Effects of substituent and solvent on the structure and spectral properties of maleimide derivatives

Journal of Molecular Structure: THEOCHEM 860 (2008) 58–63

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Effects of substituent and solvent on the structure and spectral properties of maleimide derivatives Chin-Kuen Tai a, Yih-Jiun Lin a, Pao-Ling Yeh a, Yi-Ren Tzeng b, Yu-Ma Chou c, Bo-Cheng Wang a,* a

Department of Chemistry, Tamkang University, Tamsui 251, Taiwan Institute of Nuclear Energy Research (INER), Taiwan c Department of Physics, Chinese Culture University, Taipei 110, Taiwan b

a r t i c l e

i n f o

Article history: Received 10 December 2007 Received in revised form 17 March 2008 Accepted 17 March 2008 Available online 25 March 2008 Keywords: PCM simulation Density functional theory BLA Maleimide derivatives Substituted effect

a b s t r a c t The maximum absorption wavelength ðkmax abs Þ, emission wavelength (kem) and the related oscillator strength (f) of the maleimides in the ground and first excited states were calculated by using the DFT, CIS and the time-dependent density functional theory (TD-DFT) methods, where the molecular structures were optimized by DFT/B3LYP/6-31G* calculation. Solvent effects on the maleimides were examined using the PCM simulation at DFT/B3LYP level with the 6-31G* basis set. For N-substituted maleimide, the substituent gives only a slight influence on the maleimide chromophore, while planar conformation of PhMLH leads to the improvement in p-delocalization from substituent to maleimide unit. For 3,4substituted maleimide, the steric repulsion between substituent and maleimide chromophore influences the extent of p-delocalization and the molecular conformation. The calculated ðkmax abs Þ and kem of maleimides are in good agreement with the experimental data. In the gas phase, both absorption and emission peaks are red-shift as compared to the non-substituted maleimide. Under solvent environment, the more planar conformation of PhMLH shows a blue-shift in the calculated ðkmax abs Þ and kem as compared with other N-substituted maleimides. For 3,4-substituted maleimides, the effect of substitution produces the most significant spectral red-shift as compared to other maleimides. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction With the pioneer work of Chen and co-workers on naphthylphenylamino substituted N-methyl-3,4-diphenylmaleimide [1] as an efficient and bright non-doped organic red emitter, there is a great interest in using maleimide derivatives as a new type of fluorescence dye [2–5]. Recently, Chen et al. have developed a procedure for synthesizing a series of 3,4-diaryl-maleimides, containing fluorophores [6]. They observed different substituent on diaryl-substituted maleimide that can result in a large variation in fluorescence. To understand the spectral property of various maleimides, quantum mechanic calculations have been employed to determine their geometric and electronic structures [7–9]. In one of our recent publications, the semi-empirical AM1 and the density functional theory (DFT) methods were used to generate the geometries for the ground and excited states of the 3,4-diaryl substituted maleimides. The amount of Stokes shift was calculated to be proportional to the deviations in molecular structures between the non-planar geometry in the ground state and the more planar one in the excited state [10]. * Corresponding author. Tel.: +886 226215656x2438; fax: +886 226209924. E-mail address: [email protected] (B.-C. Wang). 0166-1280/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2008.03.012

In this paper, we extend our previous calculations to consider other factors, which might also affect the geometry of maleimides and result a noticeable Stoke shift. Because the effect of functional substitution can generate a large variation in spectral properties for fluorophores, one would expect that the substituent with different electron affinity connecting to the 3,4-substituted sites should change the bond order of the carbon–carbon bond and thus affect the geometric and electronic structures of maleimides. To verify this exception, the DFT calculation was used to investigate on how the substituent affects the geometric and electronic structures of maleimides and whether those changes can be used to explain the large variations in fluorescence observed in the experiment. The solvent environment can influence the transition property of the molecule that has been observed experimentally. Two different quantum mechanical models have been used to simulate the molecule dissolved in the solvent environment: the continuum and the discrete models [11]. The continuum model treats solvent as a continuum medium, thus, the physical properties of molecule with the solvent environment can be evaluated. The discrete model regards the solvent as a molecule, and the solvent–solute interaction can be investigated to give a detail of the solvent perturbations, with some reasonable assumptions, the reaction sites with the solvent–solute coupling and the mount of the solvent.

