Substituted group and side chain effects for the porphyrin and zinc(II)–porphyrin derivatives: A DFT and TD-DFT study

Journal of Luminescence 142 (2013) 8–16

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Substituted group and side chain effects for the porphyrin and zinc(II)–porphyrin derivatives: A DFT and TD-DFT study Chin-Kuen Tai, Wen-Hua Chuang, Bo-Cheng Wang n Department of Chemistry, Tamkang University, Tamsui 251, Taiwan

art ic l e i nf o

a b s t r a c t

Article history: Received 4 January 2013 Received in revised form 18 March 2013 Accepted 25 March 2013 Available online 1 April 2013

The DFT/B3LYP/LANL2DZ and TD-DFT calculations have been performed to generate the optimized structures, electronic and photo-physical properties for the porphyrin and zinc(II)–porphyrin (metalloporphyrin) derivatives. The substituted group and side chain effects for these derivatives are discussed in this study. According to the calculation results, the side chain moiety extends the π-delocalization length from the porphyrin core to the side chain moiety. The substituted group with a stronger electron-donating ability increases the energy level of highest occupied molecular orbital (EHOMO). The side chain moiety with a lower resonance energy decreases EHOMO, the energy level of the lowest unoccupied molecular orbital (ELUMO), and the energy gap (Eg) between HOMO and LUMO in the porphyrin and zinc(II)–porphyrin derivatives. The natural bonding orbital (NBO) analysis determines the possible electron transfer mechanism from the electron-donating to -withdrawing groups (the side chain moiety) in these porphyrin derivatives. The projected density of state (PDOS) analysis shows that the electron-donating group affects the electron density distribution in both HOMO and LUMO, and the side chain moiety influence the electron density distribution in LUMO. The calculated photo-physical properties (absorption wavelengths and the related oscillator strength, f) in dichloromethane environment for porphyrin and zinc(II)–porphyrin derivatives have been simulated by using the TD-DFT method within the Polarizable Continuum Model (PCM). The present of both of the substituted group and the side chain moiety in these derivatives results in a red shift and broadening of the range of the absorption peaks of the Q/Soret band as compared to porphin. & 2013 Elsevier B.V. All rights reserved.

Keywords: Porphyrin and zinc(II)–porphyrin Substitution effect Side chain effect DFT

1. Introduction Porphyrin, metalloporphyrin, and their derivatives are common found in nature and are of much interests for material research for their electronic and photo-physical properties. These derivatives have been utilized in many commercial applications and are of interests for research in diverse fields, such as the dye science, solar energy conversion, artificial photo-synthesis, photodynamics therapy, and nonlinear optics [1–17]. Porphine, the parent compound for porphyrin, has a planar conformation of D4h symmetry and can be the subject to the meso-, β-, and inner nitrogen-substitutions to afford porphyrins. Till now, porphyrin family of designs with different substituted groups, substitution at meso-positions, and side chain moieties, substitutions at β-positions, still attracts many interests for aforementioned applications [18–22]. From the molecular design point of view, the EHOMO, ELUMO and Eg can be influenced by different substituted groups and side chain moieties on the porphyrin. In the D-π-A series of porphyrin

n

Corresponding author. Te.: +886 2 26215656; fax: +886 2 26228458. E-mail address: [email protected] (B.-C. Wang).

0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.03.037

derivatives used as solar cell materials, the molecular designs contain a porphyrin core, a side chain moiety as the electrondonating group, and a carboxyl acid (or a acrylic acid) as the electron-withdrawing group. The π-delocalization from the porphyrin core to the electron-withdrawing group (side chain moiety) was found to be more pronounced at the β- than at the mesosubstitution in the porphyrin derivatives [23]. The design and synthesis of porphyrin based compounds for the applications aimed to increase the photo-electricity conversion efficiency have been an active research subject [24–30]. However, the effects of substitution and π-conjugation in the porphyrin derivatives are still not systematically understood, and the molecular design of porphyrin compounds for better device performance remains empirical and random. Hence, it is necessary to establish the relationships between the structure of the porphyrin core, the substituted group, and the side chain moiety and the corresponding physical properties for developing new porphyrin based derivatives. Recently, the spectroscopic and electrochemical properties of metalloporphrin derivatives were intensely studied and reported. Moreover, several papers have been published on the electronic structure and bonding in metalloporphyrins with different metal ions, but their substituted group and side chain effects have not been investigated systematically [31–33].

