Theoretical Investigations on Electronic Structures and Absorption Spectra of Un

CAO Hong-Yu HAO Jun-Yun SIDun-Hui TANG Qin ZHENG Xue-Fng HAO Ce



Theoretical Investigations on Electronic Structures and Absorption Spectra of Unsymmetrical Metal-coordinated Neo-confused Porphyrin in Various Solvents①

CAO Hong-Yua,bHAO Juan-YuanaSIDuan-HuiaTANG QianbZHENG Xue-Fangb②HAO Cea①

a(116024)b(116622)

Though the photo-physical properties of free base porphyrinare attractive, there are still problems for the materials with weak and narrow range absorption of visible light. The unsymmetrical neo-confused porphyrinderivatives were introduced as novel materials for the improvement of photo-chemical and photo-physical properties. The density function theory (DFT) and time dependent density function theory (TDDFT) were applied to calculate the absorption spectrum of unsymmetrical neo-confused porphyrin(N-CP) and metal-coordinated N-CP in various solutions. The Ni and Zn coordinated neo-confused porphyrin dipole moment values are smaller than the values of prototype porphyrin (ProP) and N-CP. According to the electrophilicity index, Ni coordinated N-CP (Ni-N-CP) is susceptible to the polarity of solvents, while the Zn coordinated derivative (Zn-N-CP) is more immune to the solvent environment. Unlike the Gouterman’sfour frontier orbital model of common porphyrin materials, the electron transitions of N-CPs and metal-coordinated N-CPs from H-2 or lower molecular orbitals also contribute toultraviolet and visible absorption. Most of oscillator strengthvalues of Zn-N-CP are significantly higher than the values of Ni-N-CP, which reflects the higher absorption intensity of Q band and Soret band. The maximum wavelength at 702. 2nm in vacuum drew our attention to the novel material. The broad absorption range, intense red-shifted Q band and higher stability in solvents suggest that N-CPs, especially Zn-N-CP, can be one class of new candidate dye-sensitized material.

metal-coordinated neo-confused porphyrin, asymmetry, C-N swap strategy, photo-physical property, density function theory;

1 INTRODUCTION

Inspired by the chlorophyll from natural plants and hemefrom the proteins[1,2]with significant roles in areas such as photosynthesis, transport/storage of oxygen[3]and electron transfer[4], porphyrinmaterials have been introduced to diverse areas (e.g., photo-sensitizers, optical materials and nanotechnology) and attractedthe attention of the researchers because of its good photo and thermal stabilitiesand a wide range of light absorption[5]. In solutions at room temperature, the strong transition Soret band in the range of 380nm-450nm and the weak intensity Q bands in the range 500-700nm[6]will be observed in different porphyrin materials.

Whether the photo-physical properties of free-based porphyrinareattractive, but regardless, there is still problem with weak and narrow range absorption of visible light. The reason of the main problem is that the high symmetry of the porphyrin ring will lead to degeneracy of energy level and transition prohibition. So Frutra et al. introduced the unsym-metrical N-confused porphyrin (NCP)derivativesma-terials for the improvement of photo-chemical and photo-physical properties[7]. Neo-confused por-phyrin(N-CP) were first reported in 2011 by Lash[8]. Neo-confused porphyrins are indeed very interesting compounds but there is not a huge amount of literature on these systems[9]. This fascinating new porphyrin isomer system was continually explored by many researchers for the anticipation of novel photo physical properties and more application in different fields[10].

