Solvent vapour induced rare single-crystal-to-single-crystal transformation of stimuli-responsive fluorophore: Solid state fluorescence tuning, switching and role of molecular conformation and substituents

Journal Pre-proof Solvent vapour induced rare single-crystal-to-single-crystal transformation of stimuliresponsive fluorophore: Solid state fluorescence tuning, switching and role of molecular conformation and substituents Palamarneri Sivaraman Hariharan, Chengjun Pan, Subramanian Karthikeyan, Dexun Xie, Akira Shinohara, Chuluo Yang, Lei Wang, Savarimuthu Philip Anthony PII:

S0143-7208(19)32331-9

DOI:

https://doi.org/10.1016/j.dyepig.2019.108067

Reference:

DYPI 108067

To appear in:

Dyes and Pigments

Received Date: 3 October 2019 Revised Date:

28 October 2019

Accepted Date: 21 November 2019

Please cite this article as: Hariharan PS, Pan C, Karthikeyan S, Xie D, Shinohara A, Yang C, Wang L, Anthony SP, Solvent vapour induced rare single-crystal-to-single-crystal transformation of stimuliresponsive fluorophore: Solid state fluorescence tuning, switching and role of molecular conformation and substituents, Dyes and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.108067. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Graphical Abstract

Solvent vapour induced rare single crystal to single crystal transformation of stimuliresponsive fluorophore: Solid state fluorescence tuning, switching and role of molecular conformation and substituents

Mechanofluorochromic TPE based aggregation induced emissive fluorophores displayed substituent group dependent polymorphism, tunable emission and rare single crystal-single crystal structural transformation.

Solvent vapour induced rare single-crystal-to-single-crystal transformation of stimuliresponsive fluorophore: Solid state fluorescence tuning, switching and role of molecular conformation and substituents Palamarneri Sivaraman Hariharan[a][b], Chengjun Pan*[a], Subramanian Karthikeyan[c], Dexun Xie[d], Akira Shinohara[a], Chuluo Yang[a], Lei Wang*[a][b] and Savarimuthu Philip Anthony*[e]

[a][b] Dr. Palamarneri Sivaraman Hariharan, Prof Dr. Lei Wang Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, China. Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China. E-mail: [email protected] [a] Prof Dr. Chengjun Pan, Dr. Akira Shinohara, Prof Dr. Chuluo Yang Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, China. E-mail: [email protected] [c] Prof Dr.Subramanian Karthikeyan PG and Research department of chemistry, KhadirMohideen College, Adirampattinam, Tamil Nadu, India. [d] Prof Dr.Dexun Xie School of Chemistry, Sun Yat-sen University, Guangzhou, 510275, China; Shenzhen Research Institute, Sun Yat-sen University, Shenzhen, 518057, China.

[e] Prof Dr.Savarimuthu Philip Anthony School of Chemical & Biotechnology, SASTRA Deemed University, Thanjavur-613401, Tamil Nadu, India. E-mail: [email protected]

Keywords: Tetraphenylethylene, fluorescence switching, polymorphism, mechanofluorochromism, single crystal to single crystal transformation (sc-sc)

1

Abstract Supramolecular interactions and molecular conformations play significant role on producing fluorescent polymorphs with tunable emission in the solid state from single fluorophore. External stimuli-induced structural transformation from one polymorph to another polymorph without losing single crystalline character (single-crystal-to-single-crystal transition (SC-SC)) provides perfect model to understand the structure-property of molecules in the solid state and designing new functional fluorescent materials. Herein we have tailored the substituent groups in the aggregation-induced-emissive (AIE) tetraphenylethylene (TPE) based donor (D)-acceptor (A) fluorophores (TPPA-1-6) and explored fluorescent polymorphs formation and SC-SC transition in the solid state. TPPA-1 showed conformational fluorescent polymorphs via twisted and co-planar conformation between TPE phenyl group and cyanophenyl acceptor phenyl ring. However, single crystal analysis of other derivatives revealed that substituent groups and supramolecular interactions control the molecular conformation and SC-SC transition. Fluorophore with same substituent but positional change also show drastic effect on the fluorescent polymorphs formation and SC-SC transition. Substituents that can form strong intermolecular interactions stabilize the less stable co-planar structure whereas bulky substituents with weak intermolecular interactions produced twisted or both conformers. Photophysical, powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC) and thermogravimteric (TG) analysis were performed to substantiate and get insight on the structural transformation. The formation of fluorescent polymorphs and SCSC transition exhibited tunable and switchable solid state fluorescence. The non-planar TPE core has been made use to demonstrate high contrast stimuli induced reversible fluorescence switching. Overall, the present studies attempted to gain information on the structural design for developing SC-SC transforming functional organic fluorescent materials that could be of potential interests for optoelectronic and display devices.

