Subject: Re: Excimer Calculations

From: [email-masked] (Slawomir Blonski)
Date: Mon, 22 Nov 1993 23:54:15 -0500 (EST)

CI works well for excimers and is equivalent to combination of charge transfer
and excitation resonance. See, for example:

"Photophysics of Polystyrene. 2. Pariser-Parr-Pople Studies of Excimer
Structure"
S. Blonski, K. Sienicki, and A. Herman
Macromolecules 20 (1987) 329

and more recent references:

"Excimer geometries in bichromophoric molecules: a theoretical and experimental
study of 1,2-(bis-9-anthryl)-ethane"
G.D. Scholes, K.P. Ghiggino, and G.J. Wilson
Chemical Physics 155 (1991) 127

"Excimer interactions in poly(acenaphthalene) diads"
D.G. Scholes and K.P. Ghiggino
Chemical Physics Letters 188 (1992) 140

However, semi-empirical methods give good spectra or good geometries, but not
both, and excited-state ab initio calculations seem to be too costly so far.

I don't work on excimers now, but I am still very interested in the summary
of responds to the excimer question.

Slawomir Blonski

---

From: <[email-masked]>
Date: Tue, 23 Nov 1993 17:45:22 +1000

Calculation of excimer emission wavelengths is a particularly
non-trivial exercise.
Ab initio calculations must employ extended basis sets and proper
inclusion of correlation - the emission maximum shift with
respect to that of the "monomer" should be fairly reliably calculated
in this way, but not necessarily the actual maximum.

For more practically-oriented calculations a modified version of
QCFF/PI (used in e.g. G.D. Scholes, K.P. Ghiggino & G.J. Wilson,
Chem. Phys., 155 (1991) 127) could be used.  You may obtain that
>from me (just e-mail me personally).

Alternatively, what about CNDO/S/CI or INDO/S/CI - they should
also give reasonable results?

Regards
  Greg Scholes
  University of Melbourne

---

From: <[email-masked]>
Date: Tue, 23 Nov 93 09:02 N

Dear Netters,

  on his reply to the question concering calculation of excimer electronic
structure, Greg Scholes suggested to try CNDO and/or INDO/S-CI.

Although these procedures may look like an attractive choice, the one of them
which I know personally (INDO/S-CI such as implemented for example in Zerner's
ZINDO which is also offered in INSIGHT from Biosym) has one serious drawback
which probably makes it useless for excimers: It absurdly overestimates
through-space interactions at anything smaller than van der Waals distances.

We have been trying to do calculations on pi-complex cations (which may be
regarded as ionized excimers) and the splitting of the HOMO's and the LUMO's at
distances between 2.8 and 3.0 Angstrom (as may be typical also for pi-excimers)
were so large that the HOMO-LUMO-gap (or, conversely, the electronic transition
corresponding to HOMO-LUMO excitation) was predicted in the IR! At larger
distances, the picture becomes more realistic (for example, the splitting of
the pi-states in dodecahedradiene, where two double bonds are held at a 3.5
Angstom distance is predicted to be 0.8 eV in excellent agreement with
experiment).

Of course all this boils down to the choice of the geometry, in particular the
distance between the excimer components. We based our calculations on
geometries obtained at a correlated ab-initio level which we assume to be
reasonably realistic. Surely one could do an INDO/S calculation at some
distance where experimental excimer emission wavelengths are predicted
correctly but then the question poses itself whether we have gained any
understanding by doing this  calculation.

I think the only procedure which is liable to give correct results without any
fiddling is the CASPT2-model recently proposed by Roos et. al. and implemented
in their program system MOLCAS 2. However, anything with more than about ten
heavy atoms may take a lot of patience even with state-of-the art hardware and
the program is not of the black-box type because some human intelligence is
needed in the selection of the basis set and the active space for the MCSCF
calculation.

Therefore, I share the opinion of Greg Scholes that calculations of excimer
emissions are a non-trivial issue, but I am also waiting eagerly to hear what
other colleagues have to say on this matter

thomas bally

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