Conformationally locked salicylideneaniline derivatives with strong ESIPT fluorescence

Accepted Manuscript Conformationally locked salicylideneaniline derivatives with strong ESIPT fluorescence Jiun-Wei Hu, Hsing-Yang Tsai, Sin-Kai Fang, Chia-Wei Chang, Li-Ching Wang, KewYu Chen PII:

S0143-7208(17)31107-5

DOI:

10.1016/j.dyepig.2017.06.037

Reference:

DYPI 6058

To appear in:

Dyes and Pigments

Received Date: 12 May 2017 Revised Date:

14 June 2017

Accepted Date: 14 June 2017

Please cite this article as: Hu J-W, Tsai H-Y, Fang S-K, Chang C-W, Wang L-C, Chen K-Y, Conformationally locked salicylideneaniline derivatives with strong ESIPT fluorescence, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.06.037. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT

Conformationally locked salicylideneaniline derivatives with strong ESIPT fluorescence Jiun-Wei Hu1, Hsing-Yang Tsai1, Sin-Kai Fang, Chia-Wei Chang, Li-Ching Wang, Kew-Yu Chen*

RI PT

Department of Chemical Engineering, Feng Chia University, 40724 Taichung, Taiwan, ROC

Abstract: Three conformationally locked salicylideneaniline derivatives, 1a–1c, were synthesized and

SC

characterized by single-crystal X-ray diffraction. Compound 1a possesses a triply locked configuration, i.e., the intramolecular five-membered-ring C–H⋅⋅⋅N and six-membered-ring O–H⋅⋅⋅N hydrogen bonds and

M AN U

five-membered-ring C(1-6-7-8-9) cyclization, from which the excited-state intramolecular proton transfer takes place, resulting in a record high tautomer emission quantum yield of 0.28 in the solid state. Compared with salicylideneaniline, a substantial increase in the emission quantum yield is also observed for 1b and 1c. Furthermore, compound 1a shows pH-dependent optical properties and a highly reversible response to pH,

TE D

which makes it good candidate for potential applications in pH sensing. Time-dependent density functional theory calculations are reported on these salicylideneaniline derivatives in order to rationalize their electronic

AC C

EP

structure and optical properties.

Keywords. ESIPT; Tautomer; Salicylideneaniline derivatives; Stokes shift; X-ray diffraction; DFT calculations. *Corresponding author. Tel: +886 4 24517250 ext 3683; fax: +886 4 24510890 (K.-Y. Chen). E-mail: [email protected]. 1These authors contributed equally to this work. 1

ACCEPTED MANUSCRIPT

1. Introduction Organic fluorescent materials that exhibit excited-state intramolecular proton transfer (ESIPT)

RI PT

characteristics have continuously been attracting attention due to their unique photophysical properties [1–22]. An ESIPT reaction usually involves the transfer of a hydroxyl proton to an acceptor such as imine nitrogen

SC

through a strong intramolecular hydrogen bond. As shown in Fig. 1, molecules that show ESIPT in the ground state exist predominantly as enol (E) forms; however, upon photoexcitation, they undergo tautomerization into

M AN U

keto forms (E* → K*) via an ultra-fast ESIPT reaction occurring in the subpicosecond time domain [23]. Then the keto form in the excited state deactivates to the ground state (K* → K). Finally, the keto form (K) is transformed to the enol form (E) via a reverse ground-state proton transfer reaction. Due to the drastic structural

TE D

alternations, the keto tautomer shows considerably different optical properties compared to its normal form (E). Thus the ESIPT molecules usually emit a large Stokes-shifted fluorescence (K* → K). This unusual optical property has many important applications, typical examples of which are probes for solvation dynamics [24–26]

EP

and biological environments [27,28], chemosensors [29–33], nonlinear optical materials [34], photochromic

AC C

materials [35], and organic light-emitting diodes (OLEDs) [36–38]. To be useful in real applications, it is important to develop highly emissive ESIPT chromophores operating not only in the solution phase but also in the solid state. Unfortunately, in most cases, the emission efficiency of the large Stokes shifted K* emission is quite low due to the presence of unfavorable non-radiative deactivation pathways of K* states, which is regarded as the major drawback of the ESIPT molecules [39,40]. < Please Insert Fig. 1> Salicylideneaniline (Scheme 1), as well as its derivatives, displays a very weak ESIPT fluorescence in both 2

ACCEPTED MANUSCRIPT solution and solid state [41]. It has been widely reported that the lack of emission is chiefly due to the conformational isomerization via rotation of C(1)–C(9)–N bonds (for numbering, see scheme 1), which acts as the major non-radiative deactivation pathway. In an effort to expand the scope of the salicylideneaniline

RI PT

molecules available for highly fluorescent organic materials, the present research reports the synthesis of three conformationally locked salicylideneaniline-based chromophores (1a–1c) as well as their spectroscopic and electrochemical properties, single-crystal X-ray structures, and complementary time-dependent density

SC

functional theory (TD-DFT) calculations. Both 1a and 1b show aggregation-enhanced emission (AEE)

M AN U

characteristics and intense solid-state fluorescence, while 1c exhibits the traditional aggregation-caused quenching (ACQ). The results offer the potential for synthesizing salicylideneaniline derivatives with extended molecular architectures and attractive optical properties.

2.1 Chemicals and instruments starting

materials

such

as

7-hydroxy-1-indanone,

aniline,

2,6-diisopropylaniline,

EP

The

TE D

2. Experimental

1-aminonaphthalene, acetic acid (HOAc), and ethanol (EtOH) were purchased from Merck, ACROS and

AC C

Sigma-Aldrich. Solvents were freshly distilled according to standard procedure. Column chromatography was performed using silica gel Merck Kieselgel si 60 (40-63 mesh). 1

H and 13C NMR spectra were recorded in CDCl3 on a Bruker 400 MHz spectrometer. Mass spectra

were recorded on a VG70-250S mass spectrometer. The absorption and emission spectra were measured using a Jasco V-570 UV-Vis spectrophotometer and a Hitachi F-7000 fluorescence spectrophotometer, respectively. The single-crystal X-ray diffraction data were collected on a Bruker Smart 1000CCD area-detector diffractometer. The redox potentials were measured using cyclic voltammetry on a CHI 620 3

ACCEPTED MANUSCRIPT analyzer. The data were collected and analyzed using electrochemical analysis software. All measurements were carried out in dichloromethane containing 0.1 M tetrabutylammonium hexaflourophosphate (TBAPF6) as the supporting electrolyte under ambient conditions after purging for 15 min with N2. The

RI PT

conventional three-electrode configuration was employed, which consisted of a glassy-carbon working electrode, a platinum counter electrode, and a Ag/AgNO3 (0.1 M) reference electrode calibrated with ferrocene/ferrocenium (Fc/Fc+) as an internal reference.

