A theoretical investigation on the π-conjugation effect on the structures and spectral properties of tetra pyrrole zinc complexes

Synthetic Metals 210 (2015) 258–267

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A theoretical investigation on the p-conjugation effect on the structures and spectral properties of tetra pyrrole zinc complexes Xin Wanga,b , Fu-Quan Baia,* , Miao Xiea , Li Haoa , 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 28 July 2015 Received in revised form 11 October 2015 Accepted 13 October 2015 Available online 30 October 2015

The structures and properties of a family of different p-conjugation tetra pyrrole Zn complexes are reported. The tetra pyrrole moiety can be divided as porphyrin, one tripyrrin and one pyrrole, two dipyrrines, one dipyrrin and two pyrroles, and four pyrroles. The Zn complexes possess different geometries and tunable spectral bands depending on the mode of the different metal-coordination. For Zn porphyrin, the frontier molecular orbitals (FMOs) are the major contribution to the intense B-like band and weak Q-like band absorptions, and this is in agreement with the Gouterman’s four-orbital model. But because of the breakdown of the p-conjugation from complex 1 to 5, the FMOs are no longer separated from the other MOs in energy, the orbital with metal distribution is approaching to lowest unoccupied molecular orbital (LUMO), the absorption band is no longer intense at B-like bond and weak at Q-like bond as Gouterman's four-orbital model mentioned. The calculated fluorescence spectra in toluene solution show that fluorescence can be observed in the visible region of complexes 1–4 because of the bright higher electronic excited states. ã 2015 Elsevier B.V. All rights reserved.

Keywords: Density functional theory Optical properties Pyrrole p-Conjugation Zn complexes

1. Introduction The pyrrole based compounds have already been used for metal cation sensors in biological media, OLEDs, sensitizers for lightharvesting systems, and construction of coordination polymeric architectures [1,2]. Specially, porphyrin (H2P, composed of four modified pyrrole subunits), zinc porphyrin (ZnP), and their derivatives which are common found in nature and synthesized in lab. They are all also used as important chromophores, which play a crucial role in a number of biological processes and dyesensitized solar cells (DSSCs), etc. [3–12]. The excited electronic states of H2P and ZnP have been attracting significant interest from the theoretical and computational communities [13–21]. The interpretation of the ZnP spectra is based on the Gouterman's four orbital model which describe the charge transfer transitions from two highest occupied and two lowest unoccupied molecular orbitals [22–24]. However, if the p-conjugatian of the molecular skeleton is breached, the structures, symmetry and spectral properties of the tetra pyrrole Zn complexes would be affected. The structures and photophysical properties of zinc complexes with tripyrrin ligand were explored, the UV/Vis spectra show

* Corresponding authors. E-mail addresses: [email protected] (F.-Q. Bai), [email protected] (H.-X. Zhang). http://dx.doi.org/10.1016/j.synthmet.2015.10.012 0379-6779/ ã 2015 Elsevier B.V. All rights reserved.

strong absorptions between 300 and 700 nm [25–27]. But in our knowledge, there is a lack of comprehensive analysis of structureproperties relationship about this species based on quantum chemical simulations. Sazanovich and co-workers reported that replacement of the phenyl rings at the 5,5’-positions of bis (dipyrrinato) zinc complex with mesityl groups transforms the bis (dipyrrinato) zinc from a very weak emitter to a highly fluorescent chromophore [28]. Various types of modification of the Zn dipyrrin complexes and their photophysical properties were summarized [29–31]. The geometric and electronic structures of the fluorescent azadipyrrinato zinc(II) complex have attracted particular theoretical interest [32–36]. Zinc complexes with four pyrrolyl ligands was synthesised and characterized [37], and the ground-state structures and sequential binding energies were theoretical studied using density functional theory (DFT) methods [38]. Further works will be needed to clarify the structure-property relationship and performed to extend this family for pyrrole-based Zn complexes. However, because of different p-conjugation degree and symmetry, the structures and spectral properties of these pyrrole-based Zn complexes will be probably changed in contrast to porphyrin-based ones. For example, the geometrical structures change from planar to distorted tetrahedral in complexes bearing pyrrole ligand, dipyrrin ligand and tripyrrin ligand, respectively, the intense absorption bands shift compared to Zn porphyrin, and the lowest absorption bands no longer weak in complex with

