acceptor, complementary emissive fluorophores (blue and yellow) were achieved. These two complementary colors together span the entire range of visible spectrum (400−800 nm) and therefore when mixed in a requisite proportion generate white light in solution phase. These findings have potential for the progress of new organic white light radiating materials for applications in lighting and display devices.

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(AIEgens) have been developed that include siloles,9 cyanostilbenes,10 tetraphenylethenes,11 anthracene derivatives,12 diphenyldibenzofulvenes,13 pyrrole,14 diketopyrrolopyrrole derivatives,15 and many others.16−22 The enhanced emission of all the AIEgens in their aggregate and solid form are also traced to the nonplanar aromatic backbone that restricts formation of π stacking. However, for most of the conventional AIE materials, the emission intensity gradually increases with aggregate formation, but the emission peak maxima do not change their position. Therefore, the initial emission intensity often remains in the background affecting the fluorescence enhancement factor calculation. A better alternative would be to develop a system that changes its peak position along with emission enhancement upon aggregate formation. In the literature, several serendipitous reports have appeared where it is observed that molecules with twisted donor−acceptor (D−A) systems change emission maxima upon aggregate formation.23 The change in emission peak position is mainly attributed to the transformation of the molecule from its twisted intramolecular charge transfer (TICT) state in solution to the π−π stacking state in the aggregates. Upon aggregate formation, the molecular motion

INTRODUCTION Over the last several years, organic conjugated fluorophores have been the subject of active research1 owing to their bright prospects in materials science, optoelectronics, and biological fields.2 The rapidly expanding exploration of organic light emitting materials can be traced back to their easy synthesis, facile modification, excellent photostability, high emission efficiency in the visible region, tunable emission color, and low toxicity.3 However, conventional luminescent materials display high emission in dilute solutions but becomes weakly emissive in the solid state owing to the strong π−π stacking interaction which promotes the formation of detrimental nonradiative excimers and exciplexes.4 To mitigate this aggregation-caused quenching effect, various approaches have been adopted with only limited accomplishment.5 In 2001, Tang et al.6 reported a new class of materials known as aggregation-induced emissive (AIE) materials; wherein, upon aggregation, light emission was improved rather than being quenched. In 2002, Park et al. reported aggregation-induced emission enhancement (AIEE) materials similar to AIE materials.7 To decode the unconventional AIE phenomenon, systematic experimental measurements and theoretical calculations have been implemented, which led to the conclusion that restriction of intramolecular motions including rotation, vibration, stretching, and so forth is the main cause for the AIE effect.8 Taking cue from the aforementioned fact, a series of new AIE luminogens © 2018 American Chemical Society

Received: July 19, 2018 Accepted: October 5, 2018 Published: October 22, 2018 13757

DOI: 10.1021/acsomega.8b01706 ACS Omega 2018, 3, 13757−13771

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Scheme 1. Synthesis of 1 and 2

generate white-light emission. However, these procedures often suffer from setbacks including cumbersome low yielding synthesis, uncontrollable energy transfer, temperature sensitivity, and so forth. Although physical mixing is the most widely used way to develop white light, several aspects of these methods still require further attention. The white light generated by physical mixing strictly depends on structural similarity of the precursor. A preferable homogeneous mixture can only be achieved when the molecules are almost similar in size and shape, and they do not display a large difference in their absorption, quantum yield, photostability, and other physical properties. Also, a slight variation in their ratio drastically changes the emission of the combined mixture. Therefore, for practical applications, a binary/ternary system which can exhibit white light over a varied range of compositions is of particularly interest. We herein report a pair of isostructural fluorophores that display AIE, originated from their TICT-aggregate dual emission behavior. Besides, by regulating the electron-withdrawing ability of the acceptor, complementary emissive fluorophores (blue and yellow) were achieved. Physical mixing of these two fluorophores generates white light for a wide range of precursor ratios.

