Diiodo bodipy-rhodamine dyads: Preparation and study of the acid-activatable competing intersystem crossing and energy transfer processes

Article abstract of DOI:10.1021/jp509858t

Iodo-bodipy/rhodamine dyads with cyanuric chloride linker were prepared with the goal of achieving pH switching of the triplet excited state formation. The pH switching takes advantage of the acid-activated reversible cyclic lactam→opened amide transforma

Full text of DOI:10.1021/jp509858t

Article
pubs.acs.org/JPCB
DiiodoBodipy-Rhodamine Dyads: Preparation and Study of the Acid-
Activatable Competing Intersystem Crossing and Energy Transfer
Processes
Kejing Xu, Yun Xie,‡,§ Xiaoneng Cui, Jianzhang Zhao,* and Ksenija D. Glusac*
†
,§
†
,†
,‡
†State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, E-208 West Campus, 2
Ling Gong Rd., Dalian 116024, P. R. China
‡
Department of Chemistry, Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403, United
States
*
S Supporting Information
ABSTRACT: Iodo-bodipy/rhodamine dyads with cyanuric chloride linker were prepared
with the goal of achieving pH switching of the triplet excited state formation. The pH
switching takes advantage of the acid-activated reversible cyclic lactam↔opened amide
transformation of the rhodamine unit and the fluorescence resonance energy transfer
(
FRET). The photophysical properties of the dyads were studied with steady-state and
femtosecond/nanosecond time-resolved transient absorption spectroscopies, electro-
chemical methods, as well as TD-DFT calculations. Our results show that the model
dyad is an efficient triplet state generator under neutral condition, when the rhodamine
unit adopts the closed form. The triplet generation occurs at the iodo-bodipy moiety and
the triplet state is long-lived, with a lifetime of 51.7 μs. In the presence of the acid, the
rhodamine unit adopts an opened amide form, and in this case, the efficient FRET occurs
from iodo-bodipy to the rhodamine moiety. The FRET is much faster (τFRET = 81 ps) than
the intersystem crossing of iodo-bodipy (τISC = 178 ps), thus suppressing the triplet
generation is assumed. However, we found that the additional energy transfer occurs at the longer timescale, which eventually
converts the rhodamine-based S state to the T state localized on the iodo-bodipy unit.
1
1
INTRODUCTION
contrary, the cyclic lactam form gives absorption only in the UV
region, and it is colorless and nonfluorescent. Thus, perfect
switching of the visible light absorption (switching between
colorless and purple) and fluorescence can be achieved upon
binding with proper analytes or variation of pH of the
■
Photochemical reagents provide excellent spatial control of
chemical reactivity, which has been employed in many areas of
1
−5
material science.
For example, photodynamic therapy
(
PDT) utilizes drugs that are active only in the presence of
light to selectively target carcinogenic tissue.
1
2,13
6
−8
solution.
However, the applications of this transformation
However, due
of the molecular structure and the accompanying spectral
variation are mainly in the singlet excited state related
to unwanted photosensitivity of patients undergoing PDT
6
therapy, an additional degree of chemical control is needed.
12
manifold. To the best of our knowledge, this switching effect
has not been used for switching of triplet excited states.
Our approach toward pH-switchable triplet sensitizers is to
utilize the FRET mechanism, as shown in Scheme 1c. Model
compound B-3 consists of the iodo-bodipy (I-Bodipy) dye
Since cancerous cells exhibit lower pH than the healthy
6
,9,10
cells,
the pH-sensitive reagents for PDT are an elegant way
to increase the selectivity of PDT. Previous studies have shown
that the formation of triplet excited state in model compounds
can be controlled by a pH-switchable photoinduced electron
transfer (PET) mechanism, or a pH-switchable fluorescence
resonance energy transfer (FRET) method. , Despite this
14−17
9
(triplet generator),
and the rhodamine dye (pH-switchable
1
unit), covalently linked via the cyanuric chloride moiety. At
high pH, the rhodamine dye is present in the cyclic lactam
progress, the diversity of molecular motifs of sensitizers with
pH-switchable triplet state production is very limited.
form, whose S state is too high in energy and does not
1
interfere with the generation of the triplet state at the iodo-
bodipy moiety. At low pH, the rhodamine dye adopts an open
In this work, we explore pH-switchable triplet photo-
sensitizers that utilize rhodamine as a pH-sensitive motif. In
acidic solution (or protic solvents), rhodamine exists in its
opened amide form, but in basic solution or nonprotonic
solution, rhodamine exists in the cyclic lactam structure
amide structure, whose lower energy of S state enables the
1
FRET from I-Bodipy to the rhodamine. We postulated that this
1
1,12
(
Scheme 1a).
The two structures show drastically different
Received: September 29, 2014
Revised: February 18, 2015
Published: February 20, 2015
photophysical properties. The opened amide form gives strong
absorption at ca. 560 nm, and it is highly fluorescent. On the
©
2015 American Chemical Society
4175
DOI: 10.1021/jp509858t
J. Phys. Chem. B 2015, 119, 4175−4187

The Journal of Physical Chemistry B
Article
Scheme 1. Design of the Proposed Dyads for pH-Switchable Triplet State Generation
FRET mechanism would successfully compete with the S →
The mixture was stirred in an ice bath at 0 °C under N2
atmosphere for 5 min before addition of cyanuric chloride (31.2
mg, 0.169 mmol). The temperature of the mixture was allowed
to slowly rise to room temperature (r.t.) while being stirred.
Completion of the reaction was confirmed by TLC (silica gel,
dichloromethane). The reaction mixture was filtered, and the
solvent was evaporated under reduced pressure. The solid
residue was subjected to column chromatography (silica gel,
dichloromethane/petroleum ether 5:1, v/v), and the principal
bright orange band was collected. The desired product was
1
T intersystem crossing (ISC) at the I-Bodipy moiety, thus
1
shutting off the triplet generation. In addition to compound B-
3
, models B-1 and B-2 were prepared to investigate the
efficiency of FRET through the cyanuric chloride linker
(
Scheme 1b).
This manuscript reports a detailed photophysical study of
model compounds B-1, B-2, and B-3 using time-resolved
transient absorption spectroscopy and time-dependent density
functional theory (TD-DFT) calculations. Our results confirm
that the FRET from I-Bodipy to rhodamine indeed takes place
at low pH values. However, the surprising finding of this study
is that, at longer times, the additional singlet energy transfer
process takes place that eventually leads to the generation of
isolated as an orange powder (74.9 mg). Yield: 91%. Mp > 250
1
°
C. H NMR (CDCl , 400 MHz): δ 7.77 (d, 2H, J = 12.0 Hz),
3
7
6
.64 (s, 1H), 7.35 (d, 2H, J = 8.0 Hz), 6.00 (s, 2H), 2.56 (s,
H), 1.43 (s, 6H). 13C NMR (100 MHz, CDCl ): δ = 164.00,
3
the T state at the I-Bodipy moiety. These results are useful for
1
155.81, 142.91, 140.47, 136.86, 132.23, 131.47, 129.24, 121.39,
+
the future study on the principal photophysics of the external
stimuli-activatable triplet photosensitizers.