2

1. Introduction Switchable and tunable functional organic solid state fluorescent materials are potential candidates for optoelectronic device applications including organic light emitting-diodes (OLEDs), displays, sensors, data storage, hiding and bio-imaging[1-11]. Molecular structure, conformation and supramolecular interaction plays significant role on the fluorescence of organic molecules in the solid state[12-15]. Discovery of aggregation-induced-emission (AIE) phenomena resulted in the generation of strongly fluorescent materials using different class of organic molecules especially with non-planar molecular structure[16]. Restriction of intramolecular rotation (RIR) and rigidification of fluorophore in the solid state via supramolecular interactions facilitate radiative transitions[17-18]. The ability of non-planar conformationally flexible organic molecules to exhibit stimuli-induced conformational and phase change have been exploited to develop smart fluorescent materials[19-20]. Another important advantage of non-planar conformationally flexible AIE fluorophore is the formation of crystal polymorphs, single molecular structure displaying two or more different conformations and packing, with distinctly different solid state fluorescence that could lead to multi-colour mechanochromism[21-22]. Polymorphism considered as ideal model for understanding structure-property due to different molecular conformation and packing induced distinct property from same molecule. For example, difluoroboron avobenzone produced three different polymorphs with green, cyan and blue fluorescence via packing and conformational change[23]. Twisted angle difference in cyano substituted distyrylbenzene produced two different fluorescent polymorphs[24-25]. Phenthiazine integrated benzonitrile fluorophore showed three different fluorescent polymorphs including room temperature phosphorescent polymorph[26]. Triphenylamine based donor-acceptor-donor (D-A-D) fluorophore also produced different fluorescent polymorphs[27]. Bis(4-((9H-fluoren-9ylidene)methyl)phenyl)-

thiophene (BFMPT)

exhibited

green

and

orange emiting

confomational polymorphs with twsited central thiophene-phenylene fragment[28]. B. Z. 3

Tang et.al., reported crystallisation-induced-emissive quinoxalin based molecule with three different polymorphs showed a different molecular configuration and packing style [29]. The polymorphs that differs slightly in the stabilization energy showed phase transition upon applying external stimuli such as heat, light, solvent vapours and mechanical pressure that resulted in tunable/switchable fluorescence[30-38].But supramolecular interactions controlled organic molecular crystals are fragile and rarely preserve their single crystal integrity while undergoing transition. Sugar derived molecule showed spontaneous SC-SC transformation with large structural change[39]. Coordination complexes and organic compounds that exhibit photocycloaddition displayed SC-SC transformation[40]. Gold (I) isocyanide complexes showed excellent SC-SC transformation by modulating aurophilic interactions upon vapour exposure and mechanical pressure[41-43]. However, fluorescent organic molecular crystals that can exhibit SC-SC transformation are still rarely reported[44]. There was no systematic structure-property studies have been performed with any AIE fluorophore that might provide structural information for designing SC-SC transforming functional organic materials. Tetraphenylethylene (TPE), a typical prototype of AIE molecule that has been extensively

employed

for

generating

solid

state

fluorescent

materials

and

mechanofluorochromic materials[45]. Synthetic tailorability of TPE has enabled to generate number of AIE derivatives for optoelectronic to bio-imaging applications[46-49]. The conformational flexibility of TPE has also resulted in the formation of fluorescent polymorphs [50-52]. However, based on our knowledge only one report based on stimuli induced SC-SC transition has been reported for TPE based AIE fluorophores[44]. In this manuscript, we have synthesised TPE-cyanophenyl based donor-acceptor (D-A) compounds and explored acceptor substituent effect on the polymorphism and SC-SC transition. Solid state structural studies showed clear polymorphs for TPPA-1 and TPPA-4 with tunable fluorescence. Interestingly, both compounds showed structural transformation upon solvent exposure. Particularly, TPPA-1 preserved its single crystalline nature and exhibited SC-SC transition upon hexane 4

exposure. Solid state structural analysis, DSC, TGA, computational and detailed photophysical studies were performed to establish structure-property relationship. 2. Experimental Section Solvents (HPLC grade) were received from Alfa Aesar and used as received. 4-(1,2,2triphenylvinyl)benzaldehyde, trimethoxy

2,2'-(2-methoxy-1,4-phenylene)diacetonitrile,

phenyl)acetonitrile,

methoxyphenyl)acetonitrile,

2-(3,5-dimethoxyphenyl)acetonitrile, 2-(3,4-difluorophenyl)

2-(3,4,52-(3-fluoro-4-

acetonitrile,

2-(3,4-

dichlorophenyl)acetonitrile and sodium methoxide were purchased from Alfachem and used as received. 2.1 General procedure for the preparation of TPPA derivatives To the stirred solution of 4-(1,2,2-triphenylvinyl)benzaldehyde (0.5g,1.0 equivalent) in methanol, added 1 equivalent of respective phenylacetonitrile and 2 equivalents of sodium methoxide and stirred at room temperature for 12 hours and the products precipitated out. The yellow precipitate was filtered out and washed with cold methanol and dried in a vacuum. 2-(3,4-Dimethoxyphenyl)-3-(4-(1,2,2-triphenylvinyl)phenyl) acrylonitrile (TPPA-1) Pale yellow colored solid. Yield: (85%), mp (°C): 172, FT-IR (KBr pellet) ν: 2210, 1647, 1620, 1597, 1515, 1276, 1245, 1142, 1020, 767, 747, 700 cm−1. 1H NMR (CDCl3, 700 MHz,) δ 7.64 (d, J = 14.0 Hz, 2H), 7.31 (s, 1H), 7.23 (dd, J = 8.4, 2.1 Hz, 1H), 7.15-7.11 (m, 12H), 7.07-7.03 (m, 6H), 6.91 (d, J = 7.0 Hz, 1H), 3.94 (s, 3H), 3.92 (s, 3H).

