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Research on the prediction of absorption half-band width of some azobenzene compounds
Journal of Molecular Structure: THEOCHEM 730 (2005) 151–154 www.elsevier.com/locate/theochem
Research on the prediction of absorption half-band width of some azobenzene compounds Junna Liu*, Shenfeng Yuan, Zhirong Chen Department of Chemical Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China Received 17 January 2005; revised 16 June 2005; accepted 20 June 2005 Available online 2 August 2005
Abstract In order to predict the absorption half-band width, the ground state and the first singlet-excited state configurations of some azobenzene compounds are calculated by B3LYP/6-311G* and CIS methods, respectively. Based on the configurations obtained above, the absorption maximum and emission maximum are calculated by TD-DFT method, the results are in good agreement with observed values. Further study shows a linear relationship between Dl1/2 (the observed absorption half-band width) and S (the difference between the emission maximum and absorption maximum). According to the relationship obtained, the absorption half-band width of other azobenzene derivatives in the same series could be predicted successfully. q 2005 Elsevier B.V. All rights reserved. Keywords: Azobenzene compounds; TD-DFT; Absorption maximum; Emission maximum; Half-band width
1. Introduction The absorption bandwidth has a great influence on the color and brightness of dyestuffs. For functional dyes, such as laser dyes, fluorescent dyes and organic non-linear optical materials and so on, strict demands on absorption band have to be satisfied. Therefore, the research on absorption bandwidth of dyes is of great importance. However, owing to the complexity of the formation of absorption bandwidth and the variety of its influencing factors, such as molecular configuration, steric hindrance effect, solvent effect and so on, the prediction of absorption bandwidth is a difficult work. Usually, half-band width (Dl1/2, the width at the half height of the absorption band) is used to characterize the width of the absorption band. As pointed out in Ref. [1], the absorption bandwidth is dependent on the overall equilibrium geometry difference between the ground state and the first singlet-excited state, and great geometry differences would lead to broad * Corresponding author. Tel.: C86 57187951620; fax: C86 57187951227. E-mail address: [email protected] (J. Liu).
0166-1280/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2005.06.021
absorption bands. For example, the bandwidth of cyanine dyes is narrower than that of polyene compounds because the geometry change from the ground state to the first singlet-excited state of cyanine dyes is less than that of polyene compounds. Since both the bandwidth and Stoke’s shift (the frequency difference between the absorption maximum and fluorescence maximum) are dependent on the equilibrium geometry difference between the ground state and the first singlet-excited state, perhaps it is not surprising that there exists a simple relationship between them. In literature, M. Pestemer etc. [2] illustrated that for a wide range of fluorescent brighteners there existed a simple relationship between the width of the absorption band and the Stoke’s shift, and L. B. Cheng etc. [3] found a relationship between the half-band width and the Stoke’s shift obtained by PPP-MO method for 31 dyes. In recent years, DFT method has been developed for optimizing molecular geometry and calculating electronic spectrum, and generally speaking, better results could be obtained than semi-empirical methods. In this article, the absorption maximum and emission maximum of 10 azobenzene compounds (Fig. 1) are calculated by DFT method, and their absorption half-band width is studied on the basis of the results of predecessors [2,3].
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J. Liu et al. / Journal of Molecular Structure: THEOCHEM 730 (2005) 151–154
R5
4
5
14 3
R1
6
1 N
2 N
13
9 12
8
7
R3
R2
R4
10
N(R)2 15
11
configuration at the level 6-311G*, and then the emission maximum lmax is also calculated by TD-DFT method at the level 6-311G*. For the convenience of comparison of calculated results with observed ones in ethanol, all calculations are performed with ethanol as solvent (with key words scrf in Gaussian package).
Fig. 1. The general structure of azobenzene compounds.
3. Results and discussion 2. Calculation method
3.1. Geometry optimization
Based on Gaussian 98 package, the ground state configuration is optimized with B3LYP/ 6-311G* method, and the absorption maximum lmax is calculated with timedependent density functional theory (TD-DFT) at the level 6-311G*. The CIS method, which has been used popularly in recent years in the research of the electron excited states [4–6], is used to optimize the first singlet-excited state
Partial data of the optimized ground state and first singlet-excited state geometries are shown in Tables 1 and 2, respectively. For the convenience of comparison, the differences between them are also listed in Table 2. Viewed from the data in Table 1, the length of nitrogen–nitrogen bond (LN–N) is longer than that of common nitrogen–nitrogen double-bonds (about
Table 1 Partial data of the optimized ground state geometry No.
