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Abstract

The femtosecond up-conversion fluorescence measurement showed that DA1 and DA3 exhibited a rise process for the energetically downhill excitonic energy transfer (EET) in a timescale of sub-ps to several ps, while DA2 exhibited a decay process in such downhill EET. DA2 showed the shortest time constant of ~ 4.6 ps for torsional relaxation. This suggested that the vinylene linker has the fastest torsional relaxation from a flexible ground-state structure to a more rigid planar geometry, which may reduce conformational defects and improve exciton migration.

Scheme 1. Chemical structures and synthesis routes for DA1, DA2, and DA3.

Discussion

The fitted parameters from Figure 1 are summarized in Table 1. The fluorescence dynamics of DA1 and DA3 were both measured at the emission wavelengths of 580 nm and 620 nm, while the emission wavelengths of 600 nm and 640 nm were used for DA2. As shown in Table 1, the initial fast timescale, ranging from sub-ps to several ps, showed a rising dynamics for DA1 and DA3 at both detection wavelengths, but a decay process for DA2. This notably faster timescale was attributed to energetically downhill excitation energy transfer (EET) from high-energy to low-energy segments. It was noted that the fluorescence measured at shorter wavelengths exhibited a much faster rise or decay time than those at longer wavelengths for all the three samples. As the excitation energy migrates down to lower energy long segments, the possibility for further transfer to even lower energy long segments becomes less significant because there are fewer lower energy segments that exist to which the excitation energy can migrate when they keep moving energetically downhill. The rising dynamics at the emission of 580 nm and 620 nm for DA1 and DA3 indicated that conformational defects possibly slowed down the EET from 580 nm and 600 nm segments to even lower energy segments, therefore leading to a net rising dynamics. However, in DA2, the ultrafast (a couple of) ps timescale showed a decay process, which indicated that EET occurred very fast from 600 nm and 640 nm emission segments to even lower segments, leading to a net decay dynamics. The τ₂ processes at 38-89 ps for all samples may be assigned to the EET between the segments with comparable energy that was reported to occur on a time scale of approximately from ~10 ps to 100 ps.[1, 2]

Figure 1. Fluorescence dynamics detected at two different wavelengths for DA1 (580 nm and 620 nm), DA2 (600 nm and 640 nm) and DA3 (580 nm and 620 nm).

Table 1. The fitted parameters for fluorescence dynamics detected at two different wavelengths for DA1, DA2 and DA3, respectively.

Figure 2. Fluorescence dynamics detected at 580 nm, 640 nm, and 580 nm for DA1, DA2, and DA3, respectively.

Table 2. The fitted parameters for fluorescence dynamics detected at 580 nm, 640 nm, and 580 nm for DA1, DA2, and DA3, respectively.

The fitted parameters from Figure 2 are summarized in Table 2. As shown in Figure 2 and Table 2, the τ₁ component in DA1, DA2, and DA3 was observed at 13.2 ps, 4.6 ps and 8.1 ps with amplitude at 23.6%, 43.1%, and 17.8%, respectively. The τ₁ process in this work can be assigned to the torsional relaxation. Compared to DA1 (linked without any group) and DA3 (linked via an ethynylene group), DA2 (linked with a vinylene group) showed the shortest time constant of ~ 4.6 ps and the highest amplitude of ~ 43.1%. This suggested that the vinylene linker showed the fastest torsional relaxation from a flexible ground-state structure to a more rigid planar geometry, which could reduce conformational defects and improve exciton migration. The slow decay (τ₂) around several hundred ps in Figure 2 and Table 2 may be assigned to fluorescence from S1 to S0. Some other groups also reported that hundreds of ps can be assigned to the relaxation from S₁ to S₀ in oligothiophenes and polythiophenes.[2, 3]

Impact of my career

The ACS PRF DNI grant has helped the PI to successfully establish his laboratory and have provided critical startup funds to obtain preliminary data to seek additional grants in his early career. With the assistance of this ACS PRF DNI grant, the PI has been awarded the NSF CAREER award. In addition, the postdoc who has been supported by this ACS grant has also been greatly benefited by publishing results from this grant and has established a strong foundation for his career.

Acknowledgement

Acknowledgment is made to Donors of the American Chemical Society Petroleum Research Fund for support of this research.

References

[1] I.G. Scheblykin, A. Yartsev, T. Pullerits, V. Gulbinas, and V. Sundström, Excited State and Charge Photogeneration Dynamics in Conjugated Polymers. J. Phys. Chem. B. 111(2007) 6303-6321.

[2] Y. Xie, Y. Li, L. Xiao, Q. Qiao, R. Dhakal, Z. Zhang, Q. Gong, D. Galipeau, and X. Yan, Femtosecond Time-Resolved Fluorescence Study of P3HT/PCBM Blend Films. J. Phys. Chem. C. 114(2010) 14590-14600.

[3] T. Nakamura, Y. Araki, O. Ito, K. Takimiya, and T. Otsubo, Fluorescence Up-Conversion Study of Excitation Energy Transport Dynamics in Oligothiophene-Fullerene Linked Dyads. J. Phys. Chem. A. 112(2008) 1125-1132.