C.-K. Tai et al. / Journal of Luminescence 142 (2013) 8–16

In this study, the effects of the substituted group and side chains are investigated and discussed for various porphyrin and zinc(II)–porphyrin derivatives (Figs. 1 and 2). Various groups and side chain moieties are designed to substitute in the meso- and βpositions, respectively, of porphyrin. The substituted groups of phenyl- (Ph-), furyl- (Fu-), and fluorenyl- (Fl-) are used for the tetra-substituted porphyrins. Ethenyl phenyl acrylic acid (A) and ethenyl thiophenyl acid (B) are used as the side chain moieties. In addition, we have also investigated the di-substitution (DPhH2P) pattern and N2S2 system with the substitution of two nitrogen atoms inner rim of the porphyrin core with two sulfur atoms (TFuS2P) (Fig. 2). The geometrical and electronic properties (e.g. EHOMO, ELUMO and Eg) of the porphyrins (DPhH2P, TFuH2P, TPhH2P, TFuS2P) and zinc(II)–porphyrin derivatives (TPhZnPA, TFlZnPA, TPhZnPB, and TFlZnPB) will be discussed. The NBO analysis is used to investigate the potential of the electron transfer mechanism based on the charge separation in zinc(II)-–porphyrin derivatives at ground state. The PDOS analysis provides electron density distributions of HOMO and LUMO. Finally, the absorption spectra of porphyrin and Zinc(II)–porphyrin derivatives are calculated in dichloromethane solution environment by using the TD-DFT method with Polarizable Continuum Model (PCM) model and the results are compared with experimental data. Herein, we attempt to establish the influence of the substituted group and the side chain moiety on the optimized geometries, electronic properties, and absorption spectra. These calculation results shall provide information for the better molecular design of dyes used in dye sensitized solar cell (DSSC) devices.

2. Computational methods For the porphyrin and zinc(II)–porphyrin derivatives, the optimized geometries and electronic properties were calculated using the Kohn–Sham density functional theory (DFT) and Becke's threeparameter hybrid exchange functional combined with the Lee, Yang, and Parr's correlation functional (B3LYP) with the LANL2DZ basis set [34,35]. The PDOS and NBO analyses were based on the calculated orbital populations at the DFT/B3LYP/LANL2DZ optimized structures. Time-dependent density functional theory (TD-DFT) calculations (nstates¼100) are used to compute the absorption spectra and related oscillator strengths (f) within the visible to nearinfrared region. The optimized structures of porphyrin and zinc (II)–porphyrin for TD-DFT calculations were obtained by the DFT/ B3LYP/LANL2DZ method [36,37]. The solvent effect simulation was achieved by using the Polarizable Continuum Model (PCM) model [38,39]. All of the computations in this study were performed using

porphyrin derivatives

9

the Gaussian 03 package to determine the optimized structures and the electronic and photo-physical properties of the porphyrin and zinc(II)–porphyrin derivatives [40].

