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Emissive Nanomaterials and Liquid Crystals
Published in Klaus D. Sattler, 21st Century Nanoscience – A Handbook, 2020
Marianne E. Prévôt, Julie P. Vanegas, Elda Hegmann, Torsten Hegmann, Julia Pérez-Prieto, Yann Molard
Fluorescence vs. Phosphorescence: Distinction between fluorescence and phosphorescence mainly concerns the transition kinetics and the Stokes displacement. It has been found experimentally that the transitions between states of different multiplicity are 103 –105 times slower than those of the same multiplicity [17]. In the case of fluorescence, the excited species return very quickly (10-10 –10-7 s) to their original energy state. During phosphorescence, they pass through an intermediate energy state, or they remain a certain time in the excited state before returning to the initial state. The intersystem crossing may be fast enough (10-9 –10-7s) to compete with other de-excitation pathways, namely fluorescence and internal conversion. The characteristic time, τL, of a phosphorescent transition extends from 10-6 s to several hours. Due to the de-excitation pathway implied in fluorescence and phosphorescence, less Stokes displacement is expected in the case of fluorescent emission compared with phosphorescent emission.
Basic Principles of Fluorescence
Published in Guy Cox, Fundamentals of Fluorescence Imaging, 2019
The Jablonski diagram in Fig. 1.11 shows the general idea. In this diagram, the molecule absorbs a photon and is excited to an excited electronic state. It sheds vibrational energy through NRD. Before it emits, the molecule converts the electronic energy into vibrational energy of a lower electronic state in a radiationless transition. When the spin state is preserved, this electronic relaxation is called internal conversion (IC) and when the spin state changes it is called intersystem crossing (ISC). As this transition is not accompanied by a photon to remove the energy, the transition must conserve energy. Therefore, radiationless transitions are drawn as horizontal arrows (same energy) on a Jablonski diagram. The ensuing vibrational energy can be removed by NRD as before. The highly vibrationally excited S0 molecule will return to the ground state, without ever emitting light. Following ISC, the T1 molecule can either phosphoresce, or undergo ISC again, converting T1 electronic energy into S0 vibrational energy. Again, the molecule sheds the excess vibrational energy, returning to the ground state.
Molecular Structure
Published in Thomas C. Weinacht, Brett J. Pearson, Time-Resolved Spectroscopy, 2018
Thomas C. Weinacht, Brett J. Pearson
Spin–orbit coupling generally leads to slow, intersystem crossing when exciting a molecule from an initial state with a well-defined total spin to a final state with the same total spin but which is not an eigenstate of the total Hamiltonian. For example, the final state can be described as a superposition of two eigenstates that each have a mixture of singlet and triplet character. Since the energy difference between these product states is small, the timescale over which a coherent superposition evolves is relatively long (this is the time over which the phase difference between states changes by an appreciable fraction of 2π). Thus, for the short timescales probed by most time-resolved experiments, spin–orbit coupling is usually neglected.
Effects of the carboxylic acid substituents on the photophysical and nonlinear optical properties of asymmetrical Zn(II) phthalocyanines–quantum dots conjugates
Published in Inorganic and Nano-Metal Chemistry, 2018
Sithi Mgidlana, David O. Oluwole, Tebello Nyokong
The triplet quantum yields of the Pcs increase in the presence of QDs due to the heavy atom effect discussed above.[19]There is a larger increase in ΦT value for 2-CdTe/ZnSe (0.86) as compared to 2-CdTe/ZnSe/ZnO (0.61) most likely due to the larger loading in the former. For 1-CdTe/ZnSe (0.71) as compared to 1-CdTe/ZnSe/ZnO (0.78) and for 3-CdTe/ZnSe (0.70) as compared to 3-CdTe/ZnSe/ZnO (0.81), the ΦT values are larger for CdTe/ZnSe/ZnO containing conjugates. But for 2-CdTe/ZnSe compared to 2-CdTe/ZnSe/ZnO the ΦT value is larger for the former. This trend is in agreement with DLS sizes where the CdTe/ZnSe/ZnO containing conjugates were larger than the CdTe/ZnSe conjugates for complexes 1 and 3 and not for 2, suggesting that the size of the nanoconjugate affects its triplet state parameters. The lengthening of τT for the Pcs in the presence of QDs could be due to the shielding effect by the QDs. The lifetimes of intersystem crossing (ISC) were determined from the relationship (τisc = τF/ΦT) where τF and ΦT are fluorescence lifetime and triplet quantum yield, respectively. The values are lower where ΦT values are high.
Photoinduced relaxation dynamics of nitrogen-capped silicon nanoclusters: a TD-DFT study
Published in Molecular Physics, 2018
Xiang-Yang Liu, Xiao-Ying Xie, Wei-Hai Fang, Ganglong Cui
In order to treat intersystem crossing transitions, the electronic Hamiltonian must include the spin-orbit interaction, which drives radiationless transitions between electronic states with different spins. We add a spin-orbit operator to the zero-order electronic Hamiltonian and consider it as a perturbation: where r and s correspond to spatial and spin coordinates of electrons. The time-dependent Schrödinger equation now reads
Theoretical insights on the luminescent mechanism of a highly efficient green-activated delayed fluorescence emitter using the QM/MM method
Published in Molecular Physics, 2023
Hai-Yang Sun, Zi-Yue Yu, Ai-Ping Zhou, Shu-Li Wei, Qiang Chang, Tian Zhang, Yu-Ping Sun
To further quantitatively characterise the geometric changes of the molecules during excitation, the root-mean-square displacement (RMSD) is calculated using the Multiwfn programme. The geometry changes and the corresponding RMSD values are shown in Figure 2. It can be seen that the geometric changes during the state transitions are mainly concentrated in the dihedral angle between the donor and acceptor groups. Moreover, except for the S1-T3 transition of molecule PPZTPI, the RMSD values of the solid phase state are much smaller than the corresponding values in toluene, suggesting that the action of surrounding molecules in aggregation can limit the rotations of the dihedral angles. The RMSD values of PPZPPI between S0 and S1, between S1 and T1, and between S1 and T2 in toluene are 0.661, 0.665, and 0.791, respectively, while the corresponding values in the solid phase decrease to 0.075, 0.046, and 0.150, respectively, about one-fifth to one-fifteenth of values in toluene. A similar trend can be observed for PPZTPI from liquid phase to solid phase. In general, the degree of geometry changes in transitions is directly related to non-radiative processes. Smaller geometry changes would predict smaller recombination energies, smaller HR factors, and slower nonradiative transition rates. Furthermore, the occurrence of intersystem crossing and reverse intersystem crossing processes is related to the geometrical difference between the two electronic states with different spins involved. Comparing the geometric structures of the two molecules in different environments predicts that both PPZPPI and PPZTPI will exhibit aggregation-induced emission properties. The resulting changes in the electronic structure, molecular vibrations, and molecular optical properties are discussed in the following subsections.