Self-Reversible Mechanochromism and Thermochromism of a

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Self-Reversible Mechano and Thermochromism of Triphenylamine based Molecule: Tunable Fluorescence and Nanofabrication Studies Palamarneri Sivaraman Hariharan, Natarajan Sathiyamoorthy Venkataramanan, Dohyun Moon, and Savarimuthu Philip Anthony J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 03 Apr 2015 Downloaded from http://pubs.acs.org on April 3, 2015

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The Journal of Physical Chemistry

Self-Reversible Mechano and Thermochromism of Triphenylamine based Molecule: Tunable Fluorescence and Nanofabrication Studies P. S. Hariharan,a N. S. Venkataramanan,a Dohyun Moonb* and Savarimuthu Philip Anthony a* a)

School of Chemical & Biotechnology, SASTRA University, Thanjavur-613401, Tamil Nadu, India. Fax: +914362264120; Tel: +914362264101; E-mail: [email protected]

b)

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

ABSTRACT. A triphenylamine based fluorophore, 4-((4-methoxyphenyl)(phenyl)amino) benzaldehyde (1) exhibits external stimuli-responsive self-reversible solid state fluorescence switching, tunable fluorescence and a rare phenomenon of temperature dependent fluorescence. Mechanically grinding crystalline powder of 1 converts the blue fluorescence (λmax = 457 nm) to green (λmax = 502 nm) and blue fluorescence robustly self-recovered within 8 minutes. X-ray analysis and theoretical studies suggest that change of highly twisted molecular conformation and crystalline form into amorphous phase with more planar conformation responsible for fluorescence switching. Self-reversible fluorescence switching did not show significant change in fluorescence up to several cycles of measurement. Interestingly 1 in toluene showed a rare phenomenon of fluorescence enhancement with increasing temperature via activating more vibrational bands that lead to stronger TICT emissions. Morphological change mediated fluorescence tuning has also been demonstrated by fabricating nanoparticles of 1. The conversion of highly polydispersed featureless different shaped nanoparticles ACS Paragon Plus Environment

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into nearly uniform sized spherical nanoparticles (20-25 nm) converts green (λmax = 502 nm) to blue fluorescence (λmax = 478 nm). The self-reversible multi-stimuli responsive fluorescence switching and polymorphism and nanofabrication mediated fluorescence tuning suggest its application potential in sensors particularly for fluorescence thermometer.

Introduction Organic fluorescent materials that exhibit fluorescence response to external stimuli such as pressure and heat have attracted intense attention because of their fundamental research for better understanding the mechanism and potential practical applications such as sensors, optical recording, security inks, photonics and optoelectronic devices.1-10 In recent years, the external stimuli responsive organic fluorescent materials are considered to be most smart materials since the external force can easily be controlled or modulated.11 A number of piezochromic organic molecules,7,12-18 polymers19-21 and metal complexes22-26 have been explored for over a decade. Particularly, organic molecules with twisted molecular conformation that includes triphenylamine based derivatives,27 para-phenylene vinylene oligomers,20 organoboron compounds,7 pyrene derivatives11 and 9, 10-distyrylanthracene derivatives28 have often been used to generate stimuli responsive fluorescent solids. The switching and altering of the solid state fluorescence of fluorophores are generally accompanied by molecular conformation and supramolecular packing change. The fluorescence molecules that display selfrecovery of initial fluorescence are reported rarely compared external stimuli controlled fluorescence switching.7,29-32 Hence, the search for stimuli responsive organic fluorescent system that exhibit spontaneous self-recovery of fluorescence is highly interesting. Similarly the development of fluorescent thermometers have been actively explored for imaging temperature distribution in cells and micro-fluidic devices with high spatial and temporal resolutions.3336

