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Rational Tuning of AIEE Active Coumarin Based #-Cyanostilbenes Towards Far-Red/NIR Region Using Different #-Spacer and Acceptor Units Mahalingavelar Paramasivam, and Sriram Kanvah J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b01334 • Publication Date (Web): 05 May 2016 Downloaded from http://pubs.acs.org on May 6, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Rational Tuning of AIEE Active Coumarin Based α-Cyanostilbenes towards Far-Red/NIR Region Using Different π-Spacer and Acceptor Units Mahalingavelar Paramasivam† and Sriram Kanvah†, * †

Department of Chemistry, Indian Institute of Technology Gandhinagar, Palaj, Gandhinagar

382355, India.

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Abstract A series of cyanostilbene based D-π-A derivatives, comprising of 6-(diethylamino)coumarin as a donor, benzene and benzothiazole as acceptors bridged by different π-spacers (benzene, thiophene) have been synthesized and characterized. The influence of π-spacer and acceptor units on the photophysical, electrochemical and thermal properties of the dyes was investigated in detail. The incorporation of coumarin donor results in a significant increase of the fluorescence quantum yield. In the solution phase, the absorption and emission of the dye TB show a bathochromic shift (TB > BBT) indicating dominant dynamic intramolecular rotations (IMR) behaviour of cyano group over the acceptor functionality. On the other hand, the emission is reversed (BBT > TB) in the case of aggregates. The synergistic combination of intramolecular planarization and torsional alleviation around cyanostilbenes induced by strong electron withdrawing benzothiazole unit in BBT enabled pronounced bathochromic shift upon aggregation. Upon varying the spacer/acceptor substitution, the emission wavelength of the aggregates shifts from visible to far red/NIR region with concomitant large Stokes shifts of 158-213nm. The fluorescence intensity of the aggregates gradually decreases with increased planarity of the backbone. Various experimental techniques were employed to evaluate the structure- emission phenomena relationship. The aggregation induced enhanced emission (AIEE) behaviour of the dyes was substantiated from fluorescence lifetime decay studies and SEM analysis. DFT computed band gap follows a consistent trend with the values estimated from electrochemical-optical studies. All the dyes possess excellent thermal stability. Our study demonstrated far-red/NIR AIEE cyanostilbene derivatives exhibiting large Stokes shift having excellent thermal properties and fluorescence quantum yields could act as potential candidates for optoelectronic or imaging applications.


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Introduction Molecular engineering of organic π-conjugated materials having good luminescent properties is of great demand due to their potential applications towards optoelectronic and biological applications such as OLEDs, fluorescent probes for metal ion sensing, bioimaging and so forth.1 Most of the conventional fluorophores are fairly emissive in dilute concentrations but show quenching upon increasing the concentration owing to strong intermolecular interactions. This effect is commonly termed as aggregation caused quenching (ACQ) and doesn’t permit the fluorogens to be utilized in ‘turn-on’ mode for bioimaging applications. For biological applications, fluorophores should have intense emission at near infra-red (NIR) region, large Stokes shift, excellent thermal and photo stability. NIR fluorophores are advantageous in terms of high detection sensitivity, minimal interfering absorption, reduced emission scattering and enhanced tissue penetration.2 Although highly fluorescent at NIR region, well-established fluorophores such as squaraine, cyanine

dyes fail to meet the

aforesaid requirements and undergoes ACQ effect upon aggregation.3 To overcome this limitation, design of fluorophores based on aggregation induced emission (AIE) strategy is considered as a better alternative.4-6 In this design, structural constraints are imposed on the fluorophores that result in feeble or no emission in solution state but exhibit strong emission in their aggregated state.7 Till date, a large number of AIE luminogens have been reported using frameworks like siloles6, tetraphenylethenes8-9, cyanostilbenes10-11 and distyryl anthracenes.12-14 Closer examination of these investigations reveal that vast majority of these aggregates emit in the UV-visible region, otherwise their emission is either blue shifted or remains unchanged from the solution emission.15-17 However, these fluorophores can be finetuned to achieve the emission at NIR region by modulating various suitable functional building blocks at appropriate position while subtly balancing the AIE framework.18 One promising strategy is to design a system that can control the translation of molecules along their long axis from omni-directions. In other words, molecules with random orientations 3 ACS Paragon Plus Environment


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should be converted into favorable J-aggregation pattern during self-assembly.19-20 αCyanostilbenes are such kind of attractive building blocks owing to their characteristic twist elasticity behavior i.e., significant twist in solution phase and intramolecular planarization in the aggregated state. The main advantages of α-cyano substitution is their ease in functionalization at terminal positions21 and the dense molecular packing which may result in H or J type aggregation.22 However, the aggregation pattern of α-cyanostilbene derivatives is solely dependent on the conformation of the cyano group and can be controlled in two ways. 1) Position of α-cyano group 22-23 and 2) Incorporation of different functional building blocks proximal to the cyano group.24-25 Thus, molecular level knowledge of the conformation of αcyanostilbenes is a prerequisite for better understanding of the photophysical properties upon molecular packing. A variety of AIEE luminogens based on α-cyanostilbenes have been reported where the α-cyano substitution is at the center position.25-26 Single X-ray crystal structure analysis reveals that central substitution leads to formation of dimer aggregates through intermolecular interactions of the polar cyano group with inner hydrogens of neighboring terminal aromatic units.26 The resulting H-aggregation, leads to blue shifted emission of the aggregates from the monomers. To obtain remarkable bathochromic shift emission, it is thus essential to develop molecules with well patterned head-to-tail arrangement which yields extended intermolecular interactions upon aggregation.27-29 Furthermore, to achieve desirable optical properties, the interaction of cyanostilbenes should not be perturbed by the introduction of new functional building block in terms of steric hindrance around the cyano group and can limit the interaction of cyano group with adjacent molecules.35 Keeping these considerations in mind, we have introduced α-cyano substitution at the terminal position. Unlike the central position, the strategy of terminal α-cyano substitution is anticipated to result in significant changes in terms of ability to migrate the excitons during the aggregate formation.30-31 Herein, we report the synthesis and photophysical studies of four coumarin containing α-cyanostilbene derivatives with various 4 ACS Paragon Plus Environment