C.-K. Tai et al. / Journal of Molecular Structure: THEOCHEM 860 (2008) 58–63

The influence of spectral properties of maleimides in different solvent polarity environments was determined by the PCM simulation. The PCM simulation is an extension of the solvent reaction field model, which was introduced by Born, Kirkwood and Onsager for the charge distribution in a spherical cavity surrounded by a dielectric medium [12,13]. Moreover, the PCM uses a more realistic shape of the cavity to simulate the dielectric response with discrete charges on the cavity surface, and introduce non-polar contributions to the free energy of solvation as well as the electrostatic component. There are several formalisms of PCM in the literature and most of their differences can be attributed to the empirical parameters used (for example, atomic van der Waals radii, etc.) [14]. Since the optical spectra of maleimides were recorded under the solvent environment, thus the PCM simulation was adopted to calculate the spectral properties of maleimides. The bond alternation analysis (BLA) was used to examine the extent of p-electron delocalization of maleimides under the influence of various substituents. The aim of this work is to clarify how the physical and chemical properties of the substituents are related to the observed fluorescence. First, a brief review of the theoretical methods will be presented carried out. Second, the calculated geometries, electronic structures and spectral properties of maleimide derivatives with the objective of clarifying the effects of substituent and solvent will be discussed. 2. Computational method The geometries of maleimides were optimized by using the STO-3G, 3-21G, 6-31G, 6-31G*, 6-31+G* and 6-31++G** basis sets with the DFT/B3LYP and the Hartree–Fock (HF) method. The calculated results were compared with the experimental data to select an appropriate basis set for this molecular system. As shown in Table 1, the calculated bond lengths converge for the 6-31G* basis set that give nearly the same results as the 6-31++G**. The normalized deviations of Di ¼ ½di ð6-31G Þ  di ð6-31 þ þG Þ=di ð6-31 þ þG Þ with di being the bond length of the ith row in Table 1 are all smaller than 0.002 Å. In general, the calculated bond angles (last two rows in Table 1) are independent of the basis set used. The bond lengths and bond angles of maleimides calculated by using DFT/ B3LYP method with the 6-31G*, 6-31+G* and 6-31++G** basis sets are in good agreement with the experiment. Thus, the 6-31G* basis

59

set was selected to perform the calculations with a reasonable CPU time requirement. Theoretically, the solvent–solute simulations are described by a solvent reaction field that can be partitioned into many contributions of different physical origin, such as dispersion, repulsive and electrostatic forces between solvent and solute molecules [15]. The PCM simulation was based on the quantum mechanical (QM) method, where the solute molecule is first calculated by ab initio technique. Then, the interactions between solute and solvent ^ R , which molecules are considered through the reaction potential, V acts as a perturbation on the solute Hamiltonian as follows: ^ 0 W0 ¼ E0 W0 in vacuo H ^0 þ V ^ R W ¼ E0 W in solution ½H ^ 0 is the Hamiltonian of the solute in vacuo (including nuclewhere H ar repulsion terms), W0 and W are the solute wavefunctions in vacuo and in solution, respectively. Among all QM reaction field methods, the PCM has been popularly used in molecular simulations because of its adaptability and accuracy [11,15,16]. In PCM method, the elec^0 trostatic component of the reaction potential is needed to add to H to obtain an effective Hamiltonian. The electrostatic component is generally described by a set of induced point charge {qi} placed at the center of small elements covering the cavity in the dielectric medium, where the solute is embedded. The Gaussian 03 package was used [17] to study the substituent effect and the spectral properties of maleimides. The geometries of non-substituted maleimide and its derivatives at the ground state are optimized by the DFT/B3LYP level using the 6-31G* basis sets. For the first excited state geometry, the calculation was started with the optimized ground state geometry and then modified by using the configuration interaction singles (CIS) method. The maximum absorption wavelength ðkmax abs Þ, emission wavelength (kem) and related oscillator strength (f) were calculated using the timedependent density functional theory (TD-DFT) method. For the representation that the different substituents influence the extent of p-electron conjugations on the cyclic five-member ring of maleimide chromophore, the bond length alternation (BLA) method was computed [18]. BLA value is defined as the bond length difference between the single and double bonds, i.e., R(CAC) – R(C@C). 3. Results and discussions

Table 1 Calculated and experimental geometric parameters (Å and degree) of non-substituted maleimidea

N1AC2 N1AC5 C2AC3 C4AC5 C3AC4 C2AO6 C5AO7 O6AC2AN1 O7AC5AN1

STO-3G

3-21G

6-31G

6-31G*

6-31+G*

6-31++G**

Expt.b

1.451 1.424 1.451 1.424 1.539 1.523 1.539 1.523 1.344 1.314 1.254 1.217 1.254 1.217 126.3 126.4 126.3 126.4

1.406 1.389 1.406 1.389 1.512 1.508 1.512 1.508 1.335 1.317 1.227 1.202 1.227 1.202 126.7 126.8 126.7 126.8

1.404 1.386 1.404 1.386 1.501 1.495 1.501 1.495 1.344 1.326 1.236 1.210 1.236 1.210 126.3 126.5 126.3 126.5

1.398 1.381 1.398 1.381 1.504 1.502 1.504 1.502 1.337 1.319 1.212 1.185 1.212 1.185 126.5 126.6 126.5 126.6

1.397 1.381 1.397 1.381 1.504 1.502 1.504 1.502 1.339 1.321 1.214 1.187 1.214 1.187 126.3 126.6 126.3 126.6

1.397 1.380 1.397 1.380 1.504 1.502 1.504 1.502 1.339 1.321 1.214 1.187 1.214 1.187 126.3 126.5 126.3 126.5

1.407 1.381 1.467 1.451 1.301 1.200 1.235 123.0 123.6

a The calculation results are used B3LYP, and Hartree–Fock method (shown in bold). b The experimental data from Ref. [19].

