Molecular Engineering of Triphenylamine Based Aggregation

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Molecular Engineering of Triphenylamine based Aggregation Enhanced Emissive Fluorophore: Structure Dependent Mechanochromism and Self-Reversible Fluorescence Switching Palamarneri Sivaraman Hariharan, Viki Kumar Prasad, Surajit Nandi, Anakuthil Anoop, Dohyun Moon, and Savarimuthu Philip Anthony Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01363 • Publication Date (Web): 21 Nov 2016 Downloaded from http://pubs.acs.org on November 22, 2016

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Crystal Growth & Design

Molecular Engineering of Triphenylamine based Aggregation Enhanced Emissive Fluorophore: Structure Dependent Mechanochromism and Self-Reversible Fluorescence Switching Palamarneri Sivaraman Hariharan,a) Viki Kumar Prasad,b) Surajit Nandi,b) Anakuthil Anoop,b) Dohyun Moon*c) and Savarimuthu Philip Anthony*a) a)

Department of Chemistry, School of Chemical & Biotechnology, SASTRA University, Thanjavur – 613401, Tamil Nadu, INDIA. E-mail: [email protected] b)

Department of Chemistry, Indian Institute of Technology Kharagpur, West Bengal 721302 INDIA

c)

Beamline Department, Pohang Accelerator Laboratory, 80 Jigokro-127beongil, Nam-gu, Pohang, Gyeongbuk, Korea. E-mail: [email protected]

KEYWORDS: stimuli-responsive organic fluorescent materials, smart fluorescent materials, triphenylamine, aggregation enhanced emission

Triphenylamine (TPA), a propeller shaped optoelectronic molecule, has been used to generate stimuli-responsive smart fluorescent organic materials and correlate the effect of subtle structural change on the molecular packing and mechanochromic fluorescence (MCF). The substituent (OCH3) position in the TPA phenyl ring and acceptors (malononitrile, cyanoacetamide, cyanoacetic acid, ethyl cyanoacetate and diethylmalonate) influenced strongly on the solid state and mechanochromic fluorescence as well as molecular packing. The structure-property studies revealed that (i) TPA derivatives without OCH3 substituent exhibit strong fluorescence (Φf = 85 % (TCAAD-1, 55 % (TDEM)), (ii) higher dihedral angle (τ) between donor (aminophenyl) and acceptor lead to weak/non fluorescent material, (iii) substituent at ortho position to acceptor increased the dihedral angle (τ = 26.49 (TCAAD-2), τ = 27.14 (TDMM)) and (iv) the increase of alkyl groups produced self-reversible high contrast off-on fluorescence switching materials (TDEM). PXRD studies indicate that stimuli induced reversible phase transformation from crystalline to amorphous and vice-versa was responsible for fluorescence switching. The computational studies also supported that OCH3 substitution at ortho to acceptor increased the dihedral angle and optical band gap. Thus, the present studies provide a structural insight for designing TPA based organic molecules for developing new smart organic materials. ABSTRACT:

Introduction Stimuli-active smart fluorescent materials have been received significant interest because of their fundamental importance and application potential in sensors, security inks, data storage and optoelectronic devices.1-8 Particularly, developing organic mechanochromic fluorescence (MCF) materials that exhibit reversible fluorescence changes without undergoing any structural change have made rapid progress in recent years.9-13 For example, fluorophores with non-planar twisted molecular conformation whose structures adopt more planar conformation and exhibit phase transition from crystalline to amorphous phase by mechanical grinding have often been reported for MCF.14-20 Moreover, subtle alternation of molecular packing by disrupting weak intermolecular inter-

actions substantially modulates the excitonic interactions and solid state fluorescence.21-25 Derivatives of tetraphenylpyrene, tetraphenylethenes, cyanostilbene, phenanthroimidazole, boron-diketonate and dendritic molecules are some of the molecular building blocks extensively employed for mechano and vapochromism.26-30 In most of the MCF materials, reversible fluorescence switching was achieved by applying secondary force such as heating/vapor exposing. Only few classes of materials exhibited self-reversible fluorescence without the need of secondary force.31-34 Nevertheless, it is still a great challenge to predict structural design for organic molecules to exhibit MCF especially self-reversible and high contrast fluorescence switching. Hence, there is a strong interest in exploring MCF molecules with subtle structural change to

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understand the relationship between molecular structures and resulting MCF properties at the molecular level.35-37 Triphenylamine (TPA), a propeller shaped nonplanar optoelectronic molecule, has been employed as electron donor (D) moiety in fabricating dyes for dye-sensitized solar cells (DSSC), organic field effect transistor and solid state fluorescent materials for OLED device applications.38-40 The nonplanar twisted structure that prevented close stacking often produced aggregation induced emissive materials whereas synthetic tailorability allowed to engineer the molecular structure with different substituent or acceptor (A) groups.41-45 The nonplanar molecular structure with D-π-A configuration provided the basis for developing solid state fluorescent materials with MCF properties. TPA substituted with dicyanodistyrylbenzene π-conjugated molecule reported for high contrast fluorescence switching.46 TPA based D-A-D molecular structure revealed temperature dependent fluorescence in solution and phase transition dependent mechanochromism.47

