In this work we show the potentialities of applying impulsive coherent vibrational spectroscopy to conjugated systems relevant for applications. We studied films of sexithiophene, a candidate for large area molecular electronics, poly-phenylene vinylene, a prototype electroluminescent material, and polydiacetylene, very promising for applications in photonic devices. These experiments demonstrate the possibility of studying coherent molecular dynamics in organic systems with extremely high time resolution

Cerullo, Giulio

De Silvestri, Sandro

Lanzani, Guglielmo

Svelto, Orazio

English

UnissResearch

THURSDAY MORNING / QELs'99 / 183 
QThB8 Fig. 1. SHG scattering indicatrix for 
Gd LB film. 
nm. A single beam SHG interferometry' is per- 
formed as indium-tin oxide being a source of a 
reference SHG signal. The DC-magnetic field 
up to 1.5 kOe is applied to the sample in the 
longitudinal geometry. 
Figure 1 shows a typical scattering indica- 
trix, i.e. dependence of the SHG intensity on 
the angle of reflection for the fixed angle of 
incidence (45') measured in p-in, p-out polar- 
ization combination of the fundamental and 
SH waves. Similar SHG indicatrices are ob- 
served for all the polarization combinations. 
The SHG radiation is scattered into a broad 
angular interval, that indicates an inhomoge- 
neity of spatial distribution of nonlinear 
sources. This inhomogeneity should be attrib- 
uted to the presence of 2D islands of Gd ions 
with noncentrosymmetric geometrical shape. 
Apart from diffuse SHG, a noticeable peak of 
the specular reflection is observed (inset in Fig. 
1). It can appear due to the predominant in- 
plane orientation of these 2D Gd islands 
caused by the procedure of the film deposition. 
The magnetoinduced effects are studied for 
the specular SHG component. The SHG polar- 
ization diagrams show an angular shift of 12 
degrees as the DC-magnetic field of 1 kOe and 
of the opposite directions is applied. Figure 2 
shows the results of the SHG phase measure- 
ments that reveal the magnetoinduced shift of 
the SH wave phase of approximately 115 de- 
_-. I 
C I  I 
15 20 25 30 
R e f e r e n c e  P o s i t i o n  ( e m )  
QThB8 Fig. 2. The SHG interference patterns 
from Gd-containing LB film for the longitudinal 
application ofthe DC-magnetic field in s-in, s-out 
polarization combination: open circles-1 kOe, 
filled circles-1 kOe. 
grees for the s-in, s-out polarization combina- 
tion, while for the p-to-p polarization combi- 
nation it is less than 5 degrees. Significant 
magnitude of magnetization induced effects in 
SHG from Gd 2D layers in LB films which 
manifest themselves in sufficient angles of ro- 
tation of the SH wave polarization and varia- 
tions of the SH wave phase indicate a strong 
coupling between nonmagnetic and magneto- 
induced nonlinear polarizations of the Gd 
layer. 
This work was supported by INTAS Fellow- 
ship grants for Young Scientists YSF-8,IO. 
1. R. Stolle, G. Marowsky, E. Schwarzberg, 
and G. Berkovic, Appl. Phys. B 63, 491 
(1996). 
QThC 8:OO am-10:OO am 
Rooms 341/342 
Optical Processes in Organic 
Materials 
Misha Y. Ivanov, SIMS NRC, Canada, 
Presider 
QThCl (Invited) 
~~ 
8:OO am 
Morphologydependent photophysics in 
conjugated polymers 
Benjamin J. Schwartz, Chemistry Dept., UCLA 
405 Hilgard Ave., Los Angeles, CA, 900095 
USA; E-mail: schwartz@chem. ucla.edu 
We present evidence that the degree of inter- 
chain interactions and morphology in conju- 
gated polymer films can be controlled by alter- 
ing the chain conformation in the solution 
from which the film is cast. Light scattering 
experiments show that physical size of polyE2- 
methoxy-5-(2'-ethyl-hexyloxy)- 1,4-phenylene 
vinylene] (MEH-PPV) chains can vary by a 