SC

2.2 Synthesis and Characterization 2.2.1 General procedure for condensation

M AN U

The general procedure for the synthesis of Schiff bases (1a–1c): To a stirred mixture of 7-hydroxy-1-indanone (4.7 mmol) and molecular sieves 4Å (0.5 g) in ethanol (25 mL) was added 2,6-diisopropylaniline (aniline or 1-aminonaphthalene, 4.7 mmol) and acetic acid (0.1 mL) at ambient temperature. The mixture was refluxed for 12 h. After cooling, the mixture was poured into the cold water and

TE D

extracted with CH2Cl2 and dried with anhydrous MgSO4. After solvent was removed, the crude product was purified by silica gel column chromatography with eluent CH2Cl2 to afford 1a (80% yield)/1b (86% yield)/1c (72% yield). Characterization data: 1a: M.p. 71–72 ℃; 1H NMR (400 MHz, CDCl3) δ 11.25 (br, 1H), 7.33 (t, J

EP

= 8.0 Hz, 1H), 7.10–7.16 (m, 3H), 6.88 (d, J = 7.6 Hz, 1H), 6.83 (d, J = 8.7 Hz, 1H), 3.05 (t, J = 5.7 Hz, 2H), 2.91 (m, 2H), 2.44 (t, J = 5.8 Hz, 2H), 1.17 (d, J = 6.2 Hz, 6H), 1.13 (d, J = 7.3 Hz, 6H); 13C NMR (100 MHz,

AC C

CDCl3) δ 179.21, 158.08, 150.87, 145.17, 137.29, 134.07, 124.58, 123.22, 116.14, 113.32, 30.08, 28.27, 28.12, 23.71, 22.82; IR (KBr): 3506, 3058, 2962, 2927, 1635, 1465, 1295, 1214, 1184, 821, 767, 725 cm-1; MS (FAB) m/z (relative intensity) 308 (M + H+, 100); HRMS calcd. for C21H26NO 308.2014, found 308.2012. Selected data for 1b: M.p. 56–57 ℃; 1H NMR (400 MHz, CDCl3, in ppm): δ 11.14 (br, 1H), 7.30–7.32 (m, 3H), 7.16 (t, J = 7.6 Hz, 1H), 7.03 (m, 2H), 6.85 (d, J = 7.6 Hz, 1H), 6.80 (d, J = 8.0 Hz, 1H), 3.08 (t, J = 5.8 Hz, 2H), 2.80 (t, J = 5.8 Hz, 2H); 13C NMR (100 MHz, CDCl3) δ 178.44, 157.96, 150.55, 149.42, 134.03, 129.10, 124.61, 123.95, 121.06, 115.97, 113.31, 29.31, 28.41; IR (KBr): 3355, 3054, 2923, 2854, 1631, 1592, 1469, 1265, 1207,

4

ACCEPTED MANUSCRIPT

825, 767, 701 cm-1; MS (FAB) m/z (relative intensity) 224 (M + H+, 100); HRMS calcd. for C15H14NO 224.1075, found 224.1073. Selected data for 1c: M.p. 127–128 ℃; 1H NMR (400 MHz, CDCl3) δ 11.28 (br, 1H), 7.97 (d, J = 8.0 Hz, 1H), 7.86 (d, J = 8.0 Hz, 1H), 7.66 (d, J = 8.0 Hz, 1H), 7.35–7.51 (m, 3H), 7.02 (d, J = 7.5 Hz, 1H), 6.89 (d, J = 8.0 Hz, 1H), 6.86 (m, 2H), 3.07 (t, J = 5.8 Hz, 2H), 2.72 (t, J = 5.8 Hz, 2H); 13C NMR

RI PT

(100 MHz, CDCl3) δ 179.64, 157.94, 150.89, 146.01, 134.16, 127.95, 126.73, 126.21, 125.76, 125.68, 124.53, 123.81, 123.27, 116.07, 114.82, 113.29, 29.64, 28.18; IR (KBr): 3359, 2919, 2850, 1627, 1465, 1388, 1261, 1214, 779, 713 cm-1; MS (FAB) m/z (relative intensity) 274 (M + H+, 100); HRMS calcd. for C19H16NO

SC

274.1232, found 274.1240. 2.3 Computational methods

M AN U

All the electronic structure calculations were carried out using the Gaussian 03 program [42]. All the geometry optimizations for compounds 1a–1c in the ground and the first excited states were performed using density functional theory (DFT) and time-dependent DFT (TDDFT) with the 6-31G* basis set and the B3LYP functional. The hybrid DFT functional B3LYP has proven to be a suitable DFT functional to describe hydrogen

TE D

bonding [43]. The stability (EHB) caused by the intramolecular hydrogen bond of the enol form can be calculated through the difference in energy between the structure containing the hydrogen bond (closed form) and that in which the C1C2OH dihedral angle is rotated to 180° (open form) without further geometry

EP

optimization. Vibrational frequencies were also performed to check whether the optimized geometrical structures for all compounds were at energy minima, transition states, or higher order saddle points. After

AC C

obtaining the converged geometries, the TD-B3LYP/6-31G* was used to calculate the vertical excitation energies. Emission energies were obtained from TDDFT/B3LYP/6-31G* calculations performed on S1 optimized geometries. As observed by the only slightly solvent polarity dependent shift of the emission (absorption) spectra in 1a–1c, the charge transfer character of tautomer emission for 1a–1c is slim. Thus, solvent effects are not considered throughout these computations.

3. Results and Discussion 5

ACCEPTED MANUSCRIPT 3.1 Synthesis and characterization Scheme 1 shows the chemical structures and the synthetic route to the conformationally locked salicylideneaniline derivatives 1a–1c. These Schiff bases were prepared through condensation reactions between 7-hydroxy-1-indanone (2) and aromatic amines [17]. Detailed synthetic procedures and product

RI PT

characterizations are provided in the Experimental section and Supplementary data. All obtained products are soluble in common organic solvents, such as acetone, tetrahydrofuran (THF) and dichloromethane, whereas they are insoluble in water.

SC

< Please Insert Scheme 1> 3.2 Hydrogen bond studies

M AN U

The dominance of an enol form for salicylideneaniline [44] and 1a–1c, namely the intramolecular hydrogen-bond formation between O–H and N, is supported by a combination of 1H NMR and X-ray single-crystal analyses. In the 1H NMR studies, the existence of a strong hydrogen bond between O–H and N is supported by the observation of a large downfield shift of the proton peak at δ > 11 ppm for all Schiff bases, the

TE D

values of which are in the order salicylideneaniline (13.26 ppm) > 1c (11.28 ppm) ≈ 1a (11.25 ppm) > 1b (11.14 ppm) in CDCl3 (Table 1). The hydrogen bonding energy (∆E in kcal/mol) of salicylideneaniline and 1a–1c can be empirically calculated by introducing Schaefer’s correlation [45], expressed as ∆δ = (–0.4 ± 0.2) + ∆E, where

EP

∆δ is given in parts per million for the difference between chemical shift in the O–H peak of salicylideneaniline

