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Theoretical investigation on the spectroscopic properties of Zn porphyrin and Zn tetrapyrrin
Synthetic Metals 213 (2016) 18–24
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Theoretical investigation on the spectroscopic properties of Zn porphyrin and Zn tetrapyrrin Xin Wanga,b , Fu-Quan Baia,* , Ying-Tao Liub , Jian Wanga , Hong-Xing Zhanga,* a b
International Joint Research Laboratory of Nano-Micro Architecture Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, China School of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 4 November 2015 Received in revised form 16 December 2015 Accepted 25 December 2015 Available online 7 January 2016
Exploring the relationship between the geometrical structures and spectral properties has great significance to design some desirable materials. For important pyrrole contained macromolecules, there is a new complex of Zn tetrapyrrin with opened chain ligand derived from breaking one methine bridged of Zn porphyrin. The frontier molecular orbitals, absorption and emission properties of such Zn tetrapyrrin complexes are investigated by density functional theory (DFT) and its time-dependent density functional theory (TD-DFT) methods. Compared to Zn porphyrin, the HOMOs and LUMOs are no longer degenerate, the lowest lying absorption and emission of Zn tetrapyrrin start to increase in intensity with the shifting of charge transfer transition band to near-infrared region. We hope these theoretical studies will assist the design of novel molecular materials. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Density functional theory Spectral properties Tetrapyrrin Porphyrin Zn complexes
1. Introduction Pyrrole is a well-know component for building p-conjugated molecules. For example, dipyrrin, tripyrrin and porphyrin can be formed when two, three and four pyrroles liked by methine bridge, respectively, and they are all flat molecules with fully p-conjugated [1–14]. Except closed macrocycle porphyrin, opened chain tetrapyrrin can also be formulated when four pyrrole units connect with methine bridged. Based on porphyrin, tripyrrin [1–3] and dipyrrin [4], tetrapyrrin is designed in this work. There are two forms, one is containing one hydrogen proton at the four nitrogen elements, a, the other is containing three, b (see Fig. 1). And the carbon atom at terminal a position contains only one hydrogen atom as dipyrrin and tripyrrin to ensure the tetrapyrrin fully p-conjugated. Recently, Zn porphyrin (ZnP) and its derivatives have been widely studied as active materials in dye-sensitized solar cells (DSSCs) because of the high absorption coefficients, facile synthesis and lower costs [5–14]. The structure and absorption properties of linear tetrapyrroles and opened chain Zn tetrapyrroles were also studied owing to the extended p-electron system and the flexible framework, and they can be used as biomaterials, photonic and electronic materials [15–24]. But to the best of our
* Corresponding authors. Fax.: +86 431 88498966. E-mail addresses: [email protected] (F.-Q. Bai), [email protected] (H.-X. Zhang). http://dx.doi.org/10.1016/j.synthmet.2015.12.023 0379-6779/ ã 2015 Elsevier B.V. All rights reserved.
knowledge, reports on the structures and photophysical properties of Zn complexes consisting with opened chain form tetrapyrrin are scarce. So in this article, the Zn tetrapyrrin complexes are designed and studied to compare with Zn porphyrin. Our interesting in those peculiar complexes arises from the expectation that Zn tetrapyrrins might serve as new potential materials. When the four nitrogen atoms coordinated to zinc ion, the hydrogen at the N H group can be removed, so the fourcoordinate Zn tetrapyrrin complexes is monocationic or monoanionic, named 2 and 3, respectively (see Fig. 2). In order to completely characterize and predict the properties of the unknown materials we designed here, the geometrical structures, the highest occupied molecular orbitals (HOMOs) properties, the lowest unoccupied molecular orbitals (LUMOs) properties, the energy gaps between the molecular orbitals (MOs), the low excitation energies and emission energies are investigated theoretically and discussed in detail. We hope our design would provide a useful guideline to the high-efficiency materials. 2. Computational details All the calculations are performed with the Gaussian09 suite of program [25]. The ground-state structures of these complexes are optimized at M06 [26]//6-31G(d)/LANL2DZ [27–30] level. In present calculations, full electron approach for all atoms is rather time consuming. In addition, the influence of electrons in inner core for the bonding and orbital characters is very weak. So it is
X. Wang et al. / Synthetic Metals 213 (2016) 18–24
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based p-conjugated Zn porphyrin complexes. After the comparison between the calculations and experimental data of geometrical structure at ground states and absorption properties, the same decision is made for the excited states and emission properties. And then the suitable method is applied to study the geometry structures and absorption properties of the other four pyrroles based p-conjugated Zn complexes. 3. Results and discussions 3.1. Ground-state and excited-state structures
Fig. 1. Structures of opened chain form tetrapyrrin.
