Singlet and triplet excitation management in a bichromophoric near-infrared-phosphorescent BODIPY-benzoporphyrin platinum complex

Article abstract of DOI:10.1021/ja108493b

Multichromophoric arrays provide one strategy for assembling molecules with intense absorptions across the visible spectrum but are generally focused on systems that efficiently produce and manipulate singlet excitations and therefore are burdened by the restrictions of (a) unidirectional energy transfer and (b) limited tunability of the lowest molecular excited state. In contrast, we present here a multichromophoric array based on four boron dipyrrins (BODIPY) bound to a platinum benzoporphyrin scaffold that exhibits intense panchromatic absorption and efficiently generates triplets. The spectral complementarity of the BODIPY and porphryin units allows the direct observation of fast bidirectional singlet and triplet energy transfer processes (k _ST(¹BDP→¹Por) = 7.8×10¹¹ s^-1, k_TT(³Por→³BDP) = 1.0×10¹⁰ s^-1, k_TT(³BDP→ ³Por) = 1.6×10¹⁰ s^-1), leading to a long-lived equilibrated [³BDP][Por]=[BDP][³Por] state. This equilibrated state contains approximately isoenergetic porphyrin and BODIPY triplets and exhibits efficient near-infrared phosphorescence (λ_em = 772 nm, φ = 0.26). Taken together, these studies show that appropriately designed triplet-utilizing arrays may overcome fundamental limitations typically associated with core-shell chromophores by tunable redistribution of energy from the core back onto the antennae.

Full text of DOI:10.1021/ja108493b

Published on Web 12/10/2010
Singlet and Triplet Excitation Management in a
Bichromophoric Near-Infrared-Phosphorescent
BODIPY-Benzoporphyrin Platinum Complex
†
Matthew T. Whited, Peter I. Djurovich, Sean T. Roberts, Alec C. Durrell,
Cody W. Schlenker, Stephen E. Bradforth, and Mark E. Thompson*
Department of Chemistry, UniVersity of Southern California, Los Angeles,
California 90089, United States
Received September 20, 2010; E-mail: met@usc.edu
Abstract: Multichromophoric arrays provide one strategy for assembling molecules with intense absorptions
across the visible spectrum but are generally focused on systems that efficiently produce and manipulate
singlet excitations and therefore are burdened by the restrictions of (a) unidirectional energy transfer and
(b) limited tunability of the lowest molecular excited state. In contrast, we present here a multichromophoric
array based on four boron dipyrrins (BODIPY) bound to a platinum benzoporphyrin scaffold that exhibits
intense panchromatic absorption and efficiently generates triplets. The spectral complementarity of the
BODIPY and porphryin units allows the direct observation of fast bidirectional singlet and triplet energy
1
1
11 -1
3
3
10 -1
3
3
transfer processes (kST( BDPf Por) ) 7.8 × 10
s
, kTT( Porf BDP) ) 1.0 × 10
s , kTT( BDPf Por)
10
-1
3
3
)
1.6 × 10
s ), leading to a long-lived equilibrated [ BDP][Por]h[BDP][ Por] state. This equilibrated
state contains approximately isoenergetic porphyrin and BODIPY triplets and exhibits efficient near-infrared
phosphorescence (λem ) 772 nm, Φ ) 0.26). Taken together, these studies show that appropriately designed
triplet-utilizing arrays may overcome fundamental limitations typically associated with core-shell chro-
mophores by tunable redistribution of energy from the core back onto the antennae.
3
Introduction
bacteria. Researchers have used dendrimers as synthetic mimics
of these assemblies, with antennae arranged around central
chromophores to optimize the energy transfer process, generat-
The design of new chromophores for application in dye-
sensitized and organic photovoltaic cells (OPVs) is an active
area of research with important implications in energy science.
Aside from meeting a variety of materials criteria, promising
candidates must exhibit high absorptivities over a range of
wavelengths to harvest the greatest possible fraction of available
light. This criterion is critically important for devices employing
organic thin films, for which exciton diffusion and carrier
conduction can impose strict limits on the ideal thickness of
ing so-called core-shell systems with a “shell” of antenna
4
chromophores surrounding a lower energy core. Although the
core-shell approach can give good spectral coverage since both
the core and shell absorbance can be used to excite the central
chromophore, it often leads to isolation of the core chromophore,
preventing effective energy-harvesting from the periphery of
the core-shell system. Thus, while F o¨ rster processes can
efficiently funnel energy to the interior, the insulating nature
of the antennae effectively confines the excitation, preventing
collection of charge or energy from the core. In this paper we
report a core-shell system that efficiently captures a broad
spectrum, using both antennae and core chromophores, and
shows quantitative energy transfer from the antennae to core
but at the same time spreads the excited state energy over both
the core and shell chromophores. This has been achieved by
selecting core and shell components having both the appropriate
1
active layers. One promising strategy for achieving this goal
is the construction of multichromophoric arrays capable of
efficient intramolecular energy transfer between two or more
2
highly absorbing components. This approach has been used in
a range of systems, in which “antenna” chromophores transfer
energy to a central chromophore, reminiscent of the energy
transfer relays in photosynthetic organisms such as purple
†
California Institute of Technology, Pasadena, CA 91125.
(
1) Peumans, P.; Yakimov, A.; Forrest, S. R. J. Appl. Phys. 2003, 93,
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Ed. 2002, 41, 1900. (c) Cotlet, M.; Vosch, T.; Habuchi, S.; Weil, T.;
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(4) (a) Adronov, A.; Frechet, J. M. J. Chem. Commun. 2000, 1701. (b)
Denti, G.; Campagna, S.; Serroni, S.; Ciano, M.; Balzani, V. J. Am.
Chem. Soc. 1992, 114, 2944. (c) Balzani, V.; Campagna, S.; Denti,
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(e) Devadoss, C.; Bharathi, P.; Moore, J. S. J. Am. Chem. Soc. 1996,
118, 9635. (f) Sato, T.; Jiang, D. L.; Aida, T. J. Am. Chem. Soc. 1999,
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Angew. Chem., Int. Ed. 2001, 40, 3194. (h) Frechet, J. M. J. J. Polym.
Sci., Part A: Polym. Chem. 2003, 41, 3713.
