Excited-state intramolecular proton transfer-based multifunctional solid

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Applications of Polymer, Composite, and Coating Materials

Excited-state intramolecular proton transfer-based multifunctional solidstate emitter: a fluorescent platform with ‘write-erase-write’ function Pragyan Pallavi, Virendra Kumar, MD WASEEM HUSSAIN, and Abhijit Patra ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14215 • Publication Date (Web): 28 Nov 2018 Downloaded from http://pubs.acs.org on November 30, 2018

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ACS Applied Materials & Interfaces

Excited-state intramolecular proton transfer-based multifunctional solid-state emitter: a fluorescent platform with ‘write-erase-write’ function Pragyan Pallavi, Virendra Kumar, MD. Waseem Hussain, and Abhijit Patra* Indian Institute of Science Education and Research, Bhopal, Bhopal by-pass road, Bhauri, Bhopal-462066, Madhya Pradesh, India, [email protected] KEYWORDS. ESIPT, solid-state emission, fluorescence switching, multifunctional, molecular ink, thin film

ABSTRACT. The excited-state intramolecular proton transfer (ESIPT)-based molecular probes have drawn significant attention owing to their environment-sensitive fluorescence properties, large Stokes shift and emerged as building blocks for the development of molecular sensors and switches. However, most of the ESIPT-based fluorophores exhibit weak emission in the solid state limiting the scope of real-time applications. Addressing such issues, herein, we presented a C3-symmetric like molecular architecture employing a simple one-step Schiff base condensation between triaminoguanidinium chloride and 3,5-di-tert-butyl-2hydroxybenzaldehyde (TGHB). The temperature-dependent fluorescence studies including at

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77 K indicated the strong emission from the keto tautomer compared to that of the enol. The facile ESIPT in TGHB in the solid-state led to a remarkable enhancement of fluorescence quantum yield of 1600-times compared to that of the solution (λem = 545 nm) by restricting the intramolecular rotation and subsequently suppressing the nonradiative deactivation. The excited–state processes were further elucidated through time-resolved fluorescence measurements. TGHB exhibited turn on-off fluorescence upon exposure to acid /base vapor in the form of powder as well as transparent, free-standing thin film. A rewritable and erasable fluorescent platform was demonstrated using TGHB as molecular ink, which offers a potential testbed for performing multiple times ‘write-erase-write’ cycles. In addition, TGHB, possessing multiple binding sites (O and N donors) involving the central core of triaminoguanidinium cation, displayed selective turn-on fluorescence with Zn2+. The structure-property relationship revealed in the present study provides insight towards the development of novel cost-effective multifunctional materials promising for stimuli-responsive molecular switches.

1. INTRODUCTION Development of solid-state luminescent organic materials exhibiting environment-sensitive, stimuli-responsive switchable fluorescence is on high demand. The increasing interest of solidstate emitters is due to their immense application potential in sensors, switches, memory and display devices.1-5 Most of the organic luminophores, e.g., perylene, pyrene, anthracene, BODIPY, etc. exhibit strong emission and interesting excited-state properties in solution or molecularly dispersed state.6-9 However, fluorescence is often completely or significantly


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diminished in the solid state limiting the scope of real-world applications. So, the challenge is tuning the molecular architecture and assembly to obtain the solid-state fluorescent materials. Additionally, a simple, cost-effective and scalable fabrication methodology is desirable. Molecular interaction plays a significant role governing the properties of materials.10 The short-range intermolecular interactions like π- stacking, hydrogen bonding (including CH···π) favors dissipation of excited state energy through radiationless pathways leading to quenching of fluorescence in the aggregated or solid state.11, 12 On the contrary, the molecular aggregation can also restrict the intramolecular rotation, hindering the nonradiative decay channels, which lead to enhancement of fluorescence in the solid state.2, 10, 13-17 Thus, the subtle balance between intra and intermolecular interactions dictated by the shape and geometry of the molecules govern the light emission properties of the resultant materials.18-20 The suppression of nonradiative deactivation of the molecular excited state is the key to achieve the strong fluorescence in the solid state. One of the approaches in this context is the restriction of molecular motions (bond rotation and vibration) by embedding in the rigid matrix.21-24 The rigidification of molecular conformation can also be achieved by the structural modifications through the synthetic route.25-27 However, such methodologies are often limited by the long-term stability of molecular assemblies and the complexity involved in a multistep synthesis. One intriguing approach circumventing concentration quenching relies on the construction of molecular architecture exhibiting excited-state intramolecular proton transfer (ESIPT).28, 29 In ESIPT fluorophores, an intramolecular hydrogen bonding takes place between a proton donor (hydroxyl or amino proton) and a neighbouring proton acceptor (imine


