Molecules excite from the ground state to the excited state after absorbing electromagnetic radiation. If the vibrational energy level of S1 overlaps with the vibrational energy level of T1, intersystem crossing (ISC) may occur. It is a spin-forbidden process for a molecule to enter the T1 state from the S1 state. And it happens because there is spin-orbital coupling (SOC) in which the magnetic field generated by the orbital motion exerts a torque on the electron spin and reverses the spin. When the molecule transitions from the T1 state to the S0 state by luminescence, the spin is reversed again, and this type of luminescence is phosphorescence. Phosphorescence is the light released by electrons returning from the first excited state of the triplet state to the ground state.
Figure 1. A) Monitoring of the phosphorescence activation as a function of the irradiation time. B) Long-term stability data of phosphorescence lifetimes and intensity. C) Image of PLTs realized with PMMA. D) Luminescence images of thin films and 3D printed (TCATPB in Exceval) objects under room light or dark with the excitation UV lamp (365 nm) on and off. (Marin, L.; et al. 2020)
Phosphorescent materials have the following advantages including long triplet life, allowing excitons to migrate over long distances, and effectively avoiding the interference of short-lived background fluorescence of organisms. Phosphorescent material therefore has broad application prospects and become a very popular research field at present.
Alfa Chemistry has established two calculation strategies to predict phosphorescence spectrum.
1. Optimize the structure of T1
We use UDFT method to optimize the structure of T1. If the T1 state of the molecule is a non-planar structure, the initial structure needs to be adjusted during optimization, otherwise a planar structure with virtual frequencies will be obtained.
There are two methods to obtain the emission energy:
2(a) Triplet (TD) calculation
TD calculation is performed based on the optimized structure obtained from the previous step, and the excitation energy of this step is used as the emission energy of phosphorescence.
2(b) Ground state calculation
We perform the ground state calculation on the structure of T1, and use the free energy of optimized structure to subtract the ground state energy to get the emission energy.
1. TD calculation
We perform TD calculation under the equilibrium structure of T1 to obtain the ground-state solvation free energy where the solvent is in equilibrium with the ground-state of the solute.
2. Ground state calculation
We then calculate the ground state based on the equilibrium structure of T1 to obtain the nonequlibrium solvation free energy when the solvent is not in equilibrium with the excited-state of the solute. The final emission energy is obtained by using the structure optimization to subtract the final energy from the ground state energy.
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