The photophysical properties of chromophores at high (100 mM and above) concentrations in polymers and as neat solids

Article abstract of DOI:10.1039/b605925b

The absorption, fluorescence, and photostability of five conjugated chromophores: perylene, 2,5,8,11-tetra-t-butyl perylene (TTBP), perylene orange (PO), perylene red (PR), and a zwitterionic Meisenheimer complex (MHC), are studied as a function of concentration in poly(methyl methacrylate) (PMMA). At 1 mM concentrations, all five molecules exhibit properties consistent with unaggregated chromophores. At higher concentrations, perylene and PO both exhibit excimer formation, while TTBP, PR, and the MHC retain their monomeric fluorescent lineshapes. In these three molecules, however, the fluorescence decay times decrease by 10% (TTBP) to 50% (MHC) at concentrations of 100 mM in PMMA. The fluorescence properties of these highly concentrated samples are sensitive to the sample preparation conditions. In the neat solid where the effective concentration is on the order of 1 M, all three molecules exhibit very fast fluorescence decays, on the order of 150 ps or less, despite the fact that they retain their basic monomeric fluorescence lineshape. In addition to the enhanced nonradiative decay at high concentrations, these three molecules also undergo a concentration-dependent photobleaching. The combined effects of intermolecular nonradiative decay channels and photobleaching appear to be a general obstacle to achieving highly concentrated dye-doped solids. the Owner Societies 2006.

Full text of DOI:10.1039/b605925b

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PAPER
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The photophysical properties of chromophores at high (100 mM and
above) concentrations in polymers and as neat solidsw
Rabih O. Al-Kaysi, Tai Sang Ahn, Astrid M. Muller and Christopher J. Bardeen*
¨
Received 26th April 2006, Accepted 31st May 2006
First published as an Advance Article on the web 19th June 2006
DOI: 10.1039/b605925b
The absorption, fluorescence, and photostability of five conjugated chromophores: perylene,
2,5,8,11-tetra-t-butyl perylene (TTBP), perylene orange (PO), perylene red (PR), and a
zwitterionic Meisenheimer complex (MHC), are studied as a function of concentration in
poly(methyl methacrylate) (PMMA). At 1 mM concentrations, all five molecules exhibit
properties consistent with unaggregated chromophores. At higher concentrations, perylene and
PO both exhibit excimer formation, while TTBP, PR, and the MHC retain their monomeric
fluorescent lineshapes. In these three molecules, however, the fluorescence decay times decrease by
10% (TTBP) to 50% (MHC) at concentrations of 100 mM in PMMA. The fluorescence
properties of these highly concentrated samples are sensitive to the sample preparation conditions.
In the neat solid where the effective concentration is on the order of 1 M, all three molecules
exhibit very fast fluorescence decays, on the order of 150 ps or less, despite the fact that they
retain their basic monomeric fluorescence lineshape. In addition to the enhanced nonradiative
decay at high concentrations, these three molecules also undergo a concentration-dependent
photobleaching. The combined effects of intermolecular nonradiative decay channels and
photobleaching appear to be a general obstacle to achieving highly concentrated dye-doped solids.
Introduction
1
D ¼ Ar4=3R60
ð1Þ
trad
Electronic energy transfer (EET) between identical chromo-
phores plays an important role in a variety of technological
where r is the chromophore number density, R0 is the Forster
radius for donor–donor FRET, trad is the radiative lifetime of
the isolated donor, and A is a constant on the order of 0.5.14
Given the diffusion constant D, the 3-D diffusion length of the
exciton LD is given by
applications of organic electronic materials. In organic solar
cells, photocurrent generation relies on exciton diffusion to
charge separation regions where the photoexcitations are
ionized into electron-hole pairs.1–3 Conjugated polymer fluor-
escence sensors rely on energy transport along the polymer
backbone, which allows a single binding event to modulate the
fluorescence output of multiple chromophore units.4–6 The
mobility of excitations is what gives rise to the superquenching
or amplification effect in such sensor systems. In both applica-
tions, one wants to optimize the EET properties of a given
material. In organic light-emitting diodes, on the other hand,
exciton diffusion can lead to luminescence quenching at defect
sites which is detrimental to device performance.7 In all three
applications, optimizing device performance depends on un-
derstanding the underlying chemical mechanism of the EET.