X. Wang et al. / Synthetic Metals 210 (2015) 258–267

tripyrrin ligand. It was reported that the color and intensity of porphyrins and their derivatives are derived from the highly conjugated p-electron systems. The absorption spectra of Zn porphyrin can be explained by Gouterman's four-orbital model. This model describes the low-lying p–p* excited states of porphyrins in terms of the transitions between the two highest occupied molecular orbitals a1u and a2u (HOMO and HOMO-1) to the two degenerate lowest energy unoccupied molecular orbitals eg (LUMOs). The absorption spectra of typical porphyrins consist of a second excited state (S0 ! S2) (the B band) with greater oscillator strength at ca. 400 nm and a first excited state (S0 ! S1) at ca. 550 nm (the Q band) with less oscillator strength. When the p-conjugation degree is breaked slightly, the quasi gouterman’s four-orbital model could be also adopted to explain absorption transitions of porphyrin derivatives, though there is no D4h symmetry, but the low-lying excited states are all nearly from the transitions between frontier four orbitals. We are inquisitive about if the other tetra pyrrole Zn complexes are also in agreement with the quasi gouterman’s four-orbital model or if the p-conjugation degree is the most important factor responsible for the Gouterman’s four-orbital model. In this paper, six different p-conjugation tetra pyrrole Zn complexes are designed along with the structures of synthesized molecules in experiment to compare the effect of p-conjugation, and the molecular structures are represented in Fig. 1. Complex 1 is Zn porphyrin without any substituent [9,11]. 2 is (trpy)Zn(pyr) (pyr = pyrrolyl, trpy = tripyrrin) based on compound [Zn(N3)(L3)] (L3 = tripyrrin, R1 = R2 = Et) where the Et side chains and N3 ligand are replaced by H and pyrrolyl respectively [25]. 3 is compound Zn(dpy)2 (dpy = dipyrrin), and the dipyrrin ligand has not any substituent group [33], complex 5 Zn2+(Pyr)4 (pyr = pyrrolyl) is based on compounds [{(TMEDA) Na}2Zn(NC4H4)4] and [{(PMDETA)Na}F2Zn(NC4H4)4] by removing [(TMEDA)Na]2 (TMEDA is N,N,N0 ,N0 -tetramethylethyl-enediamine) and [(PMDETA)Na]2 (PMDETA is N,N,N0 ,N00 ,N00 -pentamethyldiethylenetriamine) [37] for the sake of simplification, and the Pyrrolyl (NC4H4) is the ligand means where conversion of the N H bond to an NM bond has occurred. Finally, we designed complex (dpy)Zn(pyr)2 (pyr = pyrrolyl, dpy = dipyrrin) as 4 based on the complexes mentioned above, and this is a Zn complex with two pyrrolyl and a conjugated dipyrrin ligand to provide a comparison with other tetra pyrrole Zn complexes. And we can research the properties of the series more perfectly and systemly. To determine the p-conjugation effect, the structures and symmetry of the designed compounds were investigated, and the frontier molecular orbital properties, intensity variations and wavelength shifts of the Q-like and B-like bands of these complexes are also studied by quantum chemistry calculation. The so-called Q-like band is relatively weak and occurs in the visible region (450–700 nm), and the intense B-like band occurs in the near-UV region (at about 400 nm).

259

2. Computational methods In this study, the ground-state geometrical structures of all molecules are fully optimized by the density functional theory (DFT) using the M06 hybrid functional [39] combined with the 631G(d) basis set [40] for non-metal atoms and the LANL2DZ [41–43] basis set for Zn atom. The geometry optimizations are confirmed by the frequency calculations. In the calculations, D4h symmetry is adopted for 1, and Cs for 2, D2d for 3 and 5b, C2 for 4, while S4 for 5a. The appropriate configurations are obtained by using fixed-symmetry. Geometry optimizations and vibrational frequency calculations are performed by the Gaussian 09 program package (Revision D.01) with a tight self-consistent filed convergence threshold [44]. Based on of the optimized structures, the computations of the electronic structures and electronic spectra are carried out by the time-dependent density functional theory (TD-DFT) at the same level. In addition, the solvent effects are taken into account by means of the polarized continuum model (PCM) approach [45,46]. We also considered the emission spectra on the basis of the optimized excited state geometry structures by TD-M06 functional. The rational M06 functional is employed after the functional test with more than five functionals on complexes 1 and 3 to allow a comparison with the measured data [11,33]. Table S-1 and Table S-2 present the ground-state geometry parameters and the absorption results obtained from different functionals. The calculated results are compared well with the experimental results. The M06 functional is found to be more suitable for optimization of the geometries of these pyrrole based Zn complexes. The few functionals have no large deviation in excitation energies and oscillator strengths of the absorption spectra, especially for the enhanced B-like band. For maintaining consistent with the functional used in geometry optimization, M06 functional is employed in spectral properties calculation. 3. Results and discussions 3.1. Molecular design and geometry optimization The optimized structural diagrams are shown in Fig. 1. Moreover, the selected bond lengths, bond angles and torsion angles along with some single-crystal experimental values are presented in Table 1 [12,25,33,37]. The main geometrical parameters of 1–5 in S1 state are also presented in Table S-3. The selected dihedral angles of these complexes are given in Fig. S1–6, and shown in the supporting information. Complex 1 is zinc poyphyrin with D4h symmetry, and the metal ion Zn2+ lies in the center of porphyrin plane. The average Zn N bond length is 2.053 Å and average bond angle of N Zn N is 90 . Complex 2 is zinc tripyrrin complex combined by a tridentate pincer planar ligand and a deprotonated pyrrole ligand. The molecule has idealized mirror symmetry (Cs) which is close to distorted tetrahedral rather than to planar, and the main distortion

Fig. 1. Optimized geometry structures of 1–5. In 1, Zn–Nphy is 2.053 Å; In 2, Zn–Ntrpy is 2.063 Å and 2.104 Å; In 3, Zn–Ndpy is 2.042 Å, Zn–Npyr is 2.004 Å; In 4, Zn–Ndpy is 2.106 Å; In 5, Zn–Npyr is 2.060 Å; In 6, Zn–Npyr is 2.070 Å.

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X. Wang et al. / Synthetic Metals 210 (2015) 258–267 Table 1 Selected bond lengths (Å), bond angles ( ) and torsion angles ( ) of 1–5 in the ground states. complex

parameters

Expt.

