ESIPT Fluorescent Chromism and Conformational Change of 2-(2

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

ESIPT Fluorescent Chromism and Conformational Change of 2-(2’Hydroxyphenyl-5’-Sulfonic Acid)Benzothiazole by Amine Absorption Yuta Nakane, Takashi Takeda, Norihisa Hoshino, Ken-ichi Sakai, and Tomoyuki Akutagawa J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03248 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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The Journal of Physical Chemistry

ESIPT Fluorescent Chromism and Conformational Change of 2-(2’Hydroxyphenyl-5’-Sulfonic Acid)Benzothiazole by Amine Absorption Yuta Nakane,† Takashi Takeda,†, ‡ Norihisa Hoshino,†, ‡ Ken-ichi Sakai,* ± Tomoyuki Akutagawa*†, ‡ †

Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan. ‡ Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan. ± Chitose Institute of Science and Technology, Bibi, Chitose 066-8655, Japan.

ABSTRACT: Sulfonic acid (-SO3H)-substituted 2-(2’-hydroxyphenyl)benzothiazole (1) was designed as a new solid-state ESIPT (excited state intramolecular proton transfer) fluorescent chromic molecule that responds to various types of organic bases and amines as a sensing device of biological important molecules such as ammonia and histamine. Crystal 1 exhibited a reversible adsorptiondesorption behaviour with pyridine, aniline, thiazole, quinoline, ammonia, propylamine, octylamine, diethylamine, 1,4-diaminobutane, histamine, and so on. The sorption behaviour of these compounds induced the fluorescence chromism of crystal 1 from nonESIPT weak blue, to ESIPT strong green, and finally to non-ESIPT strong green emissions, which applied for the solid state sensing devices for biologically important organic bases and amines.

INTRODUCTION Fluorescent chromic materials exhibit a drastic change in emission with external stimuli. Recently, they have attracted much attention for application in chemical sensors for biological and environmental monitoring.1, 2 In these chromic materials, the molecular structure and/or packing structure of the crystals are drastically modified in response to external stimuli, resulting in a change in the electronic structure and fluorescent properties. The thermochromic,3, 4 mechanochromic,5, 6 vapochromic,7, 8 and photochromic 9, 10 responses are induced by temperature, mechanical force, molecular sorption, and light irradiation, respectively. Although fluorescence chromism in solution has been widely examined in metal-coordination compounds and supramolecular materials,11-13 solid-state fluorescent chromic materials have not been sufficiently developed for application in chemical sensors and molecular memory devices. Among the various types of emission mechanisms, excited state intramolecular proton transfer (ESIPT) is an interesting emission that arises from intramolecular proton-transferred structural isomers in the excited state. Thus, ESIPT has been observed in intramolecular hydrogen-bonded tautomeric molecules such as keto-enol and/or lactam-lactim isomers.14 One of the notable features of ESIPT fluorescence is a significant Stokes-shift over ~10,000 cm-1 with a drastic change in emission colour owing to a large-magnitude structural relaxation from the excited state. For instance, 2-(2’-hydroxyphenyl)benzothiazole (HBT) is a well-known solid-state ESIPT fluorescent molecule with intramolecular O-H•••N hydrogen bonds, which has an absorption maximum (λAmax) of around 340 nm and an emission maximum (λFmax) of around 510 nm with a Stokesshift of ~10,000 cm-1.15-17 A large number of ESIPT fluorescent molecules with intramolecular hydrogen-bonded π-molecular systems have been developed. For instance, (Z)-4-(2-hy-

