Accessing the long-lived triplet excited states in bodipy-conjugated 2-(2-hydroxyphenyl) benzothiazole/benzoxazoles and applications as organic triplet photosensitizers for photooxidations

Article abstract of DOI:10.1021/jo300943t

Bodipy derivatives containing excited state intramolecular proton transfer (ESIPT) chromophores 2-(2-hydroxyphenyl) benzothiazole and benzoxazole (HBT and HBO) subunits were prepared (7-10). The compounds show red-shifted UV-vis absorption (530-580 nm; ε up to 50000 M^-1 cm^-1) and emission compared to both HBT/HBO and Bodipy. The new chromophores show small Stokes shift (45 nm) and high fluorescence quantum yields (Φ_F up to 36%), which are in stark contrast to HBT and HBO (Stokes shift up to 180 nm and Φ_F as low as 0.6%). On the basis of steady state and time-resolved absorption spectroscopy, as well as DFT/TDDFT calculations, we propose that 7-9 do not undergo ESIPT upon photoexcitation. Interestingly, nanosecond time-resolved transient absorption spectroscopy demonstrated that Bodipy-localized triplet excited states were populated for 7-10 upon photoexcitation; the lifetimes of the triplet excited states (τ_T) are up to 195 μs. DFT calculations confirm the transient absorptions are due to the triplet state. Different from the previous report, we demonstrated that population of the triplet excited states is not the result of ESIPT. The compounds were used as organic triplet photosensitizers for photooxidation of 1,5-dihydroxylnaphthalene. One of the compounds is more efficient than the conventional [Ir(ppy)₂(phen)][PF₆] triplet photosensitizer. Our result will be useful for design of new Bodipy derivatives, ESIPT compounds, and organic triplet photosensitizers, as well as for applications of these compounds in photovoltaics, photocatalysis and luminescent materials, etc.

Full text of DOI:10.1021/jo300943t

Article
pubs.acs.org/joc
Accessing the Long-Lived Triplet Excited States in Bodipy-
Conjugated 2-(2-Hydroxyphenyl) Benzothiazole/Benzoxazoles and
Applications as Organic Triplet Photosensitizers for Photooxidations
Pei Yang, Jianzhang Zhao,* Wanhua Wu, Xuerong Yu, and Yifan Liu
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, E-208 West Campus, 2
Ling-Gong Road, Dalian 116024, P. R. China
S
* Supporting Information
ABSTRACT: Bodipy derivatives containing excited state
intramolecular proton transfer (ESIPT) chromophores 2-(2-
hydroxyphenyl) benzothiazole and benzoxazole (HBT and
HBO) subunits were prepared (7−10). The compounds show
red-shifted UV−vis absorption (530−580 nm; ε up to 50000
M−1 cm−1) and emission compared to both HBT/HBO and
Bodipy. The new chromophores show small Stokes shift (45
nm) and high fluorescence quantum yields (ΦF up to 36%),
which are in stark contrast to HBT and HBO (Stokes shift up
to 180 nm and ΦF as low as 0.6%). On the basis of steady state
and time-resolved absorption spectroscopy, as well as DFT/TDDFT calculations, we propose that 7−9 do not undergo ESIPT
upon photoexcitation. Interestingly, nanosecond time-resolved transient absorption spectroscopy demonstrated that Bodipy-
localized triplet excited states were populated for 7−10 upon photoexcitation; the lifetimes of the triplet excited states (τT) are up
to 195 μs. DFT calculations confirm the transient absorptions are due to the triplet state. Different from the previous report, we
demonstrated that population of the triplet excited states is not the result of ESIPT. The compounds were used as organic triplet
photosensitizers for photooxidation of 1,5-dihydroxylnaphthalene. One of the compounds is more efficient than the conventional
[Ir(ppy)2(phen)][PF6] triplet photosensitizer. Our result will be useful for design of new Bodipy derivatives, ESIPT compounds,
and organic triplet photosensitizers, as well as for applications of these compounds in photovoltaics, photocatalysis and
luminescent materials, etc.
atom effect.21−24 However, it should be pointed out that
exceptions do exist, for example, benzophenone and biacetyl
1. INTRODUCTION
Excited state intramolecular proton transfer (ESIPT) has
attracted much attention1−4 due to applications in fluorescent
undergo efficient ISC upon photoexcitation. For most aromatic
molecular probes,3,4 molecular logic gates,1,4,5 fluorescent
hydrocarbon, the major nonradiative decay channel of the S1
bioimaging,6 and light-emitting materials,2,3,7−9 as well as for
state is the intersystem crossing, not internal conversion, and
photophysical studies.1,9−15 The typical ESIPT chromophores
the same is also true for compounds like methylene blue.
Although sporadic heavy-atom-free organic triplet photo-
sensitizers do exist, such as porphyrins, ect,22,25a,b the
photophysics of these compounds is elusive, and it is difficult
to design an organic chromophore to show the predetermined
ISC property. Organic chromophores with triplet excited states
populated upon photoexcitation can be used for photovoltaics,
photocatalysis,26 and photodynamic therapy (PDT).25a,b,27
On the other hand, boron-dipyrromethene (Bodipy) is a
versatile fluorophore, due to its strong absorption of visible
light (ε up to 80000 M−1 cm−1 at 504 nm for the unsubstituted
Bodipy), high fluorescence quantum yields (close to unity), and
good photostability.28−31 Moreover, the rich chemistry of
Bodipy allows feasible derivatization.25,28b,32 Bodipy was widely
used for construction of molecular probes.25h,g,30 However, to
are those containing 2-(2-hydroxyphenyl) benzothiazole
(HBT) or 2-(2-hydroxyphenyl) benzoxazole units (HBO)
(Scheme 1).16 Furthermore, these ESIPT chromophores are
usually limited to those with a small π-conjugation framework,
and thus the absorption are in UV or blue range,2,7,9−11,13,16−19
which is detrimental to their applications in molecular probes
or photoluminescent dyes.2−4,12 ESIPT chromophores with a
larger π-conjugation framework are expected to show red-
shifted absorption. However, very few such ESIPT chromo-
phores have been reported.2,14,18,20
We noticed that one interesting property of ESIPT
chromophores has not yet been explored, that is, the
population of triplet excited states upon photoexcitation.12,16,18
This is a remarkable feature because very often the triplet
excited state of the heavy-atom-free organic chromophores
cannot be efficiently populated upon photoexcitation. S0→T1 is
a quantum mechanically forbidden transition, and the
intersystem crossing (ISC) is usually difficult without heavy
Received: May 10, 2012
Published: June 28, 2012
© 2012 American Chemical Society
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a
Scheme 1. Synthesis of Bodipy-Derived Chromophores 7−10
a
Key: (a) PdCl2(PPh3)2, PPh3, CuI, NEt3, ethynyltrimethylsilane, argon atmosphere, 80 °C, 3 h; then (n-Bu)4NF/THF, room temperature, 10 min;
54%. (b) 2-Aminothiophenol, CH3OH, room temperature, 12 h; 30%. (c) 2-Aminophenol, acetic acid, Pb(OAc)4, room temperature 30 min, then
110 °C, 1 h; 60%. (d) PdCl2(PPh3)2, PPh3, CuI, NEt3, ethynyltrimethylsilane, argon atmosphere, 80 °C, 4 h; then K2CO3, CH3OH, CH2Cl2, room
temperature, 6 h; 35%. (e) Pd(PPh3)4, CuI, NEt3, THF, argon atmosphere, 60 °C, 6 h; yield: 40% for compound 7, 10% for compound 8. (f)
Pd(PPh3)4, CuI, NEt3, THF, argon atmosphere, 60 °C, 8 h; 25%. (g) n-C4H9Br, DMF, 50 °C, 4 h; 86%. (h) PdCl2(PPh3)2, PPh3, CuI, NEt3, argon
atmosphere, 60 °C, 6 h; 36%.
