Bodipy-squaraine triads: Preparation and study of the intramolecular energy transfer, charge separation and intersystem crossing

Article abstract of DOI:10.1016/j.dyepig.2017.08.028

Two triads (BDP-SQ and Styryl-BDP-SQ) were prepared with Bodipy, styrylBodipy and Squaraine (SQ) units. SQ shows unexpected efficient intersystem crossing (ISC. Φ_T = 50%), which is attributed to S₁→T₁ transition. In the two triads, the F?rster Resonance Energy Transfer (FRET) direction, as well as the spatial localization of the T₁ state, was judiciously tuned. The cascade photophysical properties of the triads were studied with steady-state and time-resolved optical spectroscopies, as well as with electrochemical characterization and theoretical computations. We show that triplet state was produced in triad BDP-SQ upon photoexcitation, but in Styryl-BDP-SQ the fast FRET and the charge separation (CS) processes compete with the ISC of the SQ unit, and no triplet state was formed upon photoexcitation. The singlet energy transfer kinetics were found to be 1.6 and 0.6 ps, respectively and are solvent polarity dependent. Charge transfer was confirmed with ultrafast transient absorption spectroscopy.

Full text of DOI:10.1016/j.dyepig.2017.08.028

Accepted Manuscript
Bodipy-squaraine triads: Preparation and study of the intramolecular energy transfer,
charge separation and intersystem crossing
Dong Yu, Alessandro Iagatti, Paolo Foggi, Jianzhang Zhao, Gloria Mazzone, Kejing
Xu, Wei Ji, Mariangela Di Donato, Nino Russo
PII:
S0143-7208(17)30973-7
10.1016/j.dyepig.2017.08.028
DYPI 6190
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 27 April 2017
Revised Date: 11 July 2017
Accepted Date: 16 August 2017
Please cite this article as: Yu D, Iagatti A, Foggi P, Zhao J, Mazzone G, Xu K, Ji W, Di Donato M,
Russo N, Bodipy-squaraine triads: Preparation and study of the intramolecular energy transfer, charge
separation and intersystem crossing, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.08.028.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT
Graphical abstract
Bodipy-Squaraine Triads: Preparation and Study of the Intramolecular
Energy Transfer, Charge Separation and Intersystem Crossing
Yu Dong, Alessandro Iagatti, Paolo Foggi, b,d,e,f Jianzhang Zhao,* Gloria Mazzone,* Kejing
a
b,d
a
g
a
a
b,c,d
and Nino Russog
Xu, Wei Ji, Mariangela Di Donato*

ACCEPTED MANUSCRIPT
Bodipy-Squaraine Triads: Preparation and Study of
the Intramolecular Energy Transfer, Charge
Separation and Intersystem Crossing
a
b,d
b,d,e,f
*a
*g
Yu Dong, Alessandro Iagatti Paolo Foggi
Jianzhang Zhao, Gloria Mazzone, Kejing
a
a
*b,c,d
g
Xu, Wei Ji, Mariangela Di Donato
and Nino Russo
a
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
E-mail: zhaojzh@dlut.edu.cn
b
LENS (European Laboratory for Non-Linear Spectroscopy) via N. Carrara 1, 50019 Sesto
Fiorentino, Italy.
E-mail: didonato@lens.unifi.it
c
Dipartimento di Chimica ‘Ugo Shiff’, Università di Firenze, via della Lastruccia 3-13, 50019
Sesto Fiorentino, Italy
d
INO, Istituto Nazionale di Ottica Largo Enrico Fermi 6, I-50125 Florence, Italy
e
Dipartimento di Chimica, Università degli Studi di Perugia, via Elce di Sotto 8, 06123 Perugia,
Italy
f
ICCOM-CNR via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy
g
Dipartimento di Chimica e Tecnologie Chimiche, Università della Calabria, I-87036
Arcavacata di Rende, Italy.
E-mail: gloria.mazzone@unical.it
1