gets restricted, hence TICT get turned off and emission occurs from aggregates. Triphenylamine (TPA) derivatives have been extensively applied as the electron donor (D) in optoelectronic materials,24 and triazine (TRZ) is known as a strong electron acceptor (A) with high symmetry and AIE activity.25 Again, conjugated star-shaped donor−π−acceptor (D−π−A) systems have exhibited some special merits such as broad absorption, large stokes-shift, easily regulated fluorescence, as well as excellent nonlinear optical properties.26 By taking all these factors into consideration, in this work, we synthesized two conjugated star-burst D−π−A type molecules. The luminescence behavior of the molecules changed upon aggregate formation and aggregation-induced emission (AIE) was observed.23 Because of their difference in the extent of D− A interaction, the two molecules exhibited blue and yellow photoluminescence in polar solvents. The blue and yellow are complementary colors and together they cover the whole range of visible spectrum. It is reported in the literature that by controlling the ratio of the molecules with complementary emission, one can successfully generate white light which covers the whole range of visible spectrum. The design and advancement of white-light emissive luminescent materials with tunable emission properties have recently enticed colossal attention in modern exploration owing to their applications in optoelectronics, light emitting diodes, optical data storage, photonics, artificial photosynthesis, as well as solar cell devices, chemosensors, and biosensors.27 Therefore, it is not unexpected that tremendous research efforts have been invested for developing white-light emitting materials. 28−36 In the literature, several binary and ternary systems27m,37 with complementary photoluminescence have been combined to develop efficient white light in solution and solid. Apart from physical mixing, various other principles including fluorescence resonance energy transfer,38 inter- and intramolecular charge transfer,39 excited state intramolecular proton transfer,40 symmetry breaking,41 doping in composite structures,42 photo-induced electron transfer,43 hydrogen bonding facilitated J-aggregation,44 mixing of monomer and excimer fluorescence,45 and so forth have been used to

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RESULTS AND DISCUSSION Design Strategy, Synthesis, and Characterization. Two star-shaped D−A conjugated molecules (1 and 2) were prepared by means of the multistep synthesis as shown in Scheme 1. S1a was synthesized from p-bromoacetophenone using the p-toluenesulfonic acid catalyst (see Supporting Information). The ethynyl groups were introduced by Sonogashira coupling of S1a with trimethylsilylacetylene using [Pd(PPh3)2Cl2] and CuI to afford S2a in 84% yield. Subsequent desilylation at room temperature in a mixture of dichloromethane and methanol (2:1), in the presence of K2CO3, yielded S3a. Finally, compound 1 was synthesized in 74% yield by Sonogashira coupling of compound S3a with monobromotriphenylamine (S4) by using [Pd(PPh3)2Cl2] and CuI. Monobromotriphenylamine (S4) was synthesized follow13758


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ing a previously reported procedure.46 Compound 2 was prepared following the similar procedure. However, S1b was synthesized from 4-bromobenzonitrile using trifluoro acetic acid. The compounds 1 and 2 were characterized by 1H and 13C NMR spectroscopy (Figures S1−S9). The purity of the samples was analyzed by elemental analysis and melting point determination (see Supporting Information). Finally, from the ESI-MS (Figures S10 and S11) and single crystal Xray diffraction study (Figure 1a, CCDC 1850927), the formation of the compounds was confirmed.

Figure 2. Optimized structures of (a) 1 and (b) 2; HOMO (bottom) and LUMO (top) orbitals of (c,d) 1 and (e,f) 2, as well as their energies calculated at the B3LYP/6-31G(d) level.

