Journal of the American Chemical Society
ARTICLE
of function in response to external stimuli is also possible in
human-made nanosystems for solar energy conversion.
synchronized with a mechanical chopper modulating the pump path at
499 Hz. The observed kinetics were independent of both pump and
probe intensity. The measured transient absorption signal was fit with a
single exponential decay using least-squares analysis, and goodness of fit
was established from residuals. The fitting was restricted to the part of
the curve where the effects of the excitation pulse and coherence artifacts
were negligible.
’ EXPERIMENTAL SECTION
The details for the synthesis and characterization of all new com-
pounds reported are given in the Supporting Information. Spectroscopic
studies were carried out on dilute (∼10-5 M) solutions in dichloro-
methane, and acetic acid was added to convert the dye to the protonated
form, unless otherwise specified. Random errors associated with the
reported lifetimes were typically e5%.
Steady-State Spectroscopy. Absorption spectra were measured
on a Shimadzu UV2100U UV-vis and/or UV-3101PC UV-vis-NIR
spectrometer. Steady-state fluorescence spectra were measured using a
Photon Technology International MP-1 spectrometer and corrected for
detection system response. Excitation was provided by a 75 W xenon-arc
lamp and single grating monochromator. Fluorescence was detected 90ꢀ
to the excitation beam via a single grating monochromator and an R928
photomultiplier tube having S-20 spectral response and operating in the
single-photon counting mode.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details of the
b
synthesis and characterization of the molecules described. This
acs.org.
’ AUTHOR INFORMATION
Corresponding Author
gust@asu.edu; GRFleming@lbl.gov
Time-Resolved Fluorescence. Fluorescence decay measure-
ments were performed employing two different systems: a conventional
single-photon timing apparatus and a streak camera setup. The excita-
tion source for the single-photon timing system was a mode-locked Ti:
Sapphire laser (Spectra Physics, Millennia-pumped Tsunami) with a
130-fs pulse duration operating at 80 MHz. The laser output was sent
through a frequency doubler and pulse selector (Spectra Physics model
3980) to obtain 370-450 nm pulses at 4 MHz. Fluorescence emission
was detected at the magic angle using a double grating monochromator
(Jobin Yvon Gemini-180) and a microchannel plate photomultiplier
tube (Hamamatsu R3809U-50). The instrument response function was
35-55 ps. The spectrometer was controlled by software based on the
LabView programming language, and data acquisition was done using a
single-photon counting card (Becker-Hickl, SPC-830).
Time vs wavelength fluorescence intensity surfaces were recorded on
the streak camera system, with excitation provided by an ultrafast laser.
Laser pulses of ∼130 fs at 800 nm were generated from an amplified,
mode-locked titanium-sapphire 250 kHz laser system (Verdi/Mira/
RegA, Coherent). The excitation light wavelength was adjusted using an
optical parametric amplifier (Coherent). Fluorescence with polarization
set to the magic angle was collected 90ꢀ to the excitation beam and focused
on the entrance slit of a Chromex 250IS spectrograph, which was coupled
to a Hamamatsu C5680 streak camera with a M5675 synchroscan sweep
unit. The streak images were recorded on a Hamamatsu C4742 CCD
camera and curvature corrected (corrections for shading and system
detection sensitivity at different wavelengths were not performed). The
instrument response function was ∼5-20 ps.
’ ACKNOWLEDGMENT
This work was funded by the Helios Solar Energy Research
Center, which is supported by the Director, Office of Science,
Office of Basic Energy Sciences of the U.S. Department of Energy
under Contract No. DE-AC02-05CH11231. G.K. was supported
as part of the Center for Bio-Inspired Solar Fuel Production, an
Energy Frontier Research Center funded by the U.S. Department
of Energy, Office of Science, Office of Basic Energy Sciences
under Award Number DE-SC0001016. C.M. thanks the Science
Foundation Arizona for financial support.
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