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Donor-length dependent photoninduced charge transfer in two-photon absorption
Journal Pre-proof Donor-length dependent photoninduced charge transfer in two-photon absorption Chunhua Tian, Xinxin Wang, Xijiao Mu, Jun Quan PII:
S1386-1425(19)30921-7
DOI:
https://doi.org/10.1016/j.saa.2019.117531
Reference:
SAA 117531
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: 16 June 2019 Revised Date:
9 September 2019
Accepted Date: 12 September 2019
Please cite this article as: C. Tian, X. Wang, X. Mu, J. Quan, Donor-length dependent photoninduced charge transfer in two-photon absorption, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2019), doi: https://doi.org/10.1016/j.saa.2019.117531. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier B.V. All rights reserved.
Donor-length dependent photoninduced charge transfer in two-photon absorption
Chunhua Tian,1,+ Xinxin Wang,2,+ Xijiao Mu2,* Jun Quan1,*
1. School of Physical Science and Technology, Lingnan Normal University, Zhanjiang 524048, China 2. School of Mathematics and Physics, Center for Green Innovation, Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, University of Science and Technology, Beijing, Beijing 100083, PR China *
Corresponding
authors.
Email:
[email protected]
(X.
Mu)
and
[email protected] (J. Quan).
Abstract In this paper, we study the donor-length dependent photoinduced charge transfer in two-photon absorption (TPA). In the donor-acceptor (D-A) system of oligo-thiophene-fullerene, two kind of lengths of oligo-thiophene are chosen to study the donor-length dependent photoinduced charge transfer in TPA. It is found that donor length can significantly influence the rate of charge transfer. When the donor length is quarter-thiophene, the rate of charge transfer is less than 50%; while with the increase of donor-length to octant-thiophene, the rate of charge transfer can be reach up to 100%. The transition channels of photoinduced charge transfer in TPA are visualized with transition density matrix and charge difference density. Our results promote deeper understanding and rational design for donor-acceptor system with large rate of photoinduced charge transfer in TPA.
Key words: Donor-acceptor; Donor-length; photoinduced charge transfer; two-photon absorption
1. Introduction Photoinduced charge transfer in one-photon absorption (OPA) and two-photon absorption (TPA) is very important for the process of charge and energy transfer in the nonlinear optical absorption and fluorescence in system of biology and materials.1-24 TPA is a third order nonlinear optical phenomenon in which a molecule simultaneously absorbs two photons, which can be quantified by the TPA cross section, a quantity that is proportional to the imaginary component of the χ3 tensor (or, on a molecular level, to the imaginary component of γ) and depends on the photon energy and energies.6,21 The TPA excitation is confined to a volume on the order of the cube of the wavelength of the excitation light, which provides good resolution in three dimensions.6, 21, 25, 26 The excitation rate for TPA and the intensity of the two-photon induced fluorescence decrease as the forth power of the distance from the focal plane. Another advantage is that TPA processes can provide improved penetration of light into absorbing materials.
27
The ability to penetrate a material and to be focused
accurately in three dimensions make the TPA process ideal for fluorescence imaging of thick samples (even in vivo) or in medical applications in which, for example, a drug can be activated by TPA at a very precise location without affecting the tissue above and below the focal plane of the excitation beam. 28 It is great challenge to visualize the processes of photoinduced charge transfer in TPA, while it is very important to judge whether the electronic transitions are the charge transfer excited states in each step of TPA. Recently, Mu and coworkers developed programs to visualize the whether the electronic transitions are the charge transfer excited states in each step in every channel in TPA,21 which can provide definite and clear pictures of charge transfer in TPA, and charge transfer and electron-hole coherence in TPA are well revealed.
Figure 1. The quarter-thiophene-fullerene (a) and octant-thiophene-fullerene (b) as donor (green color) and acceptor (red color) system. In this paper, we visually investigate the photoinduced charge transfer in ologi-thiophene-fullerene dyad29~33 as donor-acceptor system of in TPA, using visualization program developed by Mu. The two molecular heads are fullerenes and the molecular tails are thiophene multimers. This molecular structure is to investigate the effect of polymer chain length on charge transfer in two-photon absorption. Firstly, the optical spectra in one-photon absorption (OPA) and two-photon absorption (TPA) of dyad are simulated. Secondly, photoinduced intra-molecular charge transfer in each channel in every step in TPA is visualized with visualization program of two dimensional (2D) and three dimensional (3D) methods. Our results can provide clearly evidence for the charge transfer in TPA.
2. Method All the quantum chemical calculations are done with Gaussian 16.34 The ground state geometries of them in Fig. 1 are optimized with density functional theory (DFT),35 dispersion correction, B3LYP functional36 and 6-31(d) basis set. The electronic transitions of them are calculated with time-dependent DFT (TD-DFT),37 CAM-B3LYP functional38 and 6-31(d) basis set.
The optical absorption spectra ( δ tp ) of them in TPA are calculated with,6, 21, 39 δtp = 8∑ j≠g j≠ f
where and
g
,
j
ω j and ω f
the lifetime.
θj
and
f µ j
2
jµ g
ωf 2 ω j − + Γ f 2 2
f
2
(1 + 2cos θ ) + 8 2
j
∆µ fg
2
f µ g
ωf 2 + Γf 2 2
2
(1 + 2cos φ ) 2
(1)
are the ground, intermediate and final states, respectively;
are the intermediate and final excitation energy in OPA, and is the angle between two transition dipole moments, and
φ
Γf
is
is the
angle between transition dipole moment and difference of excited and ground permanent dipole moments. The FWHM of the two-photon absorption spectrum was calculated to be 25 nm. The physical descriptions of 2D and 3D visualization methods of transition density matrix (TDM) and charge difference density can refer to [21].
3. Results and discussion 3.1 Optical absorption spectra in OPA and TPA Fig. 2 (a)-(d) are the optical absorption spectra of quarter-thiophene-fullerene, octant-thiophene-fullerene in OPA and TPA, respectively. Comparing Fig. 2(a) and 2(b) for in OPA It is found that strong peak of optical absorption will be red-shifted from 395 nm to 480 nm with the increase of donor length, which reveals that the donor length can efficiently tune the position of absorption peak. For quarter-thiophene-fullerene, the absorption spectra of them are similar in OPA in Fig. 2(a) and TPA in Fig. 2(c), except for the half energy for TPA. The electronic transition S18 is from S18<-S17
dipole moments between excited state and ground state in the second term in Eq. (1), ∆µ fg = 14.5
where
a.u. The second strong peak in 815 nm in TPA is contributed from
two channels transitions: S17<-S6<-S0 and S17<-S9<-S0. For the S17<-S9<-S0, the intermediate excited state S9 is strong absorption peak in OPA, which is a localized excited state; while the intermediate excited state S6 is a weak intramolecular charge transfer excited state, where the electrons transfer from ologo-thiophene to fullerene , which will be shown later.