TPA end-group as seen in Figures 2 and S15. This overlapping electron distribution between the two frontier molecular orbitals is crucial for their optoelectronic device performance. The HOMO and LUMO energies of 1 are close to −4.92 and −1.43 eV with a calculated band gap of 3.5 eV. Compound 2 exhibits a similar electronic structure for HOMO energy level. However, the LUMO is largely concentrated on 2,4,6triphenyl-1,3,5-triazine because of its highly electron-deficient nature. The contribution of the bridging acetylene bond and the phenyl moiety of TPA in LUMO is much diminished compared to compound 1. The HOMO of compound 2 is also triply degenerate and the LUMO is doubly degenerate with energy values close to −4.99 and −2.08 eV, respectively (Figure S16). The calculated band gap is 2.9 eV. The maximum difference between the HOMO energy levels of the two molecules is 0.07 eV, which indicates that the HOMO of 2 is unaltered by TRZ. However, the LUMO (−2.1 eV) is lowered compared to 1 (−1.4 eV) because of the electrondeficient nature of the TRZ unit. This low lying LUMO in compound 2 facilitates faster and greater extent of charge transfer from donor to acceptor in the excited state.48 The calculated frontier orbital energy energies and relative energy band gaps (Eg) are presented in Figure 2 and Table 1. This follows the same trend that was observed from the UV−vis absorption analysis. Photophysical Properties. UV−vis spectroscopic studies of individual compounds (benzene core containing compound 1 and TRZ core containing compound 2) were carried out in tetrahydrofuran (THF) to probe the absorption pattern. The optical characteristics of compounds 1 and 2 are summarized in Table 1. Both the compounds exhibited dual absorption bands. The dual band characteristic is quite common in twisted D−A structures because of partial localization of the conduction band.49 It is clearly observed from Figure 3 that compound 1 has UV absorption bands at 295 nm (ε = 1.380 × 105 mol L−1 cm−1) and 365 nm (ε = 1.836 × 105 mol L−1 cm−1) in THF. Compound 2 also exhibited two absorption bands at 310 nm (ε = 1.337 × 105 mol L−1 cm−1) and 400 nm (ε = 1.231 × 105 mol L−1 cm−1). The high energy absorption band can be assigned to the n−π* transition of TPA moiety, whereas the low energy structured absorption band could be attributed to intramolecular charge transfer 1ICT transitions from the electron-releasing TPA to the electron-withdrawing

Figure 1. (a) Single-crystal XRD structure of 2 (CCDC 1850927); (b) crystal packing of 2 viewed along c axis. Color codes: blue (N), dark gray (C), and light gray (H).

Suitable crystals of compound 2 were grown by slow evaporation of saturated solution of the compound in CHCl3. Structural refinements of 2 revealed that two molecules of 2 are oriented into a staggered type π−π stacking fashion in a single unit cell. Moreover, the closest distance between two molecules in a unit cell is 3.25 Å (C3[1]−N3[2], Figure S12). Crystal structure of 2 shows a planar geometry in the TRZ core having a torsional angle of 1.34° (N1−C2−C10−C11, Figure S13). The distance between the tri-phenylamine core to the TRZ core is approximately 12.5 Å (C1−N4, Figure S13). Crystal packing showed that compound 2 has layered packing when viewed along the c axis (Figure 1b). Important crystallographic data and refinement parameters of 2 are provided in Table S1. Molecular Modeling Analysis. The ground-state geometries of 1 and 2 were fully optimized using density functional theory (DFT) at the B3LYP/6-31G(d) level, as implemented in the Gaussian 09 program package.47 Figure 2 illustrates the optimized structures and the electron density distribution of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for compounds 1 and 2. As shown in Figure 2, the 1,3,5-triphenylbenzene (TPB) part of 1 shows a large deviation from planarity. The dihedral angle between the central benzene ring and the substituted phenyl is calculated to be 37.95°. On contrary, the 2,4,6-triphenyl-1,3,5-triazine part of 2 retains an almost planar geometry. The dihedral angle between the central TRZ ring and the substituted phenyl is 0.07° (≈0°), which is in close agreement with the X-ray single crystal structure analysis of 2 with dihedral angle of 1.34°. However, the TPA at the terminal motifs is twisted away from the molecular backbone. The HOMO of compound 1 is triply degenerate and mainly concentrated on electron-rich TPA and spreading up to the bridging alkyne bond (Figure S15). The LUMO is doubly degenerate, but not concentrated only on the TPB core, rather it extends mostly up to the bridging acetylenic bond with little involvement of benzene moiety of 13759


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Table 1. Optical Properties and DFT Study of Compounds 1 and 2 absorption

emission

DFT study

molecules

λmax (nm) (soln)

λmax (nm) (film)

a ΔEopt g (eV)

λmax (nm) (soln)

λmax (nm) (film)

τ (ns)

HOMO (eV)

LUMO (eV)

ΔEg (eV)

1 2

295, 365 310, 400

303, 371 311, 409

2.97 2.66

435 545

497 508

1.43 1.46

−4.92 −4.99

−1.43 −2.08

3.50 2.91

Band gap was calculated from absorption spectra of the thin film.