1
27, 9760. (d) Diring, S.; Puntoriero, F.; Nastasi, F.; Campagna, S.;
Ziessel, R. J. Am. Chem. Soc. 2009, 131, 6108. (e) Kuciauskas, D.;
Liddell, P. A.; Lin, S.; Johnson, T. E.; Weghorn, S. J.; Lindsey, J. S.;
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8
604. (f) Terazono, Y.; Kodis, G.; Liddell, P. A.; Garg, V.; Moore,
T. A.; Moore, A. L.; Gust, D. J. Phys. Chem. B 2009, 113, 7147. (g)
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88
9 J. AM. CHEM. SOC. 2011, 133, 88–96
10.1021/ja108493b  2011 American Chemical Society

Triplet Management in a Multichromophoric Array
A R T I C L E S
a
Table 1. Visible-Light Absorption Properties of Representative
9
Chromophores Referenced to AM1.5G Photon Flux
50% photon
capture
integrated
absorption
coefficient (M
AM1.5G photon
capture (1 µM,
threshold
50 ,
a c
(PCT )
-1
,
a b
complex
)
10 µM)
2
+
8
8
8
8
8
Ru(bpy)
3
0.70 × 10
3.8 × 10
6.2 × 10
1.1 × 10
11 × 10
0.9%, 7.7%
4.9%, 29%
7.0%, 32%
1.7%, 12%
13%, 53%
340 µM
29 µM
34 µM
490 µM
8.5 µM
chlorophyll B
Pt(TPBP)
Me
Pt(TPBP) +
Me BODIPY
2
BODIPY
4
2
a
b
3
50-750 nm. Percentage of incident solar photons absorbed for a
c
solution of a given concentration (1-cm path length). Concentration
required to absorb 50% of the incident solar photons (1-cm path length).
2
Pt(TPBP) and Me BODIPY are the two compounds we have
chosen to use as the core and shell, respectively, in the light-
harvesting system reported in this paper. In Figure 1b, the
extinction spectra are translated into a rate of photon absorption
9
10
under AM1.5G sunlight for a 2 µM solution of each molecule.
The percentage of photons absorbed can then be determined
from the ratio of the integrated area under each curve to the
integrated AM1.5G flux (black trace), which automatically
corrects for the fact that photon flux varies with wavelength.
Although the spectra in Figure 1 are plotted versus wavelength,
it is important to note that the values presented in Table 1 were
determined by integration versus energy rather than wavelength,
since the inverse relationship between wavelength and energy
will tend to overweight the lower-energy portion of the spectrum
if wavelengths are used.
Figure 1. (a) UV-vis spectra of Pt(TPBP) (blue solid), chlorophyll B (red
2+
dash), Ru(bpy)
3
2
(green dot), and Me BODIPY (gray dot-dash). (b) Photon
flux for AM1.5G (black) plotted with rate of photon absorption for 2 µM
2
+
solutions of Pt(TPBP) (blue), chlorophyll B (red), Ru(bpy)
Me BODIPY (gray dot-dash) in a 1-cm path length cell.
3
(green), and
2
Chart 1. Model Complexes Used in This Study
At low concentration, the percentage of photons absorbed
roughly follows the integrated absorption coefficient (Table 1),
and by this measure Pt(TPBP) appears to perform better than
2+
Ru(bpy)
3
and chlorophyll B. However, a more illuminating
comparison is given by the 50% photon capture threshold
50
(
PCT ), the concentration of a molecule required to absorb 50%
of incident solar photons between 350 and 750 nm. The
Pt(TPBP) complex exhibits a poorer PCT50 than chlorophyll B
because the relatively narrow transitions of the former span a
comparatively small portion of the visible spectrum. Thus, with
increasing concentration the absorbance of Pt(TPBP) at 450 and
singlet energies to promote efficient singlet energy transfer from
shell to core and closely matched triplet energies to enable a
balanced distribution of triplet energy between the core and
6
20 nm quickly becomes saturated, whereas the absorbance
between 450 and 550 nm is little improved.
Boron dipyrrin (BODIPY) dyes such as Me
5
shell.
2
BODIPY have
In addition to designing core and shell chromophores to
achieve efficient singlet energy transfer and triplet equilibrium
between the two components, it is important that the resultant
complex has a net absorbance showing broad coverage across
the solar spectrum. The importance and advantage of using
complexes with panchromatic transitions may be readily dem-
onstrated by comparing solar dyes in the context of solar
radiation. In Figure 1a, UV-vis extinction spectra are presented
for four chromophores: tris(bipyridyl)ruthenium(II); chlorophyll
been widely studied for imaging applications and exhibit high
11
molar absorptivities in the 450-550 nm range, precisely where
absorption by Pt(TPBP) is weakest. In addition, since these
molecules are intensely fluorescent between 500 and 600 nm,
we hypothesized that F o¨ rster resonant energy transfer (FRET)
from BODIPY to Pt(TPBP) would be fast and efficient,
consistent with previous results showing that BODIPY chro-
(
7) (a) Perez, M. D.; Borek, C.; Djurovich, P. I.; Mayo, E. I.; Lunt, R. R.;
Forrest, S. R.; Thompson, M. E. AdV. Mater. 2009, 21, 1517. (b) Perez,
M. D.; Borek, C.; Forrest, S. R.; Thompson, M. E. J. Am. Chem. Soc.
2009, 131, 9281.
6
B; platinum tetraphenyltetrabenzoporphyrin (Pt(TPBP), Chart
1
), a triplet material recently shown by our lab to perform well
7
8
in lamellar OPVs; and Me
2
BODIPY (Chart 1), a dye with an
(
8) Qin, W. W.; Baruah, M.; Van der Auweraer, M.; De Schryver, F. C.;
Boens, N. J. Phys. Chem. A 2005, 109, 7371.
absorption spectrum complementary to that of Pt(TPBP).
(
9) ASTM Standard G173, Standard Tables for Reference Solar Spectral
Irradiances: Direct Normal and Hemispherical on 37° Tilted Surface;
American Society for Testing and Materials: West Conshocken, PA,
2008.
4c
(
5) While the pioneering metallodendrimers of Balzani do easily access
triplet excited states due to fast intersystem crossing at Ru and Os,
energy transfer in these systems is still unidirectional, similar to singlet-
based arrays.
(10) A similar treatment has recently been used to describe absorption by
neat films: Schlenker, C. W.; Thompson, M. E. Chem. Commun., in
press.
(11) (a) Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed. 2008,
47, 1184. (b) Loudet, A.; Burgess, K. Chem. ReV. 2007, 107, 4891.
(
6) (a) Vernon, L. P.; Seely, G. R. The Chlorophylls; Academic Press:
New York, 1966. (b) Kalyanasundaram, K. Coord. Chem. ReV. 1982,
4
6, 159.
J. AM. CHEM. SOC. 9 VOL. 133, NO. 1, 2011 89

A R T I C L E S
Whited et al.
Scheme 1. Synthesis of Pt(BDPTPBP) (4)
2
f,12
15
mophores can serve as effective antennae for porphyrins.
BODIPY derivative 1 with a para aldehyde functionality.
Employing parameters derived from model Me
Pt(TPBP) complexes, and assuming random orientations and a
2
BODIPY and
Platination and oxidation of 2 were performed as previously
1
6
described for the related Pt(TPBP) complex, ultimately
affording 4 in pure form as a dark green-black solid in 10%
yield over two steps (see Experimental Section for full synthetic
details).
The UV-vis spectrum of 4 is presented in Figure 2 along
with the spectrum predicted from Pt(TPBP) and four
separation of 10 Å, a high efficiency (>99.9%) and rate (kFRET
1
1
-1
)
8.3 × 10 s ) of FRET is predicted using the Photochem-
13
CAD software package.