droxybenzylidene)-1,2-dimethyl-1H-imidazol-5(4H)-one exhibits red ESIPT fluorescence at λFmax = 643 nm in the nearinfrared region despite the simple π-conjugated molecular structure.18 ESIPT fluorescence in the near-infrared region, coupled to small-sized π-conjugated molecules with uncomplicated chemical designs and organic syntheses, has applications in biological and environmental sensing. In addition, the ESIPT fluorescence wavelength is sensitive to the outer environment around an ESIPT molecule. ESIPT fluorescent molecules generally exhibit solvatochromism,19 where the cleavage of intramolecular hydrogen bonds quenches ESIPT fluorescence in highly polar solvents such as alcohol, DMF, and DMSO. On the contrary, ESIPT fluorescence is activated by the formation of effective intramolecular hydrogen bonds in aprotic, low-polarity solvents such as hexane, chloroform, and acetone. The ON/OFF switching of ESIPT fluorescence can be chemically controlled by modifying the polymer matrix.20 The dispersion of 6-cyano-2-(2’-hydroxyphenyl)imidazo[1,2-a]pyridine in highly polar polyethyleneglycol (PEG) thin films exhibited non-ESIPT fluorescence at λFmax = 400 nm with a small Stokesshift because of the absence of intramolecular hydrogen bonds, whereas the intramolecular hydrogen bonds in polystyrene induced ESIPT fluorescence at λFmax = 600 nm with a large Stokes-shift. Such ON/OFF control induced by intramolecular hydrogen bonds in ESIPT molecules can cause drastic fluorescence changes from non-ESIPT to ESIPT emission based on the outer environment, which has been utilized for the new mechanism of sensor applications recently.21, 22 However, reversible ON/OFF switching control of solid-state ESIPT fluorescent materials by external stimuli such as temperature, pressure, electric field, and molecular sorption has not yet been reported. For application in sensing devices, the solid-state ON/OFF control of ESIPT fluorescence chromism should be an important feature when designing new fluorescent π-molecular devices.

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Herein, we report a new solid-state ESIPT fluorescent chromic molecule that responds to the adsorption-desorption of organic bases and amines. The well-known ESIPT fluorescent πelectronic molecule HBT was utilized for the simple design of new ESIPT fluorescent chromic molecules in which highly acidic sulfonic acid (–SO3H) was introduced into the HBT framework (1). The acidic –SO3H unit acts as the intermolecular hydrogen-bonding and proton-transfer site for the molecular sensing of organic bases. The crystal structures, optical properties, and solid-state ESIPT fluorescence chromism were evaluated for pyridine (Py), aniline (Ani), quinoline (Quino), thiazole (Thz), ammonia (NH3), propylamine (PA), octylamine (OA), diethylamine (DEA), triethylamine (TEA), 1,4-diaminobutane (DAB), histamine (HA), and amino acids. Single crystal X-ray structural analysis before and after the molecular adsorptiondesorption cycle supported the possible ON/OFF mechanism of these ESIPT fluorescent molecules in terms of the dynamic molecular conformational transformation between the intramolecular hydrogen-bonded cis-conformation and the non-hydrogenbonded trans-conformation in the crystalline state (Scheme 1). SO3H S N HO

1 Aromatic bases S N Py

N

N NH2 Ani

Quino

Thz

Amines NH2

NH3

CnH2n+1NH2 n = 3: PA

N

(C2H5)2NH

n = 8: OA DEA

N H

(C2H5)3N

TEA

HA

H2N-C4H8-NH2 DAB

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–SO3H group of 1 was transferred to the nitrogen site of the BTz-ring via intramolecular acid-base reaction and proton transfer, resulting in a zwitterionic structure that bears both the anionic –SO3- moiety and the cationic HBTz+ ring. The formation of the HBTz+ ring is forbidden in the intramolecular OH•••N hydrogen-bonded cis-conformation and does not result in ESIPT fluorescence. The solubility of molecule 1 in common organic solvents was quite low, but 1 was soluble in highly polar solvents such as DMSO and DMF. Figure 1b shows the absorption and fluorescence spectra of 1 in DMSO (blue spectra) and on KBr pellets (red spectra) at room temperature. The UV-vis spectrum of 1 in DMSO shows an λAmax = 333 nm, whereas the fluorescence spectrum indicates a main and a sub emission band at 382 and 460 nm, respectively, with blue emission and a quantum yield of 17.1% (Figures 1b and 1c). The excitation spectra of the two above mentioned fluorescence bands were confidently assigned to λAmax = 333 nm (Figure S4), suggesting emission from the same ground state molecular structure. The Stokes-shifts for these fluorescence bands at 382 and 460 nm were approximately 3,900 and 8,300 cm-1, respectively. Both the absorption and fluorescence spectra of HBT were similar to those of 1 in DMSO (Figure S5), suggesting neutral non-hydrogen-bonded trans-conformation at λFmax = 382 nm and the zwitterionic one at λFmax = 460 nm in solution.16 The solid-state absorption spectrum of 1 showed a broad band around ~350 nm, and the absorption edge was red-shifted by approximately 70 nm from the absorption peak in solution phase. The solid-state emission spectrum of the zwitterionic trans-conformation of 1 showed λFmax = 450 nm with weak blue fluorescence (Figure 1d). The excitation spectrum of crystal 1 revealed λFmax = 410 nm with a small Stokes-shift of ~2,200 cm-1, corresponding to the nonESIPT fluorescence from the trans-conformation. The solidstate fluorescence was consistent with the zwitterionic conformation obtained from the single crystal X-ray structural analysis. (b)