(a)
3.5 S9
(b) 1.25
8.0x10
5
6.0x10
5
4.0x10
5
2.0x10
5
3.0
Oscillator Strengths
1.00 5
6.0x10
0.75
5
4.0x10
0.50
5
2.0x10
0.25
S16 S15+S14
0.0 375
400
425
UV-vis spectrum
UV-vis spectrum
5
8.0x10
2.5 2.0 1.5 1.0
450
475
0.0 400
0.00
500
425
3x10
6
2x10
6
1x10
S18
sum 1 2
6
S7
S14
0 750
8x10 7x10
3
6x10
3
5x10
3
4x10
3
3x10
3
2x10
3
1x10
3
0
800
850
900
2.
The
optical
500
525
0.0 550
950
1000
4
8.0x10 (d)
1.4x10
sum 1 4 1.2x10 2
S6
S17 6
6.0x10
4
1.0x10
3
8.0x10 6
4.0x10
3
6.0x10
3
4.0x10
6
2.0x10
S9 S12
0.0 800
3
2.0x10
S7
0.0
850
Wavelength [nm]
Figure
475
6
3
Two-photo Absorption cross-section
4x10
(c)
450
Wavelength [nm]
Molar Absorption coefficient [L/cm/mol]
Molar Absorption coefficient [L/cm/mol]
Wavelength [nm]
6
0.5
S6 S7
900
950
1000
1050
Two-photo Absorption cross-section
1.0x10
6
S17
Oscillator strengths
1.0x10
1.50 6
1100
Wavelength [nm]
absorption
spectra
of
quarter-thiophene-fullerene,
octant-thiophene-fullerene in OPA (a, b) are and TPA (c, d), respectively.
3.2 Photoinduced charge transfer in OPA Optical properties of quarter-thiophene-fullerene and octant-thiophene-fullerene are analyzed with 2D and 3D visualization methods. For quarter-thiophene-fullerene in OPA, optical properties of S7, S14, S16 and S17 are analyzed, since S17 is strong absorption in OPA in Fig. 2(a), S7 and S14 are clearly observed in TPA in Fig. 2(c). 2D
visualization method of transition matrix reveal that S7 and S14 are charge transfer excited states, S16 are the localized excited state, where the quarter-thiophene and fullerene are simultaneously excited within their own localization excitation, there is no charge transfer between them. For S17, it is localized excited state, where electron-hole are localized in the quarter-thiophene. Transition density matrix can only reveal the electron-hole coherence, but can not observe the orientation of charge transfer, so we try to observe the orientation of charge transfer and results of charge transfer, using 3D charge difference density (CDD). CDD of S7 and S14 demonstrate they are charge transfer excited state, where the electrons transfer to fullerene from quarter-thiophene. CDD of S16 demonstrate that fullerene and quarter-thiophene are simultaneously excited, and there both electrons and holes, they are localized excitation themselves. While, for S17, we can see that it is localization excitation on quarter-thiophene, where all the electrons and hole are localized in quarter-thiophene. So, the 2D transition density matrix and 3D charge difference density can reveal the electron-hole coherence and orientation of charge transfer, respectively. which can characterize the optical properties in OPA. TDM
TDM
Fullerene
1.5x10-3
4-Thiophene
1.8x10-3
1.6x10-3
1.4x10-3
1.2x10-3
1.3x10-3
1.0x10-3
1.0x10-3
8.1x10-4
7.5x10-4
5.0x10-4
Fullerene
4-Thiophene
2.0x10-3
6.1x10-4
4.0x10-4
2.0x10-4
2.5x10-4
0.0
Fullerene
4-Thiophene
S7
0.0
Fullerene
4-Thiophene
S14
TDM
TDM
Fullerene
3.8x10
4-Thiophene
4.4x10
5.0x10
4.4x10
3.8x10
3.1x10
3.1x10
2.5x10
2.5x10
1.9x10
1.3x10
Fullerene
4-Thiophene
5.0x10
1.9x10
1.3x10
6.3x10
6.3x10
0.0
Fullerene
4-Thiophene
0.0
Fullerene
S16
4-Thiophene
S17
Fig. 3 Transition density matrixes for S7, S14, S16 and S17.
S7
S14
S16
S17
Fig. 4 Charge difference density for S7, S14, S16 and S17, where the purple and pink stand for hole and electron, respectively.
For the weak S6 excited state of octant-thiophene-fullerene in Fig. 2(b) in OPA, Fig. 5 reveals that it is charge transfer excited state, where electrons transfer from octant-thiophene to fullerene. The closer to fullerene, the ability of charge transfer is stronger for the octant-thiophene. For the strong absorption of S9 in OPA in Fig. 2(b), it is demonstrated that S9 is the localized excited state (see Fig. 5), where the
electron-hole are localized within octant-thiophene. TDM
TDM
8.8x10
7.5x10-4
Fullerene
6.3x10-4
3.0x10-3
8-thiophene
-4
2.6x10-3 2.3x10-3 1.9x10-3
5.0x10-4
1.5x10-3
3.8x10-4
1.1x10-3
2.5x10-4
Fullerene
8-thiophene
1.0x10-3
7.5x10-4
1.3x10-4
Fullerene
3.8x10-4
0.0
8-thiophene
Fullerene
S6
5.
Transition
0.0
S9
S6 Fig.
8-thiophene
S9 density
matrix
and
charge
difference
density
for
octant-thiophene-fullerene in OPA.
