|Other Abstract||To address the global requirement of renewable energies, the utilization and conversion of solar energy seems to be very important. Thus, the development of novel photovoltaic materials conducting solar energy conversion represents a major challenging task. As a group of promising candidates of photovoltaic compounds, dendrimeric molecules have received wide research interests because of their possible applications in the areas of artificial light harvesting systems, light emitting devices, biological and medical imaging, as well as nonlinear optical devices. The interest in dendrimers for these applications stems from the possibility of very efficient energy transfer among the branches. In this work, we mainly select two kinds of organic dendrimers for studing the excited states energy transfer mechanisms.
Firstly, the excited states of the phenylene ethynylene (PE) dendrimer are investigated comprehensively by various electronic-structure methods. Several computational methods, including SCS-ADC(2), TDHF, TDDFT with different functionals (B3LYP, BH&HLYP, CAM-B3LYP), and DFT/MRCI, are applied in systematic calculations. Through the study, we find that when the peripheral groups of the dendrimer are initially excited, the exciton is transferred along the branches from the margin to the center of the dendrimer in a unidirectional, multistep manner. Studying this energy transfer process can be very significant for synthesizing the novel photovoltaic materials. In this work, the theoretical approach based on the one electron transition density matrix is used to understand the electronic characters of excited states, particularly the contributions of local excitations and charge-transfer excitations within all interacting conjugated branches. Furthermore, the potential energy curves of low-lying electronic states as the functions of ethynylene bonds are constructed at different theoretical levels. This work provides us theoretical insights on the intramolecular excited-state energy transfer mechanism of the dendrimers at the state-of-the-art electronic-structure theories.
Secondly, we report an interesting view to understand the ultrafast excited-state energy transfer (EET) process in the D3-symmetric dendrimer tris(4-ethynylphenyl)amine (TEPA) from the perspective of the well-known E⊗e Jahn-Teller (JT) effect. Upon excitation to two lowest excited states (S1 and S2) with doubly degenerate E symmetry, two sets of e vibrational modes, dihedral angle twist and strong pyramidalization near the nitrogen core, lead to the JT distortion and symmetry lowering. Through the excited-state dynamics simulation with the on-the-fly surface hopping approach at the TDDFT level, we find that the system may either travel three equivalent minima of S1 state or undergo the nonadiabatic transitions between S1 and S2 states. These motions induce the ultrafast EET among different branches and the re-orientation of the transition dipole moments, finally leading to the ultrafast fluorescence anisotropy decay. This energy transfer mechanism can provide some new insights on the excited-state dynamics of large dendrimers with three equivalent branches and transition metal complexes with the C3 symmetry.
In summary, two kinds of organic dendrimers were selected for studying their excited states energy transfer mechanisms, and furthermore, we employ our in-house program named “JADE” for investigating the energy transfer mechanism. Meanwhile, the transition density matrix was used to analyze the electronic excited states characters. Through the dynamics, we find the energy transfer process of TEPA dendrimer is related with the well-known JT effect. Additionally, we also simulated the spectrum of fluorescence anisotropy decay for the molecule, which is consistent with the experimental result. We believe this work will provide a solid basis for understanding the excited states energy transfer mechanisms of other dendrimers.|