better to describe the inner core and outmost electrons separately for the metal Zn atoms. Under this approximation, the electrons 4s23s23p63d10 for Zn atom are depicted through the LANL2DZ basis set. And the methodology at this level of theory has been carried out by several previous investigations for Zn complexes [31–35]. The vibrational frequencies are calculated to confirm that all the structures we study here are actual minima in the whole potential energy surface. The frontier molecular orbitals properties and absorption spectra are both in dependence on TD-DFT (time dependent density functional theory)/M06 approach. Then, the excited-state geometry optimizations and the emission properties are performed with the same level. In addition, the polarizable continuum model (PCM) [36,37] was used to evaluate the solvent effects of toluene solvent. The rational M06 functional is employed after the functional test within more than four different exchange-correlation (XC) functionals on complex 1 to achieve comparable with the measured data [13,14]. The different XC functionals of B3LYP [38,39], M06 [26], M06-2X [40] and M06HF [41,42] have been used for the geometrical optimization. The results of the optimized structural parameters and the experimental values for 1 are summarized in Table S1. From our results, the M06 functional can better reproduce geometrical parameters measured in the experiment. Then, B3LYP [38,39], PBE1 [43,44], M06 [26] and M06-2X [40] are employed to the computation for absorption spectra. It can be seen from Table S2 that the maximum absorption wavelength (550 nm) obtained by TD-M06 is much closer to the experimental date (524 nm) than other tested functional models. Therefore, the M06 functional is suitable and reliable to optimize the geometry and calculate the absorptions for the four pyrroles
Fig. 2 presents the molecular structures of 1–3. The optimized structural parameters and the experimental data [14,18] are summarized in Table 1. Complexes 2 and 3 are formed by coordination of zinc ion and opened tetrapyrrin. Complex 1 is Zn porphyrin (ZnP), the central metal ion Zn2+ lies in the center of the plane formed by the closed porphyrin macrocycle. The average Zn N bond length is 2.053 Å, and the neighboring N Zn N bond angle is 90 . For complexes 2 and 3, the tetrapyrrin are present in ring-opened form, the zinc ion is poised between tetrahedral and planar, and both of them have a helical geometry. The Zn N bond lengths of 2 and 3 are almost longer than those found in ZnP (1), and two of the four Zn-N distances are slightly longer than the other two. Such as the Zn N1 distance of 2, 2.073 Å, which is equal to that of Zn N2, is longer than the Zn N3 distance, which is equal to ZnN4 at 2.050 Å. Since the four pyrroles ring is opened, the bond angles of N Zn N are no longer equivalent at 90 , and that in 3 is more deviated than that of 2. The three bond angles (N1 Zn N2, N2 Zn N3 and N3 Zn N4) which are internal to the chelate tetrapyrrin are all nearly 90 , but the other one, which is between the two terminal pyrrole units is considerably wider, at about 97. This widened angle results from the torsion along N1–N2–N3–N4 of the four pyrrole subunits. The torsion angles C N Zn N of Zn tetrapyrrins range from 26.1 to 16.1, and the Zn ion is trapped in the helical tetrapyrrin ligand. The structures of 1–3 in the lowest single excited state (S1) are optimized by TD-M06//6-31G(d)/LANL2DZ method. The main geometry parameters of them in S1 state are also presented in the brackets of Table 1. The structures of the two Zn tetrapyrrins have a similar helical shape in excited states. Compared with the ground states, the bond lengths, bond angles and torsion angles are all changed more or less. For instance, the bond lengths in S1 state are unequal for ZnP (1), the distances between Zn and N of 2 (in S1 state) become increased, while the Zn N bond lengths in 3
Fig. 2. Optimized geometry structures of 1–3, top view (top) and side view (bottom).
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Table 1 Selected bond lengths (Å), bond angles ( ) and torsion angles ( ) of 1–3 in ground states and excited states (in the brackets). Geometrical parameters
Complex 1
2
3
Expt. [14,18]
ZnN1 ZnN3 N1ZnN2 N2ZnN3 C1N1ZnN2 C3N3ZnN4
2.053 (2.052) 2.053 (2.052) 90 (90) 90 (90) 0 (0) 0 (0)
ZnN2 ZnN4 N3 Zn N4 N1 Zn N4 C2N2ZnN3 C4N4ZnN1
2.053 (2.063) 2.053 (2.063) 90 (90) 90 (90) 0 (0) 0 (0)
2.029–2.046
ZnN1 ZnN3 N1ZnN2 N2ZnN3 C1N1ZnN2 C3N3ZnN4
2.073 (2.073) 2.050 (2.063) 89.9 (89.7) 90.0 (89.9) 19.1 (15.5) 15.9 (15.2)
ZnN2 ZnN4 N3 Zn N4 N1 Zn N4 C2N2ZnN3 C4N4ZnN1
2.050 (2.063) 2.073 (2.073) 89.9 (89.7) 97.0 (97.5) 2.1 (2.5) 25.5 (24.9)
2.006–2.084
ZnN1 ZnN3 N1ZnN2 N2ZnN3 C1N1ZnN2 C3N3ZnN4
2.083 (2.062) 2.062 (2.036) 90.1 (90.9) 88.6 (90.7) 15.8 (25.3) 16.5 (17.7)
ZnN2 ZnN4 N3 Zn N4 N1 Zn N4 C2N2ZnN3 C4N4ZnN1
2.062 (2.036) 2.083 (2.062) 90.1 (90.9) 97.5 (95.9) 2.6 (1.6) 26.1 (21.1)
become reduced, and the C4 N4ZnN1 in 2 and 3 both become reduced, etc. 3.2. Frontier molecular orbitals and absorption spectra The frontier molecular orbitals (FMOs) were calculated on the basis of the optimized structures by TD-DFT. The molecular orbital compositions can be divided into two parts, Zn and tetra pyrroles. Table S3 summarizes the energy levels and compositions of partial FMOs of these complexes. Table S4 gives the energy gaps between FMOs. Fig. 3 presents the energy level diagrams of FMOs. The occupied molecular orbitals are highlighted in black, and the unoccupied molecular orbitals are in red and green. From Table S3 and Fig. 3, the FMOs of these complexes are mostly assigned to the contribution of tetra pyrroles. The lowest unoccupied molecular orbitals (LUMO) and the next lowest unoccupied molecular orbitals (LUMO+1) of Zn porphyrin are degenerate with the same energy, the highest occupied molecular orbital (HOMO) and the next highest occupied molecular orbital
Fig. 3. Energy level diagrams of partial frontier molecular orbitals. Occupied molecular orbitals are highlighted in black, unoccupied molecular orbitals are in red and green. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
89.8–90.2 5.7–2.3
88.3–102.3
(HOMO1) are nearly degenerate with a difference of 0.004 eV, and the four FMOs are well separated from the other molecular orbitals in energy. These features can be explained by the Fourorbital Model [45–47]. But when the tetra pyrroles ring is opened, the energy gaps of 2 and 3 between the two occupied molecular orbitals increase to 1.096 and 1.098 eV, the energy gaps between the two unoccupied molecular orbitals increase to 1.256 and 1.257 eV, respectively. And the energy gap between LUMO+2 and LUMO+1 of 2 and 3 is 1.134 and 1.112 eV, between LUMO+3 and LUMO+2 is 1.625 and 0.927 eV, respectively (see Table S4). It is obvious that, the four FMOs of 2 and 3 are not separated energetically from the other molecular orbitals as 1. The simulated electronic absorption spectra of 1–3 in the toluene solution are shown in Fig. 4. To better understand the electronic transitions of absorption, the wavelength, transition energies and oscillator strengths, main configurations along with the experimental values [13] are displayed in Table 2. The contours of the frontier orbitals of these complexes in absorption spectra are shown in Fig. 5. From Table 2 and Fig. 4, the absorption spectra of 1 exhibit typically Zn porphyrin absorption characteristics with intense Bbands at 372 nm and weak Q-bands at 524 nm. The so-called Q-
Fig. 4. Simulated absorption spectra with Gaussian curve based on the data calculated with the TD-DFT method in toluene for 1–3.
