物理化学学 ̄E(Wuli Huaxue Xuebao) August Acta P .一Chim.Sin.2012,28(8),1885-1891 doi:10.3866/PKU.WHXB201205301 1885 www.whxb.pku.edu.cn [Article] 给电子基团对吲哚染料电子结构和吸收光谱的影响 梁桂杰 , 钟志成 和平’ 陈美华’ 候秋飞’ 许杰。 徐卫林。 李在房’ f 湖北文理学院材料科学与工程研究中心,湖北襄阳441053; 西安交通大学金属材料强度国家重点实验室,西安710049; 武汉纺织大学新型纺织材料绿色加工及其功能化教育部重点实验室,武汉430073) 摘要:利用密度泛函理论(DFT)和含时密度泛函理论(TD.DFT),分别基于B3LYP和PBE1PBE方法研究了一 系列含有不同给电子基团的吲哚染料分子(IDI—ID3)的电子结构和吸收光谱性质.重点比较了不同电子给体对 染料的分子结构、吸收光谱以及其在电池中的光伏性能的影响.结果表明从ID1、ID2到ID3,随着电子给体中苯 环数目的增加,吲哚分子上的共轭效应逐渐增大,导致吲哚分子最高占据分子轨道.最低未占据分子轨道 (HOMO.LUMO)之间的能隙变窄,分子的吸收光谱发生红移.染料分子的吸收光谱和LUMO能级分别影响染料 的吸光效率和光电转化过程中电子的注入过程,从而使其二者成为决定电池光伏性能的重要参数.综合考虑上 述两个参数对电池性能的贡献,通过理论研究证实,在ID1一ID3系列染料中,ID3具有较长的吸收谱带、较大的 分子消光系数和合适的LUMO能级,从而表现出最为优越的光伏性能。这与实验得出的结论很好地吻合. 关键词:密度泛函理论;染料敏化太阳能电池:吲哚染料;分子结构:吸收光谱 中图分类号:O641 Effect of Donor Moieties on the Electronic Structures and Absorption Spectra of Indoline Dyes LIANG Gui—Jie’ ZHONG Zhi.Cheng’, CHEN Mei.Hua’XU Jie。XU Wei.Lin。 HE Ping’HOU Qiu.Fei’Ll Zai.Fang’ f Research Centerfor Materials Science&Engt。neering,Hubei University ofArts and Science,Xiangyang 441053,Hubei Province R.China; State Key Laboratoryfor Mechanical Behavior ofMaterials.Xi'an Jiaotong University,Xi an 710049,P R.China; Key Laboratory ofGreen Processing and Functional Textiles ofNew Textile Materials,Ministy rofEducation, Wuhan Textile Universiyt,Wuhan 430073,P R.China) Abstract:The electronic structures and absorption spectra of indoline dyes containing diferent donors flD1一lD3)were investigated by density functionaI theory(DFT)and time.dependent DF at the B3LyP and PBE1 PBE levels.respectively.The effects of the donor moieties on the molecular structures,absorption spectra.and photovoltaic performance have been compared.The results indicate the increase in the number of phenyI groups in the donor decreases the highest occupied molecular orbital—the Iowest unoccupied molecular orbitaI(HOMO—LUMO)energy gap and red.shiRs the absorption band.This is related to the increased conjugation from ID1 t0 lD2 and lD3.The absorption spectra and LUMO energy Ievel act as two criteria for the photovoltaic performance of a dye by determining the light harvesting efficiency and charge injection process.respectively.Considering the above two factors contribution to the performance of a photovoltaic cel1.1D3 with a Iong absorption band and high extinction coefficient,as welI as a favorable Received:March 15,2012:Revised:Mav 30,2012;Published on Web:Mav 30,20l2. ‘Corresponding authors.LIANG Gui.Jie,Email:lej5110l1@l63.com;Tel:+86—1597116418O. ZHONG Zhi.Cheng,Email:zczhong橱l63.com;Tel:+86—710—3590061. The project was supported by the National Natural Science Foundation ofChina(51003082),Key Project ofScience and Technology Research of Ministry ofEducation,China(208089),and Natura1 Science Foundation ofHubei Province,China(201 lCDC062). 国家自然科学基金(51003082),教育部科学技术研究重点项目(208089) ̄1湖北省自然科学基金(2011CDC062)资助 (C)Editoria1 oficc ofActfa Physico.Chimica Sinica Acta Phys.一Chim.Sin.2012 Vo128 .LUMO energy level has been confirmed theoretically to be the best dye of ID1-ID3,which is consistent with experiment results. Key Words:Density functional theo ̄;Dye-sensitized solar cell;Indoline dye;Molecular structure Absorption spectrum 1 Introduction Recently,dye.sensitized solar cells(DSSCs)have attracted much attention as possible altematives to conventional sol— id-state photovoltaic devices owing to their potentially low fab— rication cost and relatively high conversion emciencies. Act. ing as photosensitizer in the DSSCs,the dye is one of the key components for high solar-to.electric conversion efifciencies of the cells.The performances of dyes.such as light harvesting efifciency,quantum yield of electron injection on the surface of semiconductor iflm,and so on,are of great importance to de- termine the emciency of the DSSCs.In order to design and synthesize more efifcient sensitizing dyes,it is necessary to un— derstand the electronic structures ofthe existing dyes. There are several types of dyes used in DSSCs among which polybipyridyl ruthenium complex dyes exhibit the relatively beRer charge-transfer(cT)performance and higher conversion efifciency. However,the large—scale application of the Ru corn— plexes are limited due to their materials costs and environment problem.Then,many organic dyes for DSSCs,including cya— nine dyes, porphyrin dyes, phthalocyanine dyes,’and couma— rin dyes are also reported because of their lower cost,more environment harmony,and more convenient synthesis process compared with the Ru complexes.Indoline dyes have been ex— perimentally prepared as one kind of promising organic dyes with high conversion efifciency for DSSCs. 