The basic physics of EET is well-studied, especially in the limit
of the incoherent dipole–dipole interaction, or Forster reso-
nance energy transfer (FRET). In this limit, Forster and later
investigators8–13 have shown how FRET occurring between
identical chromophores in a concentrated sample leads to an
effective diffusion constant for the excitation,
pffiffiffiffiffiffiffiffiffiffiffi
LD
¼
6Dtfl
ð2Þ
where tfl is the fluorescence lifetime of the donor. These two
equations provide a straightforward way to predict energy
transport properties in organic materials, assuming that en-
ergetic disorder, which can lead to anomalous diffusion,15–18
does not play a significant role. A quick calculation demon-
strates that, given a Forster radius R0 = 5 nm (an upper limit
for most commonly used organic dyes), in order to achieve a
diffusion length of 1 nm or more requires dye molecule
densities on the order of 1018 molecules cmꢀ3 or greater, or
>1 mM (milli-molar). Such high densities are typically found
in conjugated organic solids and in self-assembled chromo-
phore aggregates, but in both cases the spectroscopy can be
complicated by the presence of intermolecular electronic spe-
cies like delocalized exciton and excimer states.19–21 The
presence of chemical impurities, like carbonyl groups, in
conjugated polymer samples can also complicate the interpre-
tation of experimental results.22,23 It is also difficult to vary r
continuously in such systems due to phase separation. Ideally,
one would be able to tune r by varying the solid-state
concentration of a monomeric chromophore, which is resis-
tant to aggregation and self-quenching. Such a system would
Department of Chemistry, University of California, Riverside, CA
92521, USA. E-mail: christopher.bardeen@ucr.edu; Fax: 951-827-
4713; Tel: 951-827-2723
w The HTML version of this article has been enhanced with additional
colour images.
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hydrocarbons. We compare the photophysical properties of
this set of molecules for dilute solutions, concentrated solu-
tions, and in the solid state. In particular, we are interested in
how the monomer emission changes at high chromophore
concentrations, whether due to the formation of new emitting
states, like excimers, or due to enhanced nonradiative quench-
ing. We also compare the photostability of the concentrated
monomer systems with that of poly[2-methoxy-5-(20-ethyl-
hexyloxy)-1,4-phenylenevinylene] (MEH-PPV), a widely stu-
died conjugated polymer. We find that although some of the
sterically hindered perylenes retain their monomeric emission
spectra and lifetimes up to very high (B100 mM) concentra-
tions, such high concentrations also lead to accelerated
photobleaching. For the molecules studied here, chromo-
phore–chromophore interactions lead to chemical reactions
which can be either reversible (excimer formation) or irrever-
sible (photobleaching), but in either case they prevent the
realization of dye/polymer solids with chromophore concen-
trations comparable to those of conjugated polymer films.
Experimental
Perylene orange (PO) and perylene red (PR) were received
from BASF. PR was purified by column chromatography
(silica gel, and dichloromethane as eluting solvent). All other
materials including perylene, were purchased from Aldrich
and used without further purification. Ultrapure water from a
MilliQ filtration was used throughout the experiments.
