间位取代卟啉及其锌卟啉的电子吸收光谱

[Article]
物理化学学报(Wuli Huaxue Xuebao )
Acta Phys.⁃Chim.Sin .2012,28(5),1085-1093
May Received:December 12,2011;Revised:February 29,2012;Published on Web:March 2,2012.∗
Corresponding author.Email:qibinchen@;Tel/Fax:+86-21-64252767.
The project was supported by the National Natural Science Foundation of China (20806025,21103047)and National 111Project of China ʹs Higher Education (B08021).
国家自然科学基金(20806025,21103047)和中国高校111计划(B08021)资助项目
ⒸEditorial office of Acta Physico ⁃Chimica Sinica
doi:10.3866/PKU.WHXB201203024
间位取代卟啉及其锌卟啉的电子吸收光谱
曹振锋
陈启斌*
卢运祥
刘洪来
胡
英
(华东理工大学,结构可控先进功能材料及其制备教育部重点实验室,上海200237)
摘要:
取代的卟啉类衍生物在气敏传感器方面具有广泛的应用前景.本文采用了密度泛函理论(DFT)和含时
密度泛函理论(TD-DFT)研究了四种不同取代基的卟啉衍生物(meso 位四硝基苯基/四氨基苯基卟啉(NO 2PP,NH 2PP)及其相应的锌金属卟啉衍生物(NO 2ZnPP,NH 2ZnPP))的紫外和近紫外光谱特征.利用两种不同的交换相关泛函(广义梯度近似泛函(PBE)和杂化密度泛函(B3LYP))优化了上述四种物质的结构,并应用TD-DFT 计算了相应的电子激发能量和振动强度.结果表明,取代卟啉的吸收光谱与大量的电子跃迁有关;与B3LYP 泛函预测的光谱相比,PBE 泛函所得B 带以及Q 带的波长位置与实验值更为接近.另外,计算所得硝基取代基卟啉的B 带相对于氨基取代基卟啉的B 带发生了红移,这与实验现象也保持一致.由于卟啉衍生物的三重激发态在电子转移中有很重要的应用,因此在PBE/6-31G(d )水平上计算了四种物质的最低三重激发态能量,分别为1.426、1.469、1.608和1.581eV.关键词:
含时密度泛函理论;电子吸收光谱;电子跃迁;红移;三重激发态
中图分类号:
O641
Electronic Absorption Spectra of Meso -Substituted
Porphyrins and Their Zinc Derivatives
CAO Zhen-Feng
CHEN Qi-Bin *
LU Yun-Xiang
LIU Hong-Lai
HU Ying
(Key Laboratory for Advanced Material and Development of Chemistry,East China University of Science and Technology,
Shanghai 200237,P .R.China )Abstract:Meso -substituted porphyrin derivatives have demonstrated great potential as sensing materials for toxic gas detection.In this paper,density functional theory (DFT)and its time-dependent DFT approach (TD-DFT)were employed to investigate the ultraviolet-visible (UV-Vis)or the near-ultraviolet-visible (near-UV-Vis)absorption spectra of meso -tetra (o -nitrophenyl/o -aminophenyl)porphyrins (NO 2PP,NH 2PP)and their corresponding zinc derivatives,NO 2ZnPP and NH 2ZnPP.The geometry optimizations for these four molecules were obtained from two different exchange-correlation functionals,the generalized-gradient approximation functional PBE (Perdew-Burke-Ernzerhof)and the hybrid functional B3LYP (Becke,three-parameter,Lee-Yang-Parr).The excitation energies and oscillation strengths were obtained from TD-DFT calculations.Calculations show that the optical absorptions are associated with numerous electronic transitions.In addition,the PBE-predicted wavelengths of the B and Q bands are more consistent with experiment than those predicted by B3LYP.The B band of NO 2-substituted derivative exhibits a bathochromic shift different from that of NH 2-containing material,also consistent with experimental results.In addition,at the PBE/6-31G(d )level of theory,the calculated energies of the lowest triplet excited states of NO 2PP,NH 2PP,NO 2ZnPP,and NH 2ZnPP are 1.426,1.469,1.608,and 1.581eV,respectively.