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perature dependent positive fluorescence enhancement by introducing aldehyde functionality and polymorphism, mechanochromism and nanofabrication induced fluorescence switching by attaching malononitrile with methoxy substituted TPA.33,50 However, systematic studies of structural modification such as acceptor groups or substituent position in the phenyl ring of TPA on their fluorescence properties have never been explored in detail and such studies are expected to provide structural insight for designing new MCF molecules with self-reversible or stimuli-induced reversible fluorescence. Herein, we have attempted to correlate the acceptor structure and position of methoxy group in the phenyl ring of TPA on the molecular packing and solid state fluorescence especially MCF properties by synthesizing TPA-based donor-πacceptor (D-π-A) molecules with different acceptors (malanonitrile, cyanoacetamide, cyanoacetic acid, ethyl cyanoacetate and diethylmalonate, Scheme-1). Interestingly, acceptor structure and substituent position exhibited strong influence on the molecular conformation and packing that significantly modulated their solid state and mechanochromic fluorescence. TCAAD-2 and TDMM that showed higher dihedral angle between donor TPA and acceptor did not show any solid state fluorescence. The comparison of solid state structure and fluorescence properties revealed that TPA derivatives without methoxy substituent in the phenyl ring showed strong fluorescence. The increase of alkyl groups produced selfreversible high contrast off-on fluorescence switching materials. Theoretical studies also confirmed the increase of dihedral angle and band gap when OCH3 is substituted at ortho to acceptor in TPA. PXRD studies indicate that the reversible phase change from crystalline to amorphous was responsible for fluorescence switching. Thus the present studies could provide a structural insight for designing new TPA based MCF organic molecules.

Experimental Section Chemicals: Acetonitrile (CH3CN, HPLC grade), tetrahydrofuran (THF, HPLC grade), dimethylformamide (DMF, HPLC grade), phosphorous oxychloride, triphenylamine, 3-methoxy-N,N-diphenylaniline, 4-methoxy triphenylamine, cyanoacetic acid, cyanoacetamide, ethylcyanoacetate, malononitrile and diethylmalonoate were purchased from Sigma-Aldrich and used without further purification. General procedure for synthesizing 1-3:

Scheme 1. Structures of TPA based D- π-A molecules.

Highly twisted benzoyl group attached TPA showed mechanical force induced off-on fluorescence switching.48 Self-reversible fluorescence switching has been observed with TPA substituted with aldehyde and methyl groups in the phenyl ring.31 Malononitrile attached TPA showed phase transition induced reversible fluorescence switching.49 Recently, we have reported self-reversible and tem-

Aldehyde was introduced into TPA by following a reported Vilsmeier–Haack formylation procedure.51 Typically, phosphorous oxychloride (1.1 mL, 12 mmol) was added drop-wise to DMF (2 mL, 24 mmol) at 0 °C (cooled in icesalt mixture) and the reaction mixture was stirred for 45 minutes. Then, TPA compound (triphenylamine/3methoxy-N,N-diphenylaniline/4-methoxy -N,Ndiphenylaniline, (0.98 g, 4 mmol)) was added slowly. Then the reaction mixture was brought to room temperature and stirred at 60°C for 2 h. After cooling, the reaction mixture was poured into ice-water (200 mL). The solution


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was neutralized with 1 M NaOH solution and extracted with CH2Cl2. The combined extract was washed with water and brine solution and finally dried over Na2SO4. The solvent was evaporated and purified using column chromatography (ethyl acetate:hexane (1:10)). 4-(diphenylamino)benzaldehyde (1): (yield 90%). Mp 132-136 °C. 1H NMR (300 MHz, CDCl3) δ 9.81 (s, 1H), 7.667.70 (d, 2H), 7.31-7.38 (m, 4H), 7.14-7.19 (m, 6H), 6.99-7.03 (d, 2H). 13C NMR (125 MHz, CDCl3) δ 190.46, 153.37, 146.16, 131.32, 129.74, 129.10, 126.33, 125.12, 119.35. 4-(diphenylamino)-2-methoxybenzaldehyde (2): (yield 93%). Mp 132-137 °C. 1H NMR (300 MHz, CDCl3) δ 10.23 (s, 1H), 7.64-7.67 (d, 1H), 7.31-7.37 (m, 4H), 7.14-7.20 (m, 6H), 6.51-6.54 (dd, 1H), 6.44-6.45 (d, 1H), 3.69 (s, 3H). 13C NMR (125 MHz, CDCl3) δ 187.85, 163.12, 154.87, 146.10, 129.91, 129.67, 126.47, 125.17, 118.21, 112.23, 101.73, 55.41. 4-(N-(4-methoxyphenyl)-N-phenylamino) benzaldehyde (3): (yield 85%). Mp 67-68 °C. 1H NMR (300 MHz, CDCl3) δ 9.78 (s, 1H), 7.63-7.67 (d, 2H), 7.31-7.34 (m, 2H), 7.11-7.18 (m, 5H), 6.88-6.95 (m, 4H), 3.82 (s, 3H). 13C NMR (125 MHz, CDCl3) δ 190.4, 157.6, 153.7, 146.2, 138.9, 131.4, 129.7, 128.5, 125.9, 124.9, 118.2, 115.2, 55.6. Synthesis of TCMN-3 3 (1.0mmol) and malononitrile (1.2mmol) were dissolved in ethanol and refluxed for 4 hrs. Upon cooling to room temperature, it produced reddish orange colored powder that was filtered, washed by cold ethanol and dried under vacuum. The product was further purified by recrystallization from acetonitrile. TCMN-3: Red colored powder and recrystallized from acetonitrile. Yield: 80%. H1 NMR (300 MHz, DMSO-d6) δ 8.24 (s, 1H), 7.85-7.88 (d, 2H), 7.31-7.53 (m, 2H), 7.27-7.29 (m, 6H), 7.06-7.12 (m, 2H), 6.82-6.87 (m, 2H), 3.84 (s, 3H). C13 NMR (125 MHz, DMSO-d6) δ 159.20, 157.71, 153.34, 144.67, 137.18, 133.13, 130.03, 128.74, 126.46, 126.08, 121.93, 116.36, 115.49, 115.44, 114.57, 72.88, 55.33. C23H17N3O (351.14): calcd. C 78.61, H 4.88, N 11.96; found C 78.80, H + 4.71, N 11.40. LCMS (ESI) calcd. [M+H] : 352.14, found: 352.2. General synthesis of TCAAD, TCAA and TCAEA These compounds were synthesized by following a reported procedure.56 Typically, triphenylamine aldehyde (1/2/3, 1mmol), cyanoacetamide/cyanoacetic acid/ethyl cyanoacetate (1.5 mmol) and ammonium acetate (150 mg) were added into glacial acetic acid (5 mL). The reaction mixture was heated at 80 °C for 4 hrs. After cooling to room temperature, it was poured into ice-water. The precipitate was filtered, washed by distilled water, cold acetonitrile and dried under vacuum. The product was further purified by recrystallization. TCAAD-1: Bright yellow colored powder and recrystallized from acetonitrile. Yield: 82%. 1H NMR (300 MHz, DMSO-d6) δ 8.03 (s, 1H), 7.82-7.85 (d, 2H), 7.76 (s,1H), 7.63 (s,1H), 7.39-7.45 (m, 4H),7.18-7.25 (m, 6H), 6.88-6.91 (d, 2H). 13C NMR (125 MHz, DMSO-d6) δ 163.27, 151.05, 149.72, 145.46, 132.07, 129.98, 126.18, 125.35, 123.48, 118.69,