The further step was put forward to design the new metal-porphyrins and metal-coordinated neo-con- fused porphyrinwhich probably possess attractive structural motifs[11]and properties for the increase of visible light absorption range and intensity. Metal element doping, surface modification, or organic sensitization approaches can be used to broaden the range of spectrum and improve the utilization efficiency of electron-hole of TiO2. The full solar spectrum absorption of zinc porphyrins will benefit it as electron donors with axial ligand which can be coupled with TiO2semiconductor as electron acceptors[12]. Coordinated with transition metal, organic dye materials can expand absorption wavelength range and enhance light absorption intensity, which increases the potential value of such molecules. Ni-coordinated complex was proved to react with a variety of electrophiles[13]and this fascinating chemical functionalization is utilized to prepare unique paramagnetic Ni(II) complex[14]and dimeric complexes[15,16].Zinc-coordinated porphyrin-sassuredly provide fresh idea for the applications of these materials indye-Sensitized solar cells with their special spectral and electrochemical properties[17]. The eminent fluorescence quantum yield and electron transfer properties of Zn(II) complex[18,19]arouses the interest of materialresearchers in catalysis, medicine,optical, solar energy conversion fields[11].It is expected that the combination of metal (e.g. Zn) coordinated porphyrin and photochromic organic materials could lead to an interesting light-controllable molecular sensor and high efficiency of photodynamic antimicrobial chemotherapy[20].

Designing porphyrin-based or neo-confused-por-phyrin-based materials for particular demands requires a rational modification of structural, electronic, or spectral parameters in various solvent conditions. These can be controlled, albeit to a limited degree, by appropriate substitution.So in this article, the density function theory (DFT)[21]and time dependent density function theory (TDDFT)[22]were applied to calculate the absorption spectrum of unsymmetrical neo-confused porphyrin(N-CP) and metal-coordinated N-CP in various solutions. The prototype porphyrin (ProP) with the same side chains of N-CP was designed for easily comparable statistics about C-N swap strategy.The energy level, chemical properties, and absorption spectra of Ni and Zn coordinated N-CP were predicted with the above approaches.

2 METHODS

The density function theory (DFT) and time dependent density function theory (TDDFT)[22,23]in the Gaussian 09 software package[24]were applied in all the theoretical calculations. The density functional (DFT) method with the Becke-Lee-Yang-Parr composite of exchange-correlation functional (B3LYP)[21]as implemented in the Gaussian 09 program was used in our research. The energy and vibrational frequency calculation of prototype por-phyrin (ProP) and three N-CP derivatives were performed at B3LYP/6-31+G(d,p) level in all cases. UV-Vis absorption spectra of these molecules were researched by TDDFT at B3LYP/6-31+G(d,p) level (N=120 states) in all cases. Vibrational frequency calculations were carried out to characterize the optimized structures. To investigate the influence of the different polar solvents, certain computations were carried out using IEF-polarizable continuum model(IEF-PCM). The molecular absorption spec-trum were generate with FWHM (the full width at half-maximum of the gaussian curves) value of 800 cm-1with Gausssum3.0[25].

Based on the finite difference approximation and Koopman’s theorem[28]working formulae,andcorrespond to ionization potential and electron affinity. The values ofandare approximate to minus energies of HOMO and LUMO (Eand E)[29], respectively. So the global hardness, chemical potentialand electrophilicity indexare derived from the following formula[27][30]:

3 RESULTS AND DISCUSSION

3.1 Geometrical structures

The selected molecular structural information of ProP, N–CP, Zn–N–CP and Ni–N–CP obtained in vacuum and chloroform solvent for ground states geometries are listed in Table 1. All theoretic results show that the main porphyrinoid cores of ProP, N–CP and Ni–N–CP keep planar and conjugate structures(Fig. S1). In vacuum, the maximum length deviation of 0.059Å(Ni–N(2)) and the maximum bond distortion of 0.3º(N(2)–Ni–N(3)) demonstrate that the theoretical data are consistent with the experimental data[31]. Since the calculated bond lengths of N–CP in vacuum rarely shifted the x-ray bond data, the structures are optimized with the same theory method and basic group in chloroform to elucidate the solvent effect on the geometries. In the chloroform solvent, the bond lengths and plane angles of Ni–N–CP porphyrinoidmoietykeep the same parameters with its x-ray crystal structure parameters[31]. The bond length deviation of 0.061Å(Ni–N(2)) and the bond angle distortion of 0.68º (C(1)–Ni–N(4))manifest that the calculation parameters of N–CP in chloroform solvent are still consistent with the experiment data.