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Key Words:TD-DFT;Electronic absorption spectrum;Electronic transition;Bathochromic shift;
Triplet state
1Introduction
Porphyrins and related metalloporphyrins have been the sub-jects of numerous experimental and theoretical studies for de-cades.1-12Due to their remarkable photochemical,electrochemi-cal,and biochemical properties,13-15there has been renewed in-terests in the use of these compounds in some fields including catalysts,optical machinery,solar cells,and non-linear optics. Exploiting their interesting properties,porphyrin films have been scrutinized for possible applications as sensitive,selec-tive,and stable active layers of transducers in gas sensor,act-ing as an electron donating or accepting unit during the process of electron transfer when interacting with the analytes,which could lead to pronounced variation of the B-bands and Q-bands in the absorption spectra,as supported by many spectroscopic experiments.16,17Chen et al.18have recently reported a new chlo-rophyll f(C55H70O6N4Mg)by replacing the methyl group at C2 position with formyl group and discovered that oxygenic photo-synthesis can be extended further into the infrared region, which clearly verifies the importance of the substituent.This implies that besides the analyte,the different substitutional(the electron donating and/or withdrawing)groups of the porphyrin derivatives likely have a significant effect on their electron transfer and transition.On the other hand,compared with free-base porphyrins,metalloporphyrins play much more important roles in biological systems such as oxygen storage and transfer, metabolism,photosynthesis and chemical sensors,and thus have more artificial applications.19Additionally,Richardson et al.20have synthesized different metalloporphyrins to detect var-ious kinds of toxic gas by in situ UV-Vis measurements.Be-cause of all those important applications,understanding the de-tailed knowledge of the structure,electronic and molecular ex-citations as well as orientation of adequate transition moments is of urgent necessity to interpret energy or electron transfer processes.In particular,considering the crucial role of the sub-stituent and the complex metal atom in the properties of por-phyrin derivatives described herein,it is indispensable to sys-tematically study these effects on the electronic properties and the optical absorption of these molecules.
In order to explore the effect of the substituents and the cen-tral metal atom on the absorption spectra,meso-tetra(o-amino-phenyl)porphyrin(NH2PP),meso-tetra(o-nitrophenyl)porphy-rin(NO2PP),and the corresponding zinc derivatives(NH2ZnPP, NO2ZnPP)were synthesized in our laboratory.21Here,―NH2and ―NO2are the most common and simple electron donating and withdrawing groups,respectively,which could clearly influ-ence the electron distribution of the porphyrin ring,leading to the variation of the absorption spectra.To our knowledge,up to the present,these four porphyrin derivatives have not been well investigated.As a whole,their experimental absorption spectra seem to be similar to those of porphine(H2P)and zinc porphine(ZnP);22,23namely,free-base porphyrins have a strong B-band in the near-UV region and four weak Q-bands in the visible region,while metalloporphyrins have a B-band and two Q-bands due to their higher symmetry.The four Q peaks are la-beled as Q0x,Q1x,Q0y,Q1y,where Q0x and Q0y peaks are generally in-terpreted as pure electronic transition,24,25while Q0and Q1 peaks always present in the metalloporphyrins(Q1is vibronic). Generally,the absorption spectra of porphine and metal-por-phine are qualitatively interpreted on the basis of Goutermanʹs four-orbital model,24-26in which the spectra mostly depend on the electron transition between two highest occupied orbitals and two lowest unoccupied orbitals.However,with the addi-tion of various substituents and the decrease of the symmetry of molecules,strong mixing of configurations should occur and thus more excited states are responsible for the absorption spectra.As a result,the detailed absorption spectra of the por-phyrin derivatives should differ from one another,which need to be analyzed carefully.