13

C NMR

(CDCl3,176 MHz) δ 150.1, 149.4, 146.3, 143.5, 143.5, 143.3, 142.4, 140.3, 140.2, 132.0, 131.9, 131.5, 131.5, 131.4, 128.7, 128.0, 128.0, 127.8, 127.7, 127.0, 126.8, 126.8, 119.1, 118.4, 111.4, 110.5, 108.9, 56.2. HRMS, m/z: [M+H]+ calcd. for C37H29NO2, 520.2271; found, 520.2280. 2-(3,5-Dimethoxyphenyl)-3-(4-(1,2,2-triphenylvinyl)phenyl) acrylonitrile (TPPA-2) Pale yellow colored solid. Yield: (83%). mp (°C): 199, FT-IR (KBr pellet) ν: 2214, 1650, 1608, 1592, 1461, 1332, 1193, 1158, 1068, 827, 747, 700 cm−1. 1H NMR (CDCl3,700 5

MHz) δ 7.66 (d, J = 7.0 Hz, 2H), 7.40 (s, 1H), 7.15-7.11 (m, 11H), 7.07-7.03 (m, 6H), 6.77 (s, 2H), 6.48 (t, J = 7.0 Hz, 1H), 3.83 (s, 6H). 13C NMR (CDCl3,176 MHz) δ 161.3, 146.7, 143.5, 143.4, 143.3, 142.5, 142.4, 140.1, 136.9, 132.1, 131.6, 131.5, 131.5, 131.4, 129.0, 128.0, 128.0, 127.8, 127.0, 126.9, 126.8, 118.3, 110.6, 104.3, 101.2, 55.7. HRMS, m/z: [M+H]+ calcd. for C37H29NO2, 520.2271; found, 520.2274. 2-(3,4-Difluorophenyl)-3-(4-(1,2,2-triphenylvinyl) phenyl)acrylonitrile (TPPA-3) Bright yellow colored solid. Yield: (82%). mp (°C): 166, FT-IR (KBr pellet) ν: 2216, 1642, 1613, 1520, 1434, 1296, 1117, 697 cm−1. 1H NMR (CDCl3, 700 MHz) δ 7.67 (d, J = 8.3 Hz, 2H), 7.47 (ddd, J = 11.0, 7.3, 2.3 Hz, 1H), 7.42–7.37 (m, 1H), 7.36 (s, 1H), 7.31–7.21 (m, 2H), 7.15 (dtd, J = 10.1, 6.6, 3.6 Hz, 10H), 7.11–7.01 (m, 6H). 13C NMR (CDCl3, 176 MHz) δ 151.4, 151.4, 151.3, 151.2, 150.0, 149.9, 149.9, 149.8, 147.0, 143.2(d, J=3.52 Hz), 143.1, 142.7, 142.7, 142.5, 139.9, 132.0, 131. 8(q, J=6.16 Hz), 131.3, 131.3, 131.3, 131.1, 128.9, 127.9(d, J=7.04 Hz), 127.7, 127.0, 126.8(d, J=3.52 Hz), 122.2(d, J=7.04 Hz), 118.0, 117.9, 117.6, 115.0, 114.9, 108.5. HRMS, m/z: [M+H]+ calcd. for C35H23F2N,496.1871; found, 496.1876 2-(3,4-Dichlorophenyl)-3-(4-(1,2,2-triphenylvinyl) phenyl)acrylonitrile (TPPA-4) Bright yellow colored solid. Yield: (80%). mp (°C): 196, FT-IR (KBr pellet) ν: 2214, 1596, 1479, 1136, 1026, 885, 822, 757, 703 cm−1. 1H NMR (CDCl3, 700 MHz) δ 7.71 (d, J = 2.2 Hz, 1H), 7.65 (d, J = 8.3 Hz, 2H), 7.51–7.45 (m, 2H), 7.39 (s, 1H), 7.15–7.10 (m, 11H), 7.07–7.01 (m, 6H).

13

C NMR (CDCl3, 176 MHz) δ 147.25, 143.17, 142.66, 139.88, 134.73,

133.48, 133.19, 132.05, 131.42, 130.99, 128.98, 127.92, 127.67, 126.99, 126.80, 125.06, 117.48, 108.23. HRMS, m/z: [M+H]+ calcd. for C35H23Cl2N,528.1280; found, 528.1280. 2-(3-Fluoro-4-methoxyphenyl)-3-(4-(1,2,2-triphenylvinyl) phenyl)acrylonitrile (TPPA-5) Pale yellow colored solid. Yield: (78%). mp (°C): 181, FT-IR (KBr pellet) ν: 2215, 1615, 1522, 1441, 1282, 1139, 1028, 824, 760, 696 cm−1. 1H NMR (CDCl3, 700 MHz) δ 7.65 (d, J = 8.3 Hz, 2H), 7.43–7.36 (m, 2H), 7.30 (d, J = 21.9 Hz, 2H), 7.18–7.11 (m, 10H), 7.11– 6

7.04 (m, 5H), 7.02 (dd, J = 10.8, 6.3 Hz, 1H), 3.95 (s, 3H).