ACCEPTED MANUSCRIPT
Abstract: Two triads (BDP-SQ and Styryl-BDP-SQ) were prepared with Bodipy, styrylBodipy
and Squaraine (SQ) units. SQ shows unexpected efficient intersystem crossing (ISC. Φ = 50%),
T
which is attributed to S
1 1
→T transition. In the two triads, the Förster Resonance Energy Transfer
(
FRET) direction, as well as the spatial localization of the T state, was judiciously tuned. The
1
cascade photophysical properties of the triads were studied with steady state and time-resolved
optical spectroscopies, as well as with electrochemical characterization and theoretical
computations. We show that triplet state was produced in triad BDP-SQ upon photoexcitation,
but in Styryl-BDP-SQ the fast FRET and the charge separation (CS) processes compete with the
ISC of the SQ unit, and no tripled state was formed upon photoexcitation. The singlet energy
transfer kinetics were found to be 1.6 and 0.6 ps, respectively and are solvent polarity dependent.
Charge transfer was confirmed with ultrafast transient absorption spectroscopy.
Keywords: Charge Separation; Charge Recombination; FRET; Intersystem crossing; Triplet
State
1
. Introduction
Triplet photosensitizers, or in other words, systems giving strong absorption of light and
efficient production of long-lived triplet excited state, are versatile compounds which have been
extensively used in photocatalysis,[1−5] photodynamic therapy,[6−12] room temperature
phosphorescence materials,[13] non-linear optics,[14,15] and triplet-triplet annihilation upcon-
version.[16] Elucidation of the photophysical properties of the triplet photosensitizers is pivotal
concerning both application and the fundamental photochemistry.[17−23] Recently this area
witnessed great developments, with the production, for example, of Bodipy-derived triplet
photosensitizers.[6,24−26] Normally the intersystem crossing (ISC) is facilitated by the heavy
2

ACCEPTED MANUSCRIPT
atom effect for these metal free organic photosensitizers, and more recently the spin converter
concept has been introduced.[24,26,27] However, there is still much room remaining for
development of heavy atom free photosensitizers.[28−31] This is not a trivial task because
without heavy atom effect, it becomes very difficult, if not impossible, to predict the ISC
capability of the organic compounds.[32]
Recently a few squaraine (SQ) dyes were reported to show ISC capability.[33] Normally
squaraine dyes show weak ISC efficiency, and the triplet state has to be populated by
photosensitizing method (a triplet photosensitizer had to be used).[34,35] The case of squaraine
is an interesting example of heavy atom-free triplet photosensitizer because the extension of π-
conjugation of a chromophore usually diminish the ISC property, even in the presence of
carbonyl groups (π-π*→n-π* ISC or vice versa, is facilitated for the aromatic carbonyl
compounds). For instance, coumarin ketones demonstrate efficient ISC,[36] but their derivatives
with large π-conjugation systems lack this property.[37] Although ISC capabilities of several
squaraine have been previously reported, the ISC kinetics and the detailed photophysical
properties of such dyes were not investigated.[33] Herein we studied the origin of the triplet state
formation of SQ, and we prepared two triads with SQ and Bodipy (BDP) chromophores (Scheme
1
). The two BDP chromophores have different structures and absorption properties, so that in
one triad, the hypothetic FRET is from BDP to SQ unit, whereas in another triad, the FRET is
directed from the SQ moiety to the Styryl-BDP moiety. By such designing rationale, we are able
to study the competition of the ISC of the squaraine moiety and FRET, as well as the effect of
charge separation (CS).[38] The photophysical properties of these systems were studied with
steady state and nanosecond/picosecond time-resolved optical spectroscopies, electrochemical
measurements as well as DFT (density functional theory) computations. In the triad BDP-SQ an
3