During the past decades,some theoretical investigations have appeared to study the excitation spectra of porphyrin de-rivatives by means of different level of theories,for examples, the symmetry-adapted-cluster configuration-interaction(SAC-CI) calculations on excited states of chlorophyll and pheophytin,4 the multiconfiguration second-order perturbation theory (CASPT2)on the excitation energy of N2(NH)2.27However,the calculated spectrum results were not in well consistency with the experimental observations.Recently,the emergence of the DFT and TD-DFT has provided new impetus in the field of computational chemistry.21,28Taking electron correlation into ac-count in an implicit and expedient manner,DFT and TD-DFT calculations on the molecular structure and excited state of por-phyrins have proved to be useful and suitable.22,29In this work, with the purpose of studying the effects of the substitute group and zinc atom on the geometries and the excited electronic tran-sition of porphyrins,we used DFT and TD-DFT with two dif-ferent functionals(PBE and B3LYP)to optimize the geome-tries of the four porphyrin derivatives and calculate the elec-tronic transitions which are relevant to absorption spectra, moreover,comparing with the experimental results.The ener-gies of the triplet excited state with respect to the ground states were also evaluated owing to its relevance to the molecular electronics.
2Theoretical and experimental procedures The synthesis of NO2PP/NH2PP and the corresponding metal-loporphyrins NO2ZnPP/NH2ZnPP was carried out according to our previous work21and the general procedure.30UV-Vis spec-tra were recorded on a Shimadzu UV-2450spectrometer
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(Japan).The samples were firstly dissolved in dichlorometh-
ane(Chemical Reagent)and a quartz cuvette of1.0cm path
length containing dichloromethane was used as reference when
measuring UV-Vis spectra of dichloromethane solution.
The molecular structures of four porphyrin derivatives were
firstly optimized via two different DFT functionals:the gener-
alized-gradient approximation(GGA)exchange-correlation
functionals PBE31-33and the hybrid functional B3LYP.34,35The
former is a reformulation of the first-principles GGA function-
al PW91(more simple but improved over PW91),whereas the
latter involves the semiempirical parameters used in DFT cal-
culations.22,36,37Since these two functionals have been success-
fully used in the TD-DFT calculations of absorption spectra of
porphine and metal porphine,22,28,29,36,37they were also applied in this work to calculate the excitation transition energies and os-cillator strengths based on the optimized structures.The first step of the TD-DFT calculation employed a self-consistent ground state Kohn-Sham(KS)computation.The second step consisted of solving the central equation of the TD-DFT re-sponse theory by using the adiabatic local density approxima-tion(ALDA)for the functional derivatives of the exchange-cor-relation potential.38
The split-valence basis set6-31G(d)was used throughout the calculations.For each molecule,a structure optimization was performed firstly,followed by a frequency calculation to confirm that the optimized structure was indeed a minimum (with no imaginary frequencies).The TD-DFT was then em-ployed to evaluate the excitation energy to predict the absorp-tion spectra.All the calculations in this work were performed using the Gaussian03package.39
3Results and discussion
3.1Optimized ground-state geometries
The structural formula and optimized structures with PBE functional used for four porphyrin derivatives are displayed in Fig.1and Fig.2.For simplicity,we employ numbers1,2,3, and4to represent four derivatives NO2PP,NH2PP,NO2ZnPP, and NH2ZnPP,respectively.Some selected structural parame-ters of these derivatives predicted with two different function-als are listed in Tables1and2.For comparison,the experimen-tal parameters for H2TPP and ZnTPP are also listed.40-42Accord-ing to the present calculations,H2P and ZnP show D2h and D4h symmetries,respectively.However,with the addition of the NO2and NH2substituents,the symmetry of the studied deriva-tives reduces to C2v symmetry.It is known that different me-so-substituents and their orientations would produce different symmetries for porphyrin skeleton.Moreover,it is found that the D2v symmetries is the most stable geometry for these four derivatives.