1
21.07, 14.65. TOF HRMS ESI: Calcd ([C H BN F Cl ] ),
22 19 6 2 2
m/z = 486.1109; found, m/z = 486.1071.
Synthesis of Compound B-1. At r.t., compound 1 (200 mg,
EXPERIMENTAL SECTION
0
.41 mmol) was dissolved in anhydrous CH Cl (80 mL) then
■
2
2
Materials and Equipment. All the chemicals are analyti-
N-iodosuccinimide (NIS, 366 mg, 1.64 mmol) was added in
one portion. The mixture was stirred for 5 h. Then the mixture
cally pure and were used as received. Solvents were dried and
1
13
distilled prior to use. H and C NMR spectra were recorded
on a Bruker 400/500 MHz spectrophotometer (CDCl as
solvent, TMS as standard for which δ = 0.00 ppm). High-
resolution mass spectra (HRMS) were determined with ESI-Q-
TOF MS spectrometer.
was washed with aqueous Na
aqueous solution). The organic layer was dried over anhydrous
Na SO . The solvent was evaporated under reduced pressure.
The crude product was purified by column chromatography
(silica gel, CH Cl ). The first band was collected to give the
2 2 6
S O solution (3 × 50 mL, 1 M
3
2
4
2
2
Synthesis and Characterization. Synthesis of Com-
pound 1. 4,4-Difluoro-8-(4-aminophenyl)-3,5-dimethyl-4-
bora-3a,4a-diaza-s-indacene (abbreviated as 8-(4′-aminophenyl)
Bodipy) (100 mg, 0.169 mmol) was added to a suspension of
anhydrous K CO (0.12 g, 0.87 mmol) in dry THF (30 mL).
product as a red solid (263.6 mg). Yield: 87.0%. Mp: 193−196
1
°C. H NMR (CDCl , 400 MHz): δ 7.81 (m, 2H), 7.74 (s,
3
13
1H), 7.32 (m, 2H), 2.65 (s, 6H), 1.45 (s, 6H). C NMR (100
MHz, CDCl ): δ = 164.02, 157.12, 145.11, 140.11, 137.36,
3
131.60, 130.91, 129.07, 128.84,121.31, 85.88, 17.28, 16.08.
2
3
4
176
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The Journal of Physical Chemistry B
TOF HRMS ESI: Calcd ([C H BN F Cl I ] ), m/z =
Article
+
light continuum generation between 420 and 800 nm. The flow
cell (Starna Cell Inc. 45-Q-2, 0.9 mL volume with 2 mm path
length), pumped by a Variable Flow Mini-Pump (Control
Company), was used to prevent photodegradation of the
sample. After passing through the cell at the magic-angle
geometry, the continuum was coupled into an optical fiber and
input into a CCD spectrograph (Ocean Optics, S2000). The
data acquisition was achieved using in-house LabVIEW
(National Instruments) software routines. The group velocity
dispersion of the probing pulse was determined using
nonresonant optical Kerr effect (OKE) measurements. Sample
solutions were prepared at a concentration needed to have
absorbance of 1.0 at the excitation wavelength.
2
2
17
6
2
2 2
7
37.9042; found, m/z = 737.9047.
Synthesis of Compound B-2. Under N atmosphere, B-1
2
(
100 mg, 0.135 mmol) and meso(4-aminophenyl)bodipy (45.9
mg, 0.135 mmol) were added to a suspension of anhydrous
K CO (93.5 mg, 0.675 mmol) in dry THF (15 mL) in a
2
3
Schlenck tube. The tube was sealed, and the mixture was stirred
at 60 °C for 24 h. At the end of the reaction, the mixture was
filtered to remove the K CO and the solvent was removed on
2
3
a rotary evaporator. The crude product was subjected to
column chromatography (silica gel, dichloromethane). The
product was isolated as a red-purple powder (63.2 mg). Yield:
1
4
5%. Mp > 250 °C. H NMR (DMSO-d , 400 MHz): δ 10.63
6
(
(
(
1
1
s, 2H), 7.95−7.85 (m, 4H), 7.40−7.37 (t, 4H, J = 12 Hz), 6.19
Nanosecond Time-Resolved Transient Absorption
Spectra. The nanosecond time-resolved transient absorption
spectra were detected by the LP920 laser flash photolysis
spectrometer (Edinburgh Instruments, U.K.) and recorded on a
Tektronix TDS 3012B oscilloscope. All the solutions in the
experiments were deaerated with argon for ca. 15 min before
measurement.
m, 2H), 2.50 (s, 6H), 2.42 (s, 6H), 1.38 (s, 12H). 13C NMR
100 MHz, CDCl ): δ = 164.02, 155.83, 154.68, 144.89,
3
42.64, 141.91, 131.19, 131.07, 128.35, 121.23, 86.23, 54.74,
−
6.73, 15.63, 14.11. TOF HRMS ESI : Calcd
+
(
[C H B N F ClI ] ), m/z = 1041.0994; found, m/z =
41
36
2
9
4
2
1
041.0909.
Synthesis of Compound B-3. Under N atmosphere, B-1
100 mg, 0.135 mmol) and compound 2 (65.6 mg, 0.135
DFT Calculations. The ground state structures of
compounds were optimized using DFT with a B3LYP
functional and 6-31G (d) basis set. The initial geometries of
the compounds were generated by GaussView. The excited-
state related calculations (UV−vis absorptions) were carried
out with the TD-DFT with the optimized structure of the
ground state [DFT/6-31G(d)]. The emission frequencies of
the fluorophores were calculated based on the TDDFT
2
(
mmol) were added to a suspension of anhydrous K CO (37.3
2
3
mg, 0.270 mmol) in dry THF (15 mL) in a Schlenck tube. The
tube was sealed, and the mixture was stirred at 45 °C for 12 h.
At the end of the reaction, the mixture was filtered to remove
the K CO . The solvent was removed by a rotary evaporator.
2
3
The crude product was subjected to column chromatography
silica gel, dichloromethane/ethyl acetate, 3:1, v/v). The
(
optimized S
1
excited state geometry. There are no imaginary
product was isolated as a red-purple powder (50.1 mg).
frequencies in frequency analysis of all calculated structures. All
18
1
Yield: 62%. Mp: 179−181 °C. H NMR (CDCl , 400 MHz): δ
of these calculations were performed with Gaussian 09W.