As seen from Table 1, Me
properties by itself as a result of having only a single, sharp
visible absorption band. However, if four Me BODIPY mol-
2
BODIPY exhibits lackluster PCT50
2
2
Me BODIPY model complexes. As expected, the absorption
ecules are coupled to Pt(TPBP), the absorption bands of the
two species are both balanced and complementary (Table 1),
leading to dramatically improved PCT values relative to
Pt(TPBP) alone. Therefore, the combination of Pt(TPBP) and
profile of 4 is a composite of those for the model complexes,
with exceptionally intense transitions across the visible spectrum.
Although the peak at 514 nm ascribed to the BODIPY units is
5
0
2
quite similar to that of Me BODIPY, the Soret and Q peaks
Me
2
BODIPY chromophores into a single complex should lead
characteristic of the benzoporphyrin are broadened relative to
Pt(TPBP). For instance, the Soret band at 433 nm exhibits a
full width at half-maximum (fwhm) of 35 nm, compared with
22 nm for Pt(TPBP), and the fwhm for the Q-band at 619 nm
has increased from 19 nm for Pt(TPBP) to 24 nm for 4. The
porphyrin peak broadening may be due to conformational
disorder and the accompanying molecular distortion of the
to a multichromophoric array with UV-vis absorption properties
that rival or exceed the best chromophores for solar energy
harvesting (e.g., chlorophyll B). We report here the synthesis
and photophysical study of such a complex consisting of one
Pt(TPBP) and four Me BODIPY moieties in a core-shell
2
arrangement. This multichromophoric complex efficiently ab-
sorbs across the visible spectrum, as illustrated in Figure 1 and
Table 1, and at the same time gives the desirable energy transfer
properties for core-shell materials described above, namely,
funneling singlet energy to the core followed by rapid inter-
system crossing and redistribution of the triplet state between
the core and shell moieties.
12a,17
BODIPY units.
This broadening leads to a small improve-
ment in photon capture properties relative to those predicted
for 4 (AM1.5G Photon Capture (1 µM) ) 14% (measured),
13% (predicted); PCT50 ) 7.0 µM (measured), 8.5 µM
(predicted)), though the integrated absorption coefficient is
unchanged.
Results and Discussion
Synthesis and Characterization of Pt(BDPTPBP). The BO-
DIPY-benzoporphyrin scaffold was assembled following the
14
strategy of Finikova et al. (Scheme 1). Cyclohexenoporphyrin
, with four BODIPY units connected to the meso positions by
2
phenylene linkers, was accessed in 35% yield via an acid-
catalyzed macrocyclization of 4,5,6,7-tetrahydroisoindole and
(
12) (a) Li, F. R.; Yang, S. I.; Ciringh, Y. Z.; Seth, J.; Martin, C. H.; Singh,
D. L.; Kim, D. H.; Birge, R. R.; Bocian, D. F.; Holten, D.; Lindsey,
J. S. J. Am. Chem. Soc. 1998, 120, 10001. (b) Lee, C. Y.; Hupp, J. T.
Langmuir 2010, 26, 3760. (c) Koepf, M.; Trabolsi, A.; Elhabiri, M.;
Wytko, J. A.; Paul, D.; Albrecht-Gary, A. M.; Weiss, J. Org. Lett.
2
005, 7, 1279. (d) Kumaresan, D.; Datta, A.; Ravikanth, M. Chem.
Phys. Lett. 2004, 395, 87. (e) Maligaspe, E.; Tkachenko, N. V.;
Subbaiyan, N. K.; Chitta, R.; Zandler, M. E.; Lemmetyinen, H.;
D’Souza, F. J. Phys. Chem. A 2009, 113, 8478. (f) Ambroise, A.;
Wagner, R. W.; Rao, P. D.; Riggs, J. A.; Hascoat, P.; Diers, J. R.;
Seth, J.; Lammi, R. K.; Bocian, D. F.; Holten, D.; Lindsey, J. S. Chem.
Mater. 2001, 13, 1023. (g) Lammi, R. K.; Wagner, R. W.; Ambroise,
A.; Diers, J. R.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Phys.
Chem. B 2001, 105, 5341.
Figure 2. Predicted (dashed) and measured (solid) UV-vis spectra for 4
in CH Cl .
2 2
Panchromatic absorption in a multichromophoric array is only
useful if efficient energy transfer between the components can
be realized, so we turned to photoluminescence (PL) measure-
ments to examine the fate of absorbed photons. At room
(
(
13) Du, H.; Fuh, R. C. A.; Li, J. Z.; Corkan, L. A.; Lindsey, J. S.
Photochem. Photobiol. 1998, 68, 141.
14) Finikova, O. S.; Cheprakov, A. V.; Beletskaya, I. P.; Carroll, P. J.;
Vinogradov, S. A. J. Org. Chem. 2003, 69, 522.
90
J. AM. CHEM. SOC. 9 VOL. 133, NO. 1, 2011

Triplet Management in a Multichromophoric Array
A R T I C L E S
Figure 3. Normalized absorption, excitation (λem ) 770 nm), and emission
(λex ) 435 nm) spectra of 4 in toluene.
Figure 4. Ultrafast transient absorption spectra of 4 in toluene after
excitation at 508 nm (0.2-10 ps) (N.B.: solvent response at short time delays
prevents the accurate collection of data from 480 to 500 nm, so these data
points have been omitted from the 200 fs trace above).
temperature in toluene, irradiation of the porphyrin at 435 nm
leads to near-infrared (NIR) phosphorescence (λem ) 772, Figure
3
) along with extremely weak porphyrin fluorescence (λem
)
The PL behavior of 4 doped (0.5%) into a poly(methyl
methacrylate) (PMMA) matrix is analogous, with efficient
energy transfer and long lifetimes (τ ) 95 µs), though the
quantum efficiency (Φ ) 0.17) is slightly lower than in fluid
6
28 nm, Φ
fl
< 0.001), similar to what has previously been
1
4,16,18
reported for metal-benzoporphyrin complexes.
An ex-
citation spectrum for this 772 nm emission closely parallels the
absorption spectrum of 4 (Figure 3), indicating that intramo-
lecular energy transfer is quite efficient, as expected. Thus, direct
excitation of the BODIPY subunit at 500 nm results in nearly
exclusive (>98%) phosphorescent emission at 772 nm.
Recent studies have indicated that the triplet state of BODIPY
resides close in energy to the observed emission at 772 nm,
suggesting that the lowest-lying excited state of BODIPY-
benzoporphyrin hybrids may be precisely controlled by inde-
toluene solution due to a decreased radiative decay rate (k )
r
3
-1
3
-1
3
1
.9 × 10 s in toluene, 1.8 × 10 s in PMMA; knr ) 1.1 ×
4 -1 3
0 s in toluene, 8.7 × 10 in PMMA). Neat films of 4, on
the other hand, only display a broad, red-shifted emission (935
nm) at 77 K (Figure S8 in Supporting Information), indicating
competitive deactivation in the neat solid by excimer formation,
7
a
as previously observed for Pt(TPBP).
1
9
pendent tuning of the BODIPY and porphyrin subunits.
