Subject: colour of molecules in the solid state

From: Anik Peeters <[email-masked]>
Date: Thu, 11 Feb 1999 14:28:10 +0100 (MET)

Dear CCLers,

I am studying molecular crystals using the supermolecule approach:
a cluster of 10-15 molecules is described by the wave function
and this cluster is surrounded by point charges simulating the other
neighbours. The level of theory we are using is Hartree-Fock.

The molecule I am studying right now has been observed in three different
polymorphic forms. One has a white colour, another one is yellow and a
third one is light yellow. I have been trying to relate the yellow and
light yellow colour to the band gap between the HOMO and LUMO. However,
the energy differences I obtained are too large.

I was wondering whether there exists a cheap method (because of the size
of the system) to get a better approximation of the band gap.

Greetings,

Anik.

*************************************************************************
*	Dr. Anik Peeters						*
*	Quantum Chemistry Group, University of Antwerp (UIA)		*
*	Universiteitsplein 1, B-2610 Wilrijk, Belgium			*
*	phone: 32-3-8202366, fax: 32-3-8202310, e-mail: [email-masked]  *
*************************************************************************

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From: Jerry Perlstein <[email-masked]>
Date: Thu, 11 Feb 1999 14:43:39 -0500

Anik Peeters wrote:
> light yellow colour to the band gap between the HOMO and LUMO. 

Anik- I doubt whether the "band gap" has anything to do with the colors
you observe. More likely they are either charge transfer transitions or
possibly Davydov splittings of the exciton states.
Before you do anything fancy why don't you visually looked at the
structures to see if there are any molecular interactions which would
suggest charge transfer transitions?
JERRY 
-- 
**************
Jerry Perlstein
Center for Photoinduced Charge Transfer
Department of Chemistry
University of Rochester
Rochester,NY 14627-0216
Tel: (716)275-2511
FAX: (716)242-9485
[email-masked]
http://www.chem.rochester.edu/~stc/perlstein.html

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From: Artem Masunov <[email-masked]>
Date: Thu, 11 Feb 1999 17:01:58 -0500 (EST)

> suggest charge transfer transitions?

 I wonder what kind of interactions in homomolecular crystals (except
conventional pi-pi stacking) could be considered as charge transfer?
Are there CT spectra for sigma-interactions?
 I would appreciate if Jerry Perlstein (and all CCLers) give me some
references for the examples and interpretations.

Artem Masunov

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