一 Experimental synthesis method is a powerful and straightforward route to get new dyes;however,the process is not only expensive but also time—consuming.Density fimctional theory(DFT)method is another effective tool for theoretical treatment of structures as well as electronic and absorption spectra.Its time.dependent extension,called time-dependent DFT(TD—DFT),can give reli- able values for the valence excitation energies with standard ex- change—correlation functionals.Ham et a1. designed and stud- ied a group of indoline dyes derived from the D149 through DFT/TD.DFT calculations.and the results provided useful clues for the molecular engineering of efifcient organic dye sensitizers.Le Bahers et a1. investigated three indoline dyes D131,D102,and D149 wiht diferent acceptors by DFT,and found that the computed results agreed well with the experi— mental results;the calculations allowed new insights into the electronic structure ofthis indoline dye family. In this work,DFT and TD-DFT calculations were employed to study the electronic structures and optical properties of a se- des of indoline dyes wiht different donors including indoline dye 1(ID 1:C“H22N20 Sz),indoline dye 2(ID2:C28H22N204S2), indoline dye 3(ID3:C37H3oN203S2).The efifcient sensitized mechanism of the indoline dyes was elucidated and the influ— ence of diferent donors on the electronic and optical proper— ties of indoline dyes was investigated in detail.Further,the re— lationships between the theoretical parameters of indoline dyes and hte experimental efifciency of cells were also discussed. 2 ComputationaI methods The structures of hte dyes ID 1,ID2,and ID3 were pre—opti— mized using the semi—empiircal AM1 method. The final ground—state geometries were obtained without any symmetry constrains at the Becke s three parameter gradient—corrected ex— change potential and the Lee-・Yang--Parr gradient--corrected cor-- relation potential(B3LYP) level of theory with the 6-3 l g(d) basis set.A full natural bond orbital(NBO)analysis was em— ployed by using the POP=NBO keyword,along with a sec— ond-order perturbation theory(SOPT)analysis.The excitation energies and oscillator strengths for the lowest 30 singlet・-sin-・ glet transitions at the optimized geometry were obtained by TDDFT calculations with the hybrid functionals B3LYP and PBE1PBE, 。respectively,by the same basis set as for the rgound state.According to the calculated results,the UV-Vis absorption spectra were simulated by means of the SWizard program(Revision 4.6)using a Gaussian convolution with the full width at half-maximum of 3500 cm~.The conductor polar— izable continuum model(CPCM) was employed in this pro— rgam to introduce the solvmion effect of ethanol on the dyes. All calculations were performed with the Gaussina 03 program package. 3 Results and discussion 3.1 Molecular geometries As shown in Fig.1.the structures oflDl—ID3 all have a typi— cal D—zr-A fD:donoL 7c"conjugated body,A:acceptor)struc— tm'e.They have the same conjugated bodies(cyclopentyl in— dole.rhodanine1 and acceptors(acetic acid),while the diiferent donors(Rx=methoxyphenyl,methoxynaphthyl,and diphenylvi— nylphenyl,for IDI,ID2,and ID3). The optimized geometircal structures in the ground state of indoline dyes are shown in Fig.2,and some of the calculated bond lengths,bond angles,and dihedral angles of the dyes are listed in Tlable 1.Most of the corresponding parameters are con— sistent with the calculated results for the indoline dyes. , re— lfeeting the reasonability of the present results.All C——C lengths in the rhodanine,indoline unit and phenyl rings are be— wteen the distnace of a single bonded C—C and a double bonded C=C,implying that there is extensive delocalization throughout N0.8 LIANG Gui—Jie et a1.:Effect ofDonor Moieties on the Electronic Structures andAbsorption Spectra ofIndoline Dyes 1889 Table 3 Computed excitation energies.electronic transition configurations,and oscillator strengths for the optical transitions with the excitation energies ofthe singlet—state for the dyes; the