Fig. 1 lists the molecular structures of maleimide and its derivatives that will be investigated. These compounds include the unsubstituted maleimide, the N-substituted maleimides and the 3,4-substituted maleimides. The N-substituted maleimides are 1methyl-maleimide (MeMLH), 1-phenyl-maleimide (PhMLH) and 1-(2,6-diisopropyl-phenyl)-maleimide (2,6-DipMLH) that are maleimide at nitrogen position with methyl, phenyl and 2,6-diisopropyl-phenyl substituents, respectively. The 3,4-substituted maleimides contain one asymmetrically substituted maleimide of 3-(1H-indol-3yl)-4-bromo-maleimide (ML-3ind-4Br) and four symmetrically substituted maleimides of 3,4-dimethyl-maleimide (MLE), 3,4-diphenyl-maleimide (MLH), 1-methyl-3,4-diphenylmaleimide (MLMe) and 1-methyl-3,4-bis-(4-methoxy-phenyl)maleimide (4CH3OMLMe). The calculated geometric parameters of maleimides are shown in Table 2. Table 3 compares the difference in the calculated geometric parameters for the ground and excited states of maleimide chromophore in the maleimide derivatives. The calculated BLAs are shown in Table 4 for the extent of p-electron delocalization in the parent maleimide chromophore as resulted from the different substituents. The ðkmax abs Þ, kem and related oscillator strength (f) for these maleimides are compared and listed in Tables 5 and 6.

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C.-K. Tai et al. / Journal of Molecular Structure: THEOCHEM 860 (2008) 58–63

Fig. 1. Molecular structures of maleimides.

Table 2 Calculated geometric parameters (Å and degree) of the maleimides using the DFT/B3LYP/6-31G* methoda

N1AC2 N1AC5 C2AC3 C4AC5 C3AC4 C2AO6 C5AO7 C3AC8 C4AC9 O6AC2AN1 O7AC5AN1 C10AC8AC3AC2 a b

Maleimide

MeMLH

PhMLH

2,6-DipMLH

ML-3ind-4-Br

MLE

MLH

MLMe

4CH3OMLMe

1.398 1.398 1.504 1.504 1.337 1.212 1.212 – – 126.5 126.5 –

1.397 1.399 1.504 1.504 1.336 1.214 1.214 – – 125.6 126.0 –

1.411 1.411 1.500 1.500 1.335 1.212 1.212 – – 126.4 126.3 –

1.406 1.406 1.502 1.502 1.336 1.212 1.212 – – 126.2 126.2 –

1.405 1.384 1.542 1.481 1.369 1.240 1.234 1.446 1.879b 124.2 125.1 25.3

1.396 1.396 1.506 1.508 1.347 1.214 1.214 1.492 1.496 126.3 126.2 –

1.392 1.395 1.514 1.514 1.363 1.214 1.214 1.470 1.470 125.4 125.4 38.6

1.392 1.393 1.504 1.520 1.356 1.218 1.218 1.483 1.482 124.9 124.9 38.4

1.391 1.392 1.512 1.512 1.367 1.215 1.215 1.467 1.467 125.1 125.1 36.1

Maleimides are defined in Fig. 1. The C4ABr9 bond.

In Table 2, the calculated geometric parameters of the maleimide chromophore in the ground state of maleimides are quite sensitive with the substitution, both in substituents and the substituted positions. For N-substituted maleimides, the geometries of the maleimide chromophore are almost the same as the unsubstituted maleimide. It indicates that the N-substitution does not affect the bonding properties of the maleimide unit in Nsubstituted maleimides. For 2,6-DipMLH, it shows a non-planar conformation, where the calculated dihedral angle between the diisopropyl substituent and maleimide unit is 86.21°. The non-planar geometry of 2,6-DipMLH is probably due to the steric repulsion between diisopropyl group and the carbonyl oxygen atom in the maleimide unit. The extent of p-electron delocalization is restricted within the maleimide chromophore. The calculated N1AC2 and N1AC5 bonds are shorten to have more double bond characters for the planar conformation of PhMLH (1.406/1.411 Å of 2,6-DipMLH/PhMLH).