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spacer-acceptor combinations: coded BB (benzene-benzene), TB (thiophene-benzene), BBT (benzene-benzothiazole) and TBT (thiophene-benzothiazole) (Fig.1). The introduction of coumarin moiety in the cyanostilbene system is expected to enhance the fluorescence quantum yield.9, 32-34 The aim of this work is to explore the influence of spacers and acceptor units on the absorption and emission profiles of monomers and dye aggregates. Our studies reveal that dyes exhibit expected behavior in organic solvents. Interestingly, all the dye aggregates show bathochromic emission shifts (87-155nm) from monomer emission and the order is reversed in the case of BBT (BBT > TB). The reasons behind these observations are detailed below. Experimental Section Materials and general procedures Unless otherwise specified, all reactions were carried out under nitrogen atmosphere using standard Schlenk techniques. All reagents utilized for the synthesis of target dyes were reagent grade and used without further purification. All chromatographic separations were carried out on silica gel (60–120 mesh). 1H NMR and 13C NMR spectra were recorded on a Bruker Avance (500 MHz) spectrometer in CDCl3 and with TMS as a standard in both cases. Mass spectra were obtained using electron spray ionization mass spectrometry (Waters Synapt G2-S).UV-Vis absorption spectra were measured on Analytik Jena, Specord 210 model UV-vis spectrophotometer. Fluorescence spectra were recorded using Horiba JobinYvonFluorolog-3spectrofluorimeter. Fluorescence quantum yields of monomers were estimated using quinine sulphate (Φ = 0.545 in 0.5 M H2SO4) as reference. For aggregates, Rhodamine B (Φ = 0.65 in ethanol) was used as the fluorescence standard. Fluorescence lifetime measurements were performed on a picosecond time-correlated single photon counting (TCSPC) setup (Edinburgh Instruments Ltd, LifespecII model) employing a picosecond light emitting diode lasers (Nano LED, λex= 405nm) as the excitation source. The



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decay curves were recorded by monitoring the fluorescence emission maxima of the dyes. The lamp profile was recorded by placing a scattered (diluted Ludox solution in water) in place of the sample. The width of the instrument response function (IRF) was limited by the full width at half maxima (FWHM) of the excitation source, 625 ps at 405nm. The quality of fits was judged by χ2 values and distribution of the residuals. Thermogravimetric analyses (TGA) were performed with a TGA/SDTA 851e (Mettler Toledo) thermal analyzer using a heating rate of 10ºC min-1under nitrogen atmosphere in the temperature range of 33–550ºC. Cyclic voltammetry measurements were performed on a PC-controlled CH instruments (CHI 620C) electrochemical analyzer using 1 mM dye solution in acetonitrile at a scan rate of 50 mV s-1. 0.1 M tetra butyl ammonium hexafluorophosphate was used as a supporting electrolyte. Glassy carbon, a standard calomel electrode (SCE) and a platinum wire were used as the working, reference and auxiliary electrodes, respectively. Scanning electron microscopy (SEM) analysis was carried out using field emission SEM (JSM 7600F JEOL). For this purpose, one drop of sample [~10-5 M aqueous solution] was deposited on a silicon wafer mounted on an aluminium stub with the help of a double-sided adhesive carbon tape. The samples were heat dried at 35ºC for 12 hours and vacuum dried for 30 minutes to ensure complete removal of any residual water and coated with platinum before being analyzed. 3-(4-bromophenyl)-7-(diethylamino)-2H-chromen-2-one (1) 4-(Diethylamino) salicylaldehyde (2g, 10.3 mmol) and 4-bromophenyl acetonitrile (2.23g, 11.4 mmol) were dissolved in the mixture of dimethyl formamide and acetic acid. Piperidine (4.09 mL, 41.5 mmol) was added slowly to the reaction mixture. The reaction solution was heated to reflux with continuous stirring for 24h under nitrogen atmosphere. After cooling to room temperature, ice water was poured into reaction mixture to quench the reaction and extracted with CH2Cl2. The combined organic layers were washed with water and dried over anhydrous Na2SO4. The solvent was removed by rotary evaporation, the residue was purified by column chromatography over silica gel with n-hexane: ethyl acetate (95:5) as the eluent to 6 ACS Paragon Plus Environment

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give the desired product as yellow solid. (2.9 g, 72%) 1H NMR (500 MHz, CDCl3): δ 7.67 (s, 1H), 7.58 (d, J = 8.1 Hz, 2H), 7.51 (d, J = 8.2 Hz, 2H), 7.30 (d, J = 8.8 Hz, 1H), 6.59 (d, J = 8.8 Hz, 1H), 6.51 (s, 1H), 3.42 (q, J = 6.9 Hz, 4H), 1.22 (t, J = 7.0 Hz, 6H).13C NMR (126 MHz, CDCl3) δ 161.39, 156.3, 150.76, 140.62, 134.77, 131.41, 129.80, 129.09, 121.71, 119.40, 109.11, 108.94, 97.07, 44.89, 12.49.HRMS [ESI] Calcd: 371.0521 found: 372.0146 [M + 1]+.

4'-(7-(diethylamino)-2-oxo-2H-chromen-3-yl)-[1,1'-biphenyl]-4-carbaldehyde (2) 3-(4-bromophenyl)-7-(diethylamino)-2H-chromen-2-one(1g, 2.69 mmol), 4-formyl benzene boronic acid (443mg, 2.95 mmol) were dissolved in 20 ml of toluene. After adding 8 ml of 2M K2CO3 solution, the reaction mixture was degassed under nitrogen atmosphere for 15 minutes, then tetrakis(triphenylphosphine)palladium (93 mg, 3 mol%) was added to the reaction mixture. The reaction solution was heated to 80°C for 24 h. After cooling the reaction mixture, it was poured into water, extracted with CH2Cl2 and dried over anhydrous sodium sulphate. The solvent was removed by rotary evaporation, purified by column chromatography over silica gel with n-hexane: ethyl acetate (95:5) as the eluent to give the desired product as yellow solid (693 mg, 73% ) 1H NMR (500 MHz, CDCl3) δ 10.06 (s, 1H), 7.96 (d, J = 7.3 Hz, 2H), 7.80 (dd, J = 27.1, 8.5 Hz, 5H), 7.68 (d, J = 7.4 Hz, 2H), 7.46 – 7.15 (m, 1H), 6.81 – 6.31 (m, 2H), 3.44 (d, J = 6.5 Hz, 4H), 1.22 (d, J = 6.2 Hz, 6H). 13C NMR (126 MHz, CDCl3) δ 191.91, 161.59, 156.34, 150.74, 146.72, 140.65, 138.79, 136.15, 135.23, 130.33, 129.10, 128.80, 127.55, 127.26, 119.85, 109.08, 97.12, 44.90, 12.50.HRMS [ESI] Calcd: 397.1678 found: 398.1316 [M + 1]+.