a

effective D−π−A delocalization leading to effective energy transfer from donor (TPA) to acceptor (TPB) moiety. Therefore, both the molecules 1 and 2 exhibit single emission peaks at 435 and 545 nm, respectively (Figure 3). Both the molecules showed considerable amounts of Stokes-shift of 70 and 145 nm and the quantum yields are 26.01 and 22.20% for compounds 1 and 2, respectively in THF. The photoluminescence lifetimes measured by the time-correlated single-photon counting technique reveal that both the compounds have comparable exciton lifetime presumably due to structural similarity (Tables 1 and S2). Solvatochromism. The large Stokes-shift observed for compounds 1 and 2 prompted us to further explore the charge transfer interaction by studying the emission behavior of the substrates in solvents with varying polarity. The UV−vis spectra of both the compounds showed that the absorption profiles changed slightly and were practically free from the effect of solvent polarity (Figures 4a and 5a). On the other hand, discrete changes in emission color were observed with alteration of solvent polarity (Figures 4b and 5b). For compound 1 in low polar solvents such as hexane and toluene, the emission maxima are at around 390 and 410 nm. Nevertheless, with increasing solvent polarity, the emission maxima gradually shifted to longer wavelength [475 nm in dimethyl sulfoxide (DMSO)]. The change in emission maxima is accompanied by decrease in fluorescence intensity, which can be attributed to pronounced TICT effect in polar solvents. The observed red-shifted (but weakened) emission intensity of fluorophores is attributed to the TICT process as the TICT

Figure 3. UV−vis absorption and emission spectra of 1 and 2 in THF (c = 10−5 M).

cores and 1π−π* transitions.50 Compound 1 has a shorter onset of absorption wavelength (λonset = 418 nm) and thus has a larger optical gap (2.97 eV). On contrary, introduction of an electron deficient TRZ unit in 2 compared to benzene unit in 1 results in the longer onset of absorption wavelength (λonset = 466 nm) and a smaller optical gap (2.66 eV). Being rigid compared to the TPB moiety, the 1,3,5-triphenyl triazine (TPT) unit gives better planarity to the system 2, thereby permitting better electron delocalization. This conclusion is further supported by the DFT calculations. Though compound 1 is more nonplanar than compound 2, still it retains electronic communication, which results an

Figure 4. (a) Absorption spectra of 1 in different solvents (c = 10−5 M); (b) fluorescence emission spectra of 1 in different solvents (λex = 370 nm, c = 10−5 M); (c) photographs of 1 in solvents with different polarity taken under the irradiation of 365 nm UV light. 13760


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Figure 5. (a) Absorption spectra of 2 in different solvents (c = 10−5 M); (b) fluorescence emission spectra of 2 in different solvents (λex = 405 nm, c = 10−5 M); (c) photographs of 2 in solvents with different polarity taken under the irradiation of 365 nm UV light.

Figure 6. (a) Absorption spectra of 1 in the THF−water mixture (top left) and (b) fluorescence spectra (top right) in the THF−water mixture (λex = 370 nm, c = 10−5 M). (c) Photographs of 1 with varying amounts of water fractions under 365 nm UV light in the THF−water mixture.

in 2 as evidenced from a greater red shift and highly quenched fluorescence in polar solvents (DMF and DMSO). AIE Study. After exploring the origin of absorption and emission bands in UV−vis and fluorescence spectra, we next investigated the effect of aggregation on the luminescence behavior of the compounds. Aggregates were generated by gradually increasing the fraction of water into the THF solution of the compounds. The formation of aggregates was further characterized by dynamic light scattering (DLS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) analyses. The absorption band of 1 did not show any appreciable change with gradual addition of water in THF up to 40% (Figure 6a). However, a noticeable diminish in absorption

state is vulnerable to various nonradiative decay processes. TICT is quite a common phenomenon in systems consisting of donor and acceptor groups linked by a single bond, where the rotation around that bond leads to decoupling of the orbitals of these two groups providing the way for almost complete electron transfer from the donor to the acceptor. The highly polar TICT state is preferentially stabilized with respect to a planar local excited (LE) state in polar solvent. In nonpolar solvents, the emission occurs from a locally excited (LE) state; however, when polar solvents are used emission occurs from TICT giving rise to red-shifted fluorescence. Because compound 2 consists of a stronger acceptor (A) unit (TRZ) compared to compound 1, the TICT effect is more profound 13761


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Figure 7. (a) Absorption spectra of 2 in the THF−water mixture (top left) and (b) fluorescence spectra (top right) in the THF−water mixture (λex = 405 nm, c = 10−5 M). (c) Photographs of 2 with varying amounts of water fractions under 365 nm UV light in the THF−water mixture.

