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Fluorescence and Theoretical Calculation of Phenylhydrazone Derivatives and Fluorine Boron Complex: Synthesis and Fluorescence Characteristics.

Byline: Ping Tang, Luping Lyu and Yujin Li

Summary: A sereis of phenylhydrazone-based derivatives and their corresponding BF2 complexes were synthesized efficiently by a three-step reaction. Photophysical performance was investigated in different organic solvents and in the solid state. Although these compounds exhibited feeble fluorescent intensity in solution-state, BF2 complexes showed weaker fluorescence in solid state compared to precursors 2, which were caused by slight geometry relaxation of upon photo excitation. Density Functional Theory calculations was carried out to confirm above inference.

Keywords: Boron Complexes; Solid State; Fluorescence; DFT Calculation.

Introduction

Phenylhydrazone-based compounds have been reported in the field of academic research and broadly applied to aggregation-induced emission enhancement [1], fluorescent molecular probes [2, 3], electroluminescence [4], photodynamic therapy [5], anticathepsin agents [6], Organocatalytic [7, 8], nonlinear optical material [9], determination of Cu(II), Zn(II) and Mn(II) [10], acetate ion recognition [11, 12], etc.

Meanwhile, their corresponding BF2 complexes have been extensively known as fluorescent dyes which have exhibited practical applications in electrochromic display systems [13, 14], solar cells [15, 16], liquid crystals [17, 18] and optical storage devices [19, 20]. In 2015 Li's group reported a series of phenylhydrazone derivatives and their corresponding BF2 complexes indicating robust aggregation-induced emission enhancement properties and high solid-state fluorescent intensity following a huge Stokes Shift [1, 21]. This research supported us to create a new range of phenylhydrazone-based derivatives and their corresponding BF2 complexes and to investigate other novel optical characters.

Hence, a new series of phenylhydrazone-based derivatives and their corresponding BF2 complexes were designed and synthesized. The compounds demonstrated an attractive fluorescence characters in the solid states. Herein, we describe the synthesis and optical properties of these compounds. In addition, Density Functional Theory(DFT) and time-dependent DFT(TD-DFT) calculations were carried out to confirm the experimental results.

Experimental

Materials and methods

In the present research, unless otherwise noted, all the chemicals were used with no purification and were purchased from commercial enterprises. Standard methods were used to purify all the solvents prior to use. Melting points (m. p.) were carried on a X-4 electro-thermal digital melting point apparatus. Nuclear magnetic resonance (1H and 13C NMR) spectra were run on a Bruker AM 500 spectrometer (Bruker) with chemical shifts reported as ppm at 500 and 125 MHz, respectively, (in CDCl3 as internal standard).

Fluorescence spectra were obtained on a F-7000 Fluorescence spectrophotometer. The emission was measured in the steady state mode by exciting at the I>>max (lower energy MLCT band) for each compound with exit and entrance slits set at 5 nm. Fluorescence spectra were recorded for 3 times and average values of the integrated areas of fluorescence were adopted to calculate If in solution. It can be used for photochemical checking at spectroscopic grade. On a UV-2550, absorption spectra of UV-Vis were measured. The If values in solution were determined by using a common method. In this method, quinine sulfate (I = 0.55 in 0.05 M H2SO4 solution) was treated as a standard and dilute solutions of the mixtures were used in organic dissolvent(1A*10-5 mol*L-1).

Density Functional Theory (DFT) and time-dependent DFT (TD-DFT) calculation with 6-31G/B3LYP level have been perfomed to optimize all the structures. All the calculations were carried out on Gaussian 09W program package [22, 23].

Synthesis

General synthesis procedure of 2a-2c. Under nitrogen atmosphere, 2,4-Dihydroxyacetophenone (10 mmol), methyl hydrazinecarboxylate 0.74g (10 mmol) were dissolved in stirred methanol (15 ml) and left for 2.5 h at room temperature. The resulting solid was filtered off and recrystallized from ethanol for three times to give the title compounds 1.95 g 2a, 1.77g 2b, 1.69 g 2c.

(E)-Methyl

N'-[1-(4-hydroxyphenyl)ethylidene]hydrazinecarboxy late (2a) White solid. Yield, 87.1%. M.P.: 208-212AdegC. 1H-NMR (500 MHz, CDCl3) I'(ppm): 13.04 (s, 1H), 10.59 (s, 1H), 9.72 (s, 1H), 7.33 (d, J = 8.7 Hz, 1H), 6.29 (dd, J = 8.7, 2.4 Hz, 1H), 6.23 (d, J = 2.4 Hz, 1H), 3.74 (s, 3H), 2.23 (s, 3H); HRMS (ESI) m/z calcd for: C10H13N2O4+(M+H)+ 225.0869, found 225.0869.