3. Results and discussions Molecular structures and the selected atomic labeling scheme for porphyrin and zinc(II)–porphyrin derivatives are shown in Figs. 1 and 2. In order to select an appropriate basis set, the geometry optimization of TPhZnP was achieved by using the DFT/ B3LYP method with a series of basis sets (3–21G, 3–21G*, 6–31G, 6–31+G, 6–31G*, 6–31+G*, and LANL2DZ). These calculated geometric parameters and the related atomic labeling graph of TPhZnP are listed in Supplementary data (S1 and S2). In S1, the calculated data exhibited a good agreement with the experimental data for TPhZnP; the MAE values for these basis sets are less than 0.03 [41]. All of the selected bond lengths obtained from DFT/ B3LYP calculation with different basis sets exhibited a similar tendency. Thus, the basis set effect does not significantly influences the results of geometric optimization calculation but cause a different time wasting in our calculation procedure. Therefore, in order to reduce the calculation time, the LANL2DZ basis set was selected as the appropriate basis set in further calculations. In Table 1, the selected calculated geometric parameters of porphyrin derivatives are listed containing selected bond lengths in the porphyrin core, bond lengths (R1 to R5), and dihedral angles (φ1 to φ5) involving connections between porphyrin core and substituted group/side chain moiety using DFT/B3LYP/LANL2DZ calculations. Since the porphine core has D4h symmetry, only quarter one of porphine structure parameters should be considered. According to the calculated geometric parameters of porphine derivatives, there are little difference in the calculated bond lengths (0.01–0.02 Å), and a larger dihedral angle (φ1 to φ4) range (about 39.5–99.91). Thus, the calculated results exhibited that a substituted group (Ph-, Fu-, and Fl-) generates a slightly influence on the optimized structure of the porphyrin core as compared to porphrin [42]. For the TFuH2P and TFuS2P porphyrin derivatives with the N2S2 substitution, the S atom (TFuS2P) has a larger atomic radius contrasted with N atom (TFuH2P), which results in a larger N–S–N angle value (114.81 for TFuH2P) in the inner layer of porphyrin core and generates a quite different atomic distribution type (in the inner layer of porphyrin core) compared with other porphyrin and zinc(II)–porphyrin derivatives. In Table 1, the calculated results demonstrate an sp3 bonding (according to the C–C bond length (about 1.47–1.50 Å of R1 to R4) between the substituted group and the porphyrin core in the porphyrin and

zinc ( ) - porphyrin derivatives

Fig. 1. Atomic labeling scheme for all porphyrin and zinc(II)–porphyrin derivatives.

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C.-K. Tai et al. / Journal of Luminescence 142 (2013) 8–16

Fig. 2. porphyrin and zinc(II)–porphyrin derivatives.

zinc(II)–porphyrin derivatives. This property indicates that steric repulsion between an H atom of the substituted group and an H atom of the porphyrin core causes a non-planar molecular conformation in the designed molecular systems. Both C–C bond lengths (R1–R4) and dihedral angles (φ1–φ4) give an abbreviation tendency for TFuH2P and TFuS2P compared to other derivatives (Table 1). Thus, the Fu-substitution has a significant π-conjugation effect from the electron-donating group to the porphyrin core compared to other porphyrin derivatives (1.503 Å, TPhH2P vs. 1.473 Å, TFuH2P). For zinc(II)–porphyrin

derivatives, the π-electrons can delocalize from the side chain moiety to the porphyrin core. Therefore, the calculated results in Table 1 shows that both C–C bond length (R5) and dihedral angle (φ5) exhibit an abbreviation characteristic. In order to discuss the substituted group and side chain moiety effects in the porphyrin and zinc(II)–porphyrin derivatives, the EHOMO, ELUMO, and Eg calculated using the DFT/B3LYP/LANL2DZ method are listed in Table 2. Relative frontier molecular orbital energy levels of porphyrin and zinc(II)–porphyrin derivatives are shown in Fig. 3. Theoretically, the behavior of EHOMO, ELUMO and Eg

C.-K. Tai et al. / Journal of Luminescence 142 (2013) 8–16

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Table 1 Selected bond lengths (Å) and dihedral angles (1) of porphyrin and zinc(II)–porphyrin derivatives calculated using the DFT/B3LYP/LANL2DZ method. Porphyrin derivatives

C1−C2 C2−C3 C2−N6 C3−C4 C4−C5 N6−C5 C5−C7 R1 R2 R3 R4 R5 φ1 φ2 φ3 φ4 φ5 *

Zinc(II)–porphyrin derivatives

DPhH2P

TPhH2P

TFuH2P

TFuS2P

TPhZnP

TFlZnP

TPhZnPA

TFlZnPA

TPhZnPB

TFlZnPB

Porphine

1.401 1.446 1.386 1.384 1.447 1.391 1.413 1.499 1.499 – – – −62.1 −62.2 – – –

1.412 1.445 1.392 1.382 1.445 1.392 1.412 1.504 1.503 1.503 1.503 – −65.7 −65.6 −65.7 −65.7 –