A few fluorescent thermometers based on organic dyes and inorganic complexes have already been

reported.37-41 In general, fluorescence and life time changes with temperature is caused by competitions between radiative (i.e. fluorescence) and non-radiative (i.e. thermal collisional) decay pathways. Most of the developed fluorescent materials exhibit reduced fluorescence intensity and life time with increasing temperature due to the thermal activation of non-radiative de-excitation pathways. CdSe quantum dots and ZnO microcrystals have also showed temperature dependent fluorescence but with negative temperature coefficient.42-43 In contrast, positive temperature coefficient fluorescent probes, whose fluorescent intensity enhanced with temperature, are particularly interesting because they can effectively suppress the background interference at high temperature and allow quantitative detection of ACS Paragon Plus Environment

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temperature with high sensitivity.44-45 Recently, Cheng et al. reported a positive temperature coefficient from N,N-dimethyl-4-((2-methylquinolin-6-yl)ethynyl)aniline organic fluorophore that exhibited an unusual fluorescence intensification with increasing temperature.46 However these systems require complicated synthetic procedures. As a result, developing new fluorescent thermometers, especially with positive temperature coefficients and a broad working range becomes an important research target. Triphenylamine (TPA) is one of the important structural motif for generating materials for several optoelectronic applications such as light emitting diodes (LED), electroluminescent materials, fluorescent sensors and dye-sensitized solar cells (DSSC) owing to the non-planar shape, strong electron donating nature, high light-to-electrical energy conversion efficiencies and good hole-transporting capability.47-52 The propeller shaped core have often been used to generate efficient solid state fluorescent materials including fluorescent host and co-crystals with tunable fluorescence.53-57 The conformational flexibility of triphenylamine has also been utilized to generate piezochromic organic fluorescence

compounds.27

Herein

we

report

external

stimuli-responsive

self-recovered

mechanochromism, tunable fluorescence and a rare phenomenon of fluorescence enhancement with increasing

temperature

from

a

simple

triphenylamine

molecule,

4-((4-methoxyphenyl)

(phenyl)amino)benzaldehyde (1, Scheme 1). 1 showed strong fluorescence in solution (Φf = 0.1456 in benzene). The crystalline powders of 1 showed blue fluorescence with λmax at 457 nm. Interestingly, mechanical grinding changes the blue fluorescence into green (λmax = 502 nm) that self-recovered to blue spontaneously within 8 min. X-ray structural analysis and theoretical studies were performed to gain more insight on the fluorescence switching mechanism. These studies suggest that switching of fluorescence was due to the change of crystalline phase with twisted molecular structure to more planar amorphous phase. The featureless polydispersed nanoparticles of 1 slowly changed to highly uniform spherical nanoparticles (20-25 nm) and the fluorescence also converted from green (501 nm) to blue (478 nm). Importantly, 1 in toluene showed fluorescence change with positive temperature coefficient. Heating of toluene solution of 1 switched the fluorescence from cyan to blue with increased intensity. The repeated reproducibility of fluorescence switching in the solid state and solution with temperature has also been demonstrated up to seven cycles without significant lose of fluorescence. The mechanochromism, tunable fluorescence and fluorescence thermometer properties of a simple triphenylamine molecule demonstrate its application potential in sensors and opto-electronic devices. Experimental Section Chemicals:

Acetonitrile

(CH3CN,

HPLC

grade),

tetrahydrofuran

(THF,

HPLC

grade),

dimethylformamide (DMF, HPLC grade), phosphorous oxychloride, 4-methoxy triphenylamine were

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purchased from Sigma-Aldrich India and used without further purification. Ultrapure water produced using a Milli-Q apparatus (Millipore), was used in all experiments. Synthesis of 4-((4-methoxyphenyl)(phenyl)amino)benzaldehyde (1): Compound 1 was synthesized following a reported Vilsmeier–Haack formylation procedure.58 In a typical procedure, 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, 4-methoxy-N,N-diphenylaniline (0.98 g, 4 mmol) was added and the resulting mixture was stirred at 60°C for 2 h. After cooling to room temperature, the reaction mixture was poured into ice-water (200 mL). The solution was neutralized with 1 M NaOH solution and extracted with CH2Cl2. The extract was washed with water and brine solution and finally dried over Na2SO4. The solvent was evaporated and the compound was purified using column chromatography (ethyl acetate:hexane (1:10)). The product was obtained as greenish yellow gel 0.93 g (yield 85%). Mp 67-68 °C. 1H NMR (500 MHz, CDCl3) δ 9.77 (s, 1H), 7.63-7.66 (d, 2H), 7.26-7.34 (m, 2H), 7.10-7.18 (m, 5H), 6.88-6.95 (m, 4H), 3.81 (s, 3H).

