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