Solvent tuned excited state configuration mixing in a π-conjugated metal-organic oligomer

Article abstract of DOI:10.1039/b403084b

The photophysical properties of a π-conjugated metal-organic oligomer vary smoothly with solvent composition. The variation is believed to arise from solvent-tuned configuration mixing of 3π,π* and ³MLCT levels.

Full text of DOI:10.1039/b403084b

Solvent tuned excited state configuration mixing in a p-conjugated
metal–organic oligomer†
Shengxia Liu and Kirk S. Schanze*
Department of Chemistry, University of Florida, Gainesville, FL, 32611-7200, USA.
E-mail: kschanze@chem.ufl.edu
Received (in West Lafayette, IN, USA) 1st March 2004, Accepted 29th April 2004
First published as an Advance Article on the web 1st June 2004
The photophysical properties of a p-conjugated metal–organic
oligomer vary smoothly with solvent composition. The variation
is believed to arise from solvent-tuned configuration mixing of
3p,p* and 3MLCT levels.
There is current interest in the optical properties of p-conjugated
oligomers and polymers that contain transition metals that interact
strongly with the p-conjugated electronic system.1 These materials
have potential for application in light-emitting diodes, solar cells,
photoconductors and luminescent sensors.2 In order to optimize
these materials for opto-electronic applications, it is necessary to
understand the factors that control their optical properties. To
achieve this objective, we are exploring the excited state properties
of p-conjugated oligomers and polymers that contain complexes of
Re(
), Ru(II), Ir(III) and Pt(II) as part of the p-conjugated system.3
I
These materials exhibit long-lived, luminescent triplet excited
states that are derived either from p,p* transitions localized on the
p-conjugated segment, or metal-to-ligand charge transfer transi-
tions localized on the metal complex chromophores (e.g., 3p,p* and
3MLCT, respectively).
We recently designed several diblock p-conjugated oligomers
that are end-capped with metal–bipyridine chromophores with the
objective of determining whether it is possible to decouple the
3
p,p* excitations from the 3MLCT states that are localized on the
metal chromophores. As part of this work T1–Ru (structure below)
was prepared and its photophysical properties characterized. This
oligomer features a p-conjugated core segment consisting of a
3-alkylthiophene that is linked to short oligo(arylene ethynylene)
(OAE) segments. The oligomer is capped on both ends with
(L)Ru(bpy)22+ chromophores. It was anticipated that in this system
there would be an energetically low lying 3p,p* state localized on
the three ring p-system (phenyl–C·C–thienyl–C·C–phenyl) in the
core of the oligomer. Because of the relatively large distance
Fig. 1 (a) Photoluminescence spectra of T1–Ru in CH2Cl2, THF and
CH2Cl2 solvent mixtures. Volume fraction of CH2Cl2 (in order of
decreasing lmax): 0, 0.25, 0.50, 0.75 and 1.0. (b) Plot showing knr, kr and tem
for T1–Ru in CH2Cl2/THF solvent mixtures. Note that the scale for kr is 104
s21, whereas that for knr is 105 s21
.
2+
between the oligomer core segment and the (L)Ru(bpy)2
photophysical properties that result are a composite of the two
contributing configurations.
chromophores, it was anticipated that the 3MLCT and 3p,p* states
might be decoupled, and therefore could be separately observed by
photoluminescence and/or transient absorption spectroscopy.
The present communication describes the effect of solvent on the
excited state properties of T1–Ru.† As described in detail below,
photoluminescence and transient absorption spectroscopy (PL and
TA, respectively) suggest that in THF solution the lowest excited
state is 3MLCT, whereas in CH2Cl2 the oligomer-based 3p,p* state
is lowest. The remarkable observation is that the excited state
properties of T1–Ru vary smoothly in THF-CH2Cl2 solvent
mixtures, from 3MLCT in THF to 3p,p* CH2Cl2, i.e., there is not a
distinct state “crossover” which occurs as the solvent is varied. This
finding strongly suggests that when the two excited state manifolds
are in close in energy they undergo configuration mixing,4 and the
The absorption spectrum of T1–Ru is dominated by an intense
transition with lmax = 399 nm in THF and 392 nm in CH2Cl2 (emax
in THF is 88,400 M21cm21). This transition arises from the long-
axis polarized p,p* transitions of the OAE chromophore. The
MLCT absorption for the (L)RuII(bpy)22+ chromophores appears as
a shoulder on the red side of the more intense p,p* band. In solution
at ambient temperature T1–Ru displays a moderately intense red
photoluminescence. While exploring the properties of the lumines-
cence, it was noticed that its wavelength maximum (lmax,PL) and
bandshape is moderately solvent dependent (Fig. 1a). In particular,
in CH2Cl2 lmax,PL = 640 nm and the emission features a well-
defined vibronic shoulder, whereas in THF lmax,PL = 659 nm and
the band is noticeably broader and the vibronic structure is less
well-defined. On the basis of previous work, we suspected that the
solvent-induced change in lmax,PL and band-shape might arise due
to a switch in the lowest excited state.3c,d The blue-shifted,
structured emission observed in CH2Cl2 has the characteristics
† Electronic supplementary information (ESI) available: complete details of
the synthesis and characterization of T1–Ru. See http://www.rsc.org/