(a)

H O -

S

SO3 N H

Non-hydrogen-bonding trans-1 (ESIPT OFF)

Hydrogen-bonding cis-1 (ESIPT ON)

(c)

+

(d)

Scheme 1. Molecular structure of 1 and its conformational change via an adsorption-desorption cycle with organic bases and amines.

RESULTS AND DISCUSSION Molecular structure and optical properties of 1. Figure 1a shows the molecular conformation of 1 obtained from an X-ray structural analysis at T = 298 K. According to the residual electron density from the differential Fourier map, the protonated nitrogen atom was confirmed to form the benzothiazolium (HBTz+) cationic ring structure. The vibrational spectra also supported the formation of a protonated N-H+ bond by the appearance of the asymmetrical N-H stretching mode (νaNH) around 2,500 cm-1 (Figure S3). The highly acidic proton on the

Figure 1. Molecular structure and optical spectra of 1. (a) Zwitterionic trans-conformation in crystal 1 (T = 298 K). (b) Absorption (solid line) and emission (dashed line) spectra in DMSO at a concentration of 3.20 × 10-5 M (blue spectra) and on KBr pellets (red spectra). The emission spectra were obtained at the excitation wavelength of 330 nm. Photographs of 1 (c) in DMSO and (d) in the solid state under the excitation wavelength of 365 nm.

Solid-state ESIPT fluorescence chromism with aromatic bases. Interestingly, crystal 1 showed a reversible adsorption-desorption behaviour for the aromatic base Py. The crystals of 1


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The Journal of Physical Chemistry were thermally stable up to 220 °C according to the TG analysis (Figure S11a). After exposure to Py vapor in a vial, the crystals indicated a weight loss of 20.5% at 200 °C (Figure S11a), corresponding to the desorption of one mole of Py from the Pyadsorbed crystals 1•(Py). A reversible Py adsorption-desorption cycle was observed, corresponding to a reversible transformation between 1 and 1•(Py). During the reversible Py sorption cycle, crystal 1 showed an interesting fluorescence chromism accompanied by a drastic change in emission colour and intensity (Figure 2). After Py adsorption, the solid-state fluorescence spectra of crystal 1 indicated a large red shift of approximately 65 nm from λFmax = 450 nm for 1 to λFmax = 515 nm for 1•(Py), where the fluorescence colour drastically changed from weak blue to strong green (Figure 2b). After desorption of Py from the 1•(Py) crystals at 200 °C, the initial fluorescence spectra with λFmax = 450 nm and weak blue emission was recovered. The solid-state absorption spectra of 1•(Py) also indicated a 60nm red-shift of the absorption edge from 300 nm for 1 to 400 nm for 1•(Py) after Py adsorption (Figure S6), suggesting the change in the electronic ground state structure. It should be noted that the large Stokes-shift over 10,000 cm-1 after Py adsorption was consistent with the strong ESIPT green fluorescence. The reversible Py adsorption-desorption cycle of crystal 1 realized ON/OFF fluorescence switching for ESIPT emission. The adsorption of aromatic bases such as Ani, Quino, and Thz also showed similar ESIPT fluorescence chromism. The formation of adsorption crystals with 1:1 formula was confirmed by the TG analyses of crystal 1 after exposure to Ani, Quino, and Thz vapours, which showed a weight loss of 23.4, 21.1, and 27.4%, respectively (Figure S11a). The 1•(Ani), 1•(Quino), and 1•(Thz) crystals exhibited strong green ESIPT fluorescence at λFmax = 498, 533, and 510 nm, respectively (Figures S10b and S10c). Such drastic ESIPT fluorescence chromism of crystal 1 was not observed with organic acids or neutral molecules.