THF−water. Subsequently, a striking fluorescence enhancement with a blue-shift of peaks was recorded. This improved emission and the blue-shifted peak could be ascribed to the aggregate formation owing to the degraded solvating power of the aqueous mixture. When larger aggregates are formed at higher water fraction, the twisting motion of the molecular subunits is totally restricted; therefore, the TICT process is disrupted. From crystal structure analysis, it is also evident that due to steric hindrance the compound can form staggered type π−π stacks in the aggregate. The final emission comes from the cumulative excited state along with a blue shift in emission maxima compared to the TICT emission.52 Unlike the conventional AIE systems, where the emission intensity only increases, both the fluorophores upon aggregation display shift in their emission peaks. For compound 1, the peak undergoes bathochromic shift due to change from LE state to aggregate with π−π interacting form, and compound 2 undergoes hypsochromic transition from TICT to aggregate form. Therefore, both the complexes exhibit AIE. In our investigation, our deliberate effort to include donor−acceptor interaction and incorporation of highly twisted TPA rings yielded in generation of different emissive species including LE, TICT, and π−π stacked aggregated state, which ultimately manifested in the AIE behavior. Particle Size Analyses. The sudden change in the emission behavior with the change in the water fraction for both the compounds prompted us to further investigate the shape, size, and morphology of the aggregates using various techniques including DLS, SEM, and TEM analyses. For TEM analysis, appropriate solutions of the complex with different THF−water compositions were drop-casted on copper grids and slowly dried to afford the final sample. In mixtures with low water content (50%), both compounds 1 and 2 selfassembled into regular nanospheres with diameters of 352 and 575 nm. However, when the water fraction was increased substantially (>70%), the rapid formation of the aggregate led to the formation of smaller size particles with diameters of 300 and 150 nm, respectively (Figure 8).

intensity was observed in 60% water in THF mixture with a red shift of about 20 nm. Furthermore, a large decrease in molar absorptivity and a maximum red shift of about 60 nm could be discerned at the water fraction (f w) of 70%. Level-off tails were observed in the visible spectral region at a water fraction higher than 40%. Such tails are proposed to originate from the Mie scattering effect when spherical nano-aggregates are formed in the medium.51 As shown in Figure 6b, luminogen 1 in pure THF (10−5 mol −1 L ) exhibited an emission band centered at 435 nm and emitted a blue fluorescence under 365 nm UV light. After adding a poor solvent (water) up to 40%, the emission intensity is slightly diminished and marginally red-shifted to 450 nm. Now, the electron density distributions of HOMO and LUMO of the geometry optimized structure of 1 (Figure 2) indicate that there is minimum extent of charge transfer in its excited state because of poor electron-accepting power of the TPB ring. Hence, the emission mostly occurs from the locally excited state (LE). Upon increasing the water fraction to 70%, the aggregate size increases and along with a further bathochromic shift in the emission spectra. It is well known that dyes upon aggregation can form π−π stacked structures of the fluorophores, leading to a reduced band gap with red shift in their emission maxima.23a Thus, it can be speculated that upon aggregation the compound 1 forms staggered type π−π stacks and the emission is red-shifted from 436 nm in pure THF to 500 nm in 90% water fraction. Compound 2 exhibited two absorption bands at 308 and 400 nm (Figure 7a). Similar to compound 1, for compound 2 a visible decrease in molar absorptivity with a red shift of about 20 nm was observed because of aggregate formation, which is also supported by the appearance of level-off tails. For 2 also, a maximum red shift was observed for water fraction (f w) of 70%. It shows yellow fluorescence with a peak maximum centered at 545 nm in dilute THF solution. After addition of water, emission intensity drastically dropped (from f w 0 to 30%) with a red shift of about 35 nm (Figure 7b). This reduction in fluorescence intensity and red shift is attributed to TICT in the gradually strengthened polar mixed solvent of 13762