1.418 1.470 1.389 1.365 1.470 1.389 1.419 1.474 1.472 1.472 1.474

1.429 1.476 1.376 1.371 1.476 1.376 1.429 1.471 1.467 1.469 1.471

−49.6 −49.3 −45.7 −46.4

−42.1 −39.5 −39.5 −42.2

1.416 1.457 1.396 1.374 1.457 1.396 1.416 1.503 1.503 1.503 1.503 – −65.1 −65.1 −65.2 −65.2 –

1.416 1.457 1.396 1.375 1.457 1.396 1.416 1.503 1.503 1.503 1.503 – −67.5 −67.3 −68.0 −67.7 –

1.415 1.456 1.396 1.374 1.459 1.394 1.419 1.508 1.503 1.504 1.505 1.463 −95.2 −66.1 −67.3 −70.0 −19.4

1.416 1.456 1.396 1.374 1.459 1.394 1.420 1.507 1.503 1.502 1.506 1.463 −99.9 −66.7 −64.8 −74.3 −20.7

1.415 1.457 1.396 1.373 1.460 1.395 1.418 1.506 1.504 1.504 1.505 1.457 −75.6 −68.5 −69.6 −69.9 −16.5

1.417 1.457 1.396 1.373 1.459 1.395 1.418 1.505 1.503 1.502 1.506 1.457 −75.7 −66.8 −64.5 −74.4 −15.6

1.406 (1.382) 1.471 (1.465) 1.386 (1.371) 1.371 (1.359) 1.471 (1.437) 1.386 (1.377) 1.406 (1.388) – – – – – – – – – –

The experimental data in parentheses were obtained from Ref. [42].

Table 2 EHOMO, ELUMO, and energy gap (Eg) of the porphyrin and zinc(II)–porphyrin derivativesa,b. Porphyrin derivatives

ELUMO EHOMO Eg a b

Zinc(II)–porphyrin derivatives

DPhH2P

TPhH2P

TFuH2P

TFuS2P

TPhZnP

TFlZnP

TPhZnPA

TFlZnPA

TPhZnPB

TFlZnPB

−2.533 −5.218 2.685

−2.478 −5.089 2.611

−2.635 −5.041 2.406

−2.910 −5.141 2.231

−2.396 −5.110 2.714

−2.369 −5.053 2.683

−2.677 −5.243 2.566

−2.636 −5.163 2.528

−3.213 −5.399 2.185

−3.137 −5.271 2.134

The ELUMO, EHOMO, and Eg of porphine is −2.548, −5.376, and 2.828 eV, respectively. Units of the EHOMO, ELUMO, and Eg: eV.

Fig. 3. Frontier orbital energy levels of the porphyrin and zinc(II)–porphyrin derivatives.

can be related to different substituted groups and side chain moieties for these derivatives compared to those for unsubstituted porphyin. A stronger electron-donating ability of the electrondonating group increases EHOMO and, a molecule with a conjugated π-linker moiety has a tendency to decrease both ELUMO and Eg values. Our calculated results in Table 2 support these conclusions. For porphyrin and zinc(II)–porphyrin derivatives, the Ph-, Fu-, and Fl-substitutions increase EHOMO values compared with porphine molecule (−5.376 eV). However, the energy difference in EHOMO for these derivatives is closer, less than about 0.33 eV. It may indicate

