I performed calculation using Gaussian to obtain the fluorescence emission of some organic molecules. The following lines are the data of the optimized singlet state geometry. why does the first excited state has lower wavelength emission than the second excited state ?
Excited State 1: Singlet-A 1.9113 eV 648.69 nm f=0.0008 =0.000
85 -> 89 -0.69679
This state for optimization and/or second-order correction.
Total Energy, E(TD-HF/TD-KS) = -1179.74328545
Copying the excited state density for this state as the 1-particle RhoCI density.
Excited State 2: Singlet-A 1.3773 eV 900.19 nm f=0.4216 =0.000
88 -> 89 0.70630
I did Td-DFT calculation opt ( singlet, nof state6,root1)
I know what type of calculation I should run
Hello Manal Alwahsh, If I may, you should check the states at every step of optimization, and you will find geometry where the states have switched or are degenerate.
Your run probably indicates a non-radiative pathway competing with the fluorescence. If any part of the molecules is rotating or twisting, it might cause the switch.
I hope this helps!