For 3,4-substituted maleimides, the substituent shows a larger effect on the geometry of the maleimide chromophore. The most noticeable change is the increase of C3AC4 bond length, e.g., 0.010 Å for MLE, 0.032 Å for ML-3ind-4Br and 0.026 Å for MLH in maleimide unit. The ML-3ind-4Br molecule is an asymmetric maleimide, where the p-electron delocalization can be extended along the C8AC3AC4AC5 manifold and led to an increase in the calculated C3AC4 bond length (1.369 Å) for the other maleimides. For the symmetric 3,4-substituted maleimides, the phenyl group has a stronger electron-donating ability than the methyl group, it could lengthen the calculated C3AC8 and C4AC9 bond lengths as compared to MLH (1.470 Å) and MLE (1.496 Å). The dihedral angles between phenyl group and maleimide unit in symmetric 3,4-substituted maleimides were calculated as 38.6°, 38.4° and 36.1° for MLH, MLMe and 4CH3OMLMe, respectively. The distorted conformation of these maleimides is caused from the steric repulsion between the phenyl group and maleimide chromo-

61

C.-K. Tai et al. / Journal of Molecular Structure: THEOCHEM 860 (2008) 58–63 Table 3 Calculated geometric parameters (Å and degree) of maleimides at the excited state using the CIS/6-31G* methoda Maleimide

MeMLH

PhMLH

2,6-DipMLH

ML-3ind-4-Br

MLE

MLH

MLMe

4CH3OMLMe

C3AC8

1.397 1.381 1.397 1.381 1.452 1.502 1.452 1.502 1.344 1.319 1.216 1.185 1.216 1.185 –

1.397 1.380 1.401 1.381 1.449 1.501 1.453 1.501 1.342 1.318 1.221 1.187 1.214 1.187 –

1.467 1.401 1.467 1.401 1.407 1.494 1.407 1.494 1.388 1.313 1.210 1.186 1.210 1.186 –

1.419 1.387 1.419 1.387 1.447 1.500 1.447 1.500 1.343 1.318 1.212 1.186 1.212 1.186 –

C4AC9

–

–

–

–

O6AC2AN1

122.0 126.6 122.0 126.6 –

121.4 125.9 122.3 126.4 –

123.6 128.0 123.6 128.0 –

122.4 126.5 122.4 126.5 –

1.376 1.375 1.388 1.379 1.470 1.519 1.451 1.495 1.419 1.331 1.205 1.185 1.200 1.183 1.410 1.456 1.855b 1.861b 124.8 125.8 125.5 126.7 27.9 37.1

1.393 1.378 1.393 1.378 1.457 1.503 1.457 1.505 1.349 1.327 1.218 1.187 1.219 1.187 1.501 1.494 1.504 1.499 122.1 126.5 122.4 126.4 –

1.380 1.376 1.381 1.376 1.481 1.510 1.482 1.510 1.442 1.336 1.200 1.187 1.201 1.187 1.415 1.479 1.416 1.479 124.3 125.9 124.3 125.9 28.1 44.9

1.380 1.375 1.382 1.376 1.472 1.509 1.473 1.510 1.434 1.334 1.202 1.189 1.202 1.189 1.421 1.479 1.421 1.479 123.6 125.1 124.2 125.6 28.0 44.60

1.381 1.375 1.380 1.377 1.483 1.509 1.480 1.509 1.441 1.336 1.203 1.189 1.203 1.189 1.413 1.477 1.412 1.477 124.2 125.0 124.2 125.5 26.5 43.21

N1AC2 N1AC5 C2AC3 C4AC5 C3AC4 C2AO6 C5AO7

O7AC5AN1 C10AC8AC3AC2 a b

Calculation results used the CIS/6-31G* method for the excited state and HF/6-31G* method for ground state (shown in bold). The C4ABr9 bond.

Table 4 Bond length alternation (BLA) parameters (Å) of maleimides at the ground state and excited state using the B3LYP/6-31G* and CIS/6-31G* methods (shown in parentheses) Compound

BLA1

BLA2

BLA3

BLA4

Maleimide MeMLH PhMLH 2,6-DipMLH ML-3ind-4-Br MLE MLH MLMe 4CH3OMLMe

0.167(0.108) 0.168(0.107) 0.165(0.019) 0.166(0.104) 0.173(0.051) 0.159(0.108) 0.151(0.039) 0.148(0.038) 0.145(0.042)

0.167(0.108) 0.168(0.111) 0.165(0.019) 0.166(0.104) 0.112(0.032) 0.161(0.108) 0.151(0.040) 0.164(0.039) 0.145(0.039)

– – – – 0.077(0.009) 0.145(0.152) 0.107(0.027) 0.127(0.013) 0.100(0.028)

– – – – 0.510(0.436) 0.149(0.152) 0.107(0.026) 0.112(0.013) 0.100(0.029)

phore. By comparing MLH and MLMe with MLE, the larger size of substituent, such as phenyl group would produce the greater elongation of the C3AC4 bond (1.363 Å for MLH, 1.356 Å for MLMe and 1.347 Å for MLE as shown in Table 2). The results suggest that the repulsion at 3,4-substituents can lead to the elongation of the C3AC4 bond. For 4CH3OMLMe, the methoxy-phenyl group has stronger electron-donating ability than the phenyl group in MLH and MLMe. It causes the bond length to slightly abbreviate in the C3AC8 and C4AC9 bonds.

