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From molecules to materials:the crucial role of supramolecular interactions
Anna Painelli
Francesca Terenziani
INSTM-ParmaParma University
D-D--A chromophores-A chromophores
•polar and polarizablepolar and polarizable molecules
•model systems for electron transferelectron transfer
CN
CN
ON
DCM
< 0.5
Q-3CNQ
> 0.5
a model for interacting D--A chromophores
layout of the work
•the same molecule behaves in qualitatively different ways depending on supramolecular architecture
collective and cooperative phenomena
bottom-up approach to molecular materials
•new physics with no counterpart at the molecular level
•molecular properties from solution to supramolecular architectures
•a case study Q-3CNQ from solution to LB film
Mulliken model
ADDAg 1
ADDAe 1
two orthogonal basis states
AD AD
two orthogonal eigenstates
measures the molecular polarity
022
20
zt
t
A model for interacting polar and polarizable molecules
jijiijV ˆˆ
two-state on-site Mulliken Hamiltonian
electrostatic intersite
interactions
For a cluster of N molecules: 2N electronic basis functions
exact diagonalization for up to 16-18 sites
)ˆ2ˆ2( 0 iii
tzH
)(22
20
0 Mzt
t
j
ijVM2
1
cooperative behavior
is the ground state of an effective self-consistent Mulliken Hamiltonian ig
mean-field approximation to the ground state
Ni ggggG ..........21
Madelung energy
the interaction with surrounding molecules affects the properties of each molecule
Nii geggE .........21
mean-field excitation energy
)1(
2
t
collective behavior
ij
ijV 1
excitonic correction to the k=0 state
creating an excitation
exciton migration
D+A-
isolated molecules
D A
squeezing a repulsive clusters, M>0
the polarity varies with supramolecular
arrangement
the optical frequency is governed by a subtle interplay of mean-field, excitonic and ultraexcitonic effects
inverse intermolecular distance
Phys.Rev.B 68, 165495 (2003)
modeling solution spectra: JCPA 106, 6286 (2003)
Chem.Phys. 295, 35 (2003)
bottom-up strategy
a case study: Q-3CNQ, the molecular rectifier
0.16
0.14
0.17sp
0.50
-0.25z0
t2
estimated ground state polarity: 0.88, 0.89, 0.92
in collaboration with Robert Metzger
LB monolayer
fluor
esce
nce
a.u.
absorbance
fluor
esce
nce
a.u.
absorbanceLB
monolayer
molecular parameters
(eV)
molecular parameters from the analysis of solution spectra
But at small tilt angles:
neutral Q-3CNQ
?inverse rectification in SA?
we can model experimental spectra of LB-monolayers of Q-3CNQ with the same parameters extracted from solution spectra
In the LB layer Q-3CNQ is more polar than in solution; the observed frequency is
governed by mf effects
Experimental frequency in
LB film
tilt angle
7 Ǻ
multielectron transfer as the primary photoexcitation event
upon absorption of a single photon several electrons are
transferred on nearby molecular sites
Extreme manifestation of
cooperative behavior
Discontinuous charge crossovers
new phenomena appear at the supramolecular level with
no counterpart at the molecular level
number of D+A- species created upon photoexcitation
excitonic regime
inverse intermolecular distance
J.Am.Chem.Soc.125, 5624 (2003)
conclusions
supramolecular interactions in molecular materials
•cooperative and collective phenomena
•discontinuous charge crossovers
• multielectron transfer
•evolution of the molecular properties from solution to condensed phases
•supramolecular structure – properties relationships
new physics
new understanding
thanks
Geoff Ashwell (Univ. Cranfield)
Alberto Girlando (Univ. Parma)
Robert Metzger (Univ. Alabama)
Zoltan Soos (Univ. Princeton)
financial support
MIUR – COFIN2001
INSTM