However, the phosphorescence efficiency (26%) and lifetime
Femtosecond Transient Absorption: Observation of Bidirec-
tional Singlet and Triplet Energy Transfer. Steady-state PL
measurements show that irradiation of 4 leads to benzoporphy-
rin-based phosphorescence but reveal no details of the energy-
transfer processes that precede emission. In particular, upon
(
τ ) 67 µs) in toluene are similar to those previously reported
16,20
for platinum benzoporphyrins,
as opposed to much lower
efficiencies and millisecond lifetimes reported for sensitized
19a
1
triplet emission from the organic BODIPY fragment. There-
fore, we assign emission to phosphorescence from the platinated
benzoporphyrin moiety. Nevertheless, the influence of the
nonemissive BODIPY triplet on the excited state properties is
still considerable (vida infra).
formation of the BODIPY singlet excited state ( BDP), the
3
1
1
porphyrin triplet state ( Por) can be populated by (a) BDPf Por
singlet energy transfer (ST) followed by intersystem crossing
3
(ISC) to give Por or (b) rapid intersystem crossing at BODIPY
3
to form BDP, followed by triplet energy transfer (TT) to give
3
21
22
The behavior of 4 in rigid matrices closely parallels what is
observed in fluid solution. At 77 K in 2-methyltetrahydrofuran
Por. Results from both our laboratory and others have
shown that the ISC rate for Pt(TPBP) is 2.5 × 10 s-1 (τ )
400 fs). On the other hand, time-resolved studies have also
shown that the ISC rate for BODIPY derivatives in the absence
12
(
2-MeTHF), a single intense NIR phosphorescent emission is
observed at 764 nm (τ ) 92 µs), with quantitative BODIPYf
porphyrin energy transfer (Figure S5 in Supporting Information).
8
-1 17
of a heavy atom is quite slow (kISC < 2 × 10 s ). It is thus
1 1
likely that BDPf Por energy transfer precedes ISC, but steady-
state measurements cannot rule out the alternative possibility
of ISC prior to energy transfer.
(
(
(
15) (a) Miao, Q.; Shin, J.-Y.; Patrick, B. O.; Dolphin, D. Chem. Commun.
2
009, 2541. (b) Shin, J. Y.; Tanaka, T.; Osuka, A.; Miao, Q.; Dolphin,
D. Chem.sEur. J. 2009, 15, 12955.
We set out to discriminate between these two pathways by using
ultrafast transient absorption to observe energy transfer and
intersystem crossing events occurring within the first hundred
picoseconds of excitation. These studies lead to the observation of
two distinct energy-transfer processes. First, upon selective excita-
16) Borek, C.; Hanson, K.; Djurovich, P. I.; Thompson, M. E.; Aznavour,
K.; Bau, R.; Sun, Y.; Forrest, S. R.; Brooks, J.; Michalski, L.; Brown,
J. Angew. Chem., Int. Ed. 2007, 46, 1109.
17) Kee, H. L.; Kirmaier, C.; Yu, L. H.; Thamyongkit, P.; Youngblood,
W. J.; Calder, M. E.; Ramos, L.; Noll, B. C.; Bocian, D. F.; Scheidt,
W. R.; Birge, R. R.; Lindsey, J. S.; Holten, D. J. Phys. Chem. B 2005,
1
09, 20433.
tion of BODIPY at 508 nm, a bleach at 515 nm is observed within
(
(
18) Lebedev, A. Y.; Filatov, M. A.; Cheprakov, A. V.; Vinogradov, S. A.
J. Phys. Chem. A 2008, 112, 7723.
1
2
00 fs due to the formation of BDP. The 515 nm bleach recovers
over a period of ca. 10 ps, accompanied by a simultaneous growth
of bleaches at 450 and 620 nm that correspond to the benzopor-
phyrin Soret and Q bands, respectively (Figure 4). A global fit to
19) (a) Galletta, M.; Campagna, S.; Quesada, M.; Ulrich, G.; Ziessel, R.
Chem. Commun. 2005, 4222. (b) Rachford, A. A.; Ziessel, R.; Bura,
T.; Retailleau, P.; Castellano, F. N. Inorg. Chem. 2010, 49, 3730. (c)
Singh-Rachford, T. N.; Haefele, A.; Ziessel, R.; Castellano, F. N.
J. Am. Chem. Soc. 2008, 130, 16164.
1
1
the data yields a rate for BDPf Por singlet energy transfer of
/kST ) 1.29 ( 0.11 ps (Figure 5b), which is quite similar to the
rate predicted for FRET using model Me BODIPY and Pt(TPBP)
1
(20) Borisov, S. M.; Papkovsky, D. B.; Ponomarev, G. V.; DeToma, A. S.;
Saf, R.; Klimant, I. J. Photochem. Photobiol., A 2009, 206, 87.
2
J. AM. CHEM. SOC. 9 VOL. 133, NO. 1, 2011 91

A R T I C L E S
Whited et al.
Figure 5. (a) Ultrafast transient absorption spectra of 4 after excitation at 508 nm (10-300 ps). (b) Time domain slices of transient absorptions at 515 nm
BODIPY) and 620 nm (porphyrin) with predicted traces based on kinetic parameters for the system.
(
complexes (1/kFRET ) 1.2 ps, vide supra). The transient spectrum
Nanosecond-to-Microsecond Transient Absorption: Character-
ization of an Equilibrated Emissive State. To further examine the
interplay of BODIPY and platinum porphyrin triplet states
observed by femtosecond transient absorption, we turned to
nanosecond-resolved TA measurements to elucidate the nature
of the long-lived, phosphorescent state of 4. As shown in Figure
obtained after 10 ps closely resembles spectra reported for the
21-23
Pt(TPBP) triplet,
consistent with previous measurements
revealing a rate of intersystem crossing for Pt(TPBP) of 1/kISC
)
1
1
400 fs and indicating that the generation of Por by ST from BDP
is followed by fast and quantitative intersystem crossing to produce
3
Por.
6
a, excitation of the BODIPY moiety at 487 nm leads to a
At longer times (10-300 ps, Figure 5a) the bleach signals for
Por decrease, accompanied by a concomitant partial return of the
transient absorption spectrum that is dominated by bleaches
corresponding to the benzoporphyrin Soret and Q bands (435
and 620 nm, respectively) but also contains a distinct bleach of
the BODIPY absorbance (515 nm) and is essentially unchanged
from the spectra recorded at 800 ps (vide supra). All features
in the transient spectrum decay at the same rate, which matches
the rate of phosphorescent decay (λem ) 768 nm) measured for
the same solution (Figure 6b). Taken together, these findings
show that the long-lived state generated upon excitation of
BODIPY consists of BODIPY and benzoporphyrin triplets that
reach thermal equilibrium within several hundred picoseconds
of excitation. Since the energy transfer leading to equilibration
3
BODIPY bleach at 515 nm. These spectral changes are assigned
3
3
to Porf BDP triplet energy transfer and were modeled to afford
a transfer rate of 1/k ) 99.6 ( 7.1 ps. The triplet transfer/exchange
F
3
3
process establishes an equilibrated [ BDP][Por]h[BDP][ Por] state
in ca. 300 ps that does not develop further on the time-scale of the
experiment (800 ps). The interplay between BODIPY and por-
phyrin excited states following selective BODIPY excitation is
illustrated in Figure 5b, where the transient absorptions at 515 nm
(BODIPY) and 620 nm (porphyrin) are plotted versus time, along
with fits associated with the kinetic parameters presented above.