The invert pyrrole ring highly changes the structure parameters of the porphyrinoids. Due to repulsion of the two inner hydrogen atoms in theProP ring,the diagonal line N(1)–N(3)distance with the value of 4.234 Å is longer than the N(2)–N(4) distance with the value of 4.082 Å in chloroform solvent. In N–CP, the diagonal line C(1)–N(3) distance with the value of 4.153 Å is similar with the N(2)–N(4) distance with the value of 4.131 Å, indicating the C–N swap reduce the repulsion effects of two inner hydrogen atomin the porphyrin derivative. The shorter C(1)–N(3) distance and N(2)–N(4) distance reveal the readjustment of the molecule structure affected by the repulsive force. In the Ni–N–CP derivative,the calculationand experiment resultsshow that both diagonals are shorter than 4 Å,proving the more compact structure in the metal coordinated neo–confused porphrin. The analytical data reveal that the metal–coordinated N–CP structure shrinks, especially the Ni–coordinated N–CP. A few larger structure of Zn–coordinated N–CP is noticed from the calculated results. The ion radius of Ni2+(0.69 Å) and Zn2+(0.74 Å) are almost the same lengths [32], but the bond lengths of C(1)–N(3) and N(2)–N(4) are enlarged from 3.920 (Ni–N–CP) to 4.035(Zn–N–CP), and from 3.937 (Ni–N–CP) to 4.201 (Zn–N–CP) in chloroform, respectively. The length difference of diagonals C(1)–N(3) and N(2)–N(4) of Zn–N–CP is obvious larger than corres-ponding length difference of Ni–N–CP. In this sense, the symmetry of new confused porphyrinis influenced by metal ion Zn2+and C–N swap strategy.

Table 1. Selected Bond Lengths and Angles of Ni-N-CP from Experiment Results and DFT B3LYP/6-31(d,p) CalculationData

Table 1. Selected Bond Lengths and Angles of Ni-N-CP from Experiment Results and DFT B3LYP/6-31(d,p) CalculationData

Ni-N-CPProPN-CPNi-N-CPZn-N-CP Cal/vacCal/CHCl3ExpCal/vacCal/CHCl3Cal/vacCal/CHCl3Cal/vacCal/CHCl3Cal/vacCal/CHCl3 Ni-C(1)1.9301.9191.915C(1)-H--1.0701.070---- Ni-N(2)1.9901.9921.931N(1)-H1.0161.016------ Ni-N(3)1.9902.0071.969N(3)-H1.0141.0151.0141.014---- Ni-N(4)1.9471.9481.968C(1)-N(3)--4.1534.1563.9203.9204.0254.035 C(1)-Ni-N(2)89.7289.9890.10N(1)-N(3)4.2344.238----- C(1)-Ni-N(4)89.9590.2189.53N(2)-N(4)4.0824.0804.1314.1283.9373.9374.1964.201 N(2)-Ni-N(3)90.1589.8590.21H-H2.2052.2082.0822.086---- N(2)-Ni-N(4)179.8179.2179.5 N(3)-Ni-N(4)90.1889.9790.16 C(1)-Ni-N(3)179.6179.0179.3