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the best of our knowledge, ESIPT chromophores containing
Bodipy units have never been reported.1−3
To address the above challenges, we selected Bodipy to
extend the π-conjugation of the conventional ESIPT
chromophores such as HBT and HBO.
Herein we devised Bodipy-conjugated 2-(2-hydroxyphenyl)
benzothiazole (HBT) or 2-(2-hydroxyphenyl) benzoxazole
units (HBO) chromophores (7−10, Scheme 1). These dyes
show strong absorption of visible light (ε up to 50000 M−1
cm−1 at 530−580 nm) and intense fluorescence (ΦF up to
36%). Long-lived triplet excited states were observed for 7−10.
Triplet excited states of bromine- or iodine-containing Bodipy
derivatives have been reported.21,23,24 However, population of
the triplet excited state was rarely reported for heavy-atom-free
Bodipy.25a,b On the basis of steady state and time-resolved
spectroscopy, as well as DFT calculations, we propose that no
ESIPT occurs for compounds 7−10.
Importantly, we found that the population of triplet excited
state is not the result of ESIPT.18 The compounds were used as
singlet oxygen (1O2) sensitizers for the photooxidation of 1,5-
dihydroxynaphthalene (DHN).26 These organic triplet photo-
sensitizers are proved more efficient than the conventional
transition metal complex triplet sensitizer, such as [Ir-
(ppy)2(phen)][PF6].26 Organic chromophores with room
temperature long-lived triplet excited states populated upon
photoexcitation are rare, and our studies are useful for
designing new organic triplet sensitizers for applications in
photovoltaics, photocatalysis,26 singlet oxygen (1O2) photo-
sensitizing,21,23,25a,b and upconversion,33,34 etc.
Figure 1. UV−vis absorption of the compounds (in toluene); c = 1.0
× 10−5 M, 20 °C.
new compounds show red-shifted absorption compared to the
unsubstituted Bodipy, and thus the π-conjugation between the
Bodipy core and the HBT and HBO moieties are efficient. It
should be pointed out that the Bodipy and the benzothiazole
moieties are at the meso position to each other, which may
diminish the π-conjugation to some extent. The absorption of
the compounds is similar to that of 2-phenylethynyl-substituted
Bodipy (compound 12), indicating the effect of HBT or HBO
units on the absorption is not significant. With alkylation of the
hydroxyl group, 11 shows slightly blue-shifted absorption
compared to 4. Interestingly, the alkylation of the hydroxyl
moiety does not change the UV−vis absorption dramatically for
the compounds with large π-conjugation framework; for
example, 10 shows absorption similar to that of 7 (Figure 1b).
The compounds 7−10 show emission in the range of 580−
620 nm (Figure 2), which are red-shifted vs HBO and HBT
2. RESULTS AND DISCUSSION
2.1. Design and Synthesis of the Compounds. Our
strategy of molecular design lies in two notions. First, with
extension of the π-conjugation framework of HBT and HBO,
the UV−vis absorption will be red-shifted. Second, we postulate
that population of the T1 excited state upon excitation may be
retained in the new compounds.
Our approach is to connect the HBT or HBO units with the
Bodipy moiety via CC triplet bonds. 4,4-Difluoro-1,3,5,7-
tetramethyl-8-phenyl-4-bora-3a,4a-diaza-s-indacene (Bodipy)
was used as the starting material (Scheme 1). Mono- and bis-
iodination of the Bodipy by N-iodo-succinimide (NIS) gave
compounds 2a and 2b. The ethynylated HBO or the HBT
units were obtained by 5-bromosacilaldehyde through Sonoga-
shira cross-coupling reactions. Target compounds 7−9 were
obtained by Pd(0)-catalyzed Sonogashira coupling of the
respective components (Scheme 1). The effect of the one-
atom substituent (O/S) was investigated with compounds 7
and 8. Compound 10 was prepared as a control compound.
The hydroxyl group in 10 was alkylated, to exclude any ESIPT
effect. All the compounds were obtained in moderate to
satisfying yields.
2.2. Steady-State UV−vis Absorption and Emission
Spectra. The UV−vis absorption of the compounds was
studied (Figure 1a). The ethynylated HBO and HBT show
absorption at 335 and 349 nm, respectively. For the Bodipy-
conjugated chromophores, however, the absorption maximum
are remarkably red-shifted.14,18 For example, the absorption of
7 is at 535 nm (ε = 57000 M−1 cm−1). Compound 8 gives the
same absorption feature, implying that the effect of the singlet
atom substitution of S vs O is small under this circumstance
(Figure 1b). With a larger π-conjugation framework, 9 shows
red-shifted absorption at 580 nm (ε = 40000 M−1 cm−1). The
Figure 2. Fluorescence emission spectra of compounds 4 and 6−12
(in toluene); c = 1.0 × 10−5 M, 20 °C.
(530 nm)16 and the unsubstituted Bodipys (λem = 510−530
nm). Compounds 7, 8, and 9 show small Stokes shifts (ca. 45
nm). On the contrary, 4 and 6 show large Stokes shifts (160−
180 nm), which are typical for ESIPT.2 Thus we propose that
ESIPT occurs for 4 and 6, but not for 7−9. This postulation
was supported by the blue-shifted emission of 11, for which the
hydroxyl group was alkylated, and thus ESIPT is fully excluded.
Compound 11 shows blue-shifted emission (374 nm)
compared to 4 (531 nm). This blue-shifted emission (and
small Stokes shift) is in agreement with the lack of ESIPT for
11. For 4, ESIPT occur upon photoexcitation (see later
section). In stark contrast, 10 show similar emission wavelength
compared to 7. Thus we propose that 7 does not undergo
ESIPT upon photoexcitation.
Compounds 4 and 6 show large Stokes shifts (180 and 160
nm, respectively),35 and low fluorescence quantum yields of
0.002 and 0.027 (Table 2). These values are close to those of
the unsubstituted HBT and HBO (0.006 and 0.02,
respectively).35 For the new Bodipy-based chromophores,
however, the fluorescence quantum yields are much higher
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(0.225, 0.202, and 0.360 for 7−9, respectively). The emissions
of 7−10 were also recorded with excitation at shorter
wavelength (330−360 nm, Figure S25), emission similar to
that with longer excitation wavelength were observed,
indicating that the emission of HBT or the HBO units were
completely quenched in compounds 7−10. The queching may
be caused by intramolecular energy transfer to the Bodipy
moiety.