The 50% recovery of the Soret and Q bleaches from 10 to 800 ps
1
0
-1
3
3
of the triplets (10 s ) is much faster than phosphorescent
decay (1010 s ), the ratio of the species remains constant over
the phosphorescent lifetime.
supports roughly equal populations of the BDP and Por states
several hundred picoseconds after excitation and indicates a
redistribution of energy from the porphyrin core back onto
BODIPY antennae in the form of a triplet excited state. This
hypothesis is supported by the fact that the transient spectrum
obtained at 300 ps exhibits the combined features of spectra
-1
The observation of fast thermal equilibration of the triplet
states allows two calculations to be made regarding the dynamics
of the molecule. First, the thermal equilibrium between por-
22,23
3
3
reported for the triplet of the Pt(TPBP) model complex
closely related BODIPY triplets.
and
phyrin triplet ( Por) and BODIPY triplet ( BDP) may be
described using a Boltzmann distribution, allowing the energy
19b,24
3
3
of BDP to be estimated if the relative populations of Por and
BDP are known. The energy difference between two states in
thermal equilibrium is given by
3
(
21) Roberts, S. T.; Schlenker, C. W.; Barlier, V.; McAnally, R. E.; Zhang,
Y.; Mastron, J. N.; Thompson, M. E.; Bradforth, S. E. J. Phys. Chem.
Lett., submitted for publication.
2
5
(
22) Rajapakse, G. V. N. Photophysical Properties of Metallotetraphe-
nyltetrabenzoporphyrins: Insights from Experimental and Theoretical
Studies. Ph.D. Thesis; Bowling Green State University: Bowling Green,
OH, 2008.
d1P2
d2P1
∆
E ) E - E ) kT ln
(1)
2
1
(
)
(
23) Singh-Rachford, T. N.; Castellano, F. N. Inorg. Chem. 2009, 48, 2541.
(
24) Although the triplet of the Me
2
BODIPY model complex could not be
where d
represents the degeneracy of state n, and P
the population of state n. Since there are four BODIPY antennae
surrounding the porphyrin core, we regard the BDP state as
represents
n
n
observed because of inefficient intersystem crossing, a closely related
BODIPY triplet could be quantitatively generated in unoxidized
intermediate 3, for which the BODIPY triplet (which is identical to
that in complex 4) is much lower in energy than the porphyrin triplet.
The transient absorption spectra of 3 are provided in the Supporting
Information (Figures S12 and S13) as a model for the contributions
3
(25) The derivation of this equation is presented in the Supporting
Information.
3
of BDP to the transient absorption spectra of complex 4.
92
J. AM. CHEM. SOC. 9 VOL. 133, NO. 1, 2011

Triplet Management in a Multichromophoric Array
A R T I C L E S
Figure 6. (a) Nanosecond transient absorption spectra of 4 following excitation at 487 nm. (b) Normalized phosphorescent decay of 4 and restoration of
bleach at 620 nm, monitored by time-resolved transient absorption spectroscopy.
2
6,27
3
P1
P2
1
quadruply degenerate,
whereas Por is doubly degenerate
)
+
(2)
3
22,28
due to its E
u
state,
giving (dBDP/dPor) ) 2. The relative
τ
obs
τ
1
τ
2
3
3
equilibrium populations of Por and BDP were estimated to
be 0.45 and 0.55, respectively, after scaling the observed
bleaches by the respective extinction coefficients for the
BODIPY and benzoporphyrin portions of complex 4 and
keeping in mind that the ground-state absorption attributed to
BODIPY corresponds to the absorption of four BODIPY units.
From these populations, ∆E was calculated to be 0.5kT, or 12
meV under experimental conditions (T ) 293 K), confirming
where P
n
and τ
n
represent, respectively, the relative population
and lifetime of state n. Based on the relative populations of
3
3
BDP and Por determined above and assuming that τBDP
.
τ
Por, the observed lifetime of 67 µs in dilute toluene represents
a 37 µs extension relative to the phosphorescent lifetime
expected for the porphyrin in the absence of an energy reservoir
(30 µs). This calculated lifetime is quite close to values recently
reported by Borisov et al. for related platinum benzoporphyrin
3
that the two triplets are nearly isoenergetic. The energy of Por,
2
0
derived from the emission maximum at 77 K, is 1.62 eV (764
complexes, adding credence to the estimation of the relative
populations of BDP and Por states and thus to the projected
energy of BDP.
The combination of femtosecond and nanosecond TA studies
with steady-state experiments allows construction of a Jablonski
3
3
3
nm). On the basis of these calculations, the energy of BDP is
3
estimated to be 1.64 eV (758 nm), quite similar to values
reported for phosphorescence from related BODIPY chro-
1
9a,b
mophores.
These values are also supported by the ca. 50%
BDP
3
diagram describing the excited-state behavior of Pt( TPBP)
restoration of the bleaches corresponding to Por between 10
and 800 ps mentioned above. Additional support for the energy
of BDP in complex 4 is provided by PL measurements of the
(
4) (Figure 7, Table 2). After irradiation of the BODIPY
1
3
fragment to form BDP, F o¨ rster energy transfer generates an
excited singlet state ( Por) on the platinated benzoporphyrin.
Rapid intersystem crossing to Por and overlap with the
photobleach of the BODIPY unit render Por invisible to our
transient absorption experiments. However, the energy of this
state can be determined from the observed fluorescent emission
at 628 nm, which is very weak (Φ < 0.001) as a result of the
extremely efficient ISC. Following intersystem crossing, thermal
equilibration occurs between the close-lying Por and BDP
states, that is, an efficient redistribution of excitation energy
1
unoxidized platinum cyclohexenoporphyrin 3 that was isolated
as an intermediate in the synthesis of 4. Although 3 is
nonemissive at room temperature, phosphorescence is exclu-
sively observed at 756 nm at 77 K (Figure S4 in Supporting
Information). This emission, which is significantly lower in
energy than those previously reported for metalated cyclohex-
3
1
1
8
enoporphyrins, has been assigned to BODIPY phosphores-
cence and is nearly identical to the energy determined using eq
3
3
1
.
3
from core to shell. Since the lifetime of the BDP state greatly
exceeds that of Por, the decay of this equilibrated state is
governed by the relative energies of these states (and conse-
quently their relative populations) as well as the lifetime of Por.
3
3
Second, the thermal equilibrium between BDP and Por
3
excited states leads to an “energy reservoir” effect, similar to
that previously described for polycyclic aromatic hydrocarbons
3
27,29
(e.g., pyrene, naphthalene) attached to metal complexes.
