Oligofluorenes as polymeric model compounds for providing insight into the triplets of ketone and ketylimine derivatives

Article abstract of DOI:10.1021/jp307781n

A series of oligofluorenes ranging between one and three repeating units were prepared as structurally well-defined representative models of polyfluorenes. The photophysics of the oligofluorene models were investigated both by laser flash photolysis and steady-state fluorescence. The effects of the ketone and ketylimine functional groups in the 9-position on the photophysical properties, notably the triplet quantum yield (Φ_TT) by intersystem crossing and the absolute fluorescence quantum yields (Φ_fl), were investigated. The singlet depletion method was used to determine both the Φ_TT and molar absorption coefficients of the observed triplets (ε_TT). Meanwhile, the absolute Φ_fl were determined using an integrating sphere. It was found that both the ketone and ketylimine substituents and the degree of oligomerization contributed to quenching the oligofluorene fluorescence. For example, the Φ_fl was quenched 5-fold with the ketylimine and ketone substituents for the bifluorenyl derivatives compared to their corresponding 9,9-dihexyl bifluorenyl counterparts. Meanwhile, the Φ_fl quenching increased 14 times with the trifluorenyl ketone and ketylimine derivatives. Measured Φ_TT values ranged between 22 and 43% for the difluorenyl derivatives with ε_TT on the order of 13 000 cm^-1 M^-1. The Φ_TT decreased to <10% concomitant with doubling of the ε_TT when the degree of oligomerization was increased to 3. A new fluorescence emission at 545 nm formed at low temperatures for the ketone and ketylimine oligofluorene derivatives. The emission intensity was dependent on the temperature, and it disappeared at room temperature.

Full text of DOI:10.1021/jp307781n

Article
pubs.acs.org/JPCA
Oligofluorenes as Polymeric Model Compounds for Providing Insight
into the Triplets of Ketone and Ketylimine Derivatives
Patricia Robert,† Andreanne Bolduc, and W. G. Skene*
́
́ ́ ́ ́ ́ ́
Laboratoire de caracterisation photophysique des materiaux conjugues, Departement de Chimie, Universite de Montreal, CP 6128,
Centre-ville, Montreal, QC
S
* Supporting Information
ABSTRACT: A series of oligofluorenes ranging between one
and three repeating units were prepared as structurally well-
defined representative models of polyfluorenes. The photo-
physics of the oligofluorene models were investigated both by
laser flash photolysis and steady-state fluorescence. The effects
of the ketone and ketylimine functional groups in the 9-
position on the photophysical properties, notably the triplet
quantum yield (ΦTT) by intersystem crossing and the absolute
fluorescence quantum yields (Φfl), were investigated. The
singlet depletion method was used to determine both the ΦTT and molar absorption coefficients of the observed triplets (εTT).
Meanwhile, the absolute Φfl were determined using an integrating sphere. It was found that both the ketone and ketylimine
substituents and the degree of oligomerization contributed to quenching the oligofluorene fluorescence. For example, the Φfl was
quenched 5-fold with the ketylimine and ketone substituents for the bifluorenyl derivatives compared to their corresponding 9,9-
dihexyl bifluorenyl counterparts. Meanwhile, the Φfl quenching increased 14 times with the trifluorenyl ketone and ketylimine
derivatives. Measured ΦTT values ranged between 22 and 43% for the difluorenyl derivatives with εTT on the order of 13 000
cm−1 M−1. The ΦTT decreased to <10% concomitant with doubling of the εTT when the degree of oligomerization was increased
to 3. A new fluorescence emission at 545 nm formed at low temperatures for the ketone and ketylimine oligofluorene derivatives.
The emission intensity was dependent on the temperature, and it disappeared at room temperature.
polyfluorene derivatives were additionally undertaken in
attempts to assign the origins of the green emission.25−27
Although ketone defects, such as 2, within polyfluorenes are
undesired because they compromise the device efficiency and
they contaminate the emission, the ketone function is
nonetheless interesting. This is in part because functionality
can be introduced into the conjugated network courtesy of the
heteroatomic site.28−32 For example, ketylimine formation is
possible by condensation with amines.33 The judicious choice
of the amine used in ketylimine formation provides the means
to adjust the polyfluorene properties such as solubility,
electronic effects, color, and emission yields. Given that
oligomeric fluorenes are generally accepted as suitable
representative models for investigating the optoelectronic
properties of their polymeric counterparts,34−36 the photo-
physics of 4−9 (Figure 1) were subsequently investigated.
These compounds were targeted because the effect of the
ketylimine on the photophysics of the oligofluorenes could be
examined. More importantly, the transient absorbance spectra
and the quantum yield of triplet formation (ΦTT) of these
model compounds could additionally be investigated and
compared to their corresponding fluorenone precursors and
INTRODUCTION
■
Fluorescent polymers have received much attention in part
owing to their optical properties that are well suited for many
uses, including sensors, lasers, and nonlinear optics.1,2 The
most important application of conjugated fluorescent polymers
has been in terms of their use in organic light emitting diodes
(OLEDs).3−5 Of the many fluorescent polymers examined,
fluorene (1) and its derivatives have attracted the most
attention.6−8 This is a result of their inherent high emission
yields that are well suited for use in OLEDs, leading to devices
with high performances.9−14
Despite their suitable photophysical properties for OLED
usage, the fluorescence quantum yield (Φfl) and color emitted
by polyfluorenes are highly contingent on both their
substitution and their environment. This was evidenced via a
series of structurally well-defined model compounds such as 7
and 8.15−17 The advantage of these models over their
corresponding statistical copolymers is that uncertainties
relating to chain-folding and randomness in the chain structure
can be avoided, resulting in accurate structure−property
studies.18−21 These models successfully demonstrated that
ketone defects in the 9-position of fluorene were in part
responsible for both the decrease in fluorescence and undesired
g-band green emission occurring at ca. 545 nm.22−24 Studies
with these representative polymeric models in addition to
Received: August 6, 2012
Revised: August 29, 2012
Published: August 29, 2012
© 2012 American Chemical Society
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Figure 1. Series of oligofluorenes examined for triplet manifold studies.