factor of two in different solvents such as chlo- 
robenzene (CB) or tetrahydrofuran (THF). 
Photoluminescence and wavelength-depend- 
ent excitation indicate that MEH-PPV forms 
aggregate species with an absorption and lumi- 
nescence spectra that are distinctly red-shifted 
from the intrachain exciton. The degree of ag- 
gregation is both concentration and solvent 
dependent; for solutions with concentrations 
typical of those used in spin-casting, aggre- 
gates comprise a significant fraction of the to- 
tal number of excited state species. The overall 
photoluminescence quantum yield is found to 
depend on both how restricted the polymer 
conformation is due to choice of solvent and 
polymer concentration due to aggregation. 
The excited state aggregates have a longer life- 
time than their intrachain exciton counter- 
parts, as evidenced by a near-infrared transient 
absorption in femtosecond pump-probe and 
anisotropy measurements. Memory of the 
chain conformation and extent of aggregation 
of MEH-PPV in solution is carried into cast 
films. Thus, many conflicting results presented 
on the degree of interchain interactions can be 
explained by noting that the film samples in 
different studies were cast from precursor so- 
lutions with different solvents and concentra- 
tions. Overall, careful choice of the solution 
(both solvent and concentration) can be used 
to produce MEH-PPV films with desired inter- 
chain interactions for particular device appli- 
cations. 
QThC2 8:30 am 
Real time vibronic coupling dynamics in 
organic conjugated systems 
G. Lanzani, G. Cerullo,* S. De Silvestri,* 
0. Svelto, Istituto Nazionale per la Fisica della 
Materia, Istituto di Matematica e Fisica, 
Universitd di Sassari, I-07100, Sassari, Italy; 
E-mail: lanzani@ssmain.uniss. i t  
Upon electronic excitation, linear 7 ~ -  
conjugated chains (both oligomers and poly- 
mers) show substantial alterations in the geo- 
metrical configuration of the carbon skeleton. 
Re-organization into a stable excited state may 
result into variations of the C-C bond length 
up to 5 pm. Bond-order alternation may be 
reduced to zero or switched, and in long chains 
it has been suggested that topological defects 
can be generated.' Two important conse- 
quences of the strong electron-phonon cou- 
pling in such systems are the presence of a 
Peierls distortion in the ground state and the 
possibly large vibronic coupling, especially in 
case of closely lying excited states.2 For these 
reasons, vibrational dynamics and vibronic 
coupling have been the subject of extensive 
studies, theoretical as well as experimental. 
Impulsive coherent vibrational spectroscopy 
(ICVS)3 is a recent approach to this challenge, 
made possible by the advent of new ultrashort 
laser sources (below 10 fs) which allow to 
monitor in real time linear chain normal 
modes with frequency up to 2000 cm-I. 
In this work we show the potentialities of ap- 
plying ICVS to conjugated systems relevant for 
applications. We studied films of sexithiophene 
(TJ,, a candidate for large area molecular elec- 
tronics,4 poly-phenylene vynilene (PPV), a pro- 
totype electroluminescent material: and polidy- 
acetylene (PDA), very promising for applications 
in photonic devices.6 The experiments were per- 
formed using a novel optical parametric ampli- 
fier based on non-collinear phase matching in 
P-barium borate: which provides pulses with 
bandwidths broader than 50 THz, sub-lo-fs du- 
ration and tunability from 490 to 700 nm. 
In Fig. 1 we show pump-probe data for T, 
excited by a pulse with 520 nm center wave- 
length; the probe wavelength, selected with an 
interference filter after the sample, is 510 nm. 
The photoinduced absorption is strongly 
