Dear Group,
The joint BU-Harvard-MIT student-sponsored theory seminar has started. I
paste the two talks (one seminar, one 2-hour talk for students) that
Sergei Tretiak will give at MIT. I encourage you to attend to both. I
will hopefully be able to go to at least one.
Here is the schedule:
** Monday talk ************************************************
Physical Chemistry Seminar: Dr. Sergei Tretiak, Los Alamos National Lab
September 11, 2006 4:30pm
Location: MIT 56-114
Title: "Photoinduced processes in conjugated polymers and carbon
nanotubes: similarities and
differences"
***************************************************************
** Wednesday talk (2-hours, with intermission) **
Joint Theoretical Chemistry Seminar - Sergei Tretiak
September 13, 2006 4:00 - 6:00p
Location: MIT, 24-121
Title: "Nonlinear optical response and photodynamics of conjugated
molecules: effects of
branching and substitution"
Photoexcited dynamics and nonlinear optical response of organic
chromophores are investigated using time-dependent density functional
theory (TDDFT). Closed expressions for frequency dependent
polarizabilities up to the third order are derived and computationally
implemented. We find that TDDFT reproduces well the energetics of both
one-photon (linear absorption) and two-photon (TPA) states in a variety
of
donor-acceptor substituted molecules. The absolute magnitudes of TPA
cross-sections have a good agreement with experiment as well. Several
structurally related large chromophores of different symmetry are
further
investigated and compared for elucidation of the combined role of
branching and charge symmetry on photoluminescence and nonlinear
optical
response. Branching is observed to lead to both cooperative enhancement
of
TPA while ensuring high fluorescence quantum yield. New strategies for
molecular engineering of nonlinear optical materials are inferred.
Finally, we develop reduced exciton scattering model, which attributes
excited states to standing waves in quasi-one-dimensional structures
branching structures (Bethe lattices), assuming quasi-particle picture
of
optical excitations. Direct quantum-chemical calculations of branched
conjugated phenylacetylene chromophores are used to verify our model
and
to derive relevant parameters. Complex and non-trivial delocalization
patterns of photoexcitations throughout the entire molecular tree can
then
be universally characterized and understood using the proposed method,
completely bypassing 'supramolecular' calculations. Frenkel-exciton
models
appear as limiting cases of our approach. This opens a new and accurate
way to model excited state dynamics and energy transfer in arbitrary
dendrimetic structures.
***************************************************************
--
Alán Aspuru-Guzik
Assistant Professor
Department of Chemistry and Chemical Biology
12 Oxford Street
Harvard University
Cambridge, MA 02138
Tel: (617)384-8188
Group URL:
http://aspuru.chem.harvard.edu