Subject: Re: Excimer Calculations

From: [email-masked]

Rovshan,

I have used Argus in the past to calculate eximers for a
bacteriochlorophyll project.  It's not a user-defined
feature of the export version, but if there is sufficient
interest, I could try to get it into the next version.

Could you explain to me exactly what you want to do?

Mark

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Mark A. Thompson                    
Sr. Research Scientist              email:  [email-masked]
Molecular Science Research Center   FAX  :  509-375-6631
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PO Box 999, Mail Stop K1-90
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Argus available via anonymous ftp from pnlg.pnl.gov (130.20.64.11) (in the
argus directory).  Download the README file first.

Disclaimer:  The views expressed in this message are solely my own and
             do not represent Battelle Memorial Institute, Pacific 
             Northwest Laboratory, or any of its clients.
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---

From: [email-masked]

I would be interested in obtaining a copy of responses you receive
Thanks
satyam
[email-masked]

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From: [email-masked] (Charles Martin)

Netters,
Rovshan G Sadygov asks:
>
> Does anyone out there know of programs to calculate excimers? I have tried
> Argus and MOPAC5. They do not seem to give satisfactory results for the
> problem. Preferably code for Sun workstations and written on C.
> 
I am just curious what the problem is?

Some old calculations have been performed
to model the excimer bond. 
See "Mechanism and Theory in Organic Chemistry,"
Lowry and Richardson, Chapter 12  Photochemistry, which references
1a) Th. F\"orster, Angwew. Chem. Int. Ed., vol 8, page 333, (1969)
1b)B. Stevens in Advances in Photochemistry, vol 8
W. A. Noyes, G. S. Hammond, and J. N. Pitts, Jr. Eds, Wiley NY 1971, page 161
1c) M. Gordan and W. R. Ware, Eds,  "The Expliex"  Academic Press NY 1975

2) E.A. Chandross and C. J. Dempster, JACS vol 92, page 3586  (1970)

3) A.S. Cherkasov and T.M. Vember, Optics and Spectroscopy, vol 6, page 319 (1959)

Lowry and Richardson claim that Quantum Mechanical calculations
 (I don't know what kind)
give good esitmate of the theoretical position of the excimer flourescence
band (I don't now how "good" good is) if the calculations account for
charge resonance and excitatiojn resonance.  That is, if the excimer
takes the form:

	A*A <-> AA* <-> A+A- <-> A-A+

Is this along the lines of what you are looking for?

On the other hand, many excimer fluorescence intensity depend
on concentration. If you are trying to model concentration dependence,
take a look at some work by Peter Rosky (Texas at Austin?)
He has some been looking at some similar problems with excited
atoms in solution.

Hope this helps
Chuck

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From: [email-masked] (Slawomir Blonski)

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

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From: <[email-masked]>

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]>

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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