13

C 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. Spectroscopy characterization: Absorption and fluorescence spectra were recorded using Perking Elmer Lambda 1050 and Jasco fluorescence spectrometer-FP-8200 instruments. Nanoparticles fabrication of 1: Typically, a 10 mL stock solution of 1 (10-3 M) in CH3CN and THF was prepared. From the stock solutions, 25 µL was rapidly introduced into 25 mL of water under stirring at room temperature. Stirring was continued for further 20 min. The samples were left undisturbed for 5 h before doing absorption, fluorescence and microscopy evaluation. The nano/microstructures of 1 were collected on the surface of an alumina membrane with a pore size of 0.02 µm (Whatman International, Ltd.) and dried under vacuum for microscopy analysis. Spectroscopic analysis of the nanoparticles was carried out while keeping the sample directly in the solution. Microscopy characterization: The morphologies and sizes of the samples were examined using field emission scanning electron microscopy ((FE-SEM) (JSM-6701F, JEOL Japan INC) with an accelerating voltage 30 kV and filament current of 20 mA for 45 seconds. The samples are stuck onto a double-face conducting carbon tape mounted on a brass stub. Prior to analysis, the samples were coated with a thin layer of gold. Structural analysis: The powder X-ray diffraction (PXRD) patterns were measured using a XRDBruker D8 Advance XRD with Cu Kα radiation (λ = 1.54050 Å) operated in the 2θ range from 10ο to 50ο. ACS Paragon Plus Environment

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Single crystal of 1 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. The ADSC Q210 ADX program59 was used for data collection (detector distance is 63mm, omega scan; ∆ω = 3º, exposure time is 2 sec per frame) and HKL3000sm (Ver. 703r)60 was used for cell refinement, reduction and absorption correction. The crystal structure of 1 was solved by the direct method with SHELX-XT (2014/4) program and refined by full-matrix least-squares calculations with the SHELX-XL (2014/7) program package.61 All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were assigned isotropic displacement coefficients U(H) = 1.2U (C) or 1.5U(Cmethyl), and their coordinates were allowed to ride on their respective atoms. Refinement of the structure converged at a final R1 = 0.0393, wR2 = 0.1116 for 4366 reflections with I > 2σ(I); R1 = 0.0449, wR2 = 0.1158 for all reflections. The largest difference peak and hole were 0.367 and -0.271 e·Å-3, respectively. A summary of the crystal and some crystallographic data is given in Table S1. CCDC-1038308 contains the supplementary crystallographic

data

for

this

paper.

The

data

can

be

obtained

free

of

charge

at

www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EX, UK. Computational Studies: All the density functional theory calculations were performed with the Gaussian 09 program package. The structural optimizations were carried without symmetry constraints using the hybrid B3LYP functional. For all structure optimizations, we have used the 6-311+G(d,p) basis set for all atoms. The use of B3LYP/6-311+G(d,p) method was able to predict the bond parameters close enough to the X-ray crystal data. . The choice of B3LYP/6-311+G(d,p) for optimization of structures in DFT calculations is justified as a compromise between reliable results and reasonable computational cost. In order to confirm the proper converges to minima, the vibrational frequencies were computed at the same level of theory and negative frequency absence was confirmed. For the optical part, the calculation on the properties have been carried out in the framework of time dependent-DFT (TD-DFT) by extracting a minimum of 300 roots with the time dependent Kohn-Sham formalism by employing PBE0/6311+G(d,p) level of theory. In the TD-DFT calculations, the use of PBE0 funcational has provided accurate results for medium and large size molecules. For comparison, the calculated discrete spectra have been normalized and their peaks broadened with Gaussian function of fwhm=0.02 eV.


