Molecular structure-intersystem crossing relationship of heavy-atom-free bodipy triplet photosensitizers

Article abstract of DOI:10.1021/acs.joc.5b00691

A thiophene-fused BODIPY chromophore displays a large triplet-state quantum yield (φ_T = 63.7%). In contrast, when the two thienyl moieties are not fused into the BODIPY core, intersystem crossing (ISC) becomes inefficient and φ_T remains low (φ_T = 6.1%). First-principles calculations including spin-orbit coupling (SOC) were performed to quantify the ISC. We found larger SOC and smaller singlet-triplet energy gaps for the thiophene-fused BODIPY derivative. Our results are useful for studies of the photochemistry of organic chromophores.

Full text of DOI:10.1021/acs.joc.5b00691

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Note
Molecular Structure-Intersystem Crossing Relationship
of Heavy Atom-Free BODIPY Triplet Photosensitizers
Shaomin Ji, Jie Ge, Daniel Escudero, Zhijia Wang, Jianzhang Zhao, and Denis Jacquemin
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.5b00691 • Publication Date (Web): 05 May 2015
Downloaded from http://pubs.acs.org on May 7, 2015
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Molecular Structure-Intersystem Crossing
Relationship of Heavy Atom-Free BODIPY Triplet
Photosensitizers
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Shaomin Ji,a,b*Jie Ge,a,b Daniel Escudero,c Zhijia Wang,b Jianzhang Zhao,b* and Denis
Jacquemin c,d*
a Key Laboratory of Low Dimensional Materials & Application Technology, Ministry of
Education, School of Materials science and engineering, Xiangtan University, Xiangtan 411105,
P. R. China. E-mail: smji@xtu.edu.cn (S. J.)
b 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. E-mail:
zhaojzh@dlut.edu.cn (J. Z.)
c
CEISAM UMR CNRS 6230, Université de Nantes, 2 rue de la Houssinière, BP 92208,
44322Nantes Cedex 3, France. E-mail : Denis.Jacquemin@univ-nantes.fr (D. J.)
d
Institut Universitaire de France, 103 boulevard St Michel, 75005 Paris Cedex 5, France.
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ABSTRACT: A thiophene-fused BODIPY chromophore displays large triplet state quantum
yield (ΦT = 63.7 %). In contrast, with the two thienyl moieties are not fused into the BODIPY
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core, intersystem crossing (ISC) becomes inefficient and the ΦT remains low (ΦT = 6.1 %). First
principle calculations including spin-orbit coupling (SOC) were performed to quantify the ISC.
We find larger SOC and smaller singlet-triplet energy gaps for the thiophene-fused BODIPY
derivative. Our results are useful for study the photochemistry of organic chromophores.
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KEYWORDS: BODIPY; Intersystem Crossing; Spin-orbit coupling; Thiophene; Triplet State
Triplet photosensitizers are versatile compounds which have been widely used in
photodynamic therapy (PDT),1 as photocatalysts in photocatalytic hydrogen production (H2) and
photoredox organic reactions,2 as well as in photoinduced charge separation,3a−3c and triplet-
triplet annihilation upconversion.3d−3g The triplet photosensitizers play the pivotal role by first
harvesting light leading to the singlet excited states and then promoting the formation of triplet
state via intersystem crossing (ISC). Finally, they may trigger intermolecular electron or energy
transfer processes. In order to facilitate the ISC, the most well-known approach is to take
advantage of the heavy atom effect, obtained with metallic coordination centers such as Ir(III),
Pt(II), Ru(II), or alternatively with iodine atoms (I).4 The heavy atom-effect is proportional to Z4
(Z is the atomic number). As a consequence, a larger heavy atom effect is expected for Pt or Ir
than for S and Br.4 It is indeed well-known that most of the Ir(III), Pt(II) or Ru(II) transition
metal complexes show efficient and ultrafast ISC.5 For example, Ru(bpy)3Cl2 (bpy = 2,2’-
biphenylpyriding) displays a quantum yield of triplet state formation of 100%. Previously, it was
also shown that iodine substitution on the π-core of the boron dipyrromethane (BODIPY)
chromophore is an effective approach to access efficient organic triplet photosensitizers.6
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Significant heavy atom effect obtained with non-metallic atoms other than iodine remains
scarce, though designing efficient triplet photosensitizers with lighter atoms, such as sulfur,