The zwitterionic Meisenheimer complex, N0,N0 0,N0 0 0-tri(isopro-
pyl)-4-oxo-6-(isopropylimino)-2-s-(2H)triazinespiro-10-20,40,60-tri-
nitrocyclohexadienylide (MHC) was synthesized and purified
according to a previously reported method.30 An overall yield
of 15.5% for the red MHC crystals was obtained. 1H NMR (300
MHz, CDCl3): d = 1.22 (d, J = 6.9 Hz, 6 H), 1.33 (d, J = 7.2
Hz, 6 H), 1.45 (d, J = 6.3 Hz, 6 H), 1.72 (d, J = 6.6 Hz, 6 H),
3.14 (septet, J = 6.9 Hz, 1 H), 3.87 (septet, J = 7.2 Hz, 1 H)
3.95–4.10 (m, 1 H), 4.18 (septet, J = 6.6 Hz, 1 H), 4.30 (d, J =
9.0 Hz, 1 H), 9.01 (s, 2 H) ppm. 13C NMR (75 MHz, CDCl3): d
= 19.90, 22.01, 23.11, 24.10, 51.05, 51.10, 53.12, 58.40, 82.28,
120.13, 125.54, 130.83, 145.01, 154.71 ppm. Elemental analysis
for MHC (C20H31N7O7) calculated: C, 49.91%; H, 6.44%; N,
20.38%; found; C, 49.86%; H, 6.51%, N, 20.09.
Fig. 1 Structures of perylene, perylene red, Meisenheimer complex,
tetra-tert-butylperylene, and perylene orange.
not only provide a way to test the validity of diffusive theories
of Forster energy transport, but would also find applications
in solid state dye lasers and organic light emitting diodes.
In principle, it should be possible to control chromo-
phore–chromophore interactions by modifying the chemical
structure of the dye, e.g. by adding bulky sidegroups, which
insulate the conjugated portion of the molecule. There are
several studies, which examine the role of such steric modifica-
tions.24–27 While valuable, these studies often include only a
partial characterization of the photophysical properties. For
example, the steady-state fluorescence lineshape of the mono-
mer in dilute solution is compared with that of a highly
concentrated sample, without examining other possible effects,
like self-quenching and photostability. In this paper, we pre-
sent a more complete study of the concentration-dependent
photophysical properties of a series of visible-absorbing chro-
mophores in poly(methyl methacrylate) (PMMA). The mole-
cules are shown in Fig. 1. In addition to perylene, we examine
three sterically hindered perylene derivatives: 2,5,8,11-tetra-
tert-butyl perylene (TTBP), perylene orange (PO), and per-
ylene red (PR). The perylenes have well-studied spectroscopic
properties and are commonly used in devices like lasers28 and
organic light-emitting diodes.29 We also study a zwitterionic
spiro-cyclical complex, denoted as the Meisenheimer complex
(MHC)30 whose nonplanar geometry should inhibit the co-
facial aggregation commonly seen in polycyclical aromatic
Synthesis of 2,5,8,11-tetra-tert-butylperylene (TTBP)
Our synthesis of TTBP involves some variations from the
procedure usually cited in the literature,31,32 and so we report
it in detail here. In a 100 mL round bottom flask perylene
(0.50 g, 0.002 moles) was added to 2-methyl-2-chloropropane
(50 mL) previously dried over activated molecular sieves. The
mixture was sonicated and agitated to brake up the large
pieces of perylene and form a finely dispersed suspension.