5-(4-(7-(diethylamino)-2-oxo-2H-chromen-3-yl)phenyl)thiophene-2-carbaldehyde (3)



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This compound was synthesized according to the procedure similar to that of (2), using 5formyl thiophene-2-boronic acid, to give the desired product as yellow solid (820mg, 76%). 1

H NMR (500 MHz, CDCl3) δ 9.90 (s, 1H), 7.93 – 7.65 (m, 6H), 7.43 (s, 1H), 7.34 (d, J = 8.6

Hz, 2H), 6.61 (d, J = 8.3 Hz, 1H), 6.54 (s, 1H), 3.44 (d, J = 6.8 Hz, 4H), 1.23 (t, J = 6.6 Hz, 6H). 13C NMR (126 MHz, CDCl3) δ 182.77, 161.44, 156.38, 153.99, 150.84, 142.43, 140.68, 137.47, 137.08, 132.16, 129.17, 128.85, 126.33, 124.15, 119.48, 109.08, 97.10, 44.92, 12.49.HRMS [ESI] Calcd: 403.1242 found: 404.0941 [M + 1]+.

(Z)-3-(4'-(7-(diethylamino)-2-oxo-2H-chromen-3-yl)-[1,1'-biphenyl]-4-yl)-2-phenyl acrylonitrile (BB) 4'-(7-(diethylamino)-2-oxo-2H-chromen-3-yl)-[1,1'-biphenyl]-4-carbaldehyde (200 mg, 0.50 mmol) and phenyl acetonitrile (65 mg, 0.55 mmol) were dissolved in dry ethanol (10 mL). Then, catalytic amount of piperidine was added to the reaction mixture and heated to reflux for 10 h. The precipitated light yellow solid was filtered, washed with ethanol. The compound was purified by silica gel column chromatography using n-hexane: ethyl acetate (80: 20) as eluent to yield the desired product as greenish yellow solid. (234 mg, 94% ) 1H NMR (500 MHz, CDCl3) δ 7.99 (d, J = 7.7 Hz, 2H), 7.83 (d, J = 7.7 Hz, 2H), 7.79 – 7.67 (m, 6H), 7.58 (s, 1H), 7.46 (t, J = 7.1 Hz, 2H), 7.41 (d, J = 6.9 Hz, 1H), 7.34 (d, J = 8.6 Hz, 1H), 6.61 (d, J = 8.4 Hz, 1H), 6.55 (s, 1H), 3.44 (d, J = 6.9 Hz, 4H), 1.23 (t, J = 6.8 Hz, 6H). 13C NMR (126 MHz, CDCl3) δ 161.64, 156.31, 150.68, 142.75, 141.72, 140.52, 139.11, 135.64, 134.59, 132.73, 129.90, 129.12, 128.76, 127.42, 126.99, 126.02, 120.09, 118.18, 111.29, 109.10, 97.16, 44.90, 12.50.HRMS [ESI] Calcd: 496.2151found: 497.1832 [M + 1]+. (Z)-3-(5-(4-(7-(diethylamino)-2-oxo-2H-chromen-3-yl)phenyl)thiophen-2-yl)-2-phenyl acrylonitrile (TB)


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This compound was synthesized according to the procedure similar to that of BB, 5-(4-(7(diethylamino)-2-oxo-2H-chromen-3-yl)phenyl)thiophene-2-carbaldehyde (200 mg, 0.49 mmol), to give the desired product as yellow solid (221 mg, 85%) 1H NMR (500 MHz, CDCl3) δ 7.81 – 7.73 (m, 3H), 7.70 (d, J = 8.1 Hz, 2H), 7.68 – 7.61 (m, 3H), 7.60 (d, J = 3.1 Hz, 1H), 7.43 (t, J = 7.4 Hz, 2H), 7.35 (dd, J = 18.4, 7.8 Hz, 3H), 6.60 (d, J = 8.7 Hz, 1H), 6.53 (s, 1H), 3.43 (q, J = 6.7 Hz, 4H), 1.23 (t, J = 6.9 Hz, 6H). 13C NMR (126 MHz, CDCl3) δ 161.49, 156.31, 150.74, 148.73, 140.46, 137.16, 136.21, 134.32, 132.51, 129.12, 128.80, 126.00, 125.66, 123.79, 119.70, 118.33, 109.10, 107.43, 97.10, 44.90, 12.50.HRMS [ESI] Calcd: 502.1715 found: 503.1384 [M + 1]+. (E)-2-(benzo[d]thiazol-2-yl)-3-(4'-(7-(diethylamino)-2-oxo-2H-chromen-3-yl)-[1,1'biphenyl]-4-yl)acrylonitrile (BBT) 2-(benzo[d]thiazol-2-yl)acetonitrile (96 mg, 0.55 mmol) and 4'-(7-(diethylamino)-2-oxo-2Hchromen-3-yl)-[1,1'-biphenyl]-4-carbaldehyde (200 mg, 0.50 mmol) were dissolved in dry ethanol (35 mL). Then, catalytic amount of piperidine was added to the reaction mixture and refluxed for 10 h. The precipitated solid was filtered, washed with ethanol. The compound was purified by silica gel column chromatography using n-hexane: ethyl acetate (80: 20) as eluent to yield the desired product as orange solid. (228mg, 82%). 1H NMR (500 MHz, CDCl3) δ 8.29 (s, 1H), 8.11 (dd, J = 14.9, 8.3 Hz, 3H), 7.93 (d, J = 8.0 Hz, 1H), 7.85 (d, J = 8.1 Hz, 2H), 7.79 (d, J = 8.2 Hz, 3H), 7.71 (d, J = 8.2 Hz, 2H), 7.54 (t, J = 7.7 Hz, 1H), 7.44 (t, J = 7.6 Hz, 1H), 7.35 (d, J = 8.7 Hz, 1H), 6.62 (d, J = 8.8 Hz, 1H), 6.55 (s, 1H), 3.60 – 3.35 (m, 4H), 1.24 (t, J = 7.0 Hz, 6H).

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C NMR (126 MHz, CDCl3) δ 146.33, 140.58,

131.06, 129.08, 128.83, 127.66, 127.02, 123.60, 121.70, 116.71, 109.10, 97.17, 44.91, 12.50.HRMS [ESI] Calcd: 553.1824 found: 554.1901 [M + 1]+.



