Generation of phosphorescent triplet states via photoinduced electron transfer: Energy and electron transfer dynamics in pt porphyrin-rhodamine b dyads

Article abstract of DOI:10.1021/jp301345h

Control over generation and dynamics of excited electronic states is fundamental to their utilization in all areas of technology. We present the first example of multichromophoric systems in which emissive triplet states are generated via a pathway involv

Full text of DOI:10.1021/jp301345h

Article
pubs.acs.org/JPCA
Generation of Phosphorescent Triplet States via Photoinduced
Electron Transfer: Energy and Electron Transfer Dynamics in Pt
Porphyrin−Rhodamine B Dyads
Tomoyasu Mani,† Dariusz M. Niedzwiedzki,‡ and Sergei A. Vinogradov*,†
†Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
‡Photosynthetic Antenna Research Center (PARC), Washington University, Saint Louis, Missouri 63130, United States
S
* Supporting Information
ABSTRACT: Control over generation and dynamics of
excited electronic states is fundamental to their utilization in
all areas of technology. We present the first example of multi-
chromophoric systems in which emissive triplet states are
generated via a pathway involving photoinduced electron transfer
(ET), as opposed to local intrachromophoric processes. In model
dyads, PtP−Phn−pRhB+ (1−3, n = 1−3), comprising platinum-
(II) meso-tetraarylporphyrin (PtP) and Rhodamine B piperazine
derivative (pRhB+), linked by oligo-p-phenylene bridges (Phn),
upon selective excitation of pRhB+ at a frequency below that of the
lowest allowed transition of PtP, room-temperature T1→S0 phosphorescence of PtP was observed. The pathway leading to the emissive
PtP triplet state includes excitation of pRhB+, ET with formation of the singlet radical pair, intersystem crossing within that pair, and
subsequent radical recombination. Because of the close proximity of the triplet energy levels of PtP and pRhB+, reversible triplet−triplet
(TT) energy transfer between these states was observed in dyads 1 and 2. As a result, the phosphorescence of PtP was extended in time
by the long decay of the pRhB+ triplet. Observation of ET and TT in the same series of molecules enabled direct comparison of the
distance attenuation factors β between these two closely related processes.
photon absorbing antenna chromophores.12−14 These molecules
INTRODUCTION
Control over emissivity and dynamics of excited states of
molecules is central to practical utilization of these states in all
■
appeared very useful as probes in two-photon phosphorescence
lifetime microscopy (2PLM) of oxygen.14−16 One studied system
comprised Pt(II) tetrabenzoporphyrin (PtTBP)−Rhodamime B
(RhB+) dyad.13 Upon excitation of RhB+, fast energy transfer
from the RhB+ singlet state (S1RhB+) onto PtTBP and
subsequent intersystem crossing (ISC) yielded local PtTBP
triplet (T1PtTBP), which was depopulated via efficient
T1PtTBP→RhB+ electron transfer (ET), followed by nonradia-
tive recovery of the ground-state species. (Note that Rhodamine
B is a cation, and therefore its one-electron reduction leads to the
neutral radical.)
Analyzing energy and electron flow in antenna (A)−triplet
core (C) dyads, we reasoned that if a radical pair state (RP)
would have its energy higher than the emissive local triplet state
of the core emitter (CT1), and the corresponding singlet state of
the core (CS1) would lie above the initially excited singlet state
of the antenna (AS1), one likely evolution pathway for the
system would be: (1) the ET between the antenna and the core,
(2) formation of the singlet radical pair (RPS), (3) intersystem
crossing (RP-ISC) with formation of the triplet RPT, and (4)
subsequent decay of RPT into CT1 and AS0 (Figure 1). Alternatively,
areas of technology and medicine. We are particularly interested in
triplet electronic spin states, whose applications encompass
medical photodynamic therapy,1,2 energy up-conversion by
triplet−triplet annihilation,3,4 organic light-emitting devices
(OLED),5,6 and biological imaging of oxygen.7,8 In imaging
applications, such as oxygen imaging by phosphorescence
lifetime,9 the ability to impose control over spatial localization
of triplet states of probe molecules, for example, by way of
applying external optical or magnetic fields, may lead to new
ways to integrate diffuse phosphorescence lifetime imaging10,11
with other imaging modalities, thereby enhancing informational
content and improving spatial imaging resolution. Similarly, in
photodymanic therapy, controlling formation of dark triplet
states for generation of singlet oxygen would be invaluable for
confining the photodynamic action selectively to the diseased
tissue, while reducing undesirable collateral damage. In view of
these applications, molecules with intricate energy and electron
transfer pathways, providing opportunities for controlling rates
and yields of formation of triplet states, are of considerable
interest.
Recently, we described molecular systems, in which emissive
triplet states of Pt(II) porphyrins were populated via a route
Received: February 9, 2012
Revised: March 7, 2012
Published: March 8, 2012
including Forster-type energy transfer from appended multi-
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© 2012 American Chemical Society
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(T1RhB and T1PtP), which is manifested by delayed
phosphorescence of PtP, presumably mediated by the
triplet−triplet (TT) energy transfer. Although observation of
dark triplet states via coupled emissive triplet states has been
previously reported,43−46 in our case the presence of both ET
and TT in the same series of bichromophoric molecules makes
it possible to directly compare distance attenuation factors β
between these two closely related processes.
EXPERIMENTAL METHODS
■
General Information. All solvents and reagents were
obtained from standard commercial sources and used as
received. Selecto silica gel (Fisher Scientific, particle size 32−
63 μm) was used for column chromatography. 1H NMR
spectra were recorded on a Varian Unity 400 MHz
spectrometer. Mass spectra were obtained on a MALDI-TOF
MS Microflex LRF instrument (Bruker Daltonics), using
α-cyano-4-hydroxycinnamic acid as a matrix. All of the photo-
physical and electrochemical measurements were performed in
anhydrous benzonitrile at 22 °C, unless otherwise noted.
Optical Measurements. UV−vis absorption spectra were
recorded on a Perkin-Elmer Lambda 35 UV−vis spectropho-
tometer. Steady-state and time-resolved phosphorescence
measurements were performed on a FS900 spectrofluorometer
(Edinburgh Instruments, UK), equipped with R2658P PMT
(Hamamatsu). Time-resolved phosphorescence measurements
were performed using either the FS900 instrument with a
xenon flash lamp as the excitation source or an in-house
constructed time-domain phosphorometer. FS900 allows
measurements of emission decays with time constants >20 μs
due to its rather long IRF. The custom-built instrument is
based on a multichannel data acquisition board (USB-6251,
National Instruments), operating at software-selectable digitiza-
tion frequencies of up to 1.25 MHz. The excitation sources in
sample are white LEDs (1W Lumiled, Phillips, <10 ns rise
time), and the detector is an avalanche photodiode (Hamamatsu).
Both the LEDs and the detector were equipped with approp-
riate optical filters (narrow-band for excitation and long-pass
for emission). The light was coupled to samples using plastic
fibers (3 mm in diameter). The IRF of the phosphorometer has
fwhm of ∼3 μs, permitting highly accurate measurements of
phosphorescence decays with time constants ranging from >3 μs
to up to 100 ms. The software for the instrument is written in
C/C++ (Qt, Nokia).
Time-resolved fluorescence measurements were performed
by the time-correlated single photon counting method (TCSPC).
The TCSPC system consisted of a picosecond diode laser
(PicoQuant LDH-C-400, λmax = 408 nm, fwhm ≈ 100 ps, 40 MHz
rep. rate), multichannel-plate PMT (Hamamatsu R2809U), and a
TCSPC board (Becker & Hickl, SPC-730).
Quartz fluorometric cells (1 cm optical path length) were
used in all optical experiments. The samples were deoxygenated
by Ar bubbling or by applying four pump−freeze−thaw cycles.
Quantum yields were measured against fluorescence of Rhod-
amine 6G in EtOH (Φfl = 0.94),47 using rigorously deoxygenated
solutions. Time-resolved emission spectra, obtained using the
FS900 instrument, were linearly decomposed into sums of their
principal components, that is, fluorescence of RhB+ and
phosphorescence of PtP, to obtain the quantum yields of these
emission types individually.