1 2 3 4 5 6 7 8 9 10 a b c
substituenta R1
R2
R3
R4
NO2 CN CHO SO2CH2COOH SO2CH2COOH SO2CH2COOH SO2CH2COOH OCH3 SO2CH2COOH NO2
H H H H CH3 OCH3 H SO2CH2COOH H H
H H H H H H OCH3 H H CN
H H H NHCOCH3 NHCOCH3 NHCOCH3 NHCOCH3 NHCOCH3 H H
LN-Nb (10K10 m)
QN15c (1.6!10K19 C)
1.3036 1.3019 1.3001 1.2994 1.2985 1.2983 1.2965 1.2973 1.2971 1.3031
K0.7071 K0.7076 K0.7078 K0.7135 K0.7134 K0.7133 K0.7126 K0.7127 K0.7006 K0.7053
RaCH2CH3, R5aH. The length of nitrogen–nitrogen bond. The net charge on atom N15.
Table 2 Partial data of the optimized first singlet-excited state geometry No.
LN–Na (10K10m)
DLb (10K10 m)
QN15c (1.6!10K19 C)
DQd (1.6!10K19 C)
1 2 3 4 5 6 7 8 9 10
1.3174 1.3089 1.3087 1.3123 1.3080 1.3085 1.3177 1.3032 1.3028 1.3184
0.0138 0.0070 0.0086 0.0129 0.0095 0.0102 0.0212 0.0059 0.0057 0.0153
K0.7083 K0.7095 K0.7085 K0.7146 K0.7145 K0.7139 K0.7133 K0.7138 K0.7012 K0.7059
K0.0012 K0.0019 K0.0007 K0.0011 K0.0011 K0.0006 K0.0007 K0.0011 K0.0006 K0.0006
a b c d
The length of nitrogen–nitrogen bond. The difference of LN–N between the ground state and the first singlet-excited state. The net charge on atom N15. The difference of QN15 between the ground state and the first singlet-excited state.
J. Liu et al. / Journal of Molecular Structure: THEOCHEM 730 (2005) 151–154 Table 3 The calculated and observed results of azobenzene compounds No.
lobsa (nm)
lmaxb (nm)
lmax c (nm)
Sd (nm)
Dl1/2e (nm)
1 2 3 4 5 6 7 8 9 10
486 468 473 494 495 500 488 481 487 536
490.62 475.13 478.55 499.37 500.27 507.76 495.45 487.34 492.61 541.95
546.43 522.35 523.67 554.04 549.59 557.91 555.54 529.76 544.84 600.61
55.81 47.22 45.12 54.67 49.32 50.15 60.09 42.42 52.23 58.66
106 97 96 106 98 101 109 93 101 107
a b c d e
As shown in Table 2, compared with the ground state, the net charge on atom N15 (QN15) in the first singlet-excited state is almost unchanged, but the length of nitrogennitrogen bond (LN–N) is about 0.011!10K10 m longer in average, which is an indication that the nitrogen-nitrogen bond is weakened. 3.2. Electronic spectrum and absorption half-band width Based on the optimized geometries of the ground state and the first singlet-excited state, TD-DFT method is used to calculate the absorption maximum and emission maximum. Table 3 presents the calculated results (absorption maximum lmax, emission maximum lmax and SZ lmax K lmax ) and the corresponding observed values (absorption maximum lobs and half-band width Dl1/2) [3–5]. As shown, the agreement between the calculated absorption maximum and the observed one is satisfactory. Using the method of regression, a linear relationship (RZ0.9837) between Dl1/2 and S is found (Fig. 2 and Eq. (1)).
The observed absorption maximum. The calculated absorption maximum. The calculated emission maximum. SZ lmax K lmax . The observed half-band width.
112
∆λ1/2 (nm)
153
108
Dl1=2 Z 54:1318 C 0:9166S
104
To illustrate the applicability of equation 1, the absorption half-band width of other azobenzene derivatives in the same series is predicted. The ground state and the first singlet-excited state geometries, the absorption maximum, the emission maximum and S value are all calculated by the methods mentioned above, Dl1/2 value is obtained from Eq. (1). The results are listed in Table 4, and the observed values [4,7,8] are given together. As indicated, the calculated half-band width by Eq. (1) coincides with the observed one satisfactorily.
100 96 92 40
44
48
52
56
60
S (nm)
(1)
Fig. 2. The relationship between Dl1/2 and S.
1.2!10K10 m) and shorter than that of common nitrogen–nitrogen single-bonds (about 1.4!10K10 m). This could be explained by the facts that the formation of a large conjugated system between the coupling component and the diazo component and that the uniformization of bond length results from p-electron delocalization. It is also found in Table 1 that the charge on atom N15(QN15) is rather negative.