that all substituted groups (Ph-, Fu-, Fl-) are weak electrondonating groups and have a quite large resonance energy. The calculation results show that the metal ion (Zn(II)) increases the calculated EHOMO and Eg values (2.611 eV, TPhH2P vs. 2.714 eV, TPhZnP) and decreases ELUMO in zinc(II)–porphyrin derivatives. Furthermore, the side chain moiety extends the π-conjugation length and lowers the calculated EHOMO, ELUMO, and Eg values. In order to discussing the effect of resonance energy for the zinc(II)–porphyrin derivatives, we estimated the resonance energy for the side chain moiety A and B by using DFT/B3LYP/ LANL2DZ method which was based on the concept of reference [47]. The estimated resonance energy of A/B is 40.5/36.1 kcal/mol, respectively. Our calculated results show that the resonance energy of side chain moiety A is smaller than that of side chain moiety B. In Table 2, it can be found that the calculated Eg of TPhZnPA/TFlZnPA (2.566/2.528 eV) is larger than TPhZnPB/ TFlZnPB (2.185/2.134), also. This property implies that the resonance energy of the side chain moiety A is larger than the side chain moiety B in these derivatives and limits the π-conjugation effect from the side chain moiety to the porphine core for designed derivatives and affects the calculated ELUMO and Eg. Therefore, the calculated results indicate that TPhZnPA and TFlZnPA have a relative higher energy values (EHOMO, ELUMO, and Eg) as compared to TPhZnPB and TFlZnPB in Table 2. To discuss the charge separation and possible electron transfer mechanism at ground state (S0) for the zinc(II)–porphyrin derivatives, the NBO analysis was used. Here, the zinc(II)–porphyrin derivatives could divided into three parts: the substituted group (SG), the porphyrin core (Por), and the side chain moiety (SC). We summarized and listed calculated NBO values of each individual part in these designed molecular system in Table 3. The charge

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C.-K. Tai et al. / Journal of Luminescence 142 (2013) 8–16

π-delocalization can extend from the substituted group to the porphyrin core for TFuH2P and TFuS2P at HOMO. For TPhZnPB and TFlZnPB, the lower resonance energy of the thiophenyl group in the side chain moiety (B) compared with the phenyl group in the side chain moiety (A) increases the π-delocalization effect from the side chain to the porphyrin core. Therefore, the PDOS values of the LUMO orbital for TPhZnPB, and TFlZnPB indicate that the electron density distribution concentrates on the side chain moiety (about 68%), just consistent with a type of a D-π-A molecule. We conclude that TPhZnPB and TFlZnPB could be useful as DSSC material. In Table 5, the calculated absorption wavelengths, oscillator strengths (f), related transition molecular orbitals, and their coefficients, in dichloromethane solution environment for the porphyrin and zinc(II)–porphyrin derivatives computed by PCM simulations are listed; their related calculated absorption spectra are shown in Fig. 5. All of the above calculated results were obtained by using the TD-DFT method with nstates¼100. In Table 5, the difference between the calculated and experimental results is around 40–140 nm (TPhH2P) to 30–145 nm (TPhZnP and TFlZnP). Therefore, these theoretical absorption spectra prediction have a good agreement with the experimental data [43–45]. In general, the absorption spectrum for porphyrin and zinc(II)– porphyrin derivatives exhibit the Q band and Soret band character in different solvent environments. The Q band with a low absorption coefficient can be found in the range between 450–700 nm, and the Soret band with a high absorption coefficient is distributed between 400–450 nm in the solvent environment [46]. Our calculated absorption spectra in Fig. 5 exhibit a similar tendency with above description. In particular, the absorption range for porphyrin and zinc(II)–porphyrin derivatives is affected by the substituted group and the side chain moiety. For porphyrin derivatives, our calculated results in Table 5 shows that the number of substituted group slightly influences for the absorption wavelength. For TPhH2P andTFuH2P, the Fu-substitution has a stronger electron-donating ability than Ph-substitution in TPhH2P and this causes the absorption wavelength to be red-shifted (e.g. the Q band for TFuH2P/TPhH2P is at 634.7/587.6 nm) compared with porphrin (548.5 nm) in the dichloromethane solution environment. This relation between the substituted group and photophysical properties also can be observed in Table 5 for our designed systems. For TFuS2P, our calculation results indicate a large difference of the absorption wavelengths and the range of absorption peaks compared with other porphyrin derivatives. This may imply that the S atom in the inner layer of the porphyrin core enhances the π-delocalization effect as compared to the other derivatives.

