In the broadest terms, excimers are examples of weak excited state bonding. Like regular bonding, shape, charge distribution, symmetry, orbital overlap etc. all matter. Since excimers are often organic, then pi effects tend to dominate...because...carbon radicals are mostly too unstable/reactive in the absence of pi effects.  For pi systems, a good start is Huckel theory, that starts to give a sense of what the electron distribution looks like. From there, SCF (self consistent field) and beyond...onwards to the frontier of knowledge about excited states.

Dear Uppari,

The following publications describe the excimer formation in organic molecules:

1-Chemical Communications Issue 91, 2014

Excimer formation in organic emitter films associated with a molecular orientation promoted by steric hindrance

Jaehyun Lee,a Beomjin Kim,a Ji Eon Kwon,b Joonghan Kim,a Daisuke Yokoyama,c Katsuaki Suzuki,d Hidetaka Nishimura,d Atsushi Wakamiya,d Soo Young Parkb and Jongwook Park*a

Show Affiliations

Chem. Commun., 2014,50, 14145-14148

White emission with two sharp strong peaks – a molecular emission peak at 455 nm and an excimer emission peak at 591 nm – was obtained by introducing a terphenyl group into a highly twisted core chromophore, which promoted a molecular orientation in the film state suitable for excimer formation.

http://pubs.rsc.org/en/Content/ArticleLanding/2014/CC/c4cc05348f#!divAbstract

2- Journal of Luminescence.

Volumes 60–61, April 1994, Pages 454-457

 International Conference on Luminescence

 Organic molecular crystals: formation of excimers

Author links open the overlay panel. Numbers correspond to the affiliation list which can be exposed by using the show more link.H. Da¨ubler a, V.I. Yudson b, P. Reineker *, a

Abstract

For pyrene and α-perylene crystals the formation of excimers is investigated taking into account the sandwich-like arrangement of the molecules in the unit cell. The excitonic band structure consists of broad and narrow bands, which couple in a weak and strong manner, respectively, to the phonons. The antisymmetric states are identified as excimers.

http://www.sciencedirect.com/science/article/pii/0022231394901902

3-Materials Science-Poland, Vol. 27, No. 3, 2009 (see attached file).

Excimers and exciplexes

in organic electroluminescence*

J. KALINOWSKI**

Department of Molecular Physics, Gdańsk University of Technology, 80-952 Gdańsk, Poland

In organic light emitting devices (LEDs) various types of emissive states are created: (i) molecular excited states (localized excitons), or bimolecular (B-M) species: excimers, electromers, exciplexes and electroplexes. The consequences of the formation of B-M excited species for optical and electrical characteristics

of organic LEDs are discussed and illustrated by various examples. While molecular excitons can be viewed in some sense as correlated electron–hole (e-h) pairs with the inter-charge mean separation less than an intermolecular spacing, the size of B-M excited states amounts usually to one or two intermolecular

spacings. The B-M species can be classified as electrically balanced states, formed under energy and charge exchange between neighbour molecules, and have either a singlet or a triplet character. The focus of the paper is on excimer and exciplex forming single phosphorescent dopant blends-based emitting

layers but characteristic features of other B-M excited species (electromers and electroplexes) and their emissions are also mentioned. Of particular interest in modern optoelectronics are white and infrared organic LEDs. It is shown how excimer and exciplex emissions can be employed in manufacturing such

devices. Examples include efficient white and near-infrared LEDs, based on single dopant emitters of an efficient N–C–N-coordinated platinum(II) complex phosphor, and their improved versions, obtained by modification of the emitter matrix materials and electron injecting electrodes. 

Hoping this will be helpful,

Rafik

Dear Uppari,

The following publications describe the excimer formation in organic molecules:

1-Chemical Communications Issue 91, 2014

Excimer formation in organic emitter films associated with a molecular orientation promoted by steric hindrance

Jaehyun Lee,a Beomjin Kim,a Ji Eon Kwon,b Joonghan Kim,a Daisuke Yokoyama,c Katsuaki Suzuki,d Hidetaka Nishimura,d Atsushi Wakamiya,d Soo Young Parkb and Jongwook Park*a

Show Affiliations

Chem. Commun., 2014,50, 14145-14148

White emission with two sharp strong peaks – a molecular emission peak at 455 nm and an excimer emission peak at 591 nm – was obtained by introducing a terphenyl group into a highly twisted core chromophore, which promoted a molecular orientation in the film state suitable for excimer formation.

http://pubs.rsc.org/en/Content/ArticleLanding/2014/CC/c4cc05348f#!divAbstract

2- Journal of Luminescence.