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Scheme 1. Schematic representation of fluorescence switching and tuning of 1 via mechanical grounding, heating, nanofabrication and polymorphism. Results and Discussion Synthesis, absorption and fluorescence studies in solution 1 was synthesized following Vilsmeier–Haack formylation reaction using DMF and POCl3 at 60 °C. The good solubility of 1 in broad range of solvent allowed us to investigate its photophysical properties in different solvents. The absorption spectra of 1 showed a peak around 355 nm, corresponding to n-π* transition (Fig. 1a). 1 in CH3CN, hexane, ethyl acetate, DMF, DMSO and acetone showed absorption at 350 nm and in toluene, MeOH, CH2Cl2 and CHCl3 exhibited small red shifted absorption to 358 – 362 nm. The small absorption changes of 1 across polar to non-polar solvents exclude the solvatochromism. In contrast, 1 showed strong fluorescence between 420 to 522 nm in different solvents (Fig. 1b,c). 1 showed broad fluorescence between 420-457 nm in hexane, strong fluorescence at 491 nm in toluene. The fluorescence was red shifted to 522 nm in tetrahydrofuran (THF) and ethyl acetate. 1 in CHCl3 and CH2Cl2 showed weak broad fluorescence across wide range between 410 to 575 nm. 1 did not show any measurable fluorescence in highly polar solvents such as CH3CN, CH3OH, DMSO and DMF. The quantum yield (ΦF) measurement of 1 compared to quinine sulphate showed 0.153 in toluene, 0.176 in benzene, 0.087 in ethyl acetate and 0.0643 in THF. The digital images also showed strong fluorescence for 1 in hexane, toluene, ethyl acetate and THF (Fig. 1b, λexc = 365 nm). The broad fluorescence spectra of 1 in CHCl3 and CH2Cl2 lead to white fluorescence in solution. The small absorption changes and strong red shift of fluorescence in different solvents suggest that 1 formed TICT (twisted intramolecular charge transfer) state in solvents.46,62 The ability of 1 to form 6 ACS Paragon Plus Environment

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different TICT in different solvent could be reason for tuning fluorescence from 420 nm to 522 nm. The white fluorescence of 1 in CHCl3 and CH2Cl2 might be due to the formation of multiple TICT state.

Figure 1. Absorption spectra (a), fluorescence digital images (b) and fluorescence spectra (c) of 1 in different solvents. The fluorescent digital images were taken by exciting at λex = 365 nm. The propeller shape of triphenylamine (donor) and aldehyde (acceptor) in 1 promote the TICT state in solvents that resulted in a large molecular dipoles and asymmetric electron population in the frontier molecular orbitals (Fig. 2a-d). The optimized as well as molecular structure of 1 obtained from single crystal analysis confirmed the twisted propeller molecular conformation (Fig. 2a,b). From the molecular orbital diagram, it can be seen that the distribution of electron cloud over the whole molecule whereas in the LUMO level the electron mostly localizes on the phenyl aldehyde unit due to its electron ACS Paragon Plus Environment

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withdrawing effect (Fig. 2c,d). The HOMO and LUMO electron distribution difference results in a large energy gap, 3.7208 eV for optimized structure and 4.015 eV for molecular structure formed in crystal structure. In the resulting TICT state, there is a total charge separation between D and A units which can be stabilized by the polar solvent molecules. The stabilization could thus reduce the LUMO energy level of the TICT state and consequently red-shift the emission bands or completely quench the fluorescence. The energy level calculation and comparison of optimized structure with molecular structure obtained from single crystal analysis (discussed in the following section) indicate that twisting of molecular structure clearly affect the band gap (Fig. 2c,d). The optimized molecular structure showed more planar conformation compared to 1 in crystal structure (Fig. 2a,b). The more planar conformation leads to smaller band gap for optimized structure (3.7208 eV) compared to 1 in crystal structure (4.0151 eV).