separation generated by using the NBO analysis for Por, SC and SG parts allows us to analyze the charge separation in these derivatives. But these smaller NBO values for SG compared with the D-πA molecular systems also point out the weak electron-donating ability of both Ph- and Fl-substitutions in the derivatives as already discussed above. The calculated results in Table 3 show that different side chain moieties for zinc(II)–porphyrin derivatives have significantly different effects in the Por and SC parts (e.g. TPhZnPA (−0.075/−0.006 e), TPhZnPB (−0.054/−0.037 e) vs. TFlZnPA (−0.079/−0.005 e), TFlZnPB (−0.047/−0.047 e)) according to the NBO analysis. It can be concluded that the side chain moiety (B) has lower resonance energy than the side chain moiety (A). For TFlZnPB, our calculated NBO results show the most significant charge separation in these zinc(II)–porphyrin derivatives (Table 3). Therefore, the NBO results of TFlZnPB demonstrate that the SG moiety of TFlZnPB can accept an electron from other molecule based on the intermolecular charge transfer mechanism consideration. Also, the Por and SC moieties of TFlZnPB can transfer an electron to the neighboring molecule. This property implies that TFlZnPB could be used in the application of DSSC solar cells. For porphyrin and zinc(II)–porphyrin derivatives, the PDOS analysis at HOMO and LUMO is presented in Table 4. The HOMO and LUMO diagrams for these derivatives are shown in Fig. 4. Similar to the above discussion for the NBO analysis, these derivatives were also divided into Por, SG, and SC parts. In Table 4, the PDOS values of SG at HOMO and LUMO show an increase with the number of substituted group increasing with diand tetra-phenyl substituted porphyrins (11.3/18.7 at HOMO, 8.0/ 11.3 at LUMO for DPhH2P/TPhH2P). In our designed molecular systems, the PDOS analysis shows that the major electron density distribution of HOMO and LUMO concentrates on the porphyrin core (81/89% for the HOMO/LUMO of TPhH2P). For TFuH2P, the calculated results at HOMO exhibit a significant PDOS difference compared with that of TPhH2P. These PDOS values in Table 4 may be explained that the Fu-substitution has a relatively lower resonance energy as compared to the Ph-substitution. Therefore,

Table 3 Natural Bond Orbital (NBO) analysis (e) of zinc(II)–porphyrin derivatives at the ground state (S0).

Por SG SC

TPhZnPA

TFlZnPA

TPhZnPB

TFlZnPB

−0.0745 0.0801 −0.0056

−0.0790 0.0836 −0.0045

−0.0541 0.0912 −0.0372

−0.0466 0.0937 −0.0471

Table 4 Projected Density of State (PDOS) analysis at HOMO and LUMO of the porphyrin and zinc(II)–porphyrin derivatives. Porphyrin derivatives DPhH2P

Por SG

TPhH2P

TFuH2P

TFuS2P

HOMO

LUMO

HOMO

LUMO

HOMO

LUMO

HOMO

LUMO

88.7 11.3

92.0 8.0

81.3 18.7

88.7 11.3

68.6 31.4

85.3 14.7

67.7 32.3

89.9 10.1

TFlZnPA

TPhZnPB

Zinc(II)–porphyrin derivatives TPhZnP

Por SG SC

TFlZnP

TPhZnPA

TFlZnPB

HOMO

LUMO

HOMO

LUMO

HOMO

LUMO

HOMO

LUMO

HOMO

LUMO

HOMO

LUMO

81.2 18.8 –

89.7 10.3 –

74.8 25.2 –

86.2 13.8 –

80.1 16.4 3.5

76.5 7.4 16.1

74.0 23.0 2.9

72.3 10.6 17.1

76.4 16.4 7.2

28.7 2.9 68.3

72.2 23.9 3.9

27.4 4.1 68.5

C.-K. Tai et al. / Journal of Luminescence 142 (2013) 8–16

HOMO

LUMO

DPhH2P

TPhH2P

TFuH2P

TFuS2P

13

HOMO

LUMO

TPhZnP

TFlZnP

TPhZnPA

TFlZnPA

TPhZnPB

TFlZnPB

Fig. 4. HOMO and LUMO diagrams of (a) porphyrin and (b) zinc(II)–porphyrin derivatives.