3
7
7
1
1
.95−7.91 (m, 1H), 7.81 (d, 1H, J = 8.0 Hz), 7.72 (b, 1H),
.49−7.46 (m, 2H), 7.23 (t, 2H, J = 16.0 Hz), 7.10−7.07 (m,
H), 6.47−6.27 (m, 6H), 3.43−3.21 (m, 12H), 2.66 (s, 6H),
RESULTS AND DISCUSSION
■
Design and Synthesis of the Switchable Dyads. The
compounds B-1, B-2, and B-3 were prepared with the aim of
investigating the acid-activatable FRET to compete with the
ISC to produce triplet excited states. The building blocks of the
compounds are as follows: (i) rhodamine unit as a pH-
switchable FRET acceptor; (ii) I-Bodipy unit as a triplet
sensitizer, and (iii) cyanuric chloride as a rigid linker. In a
previous study, we showed that the efficient triplet state
equilibrium can be established in a Bodipy/I-Bodipy dyad
13
.63 (s, 1H), 1.47 (s, 6H), 1.21−1.11 (m, 12H). C NMR
(
100 MHz, CDCl ): δ = 156.70, 153.47, 153.12, 148.79,
3
1
1
9
45.35, 145.22, 141.17, 139.60, 132.74, 131.52, 130.71, 128.80,
28.53, 128.26, 123.82, 122.95, 120.92, 120.31, 108.33, 105.09,
7.55, 85.63, 65.09, 44.45, 29.68, 17.26, 16.03, 12.48. TOF
−
+
HRMS ESI : Calcd ([C H BN F I ClO ] ), m/z =
5
2
52
10 2 2
2
1
187.2114; found, m/z = 1187.2103.
UV−vis Absorption and Fluorescence Measurement.
The steady-state UV−vis absorption and emission spectra were
recorded on a UV 2550 spectrophotometer or Agilent 8453A
UV−vis spectrophotometer and FR 5301 PC spectrofluor-
ometer, respectively. For the samples with rhodamine, all
samples were treated with TFA (50 μL, 5 mol/L), and kept for
h before measurement. The solvent is DCM/MeOH (9:1). A
stock solution of TFA dissolved in MeOH was used.
Fluorescence quantum yields were measured with rhodamine
16
connected by a flexible linker. In this work, we used a rigid
cyanuric chloride linker, due to the fact that cyanuric chloride
20
reacts readily with amines, enabling simple synthetic route
toward dyads and triads. In order to prove that the cyanuric
chloride linker allows unrestricted intramolecular triplet energy
transfer, Bodipy/I-Bodipy dyad B-2 was designed (Scheme 1).
Compound B-3 contains both the pH-switchable rhodamine
and I-Bodipy moieties and is used in this study to evaluate the
pH-switchable FRET mechanism. When rhodamine is in the
closed lactam structure (high pH), we expect the photophysics
of B-3 to be dominated by ISC at the I-Bodipy moiety to
produce triplet excited state. The excited-state behavior of B-3
is expected to change when rhodamine is present in the opened
amide form (low pH): the spectral overlap in the absorption
and emission spectra of the chromophores should lead to
efficient FRET from the I-Bodipy to the opened amide form of
rhodamine. This FRET mechanism is postulated to efficiently
compete with the ISC of I-Bodipy (Scheme 1c).
1
B (Φ = 97.0% in ethanol) as standard. Fluorescence lifetimes
F
were measured with OB920 luminescence lifetime spectrom-
eter (Edinburgh, U.K.).
Femtosecond Time-Resolved Transient Absorption
Spectra. The 800 nm laser pulses were produced at a 1 kHz
repetition rate by a mode-locked Ti:sapphire laser and
regenerative amplifier (Hurricane, Spectra-Physics). The output
from the Hurricane was split into pump (85%) and probe
(
10%) beams. The frequency of the pump beam was sent into
an optical paramagnetic amplifier (OPA-400, Spectra Physics)
to obtain the desired excitation wavelength. The energy of the
pump beam was <300 nJ/pulse. The probe beam was focused
Our synthetic procedure starts with the cyanuric chloride as
the key intermediate. It is known that cyanuric chloride reacts
20
into a two-dimensional translated 4 mm CaF crystal for white
readily with aromatic or aliphatic amines. Thus, amino groups
2
4
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The Journal of Physical Chemistry B
Article
a
Scheme 2. Synthesis of Model Compounds B-1, B-2, and B-3
a
Key: (a) anhydrous K CO , dry THF, 0−5 °C, 1 h. Yield: 91%. (b) NIS and CH Cl , 1 h. Yield: 87.0%. (c) Anhydrous K CO , dry THF, 60 °C, 24
2
3
2
2
2
3
h. Yield: 45%. (d) Anhydrous K CO , dry THF, 45 °C, 12 h. Yield: 62%.
2
3
were introduced to the Bodipy or I-Bodipy parts (Scheme 2).
Reaction of ethylenediamine with rhodamine B gives the
intermediate compound Rh-closed-H. In order to control the
stoichiometry, B-1 was prepared first. Then the reactive
intermediate Bodipy-H was used to prepare dyad B-2, while
Rh-closed-H was used to prepare dyad B-3. It should be
pointed out that all rhodamine-containing compounds are
colorless or light pink because the Rhodamine moiety is in the
closed lactam structure. All the compounds were obtained with
satisfactory yields.
UV−vis Absorption and Fluorescence Spectroscopy.
To evaluate whether FRET occurs in model dyads, we
performed UV−vis absorption and fluorescence measurements
on model compounds B-1, B-2, and B-3. The UV−vis
absorption bands of Bodipy and I-Bodipy moieties appear at
accord with the observed absorption bands of compounds B-1
21
(λmax = 535 nm, due to I-Bodipy ) and B-2 (λ = 503 and
max
535 nm, due to Bodipy and I-Bodipy, respectively). The
absorption bands at 535 nm are similar for B-1 and B-2 (Table
1), indicating that there is no significant electronic interaction
22−27
between the two chromophores in B-2 at the ground state.
The absorption profile of B-3 is pH-dependent, as shown in
Figure 1b. In the absence of trifluoroacetic acid (TFA), the
rhodamine moiety of B-3 is in its cyclic lactam form that does
12,28,29
not exhibit any absorption bands in the visible region.
In
this case, the absorption spectrum of B-3 is dominated by the I-
Bodipy moiety with its characteristic absorption band at 535
nm. In the presence of TFA, a new shoulder absorption band at
5
58 nm developed (Figure 1b), which is attributed to the
rhodamine moiety in the opened amide structure. Fitting the
absorption of B-3 in the presence of TFA gives two absorption
bands, which are in agreement with the absorption of B-3 (No
TFA) and the rhodamine compound Rh-open-H, respectively.
By comparison of the absorbance of the rhodamine component
with that of the reference compound Rh-closed-H, the ratio of
the open form/spirolactam form upon addition of TFA is >99:1
503 and 535 nm, respectively (Figure 1a). This assignment is in
(
i.e., the spirolactam → open amide form transformation is
complete).
Interestingly, we found that the transformation of rhodamine
moiety from the cyclic to the open form is relatively slow
(
Figure S10 of the Supporting Information). For B-3, the
transformation was followed by the change in the absorption at
Figure 1. UV−vis absorption spectra of (a) B-1, B-2, and (b) B-3, Rh-
−2
−1
5
58 nm, and the rate constant k = 8.97 × 10 min was
−5
closed-H; c = 1.0 × 10 M in CH Cl /MeOH (9:1, v/v), 20 °C.