3.2 Dipolemoment

The porphyrin derivatives are susceptible to external environment from polar or neutral molecules of different solvents[33]. So the dipole moment is an important property of a molecule for investigating the intermolecular interactions.The dipole mo-ment(DM) values of neo-confused porphyrinoid structures in this research are much more than the DM values of porphyrin derivatives without the –COOCH3side chain (more than 1.5 Debye)[33].The dipole moment value of ProP is 5.7283Debye, 1.1 Debye more than the value of C–N swapped neo-confused porphyrin. The C–N swap strategy remarkably reduced the dipole moments of por-phyrinoid derivatives. The decrease of N–CP dipole moment value is the result of the reduction distance between N atom and carboxylmoiety. The Ni and Zncoordinated neo-confused porphyrin dipole moment values are smaller than the value of N–CP. The dipole moment value with 8.3999 Debye in the high polar solvent water, 2.7 Debye more than that in vacuum, indicates that ProP is particularly vulnerable in various polarity solvents. The difference of the DM value of N–CP in vacuum and water reduces to only 1.9 Debye. The Ni coordinated neo–confused porphyrin derivatives further decreases the dipole moment value with 4.3724 Debye due to the good electron delocalization of the new conjugated system with orbitals of Ni. The DM value of Zn coordinated N–CP derivative is little higher than the value of Ni–N–CP in the vacuum and different solvents. These results reveal that the metal coordinated N–CPs are stable materials in different polarity solvents, while the metal free porphyrin is more sensitive to the solvent polarities. According to the dipole moment data, the solvent polarity can vary the molecular configuration and bring about new photo physical and chemical properties of these materials in different solvents.

Table 2. Dipole Moment in the Ground State (Field-independent Basis, Debye)

3.3 Frontier orbitals and energy levels

Though the carboxyl group changes the symmetry of ProP, the frontier molecular orbitals symmetrically locate on the core macrocyclic structure. Regardless the side chain, ProP has C2vsymmetry. The symmetry of frontier molecular orbitals of N–CP is reduced by the C–N swapped pyrrole. The metal has less effect on the symmetry of porphyrin derivatives, but it has more influence on orbital electron distribution and frontier molecular orbital energies. In Ni–N–CP the HOMO and H–1 mainly locate on the metal center, while the LUMO and L+1 mainly locate on the metal and the coordinate N or C atoms, indicating that Ni is an active and complicate center of the new molecule (Fig. 1). The unsymmetrical Ni divalent ion with 3d8electron distributions and one empty orbital may be the contribution to the above results. The HOMO and H–1 place centrally on the Znand four coordinated N or C atoms, while the LUMO and L+1distributions are similar to that of N–CP (Fig. 1). The symmetrical Zn divalent ion with 3d10electron distributions and its weak ligand field lead to the changes of Zn–N–CP. These results indicate that the Zn divalent ion not only exerts influences on the HOMO and H–1with its electrons, but also delocalizes electron configuration with its metal effect to improve the electron transition and absorption spectra.

Fig. 1. Four Frontier molecular orbitals of ProP, N–CP, Ni–N–CP and Zn–N–CP

Due to the high symmetry of ProP, the degenera-cies of energy levels are easily observed in the molecule, such as the degeneracy of HOMO and H-1. The less difference between LUMO and L+1 and the larger difference between L+1 and L+2 energy levels showed that the electron mainly transit from H-1and HOMO to LUMO and L+1. The Prop obviously conforms to Gouterman four orbitals theory[34]. These properties indicated that ProP has the relatively less absorption bands and is sensitive to solvents, resulting in the defects in application in optical material fields.

The degeneracy of energy levels of C-N swapped N-CP are rarely found because the symmetry reduces obviously. In the new molecule with C1vsymmetry of the core structure, theenergy levels of H-2-H-4 increases and energy level of L+3 decreases (Fig. 2). These variations offered the conclusion that electrons transit not only between the Gouterman four orbitals, but also transit from H-2 or lower orbitals to occupied orbitals (Fig. S2). So the electron transition probability obviously increases in the molecule, resulting in the enlargement of visible light absorption range.

Fig. 2. Energy level diagram from H-10 to L+10 ofporphyrin and neo-confusedprophyrins invacuum and solvents Vac: vacuum; Chl: chloroform; Wat: water

Coordinatedwith Ni, the new derivative energy levels of HOMO and LUMO almost keep the same with N-CP, but the upper unoccupied orbitals L+2 and L+3 of Ni-N-CP vary and the sub-HOMO orbitals (from H-2 to H-5) energy levels increase obviously(Fig. 2). Thus the electrons of sub-HOMO orbitals makemore contributions to electron transition in the metal-derivatives. The sub-HOMO orbitals (from H-2 to H-6) energy levels of Zn-N-CP increases significantly, indicating that electrons are more easily excited to unoccupied orbitals and makemore contributions to transition.

