Long-lived photoexcited states in polydiacetylenes: The photoinduced-absorption spectra
of PDA-4BCMU
D. Comoretto, I. Moggio, C. Cuniberti, G. F. Musso, and G. Dellepiane
Dipartimento di Chimica e Chimica Industriale, Universita ` degli Studi di Genova, via Dodecaneso 31, I-16146 Genova, Italy
A. Borghesi
Istituto Nazionale per la Fisica della Materia-Dipartimento di Scienze dei Materiali, Universita ` degli Studi di Milano,
via Emanueli 15, I-20126 Milano, Italy
F. Kajzar and A. Lorin
Commissariat a ` l’Energie Atomique, Direction des Technologies Avance ´es, LETI, DEIN/SPE/GCO, CE Saclay,
91191 Gif Sur Yvette, Cedex, France
Received 14 October 1997
We report the results of steady-state photoinduced absorption experiments on PDA-4BCMU grown on silica
substrate. We observe three photoinduced absorption bands at 0.82, 0.95, and 1.39 eV whose relaxation
dynamics are identical for the two low-energy peaks but different for the 1.39-eV band. On the basis of the
spectra, the pump intensity, modulation frequency, and signal phase dependences we conclude that the 0.82
and 0.95-eV absorption features are due to charged states while the 1.39-eV peak is originated by triplet
excitons. The photogeneration of both charged and neutral states in PDA-4BCMU as previously observed in
polycarbazolyldiacetylenes indicates that this is a general feature of polydiacetylenes. The relative weight of
one excited species over the other one is influenced by the molecular and supramolecular organization of the
polymer. S0163-18299802812-4
I. INTRODUCTION
The nature of excitations in -conjugated polymers
has been extensively studied by the continuous wave
photoinduced absorption PA technique. Among the
conjugated systems, the polydiacetylenes PDA’s,
( vCR—CwC—CR'v )
n
, have attracted particular interest
because they can be prepared in the form of macroscopic
single crystals through solid-state topochemical polymeriza-
tion of the corresponding monomers. In addition, the possi-
bility of changing the substituents of the polymer backbone
allows us to obtain a variety of materials with properly de-
signed optical and electronic properties. For practical appli-
cations, PDA’s grown on suitable substrates by the epitaxial
technique or, when soluble, cast from solution are usually
considered. Polymerization of diacetylenic crystals usually
leads to polymers in the blue phase characterized by a well-
defined singlet excitonic peak around 620 nm.
Thermochromic
1
and solvatochromic
2
phase changes are,
however, possible and in this condition a red form character-
ized by an absorption band around 540 nm is obtained. The
study of the different forms of PDA’s, due to their differ-
ent conjugation length, provides additional information
on the photophysics of these polymers. The PA spectra of
this class of polymers so far reported by other authors
usually deal with the blue form of PDA’s. A strong PA
peak around 1.4 eV has been observed for PDA-TS
( R vR vuCH
2
SO
3
C
6
H
4
CH
3
) Refs. 3 and 4 and
attributed to neutral photoexcitations on the basis of the
results of the study of its decay in the presence of an ap-
plied magnetic field.
5
For the same polymer no evidence
of photoexcited charged states in the NIR region is reported.
On the contrary, the presence of PA signals associated
only with charged photoexcitations has been found in
PDA-1OH ( R vuCH
3
, R vuCH
2
OH) and PDA-9PA
( R vR ' vuCH
2
9
OCOCH
2
C
6
H
5
).
4,6,7
In these cases the
assignment has been substantiated by the presence in the
low-frequency region of the PA spectrum of infrared acti-
vated vibrational IRAV modes, which are associated with
charged states.
6,7
To make this picture more confused, IRAV
modes are reported to be present also in PDA-TS and PDA-
4BCMU ( R vR ' vuCH
2
4
OCONHCH
2
COOC
4
H
9
).
6,7
In all these cases, no light-induced electron spin-
resonance LESR signals were detected suggesting
the photogeneration of doubly charged spinless bipo-
larons.
7
The presence of triplet excitons in the blue
form of PDA-4BCMU Ref. 8 and PDA-3BCMU
( R vR ' vuCH
2
3
OCONHCH
2
COOC
4
H
9
) Ref. 9 has
been detected around 1.4 eV with picosecond PA spectra for
probe energies above 1.2 eV.
For the red form of PDA’s only a few experimental data
are available for PDA-4BCMU Refs. 8 and 10 and PDA-
ClCin22 polybis-p -chlorocinnamate, 10, 12-docosadiyn-1,
22-diol.
10
The PA peak observed at 1.4–1.5 eV is attributed
to neutral excitations triplet excitons.
In the past few years we have been interested in studying
a class of polydiacetylenes having the carbazolyl group at-
tached directly
11–14
or indirectly,
14–20
through a CH
2
spacer,
to the main chain. Our interest in the polycarbazolyldiacety-
lenes PCzDAs is dictated by the particularly high value
of the third-order polarizability
21,22
and enhanced photo-
conductive response
23
observed for polyDCHD
( R vR ' vuCH
2
NC
12
H
8
) as well as by the unusual excita-
tion profile of the photocurrent in polyCPDO
PHYSICAL REVIEW B 15 MARCH 1998-II VOLUME 57, NUMBER 12
57 0163-1829/98/5712/70718/$15.00 7071 © 1998 The American Physical Society