2.3. Solvent-Dependency of the Emission. In nonprotic
organic solvents, compound 4 shows emission at 530 nm. In
protic solvent such as methanol, emission band at 420 nm was
observed (Figure 3). In toluene, dichloromethane and ethyl
beyond 500 nm are solvent-polarity-dependent (Table 1).37
The keto emission show remarkable red-shift from 520 nm (in
cyclohexane) to 620 nm (in chloroform) with increasing the
solvent polarity. ESIPT for naphthalene fluorophore has been
reported recently.18 Those compounds show very low
Table 1. Photophysical Properties of Compounds 4 and 6−
a
12 in Different Solvents
b
c
d
e
compd
solvents
λabs (nm) λem (nm)
ε
(M−1 cm−1
)
τF (ns)
4
PhCH3
CH2Cl2
THF
349
347
347
346
345
344
335
333
334
332
332
330
535
533
531
529
528
528
535
533
532
530
528
528
580
574
574
571
569
571
536
534
532
531
529
528
333
332
333
331
332
332
537
533
532
530
528
526
531
529/416
530
530/415
467
464
495
493
434/488
493
439
489
584
590
585
583
587
594
582
587
584
580
585
589
621
621
621
615
619
618
581
584
584
582
580
588
374
376
373
372
382
377
569
571
568
566
568
569
13900
14900
14900
14800
14700
14700
18600
19200
19300
19200
18000
18900
56800
56800
58400
58900
57700
55600
53000
52900
55800
54900
55400
53800
40200
35600
40200
37200
34200
33400
40900
44800
44000
43700
42900
43300
17800
17500
17900
18000
22400
20000
58300
57400
57400
57700
56500
57000
1.65
3.59
3.46
4.24
3.85
3.75
1.30
6.37
3.58
3.91
4.15
3.70
3.02
1.65
1.85
1.97
0.68
0.57
3.12
1.92
2.28
2.30
1.09
1.07
2.29
2.07
2.11
2.20
0.96
0.73
3.40
1.93
2.29
2.42
1.34
1.18
f
EtOAc
MeOH
CH3CN
PhCH3
CH2Cl2
THF
6
EtOAc
MeOH
CH3CN
PhCH3
CH2Cl2
THF
7
EtOAc
MeOH
CH3CN
PhCH3
CH2Cl2
THF
8
Figure 3. Fluorescence emission spectra of (a) 4 (λex = 370 nm), (b)
11 (λex = 320 nm), (c) 7 (λex = 480 nm), and (d) 10 (λex = 510 nm) in
different solvents. In order to demonstrate the relative fluorescence
intensity, the emission spectra were not normalized. c = 1.0 × 10−5 M,
20 °C.
EtOAc
MeOH
CH3CN
PhCH3
CH2Cl2
THF
9
EtOAc
MeOH
CH3CN
PhCH3
CH2Cl2
THF
acetate, dual emission bands were observed. Similar emission
profiles were found for 6 (Figure S24a), except that the
emission bands are slightly blue-shifted. These emission
properties are similar to HBT. Therefore, the emission of 4
and 6 at 530 nm can be assigned to the radiative decay of the
excited cis keto tautomer, whereas the emission at 420 nm can
be assigned to the excited enol tautomer.1,2 The emission of 11
confirmed these postulations (Figure 3b). With the hydroxyl
group alkylated, 11 shows the same emission in both protic and
aprotic solvents, indicating the absence of ESIPT.
The emissions of 7−10 were also studied (see Figure S24 for
the emission of 8 and 9). Different from 4 and 6, the emission
bands of 7−9 are not sensitive to the switch from aprotic to
protic solvents. Thus, we propose that 7−9 do not undergo
ESIPT. The lack of the ESIPT effect for 7−9 is interesting.
Previously it was proposed that π-conjugation can impart
significant effect on ESIPT of the fluorophores, but the
evidence is rare.19,36 It should be pointed out that the
fluorescence emission of unsubstituted Bodipy is not sensitive
to the polarity of the solvents (see Figure S24d Supporting
Information). Compound 12 shows similar emission (Figure
S24e) compared with unsubstituted Bodipy 1.
10
11
12
EtOAc
MeOH
CH3CN
PhCH3
CH2Cl2
THF
EtOAc
MeOH
CH3CN
PhCH3
CH2Cl2
THF
4.28
3.69
3.94
3.97
2.91
2.97
EtOAc
MeOH
CH3CN
a
The excitation wavelength for 4, 6, 11, 7−10, and 12 were 370, 360,
320, 480, 490, 490, 510, and 490 nm, respectively. 1.0 × 10−5 M, 20
Previously HBT- or HBO-based chromophores with a large
π-conjugation framework were prepared (the extended π-
conjugation is on the benzoxazole unit, different from 7−9),
which show ESIPT.14,37 Furthermore, the keto emission at
b
c
°C. Absorption wavelength. Fluorescence emission wavelength.
d
e
f
Molar extinction coefficient. Fluorescence lifetimes. Not deter-
mined.
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Figure 4. Nanosecond time-resolved transient difference absorption spectra of 7 after pulsed laser excitation (λex = 532 nm) in deaerated toluene. c =
2.0 × 10−5 M; 20 °C. (a) Transient absorption spectra. Inset: decay trace at 550 nm. (b) Calculated absorption spectrum of the triplet excited state
of 7. (c) Calculated absorption spectrum of the putative trans keto form of 7. Toluene was used as solvent in the DFT calculations. Note the
bleaching cannot be predicted by the calculations. Calculated at the DFT//B3LYP/6-31G(d) level with Gaussian 09W.
fluorescence quantum yields and large Stokes shift, which are
typical for ESIPT chromophores.18 The absence of ESIPT for
7−9 supported the recent report that the ESIPT can be
eliminated by π-conjugation variations.19
2.4. Nanosecond Time-resolved Transient Difference
Absorption Spectra. Time-resolved spectroscopy was used
for study of transient species of ESIPT dyes upon photo-
excitation.12,18 For some compounds, triplet excited state was
observed,18 and in other cases the keto forms were observed;
both show slow decay kinetics.18 The nanosecond time-
resolved TA spectroscopy was used to study the possible
population of the triplet excited state of the new compounds
(Figure 4).
Upon pulsed 532 nm laser excitation, bleaching at 535 nm
was observed for 7 (Figure 4a), which indicates depletion of the
ground state. TA bands at the 415−480 nm and 584−800 nm
range were observed. The significant bleaching is drastically
different from the normal ESIPT chromophores.18 Fitting of
the decay of the bleaching at 550 nm gives the lifetime of the
transient species as 194 μs. Under aerated condition, however,
this transient was significantly quenched (τT = 0.32 μs; Figure
S26a). On the basis of these results, we propose that the
transient species observed are due to the triplet excited state of
7, not the keto tautomers.18 Furthermore, the Bodipy unit in 7
contributes significantly to the T1 excited state, indicated by the
remarkable bleaching at 535 nm. This observation is different
from that of the HBT. Previously it was shown that the
transient species of HBT is due to the keto tautomers, not the
triplet excited state.17
In order to prove the transient of 7 is due to the triplet
excited state and not the keto tautomers, the TA of the triplet
excited state of 7 was calculated (Figure 4b). The calculated TA
bands are located at 436 and 653 nm, which are in good
agreement with the experimental results at 441 and 658 nm
(Figure 4a). Furthermore, the absorption of the putative keto
form of 7 was also calculated (Figure 4c), which is substantially
different from the experimental results (Figure 4a). Thus, the
transient absorption observed for 7 is due to its triplet excited
states. Similar transient signals were observed for 8 (see Figure
S28). The triplet excited state lifetime of 8 was determined as
195 μs. The transient can be significantly quenched by O2 (τ =
0.33 μs in aerated solution).
than that of 7. Absorption in the UV range and a short triplet
excited state lifetime are detrimental for the potential
applications of the compounds.42 Previously, a much shorter
triplet excited state lifetime was found for the unsubstituted
HBO (τT = 10.9 μs).10 In another study the TA spectra of
HBO gives a lifetime of 2.5 μs, which cannot be quenched by
O2. Thus the transient of HBO is attributed to the ESIPT
produced keto form, not the triplet excited state.16 Our results
show that by attaching the HBO moiety to the Bodipy
chromophore, the triplet excited state lifetime is greatly
extended. No significant TA was observed for the unsubstituted
Bodipy. On the other hand, observation of the triplet excited
state of the Bodipy chormophore is rare. Previously the triplet
excited states of Bodipy were observed with heavy atom effect
of iodine21,23a,24a,b and Pt(II) atoms.24c Very few heavy-atom-
free Bodipy chromophores were reported to show the triplet
excited states populated.24d,25a
TA spectra of 9 were also studied (Figure 5). Significant
bleaching at 560 nm and TA bands at 450 nm and in the range
Figure 5. Nanosecond time-resolved transient difference absorption of
9 and 10 after pulsed laser excitation (λex = 532 nm). (a) Transient
absorption of 9 and decay trace at 570 nm (inset). (b) Transient
absorption of 10 and decay trace at 550 nm (inset). In deaerated
toluene. c = 2.0 × 10−5 M; 20 °C.
of 620−800 nm were observed. The lifetime of the transient
species determined by following the bleaching at 570 nm is 121
μs. In aerated solution, the lifetime was greatly reduced to 0.51
μs (Figure S26b). Thus the transient species observed were
attributed to the triplet excited states.18 Similar to 7 and 8, the
bleaching of 9 is related to the depletion of the ground state of
the Bodipy chromophore.