9,9′-dialkylated oligofluorene counterparts. This is of impor-
tance given that the intrinsic singlet excited state deactivation of
these compounds by intersystem crossing has not been
previously investigated. The knowledge gained from investigat-
ing this often overlooked inherent fluorescence quenching
mode is valuable for further deciphering the complex
fluorescent deactivation mechanisms of polyfluorenes. Tran-
sient absorbance methods further provide vital information for
assigning intra- vs intermolecular deactivation modes and for
detecting short-lived intermediates. These are particularly
significant, since the exact quenching modes of polyfluorene
derivatives remain contentious.37−39 The absolute fluorescence
quantum yields (Φfl) of the oligofluorenes 4−9 were also
investigated to further provide information about the
deactivation modes, especially since these compounds have
not been previously investigated using such absolute measure-
ments.
Instruments FLSP-920 fluorimeter after deaerating the solvent
for 20 min. Absolute quantum yields were measured using a
commercial integrating sphere system. Lifetime measurements
were done by the time correlated single photon counting
method using a ps-pulsed laser at 375 nm.
Cryofluorescence. Cryofluorescence was performed using
cryocuvettes from NSG Precision Glass and an Optistat
cryostat from Oxford Instruments. The measurements were
done in an anhydrous and deaerated mixture of 4:1
ethanol:methanol at given temperatures after the temperature
had stabilized for 20 min. The change in solvent refractive index
with temperature was taken into account for the emission
measurements at various temperatures, according to eq 1.
2
⎛
⎝
⎞
⎠
η
xK
⎜
⎟
⎟
Φ
= Φ300K ⎜
·
xK
η
(1)
300K
Transient Absorbance Spectroscopy. Laser flash
photolysis analyses were done using a mini-laser flash
photolysis (Luzchem) system excited at 355 nm with the
third harmonic of a Nd:YAG laser. The solutions were prepared
with an absorbance between 0.3 and 0.4 at 355 nm. The
solutions were prepared in either dichloromethane or a 1:1
mixture of dichloromethane/methanol, depending on the
solubility of both the compounds and quencher. The transient
absorbance spectra were generated by averaging the transient
kinetics over 5−7 shots per wavelength that were recorded at
different intervals after the laser pulse. Quenching kinetics were
done by measuring the change in the transient absorbance
EXPERIMENTAL SECTION
■
Materials and General Methods. All reagents were
commercially available, and they were used as received unless
otherwise stated. Anhydrous and deaerated solvents were
obtained with an aluminum solvent purification system by Glass
1
Contour. H NMR and 13C NMR spectra were recorded on a
400 MHz spectrometer with the appropriate deuterated
solvents.
Spectroscopic Measurements. Absorbance measure-
ments were done on a Cary 500 spectrophotometer, and
fluorescence measurements were performed on an Edinburgh
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spectra as a function of quencher. Quenchers used for assigning
the observed transients were 1,3-cyclohexadiene, 2-methyl-
naphthalene, β-carotene, and methylviologen.
δ = 7.81−7.79 (m, 1H), 7.74 (d, J = 8 Hz, 1H), 7.64 (d, J = 2
Hz, 1H), 7.47 (dd, J = 8.2 Hz, 1H), 7.47−7.45 (m, 1H), 7.37−
7.32 (m, 2H), 2.09 (s, 2H), 1.13−0.99 (m, 12H), 0.76 (t, J =
4.7 Hz, 6H), 0.60−0.56 (m, 4H).
The quantum yields of intersystem crossing (ΦISC), which
are equal to the quantum yields of triplet formation (ΦTT),
were determined by relative actinometry via the singlet
depletion method.40−42 The molar absorption coefficients of
both the singlet (εs) and triplet (εTT) of 3−9 are required for
the singlet depletion method. Values of εs were measured by
preparing stock solutions of 3−9 of precise concentrations. The
solutions were sequentially diluted and the absorbance
measured, with the desired εs derived from the slope of
absorbance as a function of concentration. The wavelength for
the singlet studies was selected such that it did not overlap with
the transient produced in the transient absorbance spectrosco-
py measurements. The εTT value of the compounds was
determined, as per eq 2, with ΔAbs referring to the maximum
signal intensity measured at the corresponding absorbance
maximum. The kinetics of both the ground-state bleaching and
triplet were measured on short-time scales for obtaining a
maximum number of data points at the maximum signal
intensity. The signal maximum of both the ground state and
triplet were measured as a function of laser power, as per Figure
4A. The laser Q’s switch delay was adjusted to vary the laser
power output. The desired εTT was calculated from the ratio of
the two slopes, with the triplet and singlet signals being a
function of laser power.