4. Conclusion Based on the ground state and the first singlet-excited state configurations calculated by B3LYP/6-311G* and CIS method, the absorption maximum and emission maximum of some azobenzene compounds are calculated by TD-DFT methods. The results show a linear relationship between Dl1/2 and S. Using the relationship obtained, the absorption half-band width
Table 4 The predicted and observed half-band width of azobenzene compounds substituenta R1
R4
R5
R
CHaC(CN)2 SO2CH2COOH NO2
H NHCOCH3 H
H OCH3 H
CH2CH3 CH2CH3 CH2CH2CN
a b c d e f
R2aR3aH. The calculated absorption maximum. The calculated emission maximum. SZ lmax K lmax . The predicted half-band width by Eq. (1). The observed half-band width.
lmaxb (nm)
lmax c (nm)
Sd (nm)
Dl1/2e (nm)
Dlobs1/2f (nm)
540.35 475.66 488.12
611.66 587.03 541.87
71.31 111.37 53.75
119.49 156.21 103.40
114 163 110
154
J. Liu et al. / Journal of Molecular Structure: THEOCHEM 730 (2005) 151–154
of other azobenzene derivatives in the same series could be predicted successfully. Therefore, the study in this article offers a way of predicting absorption half-band width by DFT method.
References [1] J. Griffiths, Color and Constitution of Organic Molecules, Academic press, London, 1976. Chapter 3.
[2] M. Pestemer, A. Berger, A. Wangner, Textilveredlung 19 (1964) 420. [3] L.B. Cheng, X. Chen, Y.F. Hou, J. Griffiths, Dyes Pigments 10 (1989) 123. [4] A.M. Grana, J. Mol. Struct. (Theochem) 466 (1999) 145. [5] M.K. Shukla, A. Kumar, P.C. Mishra, J. Mol. Struct. (Theochem) 535 (2001) 269. [6] H.B. Yi, X.Y. Li, X.H. Duan, Acta Chim. Sinica 62 (2004) 583. [7] R.W. Lu¨, Y.L. Zhang, K.Y. Gao, Dyestuff Ind. 30 (1993) 1. [8] X. Pan, M. Wang, Chem. J. Chin. Univer. 15 (1994) 574.
Research on the prediction of absorption half-band width of some azobenzene compounds Junna Liu*, Shenfeng Yuan, Zhirong Chen Department of Chemical Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China Received 17 January 2005; revised 16 June 2005; accepted 20 June 2005 Available online 2 August 2005
Abstract In order to predict the absorption half-band width, the ground state and the first singlet-excited state configurations of some azobenzene compounds are calculated by B3LYP/6-311G* and CIS methods, respectively. Based on the configurations obtained above, the absorption maximum and emission maximum are calculated by TD-DFT method, the results are in good agreement with observed values. Further study shows a linear relationship between Dl1/2 (the observed absorption half-band width) and S (the difference between the emission maximum and absorption maximum). According to the relationship obtained, the absorption half-band width of other azobenzene derivatives in the same series could be predicted successfully. q 2005 Elsevier B.V. All rights reserved. Keywords: Azobenzene compounds; TD-DFT; Absorption maximum; Emission maximum; Half-band width
1. Introduction The absorption bandwidth has a great influence on the color and brightness of dyestuffs. For functional dyes, such as laser dyes, fluorescent dyes and organic non-linear optical materials and so on, strict demands on absorption band have to be satisfied. Therefore, the research on absorption bandwidth of dyes is of great importance. However, owing to the complexity of the formation of absorption bandwidth and the variety of its influencing factors, such as molecular configuration, steric hindrance effect, solvent effect and so on, the prediction of absorption bandwidth is a difficult work. Usually, half-band width (Dl1/2, the width at the half height of the absorption band) is used to characterize the width of the absorption band. As pointed out in Ref. [1], the absorption bandwidth is dependent on the overall equilibrium geometry difference between the ground state and the first singlet-excited state, and great geometry differences would lead to broad * Corresponding author. Tel.: C86 57187951620; fax: C86 57187951227. E-mail address: [email protected] (J. Liu).