X. Wang et al. / Synthetic Metals 213 (2016) 18–24
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Table 2 Excitation energies, oscillator strengths (f) and main configurations of the absorption spectra of these complexes calculated with TD-DFT method. Complex
Excited states
Excitation energy (eV)
Wavelength (nm)
f
Main configuration
Assignment
Exp. [13] (nm)
1
S1(EU)
2.3212
524.8
0.0036
p ! p*
550
S3(EU)
3.3270
372.7
1.3295
HOMO1 ! LUMO+1 (48%) HOMO ! LUMO (51%) HOMO1 ! LUMO (48%) HOMO ! LUMO+1 (51%)
p ! p*
423
S1(B) S4(A)
1.8304 2.8889
677.4 429.2
0.3273 0.0408
p ! p* p ! p*
S7(A)
3.4188
362.7
0.5334
HOMO ! LUMO (100%) HOMO3 ! LUMO(44%) HOMO1 ! LUMO (37%) HOMO5 ! LUMO(8%) HOMO ! LUMO+1 (9%) HOMO6 ! LUMO (49%) HOMO3 ! LUMO (28%) HOMO1 ! LUMO (10%) HOMO ! LUMO+1 (12%)
S1(B) S3(A)
1.7000 3.1813
729.3 389.7
0.2792 1.2290
2
3
bands is relatively weak and occurs in the visible region (450– 700 nm), and the intense B-bands occurs in the near-UV region (at about 400 nm). The absorption spectra of 1 can be explained clearly by Gouterman’s Four-Orbital Model [45–47]. The Q-band is welldescribed by singly excited p ! p* configurations of HOMO1 to LUMO+1 (48%) and HOMO to LUMO (51%). The B-band located at 372.7 nm, which can be described by the transitions from HOMO1 to LUMO (48%) and HOMO to LUMO+1 (51%). The LUMO and LUMO+1 of Zn porphyrin are degenerate. So both Q-band and B-band of 1 are contributed by configurations of HOMO1 to LUMO and HOMO to LUMO. Because of the energy of the configuration obtained by exciting an electron from HOMO to LUMO almost equal to that obtained by excitation from HOMO1 to LUMO, the electric dipole transition moment vectors for the one-electron transitions from HOMO ! LUMO and HOMO1 ! LUMO are similar in magnitude and parallel in their orientation. Minus () and plus (+) states can be obtained by the linear combination of the two configurations. The lowest singlet excited configurations are mixed with a different sing and the transition dipoles nearly cancel, but the high energy B-band absorption is intense because of additive combination of the transition dipoles of the configurations. However, when it comes to 2 and 3, the Q-bands are no longer weak as 1. The lowest energy absorption bands of 2 and 3 are peaking at 677 nm and 729 nm with the oscillator strengths of 0.3273 and 0.2792, respectively, which all mainly arising from HOMO to LUMO. And they are both assigned to p ! p* transitions from tetrapyrrin itself. There are two possible mechanisms for the intensified Q-band. First, the degeneracy of the HOMOs and LUMOs is removed by opening the porphyrin ring, since the molecular orbitals occupied by opened tetrapyrrin ligand are stabilized and well separated. Therefore, the Q-band is no longer a pseudoparityforbidden transition. Second, the transition dipole moment is intensified due to the electron excited from HOMO to LUMO totally, so the cancellation of transition moment vectors is lost. The B-bands of 2 and 3 around 400 nm are also simulated by TDDFT. First, in the case of complex 2, there is a strong band centered at 363 nm, which is described by the combination of HOMO6 ! LUMO (49%), HOMO3 ! LUMO (28%), HOMO1 ! LUMO (10%) and HOMO ! LUMO+1 (12%) configurations. And it is assigned to the d ! p* transitions from Zn to tetrapyrrin perturbed by p ! p* transitions from tetrapyrrin itself. A weak shoulder broad band can be found at 429 nm. TD-DFT calculation reveals that it can be assigned to the p ! p* transitions, which is described by the combination of HOMO3 ! LUMO (44%), HOMO1 ! LUMO
HOMO ! LUMO (100%) HOMO1 ! LUMO (58%) HOMO ! LUMO+1 (40%)
d ! p* p ! p*
p ! p* p ! p*
(37%), HOMO5 ! LUMO (8%) and HOMO ! LUMO+1 (9%) configurations. Furthermore, the oscillator strengths of the two transitions in the B-band absorption region are both weaker than 1. These characters of the spectra may be taken as a tool for identifying complexes with open chain structure. Now, we turn our attention to the B-bands of 3, which are similar in the UV–vis absorption spectra with 1. Transition from the HOMO1 to LUMO and HOMO to LUMO+1 lead to the intense B-like absorption centered at 390 nm, and the electronic density flow mainly on the tetrapyrrin itself. The absorption of B-band and Q-band of 3 are both intense compared to 1 and 2. By analyzing and discussing the properties of the absorption spectra, a comprehensive description can be obtained. When one C C bond of bridged methine in porphyrin is broken, the p-conjugation degree is decreasing for the tetrapyrrin compounds, and the molecular structure is changed from planar to helical sharp. Hence, the symmetry of Zn terapyrrin complexes deviates from D4h in Zn porphyrin. And the symmetry of the excited states is also changed from Eu (1) to B and A (2 and 3), respectively. Without the particular symmetry, the HOMOs and LUMOs are no longer degenerate, and the Gouterman’s FourOrbital Model could not be adopted to explain absorption transitions for the opened Zn tetrapyrrin complexes. Compared to Zn porphyrin, the intensity and peak position of B-bands and Qbands of Zn tetrapyrrin are all changed. Especially, the lowest lying absorption starts to increase in intensity with the shifting to nearinfrared region for complexes 2 and 3. Summing up the above, compared with Zn porphyrin (1), Zn tetrapyrrins (2 and 3) display broad long-wavelength absorption bands in the 550–900 nm region with intense shorter wavelength bands at about 400 nm, which mean that the designed 2 and 3 can absorb more lights in visible and near-infrared region. This result suggests that opening the ring of porphyrin ligand can help to promote the light harvesting efficiency. 