Figure 1. Energy diagram of bichromophoric antenna (A)−core (C)
systems, in which the local triplet state of the core (CT1) is populated
via excitation of the antenna, electron transfer (ET) with formation of
the singlet radical pair (RPS), intersystem crossing (RP-ISC), and
radical recombination (RT) from the resulting triplet radical pair
(RPT). Alternatively, RPS undergoes charge recombination RS, yielding
the ground-state species.
singlet radical pair RPS would recombine directly with regeneration
of the ground-state species. One attractive feature of such scheme
would be the possibility to tune the rate of the RP-ISC, for
example, by affecting the spin dynamics in the RP via modulation
of hyperfine interactions,17 and thereby to impose magnetic control
over the formation of the triplet states. On the other hand, the
ability to generate an emissive triplet state via ET, as opposed to
having to deal with ET to prevent unwanted phosphorescence
quenching, a problem we encountered in construction of two-
photon oxygen probes,13 would greatly extend the choice of
antenna chromophores suitable for construction of these useful
sensors.18
Schemes similar to the one shown in Figure 1 are common
for ET systems exhibiting magnetic field effects on the spin
dynamics in radical pairs.17,19−22 Such systems are currently
deemed central for biological magneto-sensory mecha-
nisms23−28 as well as form the basis for several spectroscopic
techniques, including Chemically Induced Dynamic Nuclear
polarization (CIDNP)29,30 and a set of methods collectively
known as Optically Detected Magnetic Resonance
(ODMR).31−33 The latter play an important role in studies
of natural photosynthetic systems34−37 and related artificial
mimics, which are important for light harvesting, molecular
electronics, and spintronics applications.22,24 Not surprisingly, a
very large number of models with energy layouts resembling
that in Figure 1 have been published to date.20,22,38 Systems
3
most related to our studies utilize emissive MLCT states of
Ru2+(bpy)3 or similar complexes usually as initial states in ET
reactions.39,40 There are also reports where phosphorescent
metalloporphyrins participate in triplet−triplet energy transfer
as acceptors.41,42 However, to the best of our knowledge,
molecules in which phosphorescent triplet states are the final
states in cascades involving ET and RP-ISC have never been
disclosed.
In this Article, we report dyads, in which the roles of the
antenna and the triplet core are played by RhB+ and a Pt(II)
tetraarylporphyrin (PtP), respectively, whereby the pathway
depicted in Figure 1 allows observation of the PtP
phosphorescence without directly exciting PtP. To investigate
our model systems, we performed optical spectroscopic studies
on the microsecond, nanosecond, and femtosecond time scales
to characterize the transient intermediates. One intriguing
feature of the reported molecules is the existence of a thermal
equilibrium between the triplet states of the dyad components
Global fitting was performed using OriginPro 8.5 (Origin-
Lab, MA) and MATLAB R2009a (MathWorks, MA).
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Decay fitting, spectral deconvolution, and maximum entropy
analysis were performed using custom written software.48
Nanosecond Transient Absorption Spectroscopy
(NSTA). The NS-TA setup consisted of a frequency-doubled
Nd:YAG laser as pump (Quanta-Ray DCR-2A, λmax = 532 nm,
pulse width 10 ns, 10 Hz rep. rate, ∼15 mW), a flash lamp
(Hamamatsu, pulse width ∼2 μs) as a white-light probe source
and a monochromator (SPEX) with a diode-array detector
(Princeton Instruments, DIDA-512). Light at 532 nm excites
both PtP and RhB+; however, the absorption at this wavelength
is strongly dominated by RhB+ (>90%). Therefore, for the
purposes of our analysis, it was reasonable to assume that the
contributions of pathway(s) resulting from direct excitation of
PtP were insignificant. The optical densities of the samples
were adjusted to be 0.3−0.5 OD at the excitation wavelength.
Femtosecond Transient Absorption Spectroscopy
(FSTA). Femtosecond time-resolved pump−probe absorption
experiments were carried out using a Helios spectrometer
(Ultrafast Systems, LCC), coupled to a femtosecond laser
system (Spectra-Physics). The system consisted of a Solstice
one-box regenerative amplifier, a Mai-Tai Ti:sapphire oscillator
as the seeding source, and an Empower diode-pumped solid-
state pulsed green laser as the pump. The pulse had fwhm ∼90 fs
at 800 nm with energy of ∼3.5 mJ/pulse at 1 kHz repetition
rate. The output of the amplifier was divided by a beam splitter.
The first beam (ca. 90% of the total power) was used as the
input for a Topas-C optical parametric amplifier, equipped with
a Berek extension (Light Conversion Ltd., Lithuania). The
second beam (10% power) was used to generate probe pulses
in the Helios spectrometer. The white light continuum pulse in
the visible (VIS) region was generated by a 3 mm sapphire
plate. The near-infrared (NIR) probe pulse was generated using
a proprietary crystalloid plate. A complementary metal−oxide−
semiconductor (CMOS) linear array sensor with 1024 pixels
was used for detection in the VIS range, while a 256 pixel
InGaAs linear diode array was used in the NIR range. To
provide isotropic excitation of the sample and to avoid pump−
probe polarization effects, the pump beam was depolarized.
Individual ΔA (OD) measurements were acquired at a
frequency of 500 Hz and averaged over 2 s periods. The
energy of the pump beam was set between 2 and 4 μJ/pulse
over a spot size of 1 mm in diameter, which corresponds to the
energy density of 1 × 1015−1.5 × 1015 photons/cm2 per pulse,
depending on the excitation wavelength (405−580 nm). The
optical densities were adjusted to be 0.8 OD at 560 nm and 0.5
OD at 510 nm (1 cm optical path length) for excitation for
RhB+ and PtP, respectively. During the measurements, the
samples were continuously stirred by a magnetic stirrer.
Synthesis. The complete synthesis is outlined in Scheme 1.
All free-base porphyrins were synthesized by the Lindsey
method.49 Platinum was inserted upon refluxing the porphyrins
with PtCl2 in anhydrous benzontitrile. Rhodamine B piperazine
amide (pRhB+) was synthesized from RhB+ following the
reported procedure.50 PtP’s and pRhB+ were coupled to each
other using the HBTU-mediated peptide coupling method. The
overall yields of compounds 1−3, starting from the assembly of
initial porphyrins, were 4.8%, 5.4%, and 0.5%, respectively.
5-(4-Methoxycarbonylphenyl)-10,15,20-triphenylpor-
phyrin (H2P−Ph1CO2Me). A mixture of pyrrole (1.6 g,
24 mmol), benzaldehyde (1.9 g, 18 mmol), and methyl 4-
formylbenozate (1.0 g, 6.1 mmol) in CH2Cl2 (600 mL) was
stirred under Ar for 30 min. BF3·Et2O (0.61 g, 4.87 mmol) was
added in one portion, and the mixture was shaded from the
ambient light and left under stirring at room temperature for an
additional 2 h. 2,3-Dichloro-5,6-dicyano-1,4-benoquinone
(DDQ) (1.38 g, 6.1 mmol) was added, and the mixture was
left overnight under stirring. The resulting solution was washed
with Na2SO3 aq (10%), NaCO3 aq (10%), HCl aq (10%),
NaOH aq (10%), and then with distilled water. The organic
phase was collected, dried over Na2SO4, and the solvent was
evaporated to dryness. The residual material was purified by
column chromatography on silica gel using CH2Cl2/hexane
mixture (2:3 by volume) as the mobile phase. The second band
was collected, the solvent was evaporated, and the remaining
solid was dried in vacuum to afford the title compound as a
purple solid (230 mg, 5.6%). The spectroscopic data matched
those previously reported.51
Pt(II) 5-(4-Methoxycarbonylphenyl)-10,15,20-triphe-
nylporphyrin (PtP−Ph1CO2Me). Free-base porphyrin H2P−
Ph1CO2Me (60 mg, 0.09 mmol) was dissolved in benzonitrile
(10 mL), PtCl2 (47 mg, 0.18 mmol) was added in one portion,
and the mixture was set to reflux under Ar. The reaction was
stopped when the absorption band near 420 nm (CH2Cl2),
characteristic of the porphyrin dication, disappeared. Complete
conversion into the Pt complex required 6−8 h. The solvent
was evaporated in vacuum, and the residue was purified by
column chromatography on silica gel, using CH2Cl2 as the
mobile phase, to afford the product as an orange powder
(76 mg, 99%). Spectroscopic data matched those previously
reported.51
Pt(II) 5-(4-Carboxylphenyl)-10,15,20-triphenylporphyr-
in (PtP−Ph1CO2H). PtP−Ph1CO2Me (75 mg, 0.086 mmol)
was dissolved in the mixture of THF (50 mL) and EtOH
(1 mL). A pellet of KOH was added to the solution, and the
mixture was stirred at room temperature for 8 h. The solvents
were removed under vacuum, and the remaining solids were
dissolved in water (ca. 20 mL). The solution was filtered,
neutralized by HCl, and the precipitate was collected by
centrifugation, washed with water, and dried in vacuum. The
title compound was obtained as an orange solid (74 mg, 99%).