Upon pulsed laser excitation, significant bleaching at 536 nm
was observed for 10 (Figure 5b). Concurrently, TA bands at
405−470 nm and 580−800 nm were observed (Figure 5b).
These transients are sensitive to O2 (τT = 0.31 μs in aerated
solution. Figure S26c), and thus the transients are due to triplet
excited state, not the keto tautomers,18 which was confirmed by
the calculated TA spectrum. DFT calculations predict transient
absorptions at 436 and 658 nm (Figure S29b), which are in
Population of the triplet excited states of organic
chromophores are important, especially for those showing
intense absorption of visible light.24,25a,b,33,34,38−43 Previously
population of the triplet excited states were observed for
naphthalene-derived ESIPT chromophores,18 but the absorp-
tions of the compounds were in the UV range (<378 nm) and
the triplet excited state lifetimes are much shorter (τT = 1.4 μs)
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good agreement with the experimental results at 445 and 630
nm (Figure 5b). Since the ESIPT is completely prohibited, thus
the population of the triplet excited state of 10 is not due to
ESIPT and formation of keto tautomer. This conclusion is
different from previous studies.12,18
Table 2. Photophysical Properties of Compounds 4 and 6−
11
a
b
c
compounds
ΦF
τT (μs)
τT (μs)
4
0.002
0.027
0.225
0.202
0.360
0.223
0.111
0.55
0.52
6
We noticed that the lifetimes of the triplet excited states of
7−10 are significantly longer than the triplet excited state
observed for a ESIPT chromophore (72 μs, 180 K).12
Previously naphthalene derivatized ESIPT dyes were re-
ported.18 The nanosecond transient difference absorption
spectra show that the chromophores undergo ESIPT upon
photoexcitation, that is, keto form was produced. Transient of
the keto form (which is not sensitive to O2) and the triplet
excited state was observed (which can be quenched by O2).18
However, the lifetime of the triplet excited state is much shorter
(τT < 2.0 μs) than that of 7−10.
7
194
195
121
136
34.1
0.32
0.33
0.51
0.31
0.36
8
9
10
11
a
Fluorescence quantum yields in THF, with compound 1 as the
b
standard (Φ = 0.720 in THF). 1.0 × 10−5 M, 20 °C. Triplet state
c
lifetimes, 1.0 × 10−5 M in deaerated toluene. Triplet state lifetimes,
2.0 × 10−5 M in air-saturated toluene. (λex = 532 nm). Compound 6
showed no signal in transient absorption.
The TA of 4 is drastically different from that of 7 (Figure 6).
For example, the bleaching of 4 is at 350 nm, with transients at
lifetime of the triplet state was determined as 34.1 μs in
deaerated solution, which is significantly reduced to 0.36 μs in
aerated solution. DFT calculation predicted transient absorp-
tion at 422 nm, which is in good agreement with the
experimental results at 426 nm (Figure S27). Thus the
production of the triplet excited state of the compounds
described in this paper is not related to ESIPT process.
To the best of our knowledge, the triplet excited state
lifetime of the derivatives are the longest observed for Bodipy
derivatives. Previously triplet excited state lifetime of 66.3−
123.2 μs were observed for Bodipy derivatives,24,43h and herein
the lifetime was extended to 136−95 μs (Table 2).
The emission of compound 7 at 77 K was studied (Figure
S32). Compared to the emission at room temperature, a new
emission band at 630 nm was observed, which is tentatively
assigned to the phosphorescence of 7. Unfortunately the
emission is too weak for accurate determination of the lifetime.
However, the closely lying S1 and T1 states may account for the
population of the triplet excited state of compound 7.
2.5. Spin Density of the Triplet States of the
Compounds. In order to study the triplet excited state of
the compounds from a theoretical perspective, the spin density
surfaces were analyzed by DFT calculations (Figure 7).24,41,43
For 7−10, the spin density surfaces are localized on the
Bodipy moieties. The HBT or the HBO subunits contribute
very little to the spin density (Figure 7). This result is in
agreement with the nanosecond time-resolved transient
difference absorption spectra of these compounds (Figures
Figure 6. Nanosecond time-resolved transient difference absorption of
4 after pulsed laser excitation (λex = 355 nm). (a) Transient absorption
with different delay times. (b) Transient absorption with different
delay times. In aerated toluene. 2.0 × 10−5 M; 20 °C. Calculated UV−
vis absorption of the (c) cis keto tautomer and (d) the triplet excited
state of 4. Toluene was used as solvent in the calculations. Calculated
at the B3LYP/6-31G(d) level with Gaussian 09W.
488 nm. Furthermore, a short lifetime of 0.55 μs was observed.
The transient lifetime is 0.52 μs in aerated solution (Figure 6b).
The large Stokes shift of 4 (ca. 180 nm) and the low
fluorescence quantum yields (Table 2) propose that ESIPT
exists for 4, which is similar to that of HBT. In addition, the
DFT calculation predicted absorption of keto form of 4 at 441
nm (experimental result is at 488 nm, Figure 6c). The
calculated absorption of the triplet excited state (Figure 6d) is
not in agreement with the experimental results.
Extension of the π-conjugation framework of 4 changes its
photophysical property completely. For example, 7 does not
show the ESIPT effect, but the triplet excited state of 7 was
populated upon photoexcitation. These properties are in stark
contrast to HBT or 4. No TA signals were observed for 6 either
in deaerated solution or in aerated solution. The lack of
transient signal of 6 is due to the absence of the triplet excited
state and the trans keto tautomer.
Figure 7. Isosurfaces of spin density of compounds 7−11 at the
optimized triplet state geometries. Toluene was used as solvent in the
calculations. Calculation was performed at the B3LYP/6-31G(d) level
with Gaussian 09W.
Compound 11 was also studied (Figure S27). Triplet excited
state was populated for 11 upon photoexcitation, and the
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3−6). Furthermore, the bleaching observed in the transient
absorption spectra of 7−9 is due to the depletion of the ground
state of the Bodipy units, which is supported by the spin density
distributions (Figure 7).
Table 3. Selected Electronic Excitation Energies (eV) and
Corresponding Oscillator Strengths (f), Main
Configurations, and CI Coefficients of the Low-Lying
a
Electronically Excited States of Compound 7
The spin density surface of 11 is localized in the
benzothioazole moiety, which is drastically different from the
spin density surfaces of 7−10. The nanosecond time-resolved
transient difference absorption spectrum of 11 (Figure S27) is
drastically different from that of 7−10. Thus our assignment of
the Bodipy moiety localized triplet excited states for 7−10 is
fully supported by the spin density analysis of these compounds
and the control compound 11.