0166-1280/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2005.06.021
absorption bands. For example, the bandwidth of cyanine dyes is narrower than that of polyene compounds because the geometry change from the ground state to the first singlet-excited state of cyanine dyes is less than that of polyene compounds. Since both the bandwidth and Stoke’s shift (the frequency difference between the absorption maximum and fluorescence maximum) are dependent on the equilibrium geometry difference between the ground state and the first singlet-excited state, perhaps it is not surprising that there exists a simple relationship between them. In literature, M. Pestemer etc. [2] illustrated that for a wide range of fluorescent brighteners there existed a simple relationship between the width of the absorption band and the Stoke’s shift, and L. B. Cheng etc. [3] found a relationship between the half-band width and the Stoke’s shift obtained by PPP-MO method for 31 dyes. In recent years, DFT method has been developed for optimizing molecular geometry and calculating electronic spectrum, and generally speaking, better results could be obtained than semi-empirical methods. In this article, the absorption maximum and emission maximum of 10 azobenzene compounds (Fig. 1) are calculated by DFT method, and their absorption half-band width is studied on the basis of the results of predecessors [2,3].
152
J. Liu et al. / Journal of Molecular Structure: THEOCHEM 730 (2005) 151–154
R5
4
5
14 3
R1
6
1 N
2 N
13
9 12
8
7
R3
R2
R4
10
N(R)2 15
11
configuration at the level 6-311G*, and then the emission maximum lmax is also calculated by TD-DFT method at the level 6-311G*. For the convenience of comparison of calculated results with observed ones in ethanol, all calculations are performed with ethanol as solvent (with key words scrf in Gaussian package).
Fig. 1. The general structure of azobenzene compounds.
3. Results and discussion 2. Calculation method
3.1. Geometry optimization
Based on Gaussian 98 package, the ground state configuration is optimized with B3LYP/ 6-311G* method, and the absorption maximum lmax is calculated with timedependent density functional theory (TD-DFT) at the level 6-311G*. The CIS method, which has been used popularly in recent years in the research of the electron excited states [4–6], is used to optimize the first singlet-excited state
Partial data of the optimized ground state and first singlet-excited state geometries are shown in Tables 1 and 2, respectively. For the convenience of comparison, the differences between them are also listed in Table 2. Viewed from the data in Table 1, the length of nitrogen–nitrogen bond (LN–N) is longer than that of common nitrogen–nitrogen double-bonds (about
Table 1 Partial data of the optimized ground state geometry No.
1 2 3 4 5 6 7 8 9 10 a b c
substituenta R1
R2
R3
R4
NO2 CN CHO SO2CH2COOH SO2CH2COOH SO2CH2COOH SO2CH2COOH OCH3 SO2CH2COOH NO2
H H H H CH3 OCH3 H SO2CH2COOH H H
H H H H H H OCH3 H H CN
H H H NHCOCH3 NHCOCH3 NHCOCH3 NHCOCH3 NHCOCH3 H H
LN-Nb (10K10 m)
QN15c (1.6!10K19 C)
1.3036 1.3019 1.3001 1.2994 1.2985 1.2983 1.2965 1.2973 1.2971 1.3031
K0.7071 K0.7076 K0.7078 K0.7135 K0.7134 K0.7133 K0.7126 K0.7127 K0.7006 K0.7053
RaCH2CH3, R5aH. The length of nitrogen–nitrogen bond. The net charge on atom N15.
Table 2 Partial data of the optimized first singlet-excited state geometry No.
LN–Na (10K10m)
DLb (10K10 m)
QN15c (1.6!10K19 C)
DQd (1.6!10K19 C)
1 2 3 4 5 6 7 8 9 10
1.3174 1.3089 1.3087 1.3123 1.3080 1.3085 1.3177 1.3032 1.3028 1.3184
0.0138 0.0070 0.0086 0.0129 0.0095 0.0102 0.0212 0.0059 0.0057 0.0153
K0.7083 K0.7095 K0.7085 K0.7146 K0.7145 K0.7139 K0.7133 K0.7138 K0.7012 K0.7059
K0.0012 K0.0019 K0.0007 K0.0011 K0.0011 K0.0006 K0.0007 K0.0011 K0.0006 K0.0006
a b c d
The length of nitrogen–nitrogen bond. The difference of LN–N between the ground state and the first singlet-excited state. The net charge on atom N15. The difference of QN15 between the ground state and the first singlet-excited state.