3.3 Optical properties in TPA For quarter-thiophene-fullerene system in TPA, the S7 and S14 are one step electronic transition S7-S0 and S14<-S0, and the second contributions are from the
∆µ fg
in the second term in Eq. (1). For these excited state, the electron-hole coherence and charge transfer have been characterized in Fig. 3 and Fig. 4. Here, we mainly consider the optical properties of S18, which is two-step transitions and in two channels, see Fig. 6. So, the contribution for this excited state in TPA is contributed from the first term in Eq. (1).The first channel is S18<-S16<-S0, and the first step is the localized excited state, where the electron-hole are localized within their own parts, so for this channel, it is localized excited state. For the channel of S18<-S17<-S0, we can see that the first step of S17<-S0, it is localized excited state, where all electrons and holes are localized
quarter-thiophene, while for the second step of S18<-S17, it is intramolecular charge transfer excited state, where electrons excited in quarter-thiophene in the first step can be efficiently transfer to fullerene in the second step. The ratio of the charge transfer channel is R
68%, according to the data in Table in Fig. 6. So, charge
transfer channel is the dominant for S18 in TPA.
Fig. 6 Transition density matrixes and charge difference density for S18 in TPA.
It is great challenge to increase the ratio of charge transfer excited state in TPA. One of potential methods is to increase the length of donor. In TPA in Fig. 2(d), S6 is of strong absorption, which is mainly contributed from the second term in Eq. (1), S6 is strong charge transfer excited state in OPA in Fig. 2(b), while for TPA, this large charge transfer from octant-thiophene to fullerene results in very large difference permanent dipole moment between excited and ground state, which is
∆µ fg = 14.5
au.
For the excited state of S17 in TPA, there are two step transitions in two channels in TPA, which are S17<-S9<-S0 and S17<-S6<-S0, so the contribution for this excited state in TPA is contributed from the first term in Eq. (1). For the channel of S17<-S6<-S0, the first step transition of S6<-S0, it is charge transfer excited state, where electrons transfer from octant-thiophene to fullerene; while for the second step transition, which is the localized excitation on octant-thiophene.
For the channel of
S17<-S6<-S0 in TPA, it is localized excitation on octant-thiophene; and for the second
transition channel of S17<-S9<-S0, it is intramolecular charge transfer excited state, wherer electrons transfer from octant-thiophene fullerene. Note that though these two transition channels in TPA are the charge transfer excited states; the sequence of charge transfer in TPA are different; which occurs in the first and second step transitions, respectively. In the octant-thiophene-fullerene system, we can see R
100%, with the increase of donor length.
Fig. 7 Transition density matrixes and charge difference density for S17 in TPA.
4. Conclusion Donor-length dependent charge transfer in quarter-thiophene-fullerene and octant-thiophene-fullerene is investigated with 2D transition density matrix and 3D charge different density. It is found that some electronic transitions in TPA are contributed the first term; while some electronic transitions in TPA are from second terms in Eq. (1). The most important conclusion is that with the increase of donor-length, the ratio of charge transfer excited state can be significantly enhanced. This is because longer conjugated donors provide a smaller energy level difference when light is incident. Our results are helpful to rationally design the strong charge transfer excited state in donor-acceptor system in TPA.
Acknowledgements: This work was supported by the National Natural Science Foundation of China (Grant No: 91436102 and 11374353), the Fundamental Research Funds for the Central Universities in China, Key Projects of Basic Research and Applied Basic Research in Universities of Guangdong Province (Grant No: 2018KZDXM046), and the Natural Science Foundation of Guangdong Province, China (Grant No: 2017A030313022). References: 1. J. L. Brédas, D. Beljonne, V. Coropceanu, A. Cornil, Charge-transfer and energy-transfer processes in π-conjugated oligomers and polymers: a molecular picture, J. Chem. Rev. 2004, 104, 4971. 2. M. Sun, Control of structure and photophysical properties by protonation and subsequent intramolecular hydrogen bonding, J. Chem. Phys. 2006, 124, 054903. 3. M. Sun, Y. Ding, G. Cui, Y. Liu, S1 and S2 Excited States of Gas-Phase Schiff-Base Retinal Chromophores: A Time-Dependent Density Functional Theoretical Investigation, The Journal of Physical Chemistry A 2007, 111, 15, 2946-2950. 4. M. Sun, L. Liu, Y. Ding, H.Xu, Do coupling exciton and oscillation of electron-hole pair exist in neutral and charged π-dimeric quinquethiophenes? J. Chem. Phys. 2007, 127, 084706. 5. M. Sun, Y. Ding, H. Xu, Direct Visual Evidence for Quinoidal Charge Delocalization in Poly-p-phenylene Cation Radical, J. Phys. Chem. B 2007 111, 13266-13270. 6. M. Sun, J. Chen, H. Xu, Visualizations of transition dipoles, charge transfer, and electron-hole coherence on electronic state transitions between excited states for two-photon absorption, J. Chem. Phys. 2008, 128, 064106.
7. Z. Yang, Y. Li, Z. Li, D. Wu, J.Kang, H. Xu, M. Sun, Surface enhanced Raman scattering of pyridine adsorbed on Au@Pd core/shell nanoparticles, J. Chem. Phys. 2009, 130, 234705. 8. S. Zhang, Y. Fang, Z. Yang, D. Wu, B. Dong, H. Xu, Near- and Deep-Ultraviolet Resonance Raman Spectroscopy of Pyrazine−Al4 Complex and Al3−Pyrazine−Al3 Junction, The Journal of Physical Chemistry C, 2009, 113, 44, 19328-19334. 9. Y. Li, T. Pullerits, M. Zhao, M. Sun, Theoretical Characterization of the PC60 BM:PDDTT Model for an Organic Solar Cell , The Journal of Physical Chemistry C, 2011, 115, 44, 21865-21873. 10. L. Xia, D. Li, Y. Li, P. Song, M. Zhao, Y. Ding, Y. Li, F. Ma, L. Liu, M. Sun, Visualizations of charge transfer for the model of poly(3,4-alkylenedioxythiophene)s in neutral and various oxidation states, RSC Adv., 2012,2, 12983-12988.
11. Y. Feng, L. Meng, J. Zhao, Y. Li, M. Sun, J. Chen, Bioorganic dye-sensitized solar cell of carotenoid–pheophytin a–TiO2, RSC Adv., 2014,4, 63016-63024. 12. P. Song, Y. Li, F. Ma, T. Pullerits, M. Sun, External Electric Field-Dependent Photoinduced Charge Transfer in a Donor–Acceptor System for an Organic Solar Cell, The Journal of Physical Chemistry C ， 2013, 117, 15879-15889. 13. P. Song, Y. Li, F. Ma, M. Sun, Insight into external electric field dependent photoinduced intermolecular charge transport in BHJ solar cell materials, J. Mater. Chem. C, 2015,3, 4810-4819. 14. Y. Li, Y. Feng, M. Sun, Photoinduced Charge Transport in a BHJ Solar Cell Controlled by an External Electric Field, Sci. Rep. 2015, 5, 13970. 15. P. Song, Y. Li, F. Ma, T. Pullerits, M. Sun, Photoinduced Electron Transfer in Organic Solar Cells, The Chemical Record, 2016, 16, 734-753. 16. P. Song, Q. Zhou, Y. Li, F. Ma, M. Sun, Vibronic quantized tunneling controlled photoinduced
17.