(9,9-Dihexyl-9H-fluoren-2-yl)boronic Acid.49 To 2-bromo-
9,9-dihexyl-9H-fluorene (1.53 g, 3.67 mmol) dissolved in
anhydrous THF (23 mL) was added n-BuLi (2.2 M in hexanes,
2 mL) at −78 °C. The reaction mixture turned orange, and it
was allowed to react for 1 h before triethylborane (1 mL, 6.9
mmol) was added dropwise. After stirring for 14 h, water (23
mL) was added to the reaction mixture and it was allowed to
further stir for 1 h. 2 M HCl (18 mL) solution was added, and
the THF was evaporated under vacuo. The crude product was
extracted with ethyl acetate (4 × 70 mL), and the organic layer
was dried with MgSO4, filtered, and then concentrated under
vacuo. The product was purified by flash chromatography using
dichloromethane (100%) to afford the title compound as a
white powder, which was used immediately without additional
characterization.
9,9,9′,9′-Tetrahexyl-9H,9′H-2,2′-bifluorene (4).50 To 2-
bromo-9,9-dihexyl-9H-fluorene (2.20 g, 5.30 mmol), (9,9-
dihexyl-9H-fluoren-2-yl)boronic acid (4 g, 10.6 mmol)
dissolved in toluene (8 mL) was added a solution (10 mL)
of sodium carbonate (28 mg, 0.18 mmol). The mixture was
refluxed for 1 h under nitrogen, and then, palladium
tetrakis(triphenylphosphine) (35.5 mg, 0.02 mmol) was
added and the mixture was protected from light. After 2 days,
the reaction mixture was poured into hexanes (125 mL). The
product was extracted with a saturated solution of sodium
chloride (3 × 50 mL). The organic layer was dried with
MgSO4, filtered, and then concentrated under vacuo. The crude
product was purified by silica gel flash chromatography using
hexanes (100%) to give the product as a white powder (4.5 mg,
0.13%). 1H NMR (acetone-d6): δ = 7.77 (d, J = 8 Hz, 2H), 7.67
(d, J = 2 Hz, 2H), 7.54 (dd, J = 2, 8 Hz, 2H), 2.12−2.07 (m,
4H), 1.14−1.01 (m, 12H), 0.77 (t, J = 7, 14 Hz, 6H), 0.64−
0.56 (m, 4H). 13C NMR (chloroform-d): δ = 151.4, 150.9,
140.8, 140.5, 140.3, 126.9, 126.8, 126.0, 122.9, 121.4, 119.8,
119.7, 55.2, 40.4, 31.5, 29.7, 23.8, 22.5, 13.9.
ΔAbsTT
εTT = εs
ΔAbss
(2)
Benzophenone (ΦTT = 1; εTT = 7200 M−1 cm−1) was taken as
the reference to determine the ΦTT values of 3−9 with the
benzophenone data being taken from literature values
measured in acetonitrile.43−45 Optically matched samples at
355 nm of benzophenone and the given compounds were
prepared in anhydrous and deaerated dichloromethane. The
transient kinetics were measured at the corresponding triplet
absorbance maxima as a function of laser power. The ΦTT was
calculated according to eq 3 (see ref 46), where ΔAbs is the
maximum signal corresponding to the triplet (λ2) that was
measured at the maximum for the corresponding transients.
This signal was measured as a function of laser power, as seen
in Figure 4B.
2,7-Dibromo-9,9-dihexyl-9H-fluorene.51 2,7-Dibromofluor-
ene (0.41 g, 1.27 mmol) was dissolved in DMSO (15 mL), and
phenyltriethylammonium chloride (188 mg, 0.82 mmol) was
added. The mixture was heated at 75 °C. After 1 h, 1-
bromohexane (1 mL, 7.25 mmol) and a sodium hydroxide
(1.74 g, 15 mL) solution were added. After 17 h, the reaction
mixture was poured into 1.44 M HCl (100 mL). The crude
product was extracted with ethyl acetate (50 mL) and water (4
× 150 mL). The organic layer was further washed with a
saturated solution of sodium chloride (3 × 50 mL) and the
organic layer was then extracted, dried with MgSO4, filtered,
and concentrated under vacuo. The crude product was purified
by silica gel flash chromatography using hexanes (100%) to give
εTRTeference(λ1) ΔAbsSample(λ2)
Sample
TT
Reference
TT
Φ
=
·Φ
εTSTample(λ2) ΔAbsReference(λ1)
(3)
Synthesis. The synthesis of 3, 5, 6, 8, and 9 was done
according to known means.33,47
2-Bromo-9,9-dihexyl-9H-fluorene.48 2-Bromofluorene (1.6
g, 6.54 mmol) was dissolved in DMSO (35 mL). 1-
Bromohexane (2.2 mL, 15.59 mmol), benzyltriethylammonium
chloride (75 mg, 0.33 mmol), and a solution (14 mL) of
sodium hydroxide (7.79 g, 0.19 mol) were added to the
mixture. The color of the mixture subsequently changed from
orange to violet, and the mixture was stirred under nitrogen for
21 h. The reaction mixture was then poured into water (80
mL). The crude product was extracted with ethyl acetate (4 ×
50 mL), and the organic layer was washed with water (4 × 100
mL). It then was dried with MgSO4, filtered, and concentrated
in vacuo. The title compound was isolated as a colorless oil
(2.24 g, 5.38 mmol, 82%) after purification by silica gel flash
chromatography using hexanes (100%). 1H NMR (acetone-d6):
1
the product as a white powder (0.81 mmol, 64%). H NMR
(acetone-d6): δ = 7.92−7.84 (m, 6H), 7.77 (dd, J = 2, 8 Hz,
2H), 7.56−7.48 (m, 2H), 7.38−7.35 (m, 4H), 2.10 (s, 8H),
1.12−1.11 (m, 24H), 0.89−0.88 (m, 12H), 0.77 (t, J = 14, 7
Hz, 8H). 13C NMR (acetone-d6): δ = 153.2, 139.8, 130.7,
126.6, 122.1, 121.7, 56.3, 40.0, 29.8, 29.7, 29.6, 29.4, 29.2, 29.0,
24.0, 22.7.