13

C NMR (CDCl3, 176 MHz) δ

153.1, 151.7, 148.4, 148.3, 146.5, 143.3(d, J=7.04 Hz), 143.2, 142.4, 140.8, 140.0, 131.9, 131.5, 131.4 131.3, 131.2, 128.7, 127.9 (d, J = 8.8 Hz), 127.8 (d, J = 7.04 Hz), 127.7, 126.7 (d, J = 3.5 Hz) 122.3, 122.2, 117.9, 113.5, 113.4, 109.2, 109.2, 56.3. HRMS, m/z: [M+H]+ calcd. for, C36H26FNO:508.2071; found, 508.2071 2-(3,4,5-Trimethoxyphenyl)-3-(4-(1,2,2-triphenylvinyl) phenyl)acrylonitrile (TPPA-6) Pale yellow colored solid. Yield: (75%). mp (°C): 208, FT-IR (KBr pellet) ν: 2217, 1584, 1508, 1334, 1252, 1126, 995, 833, 702 cm−1.1H NMR (CDCl3, 700 MHz,): δ 7.66 (d, J = 14.0 Hz, 2H), 7.34 (s, 1H), 7.15-7.11 (m, 11H), 7.07- 7.03 (m, 6H), 6.83 (s, 2H), 3.92 (s, 6H), 3.88 (s, 3H). 13C NMR (CDCl3, 176 MHz,): δ 154.7, 146.6, 143.5, 143.3, 142.5, 141.6, 140.1, 139.2, 132.1, 131.7, 131.5, 130.5, 128.8, 128.0, 128.0, 127.9, 127.8, 127.0, 126.9, 118.3, 110.7, 103.4, 61.4, 56.4. HRMS, m/z: [M+H]+ calcd. for, C38H31NO3:550.2377; found, 550.2377 3. Characterization Fluorescence spectra were recorded using Thermoscientific. Fluorescence (S.NoLF1603004) quantum yields of the solid samples were measured using Hamamatsu Absolute PL Quantum yield spectrometer (C11347). Differential scanning calorimetry (DSC) was performed using Hitachi thermal analyzer system (DSC7020). Life time measurements were measured using Horiba scientific (S.No:14464). The HOMO, LUMO and band gap of all structures are studied using B3PW91/6-31+G(d,p) level theory (Gaussian 09 package) and potential energy calculations were performed using B3PW91/6-31+G**. The Powder X-ray diffraction patterns were measured using XRD-Rigaku with Cu Kα radiation (λ=1.540500 Å) operated in the 2θ range from 5° to 50°. Solvent exposure study were performed by adding 2.5mg of acetonitrile based crystal in vial and kept inside the beaker containing the respective solvents and closed the container with a lid. Single crystal X-ray diffraction were performed

7

using Bruker D 8 Venture. CCDC Nos. – 1910330-1910340 contain the supplementary crystallographic data for this paper. 4. Results and Discussion TPE integrated cyanophenyl D-A derivatives were synthesized following reported procedure (Figure 1, Scheme S1) [53]. All derivatives exhibited clear AIE properties, strong solid state fluorescence and weak/no fluorescence in the solution that was confirmed by injecting methanol solution of TPPA into water (Figure S2, S3). TPPA-1 did not show any fluorescence between 100 and 60%. It showed weak fluorescence at 476 nm when water fraction was 70%. Further increasing water fraction increased the aggregation as well as fluorescence intensity with red shift of λmax from 476 to 515 nm. TPPA-2, a positional isomer of TPPA-1 exhibited weak fluorescence (λmax = 505 nm) even at 40% water fraction and further increase of water lead to enhancement of intensity without altering λmax. AIE properties of TPPA-3-6 have also been confirmed by similar studies (Figure S2, S3). Aggregated powders of TPPA-1-6 showed fluorescence between 469 and 496 nm (Table S1). The unsubstituted and single methoxy group substituted TPPA also showed AIE phenomena [54]. The long conjugated cyanophenyl acceptor unit can adopt either coplanar with TPE

Figure 1. Molecular structure of TPE D-A derivatives with different substituents. 8

phenyl or twisted molecular conformation (Scheme S2). Computational studies suggest that twisted conformation is more preferred than co-planar conformation. However, small differences of stabilization energy between both conformers provide possibility of realizing polymorphs with tunable solid state fluorescence. Crystallization of unsubstituted TPPA and single OCH3 group substituted TPPA did not produce any polymorphs (Figure S4) [54]. Crystals grown from different solvents (CH3CN, ethyl acetate, CH3OH) did not show any

Figure 2. Solid state fluorescence spectra of TPPA-1a, TPPA-1b and TPPA-1c (a) TPPA-2, TPPA-3 and TPPA-6 (b), TPPA-4b, TPPA-4a (c) and TPPA-5a, TPPA-5b and TPPA-5c (d) (λexc = 370 nm). significant change in the fluorescence. However, TPPA-1 crystals displayed tunable fluorescence depends on the crystallization solvent and condition (Figure 2a). Crystals grown 9

from CH3OH (TPPA-1a) showed fluorescence at 473 nm whereas 488 and 493 nm were observed when grown from CH3CN at room temperature (TPPA-1b) and 4 °C (TPPA-1c). TPPA-2 and TPPA-3 as well as TPPA-6 did not show any tunable fluorescence (Figure 2b). But TPPA-4a crystals (grown from CH3CN at room temperature) showed fluorescence at 494 nm and TPPA-4b (grown from CH3CN at 4° C) showed fluorescence at 488 nm (Figure 2c). TPPA-5 crystals grown from CH3CN (room temperature and 4 °C) and DCM/hexane showed fluorescence between 468 and 482 nm (Figure 2d). All compounds showed strong solid state fluorescence (Table 1). Excited state lifetime decay studies showed mostly single exponential decay with short lifetime (Table 1). TPPA-1a and TPPA-1b showed bi-exponential decay with short lifetime but TPPA-1c revealed three exponential decays with relatively long lifetime. Interestingly, TPPA-4a showed short lifetime but TPPA-4b exhibited long lifetime. Table 1. Excited state decay life time and absolute quantum yield (Φf) of TPPA-1-6. Compounds

τ1 (ns)

TPPA-1a TPPA-1b TPPA-1c TPPA-2 TPPA-3 TPPA-4a TPPA-4b TPPA-5a TPPA-5b TPPA-5c TPPA-6

1.02 1.45 0.78 1.37 2.55 78.85 2.16 0.61 1.74 2.26 1.71

τ2 (ns) 1.74 2.61 2.66

1.78 1.91

τ3 (ns)