For ML-3ind-4Br, the electron-withdrawing substituent, Br, generates the C4ABr9 bond having more single bond character and causes the C4AC9 bond to be elongated significantly than other maleimides (1.879 Å (ML-3ind-4Br) vs. 1.496 Å (MLE) and 1.470 Å (MLH)). On the other hand, the electron-donating substituent, 1Hindol-3yl, leads to a more double bond character in the calculated C3AC8 bond, a decrease in the C3AC8 bond length (1.446 Å), and an extension in the p-electron delocalization from the 1H-indol-3yl group to the maleimide unit. ML-3ind-4Br has a lower steric repulsion between the Br and 1H-indol-3yl groups than other 3,4substituted maleimides, and thus a decrease in the calculated dihedral angle (25.3°) between 1H-indol-3yl group and maleimide unit. It also extends the p-electron delocalization from the C4AC5 bond in the maleimide chromophore to the 1H-indol-3yl group. The optimized structures of maleimides in the first excited state (S1) have been calculated by using the CIS/6-31G* method, including the substituent effect within these maleimides. To depict the structural difference between the ground and excited states, the ground state structures were optimized by using the HF/6-31G* method for comparison. The calculated geometric parameters for these maleimides are listed in Table 3. A similar result has been noted that the distorted 2,6-DipMLH conformation seems to restrict the extent of p-electron delocalization from the substituent to maleimide unit, and lead to a decrease in the calculated

Table 5 a,b Maximum absorption wavelength ðkmax abs Þnm, and related oscillator strength (f) of maleimides by using TD-DFT with PCM simulation Maleimide

MeMLH

PhMLH

2,6-DipMLH

ML-3ind-4-Br

MLE

kmax abs

f

kmax abs

f

kmax abs

f

kmax abs

f

kmax abs

f

kmax abs

f

kmax abs

f

kmax abs

f

kmax abs

f

Gas phase Cyclohexane

274.4 279.6

0.2125 0.2446

304.6 307.6

0.1451 0.2001

301.4 298.1

0.2051 0.2660

298.11 299.07

0.1518 0.2114

460.0 432.5

0.4255 0.4636

351.1 347.8

0.2824 0.3217

401.8 360.1

0.2915 0.3341

403.1 366.1

0.2714 0.3011

0.3687 0.4126

Dichloromethane

284.8

0.2672

310.1

0.2311

0.2714

343.6

0.3651

0.2407

0.5317

342.3

0.3701

372.7 (363) 369.4

0.3459

310.7

362.0 (357) 360.5

0.3714

0.2712

441.7 (422) 443.5

0.5121

289.0

299.92 (293) 299.88

0.2412

Methanol

293.6 (314) 292.1

458.0 421.5 (387) 437.8 (404) 437.3

a b

0.2801

0.2613

The experimental data are included in parenthesis and from Refs. [2,6,20]. The TD-DFT calculation is based on the optimized structure by the DFT/B3LYP/6-31G* method.

MLH

MLMe

0.3795

4CH3OMLMe

0.3544

0.4562 0.4601

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C.-K. Tai et al. / Journal of Molecular Structure: THEOCHEM 860 (2008) 58–63

Table 6 Emission wavelength (kem, nm) and related oscillator strength (f) by using the TD-DFT method with PCM simulationa,b Maleimide

MeMLH

PhMLH

2,6-DipMLH

ML-3ind-4-Br

MLE

MLH

MLMe

4CH3OMLMe

kem

f

kem

f

kem

f

kem

f

kem

f

kem

f

kem

f

kem

f

kem

f

Gas phase Cyclohexane

303.0 307.9

0.2157 0.2514

309.6 316.3

0.1721 0.2045

370.8 361.7

0.2347 0.2614

343.7 343.8

0.2264 0.2468

496.6 521.6

0.5219 0.5536

385.7 379.7

0.2874 0.3211

0.3247 0.3665

0.2721

323.4

0.2547

351.7

0.2815

343.1

0.2789

0.6175

371.6

0.3657

Methanol

313.8

0.2812

325.4

0.2600

349.0

0.2968

343.0

0.2815

532.3 (554) 532.1

0.6301

369.3

0.3754

563.7 536.1 (506) 575.6 (548) 573.2

0.4157 0.4312

312.6

550.2 521.6 (490) 532.3 (503) 532.1

0.3144 0.3445

Dichloromethane

519.2 493.1 (471) 507.7 (493) 506.4

a b

0.4017 0.4095

0.3549 0.3671

0.4628 0.4695

The experimental data are included in parenthesis and from Refs. [2,6,20]. The TD-DFT calculation is based on the structure of excited state by the CIS/6-31G* method.