3.4 Chemical reactivity

The highest occupied molecular orbital (HOMO) energy value(E) and the lowest unoccupied molecular orbital (LUMO) energy value(E)are obtained to calculate the chemical reactivity para-meters. Global hardnessand chemical potentialare calculated for the comprehending the characters of neo confused porphyrin derivatives. The electro-philicity indexdefines the electrophilic power of molecular systems in terms of a balance between opposite effects.

Because the four porphyrin derivatives are approximate to neutral molecules, the values of chemical potentialof each molecules are almost identical before and after the replacement of C and N, while the global hardnessand the electrophilicityω value have variations (Table 3).The implication of higher electrophilicity indexvalue is the increase of reaction activity and the decrease of chemical stability[29]. The electrophilicity indexof ProP has the lower value of 2.92 in vacuum, while the N-CP has the highervalue of 3.07. The highvalue of N-CP might result from the active inner hydrogen of the macro ring. The exchange of carbon and nitrogen reformed the inner charge environment with one hydrogen connected to the inner carbon atom. The longer bond length of C-H (Table 1) decreased the stability of porphyrin molecule with the slight increment ofvalue in vacuum. According to the electrophilicity index, Ni coordinated neo-confused porphyrin is susceptible to the polarity of solvents, while the Zn coordinated derivative is more immune to the solvent environment.

Table 3. Global Hardness η, Chemical Potential μ and Electrophilicityω of for PorphyrinDerivatives

3.5 Electronic spectra

According to the experiment data, two Soret band peaks (385nm and 428nm) and two Q band peaks (534nm and 657nm) are found from absorption spectrum of Ni–N–CP in chloroform solvent[31]. Based on our TDDFT calculation results,two Soret band peaks (352nm and 423nm) and two Q band peaks (520nm and 668nm)are obtained from absorption spectrum of Ni–N–CP in chloroform solvent. The absorption peak numbers and locations are kept highly consistent with experimental data[31], proving that the theory methods and parameters applied in our calculation are stable and suitable.

From Fig.3, the narrowest absorption bands can be found among these four molecules. In the nonpolar condition, the main peaks of ProPlocateon the high intensity Soret band absorption around 400nm and two weak Q band absorptions below 600nm. Even in the polar solvent, the weak Q band peaks ofProP blueshiftwith a little increase of the intensity. The absorption spectrum of ProP conforms to Goutermantheory[34]with the main transition contributions from H–1 to L+1 in the Q band. Even in Soret band, the transition contributions only extend to H–3 orbital (Table S1). Thesmallamountof ProPpeaksin calculation spectrum might be the consequence of the high symmetry and high energy level degeneracy degree, resulting in the low applications in photo absorption materials.

The absorption spectrum of the new derivative N–CP with one C–N swapped pyrrole ring varies obviously. The Soret band splits to more peaks and enlarged the absorption range, while Q band extends to over600nm with more peaks and higher intensities. In addition, N–CP is more sensitive in the polar solvents than ProP. The Q band peaks of 640.9nm redshift to 660.1nm, while the value of oscillator strength increases nearly 4 times, from 0.0687 to 0.2542 (Table S1). The absorption spectrum is no longer comply with the four orbitals theory[34]. Besides the four frontier orbitals, electron transitions from H–3and H–2 also contribute to the Q band absorption, while electron transitions from H–4 contributes to the Soret band absorption (Table S2). The high absorption intensity and oscillator strength verify that C–N swap strategy improves the photo physical properties of porphyrin[33].