Figure 2. Molecular structure of 1 (a) optimized, (b) obtained from single crystal analysis. HOMO and LUMO calculation of optimized structure (c) and molecular structure from single crystal analysis (d) of 1. Band gap for optimized structure is 3.7208 eV and crystal structure is 4.0151 eV. ACS Paragon Plus Environment

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Further to confirm the formation of TICT state, we have carried out density functional theory (DFT) study by optimizing ground singlet and triplet state as well as excited state singlet and triplet geometry of the molecule. The fully optimized gas phase ground singlet state was 57.47 kcal mol-1 higher stable than the triplet ground state. Thus at room temperature, geometrical electronic switching between states is less likely to occur in this molecule. The computed bond lengths for the molecule at the ground state are in close agreement with the X-ray crystal bond length; however, a larger twisting of the functional groups was observed when compared with those in the crystal structure. In the ground state optimized geometry, the donor group is 126.7o A out of plane to the benzene group, while in the crystal structure the out of plane angle was 119.7 o A (Fig. S1). Such change in angles are previously observed in previous DFT studies, as DFT predicts structure at the gas phase, while the X-ray crystal experience Pi-stacking between benzene molecules.63 A potential energy curve along the twist coordinate for the ground state molecule is shown in figure S1. It indicates that the out of plane of donor ring group destabilizes the molecule. The HOMO and LUMO molecular orbital picture for the ground and twisted forms are shown in figure S2, while in table S2, the orbital contribution is provided. It is evident from the orbital picture, in the HOMO a substantial electron cloud is localized over the donor and benzene molecule. Further, the electron density in the LUMO orbital is localized mainly on the acceptor part. At the twisted configuration, the percentage contribution of acceptor at the HOMO levels increases with the increase in twist angle, while at the LUMO, no appreciable change in the orbital configuration is observed. Thus the twist configuration reduces the charge transfer (CT) while exciting the molecule. To understand the electronic transition in these compounds, we carried out TDDFT calculations on the absorption spectra. The calculated absorption and excitation wavelength, oscillator strengths and the main orbital contribution with the change in twist angle are summarized in the table S3. The computed value absorption maximum for the compound is 354 nm, which is in closed agreement with experimental value of 350 nm. The absorption maximum was at 354 nm, which originates from the HOMO – LUMO orbital transition. Besides the above bands at 330 and 324 nm were observed, which are assigned to H-2 → LUMO (86%), H-2 → L+5 (4%), H-1 → LUMO (5%) and HOMO → L+1 (96%) respectively. Upon twisting, the absorption maximum get shifted to blue region and the HOMO – LUMO orbital transition percentage decreases. Moreover, the calculated oscillator strength decreases with the increase of twist angle and indicating a transformation from spin-allowed to spin-forbidden state. Thus the present computational study clearly indicates the presence of TICT process in this molecule.


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Solid state fluorescence and Mechanochromism 1 in solid state showed different fluorescences (blue, cyan and green, Scheme 1). As-synthesized powders of 1 showed strong blue fluorescence, λmax at 457 nm (Fig. 3). To get the insight on the molecular packing in the solid state, growing single crystals of 1 was attempted from different solvent (Hexane, CH3COCH3, Toluene, CH2Cl2, CHCl3, CH3CN, CH3OH and CH3CH2OH) at room temperature. Except ethanol, all solvents produced only in gel-like precipitate that showed green fluorescence, λmax at 502 nm (Fig. 3). Slow evaporation of ethanol at room temperature produced crystalline powders at the bottom of the beaker and gel-like precipitate on the beaker wall. The crystalline powder exhibited strong blue fluorescence (λmax at 447 nm) and gel-like precipitate showed green fluorescence (λmax at 502 nm). Unfortunately, the crystals are in poor quality to perform single crystal analysis. However, cooling ethanol solution of 1 at 4°C for four days produced quality single crystals. These crystals showed blue fluorescence, λmax at 457 nm which is similar to as-synthesized powder and 10 nm red shift from crystalline powder obtained at room temperature. Excitation spectra were also recorded for different solids of 1. The as-synthesized powder showed n-π* transition at 369 nm and intramolecular charge transfer (ICT) transition at 405 nm (Fig. S3). The crystals grown at 4 °C in ethanol were also showed similar excitation spectrum and suggest that 1 could have similar molecular arrangement and conformation in both samples. There is no change in the n-π* transition for