N1AC2 and N1AC5 bonds for the PhMLH (1.419/1.467 Å of 2,6-DipMLH/PhMLH). For 3,4-substituted maleimides, two substituents at the maleimide chromophore have produced the distorted conformation for these maleimides, and the calculated C3AC4 bond length and the dihedral angle between the substituent and maleimide unit were affected. For MLE, MLH, MLMe and 4CH3OMLMe, the different bond consistencies of C3AC8 and C4AC9 bonds reveal the p-bonding character within these molecules. For ML-3ind-4Br, a similar result is also observed that the different substituents have formed a molecular conformation to extend the p-electron delocalization along the C8AC3AC4AC5 path, and increase the calculated C3AC4 bond (1.369 Å) to become the longest C3AC4 bond length in these maleimides. The calculated geometric parameters of maleimides between the ground and excited states were compared. In general, the structural values of the excited state are larger than those of the ground state. For the N-substituted maleimides, the substituent has shown to influence the extent of p-delocalization in the maleimide chromophore. The group substitutions at the nitrogen position in maleimides seem to cause a significant bond length difference in the N1AC2 and N1AC5 bonds (PhMLH and 2,6-DipMLH). For the 3,4substituted maleimides, the differences in geometrical values between the ground and first excited sates are even greater, in particular the C3AC4 bond and the dihedral angle of C10AC8AC3AC2. These geometry differences may account for the possible factors to have an observed Stoke shift in maleimide derivatives. The change in the calculated bond length indicates that the substituent effect can cause p-electron delocalization in the maleimide chromophore. To evaluate the extent of p-electron delocalization of maleimides, the BLA (bond length alternation) and structural planarity were employed to analyze this phenomenon. The planarity is defined as the dihedral angle between the substituent and the maleimide unit, and the BLA is defined as the bond length difference between various pairs of carbon atoms, where BLA1 is the bond length difference between the C2AC3 and C3AC4 bonds; BLA2 is that of the C4AC5 and C3AC4 bonds; BLA3 is that of the C3AC8 and C3AC4 bonds; BLA4 is that of the C4AC9 and C3AC4 bonds and so on. The results of BLA analysis for maleimides are listed in Table 4. According to the calculated results in Table 4, the ML-3ind-4-Br maleimide gives the lowest dihedral angle that is distinctively difference from other 3,4-substituted maleimides. The ML-3ind-4-Br extends the p-electron delocalization to the 1H-indol-3yl group. For MLH, MLMe and 4CH3OMLMe, the steric repulsion of 3,4substituted functional groups makes the maleimides to become twisted, and decrease the p-conjugation length in these molecules. For the ground state of N-substituted maleimides, BLA analysis of the substituent effect in the maleimide chromophore is not as clear cut, but the significantly smaller values of BLA1 and BLA2 in PhMLH have been observed in the excited state. The result shows that the phenyl group can improve the p-electron delocalization in PhMLH as compared to other N-substituted maleimides.