ACCEPTED MANUSCRIPT
ultrafast FRET process, followed by ISC, was observed, whereas in Styryl-BDP-SQ the ISC was
quenched by FRET and CS. Triplet state was thus observed for BDP-SQ, but not for Styryl-
BDP-SQ.
2
. Results and Discussion
2
.1. Design and Synthesis of the Compounds
In order to tune the photophysical properties of the triads, SQ moiety was used as one of the
visible light-harvesting moiety, which is also responsible for the ISC. BDP and Styryl-BDP
moieties were connected to SQ via Click reaction. BDP and Styryl-BDP are expected to be the
singlet energy donor and the acceptor in the two triads, respectively.[39−44]
In the triad BDP-SQ, the BDP moiety has shorter absorption/emission wavelength than the SQ
moiety, thus we expect a FRET process with the BDP moiety acting as the singlet energy donor
and SQ moiety as the singlet energy acceptor. However, the triplet state energy level of the SQ
moiety is unknown,[33] thus spectroscopy studies (such as the nano-second transient absorption)
or theoretical computations (spin density, etc. see later sections) may elucidate the localization of
the T
1
state in BDP-SQ. In Styryl-BDP-SQ, FRET is from the SQ to the Styryl-BDP moiety.
state.
The same interesting question exists, i.e. the determination of the localization of the T
1
Along with the hypothetic singlet energy transfer, charge separation may occur. The
nanosecond/picosecond transient absorption spectroscopic studies may unveil these photo-
physical processes. The synthesis of the compounds was carried out with routine methods. All
the compounds were obtained with satisfactory yields.
4

ACCEPTED MANUSCRIPT
N
N3
O
N
N
HO
HO
O
O
O
N
H
N
Br
N
+
a
b
c
1
N
F
N
F
B
N
F
N
F
B
5
7
O
N
N
N
N
F
F
F
N N
N
N N
B
O
O
B
N
N
N
F
O
BDP-SQ
N3
O
Br
OH
N3
O
O
O
Br
NaN3
e
Br
H
+
d
f
g
O
N
F
N
O
O
B
2
3
4
F
5
N
N3
N
N
O
O
N
HO
HO
O
O
h
i
N
N
F
B
N
N
F
F
B
F
6
8
O
O
N
N
N N
N
N
N
N
N
N
N
F
F
O
F
O
B
B
N
F
Styryl-BDP-SQ
N
N
N
Br
OH
N
3
O
O
Br
NaN3
k
O
N
Br
N
+
j
l
10
11
9
HO
HO
O
O
N N
N
N N
O
O
O
N
N
N
m
O
SQ
5

ACCEPTED MANUSCRIPT
Scheme 1. Synthesis of BDP-SQ, Styryl-BDP-SQ and SQ. (a) NaH, DMF, RT overnight, 60%;
(
b) CuSO ⋅5H O, L-ascorbic acid sodium salt, TEA, RT overnight, 70%; (c) BuOH and toluene,
4 2
refluxed, 2 h, 30%. (d) K CO , 1,2-dibromoethane, dry ethanol, Ar, 70 °C, 40%; (e) NaN , DMF,
2
3
3
1
1
00 °C, 10 h, 99%; (f) 2,4-dimethylpyrrole, TFA, anhydrous CH
2
Cl
2
3 2
, rt, 12 h; DDQ, BF ⋅Et O,
2 h, 20%; (g) benzaldehyde, acetic acid, piperidine, evaporate to dry, 15 min, 50%; (h) similar
to step b 60%; (i) similar to step c, 1 h, 50%; (j) similar to step d, 40%; (k) similar to step e, 99%;
l) similar to step b, 70%; (m) similar to step c, 2 h, 25%.
(
2
.2. UV−vis Absorption and Fluorescence Spectroscopy
The UV−vis absorption spectra of the compounds are reported in Fig. 1, where the absorption
spectra of the triads are compared with those of the respective components. Two absorption
bands at 504 nm and 566 nm were observed for BDP-SQ (Fig. 1a), which can be assigned to the
BDP and SQ moieties, respectively.[33] The lack of spectral shift or bandshape changes in the
triad spectrum, compared with those of the isolated components, indicates not significant
electronic interaction between the chromophores at the ground state. Interestingly, we found that
the molecular extinction coefficients (ε) of BDP-SQ at 566 nm is much higher than that of SQ
(Table 1). Similar results were observed for Styryl-BDP-SQ (Fig. 1b and Table 1).
2
1
1
0
0
.0
.5
.0
.5
2.5
SQ
BDP-SQ
BDP
SQ
a
b
Styryl-BDP-SQ
Styryl-BDP
2
1
1
0
.0
.5
.0
.5
.0
3
0.0
300 400 500 600 700 800
Wavelength / nm
00 400 500 600 700
Wavelength / nm
Fig. 1. UV−vis absorption spectra of (a) BDP, BDP-SQ and SQ; (b) Styryl-BDP, styryl-BDP-
−5
SQ and SQ. c =1.0 × 10 M in toluene. 20 °C.
6

