ZHAO Pu-Su JING Long LI Yu-Feng ZHU Yun WANG Jing JIAN Fng-Fng

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Comparative Studies on Two 1,8-Naphthalimide Derivatives with Experimental and Theoretical Methods①

ZHAO Pu-Sua, b②JING LongaLI Yu-FengaZHU YuanaWANG JingaJIAN Fang-Fanga

a(266042)b(223300)

Two 1,8-naphthalimide derivatives of 7-benzimidazo[2,1,-a]benz[] isoquino- lin-7-one (1) and 4-bromo-7H-benzimidazo[2,1,-a]benz[de]isoquinolin-7-one (2) have been synthesized and characterized by elemental analysis, IR,1H NMR, UV-Vis and fluorescence spectra. For the two compounds, densityfunctional theory (DFT) calculations of the structures and natural population atomic charge analysishave been performed at the B3LYP/6-311G** level of theory. Based on Onsager reaction filed model and by using TD-DFT method at the B3LYP/6-311G** level, electron spectra of 1 and 2with solvent effect in CHCl3solvent have been predicted, which are in agreement with the experimental ones. Comparative studies on 1 and 2 indicate that introducing an electron-withdrawing group of Br into the 4-position of naphthalene ring in 2 doesnot significantlymakethe molecular geometry of 2 different from that of 1,but evidently changes the atomic charge redistribution, moves the positive-negative charges center and then changes the dipole moment in 2. Additionally, for compound 2, the existence of Br atom has also influenced the peak intensity and peak locations in both electron and fluorescence spectra.

1,8-naphthalimids, DFT calculation,atomic charge distribution, electronic spectra,fluorescence spectra

1 INTRODUCTION

1,8-Naphthalimide derivatives are luminophore compounds that are widely used in various fields of scienceand technology that exhibit all the necessary opticalcharacteristics and are readilyavailable from a synthetic point of view[1-5]. Owing to their strong fluorescence and good photostability, 1,8-naphtha- limide derivatives have found applications in a number of areas including coloration of polymers[6-7], laser active media[8-9], fluorescent markers in bio- logy[10-11],anticancer agents[12],fluorescence swi- tchers[13], light emitting diodes[14],electrolumines- cent materials[15],liquid crystal displays[16]and so on. Various 1,8-naphthalimide derivatives have been reported as fluorescent sensors or electro-optical devices[17-19]. Our group have also focused our atten- tion on investigating 1,8-naphthalimide derivatives and given two reports on their structural informa- tion[20-21].7-benzimidazo[2,1,-a]benz[] isoqui-nolin-7-one (1) and 4-bromo-7-benzimidazo [2,1,-a]benz[d]isoquinolin-7-one (2) are two typical derivatives of 1,8-naphthalimide. Although their synthetic methods have been reported earlier[5, 22-24],to the best of our knowledge, there are no further investigations on them available till now by both experimental and theoretical methods. Also, there are no comparative studies about the compounds of 1 and 2. In fact, in 2, there exists an electron- withdrawing group of Br in the 4-position of naphthyl ring, which is different in that of 1 and probably leads to different electronic structure of 2 and then influences the spectra properties of 2. So, after we synthesized the two compounds and made detailed structural characterizations,we made further theoretical studies on them. Using experi- mental and DFT methods, comparison has been made between compounds 1 and 2 with respect to their electronic structure and the influence of substituent on the spectroscopic properties has been suggested. We hope that the research presented herein will provide a valuable contribution to the area of electroluminescent devices and will help move forward the design of 1,8-naphthalimide-based electroluminescent materials.

2 EXPERIMENTAL

2. 1 Physical measurements

Elemental analyses for carbon, hydrogen and nitrogen were performed using a Perkin-Elmer 240C elemental instrument.1H NMR spectroscopies (DMSO-6) were recorded on an Avance Mercury plus-400 instrument with TMS as an internal stan- dard. IR spectra were measured in the 4000～400 cm-1region using KBr pellets on a Nicolet 170SX spectrophotometer. Electronic absorption spectra were performed on a Shimadzu UV3100 in CHCl3solution and solid-statefluorescence spectra were obtained on a F96-fluorospectrophotometer.