(E)-Methyl

N'-(2,4-dihydroxybenzylidene)hydrazinecarboxylate (2b) White solid. Yield, 84.3%. M.P.: 109-113AdegC; 1H-NMR (500 MHz, CDCl3) I'(ppm): 11.07 (d, J = 42.9 Hz, 2H), 9.81 (s, 1H), 8.10 (s, 1H), 7.20 (dd, J = 8.3, 2.8 Hz, 1H), 6.44-6.20 (m, 2H), 3.69 (s, 3H); HRMS (ESI) m/z calcd for C9H11N2O4+(M+H)+ 211.0713, found 211.0713.

(E)-Methyl

N'-(4-methyoxybenzylidene) hydrazinecarboxylate (2c) White solid. Yield, 81.3%. M.P.: 140-142AdegC. 1H-NMR (500 MHz, CDCl3) I'(ppm): 12.45 (s, 1H), 7.43 (d, J = 7.7 Hz, 1H), 7.29 (dd, J = 9.1, 2.1 Hz, 1H), 7.00 (dd, J = 8.2, 1.1 Hz, 1H), 6.93-6.84 (m, 1H), 3.91 (s, 3H), 2.30 (s, 3H); HRMS (ESI) m/z calcd for C10H13N2O3+(M+H)+209.0921, found209.0921.

General synthesis procedure of 3a-3b. Under nitrogen atmosphere, to a solvent of 0.20 g 2a in toluene(10 mL), 0.7 ml(4.0 mmol)N,N-diisopropyl ethylamine was put into drop by drop within 10 min.Then 0.5 ml(4.0 mmol) Boron trifluoride diethyl etherate was slowly added into above mixture and refluxed for 4 hours. Then the reaction mixture was concentrated to product crude product under reduced pressure. Silica gel chromatography eluting (silica gel, ethyl acetate: petroleum ether = 1:1) was used to purify the crude product causing 80 mg 2a, and 95 mg 2b, as yellow solid.

methyl

(2,2-difluoro-7-hydroxy-4-methyl-2H-2l4,3l4-benzo[e ][1,3,2]oxazaborinin-3-yl)carbamate (3a) Yellow solid. Yield, 71.0%. M.P.: 233-235AdegC. 1H-NMR (500 MHz, CDCl3) I'(ppm): 8.72 (s, 1H), 7.36 (d, J = 8.6 Hz, 1H), 6.35 (dd, J = 8.5, 2.2 Hz, 1H), 6.29 (d, J = 2.1 Hz, 1H), 4.03 (d, J = 7.1 Hz, 1H), 3.44 (d, J = 7.0 Hz, 3H), 2.51 (s, 3H);13C NMR (125 MHz, CDCl3) I'(ppm): 161.27, 160.84, 155.32, 152.61, 130.79, 111.22, 107.90, 104.01, 51.3, 17.65; HRMS (ESI) m/z calcd for C10H12BF2N2O4+(M+H)+ 273.0853, found 273.0867.

methyl

(2,2-difluoro-7-hydroxy-2H-2l4,3l4-benzo[e][1,3,2]o xazaborinin-3-yl)carbamate (3b) Yellow solid. Yield, 82.1%, M.P.:217-218AdegC. 1H-NMR (500 MHz, CDCl3) I'(ppm): 8.61 (s, 1H), 8.21 (s, 1H), 7.29 (d, J = 8.5 Hz, 1H), 6.31 (dd, J = 8.6, 2.2 Hz, 1H), 6.27 (d, J = 2.3 Hz, 1H), 4.12 (d, J = 7.0 Hz, 1H), 3.41 (d, J = 6.9 Hz, 3H); 13C NMR (125 MHz, CDCl3) I'(ppm): 162.31, 161.97, 154.12, 153.72, 131.81, 110.12, 108.62, 103.21, 51.3; HRMS (ESI) m/z calcd for C9H10BF2N2O4+(M+H)+ 259.0648, found 259.0676.

Results and Discussion Synthesis

Scheme-1 shows the preparation for the substituted phenylhydrazone derivatives (precursor 2) and their corresponding boron complexes (BODIHY 3): 2a, 2b, 2c, 3a and 3b. First, mixture of dimethyl carbonate and hydrazine hydrate was refluxed for 7 hours to obtain methyl hydrazinecarboxylate 1 (1.5equiv). Then phenylhydrazone derivatives 2 were accomplished by condensation with aromatic aldehydes (ketones) and methyl hydrazinecarboxylate (1.0equiv) in CH3OH at room temperature. Finally, phenylhydrazone derivatives and an excess of BF3.Et2O in DCM with N,N-Diisopropyl ethylamine as the base gave BODIHY 3 with a yield of 71.0% (3a) and 82.1% (3b). BODIHY 3 were purified by column chromatography (silica gel, EA:PE = 1:1). All the structures were sufficiently characterized by 13C-NMR, 1H-NMR, GC-MS and HRMS analysis (Table-1).

