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Halochromic Isoquinoline with Mechanochromic Triphenylamine: Smart Fluorescent Material for Rewritable and Self-erasable Fluorescent Platform Palamarneri Sivaraman Hariharan, Ebrahim Mohamed Mothi, Dohyun Moon, and Savarimuthu Philip Anthony ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11939 • Publication Date (Web): 14 Nov 2016 Downloaded from http://pubs.acs.org on November 14, 2016
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ACS Applied Materials & Interfaces
Halochromic Isoquinoline with Mechanochromic Triphenylamine: Smart Fluorescent Material for Rewritable and Self-erasable Fluorescent Platform Palamarneri Sivaraman Hariharan,a Ebrahim Mohamed Mothi,b Dohyun Moonc* and Savarimuthu Philip Anthonya* a)
Department of Chemistry, School of Chemical & Biotechnology, SASTRA University, Thanjavur613401, Tamil Nadu, India. E-mail:
[email protected]. b) Centre for Scientific and Applied Research, PSN College of Eng & Tech.Tirunelveli-627152, Tamil Nadu, India. c) Beamline Department, Pohang Accelerator Laboratory, 80 Jigokro-127beongil, Nam-gu, Pohang, Gyeongbuk, Korea, Email:
[email protected]. KEYWORDS. Smart fluorescent materials, rewritable fluorescent platform, self-erasable fluorescent platform, mechanochromism, halochromism
ABSTRACT: Halochromic isoquinoline attached mechanochromic triphenylamine, N-phenyl-N-(4-(quinolin-2yl)phenyl)benzenamine (PQPBA) and tris(4-(quinolin-2-yl)phenyl)amine (TQPA), smart fluorescent materials exhibit thermo/mechanochromism and tunable solid state fluorescence and their unusual halochromic response in PMMA matrix have been used for fabricating rewritable and self-erasable fluorescent platforms. PQPBA and TQPA showed strong fluorescence in solution (Φf = 0.9290 (PQPBA) and 0.9160 (TQPA)) and moderate solid state fluorescence (Φf = 20 (PQPBA) and 17 % (TQPA). Interestingly, they exhibited a rare temperature (0 – 100 °C) dependent positive fluorescence enhancement via activating radiative vibrational transition. The deaggregation of PQPBA and TQPA in PMMA polymer matrix lead to the enhancement of fluorescence intensity strongly and fabricated strong blue fluorescent thin films (Φf = 58 % (PQPBA) and 54 % (TQPA). The halochromic isoquinoline has been exploited for demonstrating reversible off-on fluorescence switching by acid (TFA (trifluoroacetic acid)/HCl) and base (NH3) treatment in both solids as well as PMMA thin films. Importantly, rewritable and self-erasable fluorescent platform has been achieved by make use of unusual fluorescence responses of PQPBA/TQPA with TFA/HCl after exposing NH3. Single crystal and powder X-ray diffraction (PXRD) studies provided the insight on the solid state fluorescence and external stimuli induced fluorescence changes.