AC C

and 1a–1c and that in phenol (δ 4.29). Accordingly, the hydrogen-bonding energy is calculated to be salicylideneaniline (9.37 ± 0.2 kcal/mol) > 1c (7.39 ± 0.2 kcal/mol) ≈ 1a (7.36 ± 0.2 kcal/mol) > 1b (7.25 ± 0.2 kcal/mol), which is consistent with the theoretical calculations (Table 1 and Figs. S1–S4). It is apparent that the conformationally locked salicylideneaniline derivatives 1a–1c have a weaker intramolecular hydrogen bond than salicylideneaniline. This is possible due to the fact that the imine nitrogen locates at the five-membered-ring cyclopentanimine moiety (Figs. 2–4), such that the ∠O–H–N angle is expected to be much deviated from 120o (a perfect six-membered-ring hydrogen bonding formation), resulting in a longer O–H–N hydrogen bond distance relative to salicylideneaniline (vide infra). Additionally, an increase in the hydrogen bonding strength

6

ACCEPTED MANUSCRIPT

upon increasing electron-donating properties of the N-substituted aromatic ring is evident. Note that the substitution of the hydrogen atoms in 1b by electron-donating isopropyl groups, forming 1a, seems to increase the basicity of imine through an inductive effect. As a result, 1a exhibits a small downfield shift of the hydroxyl proton, and hence, a stronger hydrogen bond relative to 1b.

RI PT

3.3 X-ray structures Molecular structures of 1a–1c were further confirmed by single-crystal X-ray diffraction analyses. Crystallographic data and refinement details are summarized in Table 2. Compound 1a crystallizes in the

SC

monoclinic space group P21/c, while compounds 1b and 1c crystallize in the orthorhombic space groups Pna21

M AN U

and P212121, respectively. They all adopt the respective E conformations and possess an intramolecular O–H⋅⋅⋅N hydrogen bond (Table 3).

Fig. 2 shows the molecular structure of 1b. The dihedral angle between the mean planes of the phenyl ring (C10–C15) and the indaneimine ring (C1–C9/N) is 39.66(2)o. The twisted conformation diminishes the

TE D

π-conjugation to some extent, thus resulting in a blue shift in the absorption spectrum compared to salicylideneaniline (vide infra). The crystal structure is stabilized by intermolecular C–H⋅⋅⋅π interactions (Fig.

EP

2b). Pertinent measurements for C(8)–H(8A)⋅⋅⋅Cg1 and C(8)–H(8B)⋅⋅⋅Cg1 are distances of 2.80 (symmetry

AC C

code: 1–X, 1–Y, 1/2+Z) and 2.88 Å (symmetry code: 1–X, 1–Y, –1/2+Z), respectively. Moreover, molecules are linked by intermolecular C(4)–H(4)···O hydrogen bonds (2.52 Å of H(4)⋅⋅⋅O distance and 147o of C(4)–H(4)–O), symmetry code: –1/2+X, 3/2–Y, Z) to form an infinite one-dimensional chain along the [100] direction that are connected to one another via intermolecular C–H⋅⋅⋅π interactions, so linking the molecules into a continuous two-dimensional framework (Fig. 2c). < Please Insert Fig. 2>

7

ACCEPTED MANUSCRIPT Fig. 3 shows the molecular structure of 1a. Besides the intramolecular O–H⋅⋅⋅N hydrogen bond, 1a also possesses two different intramolecular C–H⋅⋅⋅N hydrogen bonds [46,47] that generate two S(5) ring motifs. Pertinent measurements for C(16)–H(16)⋅⋅⋅N and C(19)–H(19)⋅⋅⋅N are distances of 2.47 and 2.36 Å,

RI PT

respectively. The non-classical C–H⋅⋅⋅N hydrogen bonds further stabilize its structure and lead it to form a rigid configuration that can partially inhibit the rotation of the phenyl ring (C10–C15) through the N–C(10) bond. The dihedral angle between the mean planes of the phenyl ring and the indaneimine ring (C1–C9/N) is

SC

76.77(2)o, which is much larger than that of 1b (Fig. 3b). This highly twisted conformation is caused by the

M AN U

introduction of the bulky isopropyl groups at the phenyl ring and results in a blue shift of absorption edge compared to 1b (vide infra). In addition, the molecules in the crystal structure are linked by two different types of intermolecular C–H⋅⋅⋅O hydrogen bonds to form a continuous two-dimensional framework (Fig. 3c). < Please Insert Fig. 3>

TE D

Fig. 4 shows the molecular structure of 1c. Besides the intramolecular O–H⋅⋅⋅N hydrogen bond, compound 1c also possesses an intramolecular C–H⋅⋅⋅N hydrogen bond (2.53 Å of C(16)–H(16A)⋅⋅⋅N distance and 100o of C(16)–H(16A)–N) that generates an S(5) ring motif (Fig. 4a). The dihedral angle (Fig. 4b) between the mean

EP

planes of the phenyl ring (C10–C15) and the indaneimine ring (C1–C9/N) is 63.37(2)o, which is larger than that of 1b, probably due to the existence of intramolecular C–H⋅⋅⋅N hydrogen bond. In the crystal (Fig. 4c),

AC C

molecules are stabilized by intermolecular C–H⋅⋅⋅π (symmetry code: –1/2+X, 1/2–Y, 2–Z) and π⋅⋅⋅π interactions (symmetry code: 1/2+X, 1/2–Y, 2–Z). Furthermore, the distance between O and N along the O–H⋅⋅⋅N hydrogen bond is in the order of 1c (2.768(2) Å) > 1a (2.726(3) Å) ≈ 1b (2.700(3) Å) > salicylideneaniline (2.615(3) Å) and consistent with the hydrogen-bonding strength estimated from 1H NMR measurements (vide supra). < Please Insert Fig. 4> 3.4 Photophysical properties in solution

8

ACCEPTED MANUSCRIPT The absorption and fluorescence spectra of 1a–1c were measured in solvents of different polarities. Table 4 summarizes the respective photophysical data of 1a–1c. Fig. 5a reveals the absorption spectra of 1a–1c in cyclohexane. Compounds 1a–1c show the lowest lying absorption band maximized at 313, 315, and 323 nm,

RI PT

with molar absorptivities of 1.0 × 104 M-1 cm-1, 3.6 × 104 M-1 cm-1, and 1.9 × 104 M-1 cm-1, respectively. These peaks are assigned to the π → π* transitions, which are further supported by the calculated frontier orbitals (vide infra). The longest wavelength absorption band of 1b is slightly red-shifted relative to that of 1a,

SC

but it is blue-shifted relative to that of 1c. Obviously, the conjugation effect of the naphthalene ring in 1c causes

M AN U

an additional red shift. The energy bandgap is in the order of 1a > 1b > 1c, which is consistent with the theoretical calculations (Table 5). It should be noted that compounds 1a–1c have a ground-state geometry in which the N-substituted aromatic ring is twisted relative to the indaneimine unit (Figs. 2–4), which may diminish the π-conjugation to some extent.