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(E)-2-(benzo[d]thiazol-2-yl)-3-(5-(4-(7-(diethylamino)-2-oxo-2H-chromen-3-yl)phenyl) thiophen-2-yl)acrylonitrile (TBT) This compound was synthesized according to the procedure similar to that of BBT, 5-(4-(7(diethylamino)-2-oxo-2H-chromen-3-yl)phenyl)thiophene-2-carbaldehyde (200 mg, 0.49 mmol), to give the desired product as dark red solid. (234 mg, 84%) 1H NMR (500 MHz, CDCl3) δ 8.37 (s, 1H), 8.05 (d, J = 7.8 Hz, 1H), 7.90 (d, J = 7.7 Hz, 1H), 7.87 – 7.56 (m, 6H), 7.56 – 7.30 (m, 4H), 6.62 (d, J = 8.1 Hz, 1H), 6.54 (s, 1H), 3.44 (d, J = 6.8 Hz, 4H), 1.24 (s, 6H).13C NMR (126 MHz, CDCl3) δ 161.41, 156.39, 140.60, 138.68, 137.41, 129.17, 128.84, 126.91, 126.27, 125.69, 124.40, 123.28, 121.67, 109.11, 44.92, 12.50.HRMS [ESI] Calcd: 559.1388 found: 560.1467 [M + 1]+.

Computational details

Density Functional Theory (DFT) calculations were carried out using Gaussian 09 (G09) abinitio quantum chemical software package. DFT and time-dependent DFT (TDDFT) methods have been used to calculate the ground state and excited state properties respectively. Geometry optimization of molecules were performed in the gas phase using B3LYP/6-31G (d, p) level of theory with harmonic vibrational analysis to ensure the nature of the stationary point. PES scan studies were carried out to confirm the most stable conformation corresponding to global minima. The minimized geometries were then used as input to obtain the frontier molecular orbitals (FMOs) and vertical electronic excitation energies (first 15 vertical singlet–singlet transitions) using different functional such as B3LYP and CAM-B3LYP.36 The integral equation formalism polarizable continuum model 10 ACS Paragon Plus Environment

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(PCM) within the self-consistent reaction field (SCRF) theory has been used for TDDFT calculations to describe the solvation of the dyes.37 The software Gauss Sum 2.2.6.1 was utilized to simulate the major portion of the absorption spectrum and to interpret the nature of transitions.38 PDOS calculations were carried out to calculate the percentage contributions of individual units present in molecules corresponding to the respective molecular orbitals such as HOMO and LUMO. Results and Discussion Synthesis The synthesis of target dyes was obtained in a four step procedures as outlined in scheme 1. Synthesis of 3-(4-bromophenyl)-7-(diethylamino)-2H-chromen-2-one (1) was carried out by the cyclic condensation of 4-(diethylamino)salicylaldehyde with 4-bromophenylacetonitrile in presence of piperidine as a base.39 The bromide intermediate (2) undergoes Suzuki coupling with 4-formyl benzene boronic acid and 5-formyl thiophene 2-boronic acid using Pd(PPh3)4 catalyst yields the corresponding formylated benzene (2) and thiophene (3) derivatives respectively. Finally, target dyes BB and TB (Phenyl acrylonitrile series: PAC series) were synthesized by typical Knoevenagel condensation of formylated intermediates (2) and (3) with phenyl acetonitrile. The dyes BBT and TBT (Benzothiazole acrylonitrile series: BAC series) were prepared using 2-(benzo[d]thiazol-2-yl)acetonitrile in quantitative yields under the same reaction conditions. All the resulting compounds were thoroughly characterized using 1H,

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C NMR, high resolution mass spectral (HRMS) analyses and

detailed in electronic supplementary information (ESI). Molecular geometry and transitions of the dyes Knowledge of the molecular geometries and electron density distribution can assist in gaining deeper insights into the electronic and spectroscopic properties of the dyes. In general, electron density distributions are proportional to the overlap of orbital interactions and can be determined by the degree of π-conjugation in the system. If overlap of orbital interactions is 11 ACS Paragon Plus Environment


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better, charge transfer is more efficient.40 The degree of π-conjugation can be measured by the torsion angle between the individual functional building blocks such as donor, πconjugated bridge and acceptor units. In our molecules, dihedral angle between coumarin donor and benzene unit is calculated to be ~32º. The presence of ortho hydrogens between the adjacent phenyl groups distorts the molecule out-of-plane fashion to ~35º in BB and BBT. This torsional distortion diminishes the π-electron delocalization from donor to acceptor due to poor overlap of orbital interactions.41 Substitution of more planar thiophene unit in TB and TBT reduces the distortion to ~21º. The vinylene linker in all the dyes is nearly coplanar with a dihedral angle value of ~178º. The terminal phenyl group in BB and TB deviates from the plane of π-conjugated backbone and the values are predicted to be 26.9º, 21.3º respectively. The large deviation of coplanarity mainly results from the intramolecular repulsion induced by the ortho hydrogen atoms of terminal phenyl unit around the bulky cyano group. Terminal substitution of phenyl unit with benzothiazole acceptor alleviates the torsional distortion from 27º to 1º. It is interesting to note that, in the BAC series, benzothiazole substitution not only helps in attaining the coplanarity in the gas phase but also extends the conjugation owing to its strong electron withdrawing ability. This provides more freedom of rotation around the cyano group for better intermolecular interactions via non-covalent forces. In gas or solution phase, particularly in bulk media, a large number of energy minimized structures may exist on the potential energy surface due to possible rotamers. Hence, it is instructive to perform potential surface scan studies as a function of labelled torsional coordinates to ascertain the structural accuracy and most stable conformation corresponding to global minima. All the geometries were scanned with the increment of 10º steps between 0º and 180º with B3LYP/631G (d, p) level of theoretical approximations. A graphical depiction of the calculated relative rotational energy barrier values (in kcal/mol) of these derivatives plotted against dihedral angle was given in Fig. 2. As depicted, one global minima is observed for PAC series dyes with the torsion angle of 30º. On the contrary, two stable conformations are 12 ACS Paragon Plus Environment

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possible for BAC series dyes owing to the rotation of a cyano group with respect to benzothiazole ring i.e., presence of cyano group on either side of sulphur or nitrogen atom. Potential energy surface (PES) scan studies clearly suggest a more stable conformation for the structure having cyano group on the side of sulphur atom. In this conformation, bulky sulphur atom is located away from cyano group.42 Significant energy difference (0.45 kcal/mol) is noticed between these two different conformations in BAC series. Relatively high rotational barrier heights of 3.27 and 3.63 kcal/mol are estimated for BB and TB respectively due to sterically crowded cyanostilbenes. PAC series dyes exhibit a two-fold increase in rotational barrier height than the BAC series.43 The torsional alleviation around the cyano moiety in BBT and TBT results in comparatively low torsional energy barrier values of 1.50 and 1.64 kcal/mol respectively.30 DFT computed parameters are well matched with the reported experimental data.31, 44 It is better to represent the energy values in terms of free energy of activation rather than rotational barrier heights. To calculate the activation free energy, it is of crucial importance to find out the location of the transition state (TS) (firstorder saddle point on the PES) between the two minima corresponding to the different conformers. The highest point on the PES is taken as the first approximation for a corresponding transition state i.e., molecules with the dihedral angle value of 90º. The exact transition state is obtained from the approximate transition state using the Berny optimization algorithm as implemented in Gaussian 09 software package.45 Harmonic vibration frequencies were calculated for each structure to ascertain the potential energy minimum (no imaginary frequency) or the transition state (single imaginary frequency) involved and to obtain Gibbs free energy. The activation free energy barriers of the benzene series molecules BB and TB were predicted to be 3.64 and 4.27 kcal/mol respectively. The substitution, of less sterically hindered benzothiazole ring reduces the free energy of activation and the values are computed to be 1.78 and 1.93 kcal/mol for the molecules BBT and TBT respectively (Fig. S8, ESI). 13 ACS Paragon Plus Environment