a,b,c,d

Zn–N1 Zn–N3 N1–Zn–N2 N1–Zn–N3 C1–N1–Zn–N2 C3–N3–Zn–N4

2.053 2.053 90 90 0 0

Zn–N2 Zn–N4 N3–Zn–N4 N2–Zn–N4 C2–N2–Zn–N3 C4–N4–Zn–N1

2.053 2.053 90 90 0 0

2.029–2.046a

Zn–N1 Zn–N3 N1–Zn–N2 N1–Zn–N3 C1–N1–Zn–N2 C3–N3–Zn–N4

2.104 2.104 88.8 140.9 23.8 39.8

Zn–N2 Zn–N4 N3–Zn–N4 N2–Zn–N4 C2–N2–Zn–N3 C4–N4–Zn–N1

2.063 1.963 103.3 140.2 -23.2 -15.0

1.968–2.025b

Zn–N1 Zn–N3 N1–Zn–N2 N1–Zn–N3 C1–N1–Zn–N2 C3–N3–Zn–N4

2.042 2.042 94.7 117.3 0.0 180.0

Zn–N2 Zn–N4 N3–Zn–N4 N2–Zn–N4 C2–N2–Zn–N3 C4–N4–Zn–N1

2.042 2.042 94.7 117.3 -55.9 -55.9

1.999–2.011c

4

Zn–N1 Zn–N3 N1–Zn–N2 N2–Zn–N3 C1–N1–Zn–N2 C3–N3–Zn–N4

2.106 2.004 90.1 104.5 0.15 29.1

Zn–N2 Zn–N4 N3–Zn–N4 N2–Zn–N4 C2–N2–Zn–N3 C4–N1–Zn–N4

2.106 2.004 111.2 123.3 -52.7 -52.7

5a

Zn–N1 Zn–N3 N1–Zn–N2 N1–Zn–N3 C1–N1–Zn–N4 C3–N3–Zn–N2

2.060 2.060 110.3 107.8 25.6 25.6

Zn–N2 Zn–N4 N1–Zn–N4 N2–Zn–N4 C2–N2–Zn–N1 C4–N4–Zn–N3

2.060 2.060 110.3 107.8 -25.6 -25.6

1.979–2.006d

Zn–N1 Zn–N3 N1–Zn–N2 N1–Zn–N3 C1–N1–Zn–N3

2.070 2.070 104.4 120.3 28.2

Zn–N2 Zn–N4 N3–Zn–N4 N2–Zn–N4 C2–N2–Zn–N3

2.070 2.070 104.4 120.3 -28.2

1.984d

1

2

3

5b

89.8–90.2a 5.7–2.3a

93.2–148.8b 17.2–22.0b 43.7b

94.8c 111.5c

107.5–113.1d

104.6–112.7d

a

From Ref. [11]. From Ref. [25]. From Ref. [33]. d From Ref. [37]. b c

from tetrahedral angles are the bond angles between the pyrrolyls: N3 Zn N1¼140.9 , N2ZnN4¼140.2 , N3 Zn N4¼103.3 and N1ZnN2¼N2 Zn–N3¼88.8 . The Zn2+ lies slightly above the plane defined by three pincer nitrogens, but the three pyrroles in tripyrrin ligand are not on a plane. The torsion angles C1–N1– Zn–N2 = 23.8 , C2–N2–Zn–N3 = 23.2 , C3–N3–Zn– N4 = 39.8 and C4–N4–Zn–N1 = 15.0 . The dihedral angle between trpyrrin and pyrrolyl is 59.8 , and the pyrrolyl presents its face to tripyrrin ligand or to ZnN2 bond. The bond length of zinc and pyrrolyl Zn Npyr is 1.963Å, shorter than the Zn N bond length in complexes 1, and it is the shortest bond length among Zn-N bond length in this kind of four pyrrole complexes. It indicates that the repulsive interaction of metal-ligand are very small, allowing us to ascertain the designed complex is reasonable and can be synthesized. All the bond distances of Zn Ntrpy fall in a narrow range 2.063–2.104 Å, longer than Zn Npyr in this complex. Two bidentate monoanionic p-conjugated dipyrrines coordinating to Zn composes complex 3, and the geometrical structure is optimized under D2d symmetry. The features of the distortion tetrahedral complex 3 are pertinent: (1) Owing to conjugation, each dipyrrin unit is planarity. The torsion angle between the two pyrrolic rings in the dipyrrin is 0 . (2) Two chelate dipyrrin ligands are set perpendicular to one another, and the dihedral angle

between the two chelate ligand planes is 90 . But the dihedral angle between the two ligands is about 74 in the experiment of dipyrrin Zn complexes [33]. It is no substituted for 3 in our calculations, which may lead the effect of the steric hindrance diminishing. To illustrate the steric hindrance affects, the dihedral angle of bis (dipyrrinato) zinc(II) with substituted dipyrrin is calculated, shown in Table S-4. Different dihedral angles from 75 to 90 are obtained by using different functions. The experimental date is about 74 , so B97D and M062X is relatively suited for the geometrical structure optimization of bis(dipyrrinato) zinc(II). When the substituent groups on the dipyrrins are removed, the dihedral angle between the two chelate dipyrrins is changing from 75 to 90 due to the steric hindrance affects diminishing. (3) The bond distance of ZnNdpy is 2.042 Å, shorter than ZnNphy by 0.011 Å. (4) Bond angles of N1 Zn N3 and N2ZnN4 are 117.3 between the two chelate planes, and the angles of N1 ZnN2 and N3 Zn N4 are 94.7 in the chelate dipyrrin plane. The geometrical structure of Complex 4 in the ground state is optimized under C2 symmetry. Complex 4 is also a pseudotetrahedral geometry feature, the bond length of metal Zn and pyrrolyl nitrogen is 2.004 Å, which is also shorter than the distance between Zn and pyrrolyl nitrogen in Zn porphyrin, indicating the repulsive interactions of metal–ligand reduced. But Zn–Ndpy is

