螺芴齐噻吩电子结构和光谱性质的量子化学研究

1、Y.H. Kan1,2, S.Y. Yang1, S.X. Wu1, Z.M. Su1*1 Institute of Functional Material Chemistry, Northeast Normal University, Changchun 130024, P.R.China2 Jiangsu Province Key Laboratory for Chemistry of Low-Dimensional Materials, Huaiyin TeachersCollege, Huai'an 223300, P.R. ChinaAbstract:Electronic s

2、tructure and photophysical properties of oligothiophene- (9,9-spirofluorene-oligothiophene with spiro-linked structure characters have been studies systematically. The conjugated bond relaxation of singly oxidation state and triplet shows that these complexes have obvious trap-barrier-trap form. The

3、 lowest excitation transition localized on a single chain and mainly come from fluorene and thiophene branch. This excited state does not possess any charge transfer character. The calculated result of energy difference between singlet and triplet indicates that the energy of the T1 excited state is

4、 less affected by change in the conjugation length than the S1 excited state.Keywords:Spirobifluorene, Oligothiophene, Localized density matrix, Density functional theory1.IntruductionDesigning and preparing organic and macromolecular photoelectric materials and devices have gained more and more att

5、entions in the field of searching for novel photoelectric materials and devices.1,2 The unique spirobifluorene structural feature not only minimizes the parallel aggregation in the solid state but also enhances in both glass transition temperature and luminescent stability. The synthesis, optical an

6、d theoretical characterization of spiro-linked materials has been presented recently in several works.3-8 Pei et al. synthesized a series of 9,9´-spirofluorene oligothiophene derivatives and discussed their NMR spectra and electrochemistry properties. 9,10 To the best of our knowledge, there ar

7、e still no reports on the electroluminescent properties of such spirofluorene oligothiophene derivatives. In this context, a series of (9,9-spirofluorene-oligothiophenes (nTSF (see Figure 1 are systematically discussed for their electronic structure and spectral properties from theoretical aspect wi

8、th DFT and localized density matrix (LDM method. We also discuss how the spiro-type structure influenced the electronic absorption spectra and predict the splitting of singlet and triplet. n TSF n = 1-6Figure 1: Structures of the spirobifluorene oligothiophenes.2. Computational MethodologyThe geomet

9、ries of the molecules are optimized using density functional theory at the B3LYP level. The C 2 symmetry point group is adopted for all the geometry configurations discussed. Under the ground state geometric structure, ZINDO and TDDFT method is chosen to generate the vertical transition energy. All

10、calculations are carried out using the GAUSSIAN 03 program package. Additionally, localized-density-matrix (LDM 11 method is applied to analyze the essence of excited state transition.3. Results and Discussion3.1. Geometry changes of singly charged doping state and triplets.DFT optimized geometry in

11、 the ground state indicates that the upper fluorene-thiophene chain is perpendicular to the connecting fluorene moiety through the sp 3 hybrid C atom. It should be noted that the two branched chain both possess nonplanar structure. Moreover, the bond length and bond angle of fluorene moiety keep unc

12、hangeable with increasing of chain length. However, the optimized geometries in both charge doping state and triplet state have an approximately planar structure. The C-C bond length changes have been compared based on the difference of optimized singly charge doping and triplet state geometry with

13、the neutral ground state species. The C-C bond length changes for the single oxidation state and triplet with respect to the ground state along the conjugated chain of compound 2TSF, 4TSF and 6TSF are listed in Figure 2. It has similar change in both two states that the largest C-C bond length chang

14、e happens in the middle of fluorene and the bonds in the thiophene attached to fluorene ring. One marked difference from the single oxidation states of homooligomer (e.g. oligophenylethylene, oligothiophene is that the bond length change doesnt uniformly distribute along the conjugated backbone. A w

15、ell-barrier-well structure could also be observed and the charge distributes in two wells not in a well. The distribution probability is equal in the two wells, forming into a new population style for polaron. -30-20-100102030-0.020.000.02C -C b o n d c h a n g e (A n g s t r o m C-C bond number -0.