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Charge transfer such as intramolecular charge transfer/π-π* transition upon photo-excitation reveals the interrelationship of structural and electronic behaviour of the molecules. To investigate the charge transfer nature, molecular orbital amplitude plots of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) of the dyes were examined. For dyes BB and BBT, electron density of HOMO is mainly delocalized over the donor, π-spacers and to a small extent on the cyanostilbenes whereas, in TB and TBT, electron density is delocalized over the entire molecule. This evidences the better charge transfer propensity of thiophene derivatives in comparison to their benzene counterparts.

For all the molecules, LUMO is dominantly populated over spacer and

acceptor units and to a small extent on the donor. The electron density of LUMO is majorly governed by spacer and acceptor functionality and shows a distinct variation according to their substitution pattern (Fig. 3)41. As shown in Table 2, other frontier molecular orbitals such as HOMO-1 and LUMO+1 are involved in charge transfer transitions to a considerable extent and their isodensity surface are shown in Fig.S9 (ESI). The substitution of phenyl unit with thiophene/benzothiazole at spacer/acceptor positions respectively reduces the band gap by lowering the LUMO level of the dyes. The HOMO-LUMO gap computed from DFT decreases in the following order: BB (3.15 eV) > BBT (3.11 eV) > TB (2.84 eV) > TBT (2.70 eV). Nonetheless, DFT computed HOMO-LUMO gap shows the difference of 0.5-0.6 eV, it follows reasonable trend with the band gap measured from electrochemical-optical studies (Fig. 3). It should be mentioned that DFT calculations were carried out in the gas phase and the bond length alternation predicted by improper long-range correction exchange could also be attributed to the variation observed in LUMO values.46 To further scrutinize the charge transfer contribution of each individual segment in terms of electron density population in the dyes, partial density of states calculations (PDOS) was carried out. For this purpose, we parted the molecule into three segments coded as D, π and A, where D represents 6-(diethylamino) coumarin, π represents biphenyl unit in BB and BBT 14 ACS Paragon Plus Environment

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and benzene-thiophene in TB and TBT. A represents the phenyl or benzothiazole acrylonitrile unit in PAC or BAC series respectively. The generated percentage contribution of these segments from respective frontier orbitals such as HOMO and LUMO was depicted in Fig. 4a. The analysis was carried out using GaussSum 2.2.6.1 software. Lower contribution obtained from π-segment in BB and BBT reveals poor electronic coupling between biphenyl segments.47 As noted from the Table 2, thiophene spacer and benzothiazole acceptor functionalities increase the HOMO-LUMO transition to a considerable extent (1317%) corresponding to the ICT character of the dyes.

Photophysical Properties UV−visible absorption and emission spectra of molecules in dioxane is shown in Fig. S1 (ESI) and the corresponding data are listed in Table 1. Synthesized molecules have absorption wavelength in the range of 410-457nm. In PAC series, BB containing a benzene spacer absorbs at 410nm while introduction of thiophene (TB) spacer in place of benzene yields a red-shifted (+18 nm) absorption (428 nm) spectrum. Similarly, in the BAC series, BBT bearing benzene spacer absorbs at 417nm and its thiophene counterpart absorbs at 457 nm. Clearly, introduction of thiophene in place of benzene results in a red-shifted (+7 to +29 nm) absorption maxima in both PAC and BAC series. The substitution of benzothiazole leads to a greater bathochromic shift of ~34 nm in thiophene dyes (TB to TBT). This may be originated from strong electronic coupling between thiophene spacer and benzothiazole acceptor via cyano vinylene linker. Consequently, energy is transferred from coumarin donor to benzothiazole acceptor unit in an efficient manner. The absorption maximum observed for the dyes in dioxane varies in the following order: BB < BBT < TB < TBT.

The dyes exhibit intense fluorescence emission in non-polar solvents. A structured emission is observed in heptane and a single broader emission peak is observed in toluene and dioxane. 15 ACS Paragon Plus Environment


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Significant emission quenching is observed in polar aprotic and protic solvents which may be attributed to the dissipation of excitation energy through non-radiative relaxation. (Fig. S3, ESI).32, 48-49 DFT calculations reveal the charge transfer nature of the dyes by distinguished asymmetric electron density distribution from coumarin donor to cyanostilbenes acceptor corresponding to HOMO-LUMO transition.49 The variation in the emission wavelength of the dyes in dioxane is small as compared to the absorption wavelength and manifests the presence of potent intramolecular rotation of cyanostilbenes in excited state. Along with dynamic rotations of cyano group, planarity induced by thiophene spacer substitution also renders the dyes TB and TBT to be non-emissive (Fig. S4, ESI). Fluorescence quantum yield (Φf) of the dyes is high in non-polar solvents and values are obtained in the range of 0.24 0.69.49-50 As summarized in Table 1, in organic solvents, moderate Stokes shift values are obtained for all the dyes. Among these, substitution of benzothiazole in BAC series considerably reduces the Stokes shift. This result implies prolonged endurance of rigid and non-planar PAC series in excited state.51 The optical band gap (Eg) was derived from the onset absorption spectra of the dyes recorded in dioxane.