ACCEPTED MANUSCRIPT
In order to confirm the hypothetic FRET and CS processes in the triads, the fluorescence of the
triads and the reference compounds were studied with optically matched solutions (Fig. 2). For
BDP-SQ, we found that the fluorescence of the energy donor, i.e. the BDP moiety was almost
completely quenched as compared to that of BDP (Fig. 2). The quenching effect is similar in
both toluene and acetonitrile (Fig. 2b). These results indicated FRET as the most probable
quenching mechanism of the energy donor fluorescence. It is known that photo-induced electron
transfer (PIET) is often inefficient in non-polar solvents such as toluene.[17, 45−47] As such we
should observe different quenching in toluene and acetonitrile if PET is responsible for the
quenching of the fluorescence of the energy donor. In toluene, fluorescence of BDP is quenched
in the triad, but fluorescence of SQ is observed (Fig. 2a) in agreement with FRET. In acetonitrile
however (Fig. 2b) no fluorescence is observed, suggesting that even if FRET is occurring other
non-radiative decay mechanism is then taking place, in agreement with fs transient absorption
(TA). The fluorescence emission of the SQ moiety in BDP-SQ was compared with the reference
compound SQ (Fig. 2c and 2d). It was found that the fluorescence was quenched to lesser extent.
The quenching effect is similar in toluene and acetonitrile. These results indicate that the photo-
induced electron transfer in BDP-SQ is not significant.
A similar study was carried out for Styryl-BDP-SQ (Fig. 3). Efficient quenching was observed
in both toluene and acetonitrile solution for the fluorescence of the energy donor. This quenching
can be attributed to FRET or PET. The fluorescence emission of Styryl-BDP-SQ was compared
with that of Styryl-BDP. We found the fluorescence quenching is more significant in polar
solvent such as acetonitrile (Fig. 3d). Thus we propose that PET is more significant in Styryl-
BDP-SQ than that in BDP-SQ.
7

ACCEPTED MANUSCRIPT
The fluorescence lifetimes of the compounds were also measured (Fig. 4). BDP and BDP-SQ
show similar fluorescence lifetimes, with the fluorescence lifetime of BDP slightly reduced (2.9
6
4
2
00
00
00
0
a
b
6
4
2
00
00
00
0
BDP
BDP-SQ
BDP
BDP-SQ
5
00 550 600 650 700
500
550
600
650
Wavelength / nm
Wavelength / nm
c
1
00
100
50
0
d
BDP-SQ
SQ
BDP-SQ
SQ
5
0
0
6
00
700
800
600
700
800
Wavelength / nm
Wavelengh / nm
Fig. 2. The fluorescence emission spectrum of SQ, BDP and BDP-SQ, measured with optically
−5
matched solutions (A = 0.27 at 470 nm. c = ca. 1.0 × 10 M, A = 0.055 at 530 nm. c = ca. 1.0 ×
−
5
1
4
0 M). (a) BDP and BDP-SQ in toluene, λex = 470 nm, (b) BDP and BDP-SQ in MeCN, λex
=
70 nm, (c) BDP-SQ and SQ in toluene, ex = 530 nm, (d) BDP-SQ and SQ in MeCN, λex = 530
λ
nm. 20 °C.
ns) as compared with that of pristine BDP (3.5 ns). Note the fluorescence intensity of BDP is
almost completely quenched in BDP-SQ (Fig. 2a and 2b). We assume that some BDP-SQ
molecules take specific conformations for which the FRET is inhibited, thus the fluorescence
lifetime is intact. However, most of the molecules take a conformation which is beneficial for
8