modulated by a complex oscillatory pattern, 
the Fourier transform ofwhich is shown in the 
inset. Five modes are clearly identified, with 
frequency ranging from 110 cm-' to 
1450 cm-'. These data allow a conclusive as- 
signment of the vibronic structure in T, and 
shed light on the vibronic coupling mecha- 
nism between lower excited states. By taking 
the Fourier transform of different time win- 
dows of the signal, we clearly observe a shift of 
the 1450 cm-' mode towards higher frequency 
upon increasing the time delay. This is inter- 
preted as due to relaxation of the vibrational 
wavepacket within an anharmonic potential 
energy surface. 
Authorized licensed use limited to: University of Sassari. Downloaded on March 30,2010 at 06:52:35 EDT from IEEE Xplore.  Restrictions apply. 
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I 
; 
I 
I 
I 
I 
I 
I 
I 
! 
184 / QELS'99 / THURSDAY MORNING 
0.01 
0.00 
r. 
5 
-0.01 
-0.02 
0 500 1000 
QThC2 Fig. 1. Differential transmission 
AT/Tvs. delay for a T, film pumped with a center 
wavelength of 520 nm and probed at 5 10 nm. The 
inset shows the Fourier transform of the oscilla- 
tory component of the signal. 
Time Delay (fs) 
0.04 
0.02 
0.00 
-0.02 
-200 0 200 400 600 800 1000 
Time Delay (fs) 
QThCZ Fig. 2. Differential transmission 
ATIT vs. delay for a PPV film pumped with a 
center wavelength of 540 nm and probed at 
590 nm. The inset shows the Fourier transform 
of the oscillatory component of the signal. 
An example of the results obtained in PPV 
in the stimulated emission (SE) region (pump 
at 540 nm and probe at 590 nm) is given in Fig. 
2. The Fourier transform indicates the pres- 
ence of two strongly coupled modes, assigned 
to the transvinylene C-C stretching (at 
1290 cm-I) and ring quadrant C-C stretching 
(at 1570 cm-I) respectively. In PDA the SE 
signal (pump at 540 nm, probe at 610 nm), 
shown in Fig. 3, is not instantaneous, pointing 
to a thermalization process which is completed 
in about 100 fs. For the first time we observe 
real time oscillations above 2000 wavenum- 
bers, corresponding to the triple carbon bond 
stretching mode of PDA. The data allow to 
investigate the geometrical configuration of 
the excited state, which is expected to have a 
substantial contribution of butatrienic charac- 
ter. Time evolution of the vibrational fre- 
quency is discussed in view of the existing 
theoretical models. 
In conclusion, these experiments demon- 
strate the possibility of studying coherent mo- 
lecular dynamics in organic systems with ex- 
tremely high time resolution. 
*Istituto Nazionale Fisica della Materia, 
CEQSE-CNR, Dipartimento di Fisica, Politec- 
nico, 1-20133 Milano, Italy 
0.04 
0.02 
0.00 
-0.02 
-100 0 LOO 200 300 400 500 
Time Delay (fs) 
QThC2 Fig. 3. Differential transmission 
AT/T vs. delay for a PDA film pumped with a 
center wavelength of 540 nm and probed at 
610 nm. The inset shows the Fourier transform 
of the oscillatory component of the signal. 
1. 
2. 
3. 
4. 
5. 
6. 
7. 
A.J. Heeger, S. Kivelson, J.R. Schrieffer, 
and W.P. Su, Rev. Mod. Phys. 60, 781 
(1988). 
E. Ehrenfreund, Z. Vardeny, 0. Brafman, 
and B. Horovitz, Phys. Rev. B 36, 1535 
(1987); G. Orlandi, F. Zerbetto, Chem. 
Phys. 108, 187 (1986). 
S. Mukamel, Principles of nonlinear optical 
spectroscopy, Oxford University Press, 
New York, 1995. 
C. Ziegler, in Handbook of Conductive 
Molecules and Polymers, edited by H.S. 
Nalwa (Wiley, New York, 1997), Vol. 3, p. 
676. 
J.H. Burroughes, D.D.C. Bradley, A.R. 
Brown, R.N. Marks, K. Mackay, R.H. 
Friend, P.L. Burn, A. Kraft, A.B. Holmes, 
Nature 347,539 (1990). 
Relaxation in Polymers, edited by T. Koba- 
yashi (World Scientific, Singapore, 1993). 