TDDFT calculations were employed to evaluate the photophysical behavior of the dyes using PCM model with inclusion of the solvent effect. Simulated vertical excitation energies predicted from TD-B3LYP results are highly overestimated. TDDFT/ CAM-B3LYP/6-13g (d, p) level of theory using dioxane shows good agreement with a little deviation as compared to experimental findings (Fig. 4b). The simulated absorption spectra of these dyes are originated from the combined contribution of ICT and π-π* transitions. The frontier energy levels of the dyes along with vertical excitation energies, oscillation strength and transitions predicted from DFT studies are compared with electrochemical-optical data (Table 2). AIEE studies


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To investigate the influence of spacer and acceptor units on AIEE characteristics of the molecules, absorption and photoluminescence spectral studies were carried out in dioxanewater binary mixtures. Owing to the incorporation of diethylamino coumarin donor unit, all the dyes are considerably emissive in dioxane. Upon addition of increased volumes of water (fw), a gradual decrease in emission intensity with a bathochromic shift in wavelength is noted. At 50% dioxane-water proportions, a new band was noticed in the low energy region with reduction in monomer’s emission. This clearly indicates initiation of the aggregate formation. Absorption spectra also showed broadening with slight red shift at higher water fractions (Fig. S5, ESI). At 99% water fraction, enhanced emission intensity is noticed with concurrent bathochromic shift of 87-155 nm in comparison to the solution emission for the dyes (Fig. 6). This enhancement in emission intensity and wavelength shifts may be stemmed from the synergistic combination of intramolecular planarization and J-type aggregation propensity of α-cyanostilbenes.4 During aggregation, dynamic intramolecular rotations of cyanostilbenes are confined. Hence, non-radiative channels were blocked and excitation energy dissipation was promoted via radiative pathways resulting in intense emission. The order of the AIEE emission follows the trend: BB (570nm) < TB (597nm) < BBT (629nm) < TBT (670nm). The AIEE peak of BBT is shifted to lower energy region with a bathochromic shift of 32nm as compared to TB. In the monomer state or in dioxane, the observed order is BB (483 nm) < BBT (487nm) < TB (505nm) < TBT (515nm). In comparison, a reversal of the pattern is found for BBT and TB upon aggregation. This is attributed to efficient dynamic intramolecular rotation (IMR) induced by the cyano group in solution. It is worth mentioning that IMR has no influence on charge transfer propensity of thiophene spacer substitution as it is located prior to cyano group. This could be the reason for the observed bathochromic shift of TB in organic solvents. On the contrary, in the case of BBT, the terminal acceptor unit (benzothiazole) is located next to the cyanovinylene group. Thus, potent IMR of cyano group in solution minimized the effect of terminal benzothiazole acceptor functionality. In addition, 17 ACS Paragon Plus Environment


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greater twisted conformation between phenyl-phenyl segments also hampers the charge transfer. As a consequence, charge transfer in BBT is less pronounced even after the incorporation of electron withdrawing benzothiazole acceptor unit. It is obvious that photophysical properties are dependent on the position and substitution pattern of individual functional building blocks such as donor, spacer and acceptor in the π-conjugated skeleton.51 The experimental observations have been well analyzed using DFT studies. The larger bathochromic shift observed in BBT upon aggregation is assigned to two factors. 1) Efficient intramolecular planarization in solid or aggregated state, due to which, introduction of strong electron withdrawing benzothiazole acceptor functionality at terminal position facilitates the ICT character of the dyes and 2) Proximity of the five membered thiazole unit to the bulkier cyano group. As illustrated in DFT studies, the replacement of the terminal phenyl group with benzothiazole dramatically reduces internal steric repulsion around the rotatable cyano group. This provides enough space for cyano moiety to endure more rotational freedom in the solution phase and efficient intramolecular planarization in the aggregated state. In contrast to the solution state, dye aggregates exhibit large Stokes shift in the range of 158-213nm (Fig. 4a). The tail part of the emission spectra of these dye aggregates in water extends up to 800nm and reveals that these molecules can act as potential candidates for bioimaging applications.

Other interesting observations are also noted. Substitution of benzene with thiophene results in a red-shifted emission (+ ~30nm) and the substitution of benzene with benzothiazole unit at terminal positions renders the emission to longer wavelength with a bathochromic shift of ~60nm (Fig. 5). Emission wavelength variation between the dyes is relatively large (100nm) upon aggregation. The emission intensity of aggregates is gradually reduced by changing the substitution of phenyl unit with thiophene and benzothiazole at spacer and acceptor positions respectively. The result suggests the introduction of more planar thiophene and benzothiazole 18 ACS Paragon Plus Environment

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units lead to a better conjugation which in turn reduces the emission intensity. Similar kind of observations were earlier reported by Tang and his co-workers that better conjugation suppresses the aggregates emission.52 The emission properties of the dyes in the aggregated state prompted us to further investigate their pattern in the solid state. The normalized emission spectra in the solid state are displayed in Fig. 4b. PAC series dyes exhibit profound emission intensity with narrower peaks due to the increased number of non-planar and rigid phenyl units. It is documented that terminal non-planar phenyl group adopt twisted conformation and efficiently hamper the π-π stacking interactions in aggregated/solid state.53 On the other hand, planar BAC series dyes undergo tight molecular packing resulting in significant fluorescence quenching (Fig S6 (a), ESI). Nonetheless solid state emission shows a hypsochromic shift, a trend consistent with the emission of dye aggregates. It is worthwhile to note that, BAC series exhibit greater variations in emission wavelength from organic solvents to solid/aggregated state (Fig. S7, ESI). The result reveals the substitution of benzothiazole unit in solid/aggregated state dramatically increased the ICT nature of the system. The measured emission spectra in thin film state are akin to the aggregated state. (Fig. S6 (b), ESI).

SEM Images To validate and investigate the morphological nature of the dye aggregates, samples were analyzed using field emission scanning electron microscopy (FESEM) technique. Samples utilized for fluorometric titration were subjected to SEM analysis. For this, 10 µL volumes of dyes (10-3 M solution in dioxane) are added to 1 ml of water. As showed in Fig. 7, distinct morphologies such as uneven spherical and rectangular and rod like structures were noted. The particles size of the dyes BB and TBT were analyzed using ImageJ software and found to be ~225nm and ~70nm for BB and TBT respectively. The average diameter of the rod shaped aggregates in BBT was measured to be ~148nm. 19 ACS Paragon Plus Environment