2.6. DFT Calculations on the UV−vis Absorption and
Fluorescence Emission. Recently DFT calculations have
been used for study of photophysical properties of
fluorophores.44−51 Previously we proposed to use the concept
of “electronic state”, rather than the conventional approx-
imation of “molecular orbitals”, to study the emission of
fluorescent molecular probes.46a,47 We also used the concept of
“dark state” to rationalize the fluorescence OFF−ON switching
effect of thiol probes,47 as well as the d-PET mechanism of the
boronic acid molecular probes.46
TDDFT//B3LYP/6-31G(d)
electronic
b
c
d
e
transition
energy
f
composition
CI
S0→S1
2.33 eV/532 nm
3.02 eV/410 nm
0.5965
0.5503
H→L
0.6935
0.1176
0.1227
0.6746
0.1516
0.6827
0.1011
0.2234
0.4165
0.1156
0.4775
0.7014
H−1→L
H→L
S0→S3
H−1→L
H→L+1
S0→S6
3.65 eV/340 nm
3.99 eV/311 nm
0.2027
0.8755
H−1→L+1
H→L+2
S0→S11
H−5→L
H−2→L+1
H−1→L+1
H→L+2
S1→S0
1.91 eV/648 nm
0.3900
H→L
a
Calculated by TDDFT//B3LYP/6-31G(d), based on the DFT//
b
B3LYP/6-31G(d) optimized ground state geometries. Only the low-
lying excited states and some allowed transitions are presented.
The ground state geometry of 7 was optimized (Figure 8).
The Bodipy and HBT moiety take coplanar geometry and thus
c
d
Oscillator strength. H stands for HOMO, and L stands for LUMO.
e
Only the main configurations are presented. CI coefficients are in
absolute values.
S1 state geometry (Figure 8). For most fluorophores, S1 excited
state is involved in fluorescence (Kasha’s rule).53 With
optimization of the S1 state geometry, the energy level of the
LUMO decreased from −2.56 to −2.73 eV. Conversely, the
energy level of HOMO promoted from −5.23 to −5.01 eV.
The calculated emission at 648 nm is reasonably close to the
experimental results of 584 nm (Figure 1). The DFT
calculations indicate that the benzothiazole moiety is not
involved in the frontier molecular orbitals of the absorption and
emission transitions, which can be used to rationalize the lack
of ESIPT effect for the dyes.19
The same theoretical calculations were applied to 4, which
was proved to undergo ESIPT. The electron density on the
hydroxyl group decreased, whereas the electron density on the
N atom of benzothiazole moiety increased upon photo-
excitation (Figure 9). This electron density change indicates
that both the acidity of the hydroxyl group and the basicity of N
atom will increase upon excitation, which coincides with the
occurrence of ESIPT.19 The excited states geometries of both
the enol and keto of 4 were calculated (Table 4). The emission
wavelength of the enol form was calculated as 410 nm, which is
in good agreement with the experimental results of 414 nm
(Figure 3). The emission of the keto tautomer was calculated as
515 nm, which is close to the experimental results (531 nm.
Figure 3).
2.7. Energy Barrier for the Intramolecular Proton
Transfer at the Excited State and Ground State. In order
to explain the different ESIPT properties of the compounds,
the potential energy curves of the intramolecular proton
transfer (i.e., the transformation from the enol form to the keto
form) at both the ground state and the excited state were
studied (Figure 10, Table 5, and Supporting Information).
Generally, for the compounds that show the ESIPT effect, there
is substantial free energy decrease (ΔE) for the transformation
from the enol form to the keto form at the S1 state (compounds
4 and 6). Conversely, for the compounds lacking ESIPT, the
Figure 8. Selected frontier molecular orbitals involved in the excitation
and emission of compound 7. CT stands for conformation
transformation. The calculations are based on the optimized ground
state geometry (S0 state, for the excitation) and excited state geometry
(S1 state, for the emission) at the B3LYP/6-31G(d)/ level using
Gaussian 09W.
ensure a full π-conjugation between the two units. On the basis
of the optimized ground state geometry, the UV−vis
absorption of 7 was calculated by TDDFT (Franck−Condon
principle, Table 3). The calculated excitation wavelength for the
S0→S1 transition is 532 nm, which is very close to the
experimental result (535 nm, Figure 1). HOMO is distributed
on the Bodipy-ethynylphenyl moiety, whereas the LUMO is
localized on Bodipy unit. The benzothiazole moiety, especially
the N atom, is not involved in HOMO and LUMO. As a result,
the basicity of the N atom in the benzothiazole moiety does not
increase upon photoexcitation. Thus, the absence of
benzothiazole moiety in the frontier molecular orbitals may
be responsible for the lack of ESIPT of 7.19,52
The geometry of the S1 state of 7 was optimized, and the
fluorescence wavelength was calculated based on the optimized
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Table 5. Free Energy Changes for the Transformation from
Enol Form to Keto Form of the Compounds at the Excited
State and Ground State
a
b
compound
ΔE (kJ mol−1
)
ΔE (kJ mol−1
)
4
6
7
8
9
9.7
11.6
9.7
−52.7
−52.4
−14.5
−13.5
−6.6
11.6
9.3
a
b
At S0 (ground state) state. At S1 state.
observed ESIPT of 4.54 For 7, however, the ΔE value at the
ground state and excited states are 9.7 and −14.5 kJ mol−1,
respectively (Figure 10b). The small energy decrease of
compound 7 at S1 state indicated that the driving force for
the ESIPT of 7 at the S1 state is smaller than that of compound
4. Similar ΔE value profiles were found for the compounds that
do not undergo ESIPT (Table 5 and Figure S31).
2.8. Application of the Long-lived Triplet Excited
State and the Visible Light Harvesting Effect: Singlet
Oxygen (1O2) Photosensitizer for Photooxidation. To
date most of the triplet photosensitizers are transition metal
complexes or porphyrin derivatives.23,34,55 Organic triplet
photosensitizers (heavy-atom-free) are rarely repor-
ted.22,24d,25a,34,39,53 Since 7−10 show intense absorption of
visible light (ε = 50000 M−1 cm−1 at 530−580 nm) and long-
lived triplet excited states (τT > 100 μs at RT), these
compounds were used as triplet photosensitizers to initiate
photophysical processes.
Herein as a proof-of-concept, compounds 7−10, in which no
heavy atoms are presented (Scheme 1), were used as singlet O2
(1O2) photosensitizer for photooxidation with 1,5-dihydrox-
ylnaphthalene (DHN) as the substrate, to produce juglone
(Scheme 2).26 Previously Ir(III) complex ([Ir(ppy)2(phen)]-
[PF6]) was successfully used as 1O2 sensitizer for photo-
oxidation of DHN.26 However, the conventional Ir(III)
complexes show very weak absorption of visible light.56
Furthermore, these transition metal complex triplet sensitizers
are not cost-efficient. Organic triplet sensitizers are ideal
alternatives to replace these expensive transition metal complex
triplet photosensitizers.
Figure 9. Selected frontier molecular orbitals involved in the vertical
excitation and the singlet excited state (S1) (both enol form and keto
form were calculated) of compound 4. CT stands for conformation
transformation. The calculations are based on the optimized ground
state geometry (S0 state, excitation) and exited state geometry (S1
state, emission) at the B3LYP/6-31G(d)/ level using Gaussian 09W.
Table 4. Selected Electronic Excitation Energies (eV) and
Corresponding Oscillator Strengths (f), Main
Configurations, and CI Coefficients of the Low-Lying
a
Electronically Excited States of Compound 4
TDDFT//B3LYP/6-31G(d)
electronic
b
c
d
e
singlet
transition
energy
f
composition
CI
(UV−
S0→S1
3.47 eV/357 0.3677
nm
H→L
0.6959
vis)
S0→S2
4.13 eV/300 0.2553
nm
H−2→L
H−1→L
H→L
0.3376
0.6000
0.7008
(FL)-
enol
S0→S1
S0→S1
3.03 eV/410 0.3519
nm
(FL)-
keto
2.41 eV/515 0.3431
nm
H→L
0.7076
a
Calculated by TDDFT//B3LYP/6-31G(d). FL stands for fluores-
b
cence. Only selected low-lying excited states are presented.
c
d
Oscillator strength. H stands for HOMO and L stands for
e
LUMO. Only the main configurations are presented. CI coefficients
The principle of the photooxidation of DHN is outlined in
Scheme 2.26 Upon photoexcitation of the triplet photo-
sensitizers, the singlet excited state of the sensitizers was first
populated (e.g., S0→S1). Then with ISC effect, the triplet
excited states (e.g., T1) of the sensitizers are populated.