1. Introduction Smart fluorescent materials that exhibit reversible fluorescence changes to external stimuli such as heat, pressure, light, solvent vapor and pH are considered as potential candidates for smart applications such as sensors, memory devices, rewritable media and security ink.1-8 The structure, conformation and supramolecular interactions of molecular materials play important role in modulating fluorescence under external stimuli.9-12 For instance, nonplanar propeller shaped molecular structure often show reversible mechanochromism because of their ability to undergo conformational/phase change under pressure.13 Derivatives of tetraphenylpyrene, diphenyl dibenzofulvenes, tetraphenylethenes, cyanostilbene based materials, phenanthroimidazole, boron-diketonate compounds, excited state intramolecular proton transfer (ESIPT) and dendritic molecules are some of the molecular building
blocks employed for mechano and vapochromism.14-27 Organic fluorescent host materials showed fluorescence switching due to sorption and desorption of solvent molecules.28-29 Organic fluorophore with halochromic functionalities showed pH dependent fluorescence switching including white light emission.30-33 Photochromic molecules, which undergo light-induced reversible isomerization between ring-closed and ring open forms and exhibit significant color changes, have been explored for developing rewritable paper.34-40 However, the use of ultraviolet (UV) light as color switching stimuli can reduce the reversibility and stability due to photobleaching and photodegradation of organic fluorophores.41 The incorporation alkoxy functional group with phenyleneethynylene aromatic fluorophore showed water responsive reversible molecular self-assembly that has been used for fabricating self-erasable fluorescent security marker.42 The reversible fluorescent switching of organic
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materials by external stimuli (mechanical pressure/solvent vapor/acid-base) have also been exploited to demonstrate rewritable fluorescent medium.23,43,44 Huang et al. reported smart responsive iridium based phosphorescent complexes for data recording and security applications.45 Similarly, temperature responsive small organic fluorophores have been actively explored in recent years for imaging temperature distribution in chemical and biological systems.46-47 Twisted intramolecular charge transfer (TICT) organic molecules have been reported for enhancement of fluorescence intensity with temperature due to thermal activation of radiative pathways.48-50 However, temperature or mechanical pressure induced reversible fluorescence switching as well as rewritable and self-erasable fluorescent platform especially in a polymer matrix using a single organic fluorophore has never been realized according to our knowledge. Triphenylamine, a propeller shaped optoelectronic molecule, has been extensively employed for developing organic solar dyes, solid state fluorescent and mechanochromic materials.51-55 The easy structural modification of triphenylamine has been used to develop materials that exhibit aggregation enhance emission, temperature dependent fluorescence enhancement and polymorphism dependent fluorescent switching and tuning.56-58 Herein, we report the fabrication of rewritable and self-erasable fluorescent platforms using a single organic fluorophore by integrating halochromic isoquinoline with mechanochromic triphenylamine, N-phenyl-N-(4-(quinolin-2yl)phenyl)benzenamine (PQPBA) and tris(4-(quinolin-2yl)phenyl)amine (TQPA) (Scheme 1). PQPBA and TQPA also showed tunable solid state fluorescence and thermo/mechanochromism. Interestingly, rare intensification of fluorescence with increasing temperature (0–100 °C) was observed in dimethylformamide (DMF). Both PQPBA and TQPA showed strong fluorescence in solution ((Φf = 0.9290 (PQPBA) and 0.9160 (TQPA) compared to quinine sulfate) and moderate solid state fluorescence (Φf = 20 (PQPBA) and 17 % (TQPA) absolute quantum yield). However, only PQPBA showed reversible mechanochromism and tunable solid state fluorescence due to conformational differences in the phenyl groups. PXRD studies indicated phase transition of PQPBA from crystalline to amorphous state under strong crushing. All PQPBA and TQPA showed relatively weak fluorescence in the solid state, they showed strong blue fluorescence (436 nm (PQPBA) and 445 nm (TQPA) in PMMA thin films (1 wt %) due to deaggregation. The presence of halochromic isoquinoline has been utilized for off-on fluorescence switching by acid (HCl and trifluoroacetic acid (TFA) and base (NH3) treatment. Acid exposure red shift and quench the fluorescence (550 nm, turn-off) whereas NH3 exposure turn-on and blue shift the fluorescence (436 nm) with strong enhancement. Interestingly, NH3 exposed PMMAPQPBA/TQPA-TFA exhibited self-reversible fluorescence switching from turn-on to turn-off whereas PMMAPQPBA/TQPA-HCl showed steady fluorescence for long period after NH3 exposure. The fluorescence can only be switched off by HCl exposure. Single crystal studies of
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TQPA with TFA showed protonation of two isoquinoline nitrogen that formed strong intermolecular interactions with TFA oxygen. The strong H-bonding and hydrophobic fluorine could be responsible for self-reversibility. The reversible and self-reversible fluorescence switching of PMMA-PQPBA/TQPA has been exploited for fabricating self-erasable and rewritable medium on glass slide and filter paper. Thus, a single multi-stimuli responsive organic fluorophore has been synthesized by substituting halochromic isoquinoline with propeller shaped triphenylamine and used for fabricating self-erasable and rewritable fluorescent platforms.