ACCEPTED MANUSCRIPT
FRET, thus the fluorescence intensity is significantly quenched (Fig. 2a and 2b). This scenario is
similar to the 'static' quenching. The fluorescence lifetime of the Styryl-BDP moiety in Styryl-
BDP-SQ (3.5 ns) is shorter than the pristine Styryl-BDP (5.1 ns) (in toluene).
In the polar solvent acetonitrile, however, the fluorescence lifetime of Styryl-BDP moiety in
Styryl-BDP-SQ is reduced significantly to 1.0 ns, which is much shorter than that of Styryl-BD
P (5.5 ns) in the same condition. These results indicated that CS is significant for Styryl-BDP-
SQ in polar solvents. This postulation was supported by the later free energy study, and from
nanosecond and picosecond time-resolved optical spectroscopic studies (see later sections).
5
4
3
2
1
00
00
00
00
00
0
2
1
00
50
a
b
Styryl-BDP-SQ
SQ
Styryl-BDP-SQ
SQ
100
5
0
0
5
50 600 650 700 750 800
Wavelength / nm
5
50 600 650 700 750 800
Wavelength / nm
3
2
1
0
0
0
0
5
4
3
0
0
0
c
d
Styryl-BDP
Styryl-BDP-SQ
Styryl-BDP
Styryl-BDP-SQ
20
1
0
0
6
50 700 750 800
Wavelength / nm
650 700 750 800
Wavelength / nm
Fig. 3. The fluorescence emission spectra of the compounds measured with optically matched
−5
solutions (A = 0.067 at 530 nm. A = 0.23 at 600 nm. c = ca. 1.0 × 10 M). (a) Styryl-BDP-SQ
and SQ in toluene, λex = 530 nm, (b) Styryl-BDP-SQ and SQ in MeCN, λex = 530 nm, (c)
Styryl-BDP-SQ and Styryl-BDP in toluene,
in MeCN, λex = 600 nm. 20 °C.
λ
ex = 600 nm, (d) Styryl-BDP-SQ and Styryl-BDP
9

ACCEPTED MANUSCRIPT
The FRET efficiency has been very often evaluated with the fluorescence quenching of the
energy donor in the dyads or triads.[48] However, this method is not always precise because
other photophysical processes, such as electron transfer, can also induce fluorescence quenching.
Recently it was proposed that FRET efficiency can be evaluated by the comparison of the
fluorescence excitation spectra with the UV−vis absorption spectra.[49]
1
0000
1000
a
SQ τf = 2.1 ns
BDP-SQ τf = 1.7 ns
b
BDP-SQ τf = 2.9 ns
1
000
BDP τf = 3.5 ns
1
00
1
00
1
0
1
0
0
10 20 30 40 50
Time / ns
0
10 20 30 40 50
Time / ns
c
10000
Styryl-BDP-SQ τf = 1.0 ns
Styryl-BDP τf = 5.5 ns
d
Styryl-BDP τf = 5.1 ns
Styryl-BDP-SQ τf = 3.5 ns
1
000
1
000
00
1
1
00
1
0
0
10 20 30 40 50
Time / ns
0
10 20 30 40 50
Time / ns
Fig. 4. Fluorescence lifetimes of the compounds. (a) BDP (
λ
em = 518 nm) and BDP-SQ (
14 nm), λex = 445 nm, in toluene (b) SQ (λem = 580 nm) and BDP-SQ (λem = 589 nm), λex
45 nm, in toluene (c) Styryl-BDP (λem = 641 nm) and Styryl-BDP-SQ (λem = 641 nm), λex
λ
em
=
=
=
5
4
6
35 nm, in toluene (d) Styryl-BDP-SQ and Styryl-BDP (λem = 626 nm) in acetonitrile, λex = 635
−5
−6
−5
nm. (a) c = 1.0 × 10 M and c = 5.0 × 10 M. (b), (c) and (d) c = 1.0 × 10 M. 20 °C.
10