G. Cerullo, M. Nisoli, and S. De Silvestri, 
Appl. Phys. Lett. 71, 3616 (1997); G. 
Cerullo, M. Nisoli, S. Stagira, and S. De 
Silvestri, Opt. Lett. 23, 1283 (1998). 
QThC3 8:45 am 
Photogeneration of triplet states by 
polaron recombination in pari+ 
hexaphenyl 
C. Zenz,* G. CeruUo,** G. Lanzani, 
W. Graupner,t F. Meghdadi,t G. Leising,? 
S. De Silvestri,** Istituto Nazionaleper la 
Fisica della Materia, Istituto di Matematica e 
Fisica, Universitd di Sassari, Via Vienna 2, 
1-07100, Sassari, Italy; E-mail: 
f513zenz@mbox. tu-graz.ac.at 
Dissociation of photo-generated excitons into 
charge carriers (polarons) is the working prin- 
ciple of organic photovoltaic cells, whereas re- 
combination of field-injected charge carriers 
into luminescent singlet excitons and non- 
radiative triplet excitons governs the efficiency 
of organic light emitting devices (OLED). In 
this paper, we address these processes in a 
single layer OLED based on para-hexaphenyl 
(PHP)' by conventional and electric field- 
assisted femtosecond pump/probe measure- 
ments. Due to its well defined conjugation 
length and its chemical structure PHP exhibits 
well resolved photo-induced absorption (PA) 
Energy (eV) 
QThC3 Fig. 1. (a) Conventional transient 
pump/probe spectra of the polycrystalline PHP 
film for different pump probe delays (T~), mea- 
sured in an OLED-structure. (b) Field-induced 
(AT/T),,-spectrum of PHP films for pump- 
probe delay of 20 ps and an applied reverse bias of 
16V. Data points are obtained using a monochro- 
mator or interference filters. The solid line is a 
guide to the eye. 
features, that can be assigned to singlet, triplet 
and polaron absorption. This allows to moni- 
tor the dynamics of field-induced singlet dis- 
sociation* and polaron recombination in a 
single layer OLED structure consisting of alu- 
minum/PHP/Indium Tin Oxide (ITO) with 
200 fs time resolution. 
Figure l(a) shows conventional differential 
transmission spectra for 0 ps and 200 ps pump- 
probe delays (T~). We observe three PA-bands 
peaking at 1.59 eV (PA,), 1.8 eV (PA,), and 
2.09 eV (PA,), and a strong stimulated emis- 
sion (SE) band for probe energies higher than 
2.5 eV. The field-induced changes of PA under 
the same experimental conditions for an ap- 
plied reverse bias of 16 V and a pump-probe 
delay of 20 ps are depicted in Fig. l(b). From 
this spectrum, a field-induced quenching of SE 
and PA, is evident, whereas both PA, and PA, 
are increased. 
PA, is assigned to singlet exciton absorp- 
tion due to the correspondence with SE- 
dynamics3 and the field-induced quenching by 
exciton dissociation. PA, at 2.09 eV can be 
assigned to polaron absorption as indicated by 
theoretical calculations and comparison with 
doping induced measurements. This assign- 
ment is also supported by the observed field- 
induced increase of PA,. 
The absorption band at 1.8 eV (PA,) has 
been assigned to triplet absorption being the 
only persistent spectral feature for pump- 
probe delays longer than 200 ps [see Fig. l(a)] 
and based on comparison with cw-PA mea- 
surements.4 The differential transmittance 
spectrum at 0 ps delay, that already contains 
this feature, suggests a triplet exciton forma- 
tion within the pump pulse ( T ~  < 200 fs). Due 
to the observed dynamics and the energetic 
position of the pump, we exclude intersystem 
crossing (ISC) and singlet fission to be the 
dominant triplet generation mechanism under 
these experimental conditions. More insight 
into the triplet formation can be gained by 
studying the dynamics of the field-induced 
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Real time vibronic coupling dynamics in organic conjugated systems

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