2
2
obtained. In this work, all the photophysical studies were
performed only after a mixture of B-3, and TFA was left
Table 1. Photophysical Parameters of the Compounds
a
b
a
,
c
d
e
f
λabs (nm)
ε
λem (nm)
Φ (%)
τF (ns)
τT (μs)
g
F
h
h
a
Bodipy
504
8.7
7.1
517
577
71.2
3.86
3.75
−
−
c
g
Rh-closed-H (TFA)
556
47.2
2.9
a
c
a
c
B-1
B-2
535
8.70
552
0.46 /0.45
76.9 /56.6
a,
c
a
a
c
a
503/537
10.2/7.34
523/552
7.1
2.99 (λem = 523 nm)
130.6
1
2
1
.62 (λem = 552 nm)
.78 (λem = 523 nm)
c
.04 (λ = 552 nm)
em
a
c
a
c
a
c
a
c
B-3
534
7.15
552 /580
3.6 /14.5
0.54 /2.53
51.7 /34.4
a
−5
b
4
−1
−1
c
−5
In CH Cl /MeOH (v/v, 9:1) (1.0 × 10 M). Molar absorption coefficient. ε: 10 M cm . In CH Cl /MeOH (v/v, 9:1) (1.0 × 10 M) with
2
2
2
2
d
5
0 μL 5 M TFA added. Fluorescence quantum yields. For B-2, the fluorescence quantum yield was determined by taking account two emission
e
bands into the calculation (upon excitation of the energy donor). Fluorescence lifetimes, the two τ for B-2 depends on its two emission wavelength
F
f
g
h
16
(λem = 523 nm and λem = 552 nm). Triplet state lifetimes. Not applicable. Literature value.
4
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The Journal of Physical Chemistry B
Article
23
standing for a sufficient amount of time to ensure complete
transformation to the opened rhodamine form.
Akkaya. For B-2, we found that the band at 503 nm in the
fluorescence excitation spectrum is weaker than the UV−vis
absorption band. Thus, the singlet energy transfer is calculated
as 84% by fitting of the spectra. For B-3, the singlet energy
transfer efficiency is 41.7%. The nonunity FRET may be
attributed to the intramolecular electron transfer or the
unfavorable orientation of the dipole moments of the
chromophores for FRET.
The photophysical properties of the compounds are
summarized in Table 1. On the basis of the fluorescence
quantum yields and the fluorescence lifetimes, we propose
intramolecular singlet energy transfer for dyads B-2 and B-3.
Electrochemical Measurements. The proposed pH-
switchable triplet generation in B-3 can occur only if the
photoinduced electron transfer does not compete with the
desired energy transfer. In order to evaluate the thermodynamic
driving force for the photoinduced electron transfer in B-3,
cyclic voltammograms of the model’s compounds were
collected (Figure 3).
The UV−vis absorption spectra of B-3 show that the
rhodamine moiety exhibits a red-shifted absorption band
relative to the I-Bodipy unit, and there is overlap between
the emission spectrum of the I-Bodipy unit and the absorption
spectrum of the rhodamine part, suggesting that the intra-
molecular FRET from I-Bodipy to rhodamine is possible.
Similarly, the FRET from Bodipy to I-Bodipy is expected for
compound B-2.
2
9,30
The FRET efficiency in B-2 and B-3 was further studied
using fluorescence emission and excitation spectroscopy
(
Figure 2). For B-2, the fluorescence of the energy donor
Figure 2. (a) Comparison of the UV−vis absorption and fluorescence
excitation spectra of B-2 (λ = 560 nm). (b) Comparison of the UV−
em
vis absorption and fluorescence excitation spectra of B-3 and Rh-
closed-H+TFA (λem = 590 nm). Spectra are measured after adding 50
μL TFA (5 M solution in MeOH) and after standing of the solution
for 1 h. (c) Change in emission spectra of B-2 and Bodipy (λex = 480
nm). (d) Change in emission spectra of B-3 and Rh-closed-H (with
Figure 3. Cyclic voltammograms of the dyad B-3 and related
compounds: (a) Rh-closed-H, (b) I-Bodipy, (c) Rh-open-H, and (d)
B-3. Ferrocene (Fc) was used as an internal reference (E = +0.64 V
−5
TFA) (λex = 500 nm). c = 1.0 × 10 M in DCM/MeOH (9:1, v/v),
1
/2
20 °C.
+
(Fc /Fc) vs standard hydrogen electrode). Voltammograms were
collected in deaerated CH Cl solutions containing 1.0 mM model
2
2
compounds, the ferrocene and 0.10 M Bu NPF6 as supporting
(
bodipy part) is significant compared to that of the energy
4
electrolytes. Reference electrode: Ag/AgNO . Scan rate: 0.1 V/s; 20
3
acceptor (I-Bodipy). This result is different from our recent
study on the iodo-bodipy dyad, where a more efficient FRET
was observed. The less efficient FRET in B-2 and B-3 may be
due to the less favorable geometry imposed by different linkers.
However, compared to the unsubstituted Bodipy-H, the
fluorescence emission of the singlet energy donor in B-2 is
substantially quenched, confirming that FRET occurs in B-2.
°
C.
1
9
The reference compound Rh-closed-H exhibits two
reversible oxidation potentials at +1.15 and +1.35 V (Figure
a), while I-Bodipy exhibits reversible oxidation and reduction
3
potentials at +1.54 V and −0.83 V, respectively (Figure 3b).
Rh-open-H gives drastically different CV curves as compared
with that of reference compound Rh-closed-H. One irrever-
sible reduction band at −0.69 V and a pseudoreversible
oxidation band at +1.49 V were observed (Figure 3c). The
redox potentials of B-3 are almost the sum of the potentials for
the reference compounds Rh-closed-H and I-Bodipy (Figure
3d), indicating that there is no substantial interaction between
the rhodamine and the I-Bodipy units. The reduction potentials
of the compounds are compiled in Table 2.
9
−1
The rate constant was estimated as k = 4.9 × 10 s with eq
1
ET
3
1
:
⎡
⎤
Φ (bodipy)
PL
kET = ⎢
− 1⎥/τ(bodipy)
⎣
ΦPL
⎦
(1)
Compound B-3 in the presence of TFA exhibits strong
fluorescence emission attributed to the rhodamine part,
suggesting that the energy transfer from I-Bodipy to Rh-open
moiety is efficient. The FRET efficiency in B-2 and B-3 was
evaluated from fluorescence excitation spectra, as described by
The free energy changes of the electron separation (CS)
process was calculated using the Weller equation (eqs 2 and 3).
4
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a
Table 2. Redox Potentials of Bodipy Photosensitizers
However, as we will show further in the text, the electron
transfer was not observed in B-3.
Femtosecond Transient Absorption Spectroscopy.
The energy transfer between bodipy and iodo-bodipy
E(Ox) (V)
E(Red) (V)
2
+1.15, +1.35
+1.54
−
Iodo-Bodipy
Rhodamine B
B-3
−0.83
−0.69
−0.81
chromophores was previously demonstrated in a covalent
+1.49
19,33
dyad containing a flexible linker.