117.30, 101.38. C22H17N3O (339.14): calcd. C 77.86, H 5.05, N 12.38; found C 77.52, H 5.40, N 12.16. LCMS (ESI) calcd. [M+H]+: 340.14, found: 340.2. TCAAD-2: Light orange colored powder and recrystallized from DMSO and obtained as an orange colored crystal. Yield: 75%. 1H NMR (300 MHz, DMSO-d6) δ 8.33 (s, 1H), 8.00-8.04 (m, 1H), 7.62-7.66 (b,2H), 7.40-7.45 (m,4H), 7.21-7.26 (m, 6H), 6.43-6.45 (m, 2H), 3.64 (s, 3H). 13C NMR (125 MHz, DMSO-d6) δ 163.23, 159.91, 152.98, 145.33, 144.14, 129.92, 129.28, 126.34, 125.48, 117.69, 112.54, 111.29, 100.99, 100.28, 55.51. C23H19N3O2 (369.15): calcd. C 74.78, H 5.18, N 11.37; found C 74.52, H 5.31, N 11.24. LCMS (ESI) calcd. [M+H]+: 370.15, found: 370.2 TCAAD-3: Light orange colored powder and recrystallized from acetonitrile and obtained as an orange colored crystal. Yield: 82%. 1H NMR (500 MHz, DMSO-d6) δ 8.00 (s, 1H), 7.79-7.82 (d, 2H), 7.79- (b,1H), 7.72 (b, 1H), 7.377.43 (m, 2H), 7.17-7.24 (m, 5H), 6.99-7.02 (d, 2H), 6.776.82 (d, 2H), 3.77 (s, 3H). 13C NMR (125 MHz, DMSO-d6) δ 163.38, 157.26, 151.51, 149.81, 145.44, 137.93, 132.12, 129.85, 128.56, 125.75, 125.08, 122.62, 117.45, 117.33, 115.35, 100.63, 55.29. C23H19N3O2 (369.15): calcd. C 74.78, H 5.18, N 11.37; found C 74.48, H 5.06, N 11.14. LCMS (ESI) calcd. [M+H]+: 370.15, found: 370.2 TCAA-2: Bright orange colored powder and recrystallized from acetonitrile. Yield: 82%. 1H NMR (300 MHz, DMSO d6) δ 13.52 (b, 1H), 8.44 (s, 1H), 8.11-8.44 (d, 1H), 7.42-7.47 (m,4 H), 7.24-7.29 (m, 6H), 6.39-6.40 (d, 1H), 6.42-6.45 (dd, 1H), 3.65 (s, 3H). 13C NMR (125 MHz, CDCl3) δ. 163.07, 159.12, 152.52, 145.03, 143.62, 128.61, 128.21, 125.26, 124.52, 115.95, 110.44, 109.59, 98.80, 95.00, 54.22. C23H18N2O3 (370.13): calcd. C 74.58, H 4.90, N 7.56; found + C 74.72, H 5.06, N 7.42. LCMS (ESI) calcd. [M+H] : 371.13, found: 371.2 TCAA-3: Obtained as red colored gel. Yield: 85%. 1H NMR (300 MHz, CDCl3) δ 13.50 (b, 1H), 8.11 (s, 1H), 7.397.45 (m, 2H), 7.18-7.23 (m, 5H), 6.99-7.04 (d, 2H), 6.776.80 (d, 2H), 3.78 (s, 3H). 13C NMR (125 MHz, CDCl3) δ 164.19, 157.42, 153.24, 152.21, 145.16, 137.66, 129.91, 128.64, 126.04, 125.45, 122.11, 117.13, 116.90, 115.37, 97.23, 55.30. C23H18N2O3 (370.13): calcd. C 74.58, H 4.90, N 7.56; found + C 74.68, H 4.86, N 7.44. LCMS (ESI) calcd. [M+H] : 371.13, found: 371.3 TCAEA-1: Bright yellow colored powder and recrystallized from DMSO. Yield: 75%. 1H NMR (300 MHz, DMSOd6) δ 8.09 (s, 1H), 7.83-7.86 d, 2H), 7.33-7.38 (m, 4H), 7.17– 7. 21 (m, 6H), 6.96–6.99 (d, 2H), 4.31-4.39 (q, 2H), 1.36-1.40 (t, 3H). 13C NMR (125 MHz, DMSO-d6) δ 163.61, 153.97, 152.43, 145.66, 135.13, 129.76, 126.42, 125.46, 123.35, 119.09, 116.76, 97.26, 62.18, 14.21. C24H20N2O2 (368.15): calcd. C 78.24, H 5.47, N 7.60; found C 78.12, H 5.34, N 7.52. LCMS (ESI) calcd. [M+H]+: 369.15, found: 369.3 TCAEA-2: Light yellow colored powder and recrystallized from DMSO. Yield: 78%. 1H NMR (300 MHz, DMSOd6) δ 8.64 (s, 1H), 8.26-8.29 (d, 1H), 7.33-7.88 (m, 4H), 7.17–7. 21 (m, 6H), 6.53–6.57 (dd, 1H), 6.40-6.41 (d, 1H), 4.30-4.37 (q, 2H), 3.66 (s, 3H), 1.35-1.39 (t, 3H). 13C NMR