Aluminum trichloride anhydrous powder (1.0 g, 0.0075 moles)
was gradually added to the rapidly stirring mixture of perylene
in 2-methyl-2-chloropropane. The mixture was then refluxed
under argon for 12 h. An additional 25 mL of 2-methyl-2-
chloropropane was added followed by aluminum trichloride
(1.0 g, 0.0075 moles), and refluxed for 12 h more. Finally
another 20 mL of 2-methyl-2-chloropropane was added along
with aluminum trichloride (1.0 g, 0.0075 moles) and refluxed
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under argon for 12 h until a TLC (silica gel, solvent; 2 : 1
hexane : dichloromethane) of the reaction mixture gave only
one product spot. The organic phase was then agitated with
100 mL portions of distilled water until the aqueous phase
became clear. The dark viscous organic mixture was spread on
a flat glass dish and heated at 170 1C over a hot plate in a well
ventilated area. The organic mixture was allowed to bake for
24 h or until smoke ceased to evolve. The black tar residue
(B0.9 g) was dissolved in dichloromethane (5 mL) followed by
hexanes (10 mL). Purification of the crude product was
achieved through column chromatography (activated neutral
alumina, and hexanes as the eluting solvents). A black residue
was retained at the top of the column while the yellow product
was slowly eluted through and collected. The solvent was
removed under reduced pressure and a yellow powder was
obtained with a yield of 0.45 g (49%). The 1H NMR (400
MHZ, CDCl3) of the product is consistent with the reported
values, d = 1.50 (s, 36 H), 7.64 (s, 4 H), 8.25 (s, 4 H). A low
resolution mass spectrum of the purified compound yielded a
single, large molecular ion peak at the expected MW = 476.
Higher MW peaks are only a few percent or less of the parent
ion peak, with none where they would be expected for >4 tert-
butyl groups, indicating that higher molecular weight products
of the substitution reaction are negligible.
Thermal annealing of the TTBP/PMMA films
A 0.3 M TTBP in PMMA was prepared as described pre-
viously. The sample was introduced in a sealed glass container
and purged with argon. Then it was heated in an oil bath set at
130 1C (Tg of PMMA B 110 1C) for no more than 20 min. To
insure the integrity of the TTBP in PMMA, an absorption
spectrum of the TTBP/PMMA polymer sample was collected
before and after the annealing process which showed no
measurable decrease in absorption or change in spectral shape.
Heating for longer than 20 min resulted in a decrease in
absorption, and such samples were not used for the spectro-
scopic measurements.
Absorption, fluorescence spectra and photophysics
Ground state absorption spectra of the polymer films were
collected using an Ocean Optics SD2000 absorption spectro-
meter. Steady state fluorescence was determined using Spex
Fluorolog Tau-3 fluorescence spectrophotometer with front
face excitation (lex = 400 nm) of the samples. Fluorescence
lifetimes were measured by exciting the samples using pulses
centered at 400 nm, derived from the frequency doubled
output of a 40 kHz regeneratively amplified Ti:Sapphire
laser system with a pulse width of 150 fs. To purify the
polarization of the excitation beam, the laser was passed
through a calcite polarizer before exciting the sample. The
fluorescence emission of liquid samples was collected at 901
relative to the excitation, whereas for polymer and solid film
samples the incident angle of the exciting laser beam was about
101 relative to the film surface normal. The excitation pulses
exhibited linear polarization, and the angle between the polar-
ization of the collected fluorescence light and that of the
excitation light was adjustable for anisotropy measurements,
using a polarizer that was placed in the emission beam path.
The emitted light was collimated and focused into a mono-
chromator (Spectra Pro-150 spectrometer) attached to a pico-
second streak camera (Hamamatsu C4334 Streakscope),
which provides both time and wavelength-resolved fluores-
cence data, with a spectral resolution of 2.5 nm and a temporal
resolution of 15 ps.
