Journal of the American Chemical Society
Article
(Hamamatsu R928). The output was recorded on an Agilent Infinium
transient digitizer. Each decay was recorded at the emission λmax and
was the average of 5000 pulses.
phosphorescent indicators of dissolved oxygen in hypoxic
environments.21 The key to observing the reversible energy
transfer in these systems is maintaining a small (<1000 cm−1
;
Nanosecond Time-Resolved Transient Absorption. Nano-
second transient absorption measurements were performed using a
Quantel Brilliant B Q-switched Nd:YAG laser-pumped OPO
(Opotek) as the pump source at a right angle to the analyzing light
source. Excitation pulses were <5 ns and were tuned to the λmax of the
compound being interrogated. An Applied Photophyics LKS.60 laser
flash photolysis spectrometer was used for detection; the instrument is
equipped with a 150 W pulsed Xe arc lamp as the analyzing light
source, a single grating monochromator (Applied Photophysics 0.25
m) after the sample, and PMT detection (Hamamatsu R928). The
output was recorded on an Agilent Infinium transient digitizer and
decays and spectra were acquired and analyzed with Applied
Photophysics LKS.60 software.
Femtosecond Time-Resolved Transient Absorption. Femto-
second transient absorption measurements were collected on an
Ultrafast Systems Helios transient absorption spectrometer. A Spectra
Physics Solstice laser, a one-box regenerative amplifier containing a
Mai Tai femtosecond oscillator and Empower pump laser, was
employed to produce 800 nm pulses at a repetition rate of 1 kHz at 3.5
W average power and a pulse width of <100 fs. From this unit, the
beam is split (50:50) with one beam directed to an optical parametric
amplifier (TOPAS, Light Conversion) and the other to the Helios
spectrometer (HE-vis-3200) to create the pump (472 nm; TOPAS)
and probe (Helios) sources, respectively. The 800 nm probe beam
passed through a CaF2 plate to generate a white light continuum
(∼330−700 nm). The spectrum was integrated for 2 s for each scan.
The pump and probe beams were directed to a 2 mm path length
cuvette containing the sample where they were spatially overlapped.
The solution was vigorously stirred in the 2 mm path length cuvette
during data collection. Transient absorption data were corrected by
subtracting spectral background features that persisted from the
previous pulse and appeared prepulse as well as applying chirp
correction using Surface Xplorer Pro 1.1.5 software (Ultrafast
Systems).
<3 kcal) energy gap between the participating excited states,
although details of the free energy dependence of the reversible
energy transfer and the overall relaxation are lacking.
In the late 1970s, Wrighton and co-workers reported the
photophysical behavior of Ru(II) complexes having a single
4,4′-dicarboxy-2,2′-bipyridine (dcbH2) ligand.22 In aqueous
solution the luminescence energy of the complex decreases
upon decreasing the pH of the solution and protonation of the
carboxyl groups of the substituted bipyridine. The authors also
demonstrated that the average pKa of the carboxyl ligands is
3
greater when the complex is in the MLCT state, consistent
with an increase in electron density on the carboxy-bipyridine
ligand in the excited state. More recently Nazeeruddin et al.
thoroughly examined the ground and excited state acid
dissociation processes of [(bpy)Ru(dcbH2)2]Cl2 in solutions
of sulfuric acid and sodium hyrdoxide.23 While there are four
dissociable protons in the complex, only two experimentally
distinct pK values were determined, indicating that the first
deprotonation from each bpy occurs at or near the same pH.
Moreover, the authors demonstrated that, just as with
[(bpy)2Ru(dcbH2)]Cl2, the luminescence maximum was
dependent on pH over the pH range 0−7. These data show
that the energy of the luminescent excited state can be
controlled with pH.
In this manuscript, the use of pH to control the 3MLCT state
energy is exploited in examination of reversible energy transfer
3
3
between the MLCT state and the IL state of a covalently
attached aromatic hydrocarbon. The systems examined involve
Ru(II) complexes having carboxylated diimine ligands and one
of two covalently linked bipyridyl pyrene ligands. The rate of
the energy transfer process and the position of equilibrium can
be systematically controlled by varying the pH of the medium.
Through a combination of ultrafast and nanosecond time-
resolved transient absorption experiments, the dynamics of the
intramolecular energy transfer and the relaxation of the
equilibrated excited state are clearly elucidated. The importance
of pH on excited state reactivity is clearly illustrated in the
oxygen quenching sensitivity. These systems are under
investigation as in vivo phosphorescent sensors for dissolved
oxygen. This work illustrates the strong dependence of the
oxygen sensing ability to small differences in energy gap
Syntheses. The ligands and complexes 4-(1-pyrenyl)-2,2′-bipyr-
idine (pyr-bpy),24 2,2′-bipyridine-4,4′-dicarboyxlic acid (dcbH2),25 cis-
[Ru(dcbH2)Cl2],26 and [(Bz)Ru(bpy)Cl]Cl (Bz = benzene)27 were all
prepared as reported in a previous work.