1H NMR (DMSO, δ): 8.71 (br s, 8H, pyrrole-βH), 8.31 (m,
2H, ArH), 8.23 (d, J = 8 Hz, 2H, ArH), 8.14 (m, 6H, o-PhH),
7.78 (m, 9H, m,p-PhH). UV−vis (CH2Cl2) λmax: 403, 511, 542
nm. MALDI-TOF (m/z): calcd for C45H28N4O2, 851.19;
found, 851.633.
Electrochemical Methods. Cyclic voltammetry experi-
ments were performed in an anaerobic glovebox using a
potentiostat workstation (BAS 100B, Bioanalytical Systems,
Inc.) and anhydrous benzonitrile as a solvent. Tetrabutylam-
momium hexafluoro-phosphate (TBAP) was used as the
supporting electrolyte (100 mM). Pt electrode (1.6 mm
diameter) was used as the working electrode, an Ag wire as
the quasi-reference electrode, and a Pt wire as the counter
electrode. Ferrocene (Fc) was used as the internal standard; all
values are reported relative to Fc+/Fc couple (scan rate 0.1 V/s).
The three electrodes were fitted into a Claisen Adapter (Kontes
Glass Company, Vineland, NJ) with working and reference
electrodes positioned as close as possible to minimize the
ohmic resistance. At each scan rate, at least five scans were
performed to ensure stability of the system.
5-(4-Methoxycarbonylbiphenyl)-10,15,20-triphenyl-
porphyrin (H2P−Ph2CO2Me). H2P−Ph2CO2Me was synthe-
sized in a fashion similar to that of H2P−Ph1CO2Me, using
pyrrole (0.28 g, 4.2 mmol), benzaldehyde (0.33 g, 3.1 mmol),
and methyl 4-formyl-4′-methoxycarbonyl-biphenyl (0.25 g,
1.0 mmol) as starting materials. The title compound was obtained
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a
Scheme 1. Synthesis of Dyads 1−3
a
(i) NaOH/EtOAc; (ii) AlMe3/piperazine; (iii) (a) BF3·Et2O/CH2Cl2; (b) DDQ; (iv) PtCl2/PhCN; (v) KOH/THF; (vi) HBTU/DIPEA/DMF;
(vii) B2pin2/PdCl2(dppf)/DMF; (viii) 4-bromo-4′-methoxycarbonyl-biphenyl/Pd(PPh3)4/KOAc/DMF.
as a purple solid (60 mg, 7.7%). 1H NMR (CDCl3, δ): 8.92 (d,
J = 4 Hz, 2H, pyrrole-βH), 8.87 (m, 6H, pyrrole-βH), 8.61 (m,
2H, ArH), 8.33 (d, J = 8 Hz, 2H, ArH), 8.28 (d, J = 8 Hz, 2H,
ArH), 8.24 (m, 4H, ArH), 8.07−7.99 (m, 6H, o-PhH), 7.82−
7.74 (m, 9H, m,p-PhH), 4.03 (s, 3H, −COOCH3), −2.74 (s,
2H, −2NH). UV−vis (CH2Cl2) λmax: 420, 516, 551, 591, 648
nm. MALDI-TOF (m/z): calcd for C52H36N4O2, 748.28;
found, 749.469 [M + H+].
Pt(II) 5-(4-Methoxycarbonylbiphenyl)-10,15,20-
triphenylporphyrin (PtP−Ph2CO2Me). The title compound
was synthesized from H2P−Ph2CO2Me (20 mg, 0.027 mmol)
in a fashion similar to that of PtP−Ph1CO2Me (see above). The
target compound was obtained as an orange powder (20 mg,
80%). UV−vis (CH2Cl2) λmax: 404, 511, 544. MALDI-TOF
(m/z): calcd for C52H34N4O2, 941.93; found, 942.34 [M + H+].
Pt(II) 5-(4-Carboxybiphenyl)-10,15,20-triphenylpor-
phyrin (PtP−Ph2CO2H). PtP−Ph2CO2H was synthesized in
a fashion similar to that of PtP−Ph1CO2H (see above) starting
from PtP−Ph2CO2Me (20 mg, 0.021 mmol). The title
compound was obtained as an orange solid (19 mg, 96%).
1H NMR (THF, δ): 10.84, (s, 1H, COOH), 8.83 (d, J = 4 Hz,
2H, pyrrole-βH), 8.75 (d, J = 4 Hz, 2H, pyrrole-βH), 8.74 (br s,
4H, pyrrole-βH), 8.28 (d, J = 8 Hz, 2H, ArH), 8.24 (d, J =
8 Hz, 2H, ArH), 8.17 (m, 6H, o-PhH), 8.16 (d, J = 8 Hz, 2H),
8.08 (d, J = 8 Hz, 2H), 7.77 (m, 9H, m,p-PhH). UV−vis
(CH2Cl2) λmax: 403, 513, 544 nm. MALDI-TOF (m/z): calcd
for C51H32N4O2, 927.22; found, 928.45 [M + H+].
5-(4-Methoxycarbonyl-p-triphenyl)-10,15,20-triphe-
nylporphyrin (H2P−Ph3CO2Me). (1) H2P−Br (40 mg,
0.057 mmol), bis(pinacolato)diboron (37 mg, 0.144 mmol),
and KOAc (0.23 g, 0.58 mmol) were dissolved in DMF (1 mL).
PdCl2(dppf)2 (9 mg, 0.012 mmol, 20%), used as a catalyst in
this Miyaura coupling reaction, was dissolved separately in
DMF (1 mL). The solution of the catalyst was added to the
mixture of the reactants, and it was stirred vigorously at 80 °C
for 12 h. The mixture was cooled, CH2Cl2 10 mL) was added,
and the resulting solution was washed with water (3 × 10 mL)
in a separatory funnel. The organic phase was collected, dried
over Na2SO4, and the solvent was evaporated to dryness. The
remaining solid was purified by column chromatography on
silica gel (CH2Cl2/hexane = 1:1) to afford the target porphyrin-
pinacolatoboron as a purple solid after removing the solvent
and drying the product in vacuum (20 mg, 47%). MALDI-TOF
(m/z): calcd for C50H41BN4O2, 740.70; found, 741.32
[M + H+]. The spectroscopic data matched those previously
reported.53
(2) Porphyrin-pinacolatoboron, obtained at the previous step
(20 mg, 0.027 mmol), 4-bromo-4′-methoxycarbonyl-biphenyl
(12 mg, 0.041 mmol), and Cs2CO3 (22 mg, 0.067 mmol) were
dissolved in DMF (3 mL). Pd(PPh3)4 (3 mg, 0.003 mmol,
10%) used as a catalyst was dissolved separately in DMF
(1 mL). The catalyst solution was added to the mixture of the
reactants, and it was stirred vigorously at 80 °C for 12 h. The
reaction mixture was cooled, CH2Cl2 (10 mL) was added, and
the resulting solution was washed with water (3 × 10 mL). The
organic phase was collected, dried over Na2SO4, and the solvent
was evaporated to dryness. The remaining solid was purified by
column chromatography on silica gel using CH2Cl2 as the
5-(4-Bromophenyl)-10,15,20-triphenylporphyrin
(H2P−Br). H2P−Br was synthesized as previously reported52
and isolated as a purple solid (460 mg, 3.1%). MALDI-TOF
(m/z): calcd for C44H29BrN4, 694.16; found, 695.15 [M + H+].
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mobile phase. The title compound was isolated as a purple solid
(12 mg, 55%). H NMR (CDCl3, δ): 8.95 (d, J = 4 Hz, 2H,
0.002 mmol) and pRhB+ (1.2 mg, 0.003 mmol) as starting
materials. 3 was isolated as a purple solid (2 mg, 65%). H
1
1
pyrrole-βH), 8.88 (m, 6H, pyrrole-βH), 8.33 (d, J = 8 Hz, 2H),
8.24 (m, 6H, o-PhH), 8.20 (d, J = 8 Hz, 2H, ArH), 7.88 (d, J =
8 Hz, 2H, ArH), 7.82 (d, J = 8 Hz, 2H, ArH), 7.79 (m, 9H, m,p-
PhH), 7.49 (m, 4H, ArH), 4.0 (s, 3H, −COOCH3), −2.73 (s,
2H, −2NH). UV−vis (CH2Cl2) λmax: 421, 516, 550, 595, 649
nm. MALDI-TOF (m/z): calcd for C58H40N4O2, 824.96;
found, 825.33 [M + H+].