2. 2 Preparation of 1 and 2

All chemicals were obtained from commercial sources and used without further purification. The reaction path is shown in Scheme 1 for 1 and 2.

Preparation of 7-benzimidazo[2,1,-a]benz[] isoquinolin-7-one (1): 1,8-naphthalic anhydride (0.01 mmol) was dissolved in glacial acetic acid (50 mL). Then,-phenylenediamine (0.01 mmol) was added with stirring. The mixture was heated to 102～103 ℃ for refluxing. After 4 h, the reaction was stopped and the acetic acid was evaporated by rotary vacuum evaporation. Subsequently,the mix- ture was diluted with 30 mL distilled water and 3.0 mL hydrochloric acid (30%) was added in. The above mixture was maintained at 60 ℃ for 1 h and then cooled to 50℃. By filtration, the solid was obtained and washed with distilled water. Finally, the product 1 was dried at room temperature. Yield 90%. m.p.: 205～206℃, which is the same with that in literature[5].Anal. Calcd. (%) for C18H10N2O: C, 79.98; H,3.73; N, 10.37. Found (%):C, 79.90; H, 3.69; N, 10.31. IR(KBr disc)/cm-1: 3057(m), 1701(m), 1653(s), 1590(m), 1518(m), 1439(m), 1341(vs), 1261(s), 1094(vs), 1023(vs), 802(s), 773(m), 470(m).1H NMR (DMSO,, ppm): 7.48 (d, 2H, -ArH), 7.85～7.92 (m, 2H-ArH+ 1H-naphthyl ring), 8.32～8.70 (m, 5H-naphthyl ring).

Preparation of 4-bromo-7-benzimida zo[2,1,-a] benz[d]isoquinolin-7-one (2): In order to obtain 2,the same procedure as for 1 was used. 4-Bromo- 1,8-naphthalic anhydride (0.01 mmol) was used in place of (0.01 mol). Yield 95%. m.p.: 224～225℃, which is very similar with that in literature[5](224～224.5℃).Anal. Calcd. (%) for: C18H9BrN2O: C, 61.91; H,2.59; N, 8.02. Found (%):C, 61.85; H, 2.51; N, 8.07. IR(KBr disc)/cm-1: 3180(m), 1772(w), 1699(vs), 1569(m), 1545(m), 1447(m), 1356(s), 1228(s), 1096(m), 1035(m), 812(m), 768(m), 750(m), 472(m).1H NMR (DMSO,, ppm): 7.51(d, 2H, -ArH), 8.03～8.11 (m, 2H-ArH + 1H-naphthyl ring ), 8.41～8.82 (m, 4H-naphthyl ring).

2. 3 Computational methods

Initial molecular geometries were optimized using MM+molecular modeling and semi-empirical AM1 methods[25](HYPERCHEM 6.0, Hypercube, Ont., Canada). Then, DFT calculations with a hybrid functional B3LYP (Becke’s three parameter hybrid functional using the LYP correlation functional)[26-27]at basis set 6-311G**[28-29]by the Berny method[30]were performed with the Gaussian 03 software package[31]. The calculated vibrational frequencies ascertained that the structures were stable (no imaginary frequencies).

In order to investigate solvent effect on the elec- tronic spectra, Onsager reaction filed model[32-36]was utilized to optimize the solvated systems. Among all the self-consistent reaction field (SCRF) methods, the Onsager reaction field[32-33]is a classic one and has been adapted to various kinds of solu- tion calculations[34-36]. In this method, the solute occupies a fixed spherical cavity of radiusawithin the solvent field, which was derived from a tight molecular volume calculation provided in Gaussian 03 on the fully optimized gas-phase stationary states using the same quantum chemical model for consistency.

Based on the optimized geometries in the CHCl3solution, and by using time-dependent density func- tional theory (TD-DFT)[37-39]methods, electronic spectra of 1 and 2 in CHCl3were predicted at the B3LYP/6-311G** level. Natural Bond Orbital (NBO)[40]analyses were also performed based on the optimized geometries.

All calculations were performed on a DELL PE 2850 server and a Pentium IV computer using the default convergence criteria.