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nitrogen or carbonyl oxygen) in the ground state.30-33 Upon photoexcitation, the redistribution of electronic charge leads to fast proton transfer (phototautomerization) within the intramolecular hydrogen bonding site.34-36 As a result, the enol tautomer (E*) is converted to keto tautomer (K*) in the excited state. Thus, hallmark of the ESIPT probes is the significant structural change in the excited electronic state compared to that of the ground state causing a large Stokes shift in keto emission (Figure S1).28, 37 Owing to unique optical features ESIPTbased probes have been employed as chemosensors, lasing, and electroluminescent materials.4, 38-45

Even though photophysical processes of ESIPT molecules have been explored extensively, systematic investigation focusing on practical applications taking advantage of the unique ESIPT process is rather limited. One possible reason can be the low quantum efficiency and often quenching of keto emission.28 Thus, development of strategies boosting the solidstate emission of ESIPT fluorophores is a pertinent issue to address. In this context, we designed a functional fluorescent probe, TGHB comprised of triaminoguanidinium chloride (TG) and 3,5-di-tert-butyl-2-hydroxybenzaldehyde (HB). The triaminoguanidinium cation was chosen as a precursor for the construction of the proton acceptor moiety as well as a C3symmetric core imparting molecular flexibility suitable for stimuli-responsive behaviour (Figure S2).46 Additionally, the central carbocation enhances the acidic character of imine N leading to the fast ESIPT process.36 3,5-di-tert-butyl-2-hydroxybenzaldehyde was employed as a proton donor and three HB units were incorporated through simple Schiff base condensation reaction with triaminoguanidinium chloride (Scheme 1, S1, S2). The strategies


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implemented in the construction of TGHB involve (i) introducing steric crowding in the ESIPT chromophore preventing π-π stacking and (ii) strong hydrogen bonding restricting intramolecular rotation. ESIPT-based fluorescent probes have been mostly explored for the detection of metal ions in solution and tunable emission (both from the enol and the keto tautomers) leading to white-light generation.47-51 The Schiff base products of hydroxy-substituted benzaldehyde and triaminoguanidinium chloride were known to exhibit favorable binding with metal ions.46, 49 The ESIPT probe consisting of 8-hydroxy-2-ethylquinoline and triaminoguanidinium chloride was reported to display strong red-shifted fluorescence upon binding with Cd2+ in solution.49 With the aim of developing solid-state fluorescent ESIPT probe, we explored TGHB as a versatile molecular platform exhibiting pH-induced switchable fluorescence in the solid state and selective detection of Zn2+ over other mono-, di- and trivalent metal ions. A writable and erasable fluorescent platform was developed harnessing the stimuli-responsive properties of TGHB. The multifunctional properties based on ESIPT process and solid-state sensing and switching exhibited by an easily accessible molecular material as demonstrated in the present paper is unique and promising for rewritable printing media applications. 2. RESULT AND DISCUSSION C3-symmetric molecular materials have received significant attention in molecular recognition, organic light emitting diodes (OLEDs), optical sensors, chiral catalysis, etc.49, 52-55 The emerging