performed on 1•(Py) at 298 K (Figure 3). Although a zwitterionic molecular structure was observed for crystal 1, the molecular structure of the 1•(Py) crystal showed the anionic moiety 1bearing the −SO3- group. Furthermore, neutral Py was also converted to the pyridinium (HPy+) cation after adsorption, where the proton transfer occurred at the nitrogen site of Py (Figure 3a), forming the electrostatic intermolecular hydrogen-bonded N-H+•••-O3S− pair with a bond-length of dN1-O1 = 3.053 (2) Å and a 1-•(HPy+) formula. The vibrational IR spectra of 1•(HPy+) also supported the formation of the HPy+ cation with a peak at νaNH ~ 2,600 cm-1 (Figure S3). On the contrary, strong intramolecular O-H•••N hydrogen bonds (dN1-O1 = 2.610(2) Å) were observed between the –OH group and the nitrogen site of the BTz ring in the ESIPT emissive cis-conformation (Figure 3a), which was different from the trans-conformation of the zwitterionic structure in 1. The cis-conformation of anionic 1was approximately 45 kJ mol-1 stable than that of the trans-one based on the theoretical DFT calculations, which was consistent with the results of X-ray crystal structural analysis (Figure S10). Thus, Py adsorption drastically changed the molecular structure from the trans- to the cis-conformation, resulting in the ON/OFF fluorescence switching between weak blue nonESIPT and strong green ESIPT emission. (a)

N2 O2 C1 N1

(b)

O1 o

a c

(a)

(c)

1’

(b)

Rev.

1•(Py)

1 H N

1-•(HPy+)

-Py

+Py

-

+

SO3

1 H N

HO N S

S O H

-

SO3

-

+

1

SO3

S O H

Figure 2. Py adsorption-desorption cycle and ESIPT fluorescence chromism of crystal 1. (a) Solid-state emission spectra of 1 before and after Py adsorption (green) and desorption (blue). 1’ represented the desorption powder of Py absorption one of 1•(Py). (b) Photographs showing emission colour change in solid-state 1 for the Py adsorption-desorption cycle under UV irradiation at 365 nm.

To gain insight into the molecular structure after Py adsorption into crystal 1, single crystal X-ray structural analysis was

Figure 3. Reversible structural transformation of crystal 1 for the Py adsorption-desorption cycle at 298 K. (a) Intramolecular OH•••N hydrogen-bonded cis-conformation of 1- and the electro-



static hydrogen-bonded N-H+•••-O3S− pair in the (1-)•(HPy+) crystal. (b) Unit cell viewed along the b axis (HPy+ cations were omitted). (c) PXRD patterns of 1 for the Py adsorption-desorption cycle: (i) Simulated PXRD pattern of 1 based on single crystal X-ray structural analysis, (ii) PXRD pattern of powder 1, (iii) Simulated PXRD pattern of 1-•(HPy+) based on single crystal X-ray structural analysis, (iv) PXRD pattern of 1-•(HPy+), and (v) PXRD pattern of 1 after Py desorption.