When the BLA results of 3,4-substituted maleimides in the ground and excited states are compared, in general, the BLA values are quite similar and consistent, except for those of ML-3ind-4-Br. The different substituents of ML-3ind-4-Br cause the BLA2 and BLA3 values to be smaller while the BLA1 and BLA4 values are larger than other derivatives. In ML-3ind-4-Br maleimide, the unique properties of smaller BLA2 and BLA3 values (0.112 Å and 0.077 Å) represent the similar bond characters for the calculated C4AC5, C3AC4 and C3AC8 bonds that extend the p-electron delocalization from the cyclic five-membered ring to the 1H-indol-3yl group. Other significant BLA value ML-3ind-4-Br is the BLA4 (0.51 Å) that a more single bond character on the C4ABr9 bond has been produced as resulted from the electron-withdrawing Br group. When the BLA values of MLH, MLMe and 4CH3OMLMe are compared, all values are significantly differences to those of nonsubstituted MLE. This is due to the fact that 3,4-substituents on MLE can improve the extent of p-electron delocalization in maleimide chromophore, and cause the smaller BLA1 and BLA2 values of 3,4-substituted maleimides as compared to the non-substituted maleimide. Although the steric repulsions between substituents may cause a twisted conformation of 3,4-substituted maleimides, the electron-donating group can still main a large double bond character on both the C4AC9 and C3AC8 bonds and generate the smaller BLA3 and BLA4 values as compared to those of MLE. The spectral properties of maleimides were examined, where the maximum absorption wavelength ðkmax abs Þ and the emission wavelength (kem) were calculated by the TD-DFT method. The optimized structures of maleimides, in their ground and excited states were generated from DFT/B3LYP/6-31G* and CIS/6-31G* calculations, respectively. The PCM simulation was used to calculate the spectral properties of maleimides in the solvent environments. Three different solvents were employed in the PCM simulation, including cyclohexane, dichloromethane and methanol as non-polar and polar solvents, respectively. Table 5 lists the calculated maximum absorption wavelength ðkmax abs Þ and the related oscillation strength (f) for the maleimide derivatives. The calculated oscillator strengths (f) of absorption and emission spectra of maleimides in various solvent environments are summarized in Tables 5 and 6. The general trend in the calculated f values for both absorption and emission of maleimides is shown to increase as the polarity solvent environment increased. In Tables 5 and 6, the most polarity solvent used was methanol that generates the maximum f values in the PCM simulation. Different substituents on different bonding sites of maleimides were shown to influence the molecular dipole moment, and thus lead to different f values. For N-substituted maleimides, MeMLH and 2,6-DipMLH, the calculated dipole moments are 1.12 and 1.17 Debye, respectively. The results indicate that the N-substituted maleimides have a reduced molecular dipole moment with respect to non-substituted maleimide, and the f values for both absorption and emission spectra were computed to decrease accordingly. For 3,4-substituted maleimides, MLE, MLH, 4CH3OMLMe, both the calculated f values and the molecular dipole

C.-K. Tai et al. / Journal of Molecular Structure: THEOCHEM 860 (2008) 58–63

moment were shown to increase. The dipole moment, l, for MLE, MLH and 4CH3OMLMe was calculated as 2.17, 2.59 and 4.57 Debye, respectively. Both the calculated absorption and emission spectra for maleimides were in good agreement with experimental data reported in the literature [2,6]. The TD-DFT calculated ðkmax abs Þ for N-substituted maleimides is around 300 nm, which is a 30 nm red-shift related to the nonsubstituted maleimide. The calculated kmax abs for MLH is only slight difference from that of and MLMe. These results indicate that the N-substitution on maleimide has a weakly influence in the calculated kmax abs . According to the PCM calculation, the effect of solvent environment in the calculated kmax abs has displayed a red-shift for MeMLH and 2,6-DipMLH, and a blue-shift for PhMLH relative to those in the gas phase. For the asymmetric 3,4-substituted maleimides, on the other hand, the calculated kmax abs are red-shift relative to the unsubstituted maleimide in both gas phase and solvent environment. The 4CH3OMLMe has a 4-methoxy-phenyl group that can extend the p-electron delocalization from the five-membered ring of the maleimide unit to the substituent, and has generated a 174 nm red-shift relative to maleimide in the gas phase. Table 6 listed the calculated emission wavelength (kem) for maleimides together with their corresponding oscillation strength (f) by TD-DFT and PCM technique. For N-substituted maleimides, the calculated kem for MeMLH, PhMLH and 2,6-DipMLH have shown to give a 6, 68 and 40 nm red-shift with respective to maleimide, separately. In general, the N-methyl substitution (MeMLH) gives only a weak effect on the calculated kem as compared to those of PhMLE and 2,6DipMLE. However, PhMLH displays a larger red-shift in kem than 2,6-dipMLH that is probably due to a more planar structure for PhMLH than 2,6-DipMLH in the first excited state. The effect of solvent environment on kem for maleimides depends strongly on the solvent polarity, e.g., a red-shift for a polar environment as in the case of MeMLH. For PhMLH, on the other hand, the planar structure gives only a slight effect in solvent environment or almost independent of the solvent polarity employed. The kem for 2,6-DipMLH in different solvent environments were calculated to be all around 343 nm. This result strongly suggests that the solvent has a very weak effect on the calculated kem. For 3,4-substituted maleimides, the kem for MLE, MLH, MLMe and 4CH3OMLMe were calculated to display a large red-shift of 82, 216, 247 and 260 nm, respectively, relative to maleimide. Since 4CH3OMLMe and MLMe have a N-methyl substitution, they have more planar structure than MLE in the excited state, thus, both give a red-shift in kem as compared to MLE. The results of PCM simulation illustrated that the solvent environment affects only weakly on the calculated kem for 3,4-maleimides. It is worthwhile to mention that the calculated spectral data for maleimides, kmax abs and kem, were all in fair agreement with experiment results. In general, the calculated kmax abs and kem have a 10–30 nm blueshift relative to the experimental data. 4. Conclusion In this study, the HF, DFT B3LYP, CIS and TD-DFT calculations were performed to generate the optimized structures and spectral properties of maleimides in the ground and first excited states. The spectroscopic properties for maleimides under different solvent environments were calculated using the PCM simulation. To determine a suitable basis set for this calculation, a series of basis sets were used to generate the optimized structures of non-substituted maleimide and compare to the geometric parameters observed experimentally. It was determined that DFT/B3LYP/6-31G* method gave the best geometry parameters for non-substituted maleimide as compared to the experimental data. The steric repulsion in both N- and 3,4-substituted maleimides gave a strong influence in the extent of p-electron delocalization from the substituent to the maleimide chromophore in the ground