arena of tripodal

molecular materials motivated us to

employ


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triaminoguanidinium cation as a triangular building block. Triaminoguanidinium chloride was synthesized by the reaction between guanidinium chloride and hydrazine hydrate (Scheme S1).55 The three-terminal amine units of triaminoguanidinium were coupled with 3,5-di-tertbutyl-2-hydroxybenzaldehyde through a Schiff base condensation reaction (Scheme 1, S2) to yield TGHB. As the synthesis is very simple, it is possible to obtain TGHB on the gram-scale. The solid product is crystalline and thermally stable up to at least 205 °C (Figure S3). The crystal structure analysis of TGHB (triclinic space group, P-1, Table S1, CCDC No. 1861945) revealed strong intramolecular hydrogen bonding between phenolic OH and aldimine N (1.87, 1.90 and 1.92 Å, Figure S4). To substantiate specific feature of TGHB as the ESIPT type fluorophore, and to further investigate the deeper aspect of the ESIPT process, three model compounds, TGHB-1 (without tert-butyl substitution), TGHB-2 (without any –OH group), and TGHB-3 (monodentate molecule having only one –OH substitution) were synthesised (Scheme S3-S5). The comparison with TGHB-1, TGHB-2, and TGHB-3 revealed the importance of multiple coordinating sites in TGHB for ESIPT-driven stimuli-responsive properties as discussed below. 2.1 Spectroscopic investigation It is important to understand the underlying photophysical properties involving excited state enol-keto tautomerization of TGHB for the realization of a specific application. The interaction of TGHB in solution with solvents of varying polarity was investigated. Absorption spectra showed peak at 305 nm corresponding to ππ* transition and 350, 370 and a hump at 390 nm


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presumably corresponding to nπ* transitions in the solvents of different polarity (Figure 1a).56 The molar extinction coefficient of TGHB in the range of 350-390 nm was found to be 2.7 × 104 - 4.2 × 104 M-1 cm -1 (Figure S6), similar to the values reported earlier for ESIPT-based fluorophores.57, 58 The emission spectra showed large Stokes shifted peak with maximum at 550 nm when excited at 370 nm corresponding to keto (K*) emission (Figure 1b). The less intense peak in the range of 450490 nm was observed in dimethyl sulphoxide (DMSO) and methanol which corresponds to enol (E*) emission. The ratio of E*/K* depends on the solvent polarity. In nonpolar solvent, the intensity of enol emission is low and due to the facile ESIPT keto emission is predominant.59 Thus an intense peak at 550 nm was observed in cyclohexane, toluene and dioxane. The characteristic dual emission of ESIPT fluorophores was also noticeable in TGHB in high polar solvents like DMSO and methanol. Usually, ESIPT is likely to be hindered in polar solvents, particularly, in polar protic solvents due to intermolecular hydrogen bonding with solvent molecules leading to dominant enol emission.29 However, the prominent keto emission was also observed in a hydroxylic solvent such as methanol. The probable reason lies in the formation of the 8-membered chelate ring between TGHB and methanol, where proton transfer can still occur via a hydrogen-bonded solvent bridge (Scheme S6, Figure S5).59 Further, the ESIPT process was elucidated by the addition of acid in a solution of TGHB in chloroform (CHCl3) and tetrahydrofuran (THF). The addition of trifluoroacetic acid (TFA) led to the protonation of imine nitrogen reducing the proton accepting tendency of TGHB. The absorption spectra revealed that the peaks at 350, 370 and 390 nm decreased with the


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addition of TFA (Figure 1c, inset, Figure S7a). Evidently, upon excitation at 370 nm, emission spectra revealed the suppression of the peak at 550 nm due to the hindrance to ESIPT by protonation (Figure 1d, Figure S7b). The fluorescence was completely quenched at the higher concentration of TFA (Figure S8). Further, the excitation spectra of TGHB in chloroform, monitored at 450 (enol emission) and 550 nm (keto emission) were significantly different (Figure S9a, b). This observation suggests the presence of two different emitting states.50, 60 However, upon addition of acid, the excitation spectra monitored at 450 and 550 nm were found to be similar (Figure S9c, d), indicating the presence of a single ground-state species.60 The fast ESIPT process in TGHB was further investigated by time-resolved emission spectra (TRES) analysis. TRES measurement was carried out using time-correlated single photon counting (TCSPC) spectrometer using a diode laser of 373 nm as the excitation source. TRES were recorded with a wavelength range from 400 to 700 nm at 4 nm interval and at different delay time from 54 ps (first detected signal as per the resolution of the setup) to 0.7 ns (Figure 2). The signature of both enol and keto emission was observed at 54 ps. The relative intensity of keto emission progressively increased with time and reached a maximum at about 0.4 ns. Subsequently, the intensity of keto emission decreased significantly and the enol emission was found to be higher than that of keto at 0.7 ns. The results suggest that the ESIPT process proceeds in the time range from sub-picosecond to 0.4 ns.61 Interestingly, as evident from earlier reports,62, 63 molecules with more than one proton donor and proton acceptor sites usually exhibit multiple proton transfers. Accordingly, the ESIPT process in TGHB is likely to be a double or triple proton transfer phenomenon.