The crystal-to-crystal structural transformation 1 + Py ⇆ 1•(HPy+) via Py adsorption and desorption was confirmed by the PXRD patterns (Figure 3c). The PXRD pattern of crystal 1 was good agreement with the simulated pattern of the single crystal X-ray structure of 1 (i and ii in Figure 3c). The Py adsorption in crystal 1 drastically changed the PXRD pattern (iii in Figure 3c), which was also consistent with the simulated pattern of the single crystal 1-•(HPy+) (iv in Figure 3c). The Py desorption from 1-•(HPy+) at 200 °C recovered the PXRD pattern of the original crystal 1 (v in Figure 3c). The molecular structure of 1 oscillated between the highly crystalline intramolecular hydrogen-bonded cis-conformation and the zwitterionic trans-conformation with Py adsorption and desorption, respectively. ESIPT fluorescence chromism with alkylamines. A different type of reversible adsorption-desorption cycle was observed for alkylamines in crystal 1. The adsorption of propylamine (PA) into crystal 1 at 298 K increased the crystal weight by about 26.5%, corresponding to the adsorption of two moles of PA per one mole of 1 to form 1•(PA)2 crystals. The TG plot of 1•(PA)2 after PA desorption exhibited a weight loss of 14.2% around 100 °C, corresponding to the desorption of one mole of PA to form 1:1 1•(PA) crystals. The transformation from 1•(PA)2 to 1•(PA) was observed after thermal treatment at 100 °C (Figure S12a), and, the 1•(PA) crystals showed decomposition around 250 °C upon further heating. Therefore, crystal 1 underwent irreversible PA adsorption from 1 to 1•(PA)2, then reversible PA adsorption-desorption cycles between 1•(PA)2 and 1•(PA). The stepwise PA adsorption-desorption cycles of crystal 1 revealed an interesting ESIPT fluorescence chromism. Adsorption of two moles of PA into crystal 1 caused a ~30 nm blueshift of λFmax from 450 to 422 nm, whereas desorption of one mole of PA from 1•(PA)2 showed a remarkable red-shift of ~100 nm, from λFmax = 422 nm for 1•(PA)2 to λFmax = 520 nm for 1•(PA) (Figure 4b). In the PA sorption cycle, the fluorescence colour drastically changed from weak blue for 1, to strong blue for 1•(PA)2, and to strong green for 1•(PA) (Figure 4b). In addition, the solid-state absorption spectra for the PA sorption cycle showed a drastic change. The absorption edge of 1 around 400 nm was blue-shifted to 363 nm for 1•(PA)2 and to 340 nm for 1•(PA) (Figure S6). The Stokes-shifts for 1, 1•(PA), and 1•(PA)2 were approximately 2,200, 10,000, and 3,900 cm-1, respectively, suggesting a different fluorescence mechanism for each: non-ESIPT for 1•(PA)2 and ESIPT for 1•(PA). The stepwise PA adsorption-desorption cycle of crystal 1 is thus coupled with ESIPT fluorescence chromism. To obtain structural information during the stepwise PA adsorption-desorption cycle, single crystal X-ray structural analyses were performed on 1•(PA)2 and 1•(PA). Figures 5a and 5b show the molecular structures of 1- in the 1-•(HPA+) and 12•(HPA+)2 crystals. Interestingly, two acidic protons at the – SO3H and –OH groups of 1•(PA)2 were deprotonated to form the dianionic structure 12- with –SO3- and –O- groups, which was crystallized with two moles of propylammonium (HPA+). The trans-conformation of the dianionic 12- moiety in the 12-

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•(HPA+)2 crystal was consistent with the weak blue non-ESIPT fluorescence with small Stokes-shift of 3,900 cm-1 (Figure 5a). On the contrary, the 1•(PA) crystals contained monoanionic 1bearing one –SO3- group and one HPA+ cation, and the intramolecular O-H•••N hydrogen-bonded cis-conformation was consistent with the strong green ESIPT fluorescence of the 1•(HPA+) crystals. The stepwise crystal-to-crystal transformation 1 → 12•(HPA+)2 ⇆1-•(HPA+) occurred in high crystallinity before and after the PA adsorption-desorption cycles (Figure 5c). After PA adsorption into crystal 1, the PXRD pattern of 12-•(HPA+)2 (iii in Figure 5c) showed drastic changes compared to that of 1 (i in Figure 5c), which was fully consistent with the simulated pattern of a single crystal of 12-•(HPA+)2 (ii in Figure 5c). After the desorption of one mole of PA from 12-•(HPA+)2 at 100 °C, the PXRD pattern of the obtained crystal (v in Figure 5c) was consistent with the simulated pattern of a single crystal of 1•(HPA+) (iv in Figure 5c). The zwitterionic trans-conformation of crystal 1 was transformed into the dianionic trans-conformation of 12- and/or into the monoanionic intramolecular hydrogen-bonded cis-conformation of 1- during the PA adsorption-desorption cycle. (a)

(b) - PA + 2PA H N

-

-

+

+ PA

12-

1 SO3

SO3 S

S N

S O H

1-

O

-

SO3

N HO

Figure 4. Stepwise PA adsorption-desorption cycle coupled with ESIPT fluorescence chromism in crystal 1. (a) Change in the solid state emission spectra of 1 with the PA adsorption-desorption cycle. (b) Photographs of the emission colour change from 1, to 12•(HPA+)2, to 1-•(HPA+) under irradiation of 365 nm with a UV lamp and molecular conformation for each state.