9,9,9′,9′,9″,9″-Hexahexyl-9H,9′H,9″H-2,2′:7′,2″-terfluor-
ene (7).52 In toluene (2.5 mL) were dissolved 2,7-dibromo-9,9-
dihexyl-9H-fluorene (377 mg, 0.54 mmol), (9,9-dihexyl-9H-
fluoren-2-yl)boronic acid (178 mg, 0.47 mmol), and sodium
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carbonate (10.3 mg, 0.11 mmol). The reaction mixture was
refluxed for 1 h under nitrogen while it was protected from
light. Palladium tetrakis(triphenylphosphine) (14 mg, 0.014
mmol) was then added. The reaction mixture was poured into
hexanes (80 mL) after refluxing under nitrogen for 48 h. The
crude product was extracted with a saturated solution of
sodium chloride (3 × 75 mL), and the organic layer was dried
with MgSO4, filtered, and then concentrated under vacuo. The
crude product was purified by silica gel flash chromatography
using hexanes and ethyl acetate (99:1). The product was
1
isolated as a colorless viscous oil (324 mg, 86%). H NMR
(chloroform-d): δ = 7.88−7.86 (m, 4H), 7.79 (d, J = 7 Hz, 2H),
7.74−7.69 (m, 8H), 7.44−7.36 (m, 6H), 2.15−2.08 (m, 12H),
1.21−1.15 (m, 36H), 0.89−0.81 (m, 30H). 13C NMR
(chloroform-d): δ = 151.8, 151.5, 150.9, 140.8, 139.9, 126.9,
126.8, 126.1, 126.0, 122.9, 121.5, 121.4, 119.9, 119.9, 119.7,
55.3, 55.2, 40.4, 31.5, 29.7, 22.6, 22.5, 14.0.
RESULTS AND DISCUSSION
■
Transient Absorbance Spectroscopy. Transient absorb-
ance spectroscopy of the compounds was investigated via laser
flash photolysis. This was done to identify and quantify the
transients produced upon photoexcitation. Given the potential
complexity of assigning the transients for 4−9, the transient
absorbance spectrum of 2 was first investigated as a benchmark.
This is of importance because the efficiency of fluorenone
triplet formation via intersystem crossing is known to be
dependent on solvent polarity.53,54 In addition, there are
limited characterization details of these transients in either
dichloromethane or the 1:1 dichloromethane/methanol
mixtures used for the current study.41,43 Dichloromethane
was the preferred solvent because of the high solubility of 4−9
in this polar aprotic solvent. It is also the solvent of choice for
polymer studies. Therefore, the results can directly be
compared to similarly reported systems. Meanwhile, the binary
mixture was required for the mutual solubility of the quencher
and the compounds studied and for cryofluorescence measure-
ments.
Figure 2. (A) Transient absorbance spectra of 2 measured in
dichloromethane at 1.04 μs after the laser pulse in the absence (black
■
●
) and with the addition of (red ) 1,3-cyclohexadiene. Inset: kinetic
trace monitored at 420 nm in the absence (black line) and with the
addition (red line) of 1,3-cyclohexadiene. (B) Transient absorbance
spectra of 5 measured in a 1:1 dichloromethane/methanol mixture
■
●
)
excited at 355 nm and measured 28.2 (black ) and 39.5 μs (red
after the laser pulse in the absence and with the addition of 1,3-
▲
cyclohexadiene (blue ) measured 1.6 μs after the laser pulse. Inset:
kinetic decay traces of 5 measured in the absence (black line) and
presence of 1,3-cyclohexadiene (red line) monitored at 450 nm after
excitation at 355 nm.
The transient absorbance spectrum of 2 obtained in
dichloromethane is shown in Figure 2A. It is evident that a
transient strongly absorbs at approximately 440 nm concom-
itant with a weak absorbance at ca. 630 nm. These absorbances
are consistent with the known triplet of 2.55 The triplet
intermediate was in part assigned from the observed
unimolecular decay (black line inset Figure 2A) in addition
to the similar absorbances of triplet fluorenone measured in
other solvents.41,43 Absolute assignment of the transient was
done by quenching the transient with 1,3-cyclohexadiene. Not
only is this a known triplet quencher,56,57 but it is invisible in
the spectroscopic window used for the transient studies. This
leads to clean transient absorbance spectra, and it makes for
easy assignment of the transient. The resulting transient
absorbance spectrum of 2 upon the addition of 0.03 mM 1,3-
cyclohexadiene is seen in Figure 2A (red circles). The resulting
spectrum is quenched with the addition of the diene. The
quenching is confirmed by comparing the kinetic traces
measured at 420 nm in both the absence and presence of the
diene (red circles inset Figure 2A). Both the observed
unimolecular decay kinetics and the absorbance quenching
with 1,3-cyclohexadiene confirm that the transient of 2
produced upon excitation at 355 nm is the triplet, whose
maximum absorbance is at 420 nm.