Preparation of the polymer films
Poly(methyl methacrylate) (PMMA, 1.0 g, 120 000 average
molecular weight) was dissolved in tetrahydrofuran (THF,
10 mL) to make a concentration CPMMA = 0.10 g PMMA
mLꢀ1 of THF. A solution of perylene, PO, PR, MHC or
TTBP in THF (Cdye = 0.02 molar) was prepared and stored in
the dark. Note Cdye and CPMMA have different units. Different
ratios of PMMA in THF solution were combined with the
pre-prepared solutions of the dyes in THF to make a concen-
tration Cdoped in the polymer using the formula:
moles dye
Cdye ꢂ Vdye solution
Cdoped
¼
¼
ð3aÞ
volume PMMA CPMMA ꢂ VPPMA solution=d
which can be rearranged to obtain
VPMMA solution
Cdye ꢂ d
CPMMA ꢂ Cdoped
¼
ð3bÞ
Photobleaching studies
Vdye solution
The photobleaching studies were done in a manner similar
to that used previously to evaluate conjugated polymers.33
All samples were maintained under an active vacuum of 10ꢀ4
Torr in a cryostat. The samples were irradiated with the 400
nm femtosecond laser pulses, with an average power of 10 mW
focused to an 80 mm diameter spot. Since PR samples have
low absorbance at 400 nm, the photobleaching was also
studied using 532 nm cw excitation. Data from both types of
experiments were consistent. Increasing or decreasing the
laser pulse energy by a factor of 3 did not change the observed
photobleaching rates; in other words, the photobleaching
depended only on the number of absorbed photons, as
calculated from eqn (5), and not on the peak power of the
pulse. Thus effects due to multiphoton absorption from
the excited state appear to be negligible given the powers
used here.
where V is either the volume of dye solution or PMMA
solution in mL and d = 1.19 g cmꢀ3 is the density of the
polymer. Glass microscope slides (cut to 2.5 ꢂ 2 cm) were
washed with acetone, wiped clean and placed on a spin coater
(Laurell, WS-400B-6TFM-LITE). While the glass slide was
spinning at 3600 rpm, 0.1 mL of the dye/PMMA mixture in
THF was added using a pipette. The sample was allowed to
spin for 1 min, then immediately loaded into a Janis ST-100
continuous-flow cryostat and placed under vacuum (less than
10ꢀ4 Torr) to prevent photooxidative damage. The solid
samples were prepared by spin-casting the dye/THF solution
directly onto the microscope slide. The liquid samples were
prepared by diluting the dye/THF solution until their optical
densities in a 1 cm length cuvette were less than 0.1 at and
above the excitation wavelength.
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The photon flux F, which is the number of photons hitting a
unit area of the sample per second, is given by
It should be noted that even though the excimer emission
contribution comprises 96% of the steady state emission in PO
(Table 1), this does not necessarily mean that 96% of the PO
molecules are bound in excimers. Rapid EET from monomers
to the low-lying excimer states, which act as luminescence
traps, can lead to the observed enhancement even at relatively
low excimer populations.36 In contrast to the end-substituted
perylenes, ring-substituted perylenes like TTBP and PR
showed no identifiable excimer component at the highest
concentrations, as can be seen from Fig. 2c and d. In fact,
even solid films of these two molecules, cast from a THF
solutions onto a glass surface, retained their monomeric
fluorescence lineshape. This type of monomer emission is also
seen in concentrated solutions and solid films of the nonplanar
MHC, which is shown in Fig. 2e.
Ilaser
P=pr2
hc=l
F ¼
¼
ð4Þ
Ephoton
hc
l
where Ephoton
¼
is the energy of the laser photons, l is the
laser wavelength, and P is the measured laser power. The
radius r of the focused laser beam at its focus is obtained by
translating a knife-edge across the focal point. The rate R at
which each molecule absorbs the photons is
R = Fs
(5)
where s is the absorption cross section of the dye molecule at
the laser wavelength, obtained from the absorption coeffi-
cient.34 The highest value of R in our measurements was a
few MHz. Since the fluorescence lifetimes are less than 10 ns, it
is unlikely that the molecule has a chance to see another
photon while it is in its electronically excited state. The average
number of photons a molecule will have absorbed after a time
t would be Rt. The fluorescence signal is proportional to the
number of molecules in the region illuminated by the laser
beam. The ratio of the fluorescence signal relative to its initial
value gives the probability that the dye molecule has photo-
bleached after absorbing Rt photons.