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white-light emission. DCM was selected as the solvent for white-light generation because both the molecules were soluble in any proportions and produce white light in a remarkably wide range of ratios (Table S4, Figure S27). We initially examined the absorption properties of the luminogens in DCM solutions (Figures S23 and S24). In DCM, the compounds exhibited two sets of absorption bands which are almost identical to that in THF. In the initial check, we found a significant spectral overlap of the absorption spectrum of 2 with the emission spectrum of 1 in the solution state (Figure S25). Thus, there is a strong probability of fluorescence energy transfer cascading from donor (1) to acceptor (2). Again, the CIE chromaticity coordinate for 1 appears in blue region A (0.15, 0.11) and the coordinate for 2 appears in yellow region H (0.42, 0.54) in the CIE diagram (Figure 10d). These results acclaim that 1 and 2 can be effectively mixed to effect efficient white-light emission. To evaluate our proposition, engendering white-light emission by mixing molecular systems with complementarily emission colors, compounds 1 and 2 were mixed in varied proportions and the subsequent emissions were recorded. Consequently, a series of solutions was prepared by varying the relative concentrations of 1 and 2 (Table 2). Upon increasing the molar ratio of 2 against 1 from 0.0 to 0.9 equivalents, the intensity of higher energy band decreased gradually, and the intensity of the lower energy band increased progressively (Figure 10a). White light was observed within a certain range of ratios of 1 and 2 with the best result obtained when the ratio of 1 and 2 was 1:4.5 (Figure 10b). On the other side, different emissive colors were obtained when 1 and 2 were mixed in different other ratios. Interestingly, the compound which is present in higher quantities dominates the resultant fluorescence of the mixture of 1 and 2. For example, when the concentration of 1 is higher than that of 2 the resultant PL is blue color emission, which is the luminescence color of pure 1. When the concentration of 2 is greater than that of 1, the resultant PL is a yellow color emission corresponding to 2 (Figure 10c). The emission spectrum corresponding to point E covered the entire visible region (400−700 nm) (Figure 10b) with two equal intensity emission maxima at 457 and 550 nm. CIE coordinates were also calculated using luminescence data to check the purity of the white-light; and this falls in the range of 0.29, 0.33 at λex = 365 nm for the 1:4.5 mixture of 1 and 2, which was very close to pure white-light (0.33, 0.33). Previously, most of the reports on generation of white light by physical mixing of two components suffer from the fact that a minute change in their molar ratio totally changes their emission profile.53 This discrepancy mostly occurs due to uncontrollable energy transfer, formation of inhomogeneous mixture due to mismatch in size and difference in solubility. In our investigation, we succeeded in developing two separate fluorophores which are structurally similar and have comparable solubility, and because of the difference in their extent of donor−acceptor interaction, they display complementary emission color. Thus, just by adjusting their molar ratio, the energy transfer among the fluorophores is controlled and the white-light emission can be achieved. In our investigation, we found that the mixture exhibits white light over a large span of molar ratios of compounds 1 and 2 ranging from (3:17) to (7:13) with CIE coordinates of (0.30, 0.35) to (0.24, 0.29) which are generally acceptable. This upsurges the reproducibility of the work, where a minor change in the proportion

Figure 8. TEM images of aggregates with various THF/H2O fractions. Compound 1 in (a) 50% and (b) 70% H2O in THF; compound 2 in (c) 50% and (d) 80% H2O in THF.