Figure 3. Solid state fluorescence spectra of 1 in different form. Digital fluorescent images of 1 at different forms are shown in the inset. λex = 365 nm. ACS Paragon Plus Environment

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as-synthesized powder, crystals grown at 4 °C and gel-like precipitate, however, the ICT peak of gel precipitate is clearly different from other two. It showed a red shift from 405 nm to 420 nm for gelprecipitate. Similarly, crystalline product obtained at room temperature showed a blue shift in ICT transition from 405 nm to 394 nm. These studies suggest that the different forms of 1 could be having different molecular packing or molecular conformation. The red-shift of ICT in the gel indicates more planar conformation for 1 whereas highly twisted molecular conformation in crystalline powder obtained at room temperature that showed blue-shift of ICT.64

Figure 4. Molecular structure (a), dimer formation via H-bonding and π-π interactions (b) and molecular packing (c) of 1 in the crystal lattice. C (grey), N (blue), O (red), H (white); H-bonds (broken line), π-π interactions (solid white line). dH…A distances are marked. Single crystal x-ray analysis of 1 grown from ethanol at 4°C was solved in monoclinic space group P21/c (Table S1). 1 displayed clearly twisted propeller conformation in the crystal lattice (Fig. 4a). Strong H-bonding between oxygen atom of aldehyde and phenyl hydrogen (d(D...A) = 3.2575 Å, θD…A = 139.9) was observed in the crystal lattice of 1 that lead to the formation dimer in exactly opposite ACS Paragon Plus Environment

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dipoles (Fig. 4b). The phenyl rings of the dimer further supported by weak π-π interactions (3.463 Å). The molecular packing of 1 did not show any other interactions in the crystal lattice and the dimer molecules are well separated in other directions (Fig. 4c). The perfect matching of PXRD pattern simulated from single crystal data with experimental pattern (crystals grown from ethanol at 4 °C) confirms the phase purity of the samples (Fig. S4). However, PXRD patterns of crystalline product obtained at room temperature and crystals grown at 4°C clearly differed (Fig. S5). This indicates that both could be polymorphs of 1. The PXRD studies of gel precipitate did not show any peak (Fig. S6). Hence, the different structural arrangements of 1 in the solid state resulted in different fluorescence.

Figure 5. Self-reversible mechanochromism (a) digital images, (b) fluorescence spectra and (c) reproducible self-reversible fluorescence switching cycle of 1. λex = 365 nm (for digital images), 370 nm (fluorescence spectra). ACS Paragon Plus Environment

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Interestingly, mechanical grinding of as-synthesized 1 showed red-shift of fluorescence from 457 to 502 nm (blue to green, Fig. 5a, b). Importantly, the red-shifted fluorescence was robustly selfrecovered back to 457 nm within 8 minutes (Fig. S7). The repeated cycle of fluorescence switching by mechanical grinding and self-recovery is shown in Fig. 5c. The switching of fluorescence from 457 to 502 nm and vice versa was observed without any significant lose of fluorescence intensity up to seven cycles. Differential calorimetric experiment of 1 showed a phase transition at 53 °C before melting at 66-67 °C (Fig. S8). The controlled heating of 1 at 55-57 °C has also exhibited clear fluorescence switching from blue to green (Fig. S9). However, it did not show complete reversibility. The cooled crystals showed fluorescence at 479 nm. Even after a week also it did not show complete reversibility. It is noted that mechanical grounded powder exhibited complete self-recovery of fluorescence from green to blue. The reason for the fluorescence red shift by mechanical grinding 1 could be due to loss of