A fluorescence spectrum similar to that of the monomer
does not by itself prove the absence of intermolecular inter-
actions. Such interactions can also manifest themselves as
nonradiative decay channels, which can be investigated using
time-resolved fluorescence measurements. Fig. 3 and Table 1
summarize the fluorescence lifetime data for the non-excimer-
forming molecules. In Fig. 3a, the fluorescence decay of
different concentrations of TTBP in PMMA is shown. The
lifetime of the 1 mM concentration, 4.4 ns, is similar to that
measured previously in other polymers and in dilute solu-
tion.38,39 At 100 mM concentration, the lifetime of 4.1 ns is
only slightly shorter. For concentrations greater than 100 mM,
the lifetime rapidly decreases and becomes multi-exponential.
Similar behavior at these concentrations was observed for
TTBP in other polymers, including polystyrene and Zeonex.
This suggests that the concentration-dependent quenching is
not sensitive to the details of the structure of the polymer host,
but is primarily a function of the proximity of the dopant
chromophores to each other. The multiexponential nature of
the decay probably reflects the presence of several different
types of aggregate quenching conformations. It is worth
noting that the spectroscopic properties of the TTBP/PMMA
Results and discussion
Fig. 2 shows the absorption and emission spectra of the five
molecules in both dilute (1 mM) and concentrated (100 mM)
PMMA films. Both perylene and PO show clear signatures of
the broad, redshifted excimer emission at higher concentra-
tions. Excimer formation has been observed previously for
both molecules in concentrated PMMA solids.35,36 The life-
times of the excimer emissions are generally longer than those
of the monomer singlet states, but their fluorescence quantum
yields are generally lower, and thus excimer formation is
usually regarded as detrimental to luminescence efficiency.37
Fig. 2 Fluorescence (solid) and absorption (dashed) spectra of different chromophores in PMMA at different concentrations. (a) Perylene, 1 mM
and 100 mM from left to right, respectively. (b) PO, 1 mM and 100 mM from left to right, respectively. (c) TTBP, 1 mM, 100 mM and fluorescence
of the solid film from left to right, respectively. (d) PR, 1 mM, 100 mM and fluorescence of the solid film from left to right, respectively. (e) MHC,
1 mM, 100 mM and fluorescence of the solid film from left to right, respectively.
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Table 1 All fluorescence decays are measured using 400 nm excitation. The normalized biexponential decays are parameterized by the time
constants t1 and t2 (in nanoseconds) with pre-exponential factors A1 and A2, respectively. The % excimer is calculated from the relative areas of
the monomer and excimer lineshapes in the steady-state fluorescence spectra
100 mM
Solid
1 mM
tns
A1
t1
A2
t2
A1
t1
A2
t2
Name of compound
Excimer formation at 100 mM?
% excimer
Perylene
Perylene orange
TTBP
Perylene red
Meisenheimer
Dimethyl-POPOP
Yes
Yes
No
No
No
No
85%
96%
—
—
—
4.4
5.6
7.1
1.3
1
1
0.6
0.64
4.1
4.8
2.5
0.8
0.62
0.64
0.72
0.06
0.08
0.15
0.38
0.36
0.28
0.53
0.41
0.87
0.4
0.36
6.8
2.67
—
Fig. 3 Natural log of normalized decay of different chromophore. (a) TTBP: 1 mM in PMMA upper line, 100 mM in PMMA middle line and 200
mM in PMMA bottom line. (b) PR: 1 mM in PMMA upper line, 100 mM in PMMA bottom line. (c) MHC: 1 mM in PMMA upper line, 100 mM
in PMMA bottom line.