Collision between the sensitizers at the triplet excited state with
the O2 molecules (for which the ground state is the triplet state,
3O2) imparts triplet−triplet energy transfer (TTET), and thus
are in absolute values.
1
1
singlet O2 was produced. DHN is oxidized by O2 to give an
intermediate peroxide and then to produce the final product of
juglone. The progress of photooxidation of DHN can be
followed by the decreasing of the UV−vis absorption at 301
nm.26
Figure 10. Potential energy curves from enol form to keto form of the
compounds 4 (a) and 7 (b) at the excited state and ground state. The
calculations are based on the optimized ground state geometry (S0
state) at the B3LYP/6-31G(d)/ level using Gaussian 09W.
1
Compounds 7−10 were used as O2 photosensitizer for
photooxidation of DHN (Figure 11). The performance of these
organic sensitizers was compared with the model triplet
photosensitizer ([Ir(ppy)2(phen)][PF6]) (Ir-1). For 8 (Figure
S33a), the UV−vis absorption of the sensitizer/DHN solution
upon photoirradiation does not change significantly. For 9,
however, the change is more remarkable, with decreased
absorption at 301 nm and the development of the new
ΔE values are much smaller and are similar to those at ground
states (Table 5). For example, the ΔE value of 4 is 9.7 kJ/mol
at S0 state but changes to −52.7 kJ/mol at S1 state (Figure 10a).
This energy changing profile at excited state coincides with the
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Scheme 2. Mechanism for the Photooxidation of DHN with a Triplet Photosensitizer; BDP = Bodipy Compounds
Table 6. Pseudo-first-order Kinetics Parameters for
Compounds 7−10 and the Control Compounds Ir-1 and
TPP
a
b
c
compound
kobs (min−1
)
νi
yield (%)
7
0.8
2.2
1.6
4.4
18.1
0.2
8
9
9.8
19.6
5.2
59.2
25.8
33.5
89.1
10
Ir-1
TPP
2.6
4.3
8.6
25.2
50.4
a
Pseudo-first-order rate constant kobs was calculated by the rule: ln(Ct/
b
C0) = −kobst. In 10−3 min−1. Initial consumption rate of DHN, νi =
c
kobs[DHN]. In 10−7 M min−1. Yield of juglone after reaction for 50
min.
1
that the efficient O2 production with 9 may be due to its
efficient ISC property. Two benzothiazole units exist in this
compound, which is different from other compounds. The
performance of the organic triplet sensitizers was compared
with meso-tetraphenylporphine (TPP, Figure 11d; see Figure
S33c for the results of TPP).57 We found that the performance
of 9 is close to TPP. The yield of juglone against the irradiation
time was also studied (Figure 12).
2.9. Conclusions. In summary, new Bodipy fluorophores
conjugated with 2-(2-hydroxyphenyl) benzothiazole (HBT)/
benzoxazole (HBO) units were prepared, with the intention to
access ESIPT compounds that give red-shifted absorption
compared to the conventional ESIPT chromophores such as
HBT and HBO, which show absorption in the UV or blue
range. The new compounds show much red-shifted absorption
(530−580 nm) compared to HBT and HBO (ca. 350 nm).
Interestingly, the new compounds show small Stokes shifts (45
nm) and high fluorescence quantum yields (ΦF >20%), which
are in stark contrast to the typical features of the conventional
ESIPT dyes (Stokes shift >100 nm and ΦF < 1%). By
comparison with the compounds that with the hydroxyl group
Figure 11. Absorption spectra for the photooxidation of DHN using
the BODIPY derivatives as the triplet sensitizers. Compounds (a) 7,
(b) 9, and (c) Ir-1. (d) Plots of ln(At/A0) vs irradiation time (t) for
the photooxidation of DHN with different triplet photosensitizers. c
[sensitizers] = 2.0 × 10−5 M, c [DHN] = 2.0 × 10−4 M, in CH2Cl2/
MeOH (9:1, v/v), 20 °C.
absorption band of juglone at 427 nm. Similar changes were
observed with Ir-1 (Figure 11c). The UV−vis absorption of
these organic triplet photosensitizers are much red-shifted
compared to that of the model Ir(III) complexes.26
The performance of the sensitizers can be quantified by
plotting the absorption variation (ln(At/A0)) against the
irradiation time (Figure 11d). It is clear that 9 is the most
efficient triplet photosensitizer. The rate constant kobs is 9.8 ×
10−3 min−1, which is more than 2-fold that of of Ir-1 (kobs = 4.3
× 10−3 min−1). Ir-1 was used as 1O2 photosensitizer for
photooxidation of DHN (Table 6).26 Thus our results show
that organic triplet photosensitizers could be more efficient
than the traditional transition metal complexes. We propose
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3.4. 2-(5′-Ethnyl-2′-butoxyphenyl)benzothiazole (11). A
mixture of 4 (200.0 mg, 0.8 mmol), K2CO3 (200.0 mg, 1.4 mmol),
and dimethyl formamide (1 mL) was stirred at rt for 10 min, and then
n-butyl bromide (0.4 mL, 3.7 mmol) was added. The mixture was
stirred at 50 °C for 4 h and then poured into 300 mL of ice−water
mixture. The product was extracted with CH2Cl2 (9 × 30 mL), and the
organic layers were dried over Na2SO4. After removal of solvent under
reduced pressure, the residue was purified with column chromatog-
raphy (silica gel, CH2Cl2/petroleum ether, 1:1, v/v) to give 211.0 mg
1
(yield 86%) of a white powder. Mp 134.2−135.4 °C. H NMR (400
MHz, CDCl3) δ 8.73 (s, 1H), 8.11 (d, 1H, J = 8.0 Hz), 7.95 (d, 1H, J =
8.0 Hz), 7.56 (d, 1H, J = 8.8 Hz), 7.52 (t, 1H, J = 7.2 Hz), 7.40 (t, 1H,
J = 7.2 Hz), 7.00 (d, 1H, J = 8.8 Hz), 4.24−4.21 (m, 2H), 3.04 (s, 1H),
1.99−2.04 (m, 2H), 1.61−1.66 (m, 2H), 1.06 (t, 3H, J = 7.2 Hz). 13C
NMR (100 MHz, CDCl3) δ 162.2, 157.0, 152.1, 136.3, 135.4, 133.6,
126.2, 125.0, 123.0, 122.4, 121.4, 114.9, 112.3, 83.1, 76.6, 69.4, 31.3,
19.7, 14.1. TOF HRMS EI+: calcd [C19H17NOS]+ m/z 307.1031,
found m/z 307.1031.
Figure 12. Plots of chemical yield as a function of irradiation time for
the photooxidation of DHN using 7−10, Ir-1, and TPP as sensitizers.
Blank experiment without any sensitizer is also presented (without
sens.). c [sensitizers] = 2.0 × 10−5 M, c [DHN] = 2.0 × 10−4 M, in
CH2Cl2/MeOH (9:1, v/v), 20 °C.
3.5. 2-(5′-Ethnyl-2′-hydroxyphenyl)benzoxazol (6). Under Ar
atmosphere, compound 5 (120.0 mg, 0.4 mmol), PdCl2(PPh3)2 (8.4
mg, 0.012 mmol), PPh3 (5.2 mg, 0.02 mmol), and CuI (3.8 mg 0.02
mmol) were dissolved in triethylamine (12 mL). After stirring,
ethynyltrimethylsilane (1.5 mL, 0.69 mmol) was added via syringe.