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Fluorescence lifetime studies in solution and aggregation Fluorescence lifetime decay measurements for the monomers and aggregates of the dyes were carried out using time-correlated single photon counting technique (TCSPC). Fig. 8 illustrates excited-state decay curves of the dyes measured in dioxane and water. The measurement of fluorescence lifetimes was performed by deconvolution of the response function from the decay curves. The detailed data of the corresponding decay parameters is compiled in Table 3. In dioxane, monomers of the dyes exhibited the fluorescence life times in the range of 1.17-1.82 ns. For dyes BB and BBT, fluorescence decay is fitted into mono-exponential expression with the lifetime of 1.52 ns and 1.82 ns respectively. On the other hand, decay profiles of TB and TBT are fitted to biexponential expression.54 It is established that molecules with high flexibility are susceptible to more conformational fluctuations in excited state.55 TB and TBT are less symmetrical with respect to spacer combination of phenylthiophene and may be prone to various possible conformations in excited state. Conversely, rigid and symmetrical BB and BBT favour to dissipate the most part of their excitation energy via radiative channels resulting in high fluorescence decay lifetimes.56 Radiative (kr) and non-radiative decay constant (knr) values of the monomers in dioxane were calculated ః

according to the equations ‫ܭ‬௥ = ቀ ఛ ቁ and ‫ܭ‬௡௥ = ቀ

ଵିః ఛ

ቁ. For these calculations, lifetimes with

a larger contribution to the emission profiles were considered. The larger non-radiative decay constant values of TB and TBT manifests the dominant dissipation of excitation energy via non-radiative channels (Table 3). The average life time of the dyes BB, TB, BBT and TBT is found to be 1.52, 0.48, 1.19 and 1.05 ns respectively. Time resolved fluorescence spectra of dye aggregates show that their decay profiles fit into three exponential expression. Lifetime of BB aggregates exhibits greater lifetime as compared to other dyes. The substitution of nonplanar terminal phenyl group with planar benzothiazole renders substantial reduction in lifetime. Same pattern was found with more planar thiophene dyes such as TB and TBT. The 20 ACS Paragon Plus Environment

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mean life time of the dye aggregates BB, TB, BBT and TBT is measured to be 21.7, 5.85, 7.66 and 2.59 ns respectively. Thus, it can be concluded that greater planarity increases the charge transfer but significantly suppresses the intensity of aggregation induced emission which reinforces the AIEE phenomena observations.

Electrochemical Studies Redox behavior of the dyes was investigated to examine the energy levels by performing cyclic voltammetry (CV) using a standard three-electrode configuration. Oxidation potentials in voltammograms reveal that all dyes undergo irreversible oxidation. HOMO levels of the dyes were calculated from the onset oxidation potential according to the empirical formula EHOMO= -[Eox + 4.4] eV, and LUMO levels were estimated from the optical band gap (E0-0) and the HOMO values using the empirical formula ELUMO= [E0-0+ EHOMO] eV. The optical band gap is measured from the onset absorption spectra of the dyes recorded in dioxane. All the dyes show an oxidation potential in the range of 1.01−1.08 V, attributed to the removal of an electron from the diethylamino coumarin donor (Fig. 9a). The substitution of benzothiazole in place of terminal benzene unit in BBT has no remarkable difference in oxidation potentials. On the other hand, substitution of benzothiazole in TBT decreases the oxidation potential significantly (Table 2).48 The onset oxidation potentials of the dyes BB, TB, BBT and TBT are estimated to be 1.08V, 1.05V, 1.07V, 1.01V corresponding to the HOMO levels of -5.48 eV, -5.45eV, -5.47 eV, -5.41 eV, respectively and LUMO values are calculated to be 2.80 eV, 2.92 eV, 2.86 eV, and 3.08 eV respectively.

Thermal stability Owing to the dense molecular packing propensity of cyanostilbene motifs, these dyes are expected to have excellent thermal stability. For the purpose, thermal gravimetric analysis (TGA) and differential scanning calorimeter (DSC) measurements were performed at a 21 ACS Paragon Plus Environment


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scanning rate of 10ºC min-1 under N2 atmosphere. All dyes possess very high decomposition temperature (T5d) with insignificant weight loss (~5% weight loss) and values are in the range of 344-382ºC (Fig. 9b). T5d of the dyes are gradually increased in the following order: TBT < BBT < TB < BB. The high thermal stability of PAC series dyes is attributed to the presence of more number of rigid phenyl units.51 The observed melting points of the dyes, measured from the first heating scan using DSC analysis (Fig. S10, ESI), are the exact reverse of the decomposition temperature pattern. This may be ascribed to tight molecular packing of planar BAC series dyes in solid state and values are found in the range of 235-281ºC. The excellent thermal stability of the dyes reveals their utilization in optoelectronic device applications such as organic light emitting diodes (OLED), organic photovoltaics (OPV) and organic field effect transistors (OFET). Conclusions In summary, we developed a simple and efficient molecular design strategy to obtain farred/NIR AIEE luminogens by band gap tuning of four coumarin based α-cyanostilbene derivatives with various spacer and acceptor combinations. Introduction of coumarin moiety increased the fluorescence quantum yield of cyanostilbenes. Optical profiles of monomer exhibit little variation due to potent intramolecular rotation of cyanostilbenes, whereas aggregates show a profound shift in emission with a large Stokes shift. Similar pattern was also observed in solid and thin film state which reveals the J-type aggregation of cyanostilbenes. Significant reduction in AIE intensity was noticed while increasing the planarity of the system and the observation was corroborated from fluorescence lifetime decay studies. The evaluation of structure-property relationship between spacer/acceptor units and AIEE phenomena was expounded by various experimental techniques and DFT studies. Our rational functionalization of α-cyanostilbenes at various positions would give important insights to further develop NIR-AIEE luminogens for a wide range of optoelectronic applications. 22 ACS Paragon Plus Environment

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Acknowledgements MP thanks IIT Gandhinagar for post-doctoral fellowship. SK acknowledges CSIR, DST and IIT Gandhinagar for financial support. We thank Dr. K. Bhanuprakash and Dr. B. Sridhar, CSIR-IICT for their valuable suggestions. ASSOCIATED CONTENT Supporting Information Absorption and fluorescence spectra measured in different media like organic solvents, solid and thin film state, emission photograph of the dyes under UV light, absorption spectra in dioxane-water mixtures, geometrical coordinates, electron density distribution of HOMO-1 and LUMO+1 of the dyes, melting point obtained from DSC analysis, Stokes shift variation between monomers and aggregates, 1H,

13

C NMR and ESI-HRMS spectral characterisation.

This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Phone: +91-7932419500). References 1. Mei, J.; Leung, N. L. C.; Kwok, R. T. K.; Lam, J. W. Y.; Tang, B. Z., AggregationInduced Emission: Together We Shine, United We Soar! Chem. Rev. 2015, 115, 1171811940. 2. Lu, H.; Mack, J.; Yang, Y.; Shen, Z., Structural Modification Strategies for the Rational Design of Red/NIR Region BODIPYs. Chem. Soc. Rev. 2014, 43, 4778-4823. 3. Guo, Z.; Park, S.; Yoon, J.; Shin, I., Recent Progress in the Development of NIR Fluorescent Probes for Bioimaging Applications. Chem. Soc. Rev. 2014, 43, 16-29. 4. An, B.-K.; Kwon, S.-K.; Jung, S.-D.; Park, S. Y., Enhanced Emission and Its Switching in Fluorescent Organic Nanoparticles. J. Am. Chem. Soc. 2002, 124, 14410-14415. 5. Tang, B. Z.; Zhan, X.; Yu, G.; Sze Lee, P. P.; Liu, Y.; Zhu, D., Efficient Blue Emission from Siloles. J. Mater. Chem. 2001, 11, 2974-2978. 6. Luo, J.; Xie Z.; Lam J. W. Y.; Cheng L.; Chen H.; Qiu C.; Kwok H. S.; Zhan X.; Liu Y.; Zhu D. and Tang B. Z.; Aggregation-Induced Emission of 1-Methyl-1,2,3,4,5-Penta phenyl silole. Chem. Commun. 2001, 1740-1741. 7. Hong, Y.; Lam, J. W. Y.; Tang, B. Z., Aggregation-Induced Emission: Phenomenon, Mechanism and Applications. Chem. Commun. 2009, 4332-4353. 23 ACS Paragon Plus Environment