suppdata/cc/b4/b403084b/
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expected for p,p* luminescence, while the broad, structureless
those of the emission, indicating that the TA arises from the excited
state. It is notable that there are significant differences among the
TA spectra in the three solvent compositions. In CH2Cl2 the
transient bleach is broad (i.e., through the entire ground state p,p*
absorption band) and the broad transient absorption extends from
500 nm out to 1600 nm in the near-IR. By contrast, in THF the
bleach is narrower (i.e., only the red side of the ground state
absorption is bleached) and the transient absorption is mainly in the
visible and features two distinct maxima at 510 and 645 nm. In the
solvent mixture, the TA spectrum is intermediate to those in the
pure solvents. In previous work we have characterized the TA
difference spectra of metal-containing p-conjugated oligomers,3
and on the basis of this work we conclude that the TA difference
spectrum of T1–Ru in CH2Cl2 is characteristic of an oligomer-
emission seen in THF is consistent with 3MLCT emission. As
shown in Fig. 1a, the solvent effect on lmax,PL and the emission
bandshape occurs progressively as the composition of the CH2Cl2/
THF solvent mixture is varied.
More insight into the solvent effect on the photophysics of T1–
Ru comes from examination of the radiative and non-radiative
decay rates (kr and knr, respectively) determined by measuring the
emission lifetime and quantum yield (tem and fem, respectively) of
the complex as a function of solvent composition.‡ Fig. 2 shows
plots of tem, kr and knr vs. volume fraction of CH2Cl2 (fMeCl) in the
THF/CH2Cl2 solvent mixture. It is evident that these parameters
vary smoothly with solvent composition. The lifetime increases
monotonically from ≈ 1 ms in THF to ≈ 10 ms in CH2Cl2. The
increase in lifetime arises because knr and kr decrease with
increasing fMeCl. Importantly, the decay parameters observed for
T1–Ru in THF are comparable to those of Ru–polypyridine
3
based p,p* excited state, while that observed in THF is
characteristic of a 3MLCT state localized on one of the
2+
(L)Ru(bpy)2 units. It is important to emphasize that the TA
3
complexes in which MLCT is the lowest excited state (e.g., for
difference spectrum of T1–Ru in CH2Cl2 exhibits a broad near-IR
Ru(bpy)32+ in CH2Cl2: tem = 488 ns, kr = 5.9 3 104 s21 and knr
=
excited state absorption that is believed to be the signature of the
3.5 3 105 s21)5 By contrast, the values of the decay parameters
observed for the complex in CH2Cl2 solution are comparable to
those of transition metal complexes in which the lowest excited
state is based on an intraligand 3p,p* configuration.6 Notably, for
T1–Ru in CH2Cl2 solution kr < 104 s21; the low radiative rate
constant suggests a decreased contribution of spin–orbit coupling,
which likely arises due to a decrease in the contribution of metal-
p,p* exciton in OAE systems.3a,b,d,6 By contrast, the narrow
3
transient bleach, and structured visible transient absorption bands
are characteristic of an MLCT configuration.3c,d
All of the available evidence points to the fact that for T1–Ru in
THF the long lived excited state is 3MLCT, whereas in CH2Cl2 it is
an OAE based 3p,p* state. The most interesting feature, however,
is that in intermediate solvent mixtures, the photophysical proper-
3
3
based orbitals in the excited state configuration (i.e., less MLCT
ties vary smoothly between those of pure 3MLCT and p,p*
character).
character. This effect is believed to arise because the lowest excited
state in these complexes is derived from configuration mixing
Taken together, the emission spectra and decay parameters
strongly suggest that the nature of the emissive excited state in T1–
Ru varies smoothly from 3MLCT in THF solution to p,p* in
CH2Cl2. Additional evidence for this transition comes from UV/
visible/near-IR transient absorption (TA) spectroscopy experi-
ments on T1–Ru in THF, CH2Cl2 and in 1 : 1 THF/CH2Cl2 (Fig. 2).
In each case strong TA is observed and its decay kinetics match
3
between states having pure 3MLCT and OAE-based p,p*
3
character. It is likely that variation of solvent changes the energy
gap between the two pure configurations, leading to variation in the
extent of the configuration mixing. This finding is significant in the
context of transition metal containing p-conjugated systems,
because it clearly demonstrates that the photophysical properties of
the systems will be a composite of those of the p-conjugated system
and the metal-complex localized charge transfer states. In addition,
it suggests that when the two excited state manifolds have similar
energies the photophysical properties of the system may be strongly
dependent upon the environment.
We acknowledge the US National Science Foundation for
support of this work (grant No. CHE-0211252).
Notes and references
‡ For all solvent compositions the emission and transient absorption decays
are strictly first-order (i.e., they follow single exponential kinetics).
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Fig. 2 Transient absorption difference spectra of T1–Ru obtained 50 ns
following 355 nm excitation. Solvent: (a) CH2Cl2; (b) CH2Cl2 : THF (1 : 1
v:v); (c) THF.
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