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2.1.1 Effect of temperature The relative abundance of enol and keto emission was further investigated at variable temperature. The emission spectra (λexc = 370 nm) of TGHB in methanol with increasing temperature from 253 to 293 K are shown in Figure 3a. At 253 K, both enol (470 nm) and keto (550 nm) peaks were present. Both the peaks diminished upon increasing the temperature with the greater drop in emission intensity of the keto peak. Hence, the plot of the ratio of fluorescence intensities at 470 and 550 nm (E*/K*) decreases with the increase of temperature (Figure 3a, inset). In order to freeze the molecular conformation, a solution of TGHB in methanol was taken in 3 mm quartz tube and was cooled rapidly by immersing it in a quartz Dewar filled with liquid-N2. The fluorescence peak maximum was blue shifted by 20 nm at 77 K (530 nm) with respect to that at room temperature (550 nm, 293 K) and interestingly, a significant enhancement in peak intensity was observed (Figure 3b, Figure S10). The blue shift in the peak maximum observed at 77 K is attributed to the solvent cage effect as reported in other ESIPT-based systems.30,

64

The similar result was also observed in non-polar, aprotic

hydrocarbon solvent like toluene (Figure S10). Thus, the enhanced keto emission at 77 K infers the strong intramolecular hydrogen bonding leading to fast ESIPT devoid of solvent perturbations.64 2.2 Fluorescence quantum yield Having understood the intriguing ESIPT process in TGHB, we focused attention to augment the photoluminescence quantum yield. TGHB in solution is weakly emissive (𝜙𝑓
96 % in 430-800 nm) thin film (Figure S25). TGHB-PMMA thin film showed absorption peaks similar to that of TGHB in solution. Upon exposure to HCl vapor, the shoulder peak at 390 nm decreased due to the protonation (Figure S26). The strong fluorescence with a peak maximum at 545 nm as that of the powder was observed (Figure 6a). Upon exposure to acid vapor (HCl/TFA), fluorescence was quenched and was regained back upon treatment with basic vapor (NH3/Et3N), as demonstrated in Figure 6a. Reversibility as well as repeatability, are important parameters to assess the performance of materials.70 The fluorescence switching exhibited by TGHB-PMMA film upon exposure to saturated HCl (20 s) and NH3 (40 s) vapor was found to be highly reversible and repeatable (Figure 6b). We further investigated the fluorescence quenching and the excited


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state processes in a thin film in greater detail. TGHB-PMMA thin film was exposed to HCl vapor of specific concentration (Figure S27a); the approximate limit of detection was found to be 7.5±0.6 ppm. The time-resolved decay analysis of the corresponding thin film was carried out (Figure S27b). The amplitude average decay time progressively decreased with increasing concentration of acid vapor, upon excitation with a 408 nm diode laser (Figure S28, Table S3). We also explored the possibility of fabrication of highly fluorescent free-standing film, which could be promising for device applications. The free-standing film was fabricated using TGHB, PMMA and employing PVA [poly(vinyl alcohol)] as a sacrificial layer. The aqueous solution of PVA (15 wt%) was spin coated on a glass plate. Then, TGHB (5 mM) and PMMA (0.5 wt%) solution in toluene was coated over PVA layer. After subsequent drying, the glass plate was immersed in water. The sacrificial layer of PVA was dissolved in 10-15 minutes, and the free-standing film was obtained. FESEM and transmission electron microscopy (TEM) images revealed the highly smooth surface of the film (Figure 6c). The transparent freestanding film was also demonstrated to exhibit facile turn-off and turn-on fluorescence upon brief exposure to respectively, HCl and NH3 vapor (Figure 6d). 2.3.1 Write-read-write function The reversible turn on-off fluorescence of TGHB upon exposure to NH3/HCl vapor was employed for the fabrication of rewritable and erasable fluorescent platform. Nonfluorescent silica plate was dipped into the ethanol solution of TGHB and dried in air for 5 minutes. The silica plate exhibited strong greenish-yellow fluorescence under illumination at 365 nm (Figure 7). The fluorescence was completely turned-off upon exposure to HCl vapor. Desired