3.3. Emission properties The emissions properties of 1–3 are calculated in toluene solution on the basis of the excited-state structures. The relevant data together with the measured values [13] are summarized in Table 3. Fluorescence spectra of 1–3 are shown in Fig. 6. Compared to ZnP (1), the fluorescence intensity of the opened chain form Zn tetrapyrrin (2 and 3) are higher as shown in Fig. 6. Fluorescence spectrum presents that the lowest lying emission arises from S1 to S0 at 716 nm for 2 and 872 nm for 3 in toluene solvent, and they are red-shifted compared with that of 1. The
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X. Wang et al. / Synthetic Metals 213 (2016) 18–24
Fig. 5. Electronic density contours of the frontier orbitals of these complexes in absorption spectra.
Table 3 Calculated fluorescent emissions for 1–3 in toluene, together with experimental values. Complex
Excited states
Excitation energy (eV)
Wavelength (nm)
Oscillator strengths
Main configuration
Assignment
Exp. [13] (nm)
1
S1(B2U)
2.3439
528.9
0.0024
LUMO ! HOMO1 LUMO+1 ! HOMO
p* ! p
596
2
S1(B)
1.7319
715.9
0.3210
LUMO ! HOMO
p* ! p
3
S1(B)
1.4218
872.0
0.1923
LUMO ! HOMO
p* ! p
bands both arise from one-electron transitions from LUMO ! HOMO, and they can be attributed to p* ! p charge transfer from tetrapyrrin itself. It is obvious that the predicted fluorescence of Zn tetrapyrrin is located in near-infrared region with higher intensity
compared with Zn porphyrin, their strong emission indicating that they can be used as good emissive layer materials. From fluorescence emission spectra the Stokes shifts (Dl = lf la, where lf and la are the wavelength of the corresponding fluorescence
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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.synthmet. 2015.12.023. References
Fig. 6. Simulated emission spectra in toluene for 1–3.
and absorption maximum respectively) for Zn porphyrin and Zn tetrapyrrin are evaluated. The values are ca. 4 nm for 1, 38 nm for 2 and 143 nm for 3, respectively, and the Stokes shift of Zn tetrapyrrin is larger compared to Zn porphyrin. It indicates that the absorbed energy is partly transferred to non-harmful and long fluorescence for Zn tetrapyrrin. So Zn tetrapyrrin can also be used as UV protection materials. For some other applications, in particular fluorescent bioimaging, larger Stokes shifts are desirable, because enlarged Stocks shifts make the self-absorption and inner filter effect reduced greatly. During studies of the properties of absorption and emission for 1–3, we may be bold to say that ring-opening is a convenient and efficient method to intensify the absorption and emission in visible and near-infrared region. The present findings may provide additional guide to the experiments about this species. 4. Conclusions The ground-state and excited-state geometry, frontier molecular orbitals properties, low excitation energies and electronic emission spectra of 1–3 have been studied by DFT and TD-DFT methods employing the M06 hybrid functional and 6–31g(d)/ lanl2dz basis set combination. Because the four pyrroles ring is opened, the degeneracy of the HOMOs and LUMOs are shifted, the Q-bands of Zn tetrapyrrin are enhanced corresponding to the absorption bands of Zn porphyrin. And the lowest lying absorption wavelength are red-shifted in the order of 1 < 2 < 3. The intense Bbands of 1–3 are all around 400 nm, and the oscillator strengths of the transition from 2 are weaker than those of 1 and 3. Complexes 2 and 3 can fluoresce at 716 nm and 872 nm with a higher intensity compared to 1. Finally, the opened chain form Zn tetrapyrrin complexes have visible and near-infrared region absorption and red fluorescent which are relatively intense compared to Zn porphyrin, and they may be found as new functional materials for various applications. And the ring-opening methods could be applied in other areas due to its unexpected efficiency and convenience. Acknowledgements This work was supported by the State Key Development Program for Basic Research of China (Grant No. 2013CB834801) and the Natural Science Foundation of China (Grant Nos. 21364009 and 21203071) and Young Scholar Training Program of Jilin University.