TE D

< Please Insert Fig. 5>

The fluorescence spectra of 1a–1c in cyclohexane are shown in Fig. 5b. Schiff bases 1a–1c all exhibit a

EP

sole, anomalously long wavelength emission (> 500 nm) in aprotic and protic solvents of varying polarities

AC C

(Table 4). The Stokes shift of the emission, defined by peak (absorption)-to-peak (emission) gap in terms of frequency, is determined to be > 11000 cm-1 for 1a–1c (Table 4). Accordingly, the assignment of 504–528 nm emission for 1a–1c in cyclohexane to a proton-transfer tautomer emission is unambiguous, and ESIPT takes place from the phenolic proton (O–H) to the N nitrogen, forming the keto-tautomer species. Comparing the weak tautomer emission (0.7 × 10-4) of salicylideneaniline in solution [41], the 1a (1b or 1c) tautomer emission quantum yield of 7.5 × 10-2 (3.2 × 10-2 or 1.1 × 10-2) in cyclohexane is about 1071 (461 or 159) times higher than that of salicylideneaniline in the same solvent. Moreover, the fluorescence quantum yield of 1a–1c is quite 9

ACCEPTED MANUSCRIPT insensitive to solvent polarity (Table 4). The results show that the inherent rotational motion of the salicylideneaniline moiety in 1a–1c has been largely restricted by the bridge group C(9)–C(8)–C(7)–C(6) inhibiting the phenol ring twist around the C(1)–C(9) bond (Figs. 2–4). The tendency of the spectral shift can be

RI PT

explained by the fact that the addition of the bulky isopropyl groups (naphthalene ring) decreases (increases) the effective conjugation length, thereby increasing (decreasing) the energy gap (1c: 528 nm > 1b: 521 nm > 1a: 504 nm). This viewpoint can be further supported by a theoretical approach based on time-dependent density

SC

functional theory (TD-DFT, see 3.8).

M AN U

3.5 Aggregation-enhanced emission (AEE)

To investigate the emission properties of 1a–1c in the aggregated state, fluorescence spectra were measured in THF/water mixtures. The emission intensity of 1a and 1b increased gently with the formation of aggregates by the addition of water to their THF solutions, as shown in Fig 6. For 1a (1b), its emission reaches maximum at a water fraction of 70% (80%) and is lowered moderately with a further increase in the water

TE D

fraction. This phenomenon is consistent with those of previous studies on other salicylideneaniline derivatives [48], which indicate the AEE properties of both new chromophores. In contrast, compound 1c exhibits the

EP

traditional aggregation-caused quenching (ACQ). The most likely explanation is that the naphthalene ring from adjacent molecules of compound 1c can partly overlap each other in the aggregated state, rendering π–π

AC C

interactions and decreases emission efficiency. < Please Insert Fig. 6>

3.6 Solid-state emission

To further verify the AEE or ACQ characteristics of 1a–1c, their emission properties in the powder and crystal forms were recorded (Fig. 7 and Table 4). In the powder (crystal) forms, the emissions of 1a and 1b are enhanced. Compounds 1a and 1b show fluorescence peaks at 506 (507) and 522 (525) nm, respectively, with minor red-shifts relative to those in solutions. The fluorescence quantum yields are increased to 0.271 (0.285) 10

ACCEPTED MANUSCRIPT and 0.124 (0.135) for 1a and 1b, respectively, as measured by a calibrated integrating sphere. These values are much higher than that in THF solution, which further prove that 1a and 1b exhibit the AEE feature. Although the locked conformation greatly rigidifies their molecular structures, there is still a phenyl rotor in both 1a and

RI PT

1b. The phenyl group can rotate in solution, and nonradiatively deactivate the excited-state of the compound to some degree. In the solid state, the intramolecular rotation of the phenyl group is restricted, thereby allowing the chromophores to emit more strongly. On the contrary, 1c emits relatively weakly in the solid state (Φ < 0.001),

SC

which further validate that 1c possesses the traditional ACQ feature. Additionally, the inset of Fig. 7 shows

M AN U

photos of the powders of 1a and 1b under UV light (365 nm). Compounds 1a and 1b emit strong bluish-green and greenish-yellow fluorescence, respectively, which demonstrates that the subtle structural change in 1a and 1b has an appreciable effect on the optical properties.

3.7 Electrochemical properties

TE D

< Please Insert Fig. 7>

EP

The cyclic voltammograms of 1a–1c are illustrated in Fig. 8. During anodic scans between 0 and 2 V in dichloromethane, only one oxidation peak is observed at 1.09, 1.01 and 1.09 V for 1a, 1b and 1c, respectively. A

AC C

one-electron irreversible oxidation wave observed for all compounds is attributed to the oxidation of the salicylideneaniline-based conjugated system. Table 5 summarizes the oxidation potentials and the HOMO and LUMO energy levels estimated from cyclic voltammetry (CV) for 1a–1c. The HOMO/LUMO energy levels of 1a, 1b, and 1c are estimated to be –5.68/–2.03, –5.60/–2.17, and –5.68/–2.47 eV, respectively, which are in good agreement with the theoretical calculations (Table 5). < Please Insert Fig. 8> 3.8 Quantum chemistry computation 11

ACCEPTED MANUSCRIPT For a deeper insight into the molecular structures and electronic properties of 1a–1c, quantum chemical calculations were performed using density functional theory (DFT) at the B3LYP/6-31G* level. Fig. 9 shows the HOMO and LUMO of the enol and keto form of 1b, where those of 1a and 1c can be found in the

RI PT

Supplementary data (Figs. S5 and S6). Upon the photoexcitation, the electron density formerly located on the hydroxyl oxygen O decreases while that on the imine nitrogen N increases, which demonstrates that the excitation from E to E* should involve intramolecular electron density transfer from O to N. Thus, the proton

SC

acceptor N is expected to be more basic, whereas the proton donor O is more acid with respect to the ground

M AN U

state (E), driving the proton transfer from O to N. The energy level of the LUMO decreases from −1.31 to −1.77 eV with completion of the ESIPT reaction, indicating that it is thermodynamically favorable enough to drive the production of the excited-state keto tautomer. In contrast, the electron density around the intramolecular hydrogen binding site is chiefly populated at amino nitrogen N and carbonyl oxygen O at LUMO (K*) and

TE D

HOMO (K), respectively, driving the ground-state intramolecular proton transfer (GSIPT). It can be also seen that the first excited states of 1a–1c for both enol and keto forms are a dominant π → π* transition from the

EP

HOMO to the LUMO. In addition, the absorption and fluorescence spectra of 1a–1c were calculated by time-dependent DFT calculations (Franck–Condon principle, Table 6). The calculated excitation/emission

AC C

wavelengths for the S0 → S1/S1 → S0 transitions are 299/494 nm for 1a, 318/525 nm for 1b, and 358/547 nm for 1c, which is consistent with the experimental results.