oscillator strengths ofthe singlet‘state ofthe dyes ciently more positive than the conduction band energy level of strengths.The first optically allowed electronic transition of ID 1一ID3 clearly corresponds to the HOMO—LUMO transition possessing high transition intensity.As the HOMO and the LU— Ti02(丘b is一2.77 eV for the bare Ti38O76 cluster as a mode1 for nanocrystalline exposing the anatase(101)surface) to inject the electrons from the dyes to the conduction band of TiO2. MO are of the and 7r*type,the HOMO-LUMO transition Call be classified as a 一7/*intramolecular charge ransfer.It tcan be found that the calculated absorption spectra of dyes agree with those of the experiment, ’ alt。hough there is some discrepancy which may be attributed to the DFT and the solvent effects; nd the hybriad functional PBE1PBE is more suitable than B3LYP for calculation of electronic absorption spectra of the indoline dyes.consulting from Fig.5 and Table 4.So the ab- sorption spectra discussed in the following in this paper are all simulated with PBE1PBE.Fig.5 shows that ID1一ID3 have broad absorption bands rlocated in the ranges of 350—400 nm The energy level of HOMO of the three indoline dyes shift to be positive from ID1.ID2 to ID3.while those of the LUMO shift to be negative.Moreover,with the increase of phenyl number in the donors,the energy gaps between the HOMO and LUM0 decrease from 2.83,2.62,to 2.56 eV for IDl,ID2,and ID3 in order. 3.3 Optical absorption spectra In order to obtain the microscopic information about the electronic transitions,the corresponding molecular orbital prop— erties were checked,the computed excitation energies,electron— ic transition configurations,and oscillator strengths along with the main excitation configuration for the dlves are listed in Ta— ble 3.The simulated UV-Vis absorption spectra of ID1一ID3 and 450—600 irm)and high molar extinction coeficifents r4.6× 104—6.3×10 cm一・L・mol in the long wavelength range, which is advantageous for light harvesting.Ⅵ thin the scope of are given in Fig.5,and the relative calculated absorption peaks wavelength analyzed.the simulated spectra of ID 1 and ID2 show three maxima.while the simulated spectrum of ID3 is composed of two bands.Furthermore,the 1ocations of the ab— are listed in T_ab1e 4.The major electronic absorption bands are assigned to thosc excitations with significant oscillator Wavelength,nm WaVeIength,nm Fig.5 Simulated UV-Vis spectra of IDI-ID3 with hybrid functionals(a)B3LYP and(b)PBEIPBE Acta PhyS.-Chim.Sin.2012 VO1.28 Table 4 Experimental and simulated absorption peaks calculated with B3LYP and PBE1PBE hybrid funetionals measuredin ethanolinRe ̄12; not available but simil ̄tothose ofID3 inRe ̄11;。measuredintert-butyl aleohol/acetonitrilemixture solutionwiththevolume ratio ofl:1 inRe£11 sorption bands have a red—shift from ID 1,ID2 to ID3(the maxi— mum absorption peaks for them center at 469,502,and 5 12 nm.respectively)with the phenyl number increasing in the do- nor moieties.The shape variation and red—shift of the peak de- scribed above are closely related to the change of electronic structures of the dyes,which may be attributed to the enhanced conjugation in the molecules induced by the increasing phenyl moiety(confirmed by the NBO naalysis). 3.4 Theoretical parameters to judge electrochemical performance of indoline dyes In DSSCs,the incident monochromatic photo-to-current con— version efifciency(IPCE)is one of hte most important factors to evaluate the performance of the dyes.As expressed in Eq. (1),6’ IPCE=LHE・ 。 ・ (1) IPCE is composed of three segments:light harvesting efifcien— cy(LHE),quantum yield of charge injection( 。 ),and electron collection efifciency( ).LHE relates to the absorption spec— trum response of aye molecules,including the spectrum loca- tion and molra extinction coefifcient. i depends on the ener- gY gap between the LUMO energy level of dyes(EL ̄o)and the conduction band edge of Ti02(丘b),which is denoted as the driving force of the electron injection. Thus,the absorption spectra and LUMO energy level of dyes act as two criteria to the electrochemical performance of the dyes by determining the IPCE value. 3.4.1 Effect of the absorption band on the electrochemical performance It is well known that red-shitf of absorption spectra of dyes are advantageous to the lihgt harvesting in DSSCs.Further— more,as in Eq.