Figure 6. PXRD pattern of 1 at initial, self-recovered after grinding and crystalline powder obtained at room temperature from ethanol. crystallinity and attaining more planar conformation from twisted core.62 The excitation spectrum also revealed a red shift in ICT transition compared to crystalline powders and suggest that the twisted phenyl groups could have adopted more planar conformation (S3).64 It is noted that more planar structure of 1 optimized using the density functional theory calculation showed smaller band gap compared to twisted molecular structure of 1 in crystals (Fig. 2). The PXRD pattern of 1 before and grounded sample (recorded after complete self-recovering of blue fluorescence, λmax =457 nm) matches perfectly and confirms that 1 recovers back to same structure after grounding (Fig.6). Solid pellets of 1 ACS Paragon Plus Environment

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were prepared with KBr (100 mg of KBr and 5 mg of 1) at different pressure that showed red shift of fluorescence with increased pressure (Fig. S10).

Figure 7. Temperature dependent fluorescence spectra of 1 in toluene (a), change of fluorescence intensity Vs temperature (b) and reversible fluorescence switching in toluene with temperature change of 1 (c). λex = 365 nm (digital images, 370 nm (fluorescence spectra). 1 in toluene exhibited strong fluorescence (λmax = 491 nm) and showed a rare phenomenon of fluorescence enhancement with small blue shift of λmax upon heating. The fluorescence λmax blue shift 14 ACS Paragon Plus Environment

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from 491 nm to 487 nm (Fig. 7a). Although it is only 4 nm blue shift, the digital images of 1 in toluene showed clear change of fluorescence colour from cyan to blue before and after heating. This could be due to the more broadening of fluorescence spectra hypsochromically with temperature. 1 in toluene did not show significant change of fluorescence by heating up to 70 °C (Fig. S11). However, a linear enhancement of fluorescence intensity was observed from 70 to 100 °C (Fig. 7b). The enhanced fluorescence returns to initial state upon cooling within 8 minutes (Fig. S12). The fluorescence switching of 1 in toluene was highly reproducible and did not show any lose of intensity in the repeated heating and cooling (Fig. 7c). The increasing of fluorescence intensity with temperature could be due to activating more vibrational bands that leading to stronger TICT (twisted intramolecular charge transfer) emissions.48 In general, TICT state showed weak red shifted fluorescence or quenches fluorescence because fluorescence from zero vibrational level of the TICT is forbidden.62 However, the radiative transitions from TICT to ground states are possible via the assistance of higher and asymmetrical vibrational bands of the TICT state. These vibrational bands become activated upon heating and enhanced fluorescence intensity with increasing temperature. The formation of the TICT state in 1 was confirmed from the absorption and fluorescence studies in different solvents (Fig. 1). A similar fluorescence enhancement with increasing temperature for 1 was also observed in benzene. It is noted that 1 showed highest quantum yield in benzene (ΦF = 0.176). However, 1 in other solvents did not show temperature dependent fluorescence. As discussed previously, the solid state fluorescence of 1 can be switched and tuned by mechanical grinding and heating. The strong solid state, tunable and switchable fluorescence prompted us to fabricate nanoparticles of 1 and explore the optical properties. Nanoparticles of 1 were fabricated by fast injecting 1 in CH3CN into water under stirring at room temperature. 1 in CH3CN did not show any fluorescence. Similarly, injecting CH3CN solution of 1 (25 µL of 10-3 M) up to 90 % of water also did not show any fluorescence nanoparticles formation. However, further increasing of water fraction produced fluorescent nanoparticles (Fig. 8). 1 in 100 % water fraction showed strong fluorescence (λmax = 501 nm). The fabricated nanoparticles fluorescence perfectly matches with the green fluorescence of mechanically grounded powder, heated crystals and gel-like precipitate. It is noted that 1 in solid state showed blue fluorescence with λmax at 457 or 447 nm. Unlike, robust self-recovery of blue fluorescence from green of mechanical grounded sample, nanoparticles solution of 1 was quite stable for few days. Excitation spectra of nanoparticles were also showed n-π* transition at 369 nm and peak broadening to 405 nm (Fig. 8 and S3). The absorption studies of nanoparticles perfectly matches with the excitation spectra (Fig. S13). The quantum yield calculation for nanoparticles compared to quinine sulfate showed 0.0104. Although, there could be a loss of intensity due to scattering from nanoparticles, it suggests the ACS Paragon Plus Environment