NMR (DMSO, δ): 8.81 (d, J = 4 Hz, 2H), 8.71 (m, 6H), 8.26
(d, J = 4 Hz, 2H), 8.14 (m, 6H), 8.11 (d, J = 8 Hz, 2H), 7.92
(d, J = 8 Hz, 2H), 7.86 (d, J = 8 Hz, 2H), 7.80 (m, 9H), 7.73
(br s, 3H), 7.49 (m, 4H), 7.10 (m, 2H), 6.94 (m, 2H), 6.83 (br
s, 2H), 3.63 (m, 8H), 3.56 (t, J = 4 Hz, 4H), 3.20 (m, 4H),
1.23−1.18 (m, 12H). UV−vis (PhCN) λmax (log ε): 405 (5.2
0.1), 515 (4.5
0.1), 567 (5.0
0.1) nm. MALDI-TOF
(m/z): calcd for C89H73N8O3Pt+, 1497.66; found, 1498.32.
Pt(II) 5-(4-Carboxy-p-triphenyl)-10,15,20-triphenylpor-
phyrin (PtP−Ph3CO2H). H2P−Ph3CO2H (12 mg, 0.14 mmol)
was dissolved in benzonitrile (60 mL), PtCl2 (22 mg,
0.060 mmol) was added, and the mixture was set to reflux
under Ar. The reaction was stopped when the absorption band
of the porphyrin dication disappeared. Complete conversion to the
Pt complex required ca. 12 h. The solvent was evaporated in
vacuum, and the residue was purified by column chromatography
(silica gel, CH2Cl2) to afford the Pt complex as an orange powder.
The obtained Pt complex was dissolved in THF (10 mL). MeOH
(0.5 mL) and a pellet of KOH were added, and the mixture was
left under stirring at room temperature for 12 h. The solvents were
removed under vacuum, and the water was added. The solution
was filtered and neutralized by addition of HCl. The precipitate
was collected by centrifugation, washed with water, and dried in
vacuum. The title compound was obtained as an orange solid
(2.0 mg, 14%). UV−vis (CH2Cl2) λmax: 403, 512, 542 nm.
MALDI-TOF (m/z): calcd for C57H36N4O2Pt, 1004.00; found,
1004.80 [M + H+].
RESULTS AND DISCUSSION
■
Choice of Chromophores. The chromophores suitable for
the proposed above scheme must satisfy several energetic
requirements (Figure 1). First, the initially populated excited
singlet state of the antenna (AS1) must have its energy higher
than the energy of the triplet state of the core (CT1). At the
same time, AS1 must lie below the singlet state of the core (CS1)
to prevent population of CS1 either directly or via S1→CS1
A
energy transfer(s), competing with ET. Second, the energy of
the radical pair state RP, created as a result of the ET, must be
higher than that of CT1 to allow downhill radical recombination
RT. These two requirements set the limits for the entire energy
cascade, which has to fit within the singlet−triplet gap (2J) of
the phosphorescent core.
On the basis of our previous spectroscopic and electro-
chemical investigations of Pt porphyrins,18 we conjectured that
a suitable dyad could be constructed from Rhodamine B
(RhB+) and a PtP (Pt(II) tetraarylporphyrin) with appropri-
ately tuned oxidation potential. From the synthetic point of
view, tetraarylporphyrins with various substitution patterns can
be conveniently generated using well-established synthetic
protocols.49,54 A piperazine amide derivative of RhB+, pRhB+,
directly suitable for amide coupling reactions, is accessible via a
simple and reliable method.50
In Pt porphyrins, strong spin−orbit coupling is responsible
for high rates of both S1→T1 intersystem crossing and
subsequent radiative T1→S0 decay,55 thereby causing bright
phosphorescence.56 Peripheral substituents in the meso-aryl
rings practically do not affect spectroscopic features of PtP’s,
while significantly altering their electrochemical potentials.18
This feature gives the desired flexibility of tuning the level of
the RP state without affecting the rest of the chromophoric
system.
The lowest energy Q00 state of a representative PtP, Pt
tetraphenylporphyrin (PtTPP) (λmax ≈ 541 nm, 2.29 eV), is
located above the pRhB+ transition (λmax ≈ 567 nm, 2.19 eV).
In fact, the pRhB+ absorption band extends beyond 600 nm, so
it is possible to excite pRhB+ selectively without directly
populating the Q00 state (S1PtP) using wavelengths in the range
of ∼560−600 nm. The energy of the PtP triplet state, based on
its phosphorescence maximum (λmax = 670 nm, 1.85 eV for
PtTPP), is below that of S1pRhB+ state. The entire 2J gap of
PtP is ca. 0.44 eV.
PtP−Ph1−pRhB+ (1). PtP−Ph1CO2H was dissolved in
DMF (20 mL) and stirred under Ar for 5 min. Diisopropyl-
ethylamine (DIPEA; 36 mg, 0.28 mmol) and o-benzotriazole-
N,N,N′N′-tetramethyl-uronium-hexafluoro-phosphate (HBTU;
11 mg, 0.029 mmol) were added to the mixture in one portion,
and it was stirred under Ar for another 5 min. pRhB+ (15 mg,
0.029 mmol) was added, and the mixture was left under stirring
and Ar for 24 h. The solvent was removed under vacuum, and
the residue was purified by column chromatography (silica gel,
THF). The second band was collected, the solvent was
evaporated, and the product was dried in vacuum. The title
compound was isolated as a purple solid (25 mg, 81%). 1H NMR
(DMSO, δ): 8.77 (d, J = 4 Hz, 2H), 8.71 (m, 6H), 8.22 (m, 2H),
8.14 (m, 6H), 8.07 (d, J = 8 Hz, 2H), 8.04 (br s, 1H), 8.00 (d, J =
8 Hz, 2H), 7.91 (br s, 1H), 7.89 (br s, 1H), 7.78 (m, 9H), 7.55 (d,
J = 8 Hz, 2H), 7.11 (m, 2H), 6.94 (d, J = 2 Hz, 2H), 3.64 (q, J =
8 Hz, 8H), 2.85 (br s, 4H), 2.69 (br s, 4H), 1.32−1.28 (m, 12H).
UV−vis (PhCN) λmax (log ε): 405 (5.2 0.1), 513 (4.4 0.1),
566 (4.80
0.1) nm. MALDI-TOF (m/z): calcd for
C77H65N8O3Pt+, 1344.48; found, 1344.55.
PtP−Ph2−pRhB+ (2). The title compound was synthesized
in a fashion similar to that of 1 (see above) from PtP−
Ph2CO2H (20 mg, 0.021 mmol) and pRhB+ (14 mg,
0.027 mmol) as starting materials. 2 was isolated as a purple
solid (28 mg, 92%). 1H NMR (DMSO, δ): 8.82 (d, J = 4 Hz, 2H),
8.75 (d, J = 4 Hz, 2H), 8.72 (br s, 4H), 8.30 (m, 4H), 8.23 (d, J =
8 Hz, 2H), 8.14 (m, 6H), 7.81 (m, 9H), 7.71 (d, J = 8 Hz, 2H),
7.42 (br s, 1H), 7.40 (br s, 1H), 7.20 (d, J = 4 Hz, 2H), 7.17 (d, J =
4 Hz, 2H), 7.05 (br s, 1H), 6.78 (m, 2H), 3.64 (q, J = 8 Hz, 8H),
3.46 (m, 4H), 3.20 (m, 4H), 1.32−1.28 (m, 12H). UV−vis
(PhCN) λmax (log ε): 405 (5.2 0.1), 514 (4.4 0.1), 566 (4.8
0.1) nm. MALDI-TOF (m/z): calcd for C83H69N8O3Pt+, 1421.52;
found, 1421.61.
