Supplementary Materials
Stark Shifts and Exciton dissociation in CdSe Nanoparticle
grafted Conjugated polymer
Mohan Singh Mehata,*
Department of Applied Physics, Delhi Technological University, Bawana Road, Delhi
110042, India
*
Corresponding Address: msmehata@yahoo.com; msmehata@gmail.com
(A) Material and Methods:
(i) Synthesis of CdSe-S3PPV: In the first step, the sulfide-substituted poly(1,4-phenylene
vinylene) (S3PPV) was synthesized via Gilch route using potassium tert-butoxide as catalyst
and 60 mole % of M1 and 40 mol % of M2 as monomers. Then the CdSe (4-5 nm in size)
was grafted to the sulfur atom in the polymer chain by exchange reaction (Chart S1). Detailed
synthetic procedures were described elsewhere.1
O
O
O
m
+ n
O
Br
t-BuOK
O
Br
m
S
M2
M1
O
S3-PPV
O
n
O
O
O
m
O
nO
m
S
CdSe
S
O
S
CdSe
m
O
O
S
m
S
O
m
n
O
O
n
m=0.6, n=0.4
n
O
n
Br
S
Br
O
O
Chart S1: Synthesis reaction steps of CdSe-S3PPV
1
(ii) Measurements: Absorption measurements were performed with a Hitachi U-3500
spectrophotometer. In the E-A measurements, a converging light beam from Xenon lamp of
Jasco FP-777 spectrofluorometer was collimated and directed through an -barium borate (BBO) polarization prism (CASIX, JDSU) and through the sample slide on an external
photomultiplier, which was further amplified and divided into two channels. The dc
component of the transmitted light intensity was directly collected by the computer and the ac
component synchronized with the applied voltage was recorded by a lock-in amplifier
(SR830, SRS) at the second harmonic of the modulation frequency of the applied electric
field. The amplitude and the phase signals from the output of the lock-in amplifier (I(2))
were finally digitized and recorded by the computer together with the dc component. The E-A
spectra of CdSe-S3PPV were obtained at the normal angle as well as at the magic angle
between the direction of the applied electric field and the electric vector of the excitation
light. To change the angle , we rotated the sample holder following Snell’s law, which
depends on the refractive index of host polymers. Note that the refractive index of PMMA
and PVA is 1.544 and 1.506, respectively. The change in absorption intensity caused by the
application of an electric field is defined as A= - (22/ln 10) I(2v)/I, where the factor 22
converts the value of measured rms signal to it's an equivalent dc signal, and I and I
represent the transmitted light intensity and its field-induced change, respectively.
E-PL spectra were measured under vacuum conditions at temperatures near 295 K.
The photoluminescence (PL) intensity at zero field and field-induced change are represented
by IPL and IPL (=IPL(F0) – IPL(F=0)), respectively. It is noted that E-PL spectra are different
from the so-called electroluminescence (EL) spectra; E-PL spectra are obtained as a result of
the electric-field-induced change in PL spectra, while EL spectra are obtained in the presence
of electric fields without photoirradiation. In the present measurements of the E-PL spectra,
the contribution of the EL intensity was negligible as an insulator film of PMMA much
2
thicker was inserted between CdSe-S3PPV film and Al electrode. In every case, E-PL spectra
were obtained with excitation at wavelengths at which the field-induced-change in absorption
intensity was negligible. The observed change in total emission intensity in the presence of F
is therefore ascribed to the field-induced change in the total emission quantum yield, which
might result from the field-induced change in the population of the emitting state and/or the
QY of the emitting state.
(B) Theoretical background of E-A and E-PL spectra
Electric field-induced change in absorption spectra or emission spectra were described
previously,2,3 based on the theory developed by Liptay.4 Energy levels of a molecule or
molecular system are influenced by an electric field depending on the permanent electric
dipole moment  and the molecular polarizability  of the state concerned. As a result,
optical spectra show a shift or/and a broadening. The change in transition energy (E) is
given by
E    F  1 F    F ,
2
(1)
Here,  (= e-g) and  (= e-g) are the differences in dipole moment vector and
molecular polarizability tensor, respectively, between the ground g and excited e states. By
assuming an isotropic distribution of the sample, the change in absorption intensity (E-A) or
photoluminescence intensity (E-PL) as function of wavenumber, , in the presence of F, i.e.
A() or IPL() observed at the second harmonic of the modulation frequency of the applied
electric field F can be expressed as a sum of the zeroth-, first- and second-derivative of the
unperturbed absorption and photoluminescence spectra as follows:2-4

d  A( ) 
d 2  A( ) 
A( )  ( fF )  A A( )  B

  C 


d   
d 2   

(2)
 '
d IPL ( ) 
d 2 I ( ) 
'
'
IPL ( )  ( fF) 2 A IPL ( )  B  3
 3  C  3 2  PL 3 
d   
d   

(3)
2

3
in which f is the internal field factor and F is the electric field strength. Coefficients A and
A' correspond to the field-induced change in the absorption and PL intensity, respectively,
whereas coefficients B (B') and C (C') correspond to the spectral shift and spectral
broadening of the E-A/E-PL spectra resulting from the difference in molecular polarizability
() and in electric dipole moment (), respectively, following the optical transition. By
neglecting
the
field-induced
change
in
transition
moment
polarizability
and
hyperpolarizability, the coefficients A , B and C can be expressed as follows:
B 
 ( m   )(3cos2  1) 
 

2hc 
10hc

2
2
2  5  (3 cos   1)(3 cos   1) 
C    

30h 2 c 2



(4)
(5)
1
in which  = e~ g, and   r( ) = e~g, with subscripts e and g representing
3
the excited and ground states, respectively. m represents the diagonal component of 

with respect to the direction of the transition dipole moment;  is the angle between the
direction of  and the transition dipole moment.
References
1
K. B. Chen, M. H. Chen, S. H. Yang, C. H. Hsieh, C. S. Hsu, C. C. Chen, H. J. Chien, J.
Poly. Sci. A: Poly. Chem. 44, 5378 (2006).
2
M. S. Mehata, T. Iimori, T. Yoshizawa, N. Ohta, J. Phys. Chem. A 110, 10985 (2006).
3
E. Jalviste, N. Ohta, J. Photochem. Photobiol. C 8, 30 (2007).
4
W. Liptay.; in ‘Excited States’, Lim, E.C.; Ed.; Academic Press: New York, 1, 129 (1974).
4