18.
19.
20.
electron transfer in an organic solar cell subjected to an external electric field, Physical Chemistry Chemical Physics, 2017, 19, 16105. J. Wang, W. Lin, P. Song, Q. Zhou, Y. Sun, X. Zhang, W. Liang, M. Sun, E. Cao, Fluorescence Resonance Energy Transfer of Monomer via Photoisomerization, ChemistrySelect, 2017, 2 6446-6451. X. Mu, K. Cai, W. Wei, Y. Li, Z. Wang, J. Wang, Dependence of UV–Visible Absorption Characteristics on the Migration Distance and the Hyperconjugation Effect of a Methine Chain, The Journal of Physical Chemistry C 2018, 122, 7831-7837. H. Zong, X. Wang, J. Quan, C. Tian, M. Sun, Photoinduced charge transfer by one and two-photon absorptions: physical mechanisms and applications, Phys. Chem. Chem. Phys., 2018,20, 19720-19743. H. Zong, J. Wang, X. Mu, X. Xu, J. Li, X. Wang, F. Long, J. Wang, Mengtao Sun, Physical mechanism of photoinduced intermolecular charge transfer enhanced by fluorescence resonance energy transfer, Phys. Chem. Chem. Phys., 2018, 20, 13558-13565.
21. X. Mu, J. Wang, M. Sun, Visualization of Photoinduced Charge Transfer and Electron–Hole Coherence
in
Two-Photon
Absorption,
The
Journal
of
Physical
Chemistry
C, 2019, 123, 23, 14132-14143 22. X. Mu, J. Wang, G. Duan, Z. Li, J. W, M. Sun, The nature of chirality induced by molecular aggregation and self-assembly, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2019, 212, 188-198. 23. Y. Li, B. Xu, P. Song, F. Ma, M. Sun, D–A−π–A System: Light Harvesting, Charge Transfer, and Molecular Designing, The Journal of Physical Chemistry C , 2017, 121, 12546-12561. 24. H. Zong, X. Mu, L. Zhu, M. Sun, External electric field dependent photoinduced charge transfer in donor-acceptor system in two photon absorption, J. Mater. Chem. C, 2019, submitted.
25. B. H. Cumpston, S. P. S. Barlow, D. L. Dyer, J. E. L. L. Erskine, A. A. Heikal, S. M. Kuebler, I. Y. S. Lee, D. McCord Maughon, J. Qin, H. Röckel, M. Rumi, X. L. Wu, S. R. Marder, J. W. Perry, Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication, Nature , 1999, 398, 51. 26. Maruo, S.; Nakamura, O.; Kawata, S.; Three-dimensional microfabrication with two-photon-absorbed photopolymerization, Optics Letters, 22, 1997, 132-134. 27. W. Kaiser, C. G. B. Garrett, Two-Photon Excitation in CaF2:Eu2+, Phys. Rev. Lett. 1961, 7, 229 – 231. 28. W. Denk, J. H.Strickler, W. W. Webb, Two-photon laser scanning fluorescence microscopy, Science, 1990,248, 73.29. 29. M. Sun, Y.Chen, P. Song, F. Ma, Intramolecular charge transfer and locally excited states of
30. 31.
32.
33.
34. 35. 36.
the fullerene-linked quarter-thiophenes dyad, Chemical Physics Letters, 2005, 413, 110-117 M. Sun, P. Song, Y. Chen, F. Ma, Intramolecular charge transfer in the porphyrin– oligothiophene–fullerene triad, Chemical Physics Letters, 2005, 416, 94-99. Shi, X., Yang, Y., Wang, L., Li, Y. Introducing Asymmetry Induced by Benzene Substitution in a Rigid Fused π Spacer of D− π–A-Type Solar Cells: A Computational Investigation. The Journal of Physical Chemistry C, 2019,123, 4007-4021. Ren, P., Sun, C., Shi, Y., Song, P., Yang, Y., Li, Y. Global performance evaluation of solar cells using two models: from charge-transfer and recombination mechanisms to photoelectric properties. Journal of Materials Chemistry C, 2019, 7, 1934-1947. Mu, X., Chen, X., Wang, J., & Sun, M. (2019). Physical Mechanism of Electric and Magnetic Interactions for Chiral Molecules in ECD and ROA. The Journal of Physical Chemistry A. 2019, DOI: 10.1021/acs.jpca.9b06674 Frisch, M. J.; Trucks, G.W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman,J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A., Gaussian 09, 2009. Wallingford CT. W. Kohn and L. J. Sham, Self-Consistent Equations Including Exchange and Correlation Effects, Phys. Rev., 1965, 140, A1133-A38. Becke,A. D., Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior, Phys. Rev. A, 1988, 38, 3098–3100.
37. Gross, E. K. U.; Kohn, W., Local Density-Functional Theory of Frequency-Dependent Linear Response, Phys. Rev. Lett., 1986, 55, 2850–2852. 38. Yanai, T.; Tew, D. P.; Handy, N. C., A New Hybrid Exchange-Correlation Functional Using the Coulomb-Attenuating Method (CAM-B3LYP), Chem. Phys. Lett., 2004, 393, 51–57. 39. M. Sun, y. Ding, L. Zhao, F. Ma, Photoexcitation mechanisms of centrosymmetric and asymmetric fluorene derivatives in two-photon absorption, Chemical Physics, 2009, 359, 166-172.
Highlight： In this paper, we study the donor-length dependent photoinduced charge transfer in two-photon absorption (TPA). In the donor-acceptor (D-A) system of oligo-thiophene-fullerene, two kind of lengths of oligo-thiophene are chosen to study the donor-length dependent photoinduced charge transfer in TPA. We conducted a detailed study of photoinduced charge transfer in two-photon absorption using 2D and 3D visual wave function analysis methods. Two-photon absorption is a two-step transition. We analyzed the degree of charge transfer for each absorption peak in the two-photon absorption spectrum. Photoinduced charge transfer, photoinduced charge recombination, and localized excitation-enhanced charge transfer processes were found in the two-step transition. The most important conclusion is that with the increase of donor-length, the ratio of charge transfer excited state can be significantly enhanced.