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inscriptions (letters/ numbers) were written on the plate using an appropriate mask and exposing to NH3 vapor. The inscription (exposed area) became readable owing to the strong fluorescence of TGHB, turned-on by NH3 vapor (Figure 7). The information can be erased instantaneously upon exposure to HCl vapor. Subsequently, the new inscriptions can be written and read. The writing can be erased with time (~ 3 h) by keeping the plate in air. Employing TGHB as molecular ink, rewritable and self-erasable platform can also be fabricated using simply Whatman filter paper (Figure S29). In fact, the inscription can be completely self-erased within 30 s after writing. The similar results were also obtained using TFA/Et3N vapor (Figure S30). Further, in order to illustrate the fast response of TGHB upon acid/base vapor for write-erase-write function, the experiment was performed with varying concentration of NH3 vapor. The gradual self-erasing of the inscription under air was demonstrated through images captured at 5 s interval (Figure S31). Thus, TGHB-coated plate was demonstrated as an efficient platform exhibiting write-erase-write function over multiple cycles in a reversible manner under ambient conditions. 3. CONCLUSION Circumventing the weak emission in the solid state, a simple strategy was employed exploiting the concept of excited-state intramolecular proton transfer (ESIPT). One-step fabrication involving Schiff base condensation between triaminoguanidinium chloride and 3,5-di-tert-butyl-2-hydroxybenzaldehyde led to the C3-symmetric like fascinating molecular optical material TGHB. Contrary to the weak emission in solution, TGHB exhibited strong


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yellowish-green emission in an aqueous dispersion of nanoparticles and the solid state with a large Stokes shift of 175 nm. Low-temperature (77 K) fluorescence measurements revealed the dominant keto emission over that of the enol indicating facile ESIPT. The enhanced fluorescence in the solid state was attributed to the restriction of intramolecular rotation of flexible chains suppressing the radiationless deactivation pathways. The strongly fluorescent transparent free-standing thin film of TGHB was demonstrated as a reversible switch in response to the acidic/basic vapor. The stimuli-responsive properties laid the foundation for the development of molecular ink for rewritable and erasable information displays. The C3symmetric central core of triaminoguanidinium offered binding pockets imparting the selective detection of Zn2+ by TGHB in a semi-aqueous medium as well as on the solid support. A comparison with reported all-organic ESIPT-based fluorescent probes including very few with C3-symmetric architecture (Table S4), establishes TGHB as a versatile solid-state emitter with a unique combination of several materials attributes. The present study contributes to the basic understanding of tailoring the molecular properties paving the ways for the development of multifunctional molecular optical materials for rewritable media, security ink and sensor devices through cost-effective fabrication route. 4. EXPERIMENTAL SECTION 4.1 General methods 1H

and

13C

NMR spectra were recorded on Bruker Avance III 500 MHz NMR

spectrometers. High resolution mass spectrometry (HRMS) data was obtained on Bruker