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(125 MHz, DMSO-d6) δ 164.11, 161.04, 154.35, 148.09, 145.67, 130.48, 129.73, 126.60, 125.55, 117.47, 113.18, 112.55, 100.98, 95.98, 61.97, 55.54, 14.28. C25H22N2O3 (398.16): calcd. C 75.36, H 5.57, N 7.03; found C 75.40, H 5.42, N 7.12.

LCMS (ESI) calcd. [M+H]+: 399.16, found: 399.3 TCAEA-3: Orange red colored powder and recrystallized from methanol at low temperature. Yield: 80%. 1H NMR (300 MHz, DMSO-d6) δ 8.08 (s, 1H), 7.81-7.84 (d, 2H), 7.32-7.37 (m, 2H), 7.12–7. 19 (m, 5H), 6.89–6.92 (d, 4H), 4.31-4.38 (q, 2H), 3.827 (s, 3H), 1.35-1.39 (t, 3H). 13C NMR (125 MHz, DMSO-d6) δ 163.78, 157.66, 154.04, 152.78, 145.62, 138.30, 133.23, 129.67, 128.42, 125.96, 125.23, 122.72, 118.02, 116.91, 115.15, 96.58, 62.14, 55.49, 14.21. C25H22N2O3 (398.16): calcd. C 75.36, H 5.57, N 7.03; found C 75.43, H + 5.68, N 6.91. LCMS (ESI) calcd. [M+H] : 399.16, found: 399.4 Synthesis of TDEM and TDMM 1 (1.0 mmol), diethylmalonate (1.2 mmol) and piperidine (1.0 mmol) were added into ethanol (10 mL). The reaction mixture was stirred at room temperature for 10 days. Then ethanol was removed using rotary evaporator and extracted 5 times using dichloromethane and the combined extract was washed with brine solution and dried over Na2SO4. The solvent was removed using rotary evaporator to produce the product as yellow colored gel. Recrystallization from hexane gave bright greenish yellow crystals of TDEM. However, diethylmalonate did not react with In a same synthetic procedure was followed for using 2 in methanol. The diethylmalonate ester exchanged with methanol solvent and produced methyl derivative. Unexpectedly, the reaction did not progress in pure ethanol solvent. The product was obtained as yellow powder and recrystallized from acetonitrile. TDEM: Yield: 65%. 1H NMR (300 MHz, CDCl3) δ 7.62 (s, 1H), 7.28-7.33 (m, 6H), 7.09-7.14 (m, 6H), 6.94–6.97 (m, 2H), 4.25-4.38 (m, 4H), 1.30-1.35 (t, 6H). 13C NMR (125 MHz, CDCl3) δ 167.40, 164.68, 150.18, 146.61, 141.69, 131.20, 129.56, 125.72, 125.18, 124.37, 122.68, 120.81, 61.58, 61.39, 14.21, 14.00. C26H25NO4 (415.18): calcd. C 75.16, H 6.06, N 3.37; found C 75.26, H 5.92, N 3.24. LCMS (ESI) calcd. [M+H]+: 416.18, found: 416.3 1

TDMM: Yield: 68%. H NMR (300 MHz, CDCl3) δ 8.070 (s, 1H), 7.26-7.33 (m, 4H), 7.09-7.19 (m, 6H), 6.47–6.49 (d, 2H), 3.81-3.82 (s, 6H), 3.64 (s, 3H). 13C NMR (125 MHz, CDCl3) δ 168.06, 165.37, 159.46, 151.95, 146.52, 138.02, 129.93, 129.50, 125.87, 124.45, 121.43, 114.69, 113.29, 103.24, 55.41, 52.41, 52.37. C25H23NO5 (417.16): calcd. C 71.93, H 5.55, N 3.36; found C 71.82, H 5.38, N 3.42. LCMS (ESI) calcd. [M+H]+: 418.16, found: 418.3. Spectroscopy and structural characterization Absorption and fluorescence spectra were recorded using Perking Elmer Lambda 1050 and Jasco fluorescence spectrometer-FP-8200 instruments. Solid state fluorescence spectra were recorded by exciting at 370 nm with 5 nm slit width in the detection window. Fluorescence quantum yields (Φf) of solid samples were measured using a Horiba Jobin Yvon model FL3-22 Fluorolog spectrofluorim-

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eter with integrating sphere. Mass spectra were recorded with a Bruker 320-MS triple quadrupole mass spectrometer using direct probe insertion method. The powder X-ray diffraction (PXRD) patterns were measured using a XRD- Bruker D8 Advance XRD with Cu Kα radiation (λ = 1.54050 Å) operated in the 2θ range from 10ο to 50ο. Single crystals were coated with paratone-N oil and the diffraction data measured at 100K with synchrotron radiation (λ = 0.62998 Å) on a ADSC Quantum-210 detector at 2D SMC with a silicon (111) double crystal monochromator (DCM) at the Pohang

Accelerator Laboratory, Korea. CCDC Nos. – 1448374 (TCAEA-1), 1448375 (TCAEA-2), 1448377 (TCAEA-3), 1448378 (TCAA-2), 1448484 (TCAAD-3, 1448485 (TCAAD2), 1448486 (TDEM), 1448488 (TDMM) and 1448490 (TCMN-3) contains the supplementary crystallographic data for this paper.