The adsorption-desorption cycle of various types of amines on crystal 1 also exhibited ESIPT fluorescence chromism (Figure S12). The molecular adsorption of OA, DEA, and TEA on crystal 1 was evaluated by TG analysis (Figure S12a). The OA and DEA molecules showed full adsorption-desorption cycles on crystal 1, with crystal weight enhancements of 44.5 and 31.5% corresponding to the formation of 12-•(HOA+)2 and 12•(HDEA+)2, respectively. On the contrary, only 30% weight enhancement was observed in 1•(TEA)0.3 crystals following TEA re-adsorption, and strong green ESIPT fluorescence only occurred at the crystal surface. The bulky TEA molecule was insufficiently adsorbed on crystal 1. The TG analysis of 12•(HOA+)2 and 12-•(HDEA+)2 crystals indicated weight losses of 19.6 and 23.3%, respectively, which were consistent with the desorption of one mole of amine to form 1-•(HOA+) and 1-


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The Journal of Physical Chemistry •(HDEA+). Further heating of these crystals caused decomposition, similar to the PA adsorption-desorption cycle, and these OA and DEA adsorption-desorption cycles exhibited ESIPT fluorescence chromism. The 12-•(HOA+)2 and 12-•(HDEA+)2 crystals showed strong blue fluorescence at λFmax = 432 and 442 nm, respectively, which were 10 and 20 nm blue-shifted from that of crystal 1. After desorption of one mole of OA and DEA from the corresponding 1:2 crystals, strong green ESIPT fluorescence appeared at λFmax = 520 nm for both 1-•(HOA+) and 1•(HDEA+) crystals. (a)

12(b)

1(c)

low concentrations of NH3 (c < ~24,000 ppm) a strong green ESIPT fluorescence appeared at λFmax = 515 nm owing to the formation of monoanionic 1-•(NH4+). The fluorescent NH3 sensing of solid-state 1 at the detection limit, c~10 ppm, exhibited a multicolour, highly sensitive, and large dynamic range owing to the ESIPT chromic behaviour. In addition, there is an ability to reserve neutral NH3 into molecule 1 in the solid state. The high sensing ability of crystal 1 can be extended to other biologically important amines such as DAB and HA, which are well known for their putrid odour and biological activity.23, 24 Unfortunately, amino acids were inert towards switching ESIPT chromism because of their zwitterionic molecular structures (Figure S13b to 13f). When the NaCl powder containing diluted HA was put on cast films of 1, a drastic fluorescent response was observed on the contact surface area from blue emission at c ~1,000 ppm to strong ESIPT green fluorescence at c ~10 ppm (Figure 6c, right). The DAB molecule also shows strong blue fluorescence with vapour diffusion, detected as a fluorescent colour change from the non-ESIPT weak blue emission at c ~1,000 ppm to the non-ESIPT strong blue emission at c ~10 ppm. A large number of primary and secondary amines can potentially be detected by ESIPT chromism in solid state sensing devices even at low concentrations; thus, solid state sensing devices can be applied to biological and environmental systems. The stepwise ESIPT chromism from the 1, 1-, to 12- states revealed multicolour fluorescent responses for the target amines. (a)

1NH3

Thin film 1

Rev. 12Ir-

High NH3 (b)

Low NH3

1

Figure 5. Crystal structure of 12-•(HPA+)2 and 1-•(HPA+). (a) Dianionic structure of 12- bearing both a –SO3- and an –O- group in 12-•(HPA+)2 crystals. (b) Monoanionic structure of 1- bearing a – SO3- group in 1-•(HPA+) crystals. (c) PXRD patterns of crystal 1 during the PA adsorption-desorption cycles. (i) PXRD pattern of powder 1. (ii) Simulated PXRD pattern of a single crystal of 12•(HPA+)2, (iii) PXRD pattern of powder 1 after PA adsorption, (iv) Simulated PXRD pattern of a single crystal of 1-•(HPA+), and (v) PXRD pattern of powder 1-•(HPA+) after PA desorption from 12•(HPA+)2.