16、020.000.02-0.020.000.02SF6BT SF4BT (a SF2BT -30-20-1001020303.2. Electronic transition energy.The calculated the lowest transition excitation energies by ZINDO and TDDFT methods are shown in Table 1. An obvious red-shift shows for the first absorption peak with the increase of chain length. Good agr

17、eement can be found for the calculation results from two methods, showing that the configuration from the electron excitation from HOMO (H to LUMO (L contributes mostly to the lowest excitation transitions. Molecular orbital calculations reveal that from 1TSF to6TSF, the electronic densities of HOMO

18、 and LUMO mainly localize on the fluorene moiety attached to thiophene ring and thiophene chain in contrast with a little distribution on the other part of fluorene. Although the electronic density distributes all over the whole conjugated chain, it mainly localizes on the fluorene and peripheral th

19、iophene ring. The localization characters are even significant for the electron distribution of H-1 and L+1. It is in agreement with our previous DFT calculations on the bond length changes of single oxidation states and triplet states which showed a certain localization trend. It is noteworthy that

20、 there is a obviously strong characteristic excitation at 4.03 eV in for all six compounds. When the electron transition configuration is focused, it can be easily found that the transitions vary with systems. For 1TSF at 4.03 eV, this transition mainly comes from the H-1L+1 transition. In contrast,

21、 H-2L+2 transition and H-3L+3 transition are responsible for 2TSF and 3TSF, respectively. The rest may be deduced by analogy, when compound 6TSF was involved in, H-6L+6 transition contributes mostly. Based on the orbital analysis on these series of compounds, we can find that the dominant contributi

22、on to the excitation of 4.03 eV arises from the intramolecular charge transfer in spirobifluorene, which is from the occupied orbital of the mainchain to the unoccupied orbital of the fluorene moiety beneath the mainchain.Table 1: Calculated transition energies (E, in eV, oscillator strength (f and

23、excited state configuration for system n TSF (n=1-6 with TD-DFT and ZINDO methodTDDFT ZINDO E f main components E f main components65(HL48(HL-0.36(H-1L+143(HL-0.36(H-1L+139(HL-0.35(H-1L+1To confirm the above conclusion and clarify the characteristic property of each excitation peak, the transition d

24、ensities corresponding to the absorption peaks are obtained using the localized density matrix method, taking 2TSF as example, which are represented in Figure 3. From the transition densities, it should be noted that the lowest allowed optical transitions is due to the electron-hole interactions bet

25、ween electrons and holes in the same single fluorene-oligothiophene branched chain, which accords so well with the analysis on molecular orbital, theres no chargeHole index 0.0121020304010203040E l e c t r o n i c i n d e xHole indexFigure 3: Contour plots of transition density matrices correspondin

26、g to the lowest energyexcitation (left and energy excitation at 4.03 eV (right for 2TSF proportional to the NDB. Compared with the energy of S1, the energy of T1 has relatively smaller slope, indicating that the triplet exciton diffuses a little exhibiting a more localized nature, which is in accord

27、ance with the bond length changing localization. Results of linear fit show that the exchange energy decreases with the decreasing of chain length. The approximately linear relationship exits between the energy of T1 and energy of S1. The calculated slope with least square method is 2.40, which indi

28、cates that the conjugated chain length affects much more on the energy of S1 than that of T1.AcknowledgmentsThe authors thank Dr. Guanhua. Chen for the opportunity to perform calculations using the LODESTAR V1.02 program package developed in his group and Dr. Xiujun Wang for valuable discussions abo

29、ut LDM analysis. This work was financially supported by the China NSF (Grant Nos. 20571029, 20573016 and the Opening Project of Key Laboratory for Chemistry of Low-Dimensional Materials of Jiangsu Province (No. JSKC06032Referencesand A.B. Holmes, Light-Emitting-Diodes Based on Conjugated Polymers, N

30、ature 347 539-541(1990.D.A. Dos Santos, J.L. Bredas, M. Logdlund, and W.R. Salaneck, Electroluminescence inconjugated polymers, Nature 397 121-128(1999.(3 W.L. Yu, J. Pei, W. Huang, and A.J. Heeger, Spiro-functionalized polyfluorene derivatives as bluelight-emitting materials, Advanced Materials 12 828-831(2000.(4 A. Crispin, X. Crispin, M. Fahlman, D.A. dos Santos, J. Cornil, N. Johansson, J. Bauer, F.Weissortel, J. Salbeck, J.L. Bredas, and W.R. Salaneck, Influence of dopant on the electronic structure of spiro-oligophen