129.74

Φf (%) 51 33 39 38 42 45 51 48 42 38 45

Single crystal structural analysis was performed to gain insight on the molecular conformation and assembly in the solid state. Interestingly, both TPPA and single OCH3 substituted compound (TPPA-OCH3) showed co-planar molecular conformation between TPE donor phenyl and cyanophenyl acceptor unit in the crystal lattice (Figure S5) [54]. Molecular packing of TPPA showed opposite molecular arrangement with well separation of molecules in the crystal lattice whereas TPPA-OCH3 showed C-H...π interactions between 10

OCH3 and TPE phenyl groups induced dimer formation (Figure S5). TPPA-1 produced polymorphic structure when crystallized from CH3OH and CH3CN displayed twisted and coplanar molecular conformation (Figure 3a). TPPA-1a showed twisted conformation and TPPA-1b/1c displayed coplanar molecular conformation. Acetonitrile solvent molecule is included in the crystal lattice of TPPA-1b and TPPA-1c and stabilized via weak H-bonding between cyano nitrogen and ethylene hydrogen (Figure S6). Molecular packing of TPPA-1a showed opposite arrangement of molecules and methoxy groups involved in H-bonding and C-H...π interactions (Figure 3b, Figure S7a). The interaction between methoxy oxygen and methylene hydrogen and cyano nitrogen and phenyl hydrogen further interlinks the molecules in the crystal lattice (Figure S7b).

Figure 3(a) Molecular conformation of TPPA-1a-c and molecular packing and supramolecular interactions in the crystal lattice of (b) TPPA-1a and (c) TPPA-1b. C (grey), N (blue), O (red) and H (white); H-bonds (broken line). dD…A distances are marked (Å).

11

In contrast, TPPA-1b showed intermolecular H-bonding interactions between methoxy group and molecules were well separated in the crystal lattice without any other strong intermolecular interactions (Figure 3c, Figure S8). Both TPPA-1b and TPPA-1c showed similar molecular packing in the crystal lattice (Figure S9). The closer molecular packing and extensive intermolecular interactions in TPPA-1a lead to enhanced fluorescence (51 %) compared to TPPA-1b/1c (33 % and 39 %) (Table 1). TPPA-2, isomer of TPPA-1, showed only twisted molecular conformation and C-H...π and H-bonding interactions produced opposite arrangement of molecules in the crystal lattice (Figure 4, Figure S10a). Fluorine substituted TPPA-3 also did not produce any polymorphs but exhibited only coplanar molecular conformation (Figure 4). The asymmetric unit of TPPA-3 contains two molecules and both showed coplanar conformation. H-bonding interactions of F and cyano nitrogen with phenyl hydrogen connect the molecules in the crystal lattice (Figure S10b). TPPA-4 crystals grown from CH3CN at room temperature (TPPA-4a) were not quality crystals to get the single crystal data, however, quality crystals were grown at 4 °C (TPPA4b). The asymmetric unit of TPPA-4b contains three molecules and one of the molecule showed complete coplanar whereas other two exhibited slightly twisted molecular conformation (Figure 4). TPPA-4b also includes CH3CN solvent in the crystal lattice (Figure S11a). The Cl atom involved in the weak intermolecular H-bond with phenyl hydrogen and cyano nitrogen (Figure S11b). TPPA-5 that contains Fluorine and OCH3 exhibited slightly twisted molecular conformation compared to TPPA-3 (Figure 4). The cyano nitrogen and OCH3 involved in C-H...π and H-bonding interactions and produced oppositely oriented molecular packing (Figure S12). The crystals obtained from different solvents showed only small twist difference with same molecular packing and interactions (Figure S12, Table S2). The small twist angle differences between the crystals lead to tunable fluorescence (Figure S13). TPPA-6, three OCH3 group substituted molecule, showed

12

twisted molecular conformation and dimer with opposite orientation of molecules via C-H...π interactions (Figure 4, Figure S14).

Figure 4 Molecular conformation of TPPA-2, TPPA-3, TPPA-4b, TPPA-5 and TPPA-6. C (grey), N (blue), O (red) and H (white).

Overall TPPA molecules with OCH3 substitution showed both twisted as well as coplanar conformation whereas halogen substituted molecules showed predominantly coplanar conformation. It is noted unsubstituted TPPA as well as only single OCH3 substituted TPPA showed only coplanar molecular conformation (Figure S5). Hence bifunctional substitution plays important role on the formation of polymorphism and tunable solid state fluorescence. Theoretical calculation of highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) of TPPA-1-2 showed clear charge transfer from donor TPE to cyanophenyl acceptor (Figure S15). The electron density is occupied in complete structure in the HOMO of TPPA-1c compared to TPPA-1a that 13

could be attributed to coplanar conformation. Computational studies were also performed to understand the potential energy change between twisted and coplanar conformation. Interestingly, the twisted conformer of TPPA-1 was stable by 9.85 Kcal/mol compared to coplanar structure. Structural analysis also revealed that twisted conformer is stabilised by additional H-bonding interactions in the crystal lattice compared to coplanar conformer (Figure 3, Figure S7, 8). The formation of two different conformers by substitutional change and observation of fluorescent polymorphs prompted us to investigate SC-SC transformation in TPPA-1 and TPPA-4. Interestingly, TPPA-1b and TPPA-1c exhibited clear structural transformation upon hexane vapour exposure. Importantly, it retains single crystalline nature after transformation. Structural analysis confirmed complete transformation of TPPA-1 from coplanar conformation (TPPA-1c) to twisted conformation (TPPA-1a, Figure 5a, Table S3). It is noted that CH3CN solvent included in the crystal lattice of TPPA-1c also completely removed by hexane exposure. TGA analysis also showed complete removal of CH3CN after hexane exposure (Figure 5b). PXRD studies further confirm the structural transformation from coplanar (TPPA-1b, TPPA-1c) to twisted conformation (TPPA-1a) by hexane exposure (Figure 5e, Figure S16). It was also observed that not only hexane all solvents exposure induces structural transformation for both TPPA-1b and TPPA-1c crystals (Figure 5e, Figure S16). Solid state fluorescence spectra revealed tuning of fluorescence from 493 to 473 nm upon exposure of solvents vapour (Figure 5c, Figure S17). Surprisingly, heating of TPPA-1b and TPPA-1c showed red shift of fluorescence and PXRD suggest that heating did not induce SC-SC structural transformation (Figure S18). Acetonitrile included in the crystal lattice might have been evaporated faster than the reorientation of molecular conformation while heating. This might produce close packing of molecules that could lead to red shift of fluorescence. Thus SC-SC transformation of TPPA1b, TPPA1-c required mild force of solvent exposure. TPPA-1a did not show any 14