would be of tremendous interest. While some oligothiophene compounds are known to form
triplet excited states upon photoexcitation, this effect was not thoroughly studied for small
molecules which contain thiophene moieties.7a,7b During the preparation of this manuscript, a
thiophene fused BODIPY derivative was reported to show ISC capability, but the detailed
mechanism was not studied.7c
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Scheme 1. Compounds Used in the Study
S
S
S
N
N
F
N
N
N
N
F
S
B
B
B
F
F
F
F
BDP-1
BDP-3
BDP-2
Recently a thiophene-fused BODIPY dye was reported to show very weak fluorescence.8a
Normally BODIPY compounds show strong fluorescence.8b−8d We consequently envisioned that
efficient ISC could be attained with this compound (BDP-1, Scheme 1). Herein we present the
synthesis and spectroscopic characterization of this BODIPY derivative. The generation of
triplet excited states was confirmed with nanosecond transient absorption spectroscopy and
rationalized with ab initio calculations. We found that triplet formation with the heavy atom-free
BDP-1 is more efficient (triplet formation quantum yield, ΦT = 63.7%) than with the BODIPY
derivative presenting non-fused thiophene rings (BDP-3, ΦT = 6.1%). Theoretical calculations
show that for BDP-1 a higher-lying triplet state, i.e. T2, which is energetically aligned to the
spectroscopic S1 state is responsible for the enhanced ISC processes. In short, we demonstrate
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that the incorporation of sulfur atoms into the π-conjugation skeleton of BODIPY is an efficient
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approach to attain high quantum yield of triplet formation.
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BDP-1
BDP-2
BDP-3
0.9
0.6
0.3
0.0
300
400
500
600
700
Wavelength / nm
Figure 1. UV−Vis absorption of BDP-1, BDP-2 and BDP-3. c = 1.0 × 10−5 M in toluene, 20 °C.
The UV−vis absorption spectra of the compounds were studied (Figure 1). BDP-1 shows
strong absorption at 571 nm. In comparison BDP-3 shows a blue-shifted absorption band
peaking at 529 nm. BDP-1 undergoes a very slight blue shift in polar solvents compared with
non-polar solvents (Supporting Information, Figure S7). This result indicates that the ground
state presents a larger dipole moment than the first singlet excited states. Similar results were
observed for both BDP-2 and BDP-3 (see Figure S7 in the Supporting Information).
Fluorescence spectra were also recorded (Figure 2). BDP-1 shows weak fluorescence (ΦF =
4.0 %). BDP-3 shows red-shifted emission at 608 nm and a larger Stokes shift. BDP-2 fluoresces
at 516 nm. These experimental evidences are in agreement with an extended π-conjugation in
BDP-1 as compared with BDP-3.
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BDP-1
BDP-2
BDP-3
0.9
9
0.6
0.3
0.0
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500
600
700
800
Wavelength/nm
Figure 2. The normalized emission spectra of BDP-1 (black, λex = 530 nm, A530 = 0.20), BDP-2
(red, λex= 470 nm, A470 = 0.26) and BDP-3 (blue, λex= 498 nm, A498 = 0.23), in toluene. 20 °C.
0.06
0.00
a
0.00
-0.08
τT = 170.5 μs
865.8 μs
...
...
0.0 μs
-0.06
-0.16
-0.12
b
-0.24
400
500
600
700
0
300
600
900
Wavelength / nm
Time / μs
Figure 3. (a) Nanosecond transient absorption of BDP-1 after pulsed laser excitation (λex = 560
nm), (b) Decay trace of BDP-1 at 569 nm in deaerated toluene, c = 1.0 × 10−5 M, 20 °C.
To investigate the triplet state generation in BDP-1, nanosecond transient absorption spectra
were recorded (Figure 3). Upon pulsed laser excitation, a bleaching band at 566 nm was
observed. Excited state absorption (ESA) at 491 nm was also observed. The lifetime of the
transient was determined to be 170.5 μs. The transient was significantly quenched in aerated
solution, hence it is related to the formation of a triplet excited state (Supporting Information,
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Figure S18). The triplet state quantum yield (ΦT) of BDP-1 amounts to 63.7% (see Table 1,
determined with the singlet state depletion method). No significant triplet state formation was
observed in BDP-2 (ΦT is negligible). A similar nanosecond transient absorption spectrum was
observed for BDP-3 (see SI, Figure S17). However, a lower triplet excited state quantum yield
was observed for BDP-3 (ΦT = 6.1%) than for BDP-1. The photophysical properties of the
compounds are summarized in Table 1.