Scheme 1. Molecule structure of PQPBA and TQPA.
2. Experimental Section All the chemicals and solvents were received commercially from Sigma-Aldrich and used as received. All reactions were carried-out under inert atmosphere. 2aminobenzaldehyde, 4-acetyltriphenylamine and 4,4’,4’’triacetyltriphenylamine was synthesized following the reported procedure.66-68
N-phenyl-N-(4-(quinolin-2-yl)phenyl)benzenamine (PQPBA) (Scheme S1) Saturated ethanolic NaOH (2 mL) was added into a mixture of 4-acetyltriphenylamine (0.78 g, 2.74 mmol) and 2-aminobenzaldehyde (0.4 g, 3.29 mmol) in ethanol at room temperature with vigorous stirring. The mixture was refluxed for overnight (12 h). After cooling, the yellow precipitate formed was collected by filtration. It was washed with ethanol (2x50 mL), water (50 mL) and again with ethanol then dried in vacuum. The crude product was recrystallized from hot petroleum ether, 0.76 g (76%). IR (KBr, cm-1): 2924, 2851, 1593, 1495, 1430, 1316, 1278, 1181, 1053, 818, 757. 1H NMR (CDCl3, 300 MHz) δ: 8.18 (d, J = 8.7Hz, 1H), 8.13 (d, J = 8.4Hz, 1H), 8.04 (dt, J1 = 8.7Hz, J2 = 2.1 Hz, 2H), 7.79-7.84 (m, 2H), 7.68-7.71 (m, 1H),7.50-7.53 (m, 1H), 7.26-7.31 (m, 4H), 7.15-7.21 (m, 6H), 7.04-7.09 (m, 2H); 13C NMR (75 MHz, CDCl3): δ: 157.0, 149.1, 148.4, 147.4, 136.6, 133.3, 129.6, 129.6, 129.4, 128.5, 127.4, 127.0, 125.9, 124.9, 123.4, 123.2, 118.7.
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Tris(4-(quinolin-2-yl)phenyl)amine (TQPA) (Scheme S2) TQPA was synthesized following similar procedure. Typically, ethanolic solution of NaOH (2 mL) was added into a mixture of triacetyl compound (0.86 g, 2.70 mmol) and 2-aminobenzaldehyde (1.3 g) at room temperature. Then the reaction mixture was refluxed for overnight (12 h). After cooling, it also produced yellow precipitate that was collected by filtration. The precipitate was washed with ethanol (2x50 mL), water (50 mL) and again with ethanol then dried in vacuum. The crude product was subjected to column chromatography (silica, dichloromethane/methanol, 95:5) to afford 1. 2 g (70 %). IR (KBr, cm-1): 1549, 1590, 1494, 1431, 1391, 1278, 1177, 820, 752, 695, 616, 506. 1H NMR (CDCl3, 300 MHz) δ: 8.23 (d, J = 8.4Hz, 3H), 8.17 (d, J = 8.4Hz, 3H), 8.12 (d, J = 8.7Hz, 6H), 7.88 (d, J = 8.4Hz, 3H), 7.83 (br d, J = 7.8Hz, 3H),7.70-7.76 (m, 3H), 7.50-7.55 (m, 3H), 7.36 (d, J = 8.7 Hz, 6H); 13C NMR (75 MHz, CDCl3): δ: 156.8, 148.4, 148.3, 136.8, 134.6, 129.7, 129.6, 128.7, 127.5, 127.1, 126.1, 124.6, 118.8.