S1386-1425(19)30921-7
DOI:
https://doi.org/10.1016/j.saa.2019.117531
Reference:
SAA 117531
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: 16 June 2019 Revised Date:
9 September 2019
Accepted Date: 12 September 2019
Please cite this article as: C. Tian, X. Wang, X. Mu, J. Quan, Donor-length dependent photoninduced charge transfer in two-photon absorption, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2019), doi: https://doi.org/10.1016/j.saa.2019.117531. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier B.V. All rights reserved.
Donor-length dependent photoninduced charge transfer in two-photon absorption
Chunhua Tian,1,+ Xinxin Wang,2,+ Xijiao Mu2,* Jun Quan1,*
1. School of Physical Science and Technology, Lingnan Normal University, Zhanjiang 524048, China 2. School of Mathematics and Physics, Center for Green Innovation, Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, University of Science and Technology, Beijing, Beijing 100083, PR China *
Corresponding
authors.
Email:
[email protected]
(X.
Mu)
and
[email protected] (J. Quan).
Abstract In this paper, we study the donor-length dependent photoinduced charge transfer in two-photon absorption (TPA). In the donor-acceptor (D-A) system of oligo-thiophene-fullerene, two kind of lengths of oligo-thiophene are chosen to study the donor-length dependent photoinduced charge transfer in TPA. It is found that donor length can significantly influence the rate of charge transfer. When the donor length is quarter-thiophene, the rate of charge transfer is less than 50%; while with the increase of donor-length to octant-thiophene, the rate of charge transfer can be reach up to 100%. The transition channels of photoinduced charge transfer in TPA are visualized with transition density matrix and charge difference density. Our results promote deeper understanding and rational design for donor-acceptor system with large rate of photoinduced charge transfer in TPA.
Key words: Donor-acceptor; Donor-length; photoinduced charge transfer; two-photon absorption
1. Introduction Photoinduced charge transfer in one-photon absorption (OPA) and two-photon absorption (TPA) is very important for the process of charge and energy transfer in the nonlinear optical absorption and fluorescence in system of biology and materials.1-24 TPA is a third order nonlinear optical phenomenon in which a molecule simultaneously absorbs two photons, which can be quantified by the TPA cross section, a quantity that is proportional to the imaginary component of the χ3 tensor (or, on a molecular level, to the imaginary component of γ) and depends on the photon energy and energies.6,21 The TPA excitation is confined to a volume on the order of the cube of the wavelength of the excitation light, which provides good resolution in three dimensions.6, 21, 25, 26 The excitation rate for TPA and the intensity of the two-photon induced fluorescence decrease as the forth power of the distance from the focal plane. Another advantage is that TPA processes can provide improved penetration of light into absorbing materials.
27
The ability to penetrate a material and to be focused
accurately in three dimensions make the TPA process ideal for fluorescence imaging of thick samples (even in vivo) or in medical applications in which, for example, a drug can be activated by TPA at a very precise location without affecting the tissue above and below the focal plane of the excitation beam. 28 It is great challenge to visualize the processes of photoinduced charge transfer in TPA, while it is very important to judge whether the electronic transitions are the charge transfer excited states in each step of TPA. Recently, Mu and coworkers developed programs to visualize the whether the electronic transitions are the charge transfer excited states in each step in every channel in TPA,21 which can provide definite and clear pictures of charge transfer in TPA, and charge transfer and electron-hole coherence in TPA are well revealed.
Figure 1. The quarter-thiophene-fullerene (a) and octant-thiophene-fullerene (b) as donor (green color) and acceptor (red color) system. In this paper, we visually investigate the photoinduced charge transfer in ologi-thiophene-fullerene dyad29~33 as donor-acceptor system of in TPA, using visualization program developed by Mu. The two molecular heads are fullerenes and the molecular tails are thiophene multimers. This molecular structure is to investigate the effect of polymer chain length on charge transfer in two-photon absorption. Firstly, the optical spectra in one-photon absorption (OPA) and two-photon absorption (TPA) of dyad are simulated. Secondly, photoinduced intra-molecular charge transfer in each channel in every step in TPA is visualized with visualization program of two dimensional (2D) and three dimensional (3D) methods. Our results can provide clearly evidence for the charge transfer in TPA.
2. Method All the quantum chemical calculations are done with Gaussian 16.34 The ground state geometries of them in Fig. 1 are optimized with density functional theory (DFT),35 dispersion correction, B3LYP functional36 and 6-31(d) basis set. The electronic transitions of them are calculated with time-dependent DFT (TD-DFT),37 CAM-B3LYP functional38 and 6-31(d) basis set.
The optical absorption spectra ( δ tp ) of them in TPA are calculated with,6, 21, 39 δtp = 8∑ j≠g j≠ f
where and
g
,
j
ω j and ω f
the lifetime.
θj
and
f µ j
2
jµ g
ωf 2 ω j − + Γ f 2 2
f
2
(1 + 2cos θ ) + 8 2
j
∆µ fg
2
f µ g
ωf 2 + Γf 2 2
2
(1 + 2cos φ ) 2
(1)
are the ground, intermediate and final states, respectively;
are the intermediate and final excitation energy in OPA, and is the angle between two transition dipole moments, and
φ
Γf
is
is the
angle between transition dipole moment and difference of excited and ground permanent dipole moments. The FWHM of the two-photon absorption spectrum was calculated to be 25 nm. The physical descriptions of 2D and 3D visualization methods of transition density matrix (TDM) and charge difference density can refer to [21].
3. Results and discussion 3.1 Optical absorption spectra in OPA and TPA Fig. 2 (a)-(d) are the optical absorption spectra of quarter-thiophene-fullerene, octant-thiophene-fullerene in OPA and TPA, respectively. Comparing Fig. 2(a) and 2(b) for in OPA It is found that strong peak of optical absorption will be red-shifted from 395 nm to 480 nm with the increase of donor length, which reveals that the donor length can efficiently tune the position of absorption peak. For quarter-thiophene-fullerene, the absorption spectra of them are similar in OPA in Fig. 2(a) and TPA in Fig. 2(c), except for the half energy for TPA. The electronic transition S18 is from S18<-S17
dipole moments between excited state and ground state in the second term in Eq. (1), ∆µ fg = 14.5
where
a.u. The second strong peak in 815 nm in TPA is contributed from
two channels transitions: S17<-S6<-S0 and S17<-S9<-S0. For the S17<-S9<-S0, the intermediate excited state S9 is strong absorption peak in OPA, which is a localized excited state; while the intermediate excited state S6 is a weak intramolecular charge transfer excited state, where the electrons transfer from ologo-thiophene to fullerene , which will be shown later.