To further verify our observation, DLS analysis was also performed on the aggregate with varying solvent fractions (Table S3). It was found that with low water fraction the formation of particles is gradual; thus, large size particles with the diameter of 352 nm appear as the main constituent for compound 1 (Figure S20). When the water fraction was increased to 70%, the average particle size reduced to 300 nm, which clearly supports our observation from TEM analysis. Compound 2 also exhibited a similar behavior along with a large decrease in the particle size from 575 nm in 50% water fraction to 155 nm in 80% water fraction (Figure S21). Finally, the morphology of the aggregates was obtained from SEM analysis of aggregated samples deposited on carbon tape after drying. As observed in Figures 9 and S22, both the

Figure 9. SEM images of aggregates with various THF/H2O fractions. Compound 1 in (a) 50% and (b) 70% H2O in THF.

compounds showed spherical morphology in their aggregate forms at the water fraction of 50%; and when the water percentage was increased, they retained their spherical morphology, but due to fast aggregate formation their effective size decreased. Generation of White Light. Compound 1 mainly shows blue emission in nonpolar to medium polar solvents including hexane, toluene, CHCl3, and DCM. Whereas the emission of compound 2 due to pronounced charge transfer is dominated by yellow emission. Both the molecules display structural similarity, similar solubility and fluorescence lifetime. When their normalized fluorescence intensity is plotted together, it spanned the entire visible spectrum region from 380 to 720 nm. Hence, it appeared that if these two complementary fluorophores are mixed in a requisite amount, it can display 13763


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Figure 10. (a) Fluorescence spectra of the mixtures of 1 and 2 in different proportions excited at 365 nm; (b) emission spectrum of a mixture of 1 and 2 showing white-light emission at 1:4.5 ratio in DCM solvent. (c) Photographs of different color emissions from different molar ratios of 1 and 2 in DCM UV light (365 nm). (d) CIE (1931) chromaticity plot for color coordinates of 1 (Point A) and 2 (Point H). The point E indicates white-light coordinates after mixing of 1 and 2 in DCM solution. The total chromophore concentration was c = 10−5 M in all the cases.

fluorophore 2 from yellow to red to green. AIE of the luminogens was achieved by progressive addition of water into their THF solutions. The aggregates formed were characterized by DLS, TEM, and SEM analyses. On the basis of on the acceptors, the emission colors can be regulated from blue (1) to yellow (2). By tuning the ratios between these two compounds almost pure white light was achieved. When these fluorophores were utilized together in the molar ratio of 1.0:4.5 in DCM, we obtained an intense white-light emission when excited at 365 nm, with CIE values of (0.29, 0.33). As a final remark, such a white-light emitting solution has prospect for the foundation of new white-light emitting materials for their potential application in solution-processed organic electronic devices.

Table 2. CIE Coordinates for the Mixtures of 1 and 2 in DCM with Different Concentrations entry

compound 1 (equiv)

10−3 M (μL)

compound 2 (equiv)

10−3 M (μL)

CIE coordinate (x, y)

A B C D E F G H

1 0.75 0.5 0.25 0.18 0.15 0.10 0

20 15 10 5 3.6 3 2 0

0 0.25 0.5 0.75 0.82 0.85 0.90 1

0 5 10 15 16.4 17 18 20

(0.15, 0.11) (0.17, 0.15) (0.21, 0.21) (0.26, 0.30) (0.29, 0.33) (0.31, 0.37) (0.33, 0.41) (0.42, 0.54)

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EXPERIMENTAL SECTION Materials and Methods. Reagents used in the present study were obtained from different standard commercial suppliers. A Bruker make 400 MHz spectrometer was used to record the NMR spectra, and the chemical shifts (δ) in the 1 H NMR spectra are reported in ppm relative to TMS as the internal standard (0.0 ppm) or proton resonance resulting from incomplete deuteration of the solvents CDCl3 (7.26 ppm). 13C NMR spectra were recorded at 100 MHz, and the chemical shifts (δ) are reported in ppm relative CDCl3 at 77.8−77.2 ppm. ESI-MS experiments were performed in an Agilent 6538 ultra-high-definition Accurate Mass Q-TOF spectrometer. A Bruker make ALPHA FTIR spectrometer was used for recording IR spectra. Elemental analyses (C, H, and N) were performed using a PerkinElmer 240C elemental analyzer. Analab μ-ThermoCal10 instrument was used for melting point determination. UV−vis and fluorescence spectra were recorded with PerkinElmer LAMBDA-750 and HORIBA Jobin Yvon Fluoromax4 spectrophotometers, respectively. The 1931 Commission Internationale de l’Eclairage (CIE) chromaticity coordinates for the corresponding photoluminescence spectra were calculated by “GOCIE” software.54 Timeresolved fluorescence measurements were carried out on

does not lead to deviation from white-light emission (Table S4, Figure S27). Thermal Study. Thermogravimetric analysis (TGA) was carried out to investigate the thermal stability of the obtained luminogens. As shown in Figure S27, the TGA analysis revealed that the onset decomposition temperatures of 1 and 2 under N2 are 405 and 430 °C, respectively. Both the compounds showed sufficient thermal stability owing to their extensive conjugated aromatic architectures. The high thermal stability of the TPA-based luminogens possibly permits them to find extensive applications for device fabrication.