< Please Insert Fig. 9>

3.9 The pH dependence of absorption and fluorescence spectra The optical response of 1a–1c to pH was also examined. Fig. 10a shows the absorption spectra change of 1a at different pH values. Upon decreasing the pH from 7.0 to 4.0, the absorption spectra of 1a do not show any 12

ACCEPTED MANUSCRIPT substantial change. When the pH is decreased from 4.0 to 1.0, the absorbance at 314 (260) nm is reduced, and concomitantly, a new red-shifted absorption band at 362 (281) nm appears. Three clear isosbestic points are observed at 272, 308, and 330 nm, which indicate a clean transformation to a new species. The long wavelength

RI PT

absorption band (362 nm) is thus assigned to its cationic form, generated from the protonated species of 1a in the ground state (Scheme 2). The red shift of the cationic absorption band confirms that the intramolecular charge transfer (ICT) effect of 1a is enhanced with decreasing pH because of the proton binding-induced

SC

enhancement of the electron-withdrawing ability of the imine. Fig. 10b depicts the emission spectra change of

M AN U

1a at different pH values. With the variation of pH from 7.0 to 1.0, compound 1a shows the unexpected turn-on fluorescence [48]. When the pH value is between 4.0 and 7.0, compound 1a exhibits an ESIPT fluorescence centered at 506 nm. As the pH is decreased from 4.0 to 1.0, the fluorescence intensity is enhanced dramatically and the emission spectra show a slightly blue shift from 506 to 502 nm. The band at 502 nm is again assigned to

TE D

the protonated species of 1a. The inset in Fig. 10b depicts the results of sigmoidal fitting of the pH-dependent fluorescence at 502 nm, affording a pKa value of 2.45. Furthermore, the emission intensity displays excellent

EP

linearity with pH in the range 1.6–3.4, demonstrating that compound 1a could be used to detect pH

AC C

quantitatively in extreme acidity.

< Please Insert Fig. 10>

< Please Insert Scheme 2>

The reversible nature of 1a was also examined by recording the emission intensity at 502 nm with respect to the change of pH from 1.6 to 7.0. As shown in Fig. S7, the results clearly indicate that compound 1a exhibits a fully reversible response to pH, and the response and recovery times in different pH solutions are rapid (within seconds). Moreover, 1H NMR experiments were carried out to study the changes of protons. Fig. S8 shows the 1

H NMR spectra (aromatic proton peaks) of 1a under neutral and acidic conditions. In DMSO-d6/D2O mixtures, 13

ACCEPTED MANUSCRIPT 1a displays in its original form and its proton signals are well assigned with blue letters. After addition of trifluoroacetic acid (TFA) into the mixture, a significant downfield shift is observed for the chemical shift values of all aromatic protons as indicated with red letters. The downfield chemical shift of the aromatic protons is

RI PT

obviously due to H+ binding with imine nitrogen, which results in the decrease of electron density around these protons. Therefore, it is evidently that proton binding with imine nitrogen in 1a causes the significant optical response to acidic pH. Note that compounds 1b and 1c undergo irreversible reactions with acid (Figs. S9 and

SC

S11). The proposed mechanism that both 1b and 1c are hydrolyzed under acidic conditions is confirmed by 1H

M AN U

NMR titration spectra (Figs. S10 and S12).

4. Conclusions

Three conformationally locked salicylideneaniline derivatives (1a–1c) have been successfully synthesized and fully characterized. They all adopt the respective E conformations and possess an intramolecular O–H⋅⋅⋅N

TE D

hydrogen bond, from which ESIPT takes place, resulting in a proton-transfer tautomer emission. The results prove that inhibiting the inherent rotational motion of salicylideneaniline by either cyclization involving

EP

C(1)–C(9) single bond or forming the intramolecular C–H⋅⋅⋅N hydrogen bond to restrict the C(9)–N–C(10) motion will also lead to a suppression of radiationless deactivation and hence an increase in fluorescence

AC C

quantum yield. Additionally, both 1a and 1b show AEE and excellent solid-state fluorescence efficiency (0.12–0.28), while compound 1c possesses the traditional ACQ feature. Compound 1a exhibits pH-dependent optical properties and a highly reversible response to pH, which makes it good candidate for potential applications in pH sensing. Future applications of this series of ESIPT fluorescent dyes can be greatly expanded. For example, by fusing benzene or naphthalene rings at C(7)–C(8) positions [37], synthesis of white-light-emitting small molecules can be achieved, making their application in single-molecule-based white-light-emitting OLEDs more feasible. Work focusing on this issue is currently in progress.

14

ACCEPTED MANUSCRIPT Acknowledgment

The project was supported by the Ministry of Science and Technology (MOST 106-2113-M-035-001) in Taiwan.

RI PT

Appendix. Supplementary data

SC

Supplementary data associated with this article can be found, in the online version, at doi:

References

M AN U

[1] Zhao J, Ji S, Chen Y, Guo H, Yang P. Excited state intramolecular proton transfer (ESIPT): from principal photophysics to the development of new chromophores and applications in fluorescent molecular probes and luminescent materials. Phys Chem Chem Phys 2012;14:8803–17.

TE D

[2] Yang P, Zhao J, Wu W, Yu X, Liu Y. Accessing the long-lived triplet excited states in bodipy-conjugated 2-(2-hydroxyphenyl) benzothiazole/benzoxazoles and applications as organic triplet photosensitizers for photooxidations. J Org Chem 2012;77:6166–78.

EP

[3] Xie L, Chen Y, Wu W, Guo H, Zhao J, Yu X. Fluorescent coumarin derivatives with large stokes shift, dual

AC C

emission and solid state luminescent properties: An experimental and theoretical study. Dyes Pigm 2012;92:1361–9. [4]

Satam

MA,

Raut

RK,

Telore

RD,

Sekar

N.

Fluorescent

acid

azo

dyes

from

3-(1,3-benzothiazol-2-yl)naphthalen-2-ol and comparison with 2-naphthol analogs. Dyes Pigm 2013;97:32–42. [5] Park S, Seo J, Kim SH, Park SY. Tetraphenylimidazole-based excited-state intramolecular proton-transfer molecules for highly efficient blue electroluminescence. Adv Funct Mater 2008;18:726–31. 15

ACCEPTED MANUSCRIPT [6] Park S, Kwon JE, Park SY. Strategic emission color tuning of highly fluorescent imidazole-based excited-state intramolecular proton transfer molecules. Phys Chem Chem Phys 2012;14:8878–84. [7] Mutai T, Sawatani H, Shida T, Shono H, Araki K. Tuning of excited-state intramolecular proton transfer

2013;78:2842–9.

RI PT

(ESIPT) fluorescence of imidazo[1,2-a]pyridine in rigid matrices by substitution effect. J Org Chem

[8] Yi PG, Liang YH. Theoretical studies of conjugation and substituent effect on intramolecular proton transfer

SC

in the ground and excited states. Chem Phys 2006;322:382–6.

M AN U

[9] Chung MW, Lin TY, Hsieh CC, Tang KC, Fu H, Chou PT, Yang SH, Chi Y. Excited-state intramolecular proton transfer (ESIPT) fine tuned by quinoline–pyrazole isomerism: π-conjugation effect on ESIPT. J Phys Chem A 2010;114:7886–91. [10]

Satam

MA,

Raut

RK,

Sekar

N.

Fluorescent

azo

disperse

dyes

from

2013;96:92–103.