X. Wang et al. / Synthetic Metals 210 (2015) 258–267

2.106 Å, longer than the distance between Zn and pyrrolyl nitrogen in complexes 1-3. The band angles N2 Zn N3 and N2ZnN4 are 104.5 and 123.3 between the chelate plane and the pyrrolyl, the angle of N1 Zn N2 is 90.1 in the chelate  dipyrrole, and N3 Zn N4 is 111.2 between the two pyrrolyls. The dihedral angle between the pyrrolyl and dipyrrin is 62.3 , and it is 47.4 for the two pyrrolyls. The optimized Zn2(Pyr)4 complex 5 has two possible and reasonable geometry which are also synthesized in experiment [37], both in common are distorted tetrahedral coordination geometries. So in this article, we name them as 5a and 5b respectively. In 5a, possessing S4 symmetry, the four bond lengths of Zn-N are equivalent at 2.060 Å, the Pyr ligands are twisted relative to each other by about 25.6 , the dihedral angle between the neighboring pyrrolyl ring planes is 75.0 , and the bond angle between the neighboring pyrrolyl rings is around 110 . In 5b (with D2d symmetry), the average Zn N bond length is about 2.070 Å. The V-shaped two pyrrolyls are face to face, and the dihedral angle between them is 64.3 . The bond angle between the V-shaped two pyrrolyl rings is 104.4 , but N1 Zn N3¼N2 Zn N4¼120.3 . The band parameters of the two structures are in good agreement with the experimental data [37]. When the p-conjugation breaking from the 1 to 5, the molecular geometry change from square-planar to distorted tetrahedral one, and the breached conjugation induces less planarity and rigidity. 3.2. Frontier molecular orbital properties Electronic structures are fundamental for understanding electronic absorption spectra. The calculated molecular orbitals energies and detailed compositions of the selected frontier molecular orbitals (FMOs) are given in Fig. 2 and Table S5-6. The maps of the frontier orbitals of these complexes plotted in GaussView are shown in Fig. 3. For all the complexes, the frontier occupied molecular orbitals (FOMOs) is mainly located on the pyrrole rings. It is obvious that the highest occupied molecular orbital (HOMO) and the next highest occupied molecular orbital (HOMO-1) of Zn porphyrin are mainly delocalized on the porphyrin ring. The energies of HOMO1 and HOMO are nearly degenerate with a difference of 0.004 eV,

261

but the electronic structures are different. The four FMOs (HOMO1, HOMO, LUMO, and LUMO + 1) are well separated from the other molecular orbitals (MOs) in energy which are consistent with the Four-orbital Model character. The HOMO and HOMO-1 of 3 are almost degenerate since both are dominated by two dpy ligands. The gap between HOMO-1 and HOMO-2 is 1.232 eV, lager than that of 2,4 and 5. And the four FMOs of 3 are also separated from the other occupied and unoccupied MOs in energy like as Zn porphyrin. The neighboring FOMOs energy levels of 2, 4 and 5 are very close but not degenerate totally. For 2, the gaps between neighboring orbitals DE(HOMO–HOMO-1) and DE(HOMO-1– HOMO-2) are 0.8 and 0.192 eV, respectively. For 4, the HOMO (3.652 eV, 50% pyr1, 50% pyr2) and HOMO-1 (3.705 eV, 49% pyr1, 49% pyr2) are similar not only in composition but also in energy level. The HOMO-2 (4.421 eV, 59% dpy, 20% pyr1, 20% pyr2) and HOMO-3 (4.454 eV, 43% dpy, 28% pyr1, 28% pyr2) are similar and DE (HOMO-1HOMO-2) is 0.716 eV. For 5b, the energy levels of HOMO, HOMO-1, HOMO-2 and HOMO-3 are 2.378, 2.460, 2.460 and 2.491 eV, respectively. HOMO-1 and HOMO2 are degenerated, and the HOMO-4 and HOMO-5 are also degenerated. The lowest unoccupied molecular orbitals (LUMOs) of the five complexes are mainly composed by the conjugated ligand except 5a and 5b. The LUMO of Zn(pyr)4 is mastered by Zn orbitals with 78% for 5a and 86% for 5b. The Zn orbitals do not significantly contribute to the frontier orbitals until LUMO + 3 for 1, LUMO + 2 for 2 and 3, LUMO + 1 for 4, and the energy of them often much higher than the neighbouring LUMO. With the decreasing degree of the conjugated chain, the orbital with the electron density located on the metal is approaching to LUMO. It can be noted, the LUMO and LUMO + 1 of Zn porphyrin and Zn(dpy)2 are degenerated with the same energy and electronic structure. The band gaps between LUMO and LUMO + 1 of 2, 4 and 5 are 1.393, 3.133 and 0.228 eV, respectively, which are larger than 1 and 3. For 2, the LUMO and LUMO + 1 are mainly composed by tripyrrin, unlike HOMO and HOMO-1 which are mainly based on pyrrolyl. For 4, the LUMO is mainly composed by the conjugated dipyrrin, but not pyrrolyl as its HOMO and HOMO-1. For 5a, the LUMO and LUMO + 1 are degenerated with 78% Zn, and the HOMO and HOMO-1 are also degenerated. For 5b, the LUMO + 1 and LUMO + 2 are degenerated with 76% Zn, and the HOMO-1 and HOMO-2 are also degenerated. The proportion of metal distribution is decreasing from LUMO to LUMO + 4 for 5a and 5b. So the four FMOs of 2–4 (HOMO-1, HOMO, LUMO, LUMO + 1) are not separated energetically from the other MOs and the separated molecular orbitals character is not fitting the Four-orbital Model. When the tetra pyrrole connected with different conjugation, the HOMO and LUMO energy levels changed, and the energy levels of all the LUMOs are reduced in the order of 5 > 4 > 3 > 1 > 2, and while 5 >4 > 2 >1 > 3 for HOMOs. The band gap of the HOMO and LUMO is changed in the order of 5 > 3 > 1 > 4 > 2, and the gaps of 1 and 3 are almost same with a difference of 0.281 eV. The LUMO + 2 energy of the 1, 2, 3, 4 and 5 is increasing from 0.438, 0.514, 0.959, 2.618 to 4.224 eV, respectively. But the calculated energy gaps between the LUMO + 2 and LUMO + 1 are 1.679, 2.222, 2.993, 0.283 and 0 eV from 1 to 5, suggesting that LUMO + 2 is not willing to separate from the LUMO + 1 when the conjugation degree is decreasing. 3.3. Absorption spectra

Fig. 2. Diagrams of the molecular orbitals energy level and compositions of 1–5. Zn orbitals are highlighted in red, pyrrolyl orbitals are blue, dipyrrin orbitals are black, tripyrrin orbitals are green and porphyrin orbitals are violet. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The calculated dipole-allowed absorption in the UV–vis region associated with their oscillator strengths, the main configurations and their Excitation energies as well as the experimental results [11,25,28,33] are given in Table 2. The fitted Gaussian-type absorption curves based on the calculated absorption data are shown in Fig. 4.