0.0842 (PQPBA) and 0.0967 (TQPA)) in ethanol. The insignificant variation in absorption and large change in the fluorescence (λmax) suggests the formation of twisted intramolecular charge transfer state (TICT).50,59-60 Interestingly, both PQPBA and TQPA in DMF exhibited unusual positive enhancement of emission intensities with increasing temperature (Figure 1b, S7). PQPBA showed fluorescence λmax at 495 nm at 0 °C that was blue shifted to 482 nm with five times increase in the intensity at 100 °C. TQPA exhibited nearly three times fluorescence enhancement with blue shift from 496 (0 °C) to 482 nm (100 °C). Above room temperature, TQPA showed mostly blue shift of fluorescence rather than increasing intensity. Although weak, PQPBA showed enhancement of fluorescence intensity with temperature in CH3CN also (Figure S8). It is noted that absorption of PQPBA and TQPA did not show any change with temperature (Figure S9).
Spectroscopy, structural characterization and acid exposure studies Absorption and fluorescence spectra were recorded using Perking Elmer Lambda 1050 and Jasco fluorescence spectrometer-FP-8200 instruments. Fluorescence quantum yields (Φf) of solid samples and thin films were measured using a Horiba Jobin Yvon model FL3-22 Fluorolog spectrofluorimeter with integrating sphere. HCl/TFA vapor exposure was performed by keeping PQPBA/TQPA powder or PQPBA/TQPA-PMMA thin film coated glass plate upside down on a beaker containing HCl (6M, 5ml) or TFA (5ml) for 10 minutes. 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 crystal of PQPBA, TQPA and TQPA-TFA was 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. - 1476946-1476950 contains the supplementary crystallographic data for this paper.
3. Results and Discussion Absorption spectra of PQPBA and TQPA in different solvent did not show significant change in the λmax (368 to 374 nm (PQPBA) and 382-390 nm (TQPA) Figure S5,S6a). But fluorescence λmax varied widely from polar to nonpolar solvents. Blue fluorescence (425 nm (PQPBA) and 432 nm (TQPA)) was observed in toluene that was red shifted to cyan in polar DMSO, DMF and ethanol (496 nm (PQPBA) and 492 nm (TQPA)) (Figure 1a). Further PQPBA and TQPA showed intense fluorescence in nonpolar solvents compared polar solvents (Table S1). Compared to quinine sulfate, PQPBA and TQPA showed 0.9290 and 0.9160 quantum yields (Φf) in toluene, respectively. Both compounds showed weak fluorescence (Φf =
Figure 1. PQPBA (a) fluorescence spectra in different solvent and (b) temperature dependent fluorescence in DMF (10-3 M. The inset shows the digital fluorescent images (λexc = 365 nm).
This unusual effect can be explained by considering TICT emissions in polar solvents. TICT is relatively common phenomenon observed in fluorophores in which donor and acceptor groups linked by single bond.59 Polar solvents facilitate fast intramolecular charge transfer (ICT) from the donor to the acceptor part in such fluorophores. In PQPBA and TQPA, ICT can occur between aminophenyl donor to isoquinoline acceptor unit. This ICT is generally accompanied by significant molecular geometry relaxation and causes the formation of TICT state at the excited state via intramolecular twisting be-
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tween donor and acceptor around single bond that result in the formation of relaxed perpendicular structure (Scheme S3). The significant relaxation in concurrence with strongly increased polarity in the TICT state could be responsible for the large Stokes shift of PQPBA and TQPA fluorescence.61,62 TICT state return to the ground state either via red shifted weak emission or nonradiative relaxation.63,64 However, increase of temperature activate more vibrational bands at higher energy levels in the TICT state that enhance the possibility of radiative electron transitions from TICT to ground states.59 Thus, PQPBA and TQPA shows a blue shift in the fluorescence (from 495 (0 °C) to 482 nm (100 °C) for PQPBA and from 496 (0 °C) to 482 nm (100 °C) for TQPA) due to relatively stronger contribution of higher vibrational bands at higher temperature. The fluorescence of PQPBA in DMF can be reversibly switched by heating/cooling without significant change in intensity/λmax (Figure S10). PQPBA and TQPA showed moderate solid state fluorescence (Φf = 20 % (PQPBA) 17 % (TQPA) Figure 2). Single crystals of PQPBA and TQPA were grown to get the insight on the solid state fluorescence. PQPBA crystals obtained from DMSO showed fluorescence λmax at 518 nm. However, PQPBA crystals grown from ethyl acetate and dichloromethane/methanol exhibited red shifted fluorescence at 525 and 537 nm, respectively. Thus, PQPBA showed tunable solid state fluorescence from 518 to 537 nm. TQPA crystals grown from toluene showed fluorescence similar to as prepared powdered sample (λmax = 520 nm). Single crystals analysis of PQPBA and TQPA confirmed the propeller molecular shape in the solid state (Figure 3-4 and Table S2-4). PQPBA crystals grown from different solvent showed different twist between aminophenyl and isoquinoline as evidenced from torsion (τ) angle (Figure 3a, S11-S13). This could be the reason for
Figure 2. Fluorescence spectra of PQPBA and TQPA. The inset shows the digital fluorescent images of QPBA crystals (λexc = 365 nm).