TE D

3-(1,3-benzothiazol-2-yl)naphthalen-2-ol and comparison with 2-naphthol analogs. Dyes Pigm

EP

[11] Guo ZQ, Chen WQ, Duan XM. Seven-membered ring excited-state intramolecular proton-transfer in 2-benzamido-3-(pyridin-2-yl)acrylic acid. Dyes Pigm 2011;92:619–25.

AC C

[12] Zhong XL, Gao F, Wang Q, Li HR, Zhang ST. Excited state intramolecular proton transfer of novel conjugated derivatives containing hydroxy and imino groups. Chin Chem Lett 2010;21:1195–8. [13] Iijima T, Momotake A, Shinohara Y, Sato T, Nishimura Y, Arai T. Excited-state intramolecular proton transfer of naphthalene-fused 2-(2′-hydroxyaryl)benzazole family. J Phys Chem A 2010;114:1603–9. [14] Eseola AO, Li W, Sun WH, Zhang M, Xiao L, Woods JAO. Luminescent properties of some imidazole and oxazole based heterocycles: synthesis, structure and substituent effects. Dyes Pigm 2011;88:262–73. [15] Zhu Q, Wen K, Feng S, Wu W, An B, Yuan H, Guo X, Zhang J. Theoretical insights into the excited-state 16

ACCEPTED MANUSCRIPT intramolecular proton transfer (ESIPT) mechanism in a series of amino-type hydrogen-bonding dye molecules bearing the 10-aminobenzo[h]quinoline chromophore. Dyes Pigm 2017;141:195–201. [16] Patil VS, Padalkar VS, Tathe AB, Sekar N. ESIPT-inspired benzothiazole fluorescein: Photophysics of

RI PT

microenvironment pH and viscosity. Dyes Pigm 2013;98:507–17. [17] Luo MH, Tsai HY, Lin HY, Fang SK, Chen KY. Extensive spectral tuning of the proton transfer emission from green to red via a rational derivatization of salicylideneaniline. Chin Chem Lett 2012;23:1279–82.

SC

[18] Fang TC, Tsai HY, Luo MH, Chang CW, Chen KY. Excited-state charge coupled proton transfer reaction

M AN U

via the dipolar functionality of salicylideneaniline. Chin Chem Lett 2013;24:145–8. [19] Chen KY, Cheng YM, Lai CH, Hsu CC, Ho ML, Lee GH, Chou PT. Ortho green fluorescence protein synthetic chromophore; excited-state intramolecular proton transfer via a seven-membered-ring hydrogen-bonding system. J Am Chem Soc 2007;129:4534–5.

TE D

[20] Chen KY, Hsieh CC, Cheng YM, Lai CH, Chou PT. Extensive spectral tuning of the proton transfer emission from 550 to 675 nm via a rational derivatization of 10-hydroxybenzo[h]quinoline. Chem

EP

Commun 2006;42:4395–7.

[21] Piechowska J, Gryko DT. Preparation of a family of 10-hydroxybenzo[h]quinoline analogues via a

AC C

modified sanford reaction and their excited state intramolecular proton transfer properties. J Org Chem 2011;76:10220–8.

[22] Piechowska J, Huttunen K, Wróbel Z, Lemmetyinen H, Tkachenko NV, Gryko DT. Excited state intramolecular

proton

transfer

in

electron-rich

and

electron-poor

derivatives

of

10-hydroxybenzo[h]quinoline. J Phys Chem A 2012;116:9614–20. [23] Khan AU, Kasha M. Mechanism of four-level laser action in solution excimer and excited-state proton-transfer cases. Proc Natl Acad Sci U.S.A. 1983;80:1767–70. 17

ACCEPTED MANUSCRIPT [24] Chen WH, Pang Y. Excited-state intramolecular proton transfer in 2-(2′,6′-dihydroxyphenyl)benzoxazole: effect of dual hydrogen bonding on the optical properties. Tetrahedron Lett 2010;51:1914–8. [25] Zhao XD, Sun CJ, Yao QQ, Li WB. Synthesis of 3-hydroxyflavone fluorescent probes and study of their

RI PT

fluorescence properties. Chin Chem Lett 2010;21:529–32. [26] Lins GOW, Campo LF, Rodembusch FS, Stefani V. Novel ESIPT fluorescent benzazolyl-4-quinolones: synthesis, spectroscopic characterization and photophysical properties. Dyes Pigm 2010;84:114–20.

SC

[27] Maupin CM, Castillo N, Taraphder S, Tu C, McKenna R, Silverman DN, Voth GA. Chemical rescue of

M AN U

enzymes: proton transfer in mutants of human carbonic anhydrase II. J Am Chem Soc 2011;133:6223–34. [28] Lim CK, Seo J, Kim S, Kwon IC, Ahn CH, Park SY. Concentration and pH-modulated dual fluorescence in self-assembled nanoparticles of phototautomerizable biopolymeric amphiphile. Dyes Pigm 2011;90:284–9.

TE D

[29] Li T, Yang Z, Li Y, Liu Z, Qi G, Wang B. A novel fluorescein derivative as a colorimetric chemosensor for detecting copper (II) ion. Dyes Pigm 2011;88:103–8.

EP

[30] Zhang YJ, He XP, Hu M, Li Z, Shi XX, Chen GR. Highly optically selective and electrochemically active chemosensor for copper (II) based on triazole-linked glucosyl anthraquinone. Dyes Pigm 2011;88:391–5.

AC C

[31] Hong WH, Lin CC, Hsieh TS, Chang CC. Preparation of fluoroionophores based on diamine-salicylaldehyde derivatives. Dyes Pigm 2012;94:371–9. [32] Huang Q, Yang XF, Li H. A ratiometric fluorescent probe for hydrogen sulfide based on an excited-state intramolecular proton transfer mechanism. Dyes Pigm 2013;99:871–7. [33] Lin WC, Fang SK, Hu JW, Tsai H, Chen KY. Ratiometric fluorescent/colorimetric cyanide-selective sensor based on excited-state intramolecular charge transfer−excited-state intramolecular proton transfer switching. Anal Chem 2014; 86;4648-52. 18

ACCEPTED MANUSCRIPT [34] Ashraf M, Teshome A, Kay AJ, Gainsford GJ, Bhuiyan MDH, Asselberghs I, Clays K. Synthesis and optical properties of NLO chromophores containing an indoline donor and azo linker. Dyes Pigm 2012;95:455–64.

N-phenyl-2-aminotropones. Dyes Pigm 2011;89:319–23.

RI PT

[35] Ito Y, Amimoto K, Kawato T, Prototropic tautomerism and solid-state photochromism of

[36] Park S, Kwon JE, Kim SH, Seo J, Chung K, Park SY, Jang DJ, Medina BM, Gierschner J, Park SY. A

SC

white-light-emitting molecule: frustrated energy transfer between constituent emitting centers. J Am Chem

M AN U

Soc 2009;131:14043–9.