The solution was stirred at 80 °C for 4 h. After removal of solvent
under reduced pressure, the residue was purified with column
chromatography (silica gel, CH2Cl2/petroleum ether, 1:1, v/v) to
give 89.0 mg (yield 70%) of a white powder. Then the mixture of the
white powder (89.0 mg, 0.3 mmol), K2CO3 (800.0 mg, 6 mmol),
CH2Cl2 (3 mL), and CH3OH (7 mL) was stirred at rt for 6 h. The
yellow solution was washed with water. The aqueous phase was
extracted with CH2Cl2. After removal of solvent under reduced
pressure, the residue was purified with column chromatography (silica
gel, CH2Cl2/petroleum ether, 1:1, v/v) to give 34.0 mg (yield 50%) of
alkylated, we propose that the new fluorophores do not undergo
ESIPT upon photoexcitation. Nanosecond time-resolved
transient difference absorption spectroscopy demonstrated
that long-lived triplet excited states (τT is up to 195 μs) were
populated for the dyes 7−9 upon photoexcitation, as well as for
those with the hydroxyl groups alkylated (10, ESIPT is
completely prohibited). This result is different from a previous
report that the triplet excited state is the result of ESIPT. The
triplet excited states are localized on the Bodipy moiety, proved
by the time-resolved transient absorption spectra and the spin
density analysis of the triplet states. The compounds were used
as organic triplet photosensitizers for production of singlet
oxygen (1O2), demonstrated with the photooxidation of 1,5-
dihydroxynaphthalene (DHN). One of the dyes is 2-fold more
efficient for photooxidation than the conventional Ir(III)
complex triplet sensitizer ([Ir(ppy)2(phen)[PF6]2). Our result
will be useful for design of new ESIPT compounds, as well as
organic triplet photosensitizers that show the predetermined
intersystem crossing (ISC) ability and for their applications in
photovoltaics, photocatalysis and upconversions, etc.
1
white powder. The overall yield was 35.0%. Mp 144.6−145.7 °C. H
NMR (400 MHz, CDCl3) δ 8.20 (s, 1H), 7.75 (d, 1H, J = 5.6 Hz),
7.62 (d, 1H, J = 5.2 Hz), 7.56 (d, 1H, J = 8.4 Hz), 7.39−7.42 (m, 2H),
7.09 (d, 1H, J = 8.8 Hz), 3.05 (s, 1H). 13C NMR (100 MHz, CDCl3) δ
162.1, 159.1, 149.2, 139.9, 137.1, 136.3, 131.2, 129.5, 126.0, 125.3,
119.6, 117.9, 113.6, 82.9, 76.4. TOF HRMS EI+: calcd [C15H9NO2]+
m/z 235.0633, found m/z 235.0637.
3.6. Compound 7. Under Ar atmosphere, compound 2a (80.0 mg,
0.18 mmol), 4 (45.0 mg, 0.18 mmol), Pd(PPh3)4 (20.8 mg, 0.018
mmol), and CuI (1.7 mg 0.01 mmol) were dissolved in triethylamine
(2 mL) and THF (10 mL). The solution was stirred at 60 °C for 6 h.
After removal of solvent under reduced pressure, the residue was
purified with column chromatography (silica gel, CH2Cl2/n-hexane,
1:1, v/v) to give 41.0 mg (yield 40%) of a dark red powder. Mp > 250
°C. 1H NMR (400 MHz, CDCl3) δ 8.01 (d, 1H, J = 8.0 Hz), 7.93 (d,
1H, J = 8.0 Hz), 7.79 (s, 1H), 7.51−7.54 (m, 4H), 7.43−7.47 (m, 2H),
7.29−7.31 (m, 2H), 7.07 (d, 1H, J = 8 Hz), 6.04 (s, 1H), 2.73 (s, 3H),
2.59 (s, 3H), 1.54 (s, 3H), 1.41 (s, 3H). 13C NMR (100 MHz, CDCl3)
δ 168.7, 158.0, 157.8, 156.7, 151.8, 144.9, 142.8, 142.2, 135.8, 134.9,
132.8, 132.6, 131.3, 130.5, 129.4, 128.1, 127.0, 126.0, 122.4, 122.3,
121.8, 118.4, 117.0, 115.3, 115.0, 95.0, 81.2, 14.9, 14.7, 13.8, 13.4. TOF
HRMS ES+: calcd [C34H26BF2N3OS + H]+ m/z 574.1936, found m/z
574.1943.
3. EXPERIMENTAL SECTION
3.1. General Methods. Fluorescence lifetimes were measured
with a OB920 phosphorescence/fluorescence lifetime spectrometer
(Edinburgh Instruments, U.K.). Nanosecond time-resolved transient
difference absorption spectra were measured with a LP920 Laser Flash
Photolysis Spectrometer (Edinburgh Instruments, U.K.), and the data
were recorded on a Tektronix TDS 3012B oscilloscope. The lifetime
values (by monitoring the decay trace of the transients) were obtained
with the LP900 software.
3.2. 1,3,5,7-Tetramethyl-8-phenyl-4,4-difluoroboradiazain-
dacene (1).58a 2-iodo-1,3,5,7-tetramethyl-8-phenyl-4,4-difluorobora-
diazaindacene (2a),24 2,6-diiodo-1,3,5,7-tetramethyl-8-phenyl-4,4-di-
fluoro-boradiazaindacene (2b),58a 5-ethynyl-2-hydroxybenzaldehyde
(3),58b and 2-benzoxazol-2-yl-4-bromo-phenol (5)58c were synthesized
according to literature methods.
3.7. Compound 8. Under Ar atmosphere, compound 2a (75.0 mg,
0.17 mmol), compound 6 (45.0 mg, 0.19 mmol), Pd(PPh3)4 (19.6 mg,
0.017 mmol), and CuI (1.7 mg 0.01 mmol) were dissolved in
triethylamine (2 mL) and THF (10 mL). The solution was stirred at
60 °C for 6 h. After removal of the solvent under reduced pressure, the
residue was purified with column chromatography (silica gel, CH2Cl2/
n-hexane, 1:1, v/v) to give 9.5 mg (yield 10%) of a dark red powder.
3.3. 2-(5′-Ethnyl-2′-hydroxyphenyl)benzothiazole (4). A
mixture of 3 (300.0 mg, 2.1 mmol), 2-aminothiophenol (500.0 mg,
4.0 mmol), and methanol (20 mL) was stirred at room temperature
(rt) for 12 h. After removal of the solvent under reduced pressure, the
residue was purified with column chromatography (silica gel, CH2Cl2/
petroleum ether, 1:1, v/v) to give 155.0 mg of a white powder. Yield:
1
1
Mp > 250 °C. H NMR (400 MHz, CDCl3) δ 8.13 (s, 1H), 7.75 (d,
30%. Mp 143.7−144.6 °C . H NMR (400 MHz, CDCl3) δ 8.02 (d,
1H, J = 5.6 Hz), 7.63 (d, 1H, J = 3.6 Hz), 7.50−7.52 (m, 4H), 7.39−
7.41 (m, 2H), 7.29−7.31 (m, 2H), 7.09 (d, 1H, J = 8.4 Hz), 6.04 (s,
1H), 2.73 (s, 3H), 2.59 (s, 3H), 1.53 (s, 3H), 1.41 (s, 3H). 13C NMR
(100 MHz, CDCl3) δ 162.3, 158.6, 157.8, 156.7, 149.4, 144.8, 142.8,
142.2, 140.0, 136.6, 134.9, 130.1, 129.4, 128.1, 125.9, 125.4, 122.3,
1H, J = 8.4 Hz), 7.95 (d, 1H, J = 8.0 Hz), 7.87 (s, 1H), 7.43−7.55 (m,
3H), 7.08 (d, 1H, J = 8.4 Hz), 3.07 (s, 1H). 13C NMR (100 MHz,
CDCl3) δ 168.4, 158.5, 152.0, 136.3, 132.7, 132.4, 127.0, 126.0, 122.4,
121.7, 118.4, 116.9, 113.3, 83.0, 76.5. TOF HRMS EI+: calcd
[C15H9NOS]+ m/z 251.0405, found m/z 251.0408.