Application in molecular sensing devices. The switching ESIPT fluorescence chromism of crystal 1 can be applied in molecular sensing devices for biologically important amines such as NH3, HA, and DAB. Cast films of crystalline 1 were fabricated onto a glass substrate and were exposed to NH3 vapour. Figures 6a and 6b show the concentration (c)-dependent fluorescent NH3 sensing. Highly concentrated NH3 vapour (81,000 < c < 380,000 ppm) drastically changed the non-fluorescent state to strong blue emission at λFmax = 450 nm due to the formation of 1-2•(NH4+)2 upon proton transfer from 1 to NH3 with the corresponding conformational change. However, at

(c)

Film

High HA

HA powder

Low HA

Figure 6. NH3 and HA sensing using ESIPT chromism. (a) Schematic illustration of the NH3 sensing procedure and photographs of thin films of 1 with different c of NH3 at the excitation wavelength of 340 nm. c = 380,000, 150,000, 81,000, 24,000, 2,300, 230, 70,



20, 10, and 0 ppm. (b) Fluorescence spectra of thin films of 1 after NH3 sensing at the excitation wavelength of 340 nm. (c) Schematic illustration of solid-state sensing of HA dispersed into NaCl powder on thin films of 1. Photograph of thin films of 1 for HA dispersed into NaCl powder: c = 1,000 (left), 100, to 10 ppm (right).

CONCLUSIONS Sulfonic acid was introduced into the fluorescent HBT derivative 1 and formed a cis-conformation zwitterion through an intramolecular acid-base reaction, which exhibited weak blue non-ESIPT fluorescence. Crystal 1 showed reversible adsorption-desorption cycles while maintaining the high crystallinity of the structure for organic bases such as Py, Ani, Thz, and Quino, which drastically changed the emission colour from weak blue to strong ESIPT green emission. In addition, crystal 1 underwent adsorption-desorption cycles for various types of amines (AA) such as NH3, PA, OA, DEA, and DAB. The irreversible adsorption of 1 → 1•(AA)2 and the reversible adsorption of 1•(AA)2 ⇆ 1•(AA) were observed as ESIPT fluorescence chromism. The fluorescent colours successively changed from the initial weak blue of 1, to the strong blue of 1•(AA)2, and finally to the strong ESIPT green of 1•(AA). The ESIPT fluorescence chromism was achieved by the structural transformation between the cis- and trans-conformations accompanied by a dynamic crystal lattice deformation during the molecular adsorption-desorption cycles and was applied in a solid state sensing device for biologically important amines such as NH3, DAB, and HA. High sensitivity and concentration dependent detection for biological indicators is progressed in organic bases and acids based on the solid state ESIPT fluorescence switching device.

ASSOCIATED CONTENT Supporting Information. Experimental section, NMR spectra of 1, IR spectra of HBT, 1, 1•(Py), 1•(PA)2, and 1•(PA), excitation spectra of 1, absorption and emission spectra of HBT and 1, solid state UV-vis absorption spectra of 1, 1•(Py), 1•(PA)2, and 1•(PA), packing structure of crystal 1, packing structure of crystal 1•(PA)2, DFT calculations cis- and trans-conformation of 1-, packing structure of crystal 1•(PA), adsorption and fluorescent chromism for various kinds of aromatic organic bases, adsorption and fluorescent chromism for various kinds of alkylamine derivatives, sensing amino acid by thin films. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas ‘π-Figuration’ (JP26102007), KAKENHI Kibankenkyu (B) (JP15H03791), JSPS Research Fellow (16J03265) and ‘Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials’ from MEXT.

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8 ACS Paragon Plus Environment

ESIPT Fluorescent Chromism and Conformational Change of 2-(2

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