Abstract
Based on nonadiabatic molecular dynamics that integrate electronic transitions with the time-dependent phonon spectrum, this article provides a panoramic landscape of the dynamical process during the formation of photoinduced excitons in conjugated polymers. When external optical beam/pulses with intensities of 10 µJ/cm2 and 20 µJ/cm2 are utilized to excite a conjugated polymer, it is found that the electronic transition firstly triggers local lattice vibrations, which not only locally distort alternating bonds but change the phonon spectrum as well. Within the first 60 fs, the occurrence of local distortion of alternating bonds accompanies the localization of the excited-state’s electron. Up to 100 fs, both alternating bonds and the excited electronic state are well localized in the middle of the polymer chain. In the first ~200 fs, the strong lattice vibration makes a local phonon mode at 1097.7 cm−1 appear in the phonon spectrum. The change of electron states then induces the self-trapping effect to act on the following photoexcitation process of 1.2 ps. During the following relaxation of 1.0 ps, new local infrared phonon modes begin to occur. All of this, incorporated with the occurrence of local infrared phonon modes and localized electronic states at the end of the relaxation, results in completed exciton formation.
Highlights
The physical picture of the exciton formation can be viewed concisely as follows: due to external optical excitation, the electron in the highest-occupied molecular orbital (HOMO) of the conjugated polymer is excited to the lowestoccupied molecular orbital (LUMO) [1], such that each of two electronic orbitals is occupied by an electron
The occurrence of the electron-hole pair can be briefly described as follows: when a conjugated polymer is excited by an external beam whose energy just covers the bandgap between the HOMO and LUMO, the HOMO electron starts moving to the LUMO
Tracing back to previous articles [10,11,12], the experimental research finds that, when the conjugated polymer is excited by a laser beam with an intensity ranging from 6 to 40 μJ/cm2, exciton formation is seen to finish in around 1ps while the electron is localized within 100 fs
Summary
The physical picture of the exciton formation can be viewed concisely as follows: due to external optical excitation, the electron in the highest-occupied molecular orbital (HOMO) of the conjugated polymer is excited to the lowestoccupied molecular orbital (LUMO) [1], such that each of two electronic orbitals is occupied by an electron. The theoretical research again points out that, during the whole dynamical process, the relaxation of exciton formation becomes a main part of a nonadiabatic ultrafast process [14], where the external photoexcitation triggers interband electronic transitions and self-localization, leading to a singlet exciton within less than 1000 fs. Further experimental research shows that highly-efficient exciton transfer is just an intrachain process rather than interchain [15] All of this means that the development of ultrafast intrachain nonadiabatic molecular dynamics is the key step to discovering the underlying mechanism with respect to the relaxation of a hot exciton in conjugated polymeric molecules, which is the main purpose. This article will discover the ultrafast nonadiabatic process under photoexcitation and clarify the relationship among localization, phonon modes and electronic transition dynamics
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