The transient absorbance spectra of the ketone and
ketylimine oligomers were subsequently investigated in
dichloromethane. Similar to 2, 5−8 each exhibited a strong
absorbance at ca. 440 nm and a weaker absorbance at
approximately 620 nm. The observed first order kinetics
suggest that the transient is the triplet. This was confirmed by
quenching with 1,3-cyclohexadiene. As seen in both the inset
and the transient absorbance spectrum of 5 (Figure 2B), the
species is quenched with the addition of 1,3-cyclohexadiene.
This was also the case for 6−9. The transient was also
examined in the addition of methylviologen, which is an oxidant
and whose corresponding cation absorbs at 607 nm in
acetonitrile.58,59 Given the limited solubility of methylviologen
in neat dichloromethane, a 1:1 dichloromethane/methanol
mixture was used. A new absorbance at ca. 600 nm was
observed in the transient absorbance spectrum (see the
Supporting Information) for both 5 and 8. The lifetime of
the transient at 600 nm was greater than 10 μs, and it decayed
with bimolecular kinetics. This suggests that the ketyl
derivatives can be photo-oxidized, resulting in the radical
cation.
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Similar transient absorbance spectra features between 2 and
the ketylimine derivative (3) were expected. However, no
transient absorbance signal was observed with 3. To some
extent, this is not surprising, since the imine is known to rapidly
deactivate excited states.44,60−62 In contrast, a transient was
observed at 420 and 450 nm, respectively, for 6 and 9. Both
transients decayed with first order kinetics, and these transients
were also quenched with 1,3-cyclohexadiene.
The transients 4 and 7 were additionally examined to
confirm that their triplet was produced. This is of particular
interest, since the triplets of these derivatives have never been
previously investigated. As seen in Figure 3, 4 produces a
conjugation. This notwithstanding, only qualitative results
relating to the triplet state can be derived from the transients.
Given that the singlet excited state is competitively deactivated
by nonradiative intersystem crossing, quantifying the amount of
fluorescence lost due to triplet formation is desired.
Triplet quantification is typically done by relative actino-
metry. This involves comparing the amount of the triplet signal
for the given sample at its absorbance maximum (ΔAbs(λ2))
relative to a standard with a known ΦTT, which is measured at
its triplet maximum (ΔAbs(λ1)), according to eq 3. The desired
triplet quantum yield can only be derived if the molar
absorption coefficient (εTT(λ2)) of the sample’s triplet is
known at the given wavelength. This is problematic for 4−9,
given the limited previous triplet investigations of these
compounds.
Knowledge of the εTT is not required from relative
actinometry when the same triplet is produced for both the
sample and the reference. In such a case, eq 3 can be simplified
to eq 4, with ΔAbs being the signal observed for the transient
as a function of excitation energy. This is the case for triplet
quenchers such as naphthalene and β-carotene.63 These
deactivate the given triplets by energy transfer to provide
their corresponding triplets that are visible at 420 and 515 nm,
respectively.42,43,64 Unfortunately, these and other standard
triplet quenchers could not be used because of their
overlapping triplets with those of 4−9. This would lead to
significant errors in deriving the desired ΦTT. For this reason,
the εTT of the oligofluorenes had to be determined from eq 2
and the desired ΦTT had to then be calculated according to eq
3.
Figure 3. Transient absorbance spectra of 4 measured in dichloro-
methane 5.31 (black ) and 8.16 μs (red ) after the laser pulse at
■
●
355 nm. Inset: decay kinetics of 4 measured at 520 nm.
ΔAbsSample(λ1)
ΔAbsReference(λ1)
Sample
TT
Reference
TT
Φ
=
·Φ
(4)
transient that is visible at 520 nm, while that of 7 absorbs at 480
nm (Supporting Information). The transient observed for both
4 and 7 was assigned to the triplet owing to the first order
decay and its disappearance in the presence of 1,3-cyclo-
hexadiene.
Triplet Quantum Yield Determination. The collective
transient data of 5, 6, 8, and 9 confirm that the triplet is
produced upon irradiation. As seen in Table 1, the maximum
absorbance of the triplet is somewhat dependent on the degree
of oligomerization. It is further apparent from the collective
data that the triplet of the ketone and ketylimine containing
oligomers is bathochromically shifted from the all-fluorene
derivatives. This is in part owing to their different degrees of
The εTT values of 4−9 were determined by the singlet
depletion method, according to eq 2.40 This method is
applicable given that the respective ground and excited state
absorbance spectra of the compounds do not overlap. The
method further requires the absence of fluorescence in the
transient absorbance spectra. The singlet depletion method
further assumes that all the excited singlet states produced upon
excitation are either returned to the ground state (vide infra) or
form the triplet state. The ground state molar absorption
coefficients (εs) of 4−9 were measured at their corresponding
maximum. The required εTT (Table 1) were calculated from the
ratio of the two slopes from Figure 4A. The slopes were derived
Table 1. Various Spectroscopic Properties of Fluorene Derivatives Measured in Dichloromethane
a
b
c
d
e
compound
εs (cm−1 M−1
)
εTT (cm−1 M−1
)
λfl (nm)
λTT (nm)
ΦTT (λ)
Φfl
Φfl(MeOH)
f
g
g
1
n/d
40 000
302
505
478
380
560
559
395
584
566
376
430/650
−
0.03
0.70
0.02
0
n/d
h
h
2
3
4
5
6
7
8
9
1600 (309)
8500 (300)
5900
0.94
<0.02
<0.02
0.77
<0.02
0
−
−
22 000 (330)
38 000 (295)
27 000 (295)
70 000 (352)
31 000 (310)
17 000 (295)
14 000
13 000
9000
520
0.22
0.36
0.43
0.09
0.08
0.02
0.53
0.11
0.06
0.71
0.04
0.02
410/610
410/650
480
79 000
26 000
30 000
0.98
0.07
<0.02
440/580
440/520
a
b
Ground state molar absorption coefficients measured at the wavelength reported in parentheses. Triplet molar absorption coefficient at λTT
c
d
derived from the singlet depletion method. Triplet absorbance maximum. Yield of triplet formation determined from the singlet depletion method
using benzophenone as a reference.45 The values were calculated at λTT. Absolute fluorescence quantum yield by using an integrating sphere with
e
2% being the lowest Φfl that can accurately be measured. Literature values.71,72 Not determined. Literature values.73
f
g
h
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by monitoring the change in the ground state bleaching and the
triplet formed as a function of laser power.