This
Balzani and co-workers have investigated unidirectional
Dexter-type triplet energy transfer between ruthenium and
osmium polypyridyl complexes through rigid oligophenylene
effect leads to a lengthening of the observed lifetime according
to the following equation when the states are in rapid equilibrium:
3
0
linkers. For their system, the rate of triplet energy transfer
was found to be related to the number of intervening phenylene
units according to the equation
(
(
26) This predicted equilibrium shift upon adding multiple degenerate
energy reservoirs has been experimentally verified for Ru(bpy)
3
-pyrene
systems (ref 27).
27) (a) Tyson, D. S.; Castellano, F. N. J. Phys. Chem. A 1999, 103, 10955.
b) Tyson, D. S.; Henbest, K. B.; Bialecki, J.; Castellano, F. N. J.
-γn
(
kTT ∝ e
(3)
Phys. Chem. A 2001, 105, 8154. (c) McClenaghan, N. D.; Barigelletti,
F.; Maubert, B.; Campagna, S. Chem. Commun. 2002, 602.
28) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic Press:
New York, 1978; Vol. 3, p 1.
where n is the number of phenylene units and γ is an attenuation
constant. An extension of their regression to n ) 1 predicts a
(
J. AM. CHEM. SOC. 9 VOL. 133, NO. 1, 2011 93

A R T I C L E S
Whited et al.
This unusual set of interactions is made possible by the different
singlet-triplet gaps associated with the BODIPY and benzopor-
phyrin moieties, which allow the behavior of this lowest-energy
molecular excited state to be tuned without disturbing the
complementary ground-state absorption properties of the en-
semble. In light of recent results from our laboratory suggesting
that molecules with excited states residing largely on the
periphery may exhibit improved performance in organic elec-
3
2
tronics, such a property may prove highly desirable.
Unfortunately, as mentioned above, neat films of 4 only
exhibit excimer emission at 77 K, indicating an energy loss
pathway for this particular molecule that renders it less desirable
as a donor for lamellar solar cells. At the moment, it is not
clear whether this emission is attributable to BODIPY- or
porphyrin-centered excimers. Nevertheless, the findings reported
in this paper have shown that modest tuning of the singlet-triplet
gap for one of the units may disfavor this sort of deactivation.
Figure 7. Jablonski diagram summarizing the excited-state behavior of
Pt( TPBP) (4) upon selective irradiation of the BODIPY or benzopor-
BDP
phyrin units.
Conclusions
Table 2. Measured Rate Constants and Energy Levels for Figure
In conclusion, we have synthesized and characterized a NIR-
phosphorescent BODIPY-platinabenzoporphyrin hybrid complex
that exhibits broad visible-light absorption, fast and efficient
intramolecular energy transfer, and high PL efficiencies up to
7
1
1
11 -1
k
k
k
k
k
ST ( BDPf Por)
7.8 × 10
2.5 × 10
1.0 × 10
1.6 × 10
s
1
2
-1
ISC (Por) 3
s
3
10 -1
F
( Porf BDP)
s
26% at ambient temperature in fluid solution. Using femto- and
3
3
10 -1
B
( BDPf Por)
s
5
-1
nanosecond transient absorption spectroscopy, we have eluci-
dated bidirectional singlet and triplet energy transfer pathways
decay ) k
r
+ knr
1.5 × 10 s
K
eq ) k
F
/k
B
0.61
1
E( BDP)
2.41 eV (516 nm)
2.00 eV (622 nm)
1.64 eV (758 nm)
1.62 eV (764 nm)
for the array, leading to
a
long-lived, equilibrated
1
E( Por)
E( BDP)
E( Por)
3
3
[
BDP][Por]h[BDP][ Por] state due to the nearly isoenergetic
3
BODIPY and benzoporphyrin triplets. As a result, the hybrid
complex may be described as an unusual example of a
core-shell chromophore that overcomes the normal limitation
of unidirectional energy transfer by utilizing singlets to funnel
excitation energy into the core and triplets to redistribute it back
onto the antennae. Castellano and co-workers have amply
demonstrated the potential importance of intermolecular
singlet-triplet interactions through recent investigations of
3
triplet energy transfer rate (kTT) of 1.1 × 10 s-1, nearly
10
3
3
identical to the value we observe for kTT( BDPf Por) of 1.6 ×
1
0
-1
1
0
s . This finding indicates that triplet energy transfer
between the benzoporphyrin and BODIPY moieties in 4 likely
occurs across the phenylene linker via a superexchange mech-
anism in a process similar to that described by Balzani for the
polypyridyl complexes.
19c,33
photon upconversion by triplet-triplet annihilation.
In this
paper, we have shown that intramolecular singlet-triplet
interactions may be utilized for a different purpose, allowing
fine-tuning of energy-transfer pathways in multichromophoric
arrays through careful modification of the singlet-triplet gaps
of the constituent materials to couple efficient light absorption
with efficient bidirectional energy redistribution. Future work
will be aimed at modifying the properties of such triplet-utilizing
arrays to optimize their performance in organic photovoltaics.
Consequences of Using Multichromophoric Arrays for
Triplet Generation. The efficient energy cascade depicted in
Figure 7 renders complex 4 analogous to the “cascatelles”
developed by Ulrich, Ziessel, and co-workers, that are used to
3
1
increase the apparent Stokes shifts of BODIPY fluorophores.
In our case, the combination of intense panchromatic absorption
with bidirectional intramolecular energy transfer makes the
resulting phosphorescent cascatelle a unique example of a
core-shell chromophore capable of efficiently funneling energy
to the core followed by redistribution back onto the antennae.
Experimental Section
General Considerations. 4,4-Difluoro-3,5-dimethyl-8-(4-formyl-
phenyl)-4-bora-3a,4a-diaza-s-indacene (1) was prepared by the
1
5a
method of Dolphin and co-workers.
All other reagents were
(
29) (a) Ford, W. E.; Rodgers, M. A. J. J. Phys. Chem. 1992, 96, 2917. (b)
Rachford, A. A.; Goeb, S.; Ziessel, R.; Castellano, F. N. Inorg. Chem.
purchased from commercial vendors and used without further
purification. All air-sensitive manipulations were performed using
standard Schlenk techniques as needed, following the procedures
indicated below for each preparation. NMR spectra were recorded
at ambient temperature on Varian Mercury 500 and 600 MHz
2
008, 47, 4348. (c) Hissler, M.; Harriman, A.; Khatyr, A.; Ziessel, R.
Chem.sEur. J. 1999, 5, 3366. (d) Morales, A. F.; Accorsi, G.;
Armaroli, N.; Barigelletti, F.; Pope, S. J. A.; Ward, M. D. Inorg. Chem.
2
002, 41, 6711. (e) Leroy-Lhez, S.; Belin, C.; D’Aleo, A.; Williams,
R. M.; De Cola, L.; Fages, F. Supramol. Chem. 2003, 15, 627. (f)
Ziessel, R.; Hissler, M.; El-Ghayoury, A.; Harriman, A. Coord. Chem.
ReV. 1998, 178, 1251. (g) Tyson, D. S.; Luman, C. R.; Zhou, X. L.;
Castellano, F. N. Inorg. Chem. 2001, 40, 4063. (h) Lavie-Cambot,
A.; Lincheneau, C.; Cantuel, M.; Leydet, Y.; McClenaghan, N. D.