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X. Wang et al. / Synthetic Metals 210 (2015) 258–267

Fig. 3. Electronic density contours of the frontier orbitals of these complexes in absorption spectra.

The absorption spectra of 1 exhibit typically porphyrin absorption characteristics with intense B-like band at 373 nm and very weak Q-like band at 525 nm. The Q-like and B-like bands are characterised by doubly degenerated excited states of Eu symmetry. The B-like band for Zn porphyrin is assumed to be a degenerate pair and the Q-like band of Zn porphyrin is the transition of pseudoparity-forbidden which can be explained by Gouterman’s four-orbital model. Compound 2 (trpy)Zn(pyr) shows strong absorptions at 535, 351, 293 nm, accompanied by two weak bands at 652 and 1137 nm. The main contribution to the Q-like bands (535 nm) and B-like bands (351 nm) are the intra-ligand charge transfer (ILCT) character, which come from p(trpy) ! p*(trpy) transition. The weak

bands, which can be found at low-energy side of these strong bands and located at 652 and 1137 nm, is contributed by the transition from HOMO-1 to LUMO and HOMO to LUMO, respectively. The main contribution to the two bands comes from pyrrolyl to conjugated trpyrrin with ligand–ligand charge transfer (LLCT) characters. Comparing UV–vis spectra of (trpy)Zn(pyr) with Zn porphyrin, we can notice that the Q-like band at 535 nm is no longer weak and the B-like band analogy to Zn porphyrin at about 350 nm is no longer intense. The Gouterman’s four-orbital model cannot be applied to the trpyrrin Zn complexes. Maybe because the symmetry is varied, and the interaction between the a1u1eg1and a2u1eg1 excited configurations diminish. Furthermore, we find that the B-like bands of complex 2

X. Wang et al. / Synthetic Metals 210 (2015) 258–267

263

Table 2 Excitation energies (eV), oscillator strengths (f), main configurations and radiative lifetime (t ) of the spectra of these complexes calculated with TD-DFT method. Complex

Excited states

Excitation energy(eV)

Wavelength(nm)

Exp (nm)

Oscillator strengths(f)

Main configuration

Assignment

t(ns)

1

S1(Eu)

2.3624

524.8

550a

0.0036

p(phy) ! p*(phy)

1157

S3(Eu)

3.3270

372.7

423a

1.3295

H-1- > L + 1 (-48%) H- > L (51%) H-1- > L (48%) H- > L + 1 (51%)

p(phy) ! p*(phy)

2

S1(A”) S2(A0 ) S3(A”) S9(A0 )

1.0905 1.9032 2.3156 3.5300

1136.9 651.5 535.4 351.3

350b

0.0003 0.0034 0.5741 0.1103

H- > L (100%) H-1- > L (100%) H-2- > L(100%) H-6- > LUMO (18%) H-2- > L + 1 (79%)

p(pyr) ! p*(trpy) p(pyr) ! p*(trpy) p(trpy) ! p*(trpy) p(trpy) ! p*(trpy)

65185 1888 8 16

S1(E)

2.9612

418.7

485d

0.0000

S3(E)

3.1391

395.0

0.7189

H-1- > L (49%) H- > L (51%) H-1- > L (49%) H- > L (48%)

p(dpy1) ! p*(dpy1) p(dpy2) ! p*(dpy1) p(dpy1) ! p*(dpy1) 3 p(dpy2) ! p*(dpy1)

S1(A) S3(B)

2.1616 2.9472

573.6 420.7

0.0011 0.0029

S5(B)

3.1498

393.6

0.7205

H- > L (100%) H-3- > L (53%) H-2- > L (47%) H-3- > L (46%) H-2- > L (52%)

p(pyr) ! p*(dpy) p(pyr) ! p*(dpy) p(dpy) ! p*(dpy) p(pyr) ! p*(dpy) p(dpy) ! p*(dpy)

S1(B1)

5.5417

223.7

0.0000

p(pyr) ! p(zn)

S9(B)

5.6272

220.3

0.0430

H-1- > L + 1 (47%) H- > L (47%) H-2- > L + 3 (4%) H-2- > L + 2 (92%)

S1(B1) S10(E)

5.4301 5.7314

228.3 216.3

0.0000 0.0501

H- > L (98%) H-3->L + 1 (94%) H-2->L + 3 (3%)

p(pyr) ! p(zn) p(pyr) ! p(zn) p(pyr) ! p*(pyr)

2

3

4

5-a

5-b

650b

p(pyr) ! p(zn) p(pyr) ! p*(pyr)

Exp (ns)

2.1(1.9)a

3c

4525 923 3

17

14

a

from Ref. [11]. from Ref. [25]. c from Ref. [28]. d from Ref. [33]. b

Fig. 4. Simulated absorption spectra with Gaussian curve based on the data calculated by the TD-DFT method in toluene for 1–5.