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Article
120.0, 118.0, 115.2, 110.0, 95.1, 81.2, 15.0, 14.7, 13.8, 13.4. ESI-HRMS
LD+: calcd [C34H26BF2N3O2]+ m/z 557.2086, found m/z 557.2119.
3.8. Compound 9. Under Ar atmosphere, compound 2b (40.0 mg,
0.07 mmol), 4 (70.0 mg, 0.28 mmol), Pd(PPh3)4 (8.1 mg, 0.007
mmol), and CuI (0.8 mg 0.004 mmol) were dissolved in triethylamine
(3.0 mL) and THF (10 mL). The solution was stirred at 60 °C for 8 h.
After removal of solvent under reduced pressure, the residue was
purified with column chromatography (silica gel, CH2Cl2/n-hexane,
1:1, v/v) to give 14.4 mg (yield 25%) of a dark red powder. Mp > 250
°C. 1H NMR (400 MHz, CDCl3) δ 8.02 (d, 2H, J = 7.6 Hz), 7.94 (d,
2H, J = 7.6 Hz), 7.80 (s, 2H), 7.43−7.56 (m, 9H), 7.32−7.34 (m, 2H),
7.09 (d, 2H, J = 7.6 Hz), 2.77 (s, 6H), 1.54 (s, 6H). TOF HRMS LD+:
calcd [C49H33BF2N4O2S2]+ m/z 822.2106, found m/z 822.2031.
3.9. Compound 10. Under Ar atmosphere, compound 2a (30.0
mg, 0.07 mmol), 11 (40.0 mg, 0.13 mmol), PdCl2(PPh3)2 (7.0 mg,
0.01 mmol), PPh3 (3.0 mg, 0.01 mmol), and CuI (1.9 mg 0.01 mmol)
were dissolved in triethylamine (10 mL). The solution was stirred at
60 °C for 6 h. After removal of solvent under reduced pressure, the
residue was purified with column chromatography (silica gel, CH2Cl2/
n-hexane, 1:1, v/v) to give 15.0 mg (yield 36%) of dark red powder.
length) was calculated as the singlet point energy at the optimized S0
state and the vertical excitation energy (S1 state). The energy shown in
the curves are relative values, with the lowest point on the curve as
zero. All these calculations were performed with Gaussian 09W.59
ASSOCIATED CONTENT
■
S
* Supporting Information
Structural characterization data of the compounds, solvent-
dependent fluorescence spectra, nanosecond time-resolved
transient difference absorption spectra, and z-matrix of the
calculated compounds. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: zhaojzh@dlut.edu.cn.
Notes
1
The authors declare no competing financial interest.
Mp > 250 °C. H NMR (400 MHz, CDCl3) δ 8.68 (s, 1H), 8.13 (d,
1H, J = 8.0 Hz), 7.96 (d, 1H, J = 7.6 Hz), 7.49−7.53(m, 5H), 7.41 (t,
1H, J = 7.6 Hz), 7.29−7.31 (m, 2H), 7.02 (d, 1H, J = 8.8 Hz), 6.03 (s,
1H), 4.26 (t, 2H, J = 6.4 Hz), 2.73 (s, 3H), 2.59 (s, 3H), 2.00−2.03
(m, 2H), 1.61−1.67 (m, 2H), 1.53 (s, 3H), 1.40 (s, 3H), 1.06 (t, 3H, J
= 7.2 Hz). 13C NMR (100 MHz, CDCl3) δ 162.4, 157.5, 156.9, 156.5,
152.1, 144.6, 136.3, 135.8, 135.4, 134.9, 134.7, 132.5, 130.4, 129.3,
128.1, 126.2, 124.9, 123.0, 122.7, 122.5, 121.4, 116.5, 112.5, 95.4, 81.3,
69.4, 31.4, 19.7, 14.7, 14.1, 13.8, 13.4. TOF HRMS ES+: calcd
[C38H34BF2N3OS+ H]+ m/z 630.2562, found m/z 630.2543.
ACKNOWLEDGMENTS
■
We thank the NSFC (20972024 and 21073028), the Royal
Society (U.K.) and NSFC (China-UK Cost-Share Science
Networks, 21011130154), the Fundamental Research Funds
for the Central Universities (DUT10ZD212), Ministry of
Education (NCET-08-0077) and Dalian University of Tech-
nology for financial support.
3.10. Compound 12. Under Ar atmosphere, compound 2a (31.5
mg, 0.07 mmol), phenylacetylene (21.4 mg, 0.21 mmol),
PdCl2(PPh3)2 (7.0 mg, 0.01 mmol), PPh3 (3.0 mg, 0.01 mmol), and
CuI (1.9 mg 0.01 mmol) were dissolved in triethylamine (10 mL).
The solution was stirred at 60 °C for 6 h. After removal of solvent
under reduced pressure, the residue was purified with column
chromatography (silica gel, CH2Cl2/petroleum ether, 1:3, v/v) to
give 24.0 mg (yield 81%) of an orange powder. Mp 189.6−191.2 °C.
1H NMR (400 MHz, CDCl3) δ 7.51−7.50(m, 3H), 7.46−7.44 (m,
2H), 7.31−7.28 (m, 5H), 6.03 (s, 1H), 2.70 (s, 3H), 2.58 (s, 3H), 1.50
(s, 3H), 1.40 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 157.7, 156.7,
144.8, 142.9, 142.3, 134.9, 132.6, 130.5, 129.4, 128.5, 128.1, 128.0,
123.8, 122.3, 115.4, 96.0, 82.1, 14.9, 14.7, 13.7, 13.3. TOF HRMS ES+:
calcd [C27H23BF2N2]+ m/z 424.1922, found m/z 424.1921.
3.11. Photooxidation Setup. A 35 W xenon lamp was used for
irradiation. NaNO2 solution (50 g/L) was set between the lamp and
the photoreactor to block the <385 nm UV light, as well as the IR
light. The irradiation power density at the photoreactor was set at 13
mW/cm−2. The photosensitizers (2.0 × 10−5 M) and DHN (2.0 ×
10−4 M) were in CH2Cl2/MeOH mixed solvent (9:1). Oxygen (O2)
was bubbled through the solution for 10 min. Then the solution was
continuously irradiated by the xenon lamp. The UV−vis absorption of
the solutions was measured every 2 min during the first 10 min of the
photoreaction. Then the UV−vis absorption of the solution was
measured every 5 min.
3.12. DFT Calculations. The geometries of the compounds were
optimized using density functional theory (DFT) with B3LYP
functional and 6-31G(d) basis set. There are no imaginary frequencies
for all optimized structures. Time-dependent DFT (TDDFT) was
used for the calculation of the electronic spectra of the compounds.
The steady-state UV−vis absorption of the organic dyes was calculated
with the optimized S0 state geometry. The fluorescence of the
compounds was calculated with the optimized S1 state geometry.
TDDFT calculations were also used for the prediction of the UV−vis
absorption of the organic dyes at triplet state, and in our case it is the
transient absorption of the organic dyes after laser flash. Please note
that the bleaching bands in the time-resolved transient absorption
spectra can be predicted by the TDDFT calculations. The spin density
of the compounds were calculated based on the optimized triplet state
geometry. The potential energy curves of the ESIPT (the C−H bond
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