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Table 1 Optical properties of the dyes. a

Water (Aggregates)

Dioxane (monomer) λabs (nm)

λem (nm)

Stokes shift (nm)

BB

410

483

73

TB

428

505

BBT

417

TB

457

λsolide (nm)

λfilmf (nm)

0.77

521

568

165

0.45

551

596

629

208

0.62

584

631

670

213

0.38

630

667

λabs (nm)

λAIEE (nm)

Stokes shift (nm)

0.61

412

570

158

77

0.29

432

597

487

60

0.47

421

515

58

0.25

457

Φsoln

b

c

ΦAIEE

d

a

optical properties measured in dioxane in the concentration of 1 × 10−5 M at ambient temperature; bquinine sulfate (0.545 in 0.5 N H2SO4 is used as standard; coptical properties obtained from AIEE studies; drhodamine B (Φ = 0.65 in EtOH) was used as standard; eemission maxima obtained in solid state; femission maxima measured in thin film.

Table 2 Comparison of energy levels estimated from electrochemical-optical studies with theoretical data and thermal properties of the dyes DFT datad(eV)

Electrochemical-Optical data

λtheory

Eoxa (V)

HOMOb (eV)

LUMOb (eV)

Eoptc (eV)

HOMO

BB

1.08

-5.48

-2.80

2.68

-5.27

-2.12

3.15

377.8

1.63

TB

1.05

-5.45

-2.92

2.53

-5.23

-2.39

2.84

403.6

1.57

BBT

1.07

-5.47

-2.86

2.61

-5.33

-2.22

3.11

384.2

1.86

TBT

1.01

-5.41

-3.08

2.33

-5.25

-2.55

2.70

433.7

1.72

Dye

a

TDDFTe

LUMO

H-L gap

f

T5d

Tm

(ºC)

(ºC)

382

248

376

235

358

281

344

280

Composition

(nm) H-1->LUMO (30%) HOMO->LUMO (47%) HOMO->L+1 (18%) H-1->LUMO (30%) HOMO->LUMO (60%) HOMO->L+1 (5%) H-1->LUMO (13%) HOMO->LUMO (51%) HOMO->L+1 (30%) H-1->LUMO (20%) HOMO->LUMO (68%) HOMO->L+1 (6%)

measured in acetonitrile with 0.1 M NBu4PF6 as a supporting electrolyte with a scan rate of 50 mV s-1.bdeduced from the

empirical formulas HOMO = - [4.4 + Eox] and LUMO = [E0-0 – HOMO].cbandgap obtained from onset absorption spectra of the dyes using the formula Eg = (1241/λ).dcomputed values from B3LYP/6-31G (d, p) level; ecomputed values from TDDFT/CAM-B3LYP/6-31g(d, p) level of theory.T5d: decomposition temperature (corresponding to 5% weight loss).Tm: melting point.



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Table 3 Lifetime values of dyes obtained in dioxane (monomer) and in water (aggregates) b

Dioxane (monomer) Dye

a

τ1a

߯

τ2

BB

1.52 ns (100%)

-

TB

0.16 ns (68.55%)

1.17 ns (31.45%)

BBT

1.82 ns (100%)

-

TBT

0.29 ns (25.32%)

1.31 ns (74.68%)

ଶ

c

kr τavgd

1.23

1.52

1.01

0.47

1.19

1.17

1.31

1.05

Water (aggregates)

knr

9 −1

߯ଶ

τavgd

27.37 ns (68.76%)

1.10

21.7

4.76 ns (64.64%)

13.05 ns (20.06%)

1.30

5.85

0.42 ns (9.83%)

3.05 ns (26.70%)

10.14 ns (63.47%)

1.24

7.66

0.44 ns (21.56%)

2.63 ns (63.77%)

5.58 ns (14.67%)

1.18

2.59

(10 s )

(109 s−1)

τ1

τ2

τ3

0.40

0.26

1.33ns (7.93%)

7.87 ns (23.31%)

0.27

0.58

1.00 ns (15.30%)

0.25

0.30

0.24

0.53

fluorescence lifetime (percentage of fraction contribution to the emission profiles are given in parenthesis). bkr: radiative

rate constant. cknr :non-radiative rate constant, dmean lifetime.

Figures

Fig. 1 Structures of the dyes investigated


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Scheme 1 Synthetic route for the preparation of target dyes a.

b.

Fig. 2 a) Optimized molecular geometries of the dyes; b) Relative rotational energy barrier values as a function of the labelled torsion coordinates in the ground state.



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Fig. 3 Schematic diagrams representing the frontier molecular orbitals involved in S0 → S1 transitions and comparison of energy levels of the dyes estimated from electrochemicaloptical data with computed HOMO-LUMO gap using B3LYP/6-31G(d, p) level of theory. a.

b.

Fig. 4 a) Percentage contributions of electron density population on each segment from HOMO and LUMO levels of the dyes; b) Simulated absorption spectra obtained using TDDFT/CAM-B3LYP/6-31G (d, p) level of theory.


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b.

a.

Fig. 5 a) Stokes shift variation obtained from monomers and aggregates of TBT; b) Normalized emission spectra of the dyes recorded in the solid state.

Fig. 6 Photoluminescence spectra of the dyes a) BB, b) TB, c) BBT, d) TBT in different ratio of dioxane-water mixtures. Inset shows the plot of (I/I0) values versus various compositions of the aqueous fractions.



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Fig. 7 Morphological features of dye aggregates in water as obtained from SEM analysis.

a .

b.

Fig. 8 a) Time resolved fluorescence decay curves of the dyes recorded in dioxane; b) in water a.

b.

Fig. 9 a) Cyclic voltammetric curves measured in acetonitrile in the presence of n-Bu4NPF6 at a scan rate of 50 mVs–1; b) TGA thermograms measured at a heating rate of 10ºC/min under N2 atmosphere.


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