depletion method, the calculated value (ΦTT = 0.94) from eqs 2
and 3 is nonetheless consistent with previously reported values
that were measured via singlet oxygen in organic solvents of
similar polarity. The consistent ΦTT measured for 2 by the
different methods validates the singlet depletion method as a
viable means for deriving εTT of the oligofluorenes.
The effect of increasing the degree of conjugation on the
ΦTT for the oligofluorenes is evident when comparing 1, 4, and
7. The triplet is reduced by ca. 10% with the addition of a
fluorene to the structure. The same trend of triplet yield
reduction with increasing degree of conjugation is further seen
with the ketone derivatives 5 and 8. In fact, ΦTT is reduced by a
third with the subsequent addition of the fluorene relative to
the native 2. The effect of the degree of conjugation on the
triplet is further apparent from the εTT. The measured εTT
values imply that the triplet of the triads absorb much stronger
than their corresponding diads. The latter in turn absorb much
stronger than their corresponding monomers. The measured
Φ
TT values confirm that the triplet manifold is formed for all of
the compounds, with the triplet diads being formed the most.
In contrast, the triplet is formed in less than 10% for the triads.
Therefore, while the triplet is formed, it is a minor deactivation
pathway of the singlet excited state for the triads.
Fluorescence. The absolute fluorescence quantum yields
(Φfl) of the fluorenes were additionally examined using an
integrating sphere. This method is preferred over relative
actinometry because a reference having similar absorbance,
fluorescence, and quantum yield to the compound of study is
not required. The Φfl values determined with an integrating
sphere are absolute values with higher precision than values
determined by relative actinometry. Measuring the absolute Φfl
is of importance given that the fluorescence yield of the
oligofluorenes has exclusively been measured by relative means.
Absolute values are further desired for accurately understanding
the deactivation modes of the oligofluorenes. Accurate values
are of importance for indirectly determining the contribution of
other deactivation modes according to the energy conservation
law: ΦTT + Φfl + ΦIC + Φother modes = 1. The Φfl values of 2−9
were measured in dichloromethane to correlate the data with
the triplet results.
The Φfl was dependent on the degree of oligomerization
similar to what was seen with the ΦTT. The Φfl are higher for
the triads than the diads. As expected, 4 and 7 fluoresce more
than their corresponding ketone and ketylimine derivatives.
Also, the sum of Φfl and ΦTT are nearly unity, confirming the
absence of other deactivation processes. This was supported by
measuring the Φfl at low temperatures in methanol. The
absence of increased fluorescence at low temperatures further
confirms that deactivation by internal conversion is not a viable
deactivation mode.
In contrast to the all-fluorene derivatives, the combined
respective Φfl and ΦTT of 5, 6, 8, and 9 are considerably less
than unity. This confirms the presence of deactivation modes
other than fluorescence and intersystem crossing. While
fluorenone is known to readily undergo photoreduction to
form a radical anion,74,75 the presence of this intermediate can
be precluded in part owing to the absence of a visible radical
anion in the transient absorbance measurements. Moreover, the
formation of radical ions would eventually lead to product
degradation with prolonged irradiation. Such products were not
observed.
■
Figure 4. (A) The maximum ground state bleaching (black ) and
●
triplet (red ) signals of 5 measured as a function of laser power at
295 and 410 nm, respectively. (B) Variation of the triplet intensity of
■
▲
●
benzophenone (blue ), 5 (black ), and 6 (red ) measured as a
function of laser power at 525, 410, and 410 nm, respectively, of
optically matched samples at 355 nm. Inset: variation of
benzophenone triplet kinetics measured as a function of decreasing
laser power measured at 450 nm.
The ΦTT values of 4−9 were derived according to eq 3 using
benzophenone as the reference (ΦTT = 1; εTT = 7200 M−1
cm−1).43−45,65 For this, optically matched samples at 355 nm of
benzophenone and the desired oligofluorene were prepared.
The maximum signal of triplet benzophenone was measured at
525 nm as a function of laser power. Similarly, the triplets of 4−
9 were measured at the corresponding triplet absorbance
maximum. The desired ΦTT was then derived from the ratio of
the slopes of Figure 4B and by taking into account the εTT of
the reference and that of the fluorene derivatives, derived from
eq 2.