films can depend on preparation conditions at chromophore
concentrations of 100 mM and higher. Fig. 4 shows the
fluorescence decays for 1 mM TTBP in PMMA along with
300 mM TTBP in PMMA under two different preparation
conditions. In the first preparation, the polymer solution was
spin cast onto a glass substrate at 500 rpm for 1 min. The
fluorescence decay of this sample showed a biexponential
behavior in which 63% of the signal decayed with time t1 =
0.45 ns and 37% decayed with a time t2 = 2.08 ns. If the same
sample is then briefly heated to 130 1C and then cooled in air,
the fluorescence lifetime is dramatically lengthened to another
biexponential decay where 73% of the fluorescence decays
with t1 = 2.37 ns and 27% decays with a t2 = 4.96 ns, close to
that of the dilute sample. The steady state fluorescence spec-
trum, shown in the figure inset, remains the same during this
process. We attribute the longer fluorescence lifetime after
thermal annealing to the disruption of molecular aggregates in
the polymer. A less pronounced effect on the 100 mM films is
observed for different spin-coating speeds. By spinning the
sample at 3600 rpm instead of 500 rpm, the emission spectrum
sharpens slightly, although this sharpening is not accompanied
by any change in the fluorescence lifetime. This sensitivity to
preparation conditions may explain why the observed fluor-
escence spectrum for the solid TTBP obtained from solution
deposition is slightly different from that obtained by vacuum
evaporation.40
Similar trends in the fluorescence lifetime are obtained for
the other two molecules which retain their monomeric emis-
sion at high concentrations. PR also shows little change
between 1 and 100 mM concentrations in PMMA, as can be
seen from Fig. 3b. The measured lifetime of the 100 mM
sample is 4.8 ns, a 15% drop from the 5.6 ns lifetime of the
1 mM sample. The MHC shows a more dramatic decrease in
lifetime with concentration. At 100 mM, the decay becomes
biexponential, with one component at 6.8 ns being close to
that of the single decay time observed for the 1 mM samples
(7.1 ns). The other component, at 2.5 ns, is much more rapid
and composes 60% of the total decay. Thus both PR and the
MHC exhibit more self-quenching at 100 mM than TTBP,
although PR is still relatively long-lived. In the neat solids,
where the effective chromophore concentration is on the order
of 1 M, all three molecules undergo much more rapid quench-
ing, as can be seen from Table 1. For all three, the fluorescence
decay is multiexponential, with at least 60% of the decay
occurring within 150 ps or less. While there is still no sign of
Fig. 4 Normalized natural log of fluorescence decay of TTBP in
PMMA. (a): 1 mM; (b): 300 mM in PMMA annealed; (c): 300 mM in
PMMA un-annealed. Inset: steady state fluorescence of 300 mM
TTBP in PMMA, before and after annealing.
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(1) and (2). This theory predicts the anisotropy decay time t =
16 ps, given a number density of 6 ꢂ 1019 molecules cmꢀ3
,
which corresponds to 100 mM. The experimentally observed
anisotropy decay time t = 30 ps is in reasonable agreement
with this value. The deviation from the theoretical value may
be the result of anomalous subdiffusion resulting from an
inhomogeneous energetic distribution of chromophores in the
solid.15–18 But if we neglect anomalous diffusion effects, the
same theory allows us to estimate the diffusion constant and
the diffusion length for the excitation. We find the diffusion
constant D = 7.9 ꢂ 10ꢀ5 cm2 sꢀ1 and the diffusion length
LD = 14 nm. This diffusion length is comparable to that
obtained by several studies on conjugated polymers using
variable concentrations of quenchers and measuring the fluor-
escence decay as a function of quencher concentration.44–46 In
the conjugated polymers, the increased chromophore concen-
tration (1021 cmꢀ3 versus 1020 cmꢀ3) is probably offset by a
lower fluorescence lifetime (several hundred ps versus 4 ns),
leading to the similar estimates for LD.
Fig. 5 Anisotropy decay of TTBP in PMMA, (a): 1 mM TTBP in
PMMA; (b): 100 mM TTBP in PMMA.
new species in the emission, there are clearly strong interac-
tions in the neat solid which effectively deactivate the singlet
excited state and quench the fluorescence.