Figure 5 (a) SC-SC transformation of TPPA-1c to TPPA-1a, (b) TGA analysis, (c) fluorescence tuning of TPPA-1c to TPPA-1a by hexane exposure, (d) DSC and (e) PXRD of TPPA-1c and after hexane exposure.

fluorescence modulation after heating. DSC studies of TPPA-1a did not show any phase transition before melting at 172 °C (Figure 5d). However, TPPA-1b and TPPA-1c showed clear phase transition between 90 and 120 °C (Figure 5d). Interestingly, hexane exposed TPPA-1b and TPPA-1c did not show any phase transition and supports that solvent exposure induced structural transformation as well as removal of acetonitrile solvent (Figure 5d). Similar to TPPA-1b and TPPA1c, TPPA-4b also included CH3CN solvent in the crystal lattice and displayed coplanar conformation (Figure 4, Figure S11). Hexane exposure of TPPA-4b showed clear structural transformation as well as fluorescence tuning (Figure 15

S19a). However, the crystals quality was not good enough to perform single crystal structural analysis. DSC of TPPA-4a showed a phase transition closer to its melting point at 198 °C (Figure S19c). In contrast, TPPA-4b showed two clear phase transition at 92 and 168 °C before melting at 198 °C (Figure S19c). The phase transition at 92 °C was completely disappeared upon hexane exposure as well as heating at 130 and 170 °C (Figure S19c). However, TPPA-4b did not retain its single crystal nature hence we could not perform single crystal structural analysis. Other TPPA compounds, TPPA-2, TPPA-3, TPPA-5 and TPPA-6 did not exhibit any structural and fluorescence change upon solvent exposure/heating. Solid state

structural,

computational

studies

and

SC-SC

transformation

reveals

that

conformationally flexible molecular structure substituted with weak interacting functionality could be potential candidates for exhibiting SC-SC transformation. TPE based donor-acceptor derivatives are known to exhibit stimuli-induced reversible fluorescence switching upon crushing followed by heating/vapour exposure. All three forms (TPPA-1a/1b/1c) showed small red shift of fluorescence with slight reduction of intensity upon crushing (Figure S20a). Heating of crushed solids blue shifted the fluorescence with slight

enhancement

of

intensity.

Other

compounds

also

exhibited

similar

mechanofluorochromism (Figure S20b-S20f, Table S4). PXRD studies indicate that crushing converts crystalline to amorphous phase whereas heating/solvent exposure re-converts amorphous to crystalline phase (Figure S21). DSC also showed a phase transition only for the crushed solids (Figure S22). 5. Conclusion In conclusion, external stimuli-responsive TPE based AIE derivatives (TPPA) with conformationally flexible D-A units have been synthesized and substituents dependent fluorescent polymorphs formation, SC-SC transition and reversible MFC were demonstrated. Solid state structural analysis indicated that bulky methoxy substitution mostly favoured twisted conformation whereas unsubstituted and Fluorine molecules preferred coplanar 16

molecular conformation in the crystal lattice. Interestingly, TPPA-1 formed fluorescent polymorphs with twisted and coplanar conformation and exhibited tunable solid state fluorescence. Importantly, TPPA-1 exhibited SC-SC transformation from coplanar to twisted conformer without losing its single crystal nature upon solvent exposure. Computational studies further supported the stability of twisted conformer compared to coplanar conformer. Similarly TPPA-4 also exhibited SC-SC transition and tunable fluorescence. On the other hand, stronger intermolecular interactions in the crystal lattice of unsubstituted TPPA, Fluorine (TPPA-3) and single OCH3 substituted molecules favoured coplanar conformation. SC-SC transition of TPPA-1 and TPPA-4 was substantiated by PXRD, DSC and TG analysis. Thus, the present studies provided structural insight for designing SC-SC transforming tunable fluorescent polymorphs for optoelectronic applications. Acknowledgements Financial support from the Science and Engineering Research Board (SERB), New Delhi, India (SERB No. EMR/2015/00-1891) is acknowledged with gratitude. Financial support from Shenzhen Peacock Plan (Grant No. KQTD20170330110107046), Start-up Foundation of Shenzhen University (No.2016003), and Science Foundation of Guangdong Province (Nos. 2017A030310397 and 2018A0303130157) were also greatly acknowledged with gratitude.

Supplementary data Supplementary data to this article can be found online at DOI: https://doi.org/10.1016/ x0xx00000x

Research data for this article

17

[CCDC 1910330-1910340 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.]