From the calculated ΦTT in Table 1, it is evident that all the
compounds produce their triplet in appreciable amounts. To
validate the results calculated from the singlet depletion
method, the triplet of 2 was also examined. This was chosen
as a reference because its ΦISC, and hence ΦTT, has extensively
been examined.54,66−70 While the ΦTT of 2 has not previously
been measured directly in dichloromethane using the singlet
The excited state deactivation modes of the ketone and
ketylimine fluorene derivatives were further investigated using
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cryofluorescence. Deactivation processes, in particular those
occurring by internal conversion (IC) such as rotation around
the fluorene−fluorene bond, can be identified from such low
temperature fluorescence measurements, according to ΦIC = Φfl
(77 K) − Φfl (300 K).56,57,76 Deactivation by internal
conversion is suppressed at reduced temperatures, resulting in
fluorescence enhancement. This is in contrast to singlet excited
state deactivation by ISC to the triplet state that is an inherent
process, which is temperature independent. While fluorescence
and triplet measurements were done in dichloromethane, this
was not a suitable solvent for low temperature measurements.
This is because it does not form the required glass matrix at 77
K for accurate fluorescent measurements. Cryofluorescence
measurements were therefore done in a 1:4 methanol/ethanol
glass forming matrix. This solvent mixture was preferred over
the commonly used EPA solvent mixture (diethylether/
pentane/ethanol)77 because it easily forms a glass matrix and
it is easier to prepare.78 The fluorescence at room temperature
was also examined in this solvent in order to correlate the data
with the low temperature measurements. The measurements
were further done in this solvent to confirm that there is little
solvent dependent fluorescence between the two polar solvents
(dichloromethane and ethanol/methanol mixture). According
to Table 1, the Φfl values of 4 and 7 increase slightly in the
ethanol matrix compared to dichloromethane. In contrast, the
Φfl of 5, 6, and 9 decreased only marginally in the methanol/
ethanol mixture, when taking into account the uncertainties of
the absolute measurements. The measured Φfl nonetheless
confirm that the oligofluorene fluorescence is not quenched by
hydrogen bonding in the protic solvent.
the absence of deactivation modes via IC (vide supra). In
contrast, a new emission occurs between 500 and 650 nm.
While the emission increase cannot be quantified, the trend of
increasing emission with reduced temperature is evident from
the inset of Figure 5. It should be noted that the low
temperature emission centered at 545 nm is reversible and it
disappears at room temperature. Moreover, the structured
fluorescence at 400 nm did not vary with temperature or
prolonged irradiation times. This confirms the absence of
product photodegradation. While the emission at ca. 545 nm at
low temperature was found exclusively for 2, 5, 6, 8, and 9, it
was more pronounced for the fluorenone derivatives. This
indicates that the heteroatomic substitution at the 9-position
contributes to the red-shifted emission.
While the fluorescence of 2 is known to be red-shifted upon
progressing from nonpolar to polar solvents, this is not
responsible for the observed temperature induced red-shifted
emission observed in Figure 5.80,81 Instead, the new emission at
545 nm is from a charge transfer state (CTS) between the
electron accepting fluorenone/ketylimine moiety and electron
donating fluorene.82,83 This was previously confirmed by
incorporating various amounts of fluorenone into polyfluorenes
that resulted in significant increases from the low energy
emission at 545 nm.84−86 The identically shifted emissions of 5,
6, 8, and 9, taken together with previous studies, highly suggest
that the temperature induced emissions at 545 nm are the same
species. This is the first time that the temperature dependent
red-shifted emission of ketyl and ketylimine oligofluorenes has
been observed. This aside, the exact mechanism that is
responsible for this emission remains contentious, especially
whether the CTS is formed from intra- of intermolecular
interactions.24,87−90 For this reason, the fluorescence of 2 was
examined to further aid in understanding the observed
temperature dependent emission of 2, 5, 6, 8, and 9. This
was in part done by examining the concentration dependent
fluorescence of 2. A single emission was seen at 415 nm at low
concentrations of 2. This emission decreased concomitant with
the formation of an emission at 545 nm with increasing
concentration. The concentration dependent fluorescence is
evident in Figure 6. It should be noted that the emission spectra
The Φfl were corrected for the temperature dependent
solvent refractive index change using eq 1. This takes into
account the change of the solvent refractive index at the given
temperature relative to the absolute quantum yield measured at
room temperature (Φ298K).78,79 Significant fluorescence en-
hancement would be seen in the 300−450 nm range if IC was a
major mode of deactivation. As seen in Figure 5, there is little
variation in the fluorescence spectra at reduced temperatures.
In fact, the Φ77K/Φ298K ratio for 8 and 9 is unity. This confirms
Figure 5. Fluorescence spectra of 8 uncorrected for temperature
dependent refractive index change measured in a 4:1 mixture of
ethanol/methanol as a function of temperature between 300 and 90 K.
Inset: variation of fluorescence intensity of 8 measured at 529 nm as a
function of temperature.
Figure 6. The fluorescence of 2 as a function of concentration in
dichloromethane between 0.01 and 2.35 mM. Inset: fluorescence
kinetics of 5 monitored at 423 (black line) and 500 (red line) nm at
□
○
298 ( ) and 77 ( ) K.
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were not recorded beyond 545 nm because of interference from
the excitation wavelength harmonic. Nonetheless, it is obvious
from Figure 6 that the red-shifted emission from the CTS is
concentration dependent and is a result of bimolecular
interactions. This is in contrast to 8, where the red-shifted
emission was not observed under similar Stern−Volmer
quenching conditions. In fact, the 545 nm emission was not
observed even at high concentrations of 8, i.e., with an
absorbance of 4 at the excitation wavelength.