Computational Studies All the density functional theory calculations were performed with the Turbomole program package.52 All the structures were optimized fully without any symmetry constraints. For the molecules where the crystal data were available, the geometry from the crystal data were used as initial guess and for others the geometry were modeled keeping similar conformation to the available data. We used the hybrid PBE0 functional53 and def2-TZVP54 basis set for all calculations. To confirm the proper convergence to minima, the vibrational frequencies were computed at the same level of theory and the absence of imaginary frequency was confirmed. For the optical part, the calculation on the properties has been carried out with time dependent-density functional theory (TD-DFT) formalism with PBE0/def2-TZVP level of theory. The geometries optimized at the ground state were used for TD-DFT calculations. To include the solvent effect, conductor-like screening model (COSMO)55 with acetonitrile as solvent (ε=35.688) was used in the calculation of vertical excitation with TD-DFT. In the TD-DFT calculations, the use of PBE0 functional has provided accurate results for medium and large size molecules. Result and Discussion Scheme 1 shows the TPA derivatives of conjugated D-π-A molecules explored for MCF properties. The synthesized TPA derivatives did not show any measurable fluorescence in solution state. However, they exhibited strong to weak fluorescence in the solid state except TCAAD-2 and TDMM-2 (Table 1). The non-fluorescence in solution and strong solid state fluorescence indicate the aggregation enhanced emission phenomena (AEE). It is noted that TPA based D-π-A compounds are known to exhibit AEE phenomena.49 The mechanochromism of TCMN-1 has already been reported by Y. Cao et al. that showed reversible fluorescence switching between 581 to 602 nm upon strong grinding and heating.49 Recently, we have reported TCMN-2, a methoxy group substituted at ortho to malononitrile, that showed tunable fluorescence via polymorphism and mechanochromism.50 The strong grinding/heating of TCMN-2 switched solid state fluores-


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cence between 562 and 545 nm. However, TCMN-3, methoxy group at para position of phenyl ring, showed solid state fluorescence at 581 nm that was blue shifted to 565 nm by slight breaking (Fig. 1a). The blue shift of fluorescence by slight breaking might be due to disrupting long range order of molecular packing in the single crystals. The PXRD pattern of slightly broken crystals perfectly matched with simulated PXRD pattern generated from single crystal data and confirmed that there was no structural change (Fig. S1a). The strong grinding lead to weak fluorescence (turn-off) and heating turn-on the fluorescence without altering λmax. The repeated grinding/heating of TCMN-3 showed reversible off-on fluorescence switching for several cycles without significant change (Fig. S2). Thus, malononitrile with simple TPA (TCMN-1) showed fluorescence switching whereas methoxy group at ortho to malononitrile (TCMN-1) exhibited polymorphism and fluorescence switching. But TCMN-3, methoxy group at para position, showed off-on fluorescence switching with mechanical pressure and heating. PXRD studies of strongly grounded TCMN-3 powder suggested conversion of crystalline to amorphous phase (Fig. S1b). However, heating converted the amorphous phase to crystalline phase. Thus, the reversible off-on fluorescence switching of TCMN-3 was due to reversible change of material phase with pressure/heating.

TCAAD-3 crystals showed orange-yellow fluorescence with λmax at 560 nm (Fig. 1b). The slight breaking of crystals blue shifted the fluorescence from 560 to 548 nm whereas strong grinding red shifted to 560 nm with reduced intensity. Heating of grounded powder blue shifted the fluorescence to strong yellowish-green. The fluorescence can be reversibly switched between 560 and 548 nm by grinding/heating (Fig. S3). PXRD studies indicated the formation of amorphous phase upon strong grinding that was converted to crystalline phase by heating (Fig. S4). The perfect matching of PXRD pattern of crystals and heated powder with simulated pattern from single crystal data confirmed the same structure.

Table 1. Absolute quantum yield, λmax and dihedral angle displayed by the molecule in the crystals. Compound

Quantum yields (Φf)

TCMN-1

54%

λmax (nm)

Dihedral angle (τ) 6.55

a

TCMN-2

26%,a 48%b

588 535b

0.36,a 0.29b

TCMN-3

47%

581

5.49

TCAAD-1

85%

520

TCAAD-2

No fluorescence

TCAAD-3

52%

562

6.04

TCAA-1

30%

565

4.52

TCAA-2

24%

617

3.11

TCAEA-1

45%

506

8.20

TCAEA-2

23%

542

18.65

5.88 26.49, 4.12c

TCAEA-3

28%

547

15.81

TDEM

55%

504

3.09

TDMM

No fluorescence

27.14

TPA with cyanoacetamide (TCAAD-1), a strong Hbonding acceptor, showed intense green fluorescence at 520 nm (Table 1).57 It exhibited red shifting of fluorescence from 520 to 560 nm upon strong grinding and heating switched the fluorescence to initial state. Surprisingly, TCAAD-2 that has methoxy at ortho position to acceptor did not show any solid state fluorescence. However,

Fig. 1. Mechanochromic fluorescent responses of (a) TCMN-3 and (b) TCAAD-3. Digital fluorescent images are shown in the bottom of the spectra. λexc = 370 (for spectra) and 365 nm (for digital images).