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In order to rationalize the different ISC of BDP-1 – BDP-3, the relative energy level of the
singlet and triplet states, as well as the spin-orbit coupling (SOC) of the compounds were studied.
The ISC rate between the n-singlet (Sn) and m-triplet (Tm) excited states (kISC) obeys the
empirical Fermi Golden rule equation (Eq. 1),
2π
=
ˆ
(Eq. 1)
kISC =
Sn HSO Tm 2 × FCWD
[
]
where the bracket term stands for their associated SOCs and FCWD accounts for the Franck-
Condon weighted density of states. From a computational viewpoint, the calculation of ISC rates
first requires the assignment of the main photodeactivation channels, followed by accurate
calculations of (i) relative energy levels of the involved excited states, (ii) SOC matrix elements,
(iii) vibrational frequencies and (iv) Huang-Rhys factors. Computing all parameters on Eq.(1)
becomes rapidly prohibitive for large molecules.9 Instead, semi-quantitative and qualitative
strategies are often used to rationalize the efficiency of ISC processes.10 Based on the
computation of kISC values relying on accurate ab initio electronic structure data it is concluded
that two main factors, i.e. substantial electronic and/or vibronic SOC and small Tm-Sn energy
gaps, govern the efficiency of the ISC processes.9 Herein, we report theoretical estimates of
SOCs and relative Tm−Sn energy gaps to rationalize the efficient ISC in BDP-1 as compared with
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those occurring in both BDP-2 and BDP-3. Modeling the electronically excited state properties
of BODIPY dyes is still a significant challenge for quantum chemical methods due to the well
documented cyanine challenge, which roots on the need of capturing large differential electron
correlation effects in these compounds.11 Recent time-dependent density functional theory (TD-
DFT) studies of the excited states of BODIPY dyes concluded that, among the available pool of
exchange-correlation functionals, M06-2X outperforms the rest of functionals.12 However, TD-
DFT still systematically overshoots the transition energies of BODIPY dyes by ca. 0.4 eV.
Therefore correcting the TD-DFT values with transition energies obtained at the scaled-opposite-
spin configuration interaction singles with double corrections, i.e. SOS-CIS(D) was advocated as
a much more accurate approach.13
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Table 1. Photophysical Parameters of the Compounds
ε b
λem (eV) c
ΦF (%)d
4.0 i
τF(ns)e
τT(μs)f
ΦT(%)h
Φ△(%)g
a
λabs
571 nm
(2.17 eV)
500 nm
(2.48 eV)
529 nm
8.35
9.71
582 nm
(2.13 eV)
516 nm
(2.41 eV)
608 nm
1.35
170.5
58.1
63.7
BDP-1
BDP-2
BDP-3
99.0 i
5.30
4.59
-
-
-
5.65
11.2
389.9
4.93
6.1
(2.35 eV)
(2.04 eV)
a In toluene (1.0 × 10−5 M). b Molar extinction coefficient at the absorption maxima. ε: 104 M−1
c
d
cm−1 . In toluene. Fluorescence quantum yields. DiiodoBodipy (ΦF = 0.027 in CH3CN) was
used as standard. e Luminescence lifetime, λex = 405 nm, at RT, in toluene. Triplet state lifetime,
f
determined with nanosecond time-resolved transient absorption spectroscopy, BDP-1 λex = 560
nm, BDP-3 λex = 532 nm, 1.0 × 10−5 M in deaerated toluene. Quantum yield of singlet oxygen
g
(1O2), with DiiodoBodipy as standard (ΦΔ = 0.83 in CH2Cl2), 1.0 × 10−5 M in CH2Cl2. Triplet
h
quantum yields upon direct photoexcitation (462 nm). Ru(bpy)3C12 (ΦT = 1.0 in H2O) was used
as standard. i Literature value.6b, 8a
Table 2 lists the TD-M06-2X and SOS-CIS(D) vertical excitation energies for the lowest
singlet and triplet excited states of BDP-1, BDP-2 and BDP-3 at their optimized ground state
geometries (see computational details in the Supporting Information). As expected, the SOS-
CIS(D) excitation energies of the spectroscopic state (S1) are in a better agreement with the
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position of measured UV−Vis absorption bands (see Table 2). Regarding their intensities, BDP-1