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Theoretical investigation on the spectroscopic properties of Zn porphyrin and Zn tetrapyrrin Xin Wanga,b , Fu-Quan Baia,* , Ying-Tao Liub , Jian Wanga , Hong-Xing Zhanga,* a b
International Joint Research Laboratory of Nano-Micro Architecture Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, China School of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 4 November 2015 Received in revised form 16 December 2015 Accepted 25 December 2015 Available online 7 January 2016
Exploring the relationship between the geometrical structures and spectral properties has great significance to design some desirable materials. For important pyrrole contained macromolecules, there is a new complex of Zn tetrapyrrin with opened chain ligand derived from breaking one methine bridged of Zn porphyrin. The frontier molecular orbitals, absorption and emission properties of such Zn tetrapyrrin complexes are investigated by density functional theory (DFT) and its time-dependent density functional theory (TD-DFT) methods. Compared to Zn porphyrin, the HOMOs and LUMOs are no longer degenerate, the lowest lying absorption and emission of Zn tetrapyrrin start to increase in intensity with the shifting of charge transfer transition band to near-infrared region. We hope these theoretical studies will assist the design of novel molecular materials. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Density functional theory Spectral properties Tetrapyrrin Porphyrin Zn complexes
1. Introduction Pyrrole is a well-know component for building p-conjugated molecules. For example, dipyrrin, tripyrrin and porphyrin can be formed when two, three and four pyrroles liked by methine bridge, respectively, and they are all flat molecules with fully p-conjugated [1–14]. Except closed macrocycle porphyrin, opened chain tetrapyrrin can also be formulated when four pyrrole units connect with methine bridged. Based on porphyrin, tripyrrin [1–3] and dipyrrin [4], tetrapyrrin is designed in this work. There are two forms, one is containing one hydrogen proton at the four nitrogen elements, a, the other is containing three, b (see Fig. 1). And the carbon atom at terminal a position contains only one hydrogen atom as dipyrrin and tripyrrin to ensure the tetrapyrrin fully p-conjugated. Recently, Zn porphyrin (ZnP) and its derivatives have been widely studied as active materials in dye-sensitized solar cells (DSSCs) because of the high absorption coefficients, facile synthesis and lower costs [5–14]. The structure and absorption properties of linear tetrapyrroles and opened chain Zn tetrapyrroles were also studied owing to the extended p-electron system and the flexible framework, and they can be used as biomaterials, photonic and electronic materials [15–24]. But to the best of our
* Corresponding authors. Fax.: +86 431 88498966. E-mail addresses: [email protected] (F.-Q. Bai), [email protected] (H.-X. Zhang). http://dx.doi.org/10.1016/j.synthmet.2015.12.023 0379-6779/ ã 2015 Elsevier B.V. All rights reserved.
knowledge, reports on the structures and photophysical properties of Zn complexes consisting with opened chain form tetrapyrrin are scarce. So in this article, the Zn tetrapyrrin complexes are designed and studied to compare with Zn porphyrin. Our interesting in those peculiar complexes arises from the expectation that Zn tetrapyrrins might serve as new potential materials. When the four nitrogen atoms coordinated to zinc ion, the hydrogen at the N H group can be removed, so the fourcoordinate Zn tetrapyrrin complexes is monocationic or monoanionic, named 2 and 3, respectively (see Fig. 2). In order to completely characterize and predict the properties of the unknown materials we designed here, the geometrical structures, the highest occupied molecular orbitals (HOMOs) properties, the lowest unoccupied molecular orbitals (LUMOs) properties, the energy gaps between the molecular orbitals (MOs), the low excitation energies and emission energies are investigated theoretically and discussed in detail. We hope our design would provide a useful guideline to the high-efficiency materials. 2. Computational details All the calculations are performed with the Gaussian09 suite of program [25]. The ground-state structures of these complexes are optimized at M06 [26]//6-31G(d)/LANL2DZ [27–30] level. In present calculations, full electron approach for all atoms is rather time consuming. In addition, the influence of electrons in inner core for the bonding and orbital characters is very weak. So it is
X. Wang et al. / Synthetic Metals 213 (2016) 18–24
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based p-conjugated Zn porphyrin complexes. After the comparison between the calculations and experimental data of geometrical structure at ground states and absorption properties, the same decision is made for the excited states and emission properties. And then the suitable method is applied to study the geometry structures and absorption properties of the other four pyrroles based p-conjugated Zn complexes. 3. Results and discussions 3.1. Ground-state and excited-state structures
Fig. 1. Structures of opened chain form tetrapyrrin.
