Hello Mohanned Alanber

To study fluorescence and phosphorescence in Gaussian software calculations, you would typically consider the following parameters and steps:

Optimization of Ground State Geometry: Start by optimizing the molecular geometry in the ground electronic state (S0 state).

Excited State Calculations: Once the ground state is optimized, perform excited state calculations using time-dependent density functional theory (TD-DFT) or configuration interaction (CI) methods. This will give you information about the excited states (S1, S2, etc.).

Optimization of Excited State Geometry: Next, optimize the geometry of the first excited singlet state (S1) to find the equilibrium structure in that state, which is important for fluorescence.

Fluorescence Energy: Calculate the vertical emission energy (fluorescence) by taking the difference in energy between the optimized S1 state and the transition to the S0 state.

Spin-Orbit Coupling Calculations: To study phosphorescence, include spin-orbit coupling calculations which are necessary because phosphorescence involves a transition between states of different multiplicity (from a triplet excited state to the singlet ground state).

Optimization of Triplet State Geometry: Similar to step 3, but for the lowest triplet state (T1).

Phosphorescence Energy: Compute the phosphorescence energy as the difference between the optimized T1 state and the S0 state.

Rate Constants: Calculate the rate constants for radiative and non-radiative transitions to predict lifetimes of the excited states.

Spectral Properties: Use the optimized geometries and transition energy differences to calculate the absorption, fluorescence, and phosphorescence spectra.

Quantum Yields: If possible, compare the rate constants for various pathways to estimate the quantum yields of fluorescence and phosphorescence.

Non-adiabatic Couplings: May be necessary if you want to consider the possibility of intersystem crossing, which affects the phosphorescence process.

Vibrational Analysis: Perform a vibrational analysis on the ground and excited state geometries to assess potential energy surfaces.

Note that performing these calculations takes time and requires powerful systems. You can use MolQube.com to prepare the infrastructure. They help you prepare the input, perform calculations and analyze the data. Their email address: [email protected]

Maryam Farajpour Mojdehi

Thank you for your comment, and thank you for making this beautiful explanation

Greetings