The side chain moiety in the zinc(II)–porphyrin derivatives (TPhZnPA, TFlZnPA, TPhZnPB, and TFlZnPB) generates significantly influences the absorption wavelengths in the Q and Soret band and the range of absorption peaks (Table 5). In these derivatives, the calculated absorption wavelength has a larger red shift for the

Q band (27–170 nm) and a significant absorption range compared to the other derivatives without the side chain moiety. For these zinc(II)–porphyrin derivatives, our calculated results in Table 5 shows that TPhZnPB and TFlZnPB exhibit the largest red shifts in the absorption wavelength (164–184 nm) and the widest range of

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C.-K. Tai et al. / Journal of Luminescence 142 (2013) 8–16

Table 5 Calculated absorption wavelengths (nm), electronic transitions, and related oscillator strengths (f) for singlet-singlet transitions of the porphyrin and zinc(II)–porphyrin derivatives in the CH2Cl2 environment (TD-DFT/B3LYP/LANL2DZ method)a. Porphyrin derivatives Compound

Absorption wavelengths

f

MO/character

Coefficient

568.6 529.6 395.2 388.2 353.0

0.0550 0.0532 1.2138 1.5475 0.4984

H to H to H−1 H−1 H−3

L L+1 to L+1 to L to L

0.52917 0.48945 0.45395 0.42968 0.6414

587.6 545.0 405.2 399.3 359.8 348.4

0.0808 0.0833 1.3264 1.7386 0.4876 0.0690

H to H to H−1 H−1 H−3 H−3

L L+1 to L+1 to L to L to L+1

0.5908 0.5606 0.5908 0.4919 0.6493 0.6859

634.7 596.9 432.0 423.9 393.2

0.1537 0.2591 1.4310 1.3183 0.1690

H to H to H−1 H−1 H−2

L L+1 to L+1 to L to L

0.4882 0.4819 0.4246 0.4348 0.4882

708.4 624.4 458.5 457.0 449.7 424.8 414.5 321.7

0.1874 0.3267 0.8820 0.9360 0.2593 0.1645 0.3600 0.2274

H to L H to L+1 H−1 to L+1 H−1 to L H−5 to L H−6 to L H−4 to L H−10 to L

0.6211 0.6043 0.5111 0.5576 0.5112 0.4369 0.6792 0.6017

Absorption wavelengths

f

MO/character

coefficient

555.7 555.7 402.3 402.3 344.1

0.0699 0.0698 1.6486 1.6487 0.0349

H to H to H−1 H−1 H−2

L L+1 to L+1 to L to L+1

0.5627 0.5626 0.4939 0.4938 0.3515

561.1 561.1 419.7 418.9 372.9 372.8

0.1336 0.1304 1.6584 1.6498 0.4443 0.4507

H to H to H−1 H−1 H−5 H−5

L L+1 to L to L+1 to L to L+1

0.5792 0.5790 0.4597 0.4605 0.6112 0.6117

582.8 564.1 470.9 388.6 371.5

0.1306 0.0285 1.7467 1.1372 0.4315

H to H to H−1 H−2 H−1

L L+1 to L to L+1 to L+2

0.5916 0.5050 0.4019 0.3934 0.5500

592.5 569.5 477.6 387.2 368.2

0.1969 0.0266 1.9624 0.9840 0.4718

H to H to H−1 H−5 H−1

L L+1 to L to L+1 to L+2

0.6055 0.5253 0.4365 0.3900 0.4345

712.9 625.4 539.1 400.3 397.7

0.1957 0.6974 0.5908 0.8491 0.5621

H to H−1 H to H−2 H−2

L to L L+2 to L+1 to L+2

0.6806 0.6493 0.5020 0.4078 0.4437

732.0 627.2 553.4 418.9 413.2

0.1996 0.6779 0.7497 0.5563 0.4877

H to H−1 H−2 H−2 H−2

L to to to to

0.6818 0.6448 0.4590 0.3945 0.4230

DPhH2P

TPhH2P (646, 589, 548, 513, 417)b

TFuH2P

TFuS2P

Zinc(II)–porphyrin derivatives Compound TPhZnP (585, 547, 418)b

TFlZnP (594, 552, 428)b

TPhZnPA

TFlZnPA

TPhZnPB

TFlZnPB

a b

L L L+1 L+2

Experimental results were obtained from Refs. [43–45]. Calculated absorption wavelengths of porphine in CH2Cl2 solution are 548.5, 509.9, 385.1, 372.1, 337.1 nm at the TD-DFT/B3LYP/LANL2DZ level of theory.