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moderate fluorescence efficiency for 1. FE-SEM studies were performed to explore the morphology of nanoparticles. Interestingly, 1 showed the formation of featureless different shaped polydispersed nanoparticles in water (Fig. 9a). We observed a change of fluorescence for nanoparticles from green (λmax = 501 nm) to cyan (λmax = 478 nm) upon storing at room temperature in aqueous solution for more than five days (Fig. S14). PXRD studies did not show any diffraction peak and suggest the amorphous nanoparticles formation (Fig. S15). FE-SEM studies showed the formation of highly uniform spherical nanoparticles with size ranges between 20-25 nm (Fig. 9b). The fluorescence was remains stable for more than two months. The spherical nanoparticles fluorescence at 478 nm perfectly matches with the self-recovered fluorescence of heated samples (λmax = 478 nm). Nanoparticles fabrication from THF solution also showed similar fluorescence (Fig. S16). Unlike in CH3CN, 1 in THF showed moderate fluorescence with λmax at 522 nm. Nanoparticles formation leads to blue shift of fluorescence λmax at 501 nm. Excitation spectra also showed clear change from THF solution to nanoparticles. The nanoparticles of 1 exhibited red-shifted broad excitation spectra.

Figure 8. Fluorescence and excitation spectra of 1 nanoparticles fabricated by injecting CH3CN solution of 1 into water at different fractions. λex = 365 nm (digital images), 370 nm (fluorescent spectra). by monitoring excitation and emission spectra. ACS Paragon Plus Environment

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Conclusions We have demonstrated self-recoverable mechanochromism, a rare observation of fluorescence enhancement with temperature and tunable fluorescence via polymorphism using a simple triphenylamine based fluorophore, 4-((4-methoxyphenyl)(phenyl)amino) benzaldehyde (1). Assynthesized powder and crystals obtained from ethanol at 4°C showed blue fluorescence with λmax at 457 nm whereas crystalline powders obtained at room temperature from ethanol showed fluorescence with a small blue shift (λmax at 447 nm). The gel-like precipitate formed from different solvents showed green fluorescence (λmax at 502 nm). Mechanical grinding of 1 exhibited fluorescence switching from blue to green (λmax = 502 nm) and spontaneous self-recovery of blue fluorescence (λmax at 457 nm) within 8 minutes. The fluorescence switching of 1 was due to the change of crystalline phase to amorphous phase which was confirmed by PXRD studies. Importantly 1 in toluene showed fluorescence enhancement with temperature by activating more vibrational bands that leading to stronger TICT emissions. The reproducible reversible fluorescence switching without any significant lose of fluorescence suggest the application potentiality of 1 for sensors. This result indicates that simple triphenylamine molecule could be used to fabricate efficient fluorescence thermometer for sensitive detection of temperature.

Figure 9. FE-SEM images of 1 nanoparticles fabricated by fast injection of 1 in CH3CN into water (100%) (a) after 5 hr and (b) after 5 days. The corresponding fluorescence digital images are shown in the inset. λex = 365 nm.


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ACKNOWLEDGMENT: Financial supports from DST, New Delhi, India (DST Fast Track Scheme No. SR/FT/CS-03/2011 (G) and SR/FST/ETI-284/2011(c)) and CRF facility, SASTRA University are acknowledged with gratitude. PSH thanks SASTRA University for research fellowship. "X-ray crystallography at the PLS-II 2D-SMC beamline was supported in part by MSIP and POSTECH. SUPPORTING INFORMATION: Crystallographic table of 1, Fluorescence, absorption spectra, PXRD pattern, DSC data, temperature dependent fluorescence, nanoparticle fluorescence data in THF. This material is available free of charge via the Internet at http://pubs.acs.org.

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Roquet,

S.;

Cravino,

A.;

Leriche,

P.;

Alévêque,

O.;

Frère,

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Roncali,

J.

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SYNOPSIS TOC:


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Self-Reversible Mechanochromism and Thermochromism of a

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