(a)
3.5 S9
(b) 1.25
8.0x10
5
6.0x10
5
4.0x10
5
2.0x10
5
3.0
Oscillator Strengths
1.00 5
6.0x10
0.75
5
4.0x10
0.50
5
2.0x10
0.25
S16 S15+S14
0.0 375
400
425
UV-vis spectrum
UV-vis spectrum
5
8.0x10
2.5 2.0 1.5 1.0
450
475
0.0 400
0.00
500
425
3x10
6
2x10
6
1x10
S18
sum 1 2
6
S7
S14
0 750
8x10 7x10
3
6x10
3
5x10
3
4x10
3
3x10
3
2x10
3
1x10
3
0
800
850
900
2.
The
optical
500
525
0.0 550
950
1000
4
8.0x10 (d)
1.4x10
sum 1 4 1.2x10 2
S6
S17 6
6.0x10
4
1.0x10
3
8.0x10 6
4.0x10
3
6.0x10
3
4.0x10
6
2.0x10
S9 S12
0.0 800
3
2.0x10
S7
0.0
850
Wavelength [nm]
Figure
475
6
3
Two-photo Absorption cross-section
4x10
(c)
450
Wavelength [nm]
Molar Absorption coefficient [L/cm/mol]
Molar Absorption coefficient [L/cm/mol]
Wavelength [nm]
6
0.5
S6 S7
900
950
1000
1050
Two-photo Absorption cross-section
1.0x10
6
S17
Oscillator strengths
1.0x10
1.50 6
1100
Wavelength [nm]
absorption
spectra
of
quarter-thiophene-fullerene,
octant-thiophene-fullerene in OPA (a, b) are and TPA (c, d), respectively.
3.2 Photoinduced charge transfer in OPA Optical properties of quarter-thiophene-fullerene and octant-thiophene-fullerene are analyzed with 2D and 3D visualization methods. For quarter-thiophene-fullerene in OPA, optical properties of S7, S14, S16 and S17 are analyzed, since S17 is strong absorption in OPA in Fig. 2(a), S7 and S14 are clearly observed in TPA in Fig. 2(c). 2D
visualization method of transition matrix reveal that S7 and S14 are charge transfer excited states, S16 are the localized excited state, where the quarter-thiophene and fullerene are simultaneously excited within their own localization excitation, there is no charge transfer between them. For S17, it is localized excited state, where electron-hole are localized in the quarter-thiophene. Transition density matrix can only reveal the electron-hole coherence, but can not observe the orientation of charge transfer, so we try to observe the orientation of charge transfer and results of charge transfer, using 3D charge difference density (CDD). CDD of S7 and S14 demonstrate they are charge transfer excited state, where the electrons transfer to fullerene from quarter-thiophene. CDD of S16 demonstrate that fullerene and quarter-thiophene are simultaneously excited, and there both electrons and holes, they are localized excitation themselves. While, for S17, we can see that it is localization excitation on quarter-thiophene, where all the electrons and hole are localized in quarter-thiophene. So, the 2D transition density matrix and 3D charge difference density can reveal the electron-hole coherence and orientation of charge transfer, respectively. which can characterize the optical properties in OPA. TDM
TDM
Fullerene
1.5x10-3
4-Thiophene
1.8x10-3
1.6x10-3
1.4x10-3
1.2x10-3
1.3x10-3
1.0x10-3
1.0x10-3
8.1x10-4
7.5x10-4
5.0x10-4
Fullerene
4-Thiophene
2.0x10-3
6.1x10-4
4.0x10-4
2.0x10-4
2.5x10-4
0.0
Fullerene
4-Thiophene
S7
0.0
Fullerene
4-Thiophene
S14
TDM
TDM
Fullerene
3.8x10
4-Thiophene
4.4x10
5.0x10
4.4x10
3.8x10
3.1x10
3.1x10
2.5x10
2.5x10
1.9x10
1.3x10
Fullerene
4-Thiophene
5.0x10
1.9x10
1.3x10
6.3x10
6.3x10
0.0
Fullerene
4-Thiophene
0.0
Fullerene
S16
4-Thiophene
S17
Fig. 3 Transition density matrixes for S7, S14, S16 and S17.
S7
S14
S16
S17
Fig. 4 Charge difference density for S7, S14, S16 and S17, where the purple and pink stand for hole and electron, respectively.
For the weak S6 excited state of octant-thiophene-fullerene in Fig. 2(b) in OPA, Fig. 5 reveals that it is charge transfer excited state, where electrons transfer from octant-thiophene to fullerene. The closer to fullerene, the ability of charge transfer is stronger for the octant-thiophene. For the strong absorption of S9 in OPA in Fig. 2(b), it is demonstrated that S9 is the localized excited state (see Fig. 5), where the
electron-hole are localized within octant-thiophene. TDM
TDM
8.8x10
7.5x10-4
Fullerene
6.3x10-4
3.0x10-3
8-thiophene
-4
2.6x10-3 2.3x10-3 1.9x10-3
5.0x10-4
1.5x10-3
3.8x10-4
1.1x10-3
2.5x10-4
Fullerene
8-thiophene
1.0x10-3
7.5x10-4
1.3x10-4
Fullerene
3.8x10-4
0.0
8-thiophene
Fullerene
S6
5.
Transition
0.0
S9
S6 Fig.
8-thiophene
S9 density
matrix
and
charge
difference
density
for
octant-thiophene-fullerene in OPA.
3.3 Optical properties in TPA For quarter-thiophene-fullerene system in TPA, the S7 and S14 are one step electronic transition S7-S0 and S14<-S0, and the second contributions are from the
∆µ fg
in the second term in Eq. (1). For these excited state, the electron-hole coherence and charge transfer have been characterized in Fig. 3 and Fig. 4. Here, we mainly consider the optical properties of S18, which is two-step transitions and in two channels, see Fig. 6. So, the contribution for this excited state in TPA is contributed from the first term in Eq. (1).The first channel is S18<-S16<-S0, and the first step is the localized excited state, where the electron-hole are localized within their own parts, so for this channel, it is localized excited state. For the channel of S18<-S17<-S0, we can see that the first step of S17<-S0, it is localized excited state, where all electrons and holes are localized
quarter-thiophene, while for the second step of S18<-S17, it is intramolecular charge transfer excited state, where electrons excited in quarter-thiophene in the first step can be efficiently transfer to fullerene in the second step. The ratio of the charge transfer channel is R
68%, according to the data in Table in Fig. 6. So, charge
transfer channel is the dominant for S18 in TPA.