3 RESULTS AND DISCUSSION

3. 1 Optimized geometries

According to Scheme 1, for compound 2, there exist two possible geometries as shown in Fig. 1.

Scheme 1. Reaction path for 1 and 2

Fig. 1. Two possible geometries for compound 2

In order to select a proper geometry of compound 2 to make comparison with that of 1, the total energies of 2aand 2bhave been calculated at the B3LYP/6-311G** level and the results are listed in Table 1. As seen from Table 1, the total energy of 2ais only lower than that of 2bby 0.093 kJ/mol, indicating that both 2aand 2bcan exist in experi- mental conditions. So, in the following discussion, both forms of 2aand 2bhave been selected to denote 2 and compare with that of 1.

The optimized geometries of 1 and 2 have been obtained at the B3LYP/6-311G** level of theory and shown in Fig. 2. To make comparisons between compounds 1 and 2 more straightforward, atomic numbering scheme of 2 was kept the same as that in 1 except the bromine atom Br(22). Since there are no crystal structures of 1 and 2 available, we compare the predicted geometric parameters of 1 and 2 with those of hydrochloride monohydrate of 1 (1×HCl×H2O),whose crystal structure was reported by our group in 2008[21]. All calculated geometric parameters of 1 and 2 are listed in Table 2 along with the experimental data for 1×HCl×H2O.

Table 1. Total Energy of 2a and2b Calculated at the B3LYP/6-311G** Level

Table2. Selected Geometric Parameters by X-ray for 1×HCl×H2O and Theoretical Calculations at the B3LYP/6-311G** Level for 1 and 2

As seen in Table 2 for 1 and 2, most of the pre- dicted geometric parameters have higher values than those determined experimentally. This is most likely due to the fact that the experimental data describe the compounds in the solid state, whereas the calculated data correspond to the molecules in the gas phase. Comparisons of the experimental values with the predicted ones in the two compounds reveal that the biggest differences of bond length and bond angle mainly occur in the imidazolyl-ring of N(2)-C(10)-N(3)-C(4)-C(9), since the N(3) atom in the experiment (1×HCl×H2O) is protonated and involved in hydrogen bonding with Cl atom[21], while in calculations, 1 and 2 are independent molecules, respectively. The biggest bond dif- ferences are all for the N(2)-C(10) bond, with the difference being 0.0476? for 1, 0.0464 ? for 2aand0.0461 ? for 2b. Considering the bond angle, the biggest differences all take place atN(2)-C(10)-N(3), with the difference being 4.787o for 1, 4.792o for 2aand 4.825o for 2b. Comparing the calculated data of 1 with those of 2, one can find that the differences between them are very small. For 1 and 2a,the bond length difference is only 0.054 ? at C(15)-C(20)and the bond angle difference is 1.077o for C(15)-C(16)-C(17). For 1 and 2b, the bond length difference is 0.051 ? at C(15)-C(20) and bond angle difference is only 0.611o at C(19)-C(20)-C(11). The aforementioned comparisons indi- cate that, although there exists an electron-acceptor group of Br in the 4-position of naphthyl ring in 2, both the optimized geometries of 1 and 2 resemble closely the structure of 1 in the crystal structure of 1×HCl×H2O and the B3LYP/6-311G** level of theory can provide satisfying calculational precision for the system studied here. Compared with 1, the Br-substituted group does not influence the geo- metry of 2 in a significant way.

3. 2 Atomic charge distributions

Based on the two optimized geometries obtained at the B3LYP/6-311G** level of theory, the NPA atomic distributions for 1 and 2 have been calculated. Some selected non-hydrogen atomic charges and the total atomic charge distributions in phenyl and naphthyl rings are also shown in Fig. 2.