In this study, a new type of
+1.19, +1.34, +1.51
linker, derived from cyanuric acid, is explored, and the model
compound B-2 was used to evaluate the efficacy of energy
transfer between bodipy and iodo-bodipy connected by a rigid
cyanuric linker. The transient absorption spectra of B-2 were
obtained upon excitation at two wavelengths: 500 nm (where
bodipy unit absorbs) and 532 nm (where iodo-bodipy unit
absorbs). The transient spectra (Figure 4) exhibits the
following features: bleach bands at 500 and 532 nm, stimulated
emission band in the 570−620 nm range, and the excited-state
absorption (ESA) in the 620−750 nm range.
a
+
With Fc as an internal reference, for which E1/2 (Fc /Fc) = +0.64 V
vs standard hydrogen electrode. Cyclic voltammetry in Ar saturated
CH CN containing a 0.10 M BuNPF supporting electrolyte; Counter
electrode is Pt electrode; working electrode is glassy carbon electrode;
Ag/AgNO couple as the reference electrode. c[Ag ] = 0.1 M. 1.0 mM
dyad photosensitizers in CH CN, 25 °C. Conditions: 1.0 mM dyad
photosensitizers and 1.0 mM ferrocene in CH CN, 298 K. The
3
6
+
3
3
3
potentials are relative to the standard hydrogen electrode.
ΔG° = e[EOX − ERED] − E00 + ΔGS
CS
(2)
When excited at 500 nm, the ground-state bleach band at 500
nm is dominant at early times but depletes over time with a
concomitant growth of the 532 nm bleach. This process is
indicative of the singlet energy transfer from the bodipy to
e2
e2
⎞⎛
⎟⎜
⎞
⎟
⎛
⎜
1
1
1
1
ΔG = −
−
+
+
S
4
πε ε R
8πε ⎝ R
R ⎠⎝ ε
ε ⎠
S 0 CC
0
D
A
REF
S
(3)
iodo-bodipy unit. The rate of this process was evaluated to be
kFRET = 2 × 1011
s
−1
(τ = 5 ps) by monitoring the kinetics at
where ΔG is the static Coulombic energy, which is described
by eq 3, e = electronic charge, E = half-wave potential for
one-electron oxidation of the electron-donor unit, ERED = half-
wave potential for one-electron reduction of the electron-
acceptor unit (in cases when irreversible oxidation peaks
occurred in the cyclic voltammograms, the anodic and cathodic
peak potentials were used instead of half-wave potentials), E0,0
S /S energy difference approximated with the fluorescence
emission wavelength, ε = static dielectric constant of the
solvent, RCC = center-to-center separation distance determined
by DFT optimization of the geometry, RCC (B-3) = 20.9 Å, RD
is the radius of the electron donor, R is the radius of the
S
5
02 nm (Figure 5a). This value is close to the previous reports
OX
on FRET bodipy-azabodipy dyads, thus confirming that the
cyanuric moiety is an excellent covalent linker for the studies of
energy transfer processes in dyads and triads.
At later times, the stimulated emission band decays and a
growth in the absorption is observed in the 620−750 nm range.
This process is assigned to the ISC of the excited state localized
on the iodo-bodipy moiety, based on the known absorption
=
0 1
S
2
1,34
spectrum of the T state of iodo-bodipy.
The ISC rate was
1
9
−1
evaluated to be kISC = 8 × 10 s (τ = 120 ps) by following the
kinetics at 700 nm (Figure 5a).
A
The excitation at 532 nm eliminates the possibility of FRET,
since the initially formed excited state is localized on the iodo-
electron acceptor, εREF is the static dielectric constant of the
solvent used for the electrochemical studies, and ε is the
permittivity of free space. The solvent used in the calculation of
0
bodipy, whose S energy is lower than the S energy of bodipy.
1
1
Thus, the transient absorption spectra exhibit only one
predominant bleach band at 532 nm. The stimulated emission
band appears at early times, but decays with a growth of
absorption in the 620−750 nm range, which is indicative of the
ISC process of the iodo-bodipy moiety, analogous to the same
process observed upon the 500 nm excitation. The lifetime of
free energy of the electron transfer is CH Cl (ε = 8.93).
2
2
S
The driving force for the charge recombination (ΔG ) was
CR
32
calculated using eq 4:
ΔGCR = −(ΔGCS + E00)
(4)
On the basis of the free-energy changes summarized in Table
, the photoinduced electron transfer is not likely to occur in
the S state is 131 ps (Figure 5b), which is similar to the
3
1
lifetime observed upon the 500 nm excitation.
the model compound B-3. Out of 6 possible electron transfer
processes, only two processes exhibit ΔG values that are more
An interesting effect was observed at longer time delays: the
relative ratio of the 532 and 500 nm bleach bands is changed,
indicating another energy redistribution process. The 500 nm
signal increases in intensity, which is consistent with the triplet
energy transfer from iodo-bodipy to the bodipy moiety. In
order to study the kinetics of the intramolecular triplet energy
transfer, the transient absorption spectra were subjected to the
global analysis (Figure S16 of the Supporting Information). On
the basis of the fit results, we evaluated the rate constants of the
1
1
negative than −0.1 eV: BDP* → RB-c and RB-o → BDP*
charge transfers. Thus, only these two electron transfer
processes are likely to have sufficiently high rates to occur
within the excited-state lifetimes of the model chromophores.
Table 3. Driving Forces of Charge Recombination (ΔG(CR)),
Charge Separation (ΔG ) and Charge Separation Energy
(
CS)
a
States for the Rho-BDP Dyad B-3 in DCM
forward triplet energy transfers from iodo-bodipy to bodipy (k
F
8
−1
electron transfer
ΔG(
CR)
(eV)
ΔG(CS) (eV)
ECS (eV)
= 7.0 × 10 s ) and the backward triplet energy transfer from
1
8
−1
RB-c → BDP*
−
−
−
bodipy to iodo-bodipy (k = 5.6 × 10 s ). Thus, the triplet
B
1
BDP* → RB-c
−1.92
−2.12
−2.22
−2.22
−2.12
−0.33
−0.03
+0.07
−0.03
−0.13
1.92
2.12
2.22
2.22
2.12
energy transfer is much slower than the corresponding singlet
energy transfer. This behavior is expected because the triplet
energy transfer can occur only by a Dexter mechanism, which
1
RB-o* → BDP
1
BDP → RB-o*
35−37
1
BDP* → RB-o
requires a close contact between the chromophores.
1
To evaluate the energy difference between the two triplet
states, the equilibrium constant for the triplet transfer between
the bodipy and the iodo-bodipy units was calculated using kf
RB-o → BDP*
a
The arrow means the direction of charge transfer.
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Figure 4. Femtosecond transient absorption spectra of B-2 in DCM/MeOH (9/1, v/v) with excitation at (a) 500 and (b) 532 nm. Arrows indicate
−5
spectral changes as time progresses; c = 1.0 × 10 M; 20 °C.