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Cyanoacetic acid acceptor substituted TPA derivatives, TCAA-1 and TCAA-2, showed weak fluorescence at 565 and 617 nm, respectively (Fig. S5). Slight breaking of TCAA-1 single crystals blue shifted the fluorescence λmax to 550 nm with increased intensity and strong grinding reduced the intensity significantly with red shift of λmax to 565 nm (Fig. S5a). Although TCAA-1 showed blue shift of fluorescence to 550 nm by heating, it showed only slight increase of intensity. PXRD studies revealed that heating did not convert TCAA-1 amorphous phase (obtained by strong grinding) to crystalline significantly (Fig. S6). The solid state fluorescence of TCAA-1 has also been tuned by nanofabrication.58 In contrast, hard crushing blue shifted the fluorescence of TCCA-2 from 617 to 560 nm and heating strongly increased the fluorescence intensity without changing fluorescence λmax (Fig. S5b). Further, off-on fluorescence switching by repeated hard crushing/heating showed good recovery of fluorescence without significant change in the intensity or λmax (Fig. S7). PXRD studies indicated clear conversion of amorphous phase to crystalline after heating (Fig. S8). Since TCAA-3 was obtained as semi-solid, we could not perform any mechanochromic fluorescence studies.

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Ethyl cyanoacetate substituted TPA compounds, TCAEA-1, TCAEA-2 and TCAEA-3, showed strong solid state fluorescence at 506, 533 and 547 nm, respectively (Fig. 2, S9). The strong green fluorescence of TCAEA-1 was red shifted from 506 to 548 nm (yellow) with reduced intensity by hard crushing (Fig. 2a). Strong enhancement of fluorescence intensity with blue shift was observed by heating (487 nm). The fluorescence switching of TCAEA-1 from green to yellow and vice-versa have also been performed for several cycles to confirm the high reversibility (Fig. S10). PXRD studies indicate that TCAEA-1 crystalline phase was converted to amorphous phase by strong crushing and heating transformed again to crystalline (Fig. 3a). It is noted that experimental PXRD patterns perfectly matched with simulated pattern from single crystal data (Fig. S11). TCAEA-2 showed strong greenish-yellow fluorescence at 533 nm. The hard crushing lead to quenching of fluorescence intensity without changing λmax and heating turn-on the fluorescence with blue shift (533 to 504 nm (Fig. 2b). PXRD studies revealed reversible change of crystalline to amorphous and vice-versa by mechanical crushing/heating (Fig. 3b, S12). Similar to TCAEA-1, the fluorescence of TCAEA-2 can be

Fig. 3. PXRD pattern of (a) TCAEA-1 and (b) TCAEA-2. Fig. 2. Mechanochromic fluorescent responses of (a) TCAEA1 and (b) TCAEA-2. Digital fluorescent images are shown in the bottom of the spectra. λexc = 370 (for spectra) and 365 nm (for digital images).

reversibly switched by repeated crushing and heating (Fig. S13). TCAEA-3 exhibited yellow fluorescence (Fig. 4, S9). Similar to TCAEA-1 and 2, hard crushing of TCAEA-3 showed red shifts of fluorescence (547 to 570 nm). But, the red shifted fluorescence was slowly self-recovered to initial state without applying secondary force. The self-recovered fluorescence spectra showed an additional hump at 490 nm.


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However, PXRD showed similar pattern for both initial and self-recovered TCAEA-3 sample and ruled-out any structural change (Fig. S14).

Interestingly, diethylmalonate attached TPA (TDEM) showed high contrast self-reversible off-on fluorescence switching. TDEM showed strong bluish-green fluorescence at 498 nm (Table 1, Fig. 5a, S15). The fluorescence was completely quenched by strong crushing. However, the fluorescence intensity was self-recovered to initial state within 20 min without applying secondary force. TDEM fluorescence can be reversibly switched off-on for many cycles by strong crushing (Fig. S16). The complete quenching of fluorescence by strong crushing could be due to the conversion of crystalline to amorphous phase.20,33 The amorphous phase self-reversed to crystalline state with time after removing the mechanical force that turn-on the fluorescence. PXRD studies confirmed the self-recovery of crystalline phase with time that perfectly matched with the initial structure (Fig. 5b, S17). Surprisingly, TDMM, a dimethylmalonate derivative, did not show any solid state fluorescence. It is noted that diethylmalonate substituted TPA (TDEM) was easily obtained using ethanol as solvent in the reaction (TDEM). But methoxy substituted aldehyde, 2, and diethylmalonate did not produce any clear product in ethanol. Hence methanol was used as solvent that produced dimethylmalonate substituted compound (TDMM) rather diethylmalonate derivative. The reaction between aldehyde- 3, and diethylmalonate produced only viscous liquid product.

gen.49 TCMN-2 showed polymorphism and crystals obtained from methanol showed similar dimer and 1D structure formation via π…π and H-bonding interaction between cyano nitrogen and phenyl hydrogen in the crystal lattice.50 TCMN-2 crystals grown from acetonitrile did not show any π…π interactions or extended networks in the crystal lattice, though the molecules adopt opposite dipole arrangement. In TCMN-3, both π…π (3.353 Å) and Hbonding interactions between phenyl hydrogen and cyano nitrogen atoms (dD…A =3.484 Å and θD-H…A =141.84°) resulted in the formation of dimer structure with opposite dipole arrangement (Fig. 6a). The dimer was further linked to four different dimers via C-H…π (dD…A =3.347 Å) and H-bonding interactions (dD…A =3.383 Å and θD-H…A =154.35°) (Fig. 6b).