shows a larger oscillator strength than BDP-3, in accordance to the experimental evidences. TD-
06-2X systematically overestimates and underestimates by ca. +0.4 and −0.25 eV the excitation
energies of the singlet and triplet excited states, respectively, as compared to SOS-CIS(D). In
view of these evidences, we discuss only the SOS-CIS(D) values in the following. To evaluate
the main Sn→Tn ISC channels, only the excited states which are below the experimental λexc of
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photoexcitation are considered (λexc = 462 nm / 2.68 eV, see Table 1). Upon photoexcitation,
there are two possible Sn→Tn ISC channels for the spectroscopic state (S1) of BDP-1, i.e. S1→T2
and S1→T1. Due to the smaller energetic gap between T2 and S1 (i.e. 0.24 eV, see Table 2), we
propose S1→T2 to be the most important triplet deactivation funnel for BDP-1. In contrast, for
BDP-2 and BDP-3 only the S1→T1 ISC channel is energetically accessible. Table 2 also collects
the SOCs obtained with quadratic response (QR)-TD-DFT/6-31G* calculations between the
involved Tm and S1 excited states (see computational details in the Supporting Information). As
expected for organic compounds, the SOCs amount only to a few cm−1. As seen in Table S3, the
effect of increasing the basis set on the SOCs calculations is almost negligible, so that the results
are almost converged at the (QR)-TD-DFT/6-31G* level of theory. It should be recalled that
SOCs values between 0.2 and 5.0 cm−1 are considered large enough to induce ISC on a
nanosecond time scale.14
The SOCs for BDP-1 are one order of magnitude larger than in the two other dyes, since the
sulfur atom contributes to the involved lowest excited states of BDP-1 (see the assignment of the
excited states in Table 2 and the involved orbitals in Figure 4). In BDP-3 the sulfur atom located
at the peripheral ligand does not contribute to the BODIPY-like excited states (see Table 2 and
Figure 4). Therefore, since BDP-1 possesses the largest SOCs and the smallest singlet-triplet
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Figure 4. Involved Kohn-Sham orbitals (M06-2X/6-311+G(2d,p) in the lowest excited states of
BDP-1 and BDP-3.
Table 2. Lowest Vertical Singlet and Triplet Electronic Transition Energies (in eV) and
Oscillator Strengths (in parenthesis) of BDP-1, BDP-2 and BDP-3 at TD-M06-2X and SOS-
−
1
CIS(D) Levels of Theory. Vertical Singlet-Triplet Splitting (in eV) and SOCs (in cm )
between the Involved Tm and S1 States
b
ˆ
S1 HSO Tm
ΔESOS-CIS(D)
SOS-
CIS(D)
States / Assignment a
TD-M06-2X
(x- ;
y- ;
z-
(S1−Tx)
components)
2.72 (0.913) 2.32
BDP-1
BDP-2
S1 (H→L, c = 0.67;
H−1→L, c = −0.20)
−
−
T1 (H→L, c = 0.72)
1.24
2.35
1.50
2.56
0.82
(0.0; 0.0; −3.2)
T2 (H−1→L, c = 0.68)
-0.24
(−1.2; −1.3; 0.0)
−
2.99 (0.539) 2.51
1.60 1.83
2.76 (0.772) 2.29
1.50 1.73
S1 (H→L, c = 0.70)
T1 (H→L, c = 0.71)
S1 (H→L, c = 0.70)
T1 (H→L, c = 0.69)
−
0.68
(-0.3; 0.0; 0.0)
BDP-3
−
−
0.56
(−0.4; −0.2; 0.2)
a
Only the excited states which are below the experimental λexc of photoexcitation are
considered (λexc = 462 nm / 2.68 eV). b Values obtained at the QR-TD-DFT/6-31G* level of
theory at the T1 optimized geometry.
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energy gap among all the BODIPY dyes, it will easily undergo photodeactivation through ISC.
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Indeed, as discussed above, BDP-1 yields the largest quantum yields of singlet oxygen (ΦΔ) and
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triplet generation (ΦT. see Table 1). Comparison of BDP-3 with BDP-2 is more qualitative,
though the slightly increased SOCs and the lowest singlet-triplet energy gaps in BDP-3
compared with BDP-2, point to slightly increase ISC channel for the latter, in accordance with
the experimental observations.