Fig. 6 Transition density matrixes and charge difference density for S18 in TPA.
It is great challenge to increase the ratio of charge transfer excited state in TPA. One of potential methods is to increase the length of donor. In TPA in Fig. 2(d), S6 is of strong absorption, which is mainly contributed from the second term in Eq. (1), S6 is strong charge transfer excited state in OPA in Fig. 2(b), while for TPA, this large charge transfer from octant-thiophene to fullerene results in very large difference permanent dipole moment between excited and ground state, which is
∆µ fg = 14.5
au.
For the excited state of S17 in TPA, there are two step transitions in two channels in TPA, which are S17<-S9<-S0 and S17<-S6<-S0, so the contribution for this excited state in TPA is contributed from the first term in Eq. (1). For the channel of S17<-S6<-S0, the first step transition of S6<-S0, it is charge transfer excited state, where electrons transfer from octant-thiophene to fullerene; while for the second step transition, which is the localized excitation on octant-thiophene.
For the channel of
S17<-S6<-S0 in TPA, it is localized excitation on octant-thiophene; and for the second
transition channel of S17<-S9<-S0, it is intramolecular charge transfer excited state, wherer electrons transfer from octant-thiophene fullerene. Note that though these two transition channels in TPA are the charge transfer excited states; the sequence of charge transfer in TPA are different; which occurs in the first and second step transitions, respectively. In the octant-thiophene-fullerene system, we can see R
100%, with the increase of donor length.
Fig. 7 Transition density matrixes and charge difference density for S17 in TPA.
4. Conclusion Donor-length dependent charge transfer in quarter-thiophene-fullerene and octant-thiophene-fullerene is investigated with 2D transition density matrix and 3D charge different density. It is found that some electronic transitions in TPA are contributed the first term; while some electronic transitions in TPA are from second terms in Eq. (1). The most important conclusion is that with the increase of donor-length, the ratio of charge transfer excited state can be significantly enhanced. This is because longer conjugated donors provide a smaller energy level difference when light is incident. Our results are helpful to rationally design the strong charge transfer excited state in donor-acceptor system in TPA.
Acknowledgements: This work was supported by the National Natural Science Foundation of China (Grant No: 91436102 and 11374353), the Fundamental Research Funds for the Central Universities in China, Key Projects of Basic Research and Applied Basic Research in Universities of Guangdong Province (Grant No: 2018KZDXM046), and the Natural Science Foundation of Guangdong Province, China (Grant No: 2017A030313022). References: 1. J. L. Brédas, D. Beljonne, V. Coropceanu, A. Cornil, Charge-transfer and energy-transfer processes in π-conjugated oligomers and polymers: a molecular picture, J. Chem. Rev. 2004, 104, 4971. 2. M. Sun, Control of structure and photophysical properties by protonation and subsequent intramolecular hydrogen bonding, J. Chem. Phys. 2006, 124, 054903. 3. M. Sun, Y. Ding, G. Cui, Y. Liu, S1 and S2 Excited States of Gas-Phase Schiff-Base Retinal Chromophores: A Time-Dependent Density Functional Theoretical Investigation, The Journal of Physical Chemistry A 2007, 111, 15, 2946-2950. 4. M. Sun, L. Liu, Y. Ding, H.Xu, Do coupling exciton and oscillation of electron-hole pair exist in neutral and charged π-dimeric quinquethiophenes? J. Chem. Phys. 2007, 127, 084706. 5. M. Sun, Y. Ding, H. Xu, Direct Visual Evidence for Quinoidal Charge Delocalization in Poly-p-phenylene Cation Radical, J. Phys. Chem. B 2007 111, 13266-13270. 6. M. Sun, J. Chen, H. Xu, Visualizations of transition dipoles, charge transfer, and electron-hole coherence on electronic state transitions between excited states for two-photon absorption, J. Chem. Phys. 2008, 128, 064106.
7. Z. Yang, Y. Li, Z. Li, D. Wu, J.Kang, H. Xu, M. Sun, Surface enhanced Raman scattering of pyridine adsorbed on Au@Pd core/shell nanoparticles, J. Chem. Phys. 2009, 130, 234705. 8. S. Zhang, Y. Fang, Z. Yang, D. Wu, B. Dong, H. Xu, Near- and Deep-Ultraviolet Resonance Raman Spectroscopy of Pyrazine−Al4 Complex and Al3−Pyrazine−Al3 Junction, The Journal of Physical Chemistry C, 2009, 113, 44, 19328-19334. 9. Y. Li, T. Pullerits, M. Zhao, M. Sun, Theoretical Characterization of the PC60 BM:PDDTT Model for an Organic Solar Cell , The Journal of Physical Chemistry C, 2011, 115, 44, 21865-21873. 10. L. Xia, D. Li, Y. Li, P. Song, M. Zhao, Y. Ding, Y. Li, F. Ma, L. Liu, M. Sun, Visualizations of charge transfer for the model of poly(3,4-alkylenedioxythiophene)s in neutral and various oxidation states, RSC Adv., 2012,2, 12983-12988.
11. Y. Feng, L. Meng, J. Zhao, Y. Li, M. Sun, J. Chen, Bioorganic dye-sensitized solar cell of carotenoid–pheophytin a–TiO2, RSC Adv., 2014,4, 63016-63024. 12. P. Song, Y. Li, F. Ma, T. Pullerits, M. Sun, External Electric Field-Dependent Photoinduced Charge Transfer in a Donor–Acceptor System for an Organic Solar Cell, The Journal of Physical Chemistry C ， 2013, 117, 15879-15889. 13. P. Song, Y. Li, F. Ma, M. Sun, Insight into external electric field dependent photoinduced intermolecular charge transport in BHJ solar cell materials, J. Mater. Chem. C, 2015,3, 4810-4819. 14. Y. Li, Y. Feng, M. Sun, Photoinduced Charge Transport in a BHJ Solar Cell Controlled by an External Electric Field, Sci. Rep. 2015, 5, 13970. 15. P. Song, Y. Li, F. Ma, T. Pullerits, M. Sun, Photoinduced Electron Transfer in Organic Solar Cells, The Chemical Record, 2016, 16, 734-753. 16. P. Song, Q. Zhou, Y. Li, F. Ma, M. Sun, Vibronic quantized tunneling controlled photoinduced
17.