better to describe the inner core and outmost electrons separately for the metal Zn atoms. Under this approximation, the electrons 4s23s23p63d10 for Zn atom are depicted through the LANL2DZ basis set. And the methodology at this level of theory has been carried out by several previous investigations for Zn complexes [31–35]. The vibrational frequencies are calculated to confirm that all the structures we study here are actual minima in the whole potential energy surface. The frontier molecular orbitals properties and absorption spectra are both in dependence on TD-DFT (time dependent density functional theory)/M06 approach. Then, the excited-state geometry optimizations and the emission properties are performed with the same level. In addition, the polarizable continuum model (PCM) [36,37] was used to evaluate the solvent effects of toluene solvent. The rational M06 functional is employed after the functional test within more than four different exchange-correlation (XC) functionals on complex 1 to achieve comparable with the measured data [13,14]. The different XC functionals of B3LYP [38,39], M06 [26], M06-2X [40] and M06HF [41,42] have been used for the geometrical optimization. The results of the optimized structural parameters and the experimental values for 1 are summarized in Table S1. From our results, the M06 functional can better reproduce geometrical parameters measured in the experiment. Then, B3LYP [38,39], PBE1 [43,44], M06 [26] and M06-2X [40] are employed to the computation for absorption spectra. It can be seen from Table S2 that the maximum absorption wavelength (550 nm) obtained by TD-M06 is much closer to the experimental date (524 nm) than other tested functional models. Therefore, the M06 functional is suitable and reliable to optimize the geometry and calculate the absorptions for the four pyrroles
Fig. 2 presents the molecular structures of 1–3. The optimized structural parameters and the experimental data [14,18] are summarized in Table 1. Complexes 2 and 3 are formed by coordination of zinc ion and opened tetrapyrrin. Complex 1 is Zn porphyrin (ZnP), the central metal ion Zn2+ lies in the center of the plane formed by the closed porphyrin macrocycle. The average Zn N bond length is 2.053 Å, and the neighboring N Zn N bond angle is 90 . For complexes 2 and 3, the tetrapyrrin are present in ring-opened form, the zinc ion is poised between tetrahedral and planar, and both of them have a helical geometry. The Zn N bond lengths of 2 and 3 are almost longer than those found in ZnP (1), and two of the four Zn-N distances are slightly longer than the other two. Such as the Zn N1 distance of 2, 2.073 Å, which is equal to that of Zn N2, is longer than the Zn N3 distance, which is equal to ZnN4 at 2.050 Å. Since the four pyrroles ring is opened, the bond angles of N Zn N are no longer equivalent at 90 , and that in 3 is more deviated than that of 2. The three bond angles (N1 Zn N2, N2 Zn N3 and N3 Zn N4) which are internal to the chelate tetrapyrrin are all nearly 90 , but the other one, which is between the two terminal pyrrole units is considerably wider, at about 97. This widened angle results from the torsion along N1–N2–N3–N4 of the four pyrrole subunits. The torsion angles C N Zn N of Zn tetrapyrrins range from 26.1 to 16.1, and the Zn ion is trapped in the helical tetrapyrrin ligand. The structures of 1–3 in the lowest single excited state (S1) are optimized by TD-M06//6-31G(d)/LANL2DZ method. The main geometry parameters of them in S1 state are also presented in the brackets of Table 1. The structures of the two Zn tetrapyrrins have a similar helical shape in excited states. Compared with the ground states, the bond lengths, bond angles and torsion angles are all changed more or less. For instance, the bond lengths in S1 state are unequal for ZnP (1), the distances between Zn and N of 2 (in S1 state) become increased, while the Zn N bond lengths in 3
Fig. 2. Optimized geometry structures of 1–3, top view (top) and side view (bottom).
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Table 1 Selected bond lengths (Å), bond angles ( ) and torsion angles ( ) of 1–3 in ground states and excited states (in the brackets). Geometrical parameters
Complex 1
2
3
Expt. [14,18]
ZnN1 ZnN3 N1ZnN2 N2ZnN3 C1N1ZnN2 C3N3ZnN4
2.053 (2.052) 2.053 (2.052) 90 (90) 90 (90) 0 (0) 0 (0)
ZnN2 ZnN4 N3 Zn N4 N1 Zn N4 C2N2ZnN3 C4N4ZnN1
2.053 (2.063) 2.053 (2.063) 90 (90) 90 (90) 0 (0) 0 (0)
2.029–2.046
ZnN1 ZnN3 N1ZnN2 N2ZnN3 C1N1ZnN2 C3N3ZnN4
2.073 (2.073) 2.050 (2.063) 89.9 (89.7) 90.0 (89.9) 19.1 (15.5) 15.9 (15.2)
ZnN2 ZnN4 N3 Zn N4 N1 Zn N4 C2N2ZnN3 C4N4ZnN1
2.050 (2.063) 2.073 (2.073) 89.9 (89.7) 97.0 (97.5) 2.1 (2.5) 25.5 (24.9)
2.006–2.084
ZnN1 ZnN3 N1ZnN2 N2ZnN3 C1N1ZnN2 C3N3ZnN4
2.083 (2.062) 2.062 (2.036) 90.1 (90.9) 88.6 (90.7) 15.8 (25.3) 16.5 (17.7)
ZnN2 ZnN4 N3 Zn N4 N1 Zn N4 C2N2ZnN3 C4N4ZnN1
2.062 (2.036) 2.083 (2.062) 90.1 (90.9) 97.5 (95.9) 2.6 (1.6) 26.1 (21.1)
become reduced, and the C4 N4ZnN1 in 2 and 3 both become reduced, etc. 3.2. Frontier molecular orbitals and absorption spectra The frontier molecular orbitals (FMOs) were calculated on the basis of the optimized structures by TD-DFT. The molecular orbital compositions can be divided into two parts, Zn and tetra pyrroles. Table S3 summarizes the energy levels and compositions of partial FMOs of these complexes. Table S4 gives the energy gaps between FMOs. Fig. 3 presents the energy level diagrams of FMOs. The occupied molecular orbitals are highlighted in black, and the unoccupied molecular orbitals are in red and green. From Table S3 and Fig. 3, the FMOs of these complexes are mostly assigned to the contribution of tetra pyrroles. The lowest unoccupied molecular orbitals (LUMO) and the next lowest unoccupied molecular orbitals (LUMO+1) of Zn porphyrin are degenerate with the same energy, the highest occupied molecular orbital (HOMO) and the next highest occupied molecular orbital
Fig. 3. Energy level diagrams of partial frontier molecular orbitals. Occupied molecular orbitals are highlighted in black, unoccupied molecular orbitals are in red and green. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
89.8–90.2 5.7–2.3
88.3–102.3
(HOMO1) are nearly degenerate with a difference of 0.004 eV, and the four FMOs are well separated from the other molecular orbitals in energy. These features can be explained by the Fourorbital Model [45–47]. But when the tetra pyrroles ring is opened, the energy gaps of 2 and 3 between the two occupied molecular orbitals increase to 1.096 and 1.098 eV, the energy gaps between the two unoccupied molecular orbitals increase to 1.256 and 1.257 eV, respectively. And the energy gap between LUMO+2 and LUMO+1 of 2 and 3 is 1.134 and 1.112 eV, between LUMO+3 and LUMO+2 is 1.625 and 0.927 eV, respectively (see Table S4). It is obvious that, the four FMOs of 2 and 3 are not separated energetically from the other molecular orbitals as 1. The simulated electronic absorption spectra of 1–3 in the toluene solution are shown in Fig. 4. To better understand the electronic transitions of absorption, the wavelength, transition energies and oscillator strengths, main configurations along with the experimental values [13] are displayed in Table 2. The contours of the frontier orbitals of these complexes in absorption spectra are shown in Fig. 5. From Table 2 and Fig. 4, the absorption spectra of 1 exhibit typically Zn porphyrin absorption characteristics with intense Bbands at 372 nm and weak Q-bands at 524 nm. The so-called Q-
Fig. 4. Simulated absorption spectra with Gaussian curve based on the data calculated with the TD-DFT method in toluene for 1–3.