In PtP−pRhB+ pairs, S1pRhB+ (λex = 585 nm, 2.12 eV) is
expected to play the role of the electron acceptor, although the
exact energy of the RP state depends on the substituents in the
porphyirn macrocycle. For PtTPP and pRhB+, whose ground-
state oxidation and reduction potentials in DMF are +1.16 V vs
SCE (PtTPP+/PtTPP) and −0.85 V vs SCE (pRhB+/pRhB),
respectively,18,57 the ET driving force ΔG = −0.11 eV is
expected in accordance with the Rehm−Weller equation.58
PtP−Ph3−pRhB+ (3). The title compound was synthesized
in a fashion similar to that of 1 by using PtP−Ph3CO2H (2 mg,
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Thus, the energy of the radical pair (ca. 2.0 eV) is expected be
ca. 0.15 eV higher than the energy of T1PtP.
against direct excitation of PtP in the dyads include virtually zero
molar extinction at 580 nm for all our models, very low excitation
intensities employed, and, as we show below, the presence of a
rise phase in the dynamics of the PtP triplet state with the time
constant 100−1000 times lower than that of the direct S1→T1
ISC within PtP. Therefore, a different pathway exists in the dyads,
which originates in the antenna singlet state S1pRhB+ and
populates the emissive triplet states of the core PtP’s.
On the basis of these considerations, we chose to study
bichromophoric assemblies comprising PtTPP derivative (from
here and throughout the text, we refer to Pt porphyrin
components of the studied dyads simply as PtP), and pRhB+
connected by oligo-p-phenylene linkers (Chart 1, Phn, n = 1−3:
Chart 1. Studied Dyads 1−3
Phosphorescence quantum yields of the dyads were
measured upon excitation near the Q-band maximum (λex
=
510 nm, ∼60% of absorption is due to PtP) and were found to
be lower than those of the corresponding reference porphyrins
PtPn and of PtTPP. Notably, in the series of the reference
compounds (PtPn), the phosphorescence quantum yields
decrease with an increase in the length of the oligo-phenylene
linker. Yet in addition, quenching of the PtP triplet states in the
dyads appears to be enhanced by the presence of the pRhB+
fragment. This effect could be a combined result of several
phenomena. However, one can probably rule out direct
quenching of the porphyrin singlet Soret (S2) or Q (S1) states
by either energy or electron transfer onto pRhB+, because both
S2→S1 internal conversion and S1→T1 ISC in Pt porphyrins
occur on the femotosecond time scale.61,62 Competing with such
fast processes would be improbable. More likely is that the PtP
triplet state already after being formed is partially quenched by
the TT energy transfer onto pRhB+ (ΔG ≈ −0.05 eV, see
below). The evidence for this pathway was indeed obtained by
nanosecond pump−probe and time-resolved phosphorescence
measurements (see below).
Electron Transfer. The most plausible pathway leading to
the formation of the triplet state T1PtP upon population of
S1RhB+ involves ET. Energy transfer processes are much less
likely. Indeed, spin-allowed singlet−singlet energy transfer from
S1RhB+ to populate S1PtP is uphill (ΔE00(S1pRhB+) = 2.12 eV
and ΔE00(S1PtP) = 2.3 eV), while the direct singlet−triplet
energy transfer S1RhB+→T1PtP is spin-forbidden. Another
potential pathway could involve S1RhB+→T1RhB+ intersystem
crossing within pRhB+ itself, followed by the population of the
T1PtP state via triplet−triplet (TT) energy transfer. This
pathway could be operational in view of the existence of the
reversible TT transfer between PtP and pRhB+ (see below).
However, transient absorption measurements ruled out this
possibility (see below), showing that the T1PtP state precedes
the T1RhB+ state and not vice versa.
compounds 1−3) and an amide bond. Oligo-p-phenylenes have
been extensively used as bridges in ET models,59 as they can be
conveniently constructed using Suzuki−Miyaura chemistry.60
By varying the number of the phenylene fragments in the
bridge, distance dependencies of ET and TT processes can be
elucidated.
Steady-State Spectroscopy. The steady-state absorption
and phosphorescence spectra of 1 are shown in Figure 2A (see
Figures S1,S2 for the spectra of 2 and 3).
The absorption spectra of compounds 1−3 are nearly
superpositions of the spectra of the individual chromophores,
indicating negligible electronic interactions between PtP and
pRhB+ in their ground and excited electronic states. The
emission of spectra of 1 (Figure 2B−D, λex = 425, 510,
580 nm), as well as of 2 and 3 (Figure S2), change with
excitation wavelength. Assuming that the overall emission is a
linear combination of the pRhB+ fluorescence and PtP
phosphorescence, we could decompose the spectra into the
sums of their components and estimate the corresponding
quantum yields (Table 1) (see Supporting Information, p S2
for details of decomposition and analysis). The fluorescence
quantum yield of pRhB+ alone in PhCN was found to be 0.44
(λex = 510 nm), which is slightly lower than reported for the
anionic form of RhB+ in EtOH (0.65),47 but higher than that
for pRhB+ in EtOH (0.24).50 The quantum yields of pRhB+
fluorescence in the dyads were found to be 0.19, 0.26, and 0.38
for 1−3, respectively (λex = 510 nm, ∼40% of absorption is due
to pRhB+), indicating the presence of additional quenching
pathways for the pRhB+ singlet state.
The key feature of the dyads 1−3 is that the phosphor-
escence of PtP could be observed upon excitation of pRhB+ at
the frequency below that of the lowest allowed transition of
PtP, that is, λex = 580 nm. It was manifested by the long-lived
decay of the PtP phosphorescence upon excitation at 580 nm
(Figure 2E). The identity of the emitting species was confirmed
by the time-resolved spectral data (Figure 2F). The quantum
yield of the phosphorescence obtained via this pathway was not
high (Φphos ≈ 0.001), but the emission was clearly observable,
whereas for the reference porphyrins PtP−PhnCO2H (Scheme
1; abbreviated in Table 1 and further in the text as PtPn) as well
as for PtTPP, no signal could be detected at all upon excitation
in the range of 550−600 nm. Additional arguments speaking
On the other hand, the radical pair state, [PtP•+−Phn−
pRhB•0], is located downhill from the initially populated state
S1pRhB+. The Gibbs free energy of the ET (ΔGET) was
estimated to be −0.12 eV using the Rehm−Weller formula:
ΔGET = Eox(D) − Ered(A) − ΔE00 + Δw,58 where Eox(D) and
Ered(A) are the oxidation and reduction potentials of the donor
and acceptor, respectively, ΔE00 is the excitation energy of the
initially populated state, and Δw is the electrostatic work term,
which in our case is zero, because the ET leads to simple
shifting of the charge. The oxidation and reduction potentials of
the dyads, measured in benzonitrile solutions, were found to be
close for all of the compounds 1−3, Eox = +0.53 0.01 V and
E
red = −1.46 0.03 V, and similar values were obtained for the
individual chromophores, Eox for PtP+/PtP and Ered for pRhB+/
pRhB. On the basis of these measurements, the energy of the
RP state (∼2.0 eV) was estimated to be between the energies of
S1pRhB+ (ΔE = 2.12 eV) and T1PtP (ΔE = 1.85 eV).
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Figure 2. (A) Normalized absorption spectra of PtP, pRhB+, and dyad 1 in benzonitrile. Emission spectra of 1 acquired using different excitation
wavelengths (λex): 425 nm (B), 510 nm (C), and 580 nm (D). The spectra of the components were obtained by linear decomposition.
Phosphorescence decay (E) and phosphorescence spectrum registered with delay of 460 μs (F) upon pulsed excitation at 580 nm.
a
Table 1. Selected Photophysical Properties of the PtP−Phn−pRhB+ Dyads 1−3 (n = 1−3) and of Their Components
b
c
d
ce
Φph λex (425 nm)
cf
Φph λex (580 nm)
g
,
,
λabs (nm)
λem (nm) (em. type)
Φfl λex (510 nm)
τfl (ns)
τph (μs)
h
1
408, 517, 567
408, 517, 567
407, 516, 567
567
585 (f), 670 (p), 730 (p)
585 (f), 670 (p), 730 (p)
585 (f), 670 (p), 730 (p)
585 (f)
0.17 0.04
0.24 0.03
0.38 0.03
0.45 0.03
1.0
1.7
2.2
2.8
0.020 0.02
0.031 0.05
0.040 0.06
0.005 0.001
0.010 0.002
0.010 0.001
3, 200
h
2
10, 170
3
30
pRhB+
PtTPP
PtP1
PtP2
PtP3
i
407, 514, 542
406, 512, 543
407, 512, 544
406, 512, 541
670 (p), 735 (p)
0.13 0.01
53
43
35
34
1
1
1
1
i
670 (p), 735 (p)
0.08 0.01
i
670 (p), 735 (p)
0.07 0.01
i
670 (p), 735 (p)
0.05 0.01
a
All measurements were performed in deoxygenated anhydrous benzonitrile. Porphyrins PtP−PhnCO2H (Scheme 1) are abbreviated in the table as
b
c
PtPn, for n = 1−3. f, fluorescence; p, phosphorescence. Emission quantum yields were determined relative to the fluorescence of Rhodamine 6G in
EtOH (Φfl = 0.95).47 λex = 405 nm. Corresponds to the phosphorescence emitted as a result of direct excitation of PtP (Soret band), followed by
d
e
f
g
h
intrachromophoric ISC. Phosphorescence emitted as a result of the pathway involving ET and RP-ISC. λex = 510 nm. Distributions of lifetimes
i
(see text for details). λex = 510 nm.