Fig. 4. Mechanochromic fluorescent responses of TCAEA-3 with digital fluorescent images. λexc = 370 (for spectra) and 365 nm (for digital images).

Fig. 5. TDEM (a) self-reversible mechanochromic fluorescent responses with digital images and (b) PXRD pattern. λexc = 370 (for spectra) and 365 nm (for digital images).

Single crystal X-ray analysis was performed to get the insight on the solid state fluorescence and MCF properties of TPA derivatives. TPA adopted propeller shape in the crystal lattice. TCMN-1 formed dimer via π…π interaction that was further linked to 1D structure by H-bonding interaction between cyano nitrogen and phenyl hydro-

The amide functionality of TCAAD compounds was expected to form strong intermolecular H-bonding interactions in the crystal lattice. TCAAD-1 formed 1D network structure via N-H…O (dD…A =3.042 Å and θD-H…A =148.70°) and C-H…O (dD…A =3.194 Å and θD-H…A =118.36°) interactions (Fig. S18). It did not show commonly observed am-



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ide-amide complementary interactions. However, TCAAD-2 and TCAAD-3 exhibited usual amide-amide complementary H-bonding interaction (dD…A =2.761, 3.036 Å and θD-H…A =161.36°, 153.26° (TCAAD-2); dD…A =2.883 Å and θD-H…A =164.08° (TCAAD-3)) and N-H…NC (dD…A =3.337 Å and θD-H…A =152.67° (TCAAD-1); dD…A =3.120 Å and θD-H…A =145.89° (TCAAD-3)) intermolecular interaction lead to network structure formation in the crystal lattice (Fig. 6c,d). The two molecules present in the unit cell of TCAAD-2 showed different dihedral angle (τ = 26.49 and 4.12, Fig. S19, Table 1). Further, both the molecules are linked by complementary amide H-bonding interaction. TCAA-1, a cyanoacetic acid attached TPA, did not exhibit commonly observed 25 acid-acid dimer in the crystal lattice. It showed helical network structure via O-H…NC intermolecular H-bonding interactions (dD…A =2.752 Å and θD-H…A =158.43°, Fig. S20). However, methoxy group substituted TCAA-2 exhibited typical carboxylic acid-acid dimer (dD…A =2.598 Å and θD-H…A =175.07°) that was further linked by π…π (3.274 Å and 3.257 Å) interactions in the crystal lattice (Fig. 7).

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Diethylmalonate acceptor substituted TDEA showed dimer formation via π…π interactions (3.320 Å and 3.383 Å, Fig. 9a). The dimers are further linked by C-H…O intermolecular interactions between carbonyl oxygen and methylene hydrogen atoms (dD…A = 3.329 Å and θD-H…A = 142.35°, Fig. 9b). TDMM, dimethylmalonate based derivative produced dimer structure via C-H…O intermolecular interactions between carbonyl oxygen and phenyl hydrogen atoms (dD…A = 3.311 Å and θD-H…A = 149.45°; Fig. 9c). The dimer was further linked into extended network structure by another C-H…O intermolecular interactions between carbonyl oxygen and methyl hydrogen atoms of methoxy group (dD…A = 3.364 Å and θD-H…A = 151.24°).

Fig. 7. Dimer and interlinking of dimer in the crystal lattice of TCAA-2. C (grey), N (blue), O (red), H (white); H-bonds (broken line). dD…A distances are marked (Å).

Fig. 6. Dimer and linking of dimers in the crystal lattice of (a,b) TCMN-3, (c) TCAAD-2 and (d) TCAAD-3. C (grey), N (blue), O (red), H (white); H-bonds (broken line). dD…A distances are marked (Å).

TCAEA-1 showed a square like structure through π…π (3.366-3.398 Å) and C-H…O (dD…A =3.135 Å and θD-H…A = 114.49°) intermolecular interactions between four different molecules (Fig. 8a). Whereas C-H…π (3.375 Å) and C-H…O (dD…A =3.423 Å and θD-H…A = 161.13°) intermolecular interactions in the crystal lattice of TCAEA-2 resulted in dimer structure with antiparallel arrangement of molecules that was further interlinked by weak C-H…π interactions (3.458 Å, Fig. 8b). However, TCAEA-3 showed only weak C-H…O intermolecular interactions between carbonyl oxygen and methylene and phenyl hydrogen atoms in the crystal lattice (dD…A = 3.404 Å, 3.432 Å and θD-H…A = 145.77° and 156.64°, Fig. 8c).

Fig. 8. Molecular arrangement of (a) TCAEA-1 (b) TCAEA-2 and (c) TCAEA-3 by weak intermolecular interactions in the crystal lattice. C (grey), N (blue), O (red), H (white); Hbonds (broken line). dD…A distances are marked (Å). The comparison of solid state fluorescence with molecular structures of TPA based D-π-A derivatives revealed that TPA without methoxy substituent in the phenyl ring showed strong fluorescence (Table-1). The increased dihedral angle between donor phenyl and acceptor in TCAAD-2 and TDMM lead to non-fluorescence. The methoxy group at ortho position to alkene acceptors tends to increase the dihedral


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angle. This was further confirmed by computational studies (Table-S1, S2 and Fig. S21). In all the molecules, the substitution at the acceptor phenyl ring (R3) blue shifted the excitation wavelength and increased the dihedral angle (Φ1). As Φ1 represents the planarity of the alkene group with the phenyl ring, an increment in the value can causes lose of πelectron conjugation that increased the excitation energy. The increase of Φ1 leads to a higher blue shift in the absorbance spectra from TCMN-1 to TCMN-2. It is noted that TDMM (with as well as without OCH3) unusually exhibited higher twist between aminophenyl and alkene acceptor, though ortho substitution showed slightly higher dihedral angle.