X. Wang et al. / Synthetic Metals 213 (2016) 18–24
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Table 2 Excitation energies, oscillator strengths (f) and main configurations of the absorption spectra of these complexes calculated with TD-DFT method. Complex
Excited states
Excitation energy (eV)
Wavelength (nm)
f
Main configuration
Assignment
Exp. [13] (nm)
1
S1(EU)
2.3212
524.8
0.0036
p ! p*
550
S3(EU)
3.3270
372.7
1.3295
HOMO1 ! LUMO+1 (48%) HOMO ! LUMO (51%) HOMO1 ! LUMO (48%) HOMO ! LUMO+1 (51%)
p ! p*
423
S1(B) S4(A)
1.8304 2.8889
677.4 429.2
0.3273 0.0408
p ! p* p ! p*
S7(A)
3.4188
362.7
0.5334
HOMO ! LUMO (100%) HOMO3 ! LUMO(44%) HOMO1 ! LUMO (37%) HOMO5 ! LUMO(8%) HOMO ! LUMO+1 (9%) HOMO6 ! LUMO (49%) HOMO3 ! LUMO (28%) HOMO1 ! LUMO (10%) HOMO ! LUMO+1 (12%)
S1(B) S3(A)
1.7000 3.1813
729.3 389.7
0.2792 1.2290
2
3
bands is relatively weak and occurs in the visible region (450– 700 nm), and the intense B-bands occurs in the near-UV region (at about 400 nm). The absorption spectra of 1 can be explained clearly by Gouterman’s Four-Orbital Model [45–47]. The Q-band is welldescribed by singly excited p ! p* configurations of HOMO1 to LUMO+1 (48%) and HOMO to LUMO (51%). The B-band located at 372.7 nm, which can be described by the transitions from HOMO1 to LUMO (48%) and HOMO to LUMO+1 (51%). The LUMO and LUMO+1 of Zn porphyrin are degenerate. So both Q-band and B-band of 1 are contributed by configurations of HOMO1 to LUMO and HOMO to LUMO. Because of the energy of the configuration obtained by exciting an electron from HOMO to LUMO almost equal to that obtained by excitation from HOMO1 to LUMO, the electric dipole transition moment vectors for the one-electron transitions from HOMO ! LUMO and HOMO1 ! LUMO are similar in magnitude and parallel in their orientation. Minus () and plus (+) states can be obtained by the linear combination of the two configurations. The lowest singlet excited configurations are mixed with a different sing and the transition dipoles nearly cancel, but the high energy B-band absorption is intense because of additive combination of the transition dipoles of the configurations. However, when it comes to 2 and 3, the Q-bands are no longer weak as 1. The lowest energy absorption bands of 2 and 3 are peaking at 677 nm and 729 nm with the oscillator strengths of 0.3273 and 0.2792, respectively, which all mainly arising from HOMO to LUMO. And they are both assigned to p ! p* transitions from tetrapyrrin itself. There are two possible mechanisms for the intensified Q-band. First, the degeneracy of the HOMOs and LUMOs is removed by opening the porphyrin ring, since the molecular orbitals occupied by opened tetrapyrrin ligand are stabilized and well separated. Therefore, the Q-band is no longer a pseudoparityforbidden transition. Second, the transition dipole moment is intensified due to the electron excited from HOMO to LUMO totally, so the cancellation of transition moment vectors is lost. The B-bands of 2 and 3 around 400 nm are also simulated by TDDFT. First, in the case of complex 2, there is a strong band centered at 363 nm, which is described by the combination of HOMO6 ! LUMO (49%), HOMO3 ! LUMO (28%), HOMO1 ! LUMO (10%) and HOMO ! LUMO+1 (12%) configurations. And it is assigned to the d ! p* transitions from Zn to tetrapyrrin perturbed by p ! p* transitions from tetrapyrrin itself. A weak shoulder broad band can be found at 429 nm. TD-DFT calculation reveals that it can be assigned to the p ! p* transitions, which is described by the combination of HOMO3 ! LUMO (44%), HOMO1 ! LUMO
HOMO ! LUMO (100%) HOMO1 ! LUMO (58%) HOMO ! LUMO+1 (40%)
d ! p* p ! p*
p ! p* p ! p*
(37%), HOMO5 ! LUMO (8%) and HOMO ! LUMO+1 (9%) configurations. Furthermore, the oscillator strengths of the two transitions in the B-band absorption region are both weaker than 1. These characters of the spectra may be taken as a tool for identifying complexes with open chain structure. Now, we turn our attention to the B-bands of 3, which are similar in the UV–vis absorption spectra with 1. Transition from the HOMO1 to LUMO and HOMO to LUMO+1 lead to the intense B-like absorption centered at 390 nm, and the electronic density flow mainly on the tetrapyrrin itself. The absorption of B-band and Q-band of 3 are both intense compared to 1 and 2. By analyzing and discussing the properties of the absorption spectra, a comprehensive description can be obtained. When one C C bond of bridged methine in porphyrin is broken, the p-conjugation degree is decreasing for the tetrapyrrin compounds, and the molecular structure is changed from planar to helical sharp. Hence, the symmetry of Zn terapyrrin complexes deviates from D4h in Zn porphyrin. And the symmetry of the excited states is also changed from Eu (1) to B and A (2 and 3), respectively. Without the particular symmetry, the HOMOs and LUMOs are no longer degenerate, and the Gouterman’s FourOrbital Model could not be adopted to explain absorption transitions for the opened Zn tetrapyrrin complexes. Compared to Zn porphyrin, the intensity and peak position of B-bands and Qbands of Zn tetrapyrrin are all changed. Especially, the lowest lying absorption starts to increase in intensity with the shifting to nearinfrared region for complexes 2 and 3. Summing up the above, compared with Zn porphyrin (1), Zn tetrapyrrins (2 and 3) display broad long-wavelength absorption bands in the 550–900 nm region with intense shorter wavelength bands at about 400 nm, which mean that the designed 2 and 3 can absorb more lights in visible and near-infrared region. This result suggests that opening the ring of porphyrin ligand can help to promote the light harvesting efficiency. 