Volumes 60–61, April 1994, Pages 454-457

 International Conference on Luminescence

 Organic molecular crystals: formation of excimers

Author links open the overlay panel. Numbers correspond to the affiliation list which can be exposed by using the show more link.H. Da¨ubler a, V.I. Yudson b, P. Reineker *, a

Abstract

For pyrene and α-perylene crystals the formation of excimers is investigated taking into account the sandwich-like arrangement of the molecules in the unit cell. The excitonic band structure consists of broad and narrow bands, which couple in a weak and strong manner, respectively, to the phonons. The antisymmetric states are identified as excimers.

http://www.sciencedirect.com/science/article/pii/0022231394901902

3-Materials Science-Poland, Vol. 27, No. 3, 2009 (see attached file).

Excimers and exciplexes

in organic electroluminescence*

J. KALINOWSKI**

Department of Molecular Physics, Gdańsk University of Technology, 80-952 Gdańsk, Poland

In organic light emitting devices (LEDs) various types of emissive states are created: (i) molecular excited states (localized excitons), or bimolecular (B-M) species: excimers, electromers, exciplexes and electroplexes. The consequences of the formation of B-M excited species for optical and electrical characteristics

of organic LEDs are discussed and illustrated by various examples. While molecular excitons can be viewed in some sense as correlated electron–hole (e-h) pairs with the inter-charge mean separation less than an intermolecular spacing, the size of B-M excited states amounts usually to one or two intermolecular

spacings. The B-M species can be classified as electrically balanced states, formed under energy and charge exchange between neighbour molecules, and have either a singlet or a triplet character. The focus of the paper is on excimer and exciplex forming single phosphorescent dopant blends-based emitting

layers but characteristic features of other B-M excited species (electromers and electroplexes) and their emissions are also mentioned. Of particular interest in modern optoelectronics are white and infrared organic LEDs. It is shown how excimer and exciplex emissions can be employed in manufacturing such

devices. Examples include efficient white and near-infrared LEDs, based on single dopant emitters of an efficient N–C–N-coordinated platinum(II) complex phosphor, and their improved versions, obtained by modification of the emitter matrix materials and electron injecting electrodes. 

Hoping this will be helpful,

Rafik

 Dear Rafik,  thanks for your reply and your explanation is good. You mention B-M species distance of electron-hole pair is important to form an excimer, in this case the life time of B-M species also playing very important role in excimer formation.

Thank you

Dear Scott Gordon, As you mention excimer formation happens from excited state pi radical continue to this why all excited pi radicals not forming excimers.

Thank you.

The quickest answer is that molecules bond to other molecules in much the same way as atoms bond to other atoms i.e. the symmetry of interaction is important. Ground state pi-stacking provides one such example. In the simplest possible terms, one is attempting to describe excited state pi-stacking as "exciplex formation".  More subtly - one can invoke Fermi's golden rule and think of something that looks and feels a lot like Marcus theory to describe what the transient electron density looks like...if one goes down that road, one gets situations where the electron density of the supermolecule feels like electron transfer between states (fluorescence, phosphorescence, luminescence) or feels more like standing waves (exciplexes, Rydberg states, "excitons"). What is really going on, is the picture of multi-center excited state electron distribution really stretches the limits of what a 1 e- Schrodinger equation can handle. It is "better" to think more simply in terms of self consistent LCAO, or in this case LCMO (linear combination of molecular orbitals). Whether one approaches this from Valence Bond, Huckel, SCF, Group Theory/Molecular orbitals, or even old school crystal field, the key concept is still dominated by the HOMO/LUMO orbitals and how those interact, are localized or delocalized over the atoms/molecules in question.Looking forward, that path leads to molecular modeling software/algorithms (which still struggle with excited states).

Kind of a rambling answer...but in summary....orbital interactions of both atoms and molecules are dominated by the respective symmetries of the overlapping orbitals (because those determine whether the corresponding matrix elements for electron overlap/transfer are zero or not).

Dear Scott Gordon, Thanks for your respond and good explanation.