Table-1: Structures of compounds 2a, 2b, 2c, 3a, 3b.

compound###R1###R2###R3

2a###4-OH###CH3###OH

2b###4-OH###H###OH

2c###4-OCH3###H###H

3a###4-OH###CH3

3b###4-OH###H

The final products 2a, 2b, 2c, 3a and 3b demonstrated considerable stability and solubility in solvents like CH3CN, EtOH, DCM, DMF, THF, EA and Chloroform. No overt and apparent fluorescence and color fading were found in months using rigorous examination. Precursor 2c's solvent effect on absorption and fluorescence properties was tested (Table-2 and Fig. 1). As show in Fig. 1 and Table-2, precursor 2c exhibited a sharp absorption peak at approximately 320 nm. The maximum absorption wavelength (I>>max) of BODIHY 3 (368 nm) showed more bathochromic than the precursor 2c. Furthermore, it was obvious that the dissolvent polarity imposed slight effect on 2c's maximum absorption wavelength (I>>max), which indicated that the dipole moments of the molecules in both excited and ground states were more or less equal [24].

Meanwhile, it was found that the maximum fluorescence intensity of precursor 2c was reached in THF which is the best solvent and was highly dependent on solvents. Precursor 2c displayed a strongest fluorescence intensity (I>>em=352 nm) with 180 fluorescence intensity (signal response value) in THF, but the value was lower than 40 in DCM (Fig. 1). As a result, the fluorescence quantum yields (If) of Precursor 2c were below 0.01 in DCM. It hardly exhibited fluorescence in DCM. In Addition, maximum emission wavelength of 2c underwent a slight change in different organic solvents.

UV-Vis spectra and emission spectra of 2a-3b in solution

Optical data of compounds 2a-3b in THF was measured respectively to examine the substituent group effects (Table-2). As shown in Table-2 and Fig. 2, compounds 2a-3b's maximum absorption wavelength (I>>abs) were at about 317 nm, 317 nm, 317 nm, 367 nm and 360 nm, respectively. Precursors 2a, 2b and 2c exhibited almost the same absorption patterns with highest absorption peaks at 317 nm. The result also showed that it had rarely effect on the maximum absorption peak of precursors 2 under different substituents on benzene ring. BODIHY 3a displayed a slight bathochromic shift of around 7 nm compared with 3b because of the electron-donating effect of methyl. BODIHY 3a(367 nm) and 3b(360 nm) demonstrated an obvious bathochromic shift (43-50 nm) compared with the corresponding precursor 2a(317 nm) and 2b(317 nm) (Table-1). The complexation of BF2 increased the molecular rigidity and coplanarity of BODIHY 3, which caused the bathochromic shift.

Table-2: Photophysical properties for compounds 2a, 2b, 2c, 3a, 3b.

###Compounds###Matrix###a(M-1cm-1)###eabs(max)nm)###eem(max)nm)###Stokes Shift(nm)###Of

###2a###THF###10700###317###374###57###0.05

###solid###318###471###153###/

###2b###THF###6900###317###374###57###0.05

###solid###320###473###153###/

###2c###DCM###2100###321###354###33###0.01

###CHCl3###7700###318###349###31###0.03

###EA###2900###318###351###33###0.06

###EtOH###4300###319###350###31###0.05

###MeCN###3900###320###348###28###0.06

###THF###6200###317###352###35###0.08

###DMF###6200###318###351###33###0.07

###solid###320###521###201###0.20

###3a###DCM###50400###368###412###45###0.01

###CHCl3###56500###369###413###44###0.16

###EA###42900###365###403###39###0.15

###EtOH###44500###367###508###142###/

###MeCN###52200###364###410###47###/

###THF###55000###367###404###37###0.21

###DMF###53400###368###413###46###/

###solid###418###508###90###/

###3b###THF###54000###360###399###39###0.19

As detailed in Table-2 and Fig. 2, compounds 2a-3b possessed the Stokes Shifts between 35-57 nm in THF. Of all, compounds 2a and 2b was the two with the highest Stokes Shift (57 nm) with emission at 374 nm and excitation at 317 nm. Compared with 2c, precursors 2a and 2b (with hydroxyl on benzene) emerged the larger Stokes Shifts and longer wavelengths in the fluorescence emission spectra because the electron donating hydroxyl on benzene helped the 2a and 2b's expansion of Stokes Shifts. Similarly, the fluorescence emission wavelength of 3a (404 nm) was larger than 3b (399 nm) due to the presence of electron donating methyl group of 3a. All the compounds showed a blue fluorescence, but fluorescence intensity of BODIHY 3 was much higher than precursor 2a-2c. Correspondingly, compound precursors 2a-2c were observed to have lower quantum yield with 5%(2a), 5%(2b) and 8% (2c) than 3a(21%) and 3b(19%).