shifted to blue compared with those of 1, but the Q-like bands shift to red. Absorption spectra of Zn(dpy)2 in toluene features intense absorption at 395 nm with the oscillator strengths of 0.7189. This B-like bands is assigned as the p ! p* transitions from dpy2 and

dpy1 to dpy1, and the transition is calculated to be somewhat redshifted from 1 and 2 of the B-like bands. We overestimate the excitation energy due to the dihedral angle between the two dpy ligands is 90 which is larger than that in the crystal structure (dihedral angle: about 74 ) [5]. The electronic requires more

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energy to transition from dpy2 to dpy1 due to the perpendicular geometry. To illustrate the effects in dihedral angle, bis(dipyrrinato) zinc(II) with different dihedral angles (75 and 81 ) between the two chelate dipyrrinatos are calculated by different density functions (B3LYP, BP86, M06, B97D and PBE0), Zn(dpy)2 (dihedral angles between the two chelate dipyrrin is 90 ) is also calculated for comparison, and all results are shown in Table S-7. When the dihedral angles are decreasing from 90 to 75 , the B-like bands and Q-like bands all shift to lower energy. A careful inspection of the results displayed in Table 2 reveal that the vertical S0 ! S1 transition of 419 nm shows very weaker oscillator strengths. Comparing with 1 and 2, there is no Q-like bands in the spectrum of 3. This effect may be attributed to the increasing symmetry from D4h to D2d of Zn porphyrin to Zn(dpy)2 which lead to significant differences in the electronic configurations contributing to the states considered here. And the perpendicular geometry leads the electronic transition from dpy2 to dpy1 more difficult than the planar Zn porphyrin. So the Q-like band shift to blue or even disappear in the visible spectrum compared with 1. The absorption spectrum of Zn bisdipyrrin complexes can be explained by the “four-orbital” (two highest occupied p orbitals and two lowest unoccupied p* orbitals) model, but not the Gouterman's fourorbital model. The two B-like bands arise from a linear combination of these one-electron transitions from HOMO-1 ! LUMO, HOMO ! LUMO, HOMO ! LUMO + 1 and HOMO-1 ! LUMO + 1. The four FMOs (HOMO-1, HOMO, LUMO, and LUMO + 1) lead to four allowed transitions with absorption energies very close to each other (419 nm and 395 nm). The two bands are characterised by doubly degenerated excited states of E symmetry as Zn porphyrin. The lowest transition is significantly blue-shifted and the second-lowest transition is red-shifted from the corresponding transitions of Zn porphyrin. The absorption spectra are an important tool to distinguish the tetra pyrrole Zn series. Complex 4 show different feature absorption bands comparing to 3 even though both of them have dipyrrin. Transitions from HOMO-2 to LUMO and HOMO-3 to LUMO lead to the most intense B-like absorption centered at

394 nm, and the electronic density flow mainly from the dpy and pyr to dpy. A weak band can be found at 574 nm which is assigned to HOMO ! LUMO, and the electronic density flow mainly from the pyr to dpy. When the molecular symmetry is decreasing, the Q-like band in the low-lying state is not weak as 3. So the different symmetry plays an important role in the intensity and wavelength of the bands. Moreover, the UV–vis transition results reveal there is not significant excited state featuring charge transfer from the dpy to the Zn, suggesting that the energy of LUMO + 1 (mainly composed by metal) is very high. For Zn complex with four pyrrolyl ligands, the 5b, the lowest excitation energy is calculated to be 228 nm (5.4301 eV), which is assigned as the p ! p transitions from pyr to Zn. The most intense absorption is 216 nm with the oscillator strengths of 0.0430, but it is very weak compared with the intense bands of other complexes we studied. There are no B-like and Q-like bands in the absorption spectrum, and it is also for the case of 5a. Thus, when the p-conjugation is broken completely in the Zn and pyrrole complexes, the absorptions are decreasing apparently. By the above part of the analysis and discussion, a comprehensive description can be obtained. When the p-conjugation degree is breaked, the symmetry of the complex is changed from D4h for 1 to Cs for 2, D2d for 3 and 5b, C2 for 4, and S4 for 5a, respectively, and the symmetry of the excited states are changed from Eu to A00 , E, A and B. The intensity and peak position of B-like bands and Q-like bands for 2–5 are all changed. For example, the Q-like bands of 2 is no longer weak and shifts to red, and it is almost disappeared for 3, 5a and b. The B-like bands of 2 shifts to blue, but it is shifted to red for 3 and 4. For 5a and b, the transitions arise from p ! p, not p ! p* as 1–4. The absorption centered at 394 nm of 4 is contributed by the transitions from HOMO-2 to LUMO and HOMO3 to LUMO, and the absorption centered at 220 nm of 5a arises from HOMO-2 to LUMO + 2, not as 1 in which the absorption arise from the frontier four orbitals. Because the excited states arise from the transitions between more than frontier four orbitals, Gouterman’s four-orbital model is unsuitable.

Table 3 Fluorescent emissions of 1–5 in toluene from the TD-DFT calculations, together with the experimental values. Complex

excited states

Excitation energy(eV)

Wavelength (nm)

Oscillator strengths (f)

Main configuration

Assignment

Exp (nm)

1

S1(B2u)

2.3439

528.9

0.0024

p*(phy) ! p (phy)

596a

S3(B2u)

3.3041

375.2

1.3163

2

S1(A00 ) S4(A00 )

0.5317 2.2094

2331.8 561.2

0.0001 0.5457

LUMO ! HOMO-1 LUMO + 1 ! HOMO LUMO ! HOMO-1 LUMO + 1 ! HOMO LUMO ! HOMO LUMO ! HOMO-2

3

S1(B2) S2(B2)

2.0268 3.0893

611.7 401.3

0.0000 0.6851

LUMO ! HOMO LUMO ! HOMO-1

p*(dpy1) ! p(dpy2) p*(dpy2) ! p(dpy2)