The emission kinetics of 2 and 5 were measured at 298 and
77 K to further compare the two emissions at ca. 545 nm. As
seen in the inset of Figure 6, the fluorescence kinetics are not
uniquely monoexponential. The kinetics of 2 at both 414 and
544 nm could be fitted to three monoexponential lifetimes, as
per Table 2. While the lifetime of the fast component (<1 ns)
Unfortunately, these are outside the resolution of our
instrument. Nonetheless, the slight increase in the Φ77K
/
Φ
298K ratio concomitant with increased contribution of the slow
kinetics at the expense of the fast kinetics at reduced
temperature suggests that a portion of the ketylimine/ketone
oligofluorene fluorescence is quenched by CTS at room
temperature. Meanwhile, the collective data nonetheless
suggest that the intermediate at 545 nm is most likely a result
from intramolecular CTS and not from intermolecular
processes.94,95
CONCLUSION
■
The triplet quantum yields of a series of oligofluorenes and
their corresponding ketone and ketylimine analogues were
successfully measured by the singlet depletion method. It was
found that the triplet was formed in significant amounts for the
bifluorene derivatives that were substituted with a ketone or
ketylimine in the 9-position. The ΦTT was substantially reduced
for the trifluorene derivatives. However, the εTT for these
derivatives was nearly double that of their corresponding
bifluorene. It was further found that a red-shifted emission,
occurring at ca. 545 nm, was formed only at reduced
temperature and it was concentration independent. The
collective temperature, kinetics, and concentration analyses
suggest that the charge transfer state is formed from intra-
rather than intermolecular processes. This qualitative finding is
of importance given the contention surrounding the specific
origins of the green emission. It further demonstrates that vital
photophysical information of polymeric systems can be derived
from their oligomeric model counterparts.
Table 2. Fluorescence Lifetimes Measured in Ethanol/
Methanol Mixture at 298 and 77 K
414 nm
544 nm
τ (ns) (% distribution)
τ (ns) (% distribution)
compound
298 K
77 K
298 K
77 K
2
τ1
τ2
τ3
τ1
τ2
τ3
0.9 (56)
3.5 (19)
41.3 (25)
0.8 (48)
3.8 (15)
46.6 (37)
0.5 (48)
1.7 (32)
13.3 (20)
0.4 (21)
1.8 (72)
4.5 (7)
0.5 (7)
7.7 (13)
26.9 (80)
5
1.6 (88)
7.9 (12)
1.7 (88)
6.3 (12)
1.5 (4)
19.6 (96)
cannot be accurately measured because it is faster than the time
resolution of our instrument, the fast kinetics (<4 ns)
nonetheless account for the majority of the signal (>70%).
The mixed lifetimes for 2 are not surprising given the
measurements were done at high concentrations to ensure
the CTS emission. Fluorescence from the monomer of 2
should be temperature independent because fluorescence is an
intrinsic property. In contrast, the lifetimes of the CTS should
be temperature dependent with slower kinetics at reduced
temperatures. The two fast components can therefore be
ascribed to a singlet exciton (<1 ns) and fluorenone (3.5 ns)
fluorescence, respectively.91 Meanwhile, the longer kinetics is
assigned to the CTS owing to its increased contribution that is
observed at 77 K. The assignment of these species is further
corroborated by previous studies.92,93
ASSOCIATED CONTENT
* Supporting Information
NMR spectra of synthesized compounds, additional transient
absorbance spectra, transient kinetic spectra, fluorescence and
cryofluorescence spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
■
Present Address
†Laboratoire de spectroscopies des mater
́
́
iaux, Departement de
Chimie, Universite
Montreal, QC.
de Montreal, CP 6128, Centre-ville
́ ́
Notes
The authors declare no competing financial interest.
Similar to 2, a fast decay (ca. 1.6 ns) was observed for 5 at
both wavelengths. This is assigned to the fluorene emission.
While the lifetime did not vary with temperature, its
contribution to the measured kinetics decreased from 88% at
298 K to 4% at 77 K. In contrast, the longer lived CTS
increased in lifetime at 77 K. Similarly, the contribution of the
long lifetime emission increased from 12% at 298 K to 96% at
77 K. It was further found that the Φ77K/Φ298K ratio for the
emission of 5 centered at 425 nm was 1.85. The low
concentrations (<10−5 M) at which the fluorescence measure-
ments were done taken together with the absence of
temperature dependent red-shifted emission preclude a
bimolecular process for the low temperature emission for the
ketone/ketylimine derivatives. The measurements were further
done in methanol, which is also a good solvent for the
oligomers, and as such, their agglomeration at reduced
temperature can be ignored. Sub-ps kinetic measurements
would provide conclusive insight into the formation kinetics.
ACKNOWLEDGMENTS
■
NSERC Canada is thanked for Discovery, Strategic Research,
and Research Tools and Instruments grants allowing this work
to be performed in addition to additional equipment funding
from the Canada Foundation for Innovation. W.G.S. also
thanks both the Humboldt Foundation and the RSC for a JWT
Jones Travelling Fellowship, allowing this manuscript to be
completed. A.B. thanks both NSERC and Universite
́
de
Montreal for graduate scholarships. The Centre for Self-
́
Assembled Chemical Structures is also acknowledged. The
assistance of Dr. S. Dufresne and Mr. N. McGregor for
preliminary contributions are appreciated.
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