18.
19.
20.
electron transfer in an organic solar cell subjected to an external electric field, Physical Chemistry Chemical Physics, 2017, 19, 16105. J. Wang, W. Lin, P. Song, Q. Zhou, Y. Sun, X. Zhang, W. Liang, M. Sun, E. Cao, Fluorescence Resonance Energy Transfer of Monomer via Photoisomerization, ChemistrySelect, 2017, 2 6446-6451. X. Mu, K. Cai, W. Wei, Y. Li, Z. Wang, J. Wang, Dependence of UV–Visible Absorption Characteristics on the Migration Distance and the Hyperconjugation Effect of a Methine Chain, The Journal of Physical Chemistry C 2018, 122, 7831-7837. H. Zong, X. Wang, J. Quan, C. Tian, M. Sun, Photoinduced charge transfer by one and two-photon absorptions: physical mechanisms and applications, Phys. Chem. Chem. Phys., 2018,20, 19720-19743. H. Zong, J. Wang, X. Mu, X. Xu, J. Li, X. Wang, F. Long, J. Wang, Mengtao Sun, Physical mechanism of photoinduced intermolecular charge transfer enhanced by fluorescence resonance energy transfer, Phys. Chem. Chem. Phys., 2018, 20, 13558-13565.
21. X. Mu, J. Wang, M. Sun, Visualization of Photoinduced Charge Transfer and Electron–Hole Coherence
in
Two-Photon
Absorption,
The
Journal
of
Physical
Chemistry
C, 2019, 123, 23, 14132-14143 22. X. Mu, J. Wang, G. Duan, Z. Li, J. W, M. Sun, The nature of chirality induced by molecular aggregation and self-assembly, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2019, 212, 188-198. 23. Y. Li, B. Xu, P. Song, F. Ma, M. Sun, D–A−π–A System: Light Harvesting, Charge Transfer, and Molecular Designing, The Journal of Physical Chemistry C , 2017, 121, 12546-12561. 24. H. Zong, X. Mu, L. Zhu, M. Sun, External electric field dependent photoinduced charge transfer in donor-acceptor system in two photon absorption, J. Mater. Chem. C, 2019, submitted.
25. B. H. Cumpston, S. P. S. Barlow, D. L. Dyer, J. E. L. L. Erskine, A. A. Heikal, S. M. Kuebler, I. Y. S. Lee, D. McCord Maughon, J. Qin, H. Röckel, M. Rumi, X. L. Wu, S. R. Marder, J. W. Perry, Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication, Nature , 1999, 398, 51. 26. Maruo, S.; Nakamura, O.; Kawata, S.; Three-dimensional microfabrication with two-photon-absorbed photopolymerization, Optics Letters, 22, 1997, 132-134. 27. W. Kaiser, C. G. B. Garrett, Two-Photon Excitation in CaF2:Eu2+, Phys. Rev. Lett. 1961, 7, 229 – 231. 28. W. Denk, J. H.Strickler, W. W. Webb, Two-photon laser scanning fluorescence microscopy, Science, 1990,248, 73.29. 29. M. Sun, Y.Chen, P. Song, F. Ma, Intramolecular charge transfer and locally excited states of
30. 31.
32.
33.
34. 35. 36.
the fullerene-linked quarter-thiophenes dyad, Chemical Physics Letters, 2005, 413, 110-117 M. Sun, P. Song, Y. Chen, F. Ma, Intramolecular charge transfer in the porphyrin– oligothiophene–fullerene triad, Chemical Physics Letters, 2005, 416, 94-99. Shi, X., Yang, Y., Wang, L., Li, Y. Introducing Asymmetry Induced by Benzene Substitution in a Rigid Fused π Spacer of D− π–A-Type Solar Cells: A Computational Investigation. The Journal of Physical Chemistry C, 2019,123, 4007-4021. Ren, P., Sun, C., Shi, Y., Song, P., Yang, Y., Li, Y. Global performance evaluation of solar cells using two models: from charge-transfer and recombination mechanisms to photoelectric properties. Journal of Materials Chemistry C, 2019, 7, 1934-1947. Mu, X., Chen, X., Wang, J., & Sun, M. (2019). Physical Mechanism of Electric and Magnetic Interactions for Chiral Molecules in ECD and ROA. The Journal of Physical Chemistry A. 2019, DOI: 10.1021/acs.jpca.9b06674 Frisch, M. J.; Trucks, G.W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman,J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A., Gaussian 09, 2009. Wallingford CT. W. Kohn and L. J. Sham, Self-Consistent Equations Including Exchange and Correlation Effects, Phys. Rev., 1965, 140, A1133-A38. Becke,A. D., Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior, Phys. Rev. A, 1988, 38, 3098–3100.
37. Gross, E. K. U.; Kohn, W., Local Density-Functional Theory of Frequency-Dependent Linear Response, Phys. Rev. Lett., 1986, 55, 2850–2852. 38. Yanai, T.; Tew, D. P.; Handy, N. C., A New Hybrid Exchange-Correlation Functional Using the Coulomb-Attenuating Method (CAM-B3LYP), Chem. Phys. Lett., 2004, 393, 51–57. 39. M. Sun, y. Ding, L. Zhao, F. Ma, Photoexcitation mechanisms of centrosymmetric and asymmetric fluorene derivatives in two-photon absorption, Chemical Physics, 2009, 359, 166-172.
Highlight： In this paper, we study the donor-length dependent photoinduced charge transfer in two-photon absorption (TPA). In the donor-acceptor (D-A) system of oligo-thiophene-fullerene, two kind of lengths of oligo-thiophene are chosen to study the donor-length dependent photoinduced charge transfer in TPA. We conducted a detailed study of photoinduced charge transfer in two-photon absorption using 2D and 3D visual wave function analysis methods. Two-photon absorption is a two-step transition. We analyzed the degree of charge transfer for each absorption peak in the two-photon absorption spectrum. Photoinduced charge transfer, photoinduced charge recombination, and localized excitation-enhanced charge transfer processes were found in the two-step transition. The most important conclusion is that with the increase of donor-length, the ratio of charge transfer excited state can be significantly enhanced.