[37] Tang KC, Chang MJ, Lin TY, Pan HA, Fang TC, Chen KY, Hung WY, Hsu YH, Chou PT. Fine tuning the energetics of excited-state intramolecular proton transfer (ESIPT): white light generation in a single ESIPT system. J Am Chem Soc 2011;133:17738–45.

TE D

[38] Chuang WT, Hsieh CC, Lai CH, Lai CH, Shih CW, Chen KY, Hung WY, Hsu YH, Chou PT. Excited-state intramolecular proton transfer molecules bearing o-hydroxy analogues of green fluorescent protein

EP

chromophore. J Org Chem 2011;76:8189–202.

[39] Skonieczny K, Yoo J, Larsen JM, Espinoza EM, Barbasiewicz M, Vullev VI, Lee CH, Gryko DT. How to

AC C

reach intense luminescence for compounds capable of excited-state intramolecular proton transfer? Chem Eur J 2016;22:7485–96.

[40] Padalkar VS, Seki S, Excited-state intramolecular proton-transfer (ESIPT)-inspired solid state emitters. Chem Soc Rev 2016;45:169–202. [41] Ziółek M, Kubicki J, Maciejewski A, Naskręcki R, Grabowska A. An ultrafast excited state intramolecular proton transfer (ESPIT) and photochromism of salicylideneaniline (SA) and its “double” analogue salicylaldehyde azine (SAA). A controversial case. Phys Chem Chem Phys 2004;6:4682–9. 19

ACCEPTED MANUSCRIPT [42] Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, Montgomery JA, Jr. Vreven T, Kudin KN, Burant JC, Millam JM, Iyengar SS, Tomasi J, Barone V, Mennucci B, Cossi M, Scalmani B, Rega GN, Petersson GA, Nakatsuji H, Hada M, Ehara M, Toyota K, Fukuda R, Hasegawa J,

RI PT

Ishida M, Nakajima T, Honda Y, Kitao O, Nakai H, Klene M, Li X, Knox JE, Hratchian HP, Cross JB, Adamo C, Jaramillo J, Gomperts R, Stratmann RE, Yazyev O, Austin AJ, Cammi R, Pomelli C, Ochterski JW, Ayala PY, Morokuma K, Voth GA, Salvador P, Dannenberg JJ, Zakrzewski VG, Dapprich S,

SC

Daniels AD, Strain MC, Farkas O, Malick DK, Rabuck AD, Raghavachari K, Foresman JB, Ortiz JV,

M AN U

Cui Q, Baboul AG, Clifford S, Cioslowski J, Stefanov BB, Liu G, Liashenko A, Piskorz P, Komaromi I, Martin RL, Fox DJ, Keith T, Al-Laham MA, Peng CY, Nanayakkara A, Challacombe M, Gill PMW, Johnson B, Chen W, Wong MW, Gonzalez C, Pople JA. Gaussian, Inc., Pittsburgh PA, 2003. [43] Koné M, Illien B, Graton J, Laurence C. B3LYP and MP2 calculations of the enthalpies of

TE D

hydrogen-bonded complexes of methanol with neutral bases and anions: comparison with experimental data. J Phys Chem A 2005;109:11907–13.

EP

[44] Arod F, Gardon M, Pattison P, Chapuis G. The α2-polymorph of salicylideneaniline. Acta Cryst C 2005;C61:o317–20.

AC C

[45] Schaefer T. Relation between hydroxyl proton chemical shifts and torsional frequencies in some ortho-substituted phenol derivatives. J Phys Chem 1975;79:1888–90. [46] Stasyuk AJ, Cyrański MK, Gryko DT, Solà M. Acidic C–H bond as a proton donor in excited state intramolecular proton transfer reactions. J Chem Theory Comput 2015;11:1046–54. [47] Steiner T. C–H–O hydrogen bonding in crystals. Crystallogr Rev 1996;6:1–57. [48] Feng Q, Li Y, Wang L, Li C, Wang J, Liu Y, Li K, Hou H. Multiple-color aggregation-induced emission (AIE) molecules as chemodosimeters for pH sensing. Chem Commun 2016;52:3123–6. 20

MANUSCRIPT Table 1. Calculated and experimentalACCEPTED parameters for 1a–1c.

1a 1b 1c Salicylideneaniline

1

H NMRa

EHBb

11.25 11.14 11.28 13.26

EHBc

7.36 7.25 7.39 9.37

11.73 11.58 11.89 13.79

The hydroxy proton signals (in ppm).

b

The intramolecular hydrogen bonding obtained from Schaefer’s correlation (in kcal/mol ).

c

The intramolecular hydrogen bonding obtained from DFT calculation (in kcal/mol ).

AC C

EP

TE D

M AN U

SC

a

RI PT

Compound

21

ACCEPTED MANUSCRIPT Table 2. Crystallographic data and refinement details for compounds 1a–1ca. Compound

1a

1b

1c

Chemical formula

C21H25NO

C15H13NO

C19H15NO

Formula weight

307.42

223.26

273.32

Crystal system

Monoclinic

Orthorhombic

Orthorhombic

P21/c

Pna21

P212121

a (Å)

8.9400(4)

10.4043(9)

7.2039(4)

b (Å)

17.4391(8)

16.1681(13)

13.1902(5)

c (Å)

12.6274(5)

7.1817(6)

α (°)

90

90

108.901(2)

90

90

90

γ (°) 3

Volume (Å )

1862.53(14)

-3

Dcalc (g cm ) -1

µ (mm ) F000 3

1.228

0.066

0.077

664

472

2.07–26.41 –11≦h≦11 –21≦k≦20 –15≦l≦15 19747

2

GOF on F

R1 [I > 2σ (I)] wR2 [I > 2σ (I)] R1 (all data) wR2 (all data) -3

a

2.33–26.41

4

1.276

0.079

Full-matrix least-squares

576

0.43 x 0.33 x 0.31 2.72–26.52

–12≦h≦13

–8≦h≦9

–19≦k≦20

–16≦k≦16

–8≦l≦8

15293

2450 (0.0493)

Full-matrix least-squares

–18≦l≦18 13578 2914 (0.0272)

Full-matrix least-squares

1.009

1.102

1.055

0.0657

0.0466

0.0442

0.1894

0.1248

0.1093

0.0934

0.0749

0.0572

0.2250

0.1569

0.1193

0.339 and –0.188

0.249 and –0.293

AC C

Residual (e Å )

1422.45(11)

0.37 x 0.17 x 0.06

TE D

Refinement method on F

3791 (0.0255)

EP

2

1208.09(17)

1.096

θ range (°)

Independent reflections (Rint)

90

4

0.54 x 0.36 x 0.31

Reflections collected

90

4

Crystal size (mm ) Index ranges

90

M AN U

Z

14.9699(6)

SC

β (°)

RI PT

Space group

0.169 and –0.148

Complete crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC 1553755－1553757 for 1a－1c.

22

o ACCEPTED MANUSCRIPT Table 3. Hydrogen-bond geometry (Å, ).