3.3. Emission properties The emissions properties of 1–3 are calculated in toluene solution on the basis of the excited-state structures. The relevant data together with the measured values [13] are summarized in Table 3. Fluorescence spectra of 1–3 are shown in Fig. 6. Compared to ZnP (1), the fluorescence intensity of the opened chain form Zn tetrapyrrin (2 and 3) are higher as shown in Fig. 6. Fluorescence spectrum presents that the lowest lying emission arises from S1 to S0 at 716 nm for 2 and 872 nm for 3 in toluene solvent, and they are red-shifted compared with that of 1. The
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X. Wang et al. / Synthetic Metals 213 (2016) 18–24
Fig. 5. Electronic density contours of the frontier orbitals of these complexes in absorption spectra.
Table 3 Calculated fluorescent emissions for 1–3 in toluene, together with experimental values. Complex
Excited states
Excitation energy (eV)
Wavelength (nm)
Oscillator strengths
Main configuration
Assignment
Exp. [13] (nm)
1
S1(B2U)
2.3439
528.9
0.0024
LUMO ! HOMO1 LUMO+1 ! HOMO
p* ! p
596
2
S1(B)
1.7319
715.9
0.3210
LUMO ! HOMO
p* ! p
3
S1(B)
1.4218
872.0
0.1923
LUMO ! HOMO
p* ! p
bands both arise from one-electron transitions from LUMO ! HOMO, and they can be attributed to p* ! p charge transfer from tetrapyrrin itself. It is obvious that the predicted fluorescence of Zn tetrapyrrin is located in near-infrared region with higher intensity
compared with Zn porphyrin, their strong emission indicating that they can be used as good emissive layer materials. From fluorescence emission spectra the Stokes shifts (Dl = lf la, where lf and la are the wavelength of the corresponding fluorescence
X. Wang et al. / Synthetic Metals 213 (2016) 18–24
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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.synthmet. 2015.12.023. References
Fig. 6. Simulated emission spectra in toluene for 1–3.
and absorption maximum respectively) for Zn porphyrin and Zn tetrapyrrin are evaluated. The values are ca. 4 nm for 1, 38 nm for 2 and 143 nm for 3, respectively, and the Stokes shift of Zn tetrapyrrin is larger compared to Zn porphyrin. It indicates that the absorbed energy is partly transferred to non-harmful and long fluorescence for Zn tetrapyrrin. So Zn tetrapyrrin can also be used as UV protection materials. For some other applications, in particular fluorescent bioimaging, larger Stokes shifts are desirable, because enlarged Stocks shifts make the self-absorption and inner filter effect reduced greatly. During studies of the properties of absorption and emission for 1–3, we may be bold to say that ring-opening is a convenient and efficient method to intensify the absorption and emission in visible and near-infrared region. The present findings may provide additional guide to the experiments about this species. 4. Conclusions The ground-state and excited-state geometry, frontier molecular orbitals properties, low excitation energies and electronic emission spectra of 1–3 have been studied by DFT and TD-DFT methods employing the M06 hybrid functional and 6–31g(d)/ lanl2dz basis set combination. Because the four pyrroles ring is opened, the degeneracy of the HOMOs and LUMOs are shifted, the Q-bands of Zn tetrapyrrin are enhanced corresponding to the absorption bands of Zn porphyrin. And the lowest lying absorption wavelength are red-shifted in the order of 1 < 2 < 3. The intense Bbands of 1–3 are all around 400 nm, and the oscillator strengths of the transition from 2 are weaker than those of 1 and 3. Complexes 2 and 3 can fluoresce at 716 nm and 872 nm with a higher intensity compared to 1. Finally, the opened chain form Zn tetrapyrrin complexes have visible and near-infrared region absorption and red fluorescent which are relatively intense compared to Zn porphyrin, and they may be found as new functional materials for various applications. And the ring-opening methods could be applied in other areas due to its unexpected efficiency and convenience. Acknowledgements This work was supported by the State Key Development Program for Basic Research of China (Grant No. 2013CB834801) and the Natural Science Foundation of China (Grant Nos. 21364009 and 21203071) and Young Scholar Training Program of Jilin University.
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