Emission spectra of 3a-2b in solid state

Further, fluorescence Emission spectra of all compounds in the solid states were tested (Table-2). The color of compounds 2a-3b in their solid state by direct visualization and UV lamp (365 nm) were shown in Fig. 3. As reflected in Table-2 and Fig. 3, both precursor 2 and BODIHY 3 had the large Stokes shifts between about 90 nm and 201 nm in the solid-state. The maximum Stokes shift of 2c was up to 201 nm, with excitation at 320 nm and emission at 521 nm in the solid-state. The fluorescence intensity of BODIHY 3 was obviously lower than the corresponding precursor 2. Precursor 2a-2c looked bright blue under the excitation of UV light. However, BODIHY 3a and 3b hardly exhibited fluorescence in their solid state. This phenomenon could be ascribed to their high coplanarity which resulted in piling of molecules and strong intermolecular interactions, correspondingly leading to concentration quenching in the solid state [22, 23].

On the other hand, precursor 2a-2c exhibited a solid-state emission partially due to their more twisted molecular structure preventing the compact packing of molecules [22, 23]. As reflected in Table-2, solid-state emissions spectra reached to a 471 nm - 508 nm ranges and were bathochromic-shifted compared with their respective emissions in solution. Furthermore, relative to 2a and 2b, precursor 2a-2c's maximum emission spectrum was more bathochromic-shifted. Which might be possibly due to the I-I stacking and denser aggregation of molecules in the solid state [25].

Quantum chemical calculations: rationalize the excitation and emission spectra

To better comprehend photophysical properties of the precursor 2 and the BODIHY 3, such as the UV-vis absorption and fluorescence, BODIHY 3a's geometry at the S0 state and the S1 state was optimized by Density Functional Theory (DFT) Quantum calculations at the B3LYP/6-31G level, which was performed in Gaussian 09W class of programs[24, 25]. The optimized configuration of BODIHY 3a was illustrated in Fig. 5. It was found that both of the optimized configurations at S0 state and the S1 state adopted chair structure. The dihedral angles between phenyl ring and BODIHY core of S0 state and S1 state were 8.67Adeg and 9.12Adeg, respectively. Which indicated that coplanarity of S0 state was sight better than that of S1 state (Fig. 5)? The geometry of the S1 excited state, which is responsible for the fluorescence (Kasha's rule) [26], is more or less same as S0 state geometry (Fig.5).

This can explain why BODIHY 3a and 3b hardly exhibited fluorescence in solid state. As shown in Fig. 4, the energy gap between the HOMO and LUMO of the BODIHY 3a was 3.38 eV. In addition, the calculated absorption and emission peaks of the 3a were located at 334 nm and 414 nm, respectively, which were consistent with the experimental results of 368 nm and 413 nm.

Conclusion

In conclusion, a series of phenylhydrazone-based derivatives and their corresponding BF2 complexes were developed using readily procedures under mild conditions. All the compounds exhibited visible fluorescence in their solutions. However, precursors 2 showed much more enhanced fluorescence in solid state compared to corresponding BF2 complexes. The geometries of the molecules at the S0 state and the S1 excited state were optimized by DFT and TD-DFT, respectively. The results of the calculation of the UV--vis absorption and emission were in agreement with the experimental results. The small geometry relaxation was proposed to the main cause for the weak fluorescence of BODIHY 3a in solid state.

Acknowledgments

We gratefully acknowledge the financial support of Hangzhou agricultural scientific research project(20160432B25, 20180432B35), College Students in Zhejiang Province Sciences and Technology Innovation Activities Plan and Xinmiao Talents Program (No.2017R452002) and China Scholarship Council(File No.201708330572), Zhejiang Public Welfare Technology Research Program (LGN19C200014), zhejiang Philosophy and Social Science Planning Project (20NDJC337YBM).

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Author:Ping Tang, Luping Lyu and Yujin Li
Publication:Journal of the Chemical Society of Pakistan
Date:Feb 14, 2020
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