Figure 5. Kinetics of the S absorption, ground state bleach, and T excited state absorption for B-2. (a) λ = 500 nm; (b) λexc = 532 nm. c = 1.0 ×
1
1
exc
−
5
10
M in DCM/MeOH (9/1, v/v); 20 °C.
and k rates. The value for the equilibrium constant is K = 0.8,
which indicates that the triplet state is slightly more localized
(Figure 6). When the rhodamine unit adopts a closed lactam
form, the excited-state behavior of B-3 exhibits the same
photophysics as the iodo-bodipy unit, which is known to exhibit
a singlet excited state lifetime of 100−150 ps due to the ISC (as
shown in the photophysics of B-2). Indeed, this behavior was
observed for B-3 in the absence of TFA (Figure 7). The fit of
b
eq
on the iodo-bodipy unit in B-2. Determination of the K value
eq
leads to the estimation of the energy gap between the bodipy
and iodo-bodipy triplets as 0.006 eV (assuming ΔE = −RT ln
K ) (e.g., nearly degenerate).
eq
The excited-state behavior of B-3 was studied under two
experimental conditions: in the absence of TFA (when the
rhodamine unit exists in a closed form) and in the presence of
TFA (when the rhodamine unit adopts an open amide form)
the kinetic trace at 700 nm gives the ISC rate of k = 5.6 ×
10 s (τ = 178 ps).
In the presence of TFA, the photophysical behavior is more
complex due to multiple possible energy transfer processes. To
ISC
9
−1
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Figure 6. (a) The transient absorption spectra of B-3 in DCM/MeOH (9/1, v/v) with (red) and without (blue) TFA obtained 10 ns after the 565
nm excitation pulse. (b) The femtosecond transient absorption spectra of B-3 in DCM/MeOH (9/1, v/v) in the presence of TFA (λ = 565 nm); c
exc
−
5
=
1.0 × 10 M. Inset: kinetics of Rh-open ground-state bleach probed at 571 nm. The sample of B-3 was kept in TFA solution for 1 h before
measurement.
Figure 7. Transient absorption spectra of B-3 upon excitation at 532 nm in the presence (panel a) and absence (panel c) of TFA. Comparison of
kinetic trances in the presence (red traces) and absence (blue traces) of TFA probed at 700 nm (panel b) and 364 nm (panel d); c = 1.0 × 10− M.
The sample of B-3 was kept in TFA solution for 1 h before measurement.
5
facilitate data interpretation, the B-3 sample was initially excited
at 565 nm, where the Rh-open unit absorbs most of the light.
The results of this study indicate that the energy of the initially
formed S state of Rh-open migrates over time to the T state
additional generation of the T state localized on the iodo-
bodipy unit (sample concentration and pump power were kept
constant in two experiments).
1
To evaluate the kinetics of energy transfer from the S state
1
1
1
of the iodo-bodipy moiety. Figure 6a demonstrates this
assignment, by showing that the higher amount of the T1
state of iodo-bodipy is formed when the Rh-open unit is
excited. The nanosecond transient absorption spectrum of B-3
in the absence of TFA exhibits the signature bands of the T1
state of iodo-bodipy moiety. The sample of B-3 in the presence
of TFA exhibits the same transient spectrum, but the signal is
stronger, indicating that excitation of Rh-open unit leads to
of Rh-open to the T state of iodo-bodipy, the femtosecond
1
transient absorption spectra of B-3 in the presence of TFA were
collected upon the excitation at the Rh-open absorption band
(λexc = 565 nm, Figure 6b). The spectral changes are associated
with a conversion from the initially formed S state of Rh-open
1
(transient absorption band at 450 nm) to the T state of iodo-
1
bodipy (transient absorption band at 700 nm). The kinetics of
this energy transfer was monitored at 571 nm, the Rh-open
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ground-state bleach band. The signal decay can be fitted to
either biexponential decay or a stretched exponential function.
Since the stretched exponential functions are often utilized to
3
8,39
describe energy transfer in conformationally flexible dimers,
we report here the results of such fit: τ = 1.8 ns, β = 0.7. The
mechanism by which the energy migrates from the S state of
1
Rh-open to the T state of iodo-bodipy is currently not
1
understood. It is known that the ISC process of the Rh-open
1
3,47
unit is not efficient,
so it is not likely that the mechanism
involves the formation of the T state of the Rh-open unit.
1
40
Probably it is a singlet−triplet energy transfer.
When B-3 is excited at the iodo-bodipy moiety (λ = 532
exc
nm), the initially formed S state is localized on the iodo-
1
bodipy moiety, and it decays via two competing channels: (i)
the intersystem crossing to generate the T state of iodo-
1
bodipy; (ii) energy transfer to the S state of Rh-open, followed
1
by another energy transfer to the T state of iodo-bodipy. For
1
example, Figure 7b compares the kinetics probed at 700 nm
(
where the T state of iodo-bodipy absorbs) for B-3 in the
1
presence and absence of TFA. In the absence of TFA, only the
ISC process occurs and the growth of the signal occurs within
1
the stimulated emission signal of the S state of Rh-open and
the excited-state absorption signal of the iodo-bodipy T state.
At early times, the stimulated emission signal grows due to the
energy transfer from the S state of iodo-bodipy to the S state
Figure 8. (a) Nanosecond time-resolved transient absorption of B-1. c
−5
=
1.0 × 10 M in deaerated CH Cl /CH OH (9:1, v/v). (b) Decay
2 2 3
trace of B-1 at 530 nm after excitation at λex = 532 nm. (c) Transient
78 ps. In the presence of TFA, the 700 nm signal arises due to
−5
difference absorption spectra of B-2, c = 1.0 × 10 M in deaerated
1
toluene and (d) decay curve of B-2 at 500 nm after excitation at λ =
ex
1
5
32 nm; 20 °C.
1
1
of the Rh-open unit. At later times, the stimulated emission
signal decays and T absorption signal grows due to the
to the bodipy takes place. Since the bleach bands decay with the
1
same rate constants, we propose that the T states of bodipy
1
formation of the T state of iodo-bodipy.
1
and iodo-bodipy part are in the equilibrium. To evaluate the
intrinsic triplet excited state lifetime of B-2 (and eliminate the
effect of triplet−triplet annihilation), the lifetime was evaluated
at several concentrations of B-2, and the extrapolation to
infinite dilution afforded an intrinsic lifetime of 464.5 μs. This
long-lived triplet excited state is an important characteristic of
B-2, since the long-lived triplet excited states are crucial for the
application of the triplet photosensitizers in luminescent
Similar behavior is observed when kinetics was probed at 364
nm, where S states of both Rh-open and iodo-bodipy absorb.
1
In the absence of TFA, the signal decays due to ISC of the
iodo-bodipy unit. In the presence of TFA, the growth and
decay of the S state of Rh-open appears as well. The kinetic
1
traces at multiple wavelengths fit well to the following model:
(
i) the rate of energy transfer from the S state of iodo-bodipy
1
4
1−44
to the S state of Rh-open fits to the stretched exponential
oxygen sensing,
sion,
triplet−triplet annihilation upconver-
1
4
4−51
1,2,52
function with the lifetime of τ = 81 ps and the power coefficient
and photodynamic therapy, etc.