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In summary, we find that a heavy atom-free thiophene-fused BODIPY (BDP-1) shows
efficient ISC, with triplet state quantum yield (ΦT) of 63.7%. In comparison, two reference
BODIPY derivatives which either contain no sulfur atom (BDP-2) or thiophene groups not
participating directly in the π-conjugation pathway of the BODIPY core (BDP-3), do not show
any significant ISC (ΦT is negligible for BDP-2, and ΦT is only 6.1% for BDP-3). Theoretical
calculations demonstrate that the increased ISC mechanisms for BDP-1 as compared with BDP-
2 and BDP-3 are due to i) the participation of the sulfur atom on the lowest-lying excited states,
which leads to moderate SOCs and ii) a small singlet-triplet energy gap in BDP-1. These
insights are useful to design heavy atom-free triplet photosensitizers and to understand the
fundamental photochemistry of the ISC mechanisms of organic chromophores.
ꢀ EXPERIMENTAL SECTION
General Remarks. All the chemicals are analytically pure and were used as received. Solvents
were dried and distilled prior to use. Fluorescence lifetimes were measured with OB920
luminescence lifetime spectrometer (Edinburgh, U.K.). Compound BDP-1 was prepared
following the reported method.8a BDP-2 and BDP-3 were reported previously.15
Compound 2. Into a 100 mL flask (dry, under Ar) was placed CH2Cl2 (30 mL), and cooled
using an ice bath before AlCl3 (4.9 g, 36.8 mmol) was added. A dropping funnel was charged
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with acetyl chloride (2.68mL, 39.2 mmol) in CH2Cl2 (30 mL), and this solution was added to the
AlCl3 suspension over a period of 10-min. After about 30 min of being stirred at 0 °C, most of
the AlCl3 had been dissolved. A second dropping funnel was charged with 3-bromothiophene
(0.574 mL, 1.00 g, 6.13 mmol) in CH2Cl2 (30 mL), and this mixture was added to the reaction
mixture over a 10-min period. The reaction was left to proceed at 0 °C for 30 min and then
warmed slowly to room temperature (RT) for another hour. Then the reaction mixture was
cooled to 0 °C once again, and water was added carefully. The reaction mixture was diluted with
CH2Cl2 and water was added. The water layer was extracted twice with CH2Cl2, and washed
with saturated NaHCO3, brine and finally dried over anhydrous MgSO4. Evaporation of the
solvent and purification by column chromatography (hexane: CH2Cl2 = 1:1) gave compound 2 as
a yellow liquid (2.4 g, 98%). 1H NMR (CDCl3, 400 MHz): δ 7.55 (d, J = 4.9 Hz, 1H), 7.10 (d, J
= 5.2 Hz, 1H), 2.68 (s, 3H).
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Compound 3. To the mixture containing 2 (204 mg, 1 mmol), CuI (19 mg, 0.1 mmol) and
Cs2CO3 (651 mg, 2 mmol) in DMSO (1 mL), ethyl isocyanoacetate (124 mg, 1.1 mmol) was
added dropwise at RT. After stirring at 50°C for 4 h, the reaction mixture was extracted with
CH2Cl2. The organic layer was washed with brine twice, then the organic layer was dried over
MgSO4 and filtrated. The filtrate was condensed with evaporation, and the silica gel column
chromatography with hexane as a mixture eluent (hexane: ethyl acetate = 9:1) gave compound 3
as a white solid (110 mg, 53%). 1H NMR (CDCl3, 400 MHz): δ 7.64 (d, J = 5.3 Hz, 2H), 7.38 (s,
1H), 7.12 (d, J = 5.3 Hz, 2H), 2.42 (s, 6H).
Compound 4. The mixture containing 3 (340 mg, 1.63 mmol) and aq NaOH solution (1.032 g,
in 7.2 mL H2O) in 13.5 mL of ethanol was refluxed for 1 h. After cooling to RT, HCl (10%) was
added dropwise to acidify. The products were extracted with CH2Cl2, and the organic layer was
washed with brine, dried over MgSO4 and filtrated. Evaporation of the filtrate yielded compound
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4 as a dark purple solid (268 mg, 90%). 1H NMR (400 MHz, DMSO-d6): δ 8.94 (s, 1H), 7.31 (d,
J = 5.3 Hz, 1H), 6.91 (d, J = 5.3 Hz, 1H), 4.40 (q, J = 21.3 Hz, 2H), 2.52 (s, 3H), 1.41 (t, J =
14.2 Hz, 3H).
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Compound 5. The solution of 4 (150 mg, 0.83 mmol) dissolved in trifluoroacetic acid (2.7 mL)
was stirred at 50 °C for 20 min, and then triethyl orthoformate (512 mg, 3.46 mmol) was added.