Even at 100 mM, the TTBP/PMMA films demonstrate
phenomena which are reminiscent of conjugated polymer
films. Evidence for rapid interchromophore EET is obtained
from the anisotropy decays shown in Fig. 5. We calculated the
Forster radius R0 = 3.9 nm for TTBP,39 and at a concentra-
tion of 1 mM, the average interchromophore distance is 10
nm, leading to negligible EET during the excited state lifetime.
At 100 mM, however, the average interchromophore separa-
tion has decreased to about 2 nm, and intermolecular EET
leads to a rapid randomization of the transition dipole mo-
ment orientation and thus a rapid fluorescence depolarization.
The decay of the fluorescence polarization can be predicted by
a statistical mechanical approach which takes into account the
random distribution of chromophore separations.41–43
Although the concentrated dye/PMMA samples would
appear to be fair mimics of amorphous conjugated polymers,
all the samples in this study had an important practical draw-
back. We found that all of our concentrated ( Z 100 mM) dye
samples have much lower photostability than either dilute
samples or conjugated polymer samples. By measuring the
absorbance of the sample and the laser power, we can estimate
the number of absorption events per chromophore per time,
and plot the photobleaching as a function of molecular
absorptions. Fig. 6 shows the evolution of the fluorescence
intensity coming from TTBP/PMMA samples under constant
laser exposure. The initial photobleaching rate of the 100 mM
sample has a slope of 2.5 ꢂ 10ꢀ4 per molecular absorption and
is observed to lose 60% of its fluorescence intensity within a
few minutes. A 30 mM sample undergoes a slower decline, and
a 1 mM sample has no observable change over the same period
of time. The average lifetime of a TTBP molecule in a 100 mM
PMMA film is only about 104 absorptions, which is 102–104
times less than isolated perylene chromophores.47 PR also
exhibits a concentration-dependent photobleaching, but the
100 mM sample is longer-lived, with an initial slope of 4.1 ꢂ
10ꢀ7 per molecular absorption and an effective lifetime of
about 106 absorption cycles. The photobleaching kinetics of
concentrated MHC films were similar to those of the pery-
lenes, with an intermediate photostability. Neat MEH-PPV
undergoes a slight decay after 108 absorption events, but is
markedly more stable than either the TTBP or PR films. The
rate of TTBP photobleaching does not depend on the chemical
structure of the host polymer, since similar photobleaching
kinetics are observed for TTBP in polystyrene and Zeonex.
The detailed kinetics of the photobleaching are quite compli-
cated, since the concentrations and thus the photobleaching
rate change with time. But since the initial rate does not
depend on the host polymer, it is likely that irreversible
chromophore–chromophore reactions are responsible. Exam-
ination of the absorption spectrum of a photobleached PR
sample showed that the decrease in visible absorption was
accompanied by an increase in a broad, featureless absorption
in the UV region below 300 nm, suggesting that the photo-
products have lost conjugation. Related mechanisms have
h
i
pffiffiffiffiffiffi
S ðtÞ ꢀ S ðtÞ
?
jj
rðtÞ ¼
¼ exp ꢀ t=t
ð6Þ
ð7Þ
S ðtÞ þ 2S ðtÞ
?
jj
9ltfl
t ¼
2
16p2R60r2g2Gð1=2Þ
Where r is the anisotropy and S8(t) and S>(t) are the
fluorescence signal parallel and perpendicular to the excitation
polarization, respectively. G(x) is the gamma function, g is a
factor that takes orientational disorder into account, and l =
2 for donor–donor transfer. R0, tfl, and r are as defined in eqns
Fig. 6 Photobleaching of the fluorescence of selected dyes in solid
state samples. (a): 1 mM TTPB in PMMA; (b): neat MEHPPV film;
(c): 30 mM TTBP in PMMA; (d): 100 mM TTBP in PMMA.
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In this work we have explored the photophysical properties of
highly concentrated polymer solutions of sterically hindered
conjugated chromophores. We found that both ring substi-
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Acknowledgements
This research was supported by the National Science Founda-
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