Firefly and PC GAMESS-related discussion club

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sanya
sanya@photonics.ru

The energy of S1->S0* transition is exactly the energy of S0*->S1 transition (only the transition is in the opposite direction). That is, any vertical spectrum may be considered as absorption or emission, depending on your starting point (on S1 or S0 surface). Practically, you should calculate the vertical (absorption) spectrum at the S0* (that is, S1) geometry and look at the energy and the oscillator strength of the S0*->S1 transition.

Generally speaking, steady-state spectra occur from the equilibrium geometries (absorption from S0 and emission from S1), while transient absorption/emission (observed, say, in time-resolved experiments) can occur from any point on the corresponding surface. The oscillator strength will give you the intensity of the transition (either up or down).

>So, if we are talking about luminescence properties we must always study first excited state,am i right?

Kasha's rule is rarely broken, although there are some examples of its violation (AFAIR, azulene, for example). A more frequent problem is that pure-GGA (and, sometimes, hybrid) DFT gives wrong order of states (say, gives too low energy for an n->pi* transition, so that it becomes the first one, while it is not really the case). The oscillator strength of the transition to S0 from this too-low-lying (artifact) state is typically almost zero, so it cannot become the emitting state in your calculations. The real problem is that its surface can interfere with the surface of really emitting state, be careful.

One more issue is that the order of states depends on the environment (polar or nonpolar), so you should know exactly, in which solvent the experimental spectra were recorded.