Essentials of Computational Chemistry

14.7 CASE STUDY: ORGANIC LIGHT EMITTING DIODE Alq3 513
are red-shifted. This appears not to be a polarization effect, but a manifestation of improved
dispersion interactions between the excited state, which because of its more highly excited
electron(s) tends to be more polarizable than the ground state, and the solvent. Differential
hydrogen bonding interactions can also play an important role in situations where
solute–solvent interactions of this type are manifest.
Given the disparate nature of the physical interactions between the different electronic
states and the solvent, and the non-equilibrium nature of the solvation of at least one state
in the vertical process, theoretical models require a fairly high degree of sophistication in
their construction to be applicable to predicting spectroscopic properties in solution. This
requirement, coupled with the rather poor utility of available experimental data (most solution
spectra show very broad absorption peaks, making it difficult to assign vertical transitions
accurately in the absence of a very complex dynamical analysis), has kept most theory in
this area at the developers’ level. A full discussion is beyond the scope of an introductory
text, but we will briefly touch on a few of the key issues.
Continuum solvation models enjoy their usual advantage of efficiency, but the proper
computation of the reaction field for the excited state requires that first the slow component is
determined based on the ground-state charge distribution, and then the fast component based
on the excited state, the latter process being iterative in the usual SCRF sense (Aguilar,
Olivares del Valle, and Tomasi 1993; Mennucci, Cammi, and Tomasi 1998; Cossi and
Barone 2000). In the absence of a surrounding solvent shell, however, differential dispersion
and hydrogen bonding interactions must be accounted for in an ad hoc fashion after this
accounting for polarization (Rauhut, Clark, and Steinke, 1993; Li, Cramer, and Truhlar 2000).
QM/MM approaches where the solute is QM and the solvent MM are in principle useful
for computing the effect of the slow reaction field (represented by the solute point charges)
but require a polarizable solvent model if electronic equilibration to the excited state is
to be included (Gao 1994). With an MM solvent shell, it is no more possible to compute
differential dispersion effects directly than for a continuum model. An option is to make
the first solvent shell QM too, but computational costs for MC or MD simulations quickly
expand with such a model. Large QM simulations with explicit solvent have appeared using
the fast semiempirical INDO/S model to evaluate solvatochromic effects, and the results
have been promising (Coutinho, Canuto, and Zerner 1997; Coutinho and Canuto 2003).
Such simulations offer the potential to model solvent broadening accurately, since they can
compute absorptions for an ensemble of solvent configurations.
14.7 Case Study: Organic Light Emitting Diode Alq3
Synopsis of Halls and Schlegel (2001) ‘Molecular Orbital Study of the First Excited State
of the OLED Material tris(8-hydroxyquinoline)aluminum(III)’.
Many modern display technologies make use of organic light emitting diodes. These
devices typically include two layers, at least one of which is organic, through which
electrons and holes propagate. When a hole meets an electron in a single layer or at an
interface, the recombination leads to a singlet exciton that fluoresces in the light-generating
event. One small organic molecule that has proven to be useful in this regard is tris-(8hydroxyquinoline)aluminum(III),
also called Alq3 (Figure 14.8).