As can be seen from the data in Tables1and2,the car-bon-carbon bond lengths of the derivatives under study are lo-cated between those of the carbon-carbon single and double bonds,indicating that the conjugation exists in porphyrins rings,as well as between porphyrins ring and phenyl group. Generally speaking,the length of carbon-carbon single bond is 0.154nm and the length of the carbon-carbon double bond is 0.133nm.Thus,the porphyrin molecules are in the conjuga-tion condition because the carbon-carbon length is between 0.133and0.154nm according to our calculation results.Previ-ous studies on the meso-phenylporphyrins have revealed that the meso-phenyl substitution and the central metal would cause more or less the out-of-plane distortion.42,43The NCαC m Cαdihe-dral angle is often used to evaluate the extent of out-of-porphy-rin distortion.From Tables1and2,it is obvious that the NCαC m Cαangles(such as N19C24C23C3,N4C3C23C24)are very small(<3°),indicating that the meso-phenyl substitutions and zinc metal affect the out-of-distortion to a relatively less de-gree because of the suited size of zinc ion within the porphyrin core.For the in-plane-distortion,the largest distortion usually occurs around the C m atoms,which can be shown clearly by the alteration of the C m―Cαbond lengths and by comparing the distances of two opposite C m atoms of the derivatives with those of ZnP and H2P.The C m―Cαbond length and C m―C m in-ter-atomic distance tend to decrease,especially in zinc deriva-tives.For example,the C m―C m distance in1/PBE is shown to be0.694nm,whereas in3/PBE the distance shortens to0.692 nm.Moreover,it can be found that the geometry of the por-phine ring remains almost unchanged with the addition of NO2 and NH2groups.On the different behavior of the two function-als,the B3LYP functional yields slightly smaller values com-pared with those by PBE.The most remarkable difference of the two different functionals can be manifested by the C2C1C6C5 dihedral angle:B3LYP predicts much larger values compared with that by PBE,especially in NH2-substituted porphyrins. 3.2Electronic structures
As mentioned above,due to the reduction of the symmetry of the porphyrins by adding meso-phenyl substitutents,the Gautermanʹs four-orbital theory is not suitable for calculating the electronic transition and predicting the absorption spectra. More frontier orbitals are involved and more mixing of config-urations should be taken into consideration.The energies of some frontier MOs of four porphyrin derivatives are shown in Figs.3and4.The highest occupied molecular orbital
(HOMO) 1Structures and nomenclature of the four
porphyrin derivatives
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energy is used as a reference here,that is,the HOMO energy is set to zero.
From Figs.3and 4,it can be seen that for 1,2,3and 4,PBE functional yields the HOMO-LUMO (the lowest unoccupied molecular orbital)energy gaps to be 1.618,1.694,1.673,and 1.736eV ,respectively,while larger energy gaps (2.705,2.767,2.837,and 2.899eV)are obtained with the hybrid functional B3LYP.Furthermore,it is found that the orbital energies be-tween LUMO and LUMO+5of 1/PBE are almost identical and the maximum differences are smaller than 0.25eV ,whereas the energy gap between HOMO and HOMO -3appears larger,especially from HOMO -1to HOMO -2.However,opposite trends are observed for 2/PBE:the orbital energies of LUMO+2and LUMO+3are much larger than those of LUMO and LUMO+1,while the orbital energy values from HOMO -1to HOMO -5are essentially degenerate.These tendencies are al-so shown in metalloporphyrins (3/PBE and 4/PBE).Generally,with the addition of substituents (NO 2,NH 2)and metal zinc,the geometries of the considered derivatives remain almost un-changed (vide supra ),while the electronic structures are shown to be distinctly different as shown in Fig.5.This can be as-cribed to the fact that NO 2is an electron-withdrawing group
Table 1Some calculated bond lengths and bond angles in free-base porphyrins 1,2using two different functionals B3LYP and PBE
L is bond length,A is bond angle,and D is dihedral angle.a
Data are taken from X-ray crystallography.40,41
Parameter L (N —H)/nm L (N —C α)/nm L (C α—C β)/nm L (C β—C β)/nm L (C m —C α)/nm L (C β—H)/nm L (C m —C m )/nm L (C1—C6)/nm A (H —N —C α)/(°)A (C α—N —C α)/(°)A (N —C α—C β)/(°)A (C α—C β—C β)/(°)A (C α—C m —C α)/(°)D (N 4C 3C 23C 24)/(°)D (N 19C 24C 23C 3)/(°)D (N 8C 7C 6C 5)/(°)D (N 4C 5C 6C 7)/(°)D (C 2C 1C 6C 5)(°)
106.3-108.1
1/B3LYP 0.1020.1370.143-0.1460.135-0.1370.140-0.141
0.1080.6910.150124.7105.4-110.6106.6-110.0125.9-1.5240.018-0.0181.52478.64
1/PBE 0.1030.137-0.1380.144-0.1460.136-0.1380.141-0.142
0.1090.6940.150124.7104.8-110.5106.8-111.5126.0-2.0590.476-0.4762.05976.07
106.1-108.02/B3LYP 0.1020.137-0.1380.144-0.1460.135-0.137
0.1410.1080.6940.150124.6105.2-110.7106.4-111.1106.3-108.2125.1-1.5221.281-1.2811.52279.70
2/PBE 0.1030.1380.144-0.1460.136-0.1380.141-0.142
0.1090.6960.150124.6104.7-110.6106.6-111.5125.1-2.3541.956-1.9562.35474.24
106.2-108.1
H 2TPP a 0.0930.1370.1430.1350.140
125.2
125.6
Fig.2Optimized structures of the four porphyrin derivatives with PBE functional
(a)1/PBE,(b)2/PBE,(c)3/PBE,(d)4/PBE.1,2,3,4represent NO 2PP,NH 2PP,NO 2ZnPP,and NH 2ZnPP,respectively.The letter after “/”refers to
the generalized-gradient approximation functional PBE.