Chem. Soc. ReV. 2010, 39, 506. (i) Medlycott, E. A.; Hanan, G. S.;
Loiseau, F.; Campagna, S. Chem.sEur. J. 2007, 13, 2837.
30) Schlicke, B.; Belser, P.; De Cola, L.; Sabbioni, E.; Balzani, V. J. Am.
Chem. Soc. 1999, 121, 4207.
1
spectrometers; H chemical shifts were referenced to residual
solvent. UV-vis spectra were recorded on a Hewlett-Packard 4853
diode array spectrophotometer. Steady-state emission experiments
at room temperature and 77 K as well as phosphorescence decays
were performed using a Photon Technology International Quanta-
(
(
(32) Wu, C.; Djurovich, P. I.; Thompson, M. E. AdV. Funct. Mater. 2009,
19, 3157.
31) (a) Ulrich, G.; Goze, C.; Guardigli, M.; Roda, A.; Ziessel, R. Angew.
Chem., Int. Ed. 2005, 44, 3694. (b) Goze, C.; Ulrich, G.; Ziessel, R.
J. Org. Chem. 2007, 72, 313.
(33) Singh-Rachford, T. N.; Nayak, A.; Muro-Small, M. L.; Goeb, S.;
Therien, M. J.; Castellano, F. N. J. Am. Chem. Soc. 2010, 132, 14203.
94
J. AM. CHEM. SOC. 9 VOL. 133, NO. 1, 2011

Triplet Management in a Multichromophoric Array
A R T I C L E S
Master Model C-60SE spectrofluorimeter. Quantum efficiency
measurements were carried out using a Hamamatsu C9920 system
equipped with a xenon lamp, calibrated integrating sphere, and
model C10027 photonic multichannel analyzer.
minimal CHCl
causing black solids to precipitate overnight. The compound was
purified by an additional precipitation from CH Cl with Et O,
3 2
(ca. 1 mL), and the solution was layered with Et O,
2
2
2
yielding the title compound as a black solid that was isolated by
Cyclohexenoporphyrin 2 ·(HCl)
-neck round-bottom flask, dichloromethane (125 mL) was sparged
with N with stirring for 20 min. The flask was shielded from light,
,5,6,7-tetrahydroisoindole (54 mg, 0.45 mmol) and aldehyde 1 (157
mg, 0.48 mmol) were added as solids, and the solution was stirred
under N for 10 min. BF ·OEt (20 µL, 0.10 mmol) was added via
2
[H
4
(
BDPTPCHP)Cl
2
]. In a
filtration (5 mg, 15%). UV-vis (CH Cl ) λ (log ε): 281 (4.98),
2
2
max
1
3
340 (4.83), 433 (5.40), 514 (5.48), 573 (4.34), 619 (5.07). H NMR
2
(CDCl ): δ 8.45 (d, J ) 8 Hz, 8H, phenyl Ar-H), 8.06 (d, J ) 8
3
4
Hz, 8H, phenyl Ar-H), 7.33-7.36 (m, 8H, porphyrin Ar-H),
7.24-7.26 (m, 8H, porphyrin Ar-H), 7.16 (d, J ) 4 Hz, 8H,
BODIPY Ar-H), 6.50 (d, J ) 4 Hz, 8H, BODIPY Ar-H), 2.79 (s,
2
3
2
1
3
syringe, and the resulting mixture was stirred for 2.5 h. DDQ (127
mg, 0.56 mmol) was added in one portion, and the reaction was
allowed to proceed with stirring for 18 h. The resulting red-brown
24H, -CH ). C NMR (CDCl ): δ 158.6, 143.2, 142.0, 137.7, 136.1,
3
3
135.4, 134.9, 133.8, 131.6, 130.3, 125.9, 124.3, 120.2, 117.9, 15.3.
MALDI: m/z for C104H B F N Pt calcd 1878.6, found 1878.9.
72
4
8
12
solution was washed with 10% aq. Na
1 × 75 mL), dried (MgSO ), filtered, and concentrated in vacuo
to a brown film. These residues were purified by column chroma-
tography on silica gel, first eluting with copious CH Cl to remove
2
SO
3
(3 × 50 mL) and brine
Femtosecond Transient Absorption Spectroscopy. Femtosec-
ond transient absorption measurements were performed using the
output of a Ti:sapphire regenerative amplifier (Coherent Legend,
4 mJ, 35 fs) operating at a 1 kHz repetition rate. ∼10% of the
amplifier output was used to pump a type II OPA (Spectra Physics
OPA-800C) and sum frequency of the signal and residual 800 nm
pump generated 3.5 µJ excitation pulses centered at 508 nm with
6.5 nm of bandwidth. Prior to the sample, the excitation pulses
were attenuated by a neutral density filter and were focused behind
(
4
2
2
residual fluorescent BODIPY impurities. After all BODIPY impuri-
ties had eluted (as judged by UV-vis spectroscopy), the desired
2 2
product was eluted as a dark brown solution using CH Cl /THF
(
20:1), characterized by optical transitions at 347 nm (BODIPY),
4
35 and 483 nm (Soret peaks), 512 nm (BODIPY), 611 and 678
nm (Q bands). The free-base porphyrin was converted to the
2
lens. Assuming a Gaussian profile,
the sample using a 50 cm CaF
dication by washing with 5% aq HCl (2 × 50 mL) and water (1 ×
the pump spot size at the sample had a fwhm of 380 µm. Probe
pulses were generated by focusing a small amount of the amplifier
output into a rotating CaF plate, yielding a supercontinuum
2
spanning the range of 320-950 nm. A pair of off-axis aluminum
parabolic mirrors collimated the supercontinuum probe and focused
it into the sample.
The sample consisting of 4 dissolved in toluene solution was
held in a 1-cm path length quartz cuvette under a deoxygenated
atmosphere and had a peak optical density of 0.17 at 517 nm. The
polarization of the pump and probe were set perpendicular to one
another, which allowed for the suppression of scatter by passing
the probe light transmitted by the sample through a polarizer prior
to detection. A spectrograph (Oriel MS127I) was used to disperse
the supercontinuum probe onto a 256 pixel silicon diode array
7
4
5 mL). The organics were dried (MgSO ), filtered, and concen-
trated by rotary evaporation to a brown film. Porphyrin 2 was
obtained in pure form as the green-brown bis(hydrochloride) salt
by precipitation from CH
Cl
5.27), 623 (4.03), 680 (4.45). H NMR (CDCl
2 2 2
Cl upon layering with Et O (76 mg,
3
(
5%). UV-vis (CH
2
2
) λmax (log ε): 354 (4.67), 481 (5.41), 513
1
3
3
): δ 8.54 (d, JHH
3
3
)
8 Hz, 8H, Ar-H), 8.02 (d, JHH ) 8 Hz, 8H, Ar-H), 6.94 (d, JHH
3
)
4 Hz, 8H, BODIPY Ar-H), 6.44 (d, JHH ) 4 Hz, 8H, BODIPY
2
3
Ar-H), 2.77 (s, 24H, BODIPY -CH
3
), 2.62 (dt, JHH ) 17 Hz, JHH
2
3
)
6 Hz, 8H, -CH
2
), 2.16 (dt, JHH ) 17 Hz, JHH ) 6 Hz, 8H,
-
CH
2
), 1.80 (m, 8H, -CH
2
), 1.26 (m, 8H, -CH
2
), 0.86 (s, 4H, -NH).