4

S1(A) S5(B)

0.7979 3.1107

1553.8 398.6

0.0002 0.7034

LUMO ! HOMO LUMO ! HOMO-4

p*(dpy) ! p(pyr) p*(dpy) ! p(dpy)

5a

S1

4.8914

253.4

0.0000

p(zn) ! p(pyr)

S5

5.2136

237.8

0.0012

LUMO ! HOMO-1 LUMO + 1 ! HOMO-1 LUMO ! HOMO LUMO + 1 ! HOMO LUMO ! HOMO-3 LUMO + 1 ! HOMO-3 LUMO ! HOMO-2 LUMO + 1 ! HOMO-2 LUMO + 2 ! HOMO LUMO + 2 ! HOMO-1

S1(B1) S2 (E)

5.0065 5.1525

247.6 240.6

0.0000 0.0072

LUMO ! HOMO LUMO ! HOMO-2

p(zn) ! p(pyr) p(zn) ! p(pyr)

5b a

From ref. [11]. From ref. [33]

b

p*(phy) ! p (phy) p*(trpr) ! p(pyr) p*(trpr) ! p(trpy)

p(zn) ! p(pyr)

500b

X. Wang et al. / Synthetic Metals 210 (2015) 258–267

265

Fig. 5. Electronic density contours of the frontier orbitals of these complexes in fluorescence spectra.

3.4. Fluorescence spectra Inspired by our work on the absorption character of the series complexes, we envision that if they are fluorescent. The calculated lowest-energy fluorescent emissions in toluene solution of 1-5 together with the measured values [11,33] are summarized in Table 3. The contours of the frontier orbitals of these complexes in fluorescence spectra are shown in Fig. 5. Fluorescence spectra of Zn porphyrin in toluene have a weak peak at 529 nm with the oscillator strengths of 0.0024 and an intense peak at 375 nm with the oscillator strengths of 1.3163. The bands both rise from a linear combination of these one-electron transitions from LUMO ! HOMO-1 and LUMO + 1 ! HOMO. The

emission of 1 is assigned to the ILCT character, and it is attributed to

p*(phy) ! p(phy) charge transfer. The lowest-energy states of 2–4

are assigned to LUMO ! HOMO, and mainly attributed to the LLCT characters. The S1 ! S0 transitions of 2 and 4 are 1554 and 2331 nm, and the electron transitions from trpy and dpy to pyr, respectively. They are not in the range of visible spectrum. The very weak S1 ! S0 emission of 3 is calculated to be at 612 nm. The emission of 1 and 3 in experimental measurements are 596 nm and 500 nm, respectively, our calculated results are comparable with the experimental case [11,33]. The lowest excitation energy of 5a and b is calculated to be 248 and 253 nm with the no oscillator strengths, and which assigned as the p ! p transitions from Zn to pyr. However the S4 ! S0 emission at 561 nm of 2,

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S2 ! S0 emission at 401 nm of 3, S5 ! S0 emission at 399 nm of 4 are very intense with the ILCT character feature as the S3 ! S0 emission of 1. The S2 ! S0 emission at 241 nm (f = 0.0072) of 5a and S5 ! S0 emission at 238 nm (f = 0.0012) of 5b are MLCT character, and they are also very weak. According to the Kasha's rule [47,48], the internal conversion process is fast, and the higher electronic excited states will relaxes to the lowest lying excited state before any possible relaxation to the ground state. According to the results we calculated, complexes 1-5 are not fluorescent. Interestingly, Zn porphyrin and Zn dipyrrin complexes used as fluorescein were synthesized and characterized long ago [3–5,38]. But the emission transitions of other tetra pyrrole Zn complex were rarely studied. To interpret if the complexes can fluoresce, the fluorescence lifetime of 1–5 are calculated. The fluorescence lifetime (t ) is computed through Einstein transition probabilities equation (Eq. (1)) [49,50] used as a quantitative index for comparing different excitation states.

t¼

c3 2  E2  f

ð1Þ

where c is the speed of light in vacuum, E is the transition energy, and f is the oscillator strength. All the parameters are all in atomic units (au). The E, f, calculated t values and experiment data for studied 1–5 are provided in Table 2. The values calculated by Einstein transition probabilities equation are in good agreement with the lifetime of the excited-singlet states from experimental measurement [11,28]. The lowest lying excited states of 1, 2 and 4 have fluorescence lifetimes of 1157, 65185, and 4525 ns, respectively. Because of the oscillator strength is zero for 3 and 5, we do not get t value of them. Usually, emissive states correspond to the t value less than 10 ns, whereas dark states with lifetimes larger than 10 ns. So, the lowest-lying excited states S1 of 1–5 are all dark states because of the long relaxation time for the excited electrons. Considering the intense allowed excited states of complexes 1–5, the fluorescence lifetime for the transitions are 2, 8, 3, 3, 17 and 14 ns, respectively. It can be seen that, the radiative lifetimes lie below 10 ns for 1–4. Thus, the S3 of 1–3 and S5 of 4 are bright states, they may relax to S1 in a radiative manner. While in 5a and b, according to the t values, all the relaxation processes are nonradiative. The dark states mean 5a and b are nonfluorescent. In complexes 1–4, the S1 are dark states, so the relaxations to S0 are nonradiative processes. However, when the excited electron relax from the higher electronic excited states(S3 for 1–3, S5 for 4) to the lowest lying excited state S1, the complexes 1–4 can fluoresce due to the short radiative lifetimes mentioned above. So the series of the tetra pyrrole Zn complexes we studied here are fluorescent except the four pyrrolyl Zn complexes 5a and b. 4. Conclusions The ground state structures of a series of tetra pyrrole zinc complexes are successfully obtained by theoretical calculations. The computed bond lengths, bond angles and torsion angles are in agreement with the experimental data. The p-conjugation effects in molecular skeleton are discussed in this study. When the degree of p-conjugation decreased from the 1 to 5, the molecular geometry changes from square-planar to distorted tetrahedron, and the breached conjugation induces less planarity and rigidity. For Zn porphyrin, HOMO + 1, HOMO, LUMO and LUMO-1 are well separated from the rest molecular orbitals, and they are the major contribution to the intense B-like band and weak Q-like band absorption. And this is in agreement with the Gouterman's four-orbital model. However, due to the interruption of the conjugated molecular skeleton, the four FMOs (HOMO + 1, HOMO, LUMO and LUMO-1) are not separated energetically from the other