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MicrOTOF-Q-II mass spectrometer. The differential scanning calorimetry (DSC) analysis was carried out using Perkin Elmer instrument (DSC 6000). Thermogravimetric analysis (TGA) was carried out using Perkin Elmer TGA-4000, heating the samples at a rate of 10° C/ min under the nitrogen atmosphere. 4.2 Synthesis and characterization of TGHB Triaminoguanidinium chloride was synthesized following a reported procedure.55 Guanidinium chloride (1 mmol) was taken in 30 mL of isopropanol in a 100 mL round bottom flask. 50-60% hydrazine hydrate (4.5 mmol) was added to the above solution and was refluxed for 6 h. After the reaction, the precipitate was filtered and was washed with 50 mL of isopropanol to obtain triaminoguanidinium chloride (yield: 97 %). Triaminoguanidinium chloride (70 mg, 0.5 mmol) was dissolved in a hot mixture of ethanol (30 mL) and water (15 mL). After adjusting the pH of the mixture to 3 with HCl (aq.), the solution of 3,5‐di-tertbutyl-2-hydroxybenzaldehyde (400 mg, 1.7 mmol) in ethanol (10 mL) was added slowly. The resulting solution was refluxed at 90° C for 6 h. The residue was washed with water several times. The yellow solid (yield: 68 %) was characterized by 1H NMR, 13C NMR spectroscopy and mass spectrometry (Figure S32-S34). TGHB-1, TGHB-2, and TGHB-3 were synthesised using similar protocol as that of TGHB (Figure S35-S40). TGHB : 1H NMR (500 MHz, CDCl3): δ ppm 10.50 (s, 3H), 8.45 (s, 3H), 7.41 (d, J = 2.3 Hz, 3H), 7.13 (s, 3H), 1.44 (s, 27H), 1.33 (s, 27H).


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NMR (126 MHz, CDCl3): δ ppm 156.4, 155.2, 141.8, 137.2, 128.3, 127.1, 116.3, 35.1, 34.2,

31.5, 29.3. MS (HRMS): m/z calculated for C46H70N6O3: 753.5426, obtained: 753.5446. TGHB-1: 1H NMR (500 MHz, DMSO): δ 11.96 (s, 3H), 10.44 (s, 3H), 9.07 (s, 3H), 8.20 (dd, J = 7.8, 1.3 Hz, 3H), 7.37 – 7.30 (m, 3H), 7.03 (d, J = 8.1 Hz, 3H), 6.93 (t, J = 7.5 Hz, 3H). 13C

NMR (126 MHz, DMSO): δ 157.69, 149.15, 147.91, 132.91, 127.65, 119.93, 119.68, 116.72.

HRMS (ESI): m/z calculated for [TGHB-1]+: 417.1675 and found 417.1679. TGHB-2 : 1H NMR (500 MHz, CDCl3): δ 11.42 (br, 3H), 8.61 (s, 3H), 7.74 (d, J = 7.9 Hz, 6H), 7.34 (d, J = 8.0 Hz, 6H), 1.32 (s, 27H). 13C

NMR (126 MHz, CDCl3): δ 154.00, 150.96, 147.63, 130.12, 128.16, 125.10, 34.91, 30.89.

HRMS (ESI): m/z calculated for [TGHB-2]+: 537.3706 and found 537.3720. TGHB-3 : 1H NMR (500 MHz, CDCl3): δ 9.75 (s, 1H), 8.68 (s, 1H), 7.38 (s, 1H), 7.05 (s, 1H), 1.32 (s, 9H), 1.25 (s, 9H). 13C

NMR (126 MHz, CDCl3): δ 156.53, 155.12, 141.67, 137.23, 128.33, 127.01, 116.12, 35.09,

34.20, 31.44, 29.45. HRMS (ESI): m/z calculated for [TGHB-3 + Na+]+: 379.2 and found 379.198.


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4.3 Crystal structure analysis Single-crystal X-ray diffraction measurements were carried out on a Bruker SMART 1000 CCD diffractometer operating at 50 kV and 30 mA using Mo Kα radiation (λ = 0.71073 Å). The crystals of TGHB were grown in solvent mixture of dichloromethane and methanol with slow evaporation. The data was collected at 100 K with an exposure time of 8 s per frame and at the crystal-to-detector distance of 6 cm. The data collection, integration, unit cell measurements, scaling and absorption corrections were done using Bruker Smart Apex II software. 4.4 Microscopic characterizations Morphology of nanoaggregates of TGHB was analyzed by Carl Zeiss (Ultraplus) field emission scanning electron microscope (FESEM). The sample was coated with a thin layer of gold prior to imaging under an accelerating voltage of 15 kV. The surface morphology of freestanding thin film was examined under Bruker transmission electron microscope (TEM) at an accelerating voltage of 200 kV. 4.5 Computational investigation Geometry optimization of TGHB was carried out using Gaussian 09 program package. The molecular structure obtained from crystal structure analysis was taken as the input geometry. The same was optimized at the B3LYP/6-31+G(d,p) level.