From the calculation spectrum of Ni–N–CP, we can get an intuitive conclusion that this new molecule improves significantly the ultra and visible light absorption property. More Q band and Soret band absorption peaks are found in the Ni coordinated derivative. Two main peaks and three other weak oscillator strength peaks locate in the region of 500nm–600nm. The Ni in the ring complicates the transition contributions of neo–confused porphyrin and reflects the advantage of metal–ligand compound. The transition contribution obviously increases the range from H–7 and H–8 to L+3 in Q band, which enriches the absorption spectrum. The Soret band of Ni–N–CP splits to several strong absorption peaks and the intensities increase with the polarity of solvents. Similarly, Ni–N–CP is also sensitive in different polar solvents.Both the Q bands and Soret bandsabsorption intensities rise obviously and the peak location redshifts along with increasing the polarity of the solvents. The 587nm absorption peak in vacuum red shifts to 669.8nm in polar water, while thevalue of oscillator strengthincreases from 0.0432(vacuum)to 0.1785(water) (Table S3). The above data indicate that the polarity of solvent can be used for the regulation of photo physics properties of asymmetry metal–coordinated porphyrin materials.

Fig. 3. Theory electronic absorption spectrum of ProP, N–CP, Ni–N–CP and Zn–N–CP in five solvents

The widest absorption rangeis found in the theory UV–Vis spectrum of Zn–N–CP.The maximum wavelengthat 702.2nm in vacuumdrawsour attention to the new material for further studies because only Zn–N–CP possess these characters among our researched materials. Compared with Ni, the influence of metal Zn to the neo–confused porphyrin is less complicated, but more significantly. Most of oscillator strengthvalues of Zn–N–CP are signi-ficantly higher than the values of Ni–N–CP, which reflect the higher absorption intensity of Q band and Soret band.With the polarity of solvents, the maximum wavelength of Q band red shifts from 702.0nm in vacuum to 710.8 in water(Table S4).The photo physical advantage of Zn–coordinated N–CP is obviously in our theory research.

The electron distribution of Zn is 3d104s2, while the electron distribution of itsdivalent cation is 3d10. The five d orbitals are full of electrons before Zn2+coordinated to the three nitrogen atoms and one carbon atom of N–CP. So the Zn2+can stabilize the neo–confused porphyrin and provide electrons for the whole conjugated molecule. These functions and characterizations benefit the electron transition and significantly improve the visible light absorption range and intensity.The zinc–coordinated N–CP amplifies the asymmetry character from C–N swapped strategy and the wide absorption range merit of nickel–coordinated N–CP. Meanwhile, the ultra–visible absorption property of Zn–N–CP can be regulated by the variation of solvent polarity.

4 CONCLUSION

The relatively accurate ground state structures and absorption spectrum of N–CP and Ni–N–CP are reproduced with DFT and TDDFT approaches. Applied with the same methods and parameters, the structural and photophysical properties of ProP and Zn–N–CP are predicted in this research. The analytical data reveal that the C–N swap strategy remarkably changes the dipole moments, reactivity and electron transition of porphyrinoid deriva-tives.Using the C–N swap procedure, it is possible to vary the energy barriers and to perturb the symmetry of the porphyrin ring.Ni and Zn further influence the frontier molecular orbital and absorption spectrum. With the 3d10electron distribution, the divalent cation Zn2+can stabilize the neo–confused porphyrin, provide electrons for the whole conjugated molecule, and improve the visible light absorption property. All these properties make Zn–N–CP extremely attractive candidate for using in dye,photo sensitization, solar energy conversionand photo dynamic therapy fields.

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27 November 2017;

2 April 2018

① This project was supported by the National Natural Science Foundation of China (Nos. 21601025, 21677029, 21571025) and Dalian Young Science and Technology Star Project (2017-61)

Tel./fax: +86 411 84986335. E-mails: haoce@dlut.edu.cn and dlxfzheng@126.com

10.14102/j.cnki.0254-5861.2011-1901