(2), LHE:1—10一厅( ) (2) where ’is the number of moles of seneitizer per square centi‘ meter of projected surface area of the film;o-is the absorption cross section obtained from the decadic extinction toemcient (units of mol一・L・cm )by multiplication with 1000 cm ・L ; is the absorption wavelength.the molar extinction coefifcient of dye molecule is proportional to the LHE.Consequently,the location of absorption band and the corresponding molar ex- tinction coefifcient together affect the overall efifciency of cells by their contribution to the light harvesting efifciency ̄ Where,口is the function of molar extinction coefifcient of dye molecule,which is obtained from the decadic extinction coeffi— cient(units of mol~・L・cm一 、by multiplication with 1000 cm ・ I -。 Ta1)le 4 shows from ID1,ID2.to ID3,with increase of phe— nyl number in the donor,the absorption bands red.shift to lon. ger wavelength while the absorbeney of the peaks increases gradually meaning much bigger molra extinction coefifcient of the dyes in the studied wavelength( ranges.w longer ab— sorption wavelength as well as bigger molra extinction coefif— cient,ID3 has higher LHE value than ID1 and ID2,and thus higher eifciency(6.1%)than ID1 and ID2(5.5%and 5.6%) compared wich 6-3%for N3.However,the light harvesting e街. ciency of ID2 is higher than that of ID1.while the emciency of cells of ID2 is lower than that of ID1.It demonstrates that the light harvesting emciency ofthe aye is not the only factor to determine the emciency of cells.and the abnormal phenome. non between也e 1ight harvesting e衔ciency and overall efif— ciency of hte cells of ID 1 and ID2 will be elaborated below. 3.4.2 Effect of the LUM0 energy level on the electrochemical performance From Fig.4,it shows that the energy gaps between the ELUMO of indoline dycs and正 are 0.35,0.27,and 0.24 eV,respective. 1v for IDl,ID2,and ID3.The driving ofrce for electron injec. tion decreases in the same order.The driving force of ID 1 is lrager than that Of ID2.1eading to the higher light to energy el'- ifciency of ID 1.Therefore.the theoretical order of the light.to— energy efficiency of three indoline dyes isⅡ)3>ID1>1D2.con. sidering of the mutual contribution of absorption spectra and the LUMO energy levels of the dyes.which is consistent with hte experimental results“- 】listed in Tlab1e 4. 4 Conclusions The electronic structures and absorption spectra of hte dyes ID1一ID3 containing diferent donors were investigated by TD— DFT calculation.at the B3LYP and PBE1PBE levels with the 6-3 tg(d)basis set.respectively.The calculated geometric char— acters indicate that the strong conjugated effect is formed in the dyes.which is very helpfu1 for emcient electron transfer. The NB0 results suggest that the introduction of more phenyl moieties leads to a decrease in the electron.donating capability of hte donor,but an increased conjugation and stabilization in the molecules.The electron population indicates that the ener- gY level location of dyes fits the electron injection process rfom the excited dyes to the semiconductor in DSSCs.The sim. ulated UV-Ⅵs absorption spectra show qualitative agreement wiht the experimental results.The indoline dyes have broad ab. sorption bands and high molar extinction coefifcients in the long wavelength range.revealing the good light harvesting effi— No.8 LIANG Gui—Jie et a1.:Effect ofDonor Moieties on the Electronic Structures and Absorption Spectra ofIndoline Dyes 1891 mency・ uchi,T.;Miura,H.;Uchida,S. Photochem.Photobio1.A (12) HoriThe absorption spectra and LUMO energy level determine the photovoltaic performance of the dyes by affecting the light 2004,164,29.doi:10.1016/i.jphotochem.2003.12.018 Agyeman,B.;Kaneko,S.;Kumara,A.;Okuya,M.; (13) Onwona—Murakami,K.;Konno,A.;Tennakone,K. n. App1.P . Part 2 2005,44,L731. sui,M.;Ito,A.;Kotani,M.;Kubota,Y;Ftmabiki,K.;Jin,J.; (14) MatYoshida, ;Minoura,H.;Miura,H.DyesPigments 2009,80, harvesting and elec ̄on injection efifciencies,respectively. Through the comparative analysis of electronic strucRtres,spec— tra and photovoltaic properties.ID3 with long absorption band as well as favorable LUM0 energy level has been theoretically proved to be the most effective one for DSSCs among ID1—- ID3,which is consistent with the experimental results. 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