Assuming that the ET in the dyads is the only deactivation
pathway for S1pRhB+ in addition to its natural radiative and
nonradiative decays, the ET rate constants (kET) could be
calculated directly from the fluorescence lifetime data (τfl, Table 1):
kET = kfl − kfl0, where kfl and kfl0 are the rate constants
(1/τfl) of the fluorescence of pRhB+ in dyads and of free pRhB+,
respectively. (Here, we assume that the native radiative (kr) and
0
nonradiative (knr) decay rate constants (kfl = kr + knr) do not
change upon inclusion of pRhB+ into dyads.) The kET values
were found to be 6.4 × 108 s−1 for 1, 2.3 × 108 s−1 for 2, and
9.7 × 107 s−1 for 3. The rate constant of the fluorescence decay
of free pRhB+ is 3.4 × 108 s−1. The rate of the ET is higher than
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Figure 3. (A) FSTA spectra of 1 in the visible range at selected times after excitation by a femtosecond laser pulse: fwhm 90 fs, λmax = 580 nm.
(B) FSTA spectra of 1 in the near-infrared (NIR) range. (C) Decay-associated spectra obtained by global fitting of the transients shown in (A).
(D) Evolution of the spectra shown in (C) at selected wavelengths (480 and 490 nm) and the corresponding time constants obtained by single-
exponential fitting. GSB, ground-state bleaching. SE, stimulated emission.
the rate of the pRhB+ fluorescence in dyad 1 and lower than
that in dyads 2 and 3.
seemingly paradoxical result, however, can be expected for a two-
component system A/B, where both A and B decay with their
own rates constants kA and kB, and B forms directly from A: A→
B. In such a system, if kB > kA, then observing the evolution of B
one would see it rise with the rate kB, which is actually the rate of
its decay, and decay at the rate kA, which is the decay rate of its
precursor (see Supporting Information for kinetic equations).
Applied to our case, the rate of the formation of RP from S1RhB+
is apparently lower than the rate of the subsequent decay, so that
the time constant of the rise (420−430 ps) actually corresponds
to the decay of the RP. The fact that we did not observe the
decay of the absorption at 490 nm with the time constant
corresponding to the decay of S1RhB+ (Figure 3D) suggests that
the long-lived species formed from the RP has its own
absorption at the same wavelength and approximately the
same molar extinction coefficient. This species is the PtP local
triplet state (see below).
Spectroelectrochemical data support our assignment of the
RP spectra. Although the absorption spectrum of pRhB0• is not
known, the spectrum of the neutral radical of RhB (RhB0•) has
been reported to have an absorption band near 430 nm.63 The
neutral radical of Rhodamine B ethyl ester, known as
Rhodamine 3B, also has been reported,64 and it also absorbs
around 430−450 nm. PtTPP cation radical (PtTPP+•) exhibits
a broad band around 600 nm57 and absorption around 400 nm.
In addition to the absorption in the visible range, we also
observed a transient peak near 810 nm, partially overlapping
with T1→Tn absorption of PtP (broad feature near 860 nm)
(see Figures S5 and S6 for details). The peak at 810 nm rises
with the time constant of 430 ps, close to that of the peak near
490 nm, and tentatively it has been assigned to the RP as well.
Additional evidence for the formation of the RP is provided by
the negative peak near 510 nm in the decay-associated spectra
(Figure 3C), which is likely to be due to the ground-state
bleaching of the porphyrin Q-band.
Evolution of the Radical Pair State. Transient electronic
absorption spectroscopy allows detection of nonemissive short-
lived intermediates in photoinduced ET reactions. We
performed femtosecond transient absorption (FSTA) measure-
ments on dyads 1 and 2 to capture the spectroscopic signatures
and record time evolution of the RP state. In the case of
compound 3, due to the low rate of the ET and consequently a
small amount of the RP species formed, observation of the
transient spectra was hampered by noise. However, other
spectroscopic data (see below) strongly suggest that the dyad 3
undergoes the same pathway as 1 and 2.
The FSTA spectra of compound 1 upon pulsed excitation at
λmax = 580 nm (fwhm 90 fs) are shown in Figure 3A,B (see
Figure S3 for the corresponding spectra of 2). Global fitting of
the transients produced two overlapping but distinct spectral
features in the region of 400−520 nm (Figure 3C; the related
decays are shown in Figure 3D). On the basis of the
comparison with other transients in the spectra and with the
literature data, the broad feature decaying with time constant of
540 ps was assigned to the S1→Sn absorption of pRhB+. This
kinetic is similar to the one of the pRhB+ stimulated emission
(e.g., at 600 nm), 490 ps, although accuracy in determination of
the latter time constant suffered from the proximity of the laser
spectrum. In the near-infrared (NIR) region, excited-state
absorption of pRhB+ is manifested by a broad peak with the
maximum near 960 nm, decaying at the matching rate. Notably,
the decay times for S1pRhB+ in the dyads and pRhB+ alone
obtained by FSTA were shorter than determined by the
TCSPC fluorescence lifetime measurements (Table S1). We
think that the discrepancy is due to the difference in sampling
by the two methods, combined with the not perfectly single-
exponential nature of the pRhB+ decay. Decay binning in our
TCSPC system starts at 600 ps and continues up to 50 ns,
therefore weighting toward the long-lived components of the
decay. In the FSTA system, the first spectrum is acquired at
∼1 ps, and the entire window is 8 ns, enhancing the contribution
of the short-lived components.
Both nanosecond transient absorption (NSTA) and emission
measurements (see below) confirm that upon population of
S1RhB+, the dyads eventually evolve with formation of the local
triplet state on PtP. It is, therefore, necessary to consider all
plausible evolution pathways of the RP, leading to that state
(see Figure 6 for pathway identification). These pathways
include: (a) RP-ISC (S[PtP+•−Phn−pRhB0•]→T[PtP+•−Phn−
pRhB0•]), followed by the radical recombination to yield
T1PtP−Phn−pRhB+; (b) RP-ISC, followed by the recombina-
tion with formation of the triplet state of pRhB+, PtP−
Phn−T1pRhB+, and subsequent TT energy transfer PtP−
Phn−T1pRhB+→T1PtP−Phn−pRhB+; and (c) the direct transition
Interpretation of transient spectra of dyads 1−3 was
complicated by strongly overlapping absorption bands of
multiple participating species, that is, ground and excited singlet,
triplet, and radical states of the chromophores. Nevertheless, the
peaks at 490 and 810 nm could be confidently assigned to the
RP state based on the global analysis and spectroelectrochemical
data (see below). Remarkably, both peaks rise with the time
constant of 420−430 ps (∼2.35 × 109 s−1), which is faster than
the decay of the parent state S1pRhB+ (kfl(1) = 109 s−1). This
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Figure 4. (A) Nanosecond transient absorption spectra of dyad 1 (λex = 532 nm, pulse fwhm 10 ns). (B) Changes in the absorbance at 567 nm,
ground-state bleaching (GSB) of pRhB+, and 455 nm, T1→Tn absorption of PtP.
S[PtP+•−Phn−pRhB0•]→T1PtP−Phn−pRhB+. Both pathways
(a) and (b) proceed via RP-ISC, and, according to our data,
pathway (a) appears to be dominant (see below). The rate of
the spin-forbidden pathway (c), known as spin−orbit-induced
intersystem crossing (SO-ISC),35 is highly dependent on the
mutual orientation of the participating molecular orbitals of the
ET partners.65,66 Molecular dynamics simulations (see Figure
S7) revealed that our dyads exhibit substantial degree of
conformational freedom, which is due to rotations around the
amide bonds. Conformational flexibility results in a broad
distribution of angles between the orbitals of PtP and pRhB+, so
that the fraction of time spent by the chromophores in the
conformation required for the SO-ISC to occur is probably very
small. Therefore, pathway (c) is unlikely to contribute
significantly to the evolution of the RP. The other two
pathways, (a) and (b), are the key to the formation the local
triplet states T1PtP and T1RhB.