Fig. 2. Molecular structures with atomic numbering scheme for 1 and 2 along with NPA atomic charge distributions () on some non-hydrogen atoms, phenyl ring and naphthyl ring in 1 and 2

As seen from Fig. 2, introducing an electron- withdrawing group of Br at the 4-position of naphthyl ring in 2leads to its atomic charge distributions different from those in 1. For example, the Br atom in 2aand 2bhas less positive charges than H(1) and H(2) in 1,respectively and the atoms C(21) and C(10) in 2aand 2balso have less positive charges than those in 1.As for atoms O(1) and N(3), in 2aand 2b, their negative charge values are less and more respectively than those in 1. Accordingly, the N(2) atom in 2ahas more negative charges than that in 1, while in 2b, the N(2) atom has less negative charges than that in 1. By comparing the atomic charge values in 1 and 2, we can find that as an electron-acceptor group, the Br atom pulls electrons from the other parts of the molecule to itself, which leads to the phenyl ring in 2 having more positive charges (0.32161in 2aand 0.33657in 2b) than that in 1 (0.31400) and the naphthyl ring except Br in 2 having less negative charges (–0.03811in 2aand –0.02186in 2b) than the naphthyl ring in 1 (–0.1574except H(1) and –0.15691except H(2)). It is evident that such atomic charge redistributions will result in the moving of positive-negative charge center and the change of dipole moment of 1 and 2. Thedipole moment of 2 becomes smaller than that of 1 (3.5233 Debye for 1, 1.8632 Debye for 2aand 1.7231 Debye for 2b). Maybe, this kind of atomic charge distributions will also influence the electronic and fluorescence spectra of 1 and 2,just as discussed below.

3. 3 Electronic spectra

For 1 and 2, electronic absorption spectra were measured in trichloromethane (CHCl3) solution at room temperature and the results are listed in Table 3. Based on Onsager reaction filed model and by using TD-DFT method at the B3LYP/6-311G** level, electronic spectra were calculated on the basis of optimized geometries in CHCl3solvent. The values of cavity size (o) recommended by the tight molecular volume calculation on the fully optimized gas-phase stationary were 4.91 ? for 1, 5.15 ? for 2aand 5.04 ? for 2b. The dielectric constant of the CHCl3solvent was 4.9, which is the default value in the Gaussian 03 software package.All calculated values are also listed in Table 3.

Table3. Experimental and Theoretical Electronic Absorption Spectra Values with Solvent Effect for 1 and 2

As seen from Table 3, in the CHCl3solution, both 1 and 2 have three electronic transition bands, with one (263 nm) having blue shifts and the other two (296 and 394 nm)having red shifts in 2 compared with those in 1. In experiments, the intensities of these three peaks of 2 are all lower than the corres- ponding of 1. Theoretically, for 1 and 2, the B3LYP/6-311G** method respectively obtains three transition peaks which are comparable with the experimental data. In 2a,the intensities of the three predicted transition peaks are all weaker than those corresponding in 1 and 2b. Only one predicted peak- intensity at 230 nm is lower than that in 1 and intensities of the other two predicted peaks are both higher than those in 1. As for peak locations, in 2b, three predicted peak locations all have some red-shifts compared with those in 1. However, in view of 2a, two predicted bands (228 and 295 nm) have red shifts and one predicted band (329 nm) has some blue shifts compared with the corresponding peaks in 1. It is remarkable that, for the system studied here, the electron-withdrawing group of Br in 2 has influenced the electronic spectra, which leads to a few changes of peak locations and the intensities of transition bands of 2. Additionally, compared the predicted values with the correspon- ding experimental ones, although there exist some differences between them, all the differences are not big, indicating that the B3LYP/6-311G** method in Onsager reaction filed model can be used to simulate the electron spectra in CHCl3for the system studied here on the whole. Natural population analyses based on the B3LYP/6-311G**optimized geometries show that the frontier molecular orbitals of 1 and 2 are mainly composed ofatomic orbitals, so elec- tronic transitions corresponding to the above elec- tronic spectra are mainly assigned to→* and→* electronic transitions. Fig. 3 shows the sur- faces of HOMO-1, HOMO, LUMO and LUMO + 1 for 1 and 2. As seen in Fig. 3, the shapes of the frontier molecular orbitals of 1 and 2 are almost the same, indicating their electronic transition models are similar. Namely, though the electron-withdraw group of Br atom makes the atomic charges re-distri- buted and results in some changes of electron transition bands in 2, it does not change the electron transition model of 2 ultimately.