tuning solid state fluorescence from 518 to 537 nm. Further PQPBA showed weak intermolecular interactions (ππ and C-H-π) between isoquinoline units in the crystal lattice (Figure S11-S13). The three isoquinoline of TQPA adopted different conformation (Figure 3b, Table S5-6).
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The supramolecular interactions (π-π and C-H-π) observed in the crystal lattice of TQPA makes a ladder structural arrangement (Figure S14).
Figure 3. Molecular conformation (a) PQPBA and (b) TQPA in the crystals lattices. C (gray), N (blue), H (white).
Solid state fluorescence with propeller molecular shape of PQPBA and TQPA prompted us to explore mechanochromism. Interestingly, strong grinding of PQPBA showed blue shift of fluorescence (from 518 to 454 nm) and heating reversed the fluorescence (Figure 4). The fluorescence color of the sample switched from cyan to blue and vice versa (Figure 5b). To compare the intensity, fluorescence spectra of slightly broken crystals, hard crushed and heating was performed using same quantity of PQPBA. The hard crushed and heated sample showed equal and higher fluorescence intensity compared to crystals. However, TQPA did not show any mechanochromism. PXRD studies suggest that strong grinding converted PQPBA from crystalline to amorphous and heating reverts back to crystalline (Figure S15). Thus the reversible fluorescence change of PQPBA by grinding/heating could be attributed to phase transition from crystalline to amorphous and vice-versa. Differential scanning calorimetric studies of strongly crushed PQPBA showed clear phase transition at 119 °C (Figure S16). PQPBA crystals did not show any phase transition. Pure triphenylamine is known to exhibit reversible fluorescence switching from 443 to 435 nm by applying mechanical pressure due to phase transition.65 Isoquinoline solids did not show measurable fluorescence that could be due to close packing of flat isoquinoline molecule in the solid state. However, the propeller shape of triphenylamine prevented isoquinoline to form close packing in the crystals of
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PQPBA and TQPA. In the crystalline state, the molecules are well ordered and held by weak intermolecular interactions. The strong grinding disrupted the molecular packing in the crystal structure and transformed highly ordered crystalline state to a poorly ordered amorphous phase. The transformation from highly organized to poorly ordered state completely quenched the fluorescence associated with flat isoquinoline. Thus, the blue fluorescence of PQPBA at amorphous state could be from propeller shaped triphenylamine unit alone.
(methyl group of toluene is disordered), two TFA and four water molecules have been included in the crystal lattice. The protonated isoquinoline units formed strong intermolecular H-bonding with TFA oxygen and water molecules that resulted in 1-D chain in the crystal lattice (Figure S19b,S21). The intermolecular H-bonding between water molecules produced water tetramer structure (Figure S22a). The interaction between water tetramers and TFA produced macro-cyclic structure (Figure S22b). The toluene molecule included in the crystal lattice occupied the cavity created by cyclic structure and stabilized by weak intermolecular interaction with water molecules (Figure S22b,c). TQPA-TFA crystals also showed very weak fluorescence (Φf =