Local Triplet States and Triplet−Triplet (TT) Energy
Transfer. To investigate the dynamics of the local triplet states
in the dyads, we performed NSTA and time-resolved
phosphorescence measurements. Upon excitation into the red
edge of the pRhB+ band (580 nm), 1 and 2 exhibit multi-
exponential phosphorescence decays in deoxygenated benzoni-
trile solutions. Almost identical decays were observed when
dyads 1 and 2 were excited directly at the PtP Q-band maxima
(510 nm). The decays were analyzed by the maximum entropy
method (MEM),48,67 which, in the case of 1, as an example,
revealed a distribution of lifetimes with peaks near τmax ≈ 5 μs
and τmax ≈ 250 μs. The peak near 5 μs was broad and extended
toward longer lifetimes, exhibiting decays with lifetimes
reaching as high as 70−80 μs. Notably, the precursor
porphyrins (PtPn) and PtTPP under the same conditions
reveal narrow unimodal lifetime distributions, corresponding to
regular single-exponential kinetics. The decay of 3 was also
analyzed by conventional single-exponential analysis (Figure
S8), as this model was sufficient for its phosphorescence
kinetics (see below).
λmax = 567 nm). Importantly, the latter shows a fast decay (τd ≈
5 μs), corresponding to the depletion of the pRhB+ ground
state, followed by a slow rise (τr = 200−250 μs), that is,
ground-state recovery (Figure 4B). This long rise was attributed
to the T1→S0 ISC of pRhB+. Indeed, long-lived triplet decay of
the parent Rhodamine molecule has been observed previously,
also by way of NSTA, in a pyrene−Rh dyad.68
In synchrony with fast decaying initial phase of pRhB+ GSB
(Figure 4B, 567 nm trace), PtP triplet−triplet absorption (455 nm
trace) is also displaying a fast decaying component. The
lifetime distribution of the phosphorescence lifetimes con-
sistently contains a significant fraction of decay components
near 3−4 μs. These data led us to postulate the existence of the
TT energy transfer between T1PtP and T1pRhB+ states. The
directionality of the TT, that is, from T1PtP to T1pRhB+,
suggests that after the initial electron transfer the RP state
undergoes RP-ISC and recombines to yield the local triplet
state T1PtP−Phn−pRhB+, rather than PtP−Phn−T1pRhB+.
Notably, these data also confirm our central hypothesis, that
is, that the population of the T1PtP occurs via the RP route, as
opposed to direct S1pRhB→T1pRhB ISC within the pRhB
moiety, followed by TT transfer from pRhB+ to PtP. If the
latter was true, the initial phase in the T1PtP dynamics would be
a rise and not decay.
Remarkably, the GSB recovery phase of pRhB+ in 1 and 2 is
paralleled by the long-lived components in the PtP
phosphorescence (Table 1). These lifetimes are much longer
than the native decay of the PtPn triplet state (τ ≈ 30−40 μs),
suggesting that it is the slow T1pRhB+ decay that is reflected in
the emission of PtP by way of “delayed phosphorescence”.
Thus, after being populated, the T1pRhB+ state either decays to
the ground state or undergoes back TT transfer with
regeneration of emissive state T1PtP. Apparently, equilibrium
is established between these two states, forming a pool of triplet
excitons. The existence of the equilibrium is supported by the
fact that the energy of T1RhB+ (1.80−1.84 eV)69−71 is almost
equal to that of T1PtP (1.85 eV). Furthermore, phospho-
rescence measurements at 77 K (Figure S11) reveal that the
decay of the T1PtP state in, for example, dyad 1 becomes nearly
perfectly single-exponential, having the lifetime (∼90 μs)
slightly lower than that of PtP alone (∼120 μs). Hence, at
low temperature, forward TT transfer becomes slow and only
slightly shortens the native PtP decay, while the backward TT
transfer virtually stops. In contrast, at high temperature the
long-lived nonemissive pRhB+ triplet state is able to act as an
“energy reservoir”,72 lengthening the overall lifetime of the
Representative NSTA spectra (λex = 532 nm) for compound
1 are shown in Figure 4 with the earliest time point of 50 ns
(see Figure S9 for the corresponding spectra of 2 and 3).
Unfortunately, the time period between the last point of the
FSTA measurements (7 ns) and the beginning of acquisition of
the NSTA data (50 ns) could not be accessed spectroscopically.
The two main features in the NSTA spectra are the T1→Tn
absorption of PtP (positive peak near 455 nm)55 and the
ground-state bleaching (GSB) of pRhB+ (negative peak with
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phosphorescence, although some energy is inevitably lost via
the nonradiative T1pRhB+ decay. Similar cases of triplet lifetime
lengthening via reversible energy transfer have been noted
previously.46,73 Our results highlight the possibility of using
multiple isoenergetic triplet states to elongate the lifetime of a
triplet exciton, tuning it in accordance with demands of
applications.
the studied dyads is shown in Figure 7. The corresponding rate
constants are summarized in Table 2.
One interesting feature of the phosphorescence in dyad 1 is
the presence of an extra peak with λmax = 720 nm (associated
decay time constant 72 μs) that could be clearly observed in the
time-resolved phosphorescence spectra of 1 (Figure 5).
Figure 6. Pathways occurring in PtP−RhB dyads. The arrows
indicate processes whose rate constants were deduced from the
spectroscopic measurements. Dashed lines indicate routes for which
the rate constants were not experimentally obtained. kRS and kRT
refer to the recombination of the singlet and triplet radical pairs,
respectively. kTT is the rate constant for the forward TT energy
transfer.
Figure 5. Emission spectra of 1 at 60 μs after excitation pulse (λmax
=
580 nm). Inset: Decay of the emission line at 720 nm.
Because of its relatively small overall intensity, this peak
could not be seen in the steady-state emission spectra, and its
unique time signature blended into the lifetime distribution
underlying the phosphorescence decay. Compounds 2 and 3
did not show this spectral feature at all. The emission of 1 at
720 nm was quenched by molecular oxygen, as was the PtP
phosphorescence, and remained present in solvents of different
polarity, such as THF (ε = 7.5), benzonitrile (ε = 25.0), and
DMA (ε = 37.8). On the bsis of the combination of these
characteristics, this spectral band was attributed to the
intramolecular triplet exciplex between PtP and pRhB+.
Our assignment was based on exclusion of other potential
emitters, such as triplet state of pRhB,+ monomer or ground-
state dimer, or pRhB+−pRhB+ triplet excimer. The monomer of
RhB+ emits phosphorescence at 690 nm, and its phosphor-
escence is detectable only at low temperature (e.g., 77 K in
EtOH/H2O or EtOH/MeOH).71,74 The RhB+ dimer does have
an emission peak at 720 nm, but its observation also requires low
temperature. Furthermore, the dimer has significantly different
absorption spectra,71 while in our system the absorption of
pRhB+ clearly matched that of the RhB+ monomer. On the other
hand, intermolecular pRhB+−pRhB+ triplet excimer, which
would have the same emission features as the triplet formed
upon excitation of the ground-state dimer, could be excluded
because sample dilutions had no effect on the emission at 720
nm. The remaining candidate was intramolecular exciplex PtP−
pRhB+, which presumably could form directly from the triplet
RP. In dyads 2 and 3, similar exciplexes could not form due to
considerably longer interchromophoric distances. On the basis of
the steady-state and emission spectra, the contribution of the
exciplex to the overall energy pathway was minimal.
Figure 7. (A) Distance dependencies of the rate constants for the
primary ET, or HT ( , kET, PtP→pRhB+), and TT energy transfer
■
( , kTT, PtP→pRhB+). (B) Energy diagrams for HT (hole transfer)
●
and TT energy transfer in the dyads, showing relative heights of the
tunneling barriers.