Fig. 3. Isodensity surfaces of HOMO-1, HOMO, LUMO and LUMO+1 for 1 and 2

3. 4 Fluorescence spectra

Experimental solid-state fluorescence spectra of 1 and 2 measured on a F96-fluorospectro photometer are shown in Fig. 4. The emission band of 1 is obser- ved at 504 nm, which lies in the blue-green region, and the half-line width is about 71 nm. The emission band of 2 is observed at 510 nm, which also lies in the blue-green region and shows some red shifts as compared to 1, with the half-line width to be about 67 nm. The emission intensity of 2 is weaker than that of 1. Evidently, compared with 1, in 2,the presence of electron-withdraw group of Br atom in the 4-position of naphthyl ring decreases the fluorescence emission intensity and results in the red shifts of the emission peak. Maybe, this pheno- menon also results from the atomic charge re-distri- butions in 2.

Fig. 4. Experimental solid-state fluorescence emission spectra of 1 and 2 at room temperature

Table4. Thermodynamic Properties at Different Temperature at the B3LYP/6-311G** Level for 1 and 2

3. 5 Thermodynamic properties

For 1 and 2, on the basis of vibrational analyses and statistical thermodynamics, the standard thermo- dynamic functions: heat capacity (0), entropy (0) and enthalpy (0) were obtained and listed in Table 4. The scale factor for frequencies is 0.96, which is a typical factor for the B3LYP/6-311G** method.

As observed in Table 4, all the values of0,0and0increase with the increase of temperature from 100.0 to 800.0 K, which is attributed to the enhancement of molecular vibration while the temperature increases. Comparison finds that the thermodynamic function values for 2 are all larger than those corresponding values of 1,since 2 has larger atomic masses than 1. In view of 2aand 2b, although they have the same atomic masses, the different positions of Br-group make them have different vibration modes, which finally lead to their different thermodynamic function values.

The correlation equations between these thermo- dynamic properties and temperaturefor 1 and 2 are as follows:

For 1:0= –56.2136 + 1.2600–

6.277*10–42(2= 0.9999)

0= 213.0679 + 0.9943– 1.725*10–42

(2= 0.99992)

0m=–13.2884 + 0.08789+ 3.152*10–42

(2= 0.9998)

For 2a:0= –35.8482 + 1.2518–

6.329*10–42(2= 0.9999)

0= 232.5198 + 1.0833– 2.272*10–42

(2= 0.99997)

0m=–14.9425 + 0.1097+ 3.081*10–42

(2= 0.9998)

For 2b:0= –35.5239 + 1.2506–

6.3197*10–42(2= 0.9999)

0= 233.1476 + 1.0835– 2.2751*10–42

(2= 0.99997)

0m= –14.9245 + 0.1098+ 3.080*10–42

(2= 0.9998)

These equations will be useful for further studies on these two compounds.

4 CONCLUSION

Two 1,8-naphthalimide derivatives of 1 and 2 have been synthesized and characterized by ele- mental analysis, IR, UV-Vis and fluorescence spec- tra. DFT calculations show the B3LYP/6-311G** level of theory can provide satisfactory precision in optimizing molecular geometries of 1 and 2. NPA atomic charge distribution analysis indicates that the electron-withdraw group of Br atom in the 4-posi- tion of naohthyl ring in 2 plays an important role in pulling the electrons from the other parts of the molecule to itself, which leads to the re-distributions of the atomic charge and the change of dipole in 2. Predicted electron spectra of 1 and 2 in CHCl3solvent are in agreement with the experimental ones on the whole. From the view of electron and fluo- rescence spectra of 1 and 2, introducing an electron- withdraw group in the 4-position of naphthyl ring in 2 will finally change the peak intensity and peak locations, which will be helpful information for further study on the 1,8-naphthalimide based electro- luminescent materials.

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28 October 2013;

9 December 2013

Doctor Foundation of Shandong Province (No. BS2010CL021), Natural Science Foundation of Shandong Province (ZR2009AL020) and Jiangsu Key Laboratory for Chemistry of Low-dimensional Materials P. R. China (JSKC12106 and JSKC12107)

. E-mail: zhaopusu@163.com