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25

Supplementary data

Solvent vapour induced rare single-crystal-to-single-crystal transformation of stimuliresponsive fluorophore: Solid state fluorescence tuning, switching and role of molecular conformation and substituents P. S. Hariharana, b, Chengjun Pan*, a, Subramanian Karthikeyanc, Dexun Xied, Akira Shinoharaa, Chuluo Yanga, Lei Wang*,a,b and Savarimuthu Philip Anthony*,e a) Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, China. Email: [email protected];[email protected] b) Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China. c) PG and Research department of chemistry, KhadirMohideen College, Adirampattinam, Tamil Nadu, India. d) School of Chemistry, Sun Yat-sen University, Guangzhou, 510275, China; Shenzhen Research Institute, Sun Yat-sen University, Shenzhen, 518057, China. e) School of Chemical & Biotechnology, SASTRA Deemed University, Thanjavur-613401, Tamil Nadu, India. Email: [email protected]; Fax: +91-4362264120

26

Scheme 1. Synthesis of TPPA, TPPA-1-6

Scheme S2. Expected molecular conformational changes in the TPPA fluorophore.

27

Figure S2. AIE studies of TPPA-1 (a, b), TPPA-2 (c, d) and TPPA-3 (e, f) by gradual injecting CH3CN solution of fluorophore into different fractions of water (λexc = 370 nm).

28

Figure S3. AIE studies of TPPA-4 (a, b), TPPA-5 (c, e) and TPPA-6 (e, f) by gradual injecting CH3CN solution of fluorophore into different fractions of water (λexc = 370 nm).

29

Table S1. Fluorescence data of TPPA, TPPA-OMe, TPPA-1-6 aggregated powders

Molecules explored TPPA TPPA-OMe TPPA-1

Emission wavelength λemm (nm) 471 465 476

TPPA-2 TPPA-3 TPPA-4

473 496 484

TPPA-5 TPPA-6

469 479

30

Figure S4. Fluorescence spectra of TPPA and TPPA-OCH3 crystals grown from different solvents (λexc = 370 nm).

31

Figure S5. (a) Molecular conformation, supramolecular interactions and molecular packing in the crystal lattice of TPPA (a) and TPPA-OCH3 (b) The C-H...π interactions were shown by blue dotted lines). C (grey), N (blue), O (red) and H (white). The dD…A distances are marked (Å).

Figure S6. Molecular conformation of TPPA-1b and TPPA-1c. Weak H-bonding interactions between cyano nitrogen and ethylene hydrogen were shown by blue dotted lines. C (grey), N (blue), O (red) and H (white). The dD…A distances are marked (Å).

32

Figure S7. Molecular packing of TPPA-1a along the ac axis (a).The supramolecular interaction and molecular packing of TPPA-1a in the crystal lattice (b) and TPPA-1c (c). C (grey), N (blue), O (red) and H(white). The dD…A distances are marked (Å).

33

Figure S8. Supramolecular interactions and molecular packing in the crystal lattice of TPPA1b (a) and TPPA-1b(c). C (grey), N (blue), O (red) and H (white). dD…A distances are marked (Å).

Figure S9. Molecular packing of TPPA-1b along the bc axis (a) and Supramolecular interactions and molecular packing of TPPA-1b in the crystal lattice (b). C (grey), N (blue), O (red) and H (white). dD…A distances are marked (Å).

34

Figure S10. Supramolecular interactions and molecular packing in the crystal lattice of TPPA-2. C-H...π and H-bonding interaction shown by blue dotted lines (a) and TPPA-3 (b) Hydrogen bonding interactions with fluorine (H...F) and CN...phenyl hydrogen interactions were shown by blue dotted lines. C (grey), N (blue), O (red), F (yellow) and H (white). The dD…A distances are marked (Å).

35

Figure S11. Molecular conformation of TPPA-4b which includes CH3CN solvent in the crystal lattice (a) and (b)involvement of Cl atom in the weak intermolecular H-bonding interactions with phenyl hydrogen and cyano nitrogen shown by blue dotted lines C (grey), N (blue), O (red), Cl (green) and H (white). The dD…A distances are marked (Å).

36

Figure S12. Molecular conformation of TPPA-5 crystal grown from different solvents (a,b,c) and (d) Supramolecular interactions and molecular packing in the crystal lattice of TPPA-5. C-H...π and H-bonding interactions were shown by dotted lines. C (grey), N (blue), O (red), F (yellow) and H (white). The dD…A distances are marked (Å).

37

Figure S13. Solid state tunable fluorescence spectra of TPPA-5 grown from different solvents Table S2: Changes in the twist angle (Torsional angle) in the crystals grown from different solvents

τ1 τ2

τ3

CN

τ4

τ1

τ2

τ3

τ4

TPPA-5a-Methanol Crystal

10.49

11.58

14.59

8.34

TPPA-5b-DCM/Hexane vapor diffusion crystal

10.71

11.75

14.66

7.97

TPPA-5c-Acetonitrile crystal

11.68

10.91

13.56

7.11

38

Figure S14. Supramolecular interactions and molecular packing in the crystal lattice of TPPA-6.C-H...π interactions were shown by blue dotted lines. C (grey), N (blue), O (red) and H (white). The dD…A distances are marked (Å).