4-(p-(1-Pyreneyl)phenyl)-2,2′-bipyridine (pyr-phen-bpy). A
total of 0.48 g (1.45 mmol) of 4,4,5,5-tetramethyl-2-(1-pyrenyl)-1,3,2-
dioxborolane and 0.45 g (1.45 mmol) of 4-(4-bromophenyl)-2,2′-
bipyridine were dissolved in 25 mL of toluene. The mixture was
purged with N2 for 15 min. Fifteen ml of 1 M NaOH and 33.5 mg (2
mol %) Pd(PPh3)4 were then added to the mixture. The reaction was
run under reflux for 20 h. After the reaction, the organic and aqueous
phases were separated. The aqueous layer was extracted with toluene
(4 × 20 mL). The organic phases were combined and dried with
MgSO4. The solvent was then removed via rotary evaporation. The
residue was purified by column chromatography on silica gel using a
gradient of 20:1−1:1 hexanes: ethyl acetate. After purification, 0.37 g
3
3
between the emitting MLCT state and the IL state of the
pyrene. As shown here, it is the excited state lifetime and not
differences in the quenching rate constant between the 3MLCT
and 3IL states that leads to the pH sensitivity of oxygen
quenching.
1
(60% yield) of a light yellow solid was obtained. HNMR (CDCl3) δ
ppm: 8.82 (ds, J = 1.84 Hz, 0.73 Hz, 1H), 8.80 (dd, J = 5.08 Hz, 0.75
Hz, 1 H), 8.75 (ddd, J = 4.77 Hz, 1.80 Hz, 0.92 Hz, 1 H), 8.50 (td, J =
8.00 Hz, 1.05 Hz, 1.05 Hz, 1H), 8.28−8.18 (m, 4H), 8.13 (s, 2H),
8.09−8.01 (m, 3H), 7.99 (td, J = 8.4 Hz, 2 H), 7.88 (dt, J = 8 Hz, 1.8
Hz, 1 H), 7.79 (td, J = 8.4 Hz, 2 Hz, 2H), 7.68 (dd, J = 5.08 Hz, 1.88
Hz, 1 H), 7.36 (ddd, J = 7.50 Hz, 4.76 Hz, 1.19 Hz, 1H). 13CNMR
(CDCl3) δ ppm: 156.53, 155.87, 149.60, 149.24, 142.24, 137.18,
137.03, 136.83, 132.27, 132.23, 132.10, 131.53(2), 131.36, 131.01,
130.88, 128.76, 128.64, 128.54, 128.48, 127.80, 127.69, 127.54, 127.25
(2), 126.16, 125.04, 124.93, 127.78, 121.65, 121.49, 119.22. ESI-MS
(m/z): 433.1762 [M+H]+ (Calcd. 433.1699, 14.54 ppm).
[(dcbH2)2Ru(bpy)]Cl2. A total of 75 mg of [(Bz)Ru(bpy)Cl]Cl
and 95.1 mg of dcbH2 were added to 2 mL of DMF. The mixture was
refluxed under nitrogen for 4 h during which the reaction color
changed from yellow to purple to dark red. After the reaction, the
mixture was added to 50 mL of acetone and cooled in the freezer to
EXPERIMENTAL SECTION
■
Spectroscopy. NMR spectra were recorded on a Varian 400 MHz
NMR. ESI mass spectra were obtained using a Bruker microTOF
spectrometer. All UV−vis absorption spectra were obtained on a
Hewlett-Packard 8452A diode array spectrophotometer. Photo-
luminescence spectra were obtained using a Spex Fluorolog
Fluoroimeter equipped with a 450 W Xe arc lamp and a SPEX 0.34
m spectrograph/CCD detector (Andor IDUS). Photoluminescence
lifetime measurements were collected using the visible output of an
OPO (OPOTEK) which was pumped by the third harmonic of a
Quantel Brilliant B Q-switched Nd:YAG laser. Excitation pulses were
generally <5 ns and were tuned to the absorption λmax of each
chromophore. Emitted light was detected through a single grating
monochromator (Applied Photophysics 0.25 m) with PMT detection
7498
dx.doi.org/10.1021/ja300866s | J. Am. Chem. Soc. 2012, 134, 7497−7506