C.-K. Tai et al. / Journal of Luminescence 142 (2013) 8–16

DPhH 2 P

TPhH 2 P

15

TFuH2P

TPhZnP

TFlZnP

TFlZnPA

TPhZnPB

TFuS2P

TPhZnPA

TFlZnPB

Fig. 5. Calculated absorption spectra of (a) porphyrin and (b) zinc(II)–porphyrin derivatives obtained using the TD-B3LYP/LANL2DZ method in CH2Cl2 environment.

absorption peaks in contrast to the unsubstituted porphine molecule. This can be explained by the fact that the thiophenyl group of the side chain moiety (B) has a relative lower resonance energy as compared to the phenyl group of side chain moiety (A) in TPZnPA andTFlZnPA since the π-conjugation extends from the side chain moiety to the porphyrin core. Our designed molecular systems for the porphyrin and zinc(II)–porphyrin derivatives show that the substituted group and the side chain moiety affect the photophysical properties and these calculated results should provide an useful information for the development of new porphyrin derivative related materials.

4. Conclusions Optimized structures, electronic properties, and related photophysical properties for porphyrin and zinc(II)–porphyrin derivatives were obtained using the DFT/B3LYP/LANL2DZ method. Absorption spectra were calculated using the TD-DFT method, and the PCM model was utilized to simulate the dichloromethane environment for theses derivatives. According to the calculated results of geometry optimization, the Ph-, Fl- and Fu-substituted groups with weak electron-donating ability have slight effects on the optimized structure of the porphyrin core. The Fu-substitution has a relative lower resonance energy compared with Ph- and Fl-

substitution and could extend the π-conjugation length from the substituted group to the porphyrin core (TFuH2P andTFuS2P). For zinc(II)–porphyrin derivatives, the side chain moieties A and B in our designed systems increase π-delocalization from the side chain moiety to porphyrin core. For the porphyrin derivatives, the electron-donating ability of the substituted group determines the difference of the calculated EHOMO from that in the unsubstituted porphyrin molecule. The calculated ELUMO and Eg can be affected by the side chain moiety in the zinc(II)–porphyrin derivatives. For TPhZnPB and TFlZnPB, a lower resonance energy at the thiophenyl group in the side chain moiety (B) as contrasted with that of the phenyl group in the side chain moiety (A) causes a significant energy drop for ELUMO and Eg. The NBO population analysis was employed to describe the charge separation in the zinc(II)– porphyrin derivatives. The weak electron-donating ability of the Ph-, and Fl- substituted groups in zinc(II)–porphyrin derivatives affect the NBO results slightly. The side chain moiety (B) has a tendency to increase the π-delocalization length in our designed systems. Therefore, the NBO values of TPhZnPB and TFlZnPB showed an inconsistent charge separation properties compared with other derivatives. For the porphyrin and zinc(II)–porphyrin derivatives, the PDOS analysis demonstrates that the substituted groups with different electron-donating abilities affect the electron density distribution at HOMO and LUMO. In our study, a different electron density distribution at LUMO for TPhZnPB and

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TFlZnPB may imply that the zinc(II)–porphyrin derivatives with the side chain moiety (B) have a significant π-delocalization from the side chain moiety to the porphyrin core compared with other derivatives. The calculated photo-physical properties and absorption spectra for porphyrin and zinc(II)–porphyrin derivatives in dichloromethane solution were obtained by using the TD-DFT method within the PCM model based on the B3LYP/LANL2DZ optimized structures. According to our calculated results, both substituted group and side chain moiety cause a different influence on the absorption wavelengths and the range of the absorption peaks as compared to the unsubstituted porphine. For our designed molecular systems, TPhZnPB and TFlZnPB exhibit a significant red shift in the absorption wavelength and the range of the absorption peaks compared with the other derivatives. In this study, we discussed the effects of the substituted group and the side chain moiety in the porphyrin and zinc(II)–porphyrin derivatives, and these calculated results should provide a useful reference for the further molecular design. Acknowledgements We thank Prof. Alexander Mebel for reading the manuscript and the National Science Council of Taiwan for the financial support of this work. We are also grateful to the National Center for High-performance Computing for providing computer time and facilities.

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2013.03. 037.

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