β = 0.6; (ii) the rate of energy transfer from the S state of Rh-
The nanosecond transient absorption spectra of B-3 showed
a somewhat surprising behavior. The femtosecond experiment
presented earlier showed that the efficient FRET occurs in the
presence of TFA, thus moving the energy from the iodo-bodipy
to the Rh-open moiety. This result was in accord with the pH-
activatable mechanism proposed in this study. The nanosecond
experiments presented here show that another energy
redistribution takes place at a longer timescale, which results
1
open to the T state of iodo-bodipy was fit using the same rates
1
as obtained in the experiments with 565 nm excitation (Figure
6). Thus, the FRET process in B-3 efficiently competes with
the ISC process of the iodo-bodipy unit, which confirms that
the proposed pH-activatable triplet generation mechanism can
take place. At later times, the decay of the Rh-based S state
1
takes place.
Nanosecond Transient Absorption Spectroscopy. In
order to study the triplet excited states of the compounds,
nanosecond transient absorption spectra of the compounds
in the population of the T state of the iodo-bodipy moiety.
1
These results support the postulations of a singlet−triplet
energy transfer from the rhodamine unit to the iodo-bodipy
moiety, derived from the femtosecond transient absorption
spectra. It is known that rhodamine chromophore gives
7
were studied. For B-1, the transient absorption spectrum
(
Figure 8a) consists of a bleach band at 535 nm and excited-
53
state absorption bands at 450 nm and in the 600−750 nm
nonefficient ISC, thus triplet−triplet energy transfer from
the rhodamine moiety to the diiodoBodipy moiety is ruled out.
The following text outlines these results in more detail.
range. These signature bands are associated with the T state of
1
2
1,34
iodo-bodipy.
The triplet excited state lifetime of B-1 (20.9
μs, Figure 8b) is much shorter than the iodo-Bodipy alone
In the absence of TFA, the transient absorption profile of B-3
2
1
21,34
(57.1 μs), suggesting that the cyanuric acid linker increases
(Figure 9a) is typical of the T state of iodo-bodipy.
This
1
the rate of nonradiative decay in B-1.
The transient absorption spectra of B-2 (Figure 8c) exhibit
two bleach bands at 502 and 532 nm, indicating the presence of
result is expected, since the Rh-closed unit does not contribute
to the photophysics of B-3. In the presence of TFA, the spectra
hardly show any changes relative to the spectra of B-3 in the
two T states, localized on either bodipy or iodo-bodipy parts in
absence of TFA (Figure 8c), suggesting that the T state of
1
1
B-2. This result is in agreement with the femtosecond transient
absorption measurements presented earlier, which have shown
that the intramolecular triplet energy transfer from iodo-bodipy
iodo-bodipy is populated even when the Rh-unit is present in
the open amide form. The T state energy level of rhodamine
1
54
(1.78 eV) is higher than the bodipy part (1.61 eV).
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eV (460 nm), which is higher in energy than the experimental
value (2.32 eV, 534 nm), but within the known 0.3−0.5 eV
accuracy of the TD-DFT method.
When the rhodamine unit is present in an opened amide
form, the calculated excited states show the involvement of the
rhodamine unit (Figure 11). The S state is localized on the
1
rhodamine unit, while the S state is localized on the iodo-
2
bodipy moiety. This calculation is in excellent qualitative
agreement with the experimentally obtained results, where it
was demonstrated that efficient FRET occurs from iodo-bodipy
to the rhodamine-open unit. The calculated transition energies
for S (2.6 eV, 477 nm) and S states (2.7 eV, 459 nm) are in a
1
2
reasonable agreement with the experimental values of 556 nm
(
2.23 eV) and 534 nm (2.33 eV).
The order of energies is reversed for the triplet excited states
of B-3, where the T state is located on the iodo-bodipy moiety
1
(
Figure 11), while the HOMO−1 → LUMO transition is
involved in the T state, which is localized on the rhodamine
2
part. Previously with phosphorescence, it was shown that the
5
4
rhodamine is with a T state energy level of 1.78 eV, and the
1
Figure 9. (a) Nanosecond time-resolved transient difference
51
−
5
Bodipy is with the T state energy level of 1.61 eV. Thus, the
1
absorption spectra of B-3 in the absence of TFA, c = 1.0 × 10
M
theoretical result is in agreement with experimental result that
in deaerated CH Cl /CH OH (9:1, v/v). (b) Decay trace of B-3 in
2
2
3
the T state of dyad B-3 is localized on the bodipy unit.
the absence of TFA at 530 nm upon excitation at 532 nm. (c)
1
Transient absorption spectra of B-3 in the presence of TFA, c = 1.0 ×
Jablonski Diagrams. On the basis of the steady-state/time-
resolved spectroscopy and TD-DFT calculations, the photo-
physics involved in the dyads B-2 and B-3 are illustrated in
Schemes 3 and 4. For B-2, the excitation of the bodipy moiety
−
5
10
M in deaerated CH Cl /CH OH (9:1, v/v) with 50 μL 5 M TFA
2 2 3
added (the solution was left to stand for 1 h before the measurement).
d) Decay trace of B-3 in the presence of TFA at 530 nm, λex = 532
(
nm; 20 °C.
generates the S state that undergoes a FRET process to the
1
1
1 −1
iodo-bodipy moiety with a rate constant of kFRET = 2 × 10 s
τ = 5 ps). Then, the ISC of the iodo-bodipy part generates the
T state on iodo-bodipy with a rate constant of k = 8 × 10 s
Observation of the triplet state for B-3 in the presence of TFA
is surprising, considering that the efficient FRET from iodo-
bodipy to the Rh-open unit was observed at the ultrafast
timescale. However, this result is reasonable based on the
observation of the energy transfer from the singlet state of
rhodamine unit to the triplet state of iodo-Bodipy unit in the
femtosecond transient absorption spectra.
(
9
−1
1
(
τ = 120 ps). At the longer timescale, the downward triplet−
triplet energy transfer (TTET) to the bodipy part takes place
with a rate constant of k = 7.0 × 10 s (τ = 1.4 ns), while the
8
−1
reverse upward TTET occurs with the slightly slower kinetics
8
−1
of k = 5.6 × 10 s (τ = 1.8 ns). The two triplet states decay
DFT Calculations. The photophysical properties of B-3 in
two rhodamine-states were evaluated using time-dependent
density functional theory (TD-DFT) calculations. Previously,
we and others have shown that TD-DFT calculations are very
useful for study of the photochemistry and photophysics of
−5
with an equal rate constant (τ = 130.6 μs, at 1.0 × 10 M) to
T
recover the molecule in the ground state. The photophysical
behavior of B-2 shows that the efficient energy transfer
processes can occur in bodipy dyads connected by a rigid
cyanuric acid linker. The data presented here do not rule out
the possibility that the energy transfer process occurs
concurrently with the independent excitation and decay of
individual chromophores. Given the possible number of
conformations that the dimers can adopt at room temperature
solution, it is possible that some conformers favor energy
transfer more than the others.
5
1,55−59
organic chromophores and transition metal complexes.