After stirring at 50 °C for 30 min, excess amounts of diethyl ether and satd NaHCO3 aq were
poured into the reaction solution. The organic layer was washed with brine and water, dried over
MgSO4, filtrated and condensed by evaporation afforded 5 as a brown solid (110 mg, 80.4%). 1H
NMR (400 MHz, CDCl3): δ 12.43 (s, 1H), 11.51 (s, 1H), 7.47 (d, J = 5.2 Hz, 1H), 6.94 (d, J =
5.3 Hz, 1H), 2.41 (s, 3H).
Compound BDP-1. To the solution of 5 (200 mg, 1.2 mmol) in CH2Cl2, POCl3 (227 mg, 1.5
mmol) was added dropwise at 0 °C. After stirring at room temperature for 3 days in the dark,
triethylamine (0.84 mL, 6 mmol) was added dropwise at 0℃. After stirring at 0 °C for 15 min,
BF3·Et2O (1.1 mL, 8.9 mmol) was added dropwise, and then the mixture was stirred at room
temperature for 2 days. The reaction was quenched by adding 10 mL of water, and the products
were extracted with CH2Cl2. The organic layer was washed with water twice and brine, dried
over MgSO4, filtrated, and condensed by evaporation. The residue was passed through the silica
gel column with the mixture eluent (hexane: CH2Cl2 = 5:1) afforded BDP-1 as a dark purple
solid (10 mg, 5%). 1H NMR (CDCl3, 500 MHz): δ 7.64 (d, J = 5.3 Hz, 2H), 7.38 (s, 1H), 7.12 (d,
J = 5.3 Hz, 2H), 2.42 (s, 6H). TOF HRMS EI+: calcd ([C15H11BF2N2S2]+) m/z = 332.0425, found
m/z = 332.0417.
Theoretical Computations. The geometries of the singlet ground state (S0) of BDP-1, BDP-2
and BDP-3 were optimized at the M06-2X/6-31G(d) level of theory. Gas phase TD-M06-2X and
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SOS-CIS(D) vertical singlet and triplet excitation energies were obtained at this geometry using
the 6-311+G(2d,p) and the 6-31+G(d) basis set, respectively. The geometries of the lowest
singlet (S1) and triplet (T1) excited states were also optimized at the TD-M06-2X/6-31G(d) level
of theory. SOCs were computed using the quadratic-response TD-DFT approach,16 i.e. QR-TD-
DFT, as implemented in the Dalton program17 at their T1 optimized geometries. The SOC
operator makes use of a semi-empirical effective single-electron approximation.18 For the latter
calculations the B3LYP functional in combination to the 6-31G(d) basis set was used. SOS-
CIS(D) and TD-DFT calculations were carried out with the Q-Chem19 and Gaussian0920
program packages, respectively.
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Triplet State Quantum Yield (ΦT). The triplet quantum yields were determined with the
singlet state depletion method.21 The ΦT values were obtained by comparing the ΔAS of the
optically matched sample solution at 452 nm in a 1 cm cuvette to that of the reference using the
equation [Eq. (2)]:
εSRu ΔAS
(Eq. 2)
ΦT = ΦTRu
εS ΔASRu
where the superscript represents the reference, ΔAS is the absorbance change of the triplet
transient difference absorption spectrum at the minimum of the bleaching band, and εS is the
ground-state molar absorption coefficient at the UV−vis absorption band maximum.
ꢀ ASSOCIATED CONTENT
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Supporting information
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Molecular structure characterization, additional spectra, atom coordinates, absolute energies of
the optimized geometries and more computation details. This material is available free of charge
via the Internet at http://pubs.acs.org.
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ꢀ ACKNOWLEDGMENT
S. J. thank the NSFC (51202207), J. Z. thank the NSFC (21273028, 21473020 and 21421005),
Ministry of Education (SRFDP-20120041130005), the Fundamental Research Funds for the
Central Universities (DUT14ZD226), Program for Changjiang Scholars and Innovative Research
Team in University [IRT_132206] and Dalian University of Technology for financial support
(DUT2013TB07) for financial support. D. E. acknowledges the European Research Council
(ERC, Marches-278845) and the Région des Pays de la Loire for his post-doctoral grant. D.J.
acknowledges the European Research Council (ERC) and the Région des Pays de la Loire for
financial support in the framework of a Starting Grant (Marches-278845) and a recrutement sur
poste stratégique, respectively. This research used resources of 1) the GENCI-CINES/IDRIS, 2)
CCIPL (Centre de Calcul Intensif des Pays de Loire), and 3) a local Troy cluster.
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