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and NH2is an electron-donating group.Apparently,the elec-tron is well delocalized around the porphyrin ring of these de-rivatives and their HOMO shapes are relatively similar to each other.However,significant discrepancies are observed for the LUMOs:the central electrons of NO2-substitued derivatives are prone to move to NO2group,while in NH2-substituted de-rivatives these electrons are rearranged regularly around the porphyrin ring.In addition,due to the full shell of d orbital in zinc metal,there are also some differences in the distribution of the electron around the porphyrin ring of zinc porphyrins in comparison to free-base porphyrins.In summary,these dispari-ties should affect the electronic transition and absorption spec-tra of these molecules to a large degree,as shown in this work.
3.3UV-Vis absorption spectra
The experimental absorption spectra of four porphyrin deriv-atives were measured in dichloromethane solution.
3.3.1NO2PP(1)and NH2PP(2)
According to the above discussions,new orbital interactions are expected to occur for porphyrin derivatives when the ortho positions are substituted by NO2and/or NH2group.As a conse-quence,the electronic structures and the Q/B-bands in the UV-Vis absorption spectrum should be changed.The wave-lengths and oscillator strengths of TD-DFT/PBE-calculated ex-citation of NO2PP and NH2PP,which can be assigned to the Q and B bands,are complied in Table3.The experimental wave-lengths are also listed.
Due to more one-electron transitions involved,more excita-tions are obtained from the TD-DFT calculations,referring to Supporting Information.These excitations are probably related to the lower wavelength absorption bands of porphyrins,al-though they have slightly larger oscillator strengths than ex-pected.The calculated and the corresponding experimental ab-sorption spectra of NO2PP and NH2PP are presented in Fig.6 with a suitable broadening of Gaussian line
shape.
3Calculated energies of the frontier molecular orbitals of
free-base porphyrins1,2using the HOMO energy as a
reference
4Calculated energies of the frontier molecular orbitals of
metalloporphyrins3,4using the HOMO energy as a reference Table2Some calculated bond lengths and bond angles in metalloporphyrins3,4using two different functionals B3LYP and PBE
a Data are taken from literature42
.