Figure S15. HOMO and LUMO calculation of molecular structure (TPPA-1a, TPPA-1c and TPPA-2) from single-crystal analysis and their band gap

39

Table S3. Crystallographic parameters comparison of SC-SC transition (TPPA-1c to TPPA1a)

Crystal

TPPA-1c

TPPA-1a

TPPA-1c/Hexane exposure

Triclinic

Triclinic

Triclinic

P-1

P-1

P-1

name Crystal system Space group Unit cell dimensions

α=

a = 5.521(3) Å

83.172(11)° β=

b = 9.316(4)

α=

a = 9.3264(7) Å

86.757(3)°

Å

86.746(15)°

β=

b=

β=

82.100(3)°

10.075(4) Å

82.071(15)°

b = 10.0917(7)

Å

85.747(11)°

c=

γ=

c=

γ=

c=

γ=

76.980(9)°

16.0099(12) Å

74.283(3)°

16.002(6) Å

74.356(12)°

31.168(15) Å

Å

α=

a = 9.328(3)

Volume

1548.8(12) Å3

1436.44(18) Å3

1433.9(9) Å3

Z

2

2

2

Density

1.202 Mg/m3

1.201 Mg/m3

1.203 Mg/m3

(calculated)

40

Figure S16. PXRD-patterns of TPPA-1c (a), TPPA-1b (b) upon exposure to different solvents

41

Figure S17. Fluorescence tuning of TPPA-1c (a), TPPA-1b (b) by different solvents exposure

42

Figure S18. Fluorescence spectra of TPPA-1 crystals on heating (a) and their PXRD patterns on heating TPPA-1a crystals (b), TPPA-1b crystals (c) and TPPA-1c crystals (d)

43

Figure S19. (a) Fluorescence tuning of TPPA-4b by hexane exposure, (b) PXRD pattern of TPPA 4b as such crystal and hexane exposure, (c) DSC of TPPA-4b as such crystal /hexane exposure and or heating and (d) TGA of TPPA-4b as such crystal and hexane exposure.

44

Figure S20. Fluorescence spectra for mechanofluorochromism of TPPA-1 (a), TPPA-2 (b), TPPA-3 (c), TPPA-4 (d), TPPA-5 (e), TPPA-6 (f) upon crushing and heating.

45

Figure S21. PXRD pattern for mechanofluorochromism of TPPA-1 (a), TPPA-2 (b), TPPA3 (c), TPPA-4 (d), TPPA-5 (e), TPPA-6 (f) upon crushing and heating.

46

Figure S22. DSC thermograms for mechanofluorochromism of TPPA-1 (a),TPPA-2 (b), TPPA-3 (c), TPPA-4 (d), TPPA-5 (e), TPPA-6 (f) upon crushing and heating.

47

Table S4. Compiled mechanochromism data of TPPA -1-6, Crystals, grinding and heating. Molecules explored

Emission wavelength

TPPA-1a TPPA-1b TPPA-1c TPPA-2 TPPA-3 TPPA-4a TPPA-4b TPPA-5a TPPA-5b TPPA-5c TPPA-6

λemm (nm) 468, 51% 482, 33% 492,39% 474, 38% 494, 42% 483, 51% 493, 45% 468, 48% 468, 42% 482, 38% 488, 45%

Hard grinding λemm (nm)

48

Hard grinding/Heating

Φ 31% 500,

λemm (nm) 467, 36 %

500, 21% 503, 25% 515, 22%

466, 30% 493, 33% 492, 41%

495, 28% 499, 25%

469, 35% 487, 35%

1

H NMR spectrum of TPPA-1 (700 MHz, in CDCl3 solvent)

13

C NMR of spectrum TPPA-1 (176 MHz, in CDCl3 solvent)

49

1

H NMR spectrum of TPPA-2 (700 MHz, in CDCl3 solvent)

13

C NMR of spectrum TPPA-2 (176 MHz, in CDCl3 solvent) 50

1

H NMR spectrum of TPPA-3 (700 MHz, in CDCl3 solvent)

13

C NMR of spectrum TPPA-3 (176 MHz, in CDCl3 solvent) 51

1

H NMR spectrum of TPPA-4 (700 MHz, in CDCl3 solvent)

13

C NMR of spectrum TPPA-4 (176 MHz, in CDCl3 solvent) 52

1

H NMR spectrum of TPPA-5 (700 MHz, in CDCl3 solvent)

13

C NMR of spectrum TPPA-5 (176 MHz, in CDCl3 solvent) 53

1

H NMR spectrum of TPPA-6 (700 MHz, in CDCl3 solvent)

13

C NMR of spectrum TPPA-6 (176 MHz, in CDCl3 solvent)

54

HRMS spectrum of TPPA-1 (a) and TPPA-2 (b)

55

HRMS spectrum of TPPA-3 (a) and TPPA-4 (b)

56

HRMS spectrum of TPPA-5 (a) and TPPA-6 (b)

57

IR spectrum of TPPA-1 (a) and TPPA-2 (b)

58

IR spectrum of TPPA-3 (a) and TPPA-4 (b)

59

IR spectrum of TPPA-5 (a) and TPPA-6 (b)

60

Highlights • • • •

Fluorescent polymorphs with tunable solid state fluorescence. A rare solvent vapour induced single crystal to single crystal transformation High contrast mechanofluorochromism Structure-property studies

SASTRA DEEMED UNIVERSITY THANJAVUR - 613 401, TAMILNADU, INDIA. S. Philip Anthony Ph.D Associate Professor School of Chemical and Biotechnology

Ph : +91-9962799257 Email : [email protected] URL : www.sastra.edu

28th-Oct-2019 Manuscript ID: DYPI_2019_2232 Title: Solvent vapour induced rare single-crystal-to-single-crystal transformation of stimuli-responsive fluorophore: Solid state fluorescence tuning, switching and role of molecular conformation and substituents Palamarneri Sivaraman Hariharan, Chengjun Pan, Subramanian Karthikeyan, Dexun Xie, Akira Shinohara, Chuluo Yang, Lei Wang and Savarimuthu Philip Anthony*

Conflict of interest There is no conflict to declare.