MOs, the orbital with electron density located on metal Zn is approaching to LUMO, the excited states arise from transitions between more than frontier four orbitals, and the Gouterman’s four-orbital model is inapplicable. The intense B-like bands of 1–4 are around 372, 351, 395, 393 nm, respectively. The Q-like bands of 1 and 4 are very weak at 524 and 574 nm, but it is not weak for 2 at 535 nm. For 3, the lowest excited states S1 of 419 nm show weaker oscillator strengths, and the Q-like band is almost disappearing. The B-like bands shift to blue region for 2, but shift to red for 3 and 4, the Q-like bands shift to red for 2 and 4, but to blue for 3 compared with those of 1. Fluorescence spectra and fluorescence lifetime are calculated using TD-DFT and Einstein transition probabilities equation. The lowest-lying excited states S1 of 1–5 are all dark states, but the higher electronic excited states of 1–4 are bright states. According to the calculation results, complexes 1–4 can fluoresce at 375, 561, 401 and 399 nm, respectively, but in 5a and b, there are more dark states should be underwent, therefore, the absent luminescent at visible region was detected in experiment. Following work will be directed towards investigating more essential structure-property relationship of this family of the pyrrole based Zn complexes. 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 No. 21173096 and 21203071) and Young Scholar Training Program of Jilin University. 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.2015.10.012. References [1] H. Maeda, Bull. Chem. Soc. Jpn. 86 (2013) 1359–1399. [2] C.M. Carcel, J.K. Laha, R.S. Loewe, P. Thamyongkit, K.-H. Schweikart, V. Misra, D. F. Bocian, J.S. Lindsey, J. Org. Chem. 69 (2004) 6739–6750. [3] A. Yella, H.-W. Lee, H.N. Tsao, C. Yi, A.K. Chandiran, M.K. Nazeeruddin, E.W.-G. Diau, C.-Y. Yeh, S.M. Zakeeruddin, M. Grätzel, Science 334 (2011) 629–634. [4] W.M. Campbell, K.W. Jolley, P. Wagner, K. Wagner, P.J. Walsh, K.C. Gordon, L. Schmidt-Mende, M.K. Nazeeruddin, Q. Wang, M. Grätzel, J. Phys. Chem. C 111 (2007) 11760–11762. [5] T. Bessho, S.M. Zakeeruddin, C.Y. Yeh, E.W.G. Diau, M. Grätzel, Angew. Chem. Int. Ed. 49 (2010) 6646–6649. [6] M. Tanaka, S. Hayashi, S. Eu, T. Umeyama, Y. Matano, H. Imahori, Chem. Commun. (2007) 2069–2071. [7] C.-W. Lee, H.-P. Lu, C.-M. Lan, Y.-L. Huang, Y.-R. Liang, W.-N. Yen, Y.-C. Liu, Y.-S. Lin, E.W.-G. Diau, C.-Y. Yeh, Chem.–A Eur. J. 15 (2009) 1403–1412. [8] J.E. Rogers, K.A. Nguyen, D.C. Hufnagle, D.G. McLean, W. Su, K.M. Gossett, A.R. Burke, S.A. Vinogradov, R. Pachter, P.A. Fleitz, J. Phys. Chem. A 107 (2003) 11331–11339. [9] B.S. Kalnoor, P.B. Bisht, K.C. Jena, V. Velkannan, P. Bhyrappa, J. Phys. Chem. A 117 (2013) 8216–8221. [10] D. Fan, M. Taniguchi, Z. Yao, S. Dhanalekshmi, J.S. Lindsey, Tetrahedron 61 (2005) 10291–10302. [11] E. Hindin, C. Kirmaier, J.R. Diers, K. -y. Tomizaki, M. Taniguchi, J.S. Lindsey, D.F. Bocian, D. Holten, J. Phys. Chem. B 108 (2004) 8190–8200. [12] W.R. Scheidt, J.U. Mondal, C.W. Eigenbrot, A. Adler, L.J. Radonovich, J.L. Hoard, Inorg. Chem. 25 (1986) 795–799. [13] T. Hashimoto, Y.-K. Choe, H. Nakano, K. Hirao, J. Phys. Chem. A 103 (1999) 1894–1904. [14] A. Kerridge, Phys. Chem. Chem. Phys. 15 (2013) 2197–2209. [15] C.-K. Tai, W.-H. Chuang, B.-C. Wang, J. Lumin. 142 (2013) 8–16. [16] K. Nakai, R. Sahnoun, T. Kato, H. Kono, Y. Fujimura, J. Phys. Chem. B 109 (2005) 13921–13927. [17] B. Minaev, Y.-H. Wang, C.-K. Wang, Y. Luo, H. Ågren, Spectrochim. Acta Part A: Mol. Biomol. Spectrosc. 65 (2006) 308–323. [18] H. Nakatsuji, J. y. Hasegawa, M. Hada, J. Chem. Phys. 104 (1996) 2321–2329. [19] K.A. Nguyen, P.N. Day, R. Pachter, J. Chem. Phys. 110 (1999) 9135–9144.

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