L(N—Cα)/nm
L(Cα—Cβ)/nm
L(Cβ—Cβ)/nm
L(C m—Cα)/nm
L(Cβ—H)/nm
L(C m—C m)/nm
L(C1—C6)/nm
A(N—Zn—N)/(°)
A(Zn—N—Cα)/(°)
A(Cα—N—Cα)/(°)
A(Cα—Cβ—Cβ)/(°)
A(Cα—C m—Cα)/(°)
D(N19C24C23C3)/(°)
D(N4C3C23C24)/(°)
D(N4C5C6C7)/(°)
D(N8C7C6C5)/(°)
D(C2C1C6C5)/(°)
0.138
0.144-0.145
0.136
0.140
0.108
0.690
0.150
89.8
126.6-126.8
106.4-106.5
106.9
125.4
0.627
-2.158
2.158
-0.627
77.46
0.138
0.145
0.137
0.141
0.109
0.692
0.150
89.8
126.9-127.1
105.8-105.9
106.8
125.3
0.056
-2.624
2.624
-0.056
75.36
0.138
0.145
0.136
0.141
0.108
0.692
0.150
90.0
126.7-126.8
106.4-106.6
107.0
124.6
1.174
-1.228
1.228
-1.174
82.44
0.139
0.145
0.137
0.141
0.109
0.695
0.150
90.0
126.8-127.0
105.8-106.1
106.9
124.4
2.138
-2.603
2.603
-2.138
73.57
a
0.138
0.144
0.135
1.400
106.6
107.4
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As evident from Table 3and Fig.6,the calculated electronic
excitations of Q bands are at 629.8and 586.8nm for 1/PBE and 665.8and 560.1nm for 2/PBE,which agree well with ex-perimental wavelengths (650.8and 551.2nm for 1,649.4and 550.1nm for 2).Calculations also show that there are several excitations between 500and 700nm assigned to Q band;how-ever,their oscillator strengths are very small (<10-3),and no ex-perimental absorptions could be related to these excitations.Fur-thermore,Q bands of 1/PBE originate from only two HOMOs (HOMO and HOMO -1)and more unoccupied MOs,because its HOMO and HOMO -1are identical and the unoccupied MOs are almost degenerate (see above).On the contrary,Q bands of 2/PBE result only from two LUMOs (LUMO and LUMO+1)and more occupied MOs.As far as the B band con-cerned,more electronic excitations are also obtained in the wavelength of 400-450nm (see Supporting Information).
The
Fig.5HOMO and LUMO isoamplitude surfaces of four porphyrin derivatives with PBE functional
Table 3TD-DFT/PBE-calculated and experimental wavelength (λ),and the corresponding one-electron transition and
oscillator strengths of free-based porphyrins 1,2
H:HOMO;L:LUMO;Abs:absorbance
Molecule NO 2PP
NH 2PP
Band Q
0x
Q 0y
B
Q 0x Q 0y
B
TD-DFT/PBE
λ/nm 629.8586.8438.3665.1560.1408.7
Excitation energy/eV
1.97
2.112.831.892.21
3.03
One-electron transition
H -1→L;H -1→L+5;H →L+1;H →L+4
H →L+4;H -1→L;H -1→L+5H -3→L+4;H -1→L;H -1→L+5;
H →L+4;H →L+9H -1→L+1;H -4→L;H →L H -5→L+1;H -4→L;H →L H -11→L+2;H -9→L;H -7→L+1;
H -5→L;H -3→L+1
Oscillator strength
0.0170.0510.2540.0280.0160.247
Experiment
λ/nm 650.8551.2421.2649.4550.8419.2
Excitation energy/eV 1.912.252.941.912.252.95
Abs
0.0330.0861.0020.0440.105
0.896
Fig.6Experimental and calculated absorption spectra of NO 2PP and NH 2PP
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large number of excitations in the B band region can be attribut-ed to more one-electron transitions taking place between many occupied and unoccupied molecular orbitals.The addition of the NO2and NH2groups reduces the symmetry of the mole-cules,and therefore,more electronic excitations are allowed. In addition,Fig.6shows that the B band of NO2PP is more bathochromic than that of NH2PP in the calculated spectra, which is also observed in the experimental spectra.The red-shift can be explained by the characteristics of the NO2 group,which is electron withdrawing group and makes the electron of porphyrin ring delocalized well,leading to the slightly smaller gap between HOMO and LUMO than that of NH2PP(see Fig.3).
The TD-DFT/B3LYP calculated results of the excitations are given in parison of Q and B bands in Tables3and 4reveals that the B3LYP absorption bands are all shifted to-ward lower wavelengths compared to those of PBE.This might be due to the sensitivity of the TD-DFT calculations to the as-ymptotic decay of the exchange-correlation potential,which in turn impacts the quality of the orbital energy differences that enter the TD-DFT formalism as a first approximation to the ex-citation energy.38,44It can be deduced from comparing with Fig.3that for1/B3LYP and2/B3LYP,the gap between the or-bital energies are2.705and2.767eV,respectively,which are about1eV larger than those obtained by PBE functional.Al-though as anticipated,hybrid functionals,which in general ex-hibit an improved asymptotic decay of the exchange-correla-tion potential over GGA functionals,would yield more accu-rate excitation energies.However,the hybrid functionals are improved by mixing of exact Hartree-fock exchange with the semilocal functionals,and the optimum amount of mixing is far from universal,45whereas no semiempirical parameters are involved for the PBE functional.From the reality of calcula-tions,the PBE functional is shown to be more accurate for pre-dicting the absorption spectra than B3LYP functional,and this is also true for the calculation of the metalloporphyrinʹs spectra (vide infra).