Fig. 9. The dimer and network structure formation by weak interactions in the crystal lattice of (a, b) TDEM and (c, d) TDMM. C (grey), N (blue), O (red), H (white); H-bonds (broken line). dD…A distances are marked (Å).

In contrast, substitution at the donor phenyl ring did not show significant influence on the dihedral angle of alkene group. However, it does influence on the excitation energy of TPA derivatives. The substitution at the donor phenyl ring of TCMN, TCAAD, and TCAA series showed red shift of excitation energy. From the MO diagrams, it is clear that the substituted methoxy group acts as donor in all the cases when it is in the donor phenyl group. Therefore, it decreased the HOMO-LUMO separation. Hence bulky group substitution closer to acceptor group in the TPA molecules might be unfavorable for developing strong solid state fluorescent material. Further, TCAA-2 crushed sample showed good recovery of fluorescence after heating compared to TCAA-1. TCAA-1 showed helical structure via OH…NC intermolecular H-bonding interactions whereas TCAA-2 exhibit commonly observed acid-acid dimer formation (Fig. 7, S20). The strong crushing of TCAA-1 might have forced the molecules to adopt acid-acid dimer structure along with phase change that could not be completely reversed by heating. Nanofabricated TCAA-1 that showed acid-acid dimer interactions showed blue shifted fluorescence compared to crystals.54 In contrast, TCAA-2 that showed strong acid-acid dimer in the crystal lattice undergoes complete reversible phase transition by crushing/heating and switched the fluorescence. The self-recovery of fluorescence with alkyl groups substituted TPA molecules after strong crushing could be

due to weak intermolecular interactions and relaxation of alkyl groups. It is noted that TCAAD-2 and TCAAD-3 exhibit similar amide-amide complementary interactions in the crystal lattice, however, TCAAD-3 showed strong fluorescence whereas TCAAD-2 is non-fluorescence. Similarly, TDEM and TDMM exhibited similar extended network structure by weak interactions but TDEM alone showed strong solid state fluorescence. This was due to the higher twist of alkene acceptor group in both TCAAD-2 and TDMM.

Conclusion In conclusion, we have synthesized TPA based D-π-A derivatives and systematically explored effect of subtle structural change on the molecular conformation and packing in the solid state and their influence on the stimuli-responsive fluorescence. TPA derivatives except TCAAD-2 and TDMM exhibited aggregation enhanced emission in the solid state. Importantly, all fluorescent solids showed stimuli-induced reversible fluorescence changes. The comparison of solid state structure and fluorescence indicated that position of methoxy group and dihedral angle between donor TPA and alkene acceptor played significant role on the MCF properties. TPA derivatives without methoxy substituent showed strong fluorescence (Φf = 85 % (TCAAD-1, 55 % (TDEM)) that could be due to the better packing and rigidification of fluorophore in the solid state. Higher dihedral angle leads to weak/non-fluorescence. TCAAD-2 and TDMM that showed high dihedral angle (26.49, 27.14) did not exhibit any fluorescence in the solid state. The methoxy group at ortho position to alkene acceptor increased the dihedral angle between donor and acceptor. The increase of alkyl groups at TPA structure lead to self-reversible high contrast off-on fluorescence switching. Subtle intermolecular H-bonding change in the crystal lattice of TCAA-1 displayed poor fluorescence recovery after strong crushing whereas typical off-on fluorescence switching was observed with TCAA-2. The theoretical studies further substantiated the role of substituent position on the solid state fluorescence. PXRD studies suggest that the fluorescence switching was due to the reversible phase conversion of materials from crystalline to amorphous and viceversa. Thus, the present studies provide better structural insight for designing new smart organic fluorescent materials.

Acknowledgement Financial support from the Science and Engineering Research Board (SERB), New Delhi, India (SERB No. EMR/2015/00-1891, SB/FT/CS-182/2011) is acknowledged with gratitude. The CRF facility of SASTRA University is also acknowledged for absorption spectroscopy. "X-ray crystallography at the PLS-II 2D-SMC beamline was supported in part by MSIP and POSTECH. We thank Department of Chemistry, IIT Kharagpur for computational facility and LCMS". The mass spectra were supported by the Basic Science Research Program of the National Research Foundation of Korea



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(NRF) funded by the Ministry of Education, Science and Technology (NRF-2014R1A1A2058815).

SUPPORTING INFORMATION Fluorescence spectra, fluorescence switching cycle, PXRD patter and crystal structures.

AUTHOR INFORMATION Corresponding Author * Savarimuthu Philip Anthony, [email protected].

* Dohyun Moon, [email protected].

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and Tunable Luminescence. J. Phys. Chem. C, 2010, 114, 11708– 11716.


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Crystal Growth & Design “For Table of Contents Use Only”

Molecular Engineering of Triphenylamine based Aggregation Enhanced Emissive Fluorophore: Structure Dependent Mechanochromism and Self-Reversible Fluorescence Switching

Palamarneri Sivaraman Hariharan, Viki Kumar Prasad, Surajit Nandi, Anakuthil Anoop, Dohyun Moon and Savarimuthu Philip Anthony

Stimuli responsive smart fluorescent triphenylamine derivatives have been synthesized that exhibited reversible and self-reversible mechanochromism as well as non fluorescence solids depend on the position of substitution and acceptor structure.


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Molecular Engineering of Triphenylamine Based Aggregation

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