Table 2. Rate Constants Associated with the Pathways
a
Displayed in Figure 6
b
c
kfl × 108
kET
×
kRT
×
kTT
×
kph
×
kisc
×
PtP
pRhB+
s−1
108 s−1
109 s−1
105 s−1
105 s−1
103 s−1
1
2
3
10.0
5.9
6.7
2.4
1.2
3.4
3.3, 0.05
1.0, 0.06
0.33
5.0
6.7
2.3
1.1
4.5
0.97
0.33
∼5.0
a
All rate constants correspond to the measurements performed in
b
deoxygenated benzonitrile solutions at room temperature. Obtained
from NSTA data. Peak maxima in the lifetime distributions.
c
The dominant pathway occurring in the dyads upon selective
excitation of pRhB+ is the following: PtP−Phn−S1pRhB+ →
Energy and Electron Transfer Dynamics. The complete
diagram describing the energy and electron transfer pathways in
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the adjacent aromatic rings.78,79 The distance attenuation
parameter βTT for the dyads was found to be 0.30 Å−1, which is
well within the ranges of values typically observed for TT
energy transfer across conjugated bridges.77,80−82 Thus, the
experimental βET and βTT in our systems are rather close to
each other, βTT/βET = 1.3, that is, significantly less than 2.
Both ET and TT energy transfer in the dyads are likely to be
mediated by through-bond couplings and occur via the super-
exchange mechanism, as opposed to incoherent hopping.83
Indeed, for either electron, hole, or triplet migration, the gaps
between the initial states (PtP−Phn−S1pRhB+ for ET and HT;
T1PtP−Phn−pRhB+ for TT) and the hypothetic intermediate
states involving the bridge (PtP+•−Phn−•−pRhB+ for ET, PtP−
Phn+•−pRhB0• for HT, and PtP−T1Phn−pRhB) are positive and
quite high (Figure 7B), which is a condition required by the
superexchange mechanism. It is the height of the tunneling
barrier that determines the exponential distance attenuation
factors in both ET and TT energy transfer process.84,85 Estima-
tions of the levels in Figure 7B were based on our own electro-
chemical measurements and relevant literature data for oligo-
phenylenes.85,86 It was more appropriate in our case to consider
HT (i.e., positive charge transfer from pRhB+ to PtP) rather
than ET, because the gap between the initial state and the
hypothetically oxidized bridge (Phn+●) was lower than for the
corresponding reduced bridge (Phn−●) (see Supporting Informa-
tion, p S12 for details).
S[PtP+•−Phn−pRhB0•] (SRP) → T[PtP+•−Phn−pRhB0•] (TRP) →
T1PtP−Phn−pRhB+ → PtP−Phn−pRhB+ (ground state). TT
energy transfer from T1PtP to T1RhB plays an important role
in elongating the lifetime of the resulting phosphorescence
emission, although the intersystem crossing to the ground
state of pRhB+ (kiscpRhB+) provides an extra escape passage for
the excitation energy. The main competing pathway to the
phosphorescence in the dyads is the singlet radical recombi-
nation. Its rate (kRS) was not assessed directly, but it is
presumed to be comparable to the rate of the triplet recom-
bination times the rate of RP-ISC (kRP‑ISC × kRT), because the
phosphorescence quantum yields in the dyads are lower than
that of the parent porphyrins. Further work on optimization of
such emissive triplet systems will need to focus on
minimization of this alternative pathway.
Distance Attenuation Factors. The dyads prepared in this
study are unique in that they feature both ET and TT energy
transfer processes. This property is interesting in view of the
relationship between the corresponding distance attenuation
factors, βET and βTT, which has been debated in the literature.
Closs et al. have originally suggested that the attenuation factor
for TT energy transfer (βTT), either via direct “through-space”
exchange or through saturated bridges, should be 2 times
higher than that for ET. Their TT versus ET studies were based
on systems with similar, but not identical, donor (D) and
acceptor (A) chromophores.75,76 The theoretical interpretation
relied on a simple fact that the Dexter-type TT exchange can be
viewed as a concerted transition of one electron and one hole,
moving in the same direction. The electron travels from the
donor LUMO to the acceptor LUMO, while the hole travels
from donor HOMO to the acceptor HOMO. The two
transitions are represented by their own electronic coupling
matrix elements VET and VHT, for ET and hole transfer (HT),
respectively, which are considered to exhibit very close distance
attenuation factors (βET ≈ βHT). Therefore, the coupling matrix
element for the TT transfer (VTT) is approximated by the
product of the two: VTT ≈ |VET||VHT| = C × exp(−2βETRDA),
where C is a system-specific constant, and RDA is the donor−
acceptor distance. It follows that: βTT/βET = 2.
Later Welter et al. analyzed the data for various D−B−A
systems with p-phenylene bridges and pointed out that βTT
values for bridge-mediated TT energy transfer reactions are not
so different from the corresponding βET values.77 Still, the data
analyzed were obtained using systems featuring not identical
chromophore pairs. In this regard, our dyads exhibit ET and
TT processes separated in time, but occurring in exactly the
same molecules, thus providing opportunity for the direct
comparison between βET and βTT.
The distance dependencies of the ET and TT transfer rate
constants for dyads 1−3 are shown in Figure 7A. Their absolute
values are different by ∼3 orders of magnitude, presumably due
to the difference between the corresponding driving forces, that
is, ΔGET = −0.12 eV and ΔGTT < −0.05 eV.
In nonadiabatic high temperature regime, the ET rate
constant has exponential dependence on the donor−acceptor
distance, that is, kET ≈ exp(−βETRDA).75,76 Considering that the
average distances RDA in our dyads (i.e., interchromophoric
edge-to-edge distances) are 13.2 and 17.2 and 21.2 Å for 1−3,
respectively, the attenuation factor βET for the initial forward
electron transfer (kET in Figure 6) was determined to be
0.23 Å−1. Notably, βET values for superexchange-dominated ET
reactions across oligo-p-phenylene bridges usually fall into the
range of 0.4−0.8 Å−1, depending on the dihedral angle between
Growing evidence suggests that for through-bond super-
exchange processes, distance attenuation factors are not
characteristics exclusively of the bridge, but rather depend on
the combination of factors, including donor and acceptor and
tunneling gap.59,86,87 All three are almost always different for
ET (or HT) and TT energy transfer processes. For example, in
our systems (Figure 7B), the actual donor and acceptor states
are PtP−Phn−S1pRhB+ for HT and T1PtP−Phn−pRhB+ for TT,
even though the individual chromophores and bridges are the
same. Similarly, the bridge states are Phn for HT and T1Phn
+•
for TT. Thus, valid comparison between distance attenuation
factors should probably involve systems with energetically
comparable initial, final, and bridge states, rather than systems
based on identical chromophores. In this regard, the ratio βTT/
βET = 1.3 obtained for our dyads proves that ambiguity
stemming from the use of different chromophores does not
fully account for deviations from the theoretically predicted
βTT/βET = 2 ratio.
CONCLUSIONS
■
We have demonstrated that in bichromophoric systems,
consisting of phosphorescent triplet cores and auxiliary antenna
dyes, emissive triplet states can be generated via electron
transfer/radical recombination pathway, rather than directly as
a result of the core-centered processes. The described PtP−
pRhB+ systems can serve as a prototype for future development
of molecules, in which emission from triplet states is potentially
susceptible to the control over the dynamics of intermediate
radical pair states. Further optimization will need to focus on
maximizing the efficiency of the ET/RP-ICS pathway relative to
the direct charge recombination. As an extra benefit, the studied
PtP−Phn−pRhB+ dyads featured both ET and TT energy
transfer processes in the same molecular systems, which
allowed direct comparison between the corresponding distance
attenuation factors βET and βTT. Their ratio (βTT/βET = 1.3)
was found to be much lower than theoretically predicted.
Because of the flexibility in tuning the porphyrin redox potentials
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ASSOCIATED CONTENT
■
S
* Supporting Information
Quantum yield calculations, absorption and emission spectra,
phosphorescence decay curves, measurements at 77 K, NSTA
data for compounds 2 and 3, FSTA data for compound 2,
simulations, and rate constants table. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: vinograd@mail.med.upenn.edu.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
Support by grants from the National Institutes of Health
(EB007279) and from the Research Foundation for Opto-
Science and Technology (Hamamatsu, Japan) is gratefully
acknowledged. Nanosecond transient absorption and fluores-
cence lifetime measurements were performed in the Regional
Laser and Biomedical Technology Laboratories (RLBL) (NIH/
NCRR grant P41RR001348) with the assistance of Dr. Thomas
Troxler. Femtosecond transient absorption measurements were
performed in the Ultrafast Laser Facility of the Photosynthetic
Antenna Research Center (PARC), 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-SC0001035. We are grateful to Professors Robin
M. Hochstrasser (UPenn) and Dewey Holten (Washington
University) for helpful discussions.
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