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4.6 Absorption and steady-state fluorescence spectroscopy UV–visible absorption spectra were recorded on Cary 100 spectrophotometer using 10 mm path length quartz cuvettes. Steady-state fluorescence measurements were carried out on Jobin Yvon Horiba Model Fluorolog-3–21; solid-state spectra were recorded in the front-face geometry. All the fluorescence spectra were corrected with respect to the excitation light intensity and photomultiplier tube (PMT) response using the correction files integrated with Horiba software. The optical densities of the samples were kept low to avoid any inner filter effect. The absolute quantum yield measurements were carried out using Fluorolog-3–21 spectrophotometer equipped with a BaSO4-coated, calibrated integrating sphere, Model F3029, Quanta-Phi 6 (Jobin Yvon Horiba). 77 K fluorescence measurement was carried out using a Dewar flask filled with liquid-N2 (FL-1013, Horiba Instruments Inc.). 4.7 Time-resolved fluorescence spectroscopy Time-resolved fluorescence measurements were done using time-correlated single photon counting (TCSPC) spectrometer (Delta Flex-01-DD/HORIBA). The Delta diode lasers of 373 and 408 nm of FWHM = 154 and 198 ps were used as excitation source. The instrument response function was measured using the aqueous dispersion of colloidal silica (LUDOX). The photomultiplier tube of picosecond photon detection module was used as a detector. All decay curves were analyzed by nonlinear least-squares iteration using IBH DAS6 (version 6.8) decay analysis software. The quality of the fit was evaluated by the fitting parameters (χ2), and the


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visual inspection of the residuals. Time-resolved emission spectra (TRES) was recorded using a 373 nm diode laser. The emission spectra were collected at different delay time with the wavelength range from 400 to 700 nm at 4 nm interval. ASSOCIATED CONTENT Supporting Information. Details of design strategy, synthesis and characterizations, crystal structure analysis, steady-state and time-resolved spectroscopic measurements, computational investigation, chelation and aggregation behavior, sensing and switching, fabrication of rewritable and self-erasable platform and the comparative account of multifunctional properties of TGHB with notable ESIPT-based molecular materials. “This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources BRNS, DAE (No. 37(2)/14/06/2016-BRNS/37020) and SERB (PDF/2016/001202)


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ACKNOWLEDGMENT The infrastructural support from IISERB and financial support from BRNS, DAE and SERB, DST is gratefully acknowledged. PGP thanks UGC and VK thanks NPDF scheme of SERB, DST (PDF/2016/001202) for fellowship. We thank Mr. Subhankar Kundu for fruitful discussion and timely help. REFERENCES (1) Sasabe, H.; Kido, J. Multifunctional Materials in High-Performance OLEDs: Challenges for Solid-State Lighting. Chem. Mater. 2011, 23 (3), 621-630. (2) Mei, J.; Leung, N. L. C.; Kwok, R. T. K.; Lam, J. W. Y.; Tang, B. Z. Aggregation-Induced Emission: Together We Shine, United We Soar! Chem. Rev. 2015, 115 (21), 11718-11940. (3) Derue, L.; Olivier, S.; Tondelier, D.; Maindron, T.; Geffroy, B.; Ishow, E. All-SolutionProcessed Organic Light-Emitting Diodes Based on Photostable Photo-cross-linkable Fluorescent Small Molecules. ACS Appl. Mater. Interfaces 2016, 8 (25), 16207-16217. (4) Mamada, M.; Inada, K.; Komino, T.; Potscavage, W. J.; Nakanotani, H.; Adachi, C. Highly Efficient Thermally Activated Delayed Fluorescence from an Excited-State Intramolecular Proton Transfer System. ACS Cent. Sci. 2017, 3 (7), 769-777. (5) La, D. D.; Bhosale, S. V.; Jones, L. A.; Bhosale, S. V. Tetraphenylethylene-Based AIE-Active Probes for Sensing Applications. ACS Appl. Mater. Interfaces 2018, 10 (15), 12189-12216.