1
3
C NMR (CDCl
3
): δ 158.4, 143.8, 141.3, 139.7, 136.2, 135.5,
135.2, 134.5, 130.6, 130.1, 120.0, 117.1, 24.7, 22.4, 15.1. MALDI:
90 4 8
m/z for C104H B F N12 calcd 1703.8, found 1702.1.
(Hamamatsu), allowing multiplex detection of the transmitted probe
Platinum Cyclohexenoporphyrin 3 [Pt(BDPTPCHP)]. Platinu-
m(II) chloride (40 mg, 0.15 mmol) was added to dry, degassed
benzonitrile (40 mL), and the mixture was heated with stirring under
as a function of wavelength. Pump induced changes in the probe
were determined by using an optical chopper to block every other
pump pulse. At early time delays, a strong nonresonant signal from
the sample cell and solvent is observed and relaxes within 300 fs.
Careful measurement of this nonresonant signal enabled its partial
subtraction from the transient data and allowed the data to be
corrected for the temporal dispersion of the supercontinuum probe
N
2
at 100 °C, causing the platinum salts to dissolve as the solution
turned yellow. The cyclohexenoporphyrin dication 2·(HCl) (40
2
mg, 0.023 mmol) was added as a solid, and the resulting solution
was heated with stirring at reflux for 3 h. The mixture was cooled
to ambient temperature and benzonitrile removed by vacuum
2
resulting from propagation through the CaF plate and sample.
2 2
distillation. The residues were dissolved in CH Cl and filtered to
remove solids. The filtrate was dried in vacuo to afford a dark brown
powder, which was further purified by washing with methanol (3
The data presented in the text were measured for a pump fluence
2
of 150 µJ/cm . Based on the measured absorption cross section of
4
, at this fluence we expect at most only a single excitation per
×
10 mL). The powder was subjected to column chromatography
SiO gel, CH Cl eluent), and the cleanest fractions were combined,
dried by rotary evaporation, and crystallized as green plates by
layering a concentrated solution of 3 in CH Cl with Et O (33 mg,
9%). UV-vis (CH Cl ) λmax (log ε): 340 (4.8), 410 (5.4), 512
5.5), 569, (4.4). H NMR (CDCl ): δ 8.22 (d, J ) 8 Hz, 8H, phenyl
Ar-H), 7.83 (d, J ) 8 Hz, 8H, phenyl Ar-H), 6.94 (d, J ) 4 Hz,
molecule. However, to verify that annihilation processes do not
contribute to the signal, we also measured transient spectra using
a pump fluence of 75 µJ/cm and found that the signal scaled
linearly between the two data sets.
(
2
2
2
2
2
2
2
6
(
2
2
1
Nanosecond Transient Absorption Spectroscopy. Time-
resolved spectroscopic measurements were carried out at the
Beckman Institute Laser Resource Center. Samples were degassed
prior to experiments by three freeze-pump-thaw cycles. Laser
excitation was achieved with 8-ns pulses from a 10 Hz Q-switched
Nd:YAG laser (Spectra-Physics Quanta-Ray PRO-Series). The third
harmonic was used to pump an optical parametric oscillator
(Spectra-Physics Quanta-Ray MOPO-700) that provided excitation
at 487 nm.
A pulsed 75-W arc lamp (PTI model A 1010) supplied probe
light for transient absorption measurements. A double monochro-
mator (Instruments SA DH-10) with 1 mm slits was used to select
probe wavelengths. Transmitted light was detected with a photo-
multiplier tube (PMT, Hamamatsu R928). The PMT current was
amplified and recorded with a transient digitizer (Tektronix DSA
3
8
H, BODIPY Ar-H), 6.41 (d, J ) 4 Hz, 8H, BODIPY Ar-H), 2.75
13
(
s, 24H, -CH
CDCl ): δ 158.2, 143.0, 140.2, 139.8, 134.9, 134.3, 133.9, 133.8,
30.2, 129.8, 119.9, 118.7, 26.9, 23.7, 15.2. MALDI: m/z for
3 2 2
), 2.48 (s, 16H, -CH ), 1.61 (s, 16H, -CH ). C NMR
(
1
3
104 88 4 8
C H B F N12Pt calcd 1895.7, found 1894.1.
Platinum Benzoporphyrin 4 [Pt(BDPTPBP)]. Platinum cyclo-
BDP
hexenoporphyrin 3 (Pt( TPCHP), 33 mg, 0.017 mmol) was
dissolved in toluene (30 mL). DDQ (40 mg, 0.18 mmol) was added,
and the solution was heated at reflux with stirring for 45 min,
causing the color to change from brown to olive. The resulting
solution was washed with sodium sulfite (10% aq, 2 × 50 mL)
and brine (1 × 50 mL), dried (MgSO
4
), filtered, and concentrated
in vacuo to a dark solid. These dark residues were dissolved in
J. AM. CHEM. SOC. 9 VOL. 133, NO. 1, 2011 95

A R T I C L E S
02). Short and long pass filters were employed to remove scattered
excitation light. Decay kinetics were averaged over 500 laser pulses.
All instruments and electronics in these systems were controlled
by software written in LabVIEW (National Instruments). Data
manipulation was performed with MATLAB R2008a (Mathworks,
Inc.) and graphed with Origin software.
Whited et al.
6
0937015), and A.C.D. acknowledges support from the NSF Center
for Chemical Innovation (CCI Powering the Planet, Grants CHE-
802907 and CHE-0947829). We are also grateful to Dr. Jay
Winkler and the Beckman Institute Laser Resource Center at the
California Institute of Technology for assistance with nanosecond
transient absorption measurements. We dedicate this manuscript
to Professor Harry B. Gray, pioneer of inorganic photochemistry,
on the occasion of his 75th birthday.
0
Acknowledgment. Funding for this research was provided by
the Center for Advanced Molecular Photovoltaics (CAMP) (KUS-
C1-015-21) of the King Abdullah University of Science and
Technology (KAUST) and the Global Photonic Corporation. The
quantification of absorbance at AM1.5G illumination and femto-
second transient absorption measurements were carried out with
support from the Department of Energy’s Energy Frontier Research
Center program (Center for Energy Nanoscience, Award DE-
SC0001011). S.T.R. acknowledges support from the National
Science Foundation in the form of an ACC-F fellowship (CHE-
1
Supporting Information Available: Derivation of eq 1; H
NMR spectrum of 4; absorption and photoluminescence spectra
for 3, 4, and model complexes; femtosecond-resolved transient
absorption spectra for 3. This material is available free of charge
via the Internet at http://pubs.acs.org.
JA108493B
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J. AM. CHEM. SOC. 9 VOL. 133, NO. 1, 2011