J. Liu et al. / Journal of Molecular Structure: THEOCHEM 730 (2005) 151–154 Table 3 The calculated and observed results of azobenzene compounds No.
lobsa (nm)
lmaxb (nm)
lmax c (nm)
Sd (nm)
Dl1/2e (nm)
1 2 3 4 5 6 7 8 9 10
486 468 473 494 495 500 488 481 487 536
490.62 475.13 478.55 499.37 500.27 507.76 495.45 487.34 492.61 541.95
546.43 522.35 523.67 554.04 549.59 557.91 555.54 529.76 544.84 600.61
55.81 47.22 45.12 54.67 49.32 50.15 60.09 42.42 52.23 58.66
106 97 96 106 98 101 109 93 101 107
a b c d e
As shown in Table 2, compared with the ground state, the net charge on atom N15 (QN15) in the first singlet-excited state is almost unchanged, but the length of nitrogennitrogen bond (LN–N) is about 0.011!10K10 m longer in average, which is an indication that the nitrogen-nitrogen bond is weakened. 3.2. Electronic spectrum and absorption half-band width Based on the optimized geometries of the ground state and the first singlet-excited state, TD-DFT method is used to calculate the absorption maximum and emission maximum. Table 3 presents the calculated results (absorption maximum lmax, emission maximum lmax and SZ lmax K lmax ) and the corresponding observed values (absorption maximum lobs and half-band width Dl1/2) [3–5]. As shown, the agreement between the calculated absorption maximum and the observed one is satisfactory. Using the method of regression, a linear relationship (RZ0.9837) between Dl1/2 and S is found (Fig. 2 and Eq. (1)).
The observed absorption maximum. The calculated absorption maximum. The calculated emission maximum. SZ lmax K lmax . The observed half-band width.
112
∆λ1/2 (nm)
153
108
Dl1=2 Z 54:1318 C 0:9166S
104
To illustrate the applicability of equation 1, the absorption half-band width of other azobenzene derivatives in the same series is predicted. The ground state and the first singlet-excited state geometries, the absorption maximum, the emission maximum and S value are all calculated by the methods mentioned above, Dl1/2 value is obtained from Eq. (1). The results are listed in Table 4, and the observed values [4,7,8] are given together. As indicated, the calculated half-band width by Eq. (1) coincides with the observed one satisfactorily.
100 96 92 40
44
48
52
56
60
S (nm)
(1)
Fig. 2. The relationship between Dl1/2 and S.
1.2!10K10 m) and shorter than that of common nitrogen–nitrogen single-bonds (about 1.4!10K10 m). This could be explained by the facts that the formation of a large conjugated system between the coupling component and the diazo component and that the uniformization of bond length results from p-electron delocalization. It is also found in Table 1 that the charge on atom N15(QN15) is rather negative.
4. Conclusion Based on the ground state and the first singlet-excited state configurations calculated by B3LYP/6-311G* and CIS method, the absorption maximum and emission maximum of some azobenzene compounds are calculated by TD-DFT methods. The results show a linear relationship between Dl1/2 and S. Using the relationship obtained, the absorption half-band width
Table 4 The predicted and observed half-band width of azobenzene compounds substituenta R1
R4
R5
R
CHaC(CN)2 SO2CH2COOH NO2
H NHCOCH3 H
H OCH3 H
CH2CH3 CH2CH3 CH2CH2CN
a b c d e f
R2aR3aH. The calculated absorption maximum. The calculated emission maximum. SZ lmax K lmax . The predicted half-band width by Eq. (1). The observed half-band width.
lmaxb (nm)
lmax c (nm)
Sd (nm)
Dl1/2e (nm)
Dlobs1/2f (nm)
540.35 475.66 488.12
611.66 587.03 541.87
71.31 111.37 53.75
119.49 156.21 103.40
114 163 110
154
J. Liu et al. / Journal of Molecular Structure: THEOCHEM 730 (2005) 151–154
of other azobenzene derivatives in the same series could be predicted successfully. Therefore, the study in this article offers a way of predicting absorption half-band width by DFT method.
References [1] J. Griffiths, Color and Constitution of Organic Molecules, Academic press, London, 1976. Chapter 3.
[2] M. Pestemer, A. Berger, A. Wangner, Textilveredlung 19 (1964) 420. [3] L.B. Cheng, X. Chen, Y.F. Hou, J. Griffiths, Dyes Pigments 10 (1989) 123. [4] A.M. Grana, J. Mol. Struct. (Theochem) 466 (1999) 145. [5] M.K. Shukla, A. Kumar, P.C. Mishra, J. Mol. Struct. (Theochem) 535 (2001) 269. [6] H.B. Yi, X.Y. Li, X.H. Duan, Acta Chim. Sinica 62 (2004) 583. [7] R.W. Lu¨, Y.L. Zhang, K.Y. Gao, Dyestuff Ind. 30 (1993) 1. [8] X. Pan, M. Wang, Chem. J. Chin. Univer. 15 (1994) 574.