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CONCLUSIONS Two new fluorescent, star-shaped donor−π−acceptor molecules with electron-accepting central core bridged by ethyne as π-conjugation arms and TPA as the peripheral end-capped electron-donating moiety have been synthesized and their photophysical properties are examined. These luminogens unveil numerous functional properties. They reveal large molar absorptivity, exhibit strong emission intensity, and demonstrate perceptible positive solvatochromic effect. Moreover, these fluorophores are thermally stable. Spectroscopic studies reveal that the fluorophores show AIE, which is very prominent for 13764


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Author Contributions

Fluorolog 04 using 370 and 395 nm nano-LED source. Quinine sulfate in 0.1 M H2SO4 was used to determine the experimental quantum yields at an excitation wavelength of 365 nm with Φ = 0.56 for all assemblies. The quantum yield measurements were done at least triplicate with values that were within 10% error being averaged. TEM was performed with a JEOL JEM-2100F instrument operating at 200 kV. DLS measurements were performed with a Zetasizer instrument ZEN3600 (Malvern, UK) with a 1738 back scattering angle and He−Ne laser (l = 633 nm). SEM images were obtained using the Zeiss Ultra-55 SEM instrument with the sample deposited on a silicon vapor. The THF−water mixtures with various water fractions were prepared by slowly adding ultrapure water into the THF solution of samples. TGA was performed in the Mettler Toledo TGA SDTA 851 instrument at a heating rate of 10 °C/min under an atmosphere of nitrogen. Single crystal X-ray data were collected on a Bruker D8 QUEST diffractometer using the SMART/SAINT software. Computational Methodology. Geometry optimizations were performed using the Gaussian 09 package.47 In all calculations, the hybrid B3LYP functional was used, as implemented in the Gaussian 09 package, mixing the exact Hartree−Fock-type exchange with Becke’s expression for the exchange functional55 that was proposed by Lee, Yang, and Parr for the correlation contribution.56 For all the calculations, the 6-31G basis set was used. Frequency calculations carried out on the optimized structures confirmed the absence of any imaginary frequencies. Quantum Yield Calculation. For fluorescence quantum yield measurements, quinine sulfate was chosen as a reference. The quantum yields were calculated by using eq 1. A ji η zy I ΦF = Φ × × R × jjjj zzzz IR A k ηR {

§

P.D. and A.K. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.S.M. is grateful to CSIR-India for financial support. P.D. is grateful to UGC (New Delhi) for the Dr. D. S. Kothari postdoctoral fellowship. We are thankful to Dr. Suman Ray (SSCU, IISc) for helpful suggestion on data analysis, TGA measurements, and solid-state UV experiments.

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ABBREVIATIONS TPA, triphenylamine; TRZ, triazine; TPB, 1,3,5-triphenylbenzene; TPT, 1,3,5-triphenyl triazine

2

(1)

ΦF and Φ(0.56) are the radiative quantum yields of the compounds and reference, respectively; A is the absorbance of the compound; AR is the absorbance of the reference; I is the area of emission of the compound; IR is the area of emission of the reference; and η and ηR are the refractive indices of the compound and reference solutions, respectively.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b01706.

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REFERENCES

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Synthesis of the compounds; 1H and 13C NMR spectra and ESI-MS data of compounds; X-ray data collection and structure refinements of compound 2; characterization data of the compounds; and coordinates of compounds 1 and 2 from computational studies (CCDC: 1850927) (PDF) Crystallographic information file (TXT)

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Corresponding Author

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Partha Sarathi Mukherjee: 0000-0001-6891-6697 13765


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