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Scheme 1. Synthetic scheme of TGHB by a one-pot Schiff base condensation reaction between 3,5-di-tert-butyl-2-hydroxybenzaldehyde and triaminoguanidinium chloride.


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Figure 1. (a) Normalized absorption spectra and (b) emission spectra (λexc = 370 nm) of TGHB (maintaining similar optical density at 370 nm) in the solvents of varying polarity. Spectroscopic features of TGHB (3 µM in CHCl3) upon addition of trifluoroacetic acid (TFA): (c) absorption spectra; inset: TGHB in the presence of 0 and 1 mM of TFA, (d) emission spectra of TGHB (λexc = 370 nm) with increasing concentration of TFA.


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Figure 2. Time-resolved emission spectra (TRES) of TGHB in CHCl3 normalized at 430 nm (λexc = 373 nm) with a wavelength range from 400 to 700 nm at a delay time of 54 ps, 0.4 ns, and 0.7 ns; inset: emission spectra of TGHB from 54 ps to 0.7 ns delay time.


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Figure 3. (a) Emission spectra (λexc = 370 nm) of TGHB with increasing temperature (253 K to 293 K); inset: the ratio of fluorescence intensities at 470 and 550 nm (E*/K*) at different temperature. (b) Emission spectra of TGHB at 293 and 77 K.


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Figure 4. (a) The fluorescence spectra of TGHB (λexc = 370 nm) solution (3 µM, ethanol : water = 9 : 1) with the addition of 3 eqv. of different metal ions depicting the strong enhancement in the presence of Zn2+; inset: digital photographs of pristine TGHB solution and that added with Zn2+ under the illumination at 365 nm. (b) Fluorescence responses (ratio of intensities at 490 and 550 nm, λexc = 370 nm) of TGHB (3 µM) to various metal ions (9 µM, orange bars) and the fluorescence change of the mixture of TGHB and an excess of other metal ions (30 µM) after the addition of Zn2+ (9 µM, cyan bars) in ethanol-water (9 : 1, v/v). (c) The digital photographs of nonfluorescent thin layer silica chromatographic plate drop-casted with 2 µL of TGHB (5 mM, in ethanol) followed by the addition of 2 µL of 10 eqv. of respective metal ion solution under the irradiation of UV light at 365 nm.


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Figure 5. (a) Emission spectra (λexc = 370 nm) of TGHB (3 µM) upon increasing the volume fraction of water-THF; inset: the plot of I/I0 (I0: intensity in pure THF) with increasing water fraction and photographs of TGHB at 0 and 90 % water fraction illuminated under the UV light at 365 nm. (b) FESEM image of TGHB nanoaggregates obtained in 90 % water-THF (v/v).


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Figure 6. (a) Emission spectra (λexc = 370 nm) of TGHB-PMMA [poly(methyl methacrylate)] thin film upon exposure to HCl and NH3 vapor; inset: the digital photographs of thin film exposed to HCl and NH3 vapor under UV illumination at 365 nm. (b) On-off fluorescence (λem = 545 nm) switching of TGHB thin film by successive exposure to saturated HCl (20 s) and NH3 (40 s) vapor (λexc = 410 nm). (c) FESEM image of the transparent free-standing thin film; inset: the flat surface of the thin film revealed through TEM image. (d) Photographs of free- standing thin film under (i) day light, (ii) UV illumination at 365 nm, (iii) upon exposure to HCl followed by exposure with (iv) NH3 vapor under UV light.


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Figure 7. Demonstration of TGHB as erasable and rewritable platform: nonfluorescent silica plate coated with TGHB (~ 10 mM) exposed to HCl and ammonia (NH3) vapor respectively, for erasing and writing the desired inscriptions. All the images were taken under illumination at 365 nm.


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Table of Contents


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Excited-state intramolecular proton transfer-based multifunctional solid

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