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and excited states. For the 3,4-substituted maleimide, the effect of substitution causes the elongation of the C3AC4 bond, and leads to a non-planar conformation. In ML-3ind-4Br, the different substituent influences the p-electron delocalization from the 1H-indol-3yl substituent to the maleimide unit. The BLA analysis shows the extent of p-electron delocalization in the maleimides. Basically, the smaller BLA value shows the more significant p-electron delocalization. For N-substituted maleimides, the similarity in BLA values suggests that the substituent influences only slightly the p-delocalization at the ground and excited states. For 3,4-substituted maleimides, the asymmetric substituents have shown to affect the p-electron delocalization. The different functional groups of 3,4-substituted maleimide, ML3ind-4Br, generate the longest p-conjugation length at the ground and excited states in maleimides. The results of TD-DFT calculation for maleimides in the ground and excited states are in good agreement with the experimental data. For the maleimides in the gas phase and solvent environment, both calculated kmax abs and kem are red-shift for non-substituted maleimide, MLH. For N-substituted maleimide, such as PhMLH, the planar conformation improves the extent of p-delocalization from the maleimide chromophore to the phenyl group, and leads to the observed blue-shift in both kmax abs and kem values. For 3,4-substituted maleimides, the asymmetric substituent has given the most significant in spectral changes as compared to other maleimides. Acknowledgements We thank Prof. Chhiu-Tsu Lin for reading the manuscript and the National Science Council of Taiwan for supporting this research. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

[18] [19] [20]

W.-C. Wu, H.-C. Yeh, L.-H. Chan, C.-T. Chen, Adv. Mater. 14 (2002) 1072. B.K. Kaletas, R.M. Williams, B. König, L.D. Cola, Chem. Commun. (2002) 776. X. Zhang, Z.-C. Li, K.-B. Li, F.-M. Li, J. Am. Chem. Soc. 126 (2004) 12200. T.-Z. Liu, Y. Chen, Polymer 46 (2005) 10383. Q. Peng, Z. Lu, Y. Huang, M. Xie, D. Xiao, D. Zou, J. Mater. Chem. 13 (2003) 1570. H.-C. Yeh, W.-C. Wu, C.-T. Chen, Chem. Commun. (2003) 404. C. Teresa, G.-L. Remedios, M. Manuela, J. Phys. Chem. A 107 (2003) 6995. S.F. Parker, Spectrochim. Acta Part A 63 (2006) 544. C.W. Miller, C.E. Hoyle, E.J. Valente, D.H. Magers, E.S. Jönsson, J. Phys. Chem. A. 103 (1999) 6406. B.-C. Wang, H.-R. Liao, H.-C. Yeh, W.-C. Wu, C.-T. Chen, J. Lumin. 113 (2005) 321. S. Miertuš, E. Scrocco, J. Tomasi, Chem. Phys. 55 (1981) 117. A. Masunov, S. Tretiak, J.W. Hong, B. Liu, G.C. Bazan, J. Phys. Chem. 122 (2005) 1. C. Adamo, V. Barone, Chem. Phys. Lett. 330 (2000) 152. C. Zazzaa, L. Bencivenni, A. Grandia, A. Aschi, J. Mol. Struct. (THEOCHEM) 680 (2004) 117. J. Tomasi, M. Persico, Chem. Rev. 90 (1994) 2027. R. Cammi, J. Tomasi, Comput. Chem. 16 (1995) 1449. M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K.Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.-C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, J.L. Andres, M. Head-Gordon, E.S. Replogle, J.A. Pople, Gaussian 03; Gaussian, Inc., Pittsburgh, PA, 2003. M.R. Boon, Theor. Chim. Acta 23 (1971) 109. P.J. Cox, S.F. Parker, Acta Crystallogr. Sect. C: Cryst. Struct. Commun. 52 (1996) 2578. C.W. Miller, E.S. Jönsson, C.E. Hoyle, K. Viswanathan, E.J. Valente, J. Phys. Chem. B 105 (2001) 2707.