When rhodamine unit is present in the closed lactam form, the
calculations predict no low-lying singlet excited state localized
on the rhodamine part (Figure 10). The first two singlet and
triplet states are localized on the iodo-bodipy moiety, which is
consistent with the time-resolved measurements presented in
the previous sections. The calculated lowest absorption is at 2.7
Figure 10. Selected frontier molecular orbitals involved in the excitation and emission of B-3 (with rhodamine in closed form). CH Cl was used as
2
2
the solvent in the calculation (PCM model). The calculations are at the B3LYP/6-31G(d)/LANL2DZ level using Gaussian 09W.
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Figure 11. Selected frontier molecular orbitals involved in the excitation and emission of B-3 (Rh-open). Dichloromethane was used as the solvent
in the calculation (PCM model). The calculations are at the B3LYP/6-31G(d)/LANL2DZ level using Gaussian 09W.
Scheme 3. Simplified Jablonski Diagram Illustrating the
Photophysical Processes Involved in B-2
expected in this compound and was postulated as a mechanism
for pH-activatable triplet generation. However, our exper-
imental results show that the energy eventually migrates from
the S state of rhodamine to the T state of the iodo-bodipy
a
1
1
moiety. It is currently not well-understood how this energy
transfer process occurs. The energy migration might involve
higher triplet states (i.e., T state) of the iodo-bodipy that are
2
close in energy with the S state of the rhodamine unit. This
1
finding indicates that the design of pH-activatable triplet
generators requires a careful consideration of energetics of first
and higher excited states of the two chromophores.
CONCLUSION
■
In order to switch the triplet excited state of organic triplet
photosensitizers by external stimuli, such as acid or base, we
prepared a series of dyad triplet photosensitizers based on
rhodamine and iodo-bodipy chromophores, with rhodamine as
the pH-activatable moiety to perturb both the singlet and the
triplet excited state of iodo-bodipy. The iodo-bodipy moiety
serves as the singlet energy donor, as well as the spin converter
with the intersystem crossing (ISC) capability. A rigid linker of
cyanuric chloride was used for connection of the components
in the dyads. The molecular design rationale is to establish an
acid-activatable competition between two major photophysical
processes of the dyads [i.e., the fluorescence resonance energy
transfer (FRET) and ISC]. Using steady-state and time-
resolved transient absorption spectroscopy, we confirmed that
the FRET is indeed acid-activatable. Femtosecond time-
resolved transient absorption spectroscopy confirmed the
ultrafast FRET in the dyads (81 ps) and the much slower
a
kF is the rate constant of forward triplet energy transfer from the
iodo-bodipy unit to the bodipy unit. k is the backward triplet energy
transfer from the bodipy to the iodo-bodipy unit.
B
The photophysical behavior of B-3 is pH-dependent. When
the rhodamine moiety adopts the closed lactam structure, the
S and T states of the rhodamine unit are significantly higher
than the corresponding states localized on the iodo-bodipy part.
Thus, the photophysical behavior of B-3 is dominated by the
ISC process of the iodo-bodipy part (Scheme 4a). When the
rhodamine unit adopts an open amide form, the S energy of
the rhodamine falls below the S energy of iodo-bodipy, and the
efficient FRET is observed. The FRET mechanism was
1
1
1
1
Scheme 4. Simplified Jablonski Diagram Illustrating the Photophysical Processes Involved in (a) B-3 in the Absence of TFA and
a
(
b) B-3 in the Presence of TFA
a
[
BDP-RB-o] stands for B-3 with the rhodamine part in the open amide structure. [BDP-RB-c] stands for B-3 with the rhodamine part in the close
amide structure. The component at the excited state was designated with red color. The number of the superscript designated either the singlet or
1
the triplet excited state. Note in [BDP-RB-o], the ISC process of the iodo-Bodipy part is with larger rate constant than the FRET process, with the
rhodamine part as the singlet energy acceptor.
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Article
ISC of the iodo-bodipy (178 ps). Thus, the ISC of the iodo-
bodipy unit in the dyads is substantially inhibited by the
competitive FRET. However, the femtosecond and nanosecond
time-resolved transient absorption spectroscopy show that the
triplet state of the iodo-bodipy unit is formed at the longer
timescale. The triplet state formation is postulated to involve
higher triplet excited states of the iodo-bodipy and probably
singlet−triplet energy transfer. Our findings shed a light on an
almost unexplored area of the switching of triplet excited states
of dyad organic triplet photosensitizers. This study will be
useful for future molecular design as well as the investigation on
the fundamental photophysical properties of the assemblies of
organic chromophores.
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ASSOCIATED CONTENT
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Anilinomethylrhodamines: pH Sensitive Probes with Tunable Photo-
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Supporting Information
Experimental procedures, molecular structure characterization,
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(13) Lee, M. H.; Han, J. H.; Lee, J. H.; Park, N.; Kumar, R.; Kang, C.;
Kim, J. S. Two-Color Probe to Monitor a Wide Range of pH Values in
Cells. Angew. Chem., Int. Ed. 2013, 52, 6206−6209.
■
Corresponding Authors
E-mail: zhaojzh@dlut.edu.cn.
E-mail: kglusac@bgsu.edu.
*
(14) Yogo, T.; Urano, Y.; Ishitsuka, Y.; Maniwa, F.; Nagano, T.
*
Highly Efficient and Photostable Photosensitizer Based on BODIPY
Author Contributions
K.X. and Y.X. contributed equally to this work.
Chromophore. J. Am. Chem. Soc. 2005, 127, 12162−12163.
§
(
15) Awuah, S. G.; You, Y. Boron Dipyrromethene (BODIPY)-Based
Photosensitizers for Photodynamic Therapy. RSC Adv. 2012, 2,
1169−11183.
16) Kamkaew, A.; Lim, S. H.; Lee, H. B.; Kiew, L. V.; Chung, L. Y.;
Notes
1
(
The authors declare no competing financial interest.
Burgess, K. BODIPY Dyes in Photodynamic Therapy. Chem. Soc. Rev.
2013, 42, 77−88.
ACKNOWLEDGMENTS
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(
17) Zhao, J.; Wu, W.; Sun, J.; Guo, S. Triplet Photosensitizers: From
J.Z. thanks the NSFC (Grants 20972024, 21273028, 21421005,
and 21473020), the Royal Society (UK) and NSFC (China-UK
Cost-Share Science Networks, 21011130154), Ministry of
Education (SRFDP-20120041130005) Program for Changjiang
Scholars and Innovative Research Team in University
Molecular Design to Applications. Chem. Soc. Rev. 2013, 42, 5323−
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Wallingford, CT, 2009.
19) Zhang, C.; Zhao, J.; Wu, S.; Wang, Z.; Wu, W.; Ma, J.; Guo, S.;
5
(
(
[
IRT_13R06], the Fundamental Research Funds for the
Huang, L. Intramolecular RET Enhanced Visible Light-Absorbing
Bodipy Organic Triplet Photosensitizers and Application in Photo-
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Central Universities (DUT14ZD226), and Dalian University
of Technology (DUT2013TB07) for financial support. K.D.G.
thanks NSF (CHE-1055397) for financial support.
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