Compound

d(H⋅⋅⋅A)

d(D⋅⋅⋅A)

∠DHA

0.89(4) 1.07(3) 0.90(3)

1.95(4) 1.73(3) 1.99(4)

2.726(3) 2.700(3) 2.768(2)

146(3) 149(3) 144(3)

EP

TE D

M AN U

SC

RI PT

O–H(0A)⋅⋅⋅N O–H(0A)⋅⋅⋅N O–H(0A)⋅⋅⋅N

d(D–H)

AC C

1a 1b 1c

D–H⋅⋅⋅A

23

MANUSCRIPT Table 4. The photophysical propertiesACCEPTED of 1a–1c in various solvents and solid state.

THF

CH3CN

CH3OH

EP AC C

Crystal

TE D

Powder

313 504 12108 0.075 311 503 12274 0.088 312 508 12366 0.074 310 502 12338 0.079 310 500 12258 0.066 506 0.271 507 0.285

315 521 12552 0.032 313 523 12828 0.039 314 526 12836 0.030 313 523 12828 0.031 313 517 12607 0.032 522 0.124 525 0.135

323 528 12020 0.011 324 531 12032 0.014 326 533 11913 0.012 324 529 11961 0.011 323 522 11803 0.010 531 < 0.001 533 < 0.001

SC

CH2Cl2

λabs (nm) λem (nm) Stokes shift (cm-1) Φf λabs (nm) λem (nm) Stokes shift (cm-1) Φf λabs (nm) λem (nm) Stokes shift (cm-1) Φf λabs (nm) λem (nm) Stokes shift (cm-1) Φf λabs (nm) λem (nm) Stokes shift (cm-1) Φf λem (nm) Φf λem (nm) Φf

1c

M AN U

Cyclohexane

1b

RI PT

1a

24

MANUSCRIPT Table 5. Calculated and experimentalACCEPTED parameters for 1a–1c. E+1a

1a 1b 1c

1.09 1.01 1.09

Eg (eV)b 3.65 3.43 3.21

EHOMO / ELUMO (eV)c –5.68/–2.03 –5.60/–2.17 –5.68/–2.47

EHOMO / ELUMO (eV)d –5.88/–1.22 –5.84/–1.31 –5.79/–1.76

RI PT

Compound

Measured in a solution of 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in dichloromethane versus SCE (in V).

b

Estimated from the onset of the absorption spectra (Eg = 1240/λ).

c

Calculated from EHOMO = –4.88 – (Eoxd –EFc/Fc+), ELUMO = EHOMO + Eg.

d

Calculated by DFT/B3LYP.

AC C

EP

TE D

M AN U

SC

a

25

ACCEPTED MANUSCRIPT

Table 6. Selected electronic excitation energies and corresponding oscillator strengths (f), main configurations, and CI coefficients of the low-lying electronically excited states of compounds 1a–1ca.

1a 1b 1c

UV–vis FL UV–vis FL UV–vis FL

S0 → S1 S1 → S0 S0 → S1 S1 → S0 S0 → S1 S1 → S0

energy

compositionb

f

4.15 eV/299 nm 2.51 eV/494 nm 3.89 eV/318 nm 2.36 eV/525 nm 3.46 eV/358 nm 2.26 eV/547 nm

0.0902 0.0945 0.2096 0.1200 0.2111 0.1168

H→L H→L H→L H→L H→L H→L

Calculated by TDDFT/B3LYP/6-31G*. FL stands for fluorescence.

b

H stands for HOMO and L stands for LUMO. Only the main configurations are presented.

c

CI coefficients are in absolute values.

M AN U TE D EP AC C

0.67651 0.70106 0.60850 0.70527 0.68677 0.70500

SC

a

26

CIc

RI PT

Dye singlet electronic transition

ACCEPTED MANUSCRIPT Figure and Scheme Captions Scheme 1. The synthetic route and the structures of 1a–1c. The Non-IUPAC atom label is for the convenience of discussion.

Fig. 1. Characteristic four-level photocycle scheme of the ESIPT process.

RI PT

Scheme 2. The acid-base form equilibrium of 1a.

Fig. 2. (a) The molecular structure of 1b, showing the atom-labelling scheme. Displacement ellipsoids are

SC

drawn at the 50% probability level. Red dashed line denotes the intramolecular O–H⋅⋅⋅N hydrogen bond. (b) Part of the crystal structure in the unit cell of 1b. Green dashed lines denote the intermolecular C–H⋅⋅⋅π

M AN U

interactions. Cg1 (blue circles) is the centroid of the C1–C6 ring. (c) Stereoview of part of the crystal structure of 1b. Blue dashed lines denote the intermolecular C–H⋅⋅⋅O hydrogen bonds.

Fig. 3. (a) The molecular structure of 1a, showing the atom-labelling scheme. Displacement ellipsoids are

TE D

drawn at the 50% probability level. Red and green dashed lines denote the intramolecular O–H⋅⋅⋅N and C–H⋅⋅⋅N hydrogen bonds, respectively. (b) Side view of the crystal structure of 1a. (c) Stereoview of part of the crystal structure of 1a. Blue and green dashed lines denote two different types of intermolecular C–H⋅⋅⋅O hydrogen

EP

bonds.

AC C

Fig. 4. (a) The molecular structure of 1c, showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level. Red and green dashed lines denote the intramolecular O–H⋅⋅⋅N and C–H⋅⋅⋅N hydrogen bonds, respectively. (b) View along the short axis of the naphthalene ring. (c) Stereoview of part of the crystal structure of 1c. Green and blue dashed lines denote the intermolecular π⋅⋅⋅π and C–H⋅⋅⋅π interactions, respectively. Cg1 (red circles) and Cg2 (green circles) are the centroids of the C1–C6 and C10–C15 rings, respectively. Fig. 5. Normalized absorption (a) and emission (b) spectra of 1a–1c in cyclohexane solution. Fig. 6. Emission spectra of 1a (a) and 1b (b) in THF/water mixtures (10 µM) with different water fractions. The 27

ACCEPTED MANUSCRIPT plot of the emission intensity versus the composition of the aqueous mixture of 1a (c) and 1b (d). Fig. 7. Normalized emission spectra of 1a–1c in solid-state. Inset: real-color photographs of the powders of 1a (left) and 1b (right) under UV light. Fig. 8. The cyclic voltammograms of 1a–1c measured in dichloromethane solution with ferrocenium/ferrocene,

RI PT

at 200 mV/s.

Fig. 9. Selected frontier molecular orbitals involved in the excitation and emission of 1b.

Fig. 10. (a) Change of absorption spectra of 1a as pH decreased from 7.0 to 1.0. (b) Change of fluorescence

SC

spectra of 1a as pH decreased from 7.0 to 1.0 (λex = 330 nm). Inset: Sigmoidal fitting of pH-dependent

AC C

EP

TE D

M AN U

fluorescence intensity at 503 nm.

28

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Highlights 1. Three conformationally locked salicylideneaniline were synthesized. ACCEPTED derivatives MANUSCRIPT 2. These molecules undergo an excited-state intramolecular proton transfer reaction.

AC C

EP

TE D

M AN U

SC

RI PT

3. Compound 1a can be used as a fluorescent pH sensor.