3.3.2NO2ZnPP(3)and NH2ZnPP(4)
The calculated and experimental spectra of NO2ZnPP and NH2ZnPP are presented in Fig.7with a suitable broadening of Gaussian line shape.Examination of this figure discloses that PBE-calculated wavelengths of Q bands for3/PBE and4/PBE are at582.9and562.9nm,respectively,with oscillator strengths of0.028and0.018,which coincides well with the ex-perimental values(601.8and599.2nm).In particular,in the absorption spectrum of NH2ZnPP,a peak lies at637nm with relatively strong oscillator strength of0.025,which may also be assigned to Q band.In addition,there are many other excita-tions with low oscillator strengths within the region of Q band, which can not be observed in experimental spectra.PBE-calcu-lated electronic excitations at416.4and415.1nm with maxi-mum oscillator strength of0.673and0.504for NO2ZnPP and NH2ZnPP,respectively,are assigned to B band,again in accor-dance with the experimental spectra in which the B band lo-cates at424.5and422.0nm,respectively.
As shown in Fig.7(c,d),in comparison with the spectra cal-culated with PBE functional,the B3LYP-calculated spectra are not well in agreement with experimental spectra.Besides, there are two peaks between450and550nm which could be assigned to Q bands.This can be mainly attributed to the fact that the gap between the HOMO and LUMO calculated by B3LYP functional is1eV larger than that of PBE. Furthermore,similar to NO2PP and NH2PP,the red-shift is al-so observed in the absorption spectrum of NO2ZnPP with NH2ZnPP.In the experimental absorption spectra,the red-shift is about2nm for NO2PP and NH2PP and2.5nm for NO2ZnPP and NH2ZnPP.In the PBE-calculated absorption spectra,the difference between the B bands is30and1.3nm,respectively. The calculated red-shift follows the same trend as that revealed by the experiment,although there appear some disparities.The red-shift can be ascribed to the addition of the metal zinc in the center of the porphyrin ring and the meso-subsitituted groups, which influence the electron distribution of the porphyrins ring.
3.4Triplet states
As a potential material in optoelectronic field,such as organ-ic photovoltaic,organic field effect transistor,optical limiting, and artificial photosynthesis mimic,46,47porphyrin derivatives are usually investigated as an electron donor(in donor-accep-tor systems)to mimic the multi-step electron-transfer process. Generally speaking,the electron firstly transits from ground state to singlet state,then relaxes to triplet state,and eventual-ly,the excitation energy of triplet state transfers to the accep-
Table4TD-DFT/B3LYP-calculated and experimental wavelength,and the corresponding one-electron transition and
oscillator strengths of free-based porphyrins1,2
Molecule
NO2PP NH2PP Q0x
Q0y
B
Q0x
Q0y
B
TD-DFT/B3LYP
λ/nm
580.2
542.8
377.1
569.2
533.5
373.4
Excitation energy/eV
2.14
2.28
3.29
2.18
2.32
3.32
One-electron transition
H-1→L+1;H→L
H-1→L;H→L+1
H-3→L+1;H-1→L;H-1→L+5;H→L+1
H-1→L;H→L+1
H-1→L+1;H→L
H-7→L;H-5→L;H-3→L+1;
H-1→L+1;H→L
Oscillator strength
0.012
0.026
1.241
0.004
0.023
0.949
Experiment
λ/nm
650.8
551.2
421.2
649.4
550.8
419.2
Excitation energy/eV
1.91
2.25
2.94
1.88
2.21
2.96
Abs
0.033
0.086
1.002
0.044
0.105
0.896
1091。