ACCEPTED MANUSCRIPT
The comparison of the excitation spectra and the UV−vis absorption spectra indicated efficient
intramolecular energy transfer, the energy transfer quantum yields are 72.7% and 100% for BDP-
SQ and Styryl-BDP-SQ (in toluene), respectively (Figure 5). However, it should be pointed out
that the quantitative energy transfer indicated by the fluorescence excitation spectra of Styryl-
BDP-SQ does not necessarily mean that there is no charge separation (CS) for this molecule. The
CS may occur after the FRET. The quantitative energy transfer indicates that the ISC of the SQ
unit is completely out-competed by the fast FRET process. In the following we show by ultrafast
transient absorption that FRET is faster than the ISC and CS processes.
1
1
0
0
0
0
.25
.00
.75
.50
.25
Abs
Exc
Abs
Exc
1.25
a
b
Abs
Exc
1
0
0
.00
.75
.50
c
1
0
0
0
.00
.75
.50
.25
0.25
.00
3
0
.00
300
0.00
00
400
500
600
700
400
500
600
700
300 400 500 600 700
Wavelength / nm
Wavelength / nm
Wavelength / nm
Fig. 5. Comparison of the UV−vis absorption and fluorescence excitation spectra. (a) BDP-SQ,
−6
−6
λ
em= 630 nm. c = 3.3 × 10 M in toluene. (b) Styryl-BDP-SQ,
λ
em= 705 nm. c = 1.0 × 10 M in
−6
toluene. (c) BDP-SQ, λem= 630 nm. c = 3.3 × 10 M in acetonitrile. 20 °C.
11

ACCEPTED MANUSCRIPT
Table 1. Photophysical Parameters of the Compounds.
h
i
ΦT
/
a
b
c
d
e
f
g
λabs /nm
ε
λem/nm
ΦF / % Φ∆ / % τF /ns τT / µs
i
i
%i
BDP
504
1.02
518
83.0
53.4
9.30
27.7
51.0
−
−
3.5
5.1
−
−
−
i
i
Styryl-BDP
BDP-SQ
350/628 0.87/1.13
504/566 1.74/1.44
565/629 1.78/2.13
641
−
517/589
641
24.0/15.3 2.9/1.7 −/197.8
31.4
−
i
−
i
−
i
Styryl-BDP-SQ
3.5
2.0
SQ
563
0.90
580
50.1
126.1
57.8
a
b
5
−1
−1
c
The maximum absorption wavelength.
ε
are molar absorption coefficient in toluene. In 10 M cm . The
−5
d
maximum emission wavelength in toluene (c = 1.0 × 10 M). Fluorescence quantum yields determined with
Styryl-BDP as reference (Φ = 59.0 % in toluene), BDP determined with Bodipy as reference, BDP-SQ
determined with diiodo (2,6-diiodo substituted)-BDP as reference (Φ = 3.6 % in toluene) The singlet
F
e
F
oxygen quantum yield determined with Rose Bengal (RB) as reference (Φ = 75.0 % in methanol). SQ
∆
f
g
determined with diiodo-BDP (Φ = 85.0 % in toluene) as reference Fluorescence lifetimes. Triplet state
∆
−5
h
lifetimes, measured by transient absorptions at c = 1.0 × 10 M in toluene. Triplet state quantum yields
determined with RB as reference (Φ = 90.0 % in methanol), SQ determined with diiodo-BDP as reference
T
i
(
Φ = 88.0 % in toluene). Not observed.
T
2
.3. Nanosecond Time-Resolved Transient Absorption Spectroscopy: the Lifetime and the Spatial
Localization of the T Excited State
1
In order to study the triplet state properties of the triads, nanosecond transient absorption
spectra were measured (Fig. 6). Firstly we studied the property of SQ. Upon pulsed excitation at
5
32 nm, a bleaching band at 563 nm was observed, which is attributed to the depletion of the
ground state of the molecule (Fig. 6d). Excited state absorption (ESA) bands in the range of 350
500 nm and 600−700 nm were observed. It should be pointed out that there is superposition of
the ESA band and the bleaching band, thus the ESA of T state of SQ may cover a broad range
−
1
(
entire visible range of the spectra). These properties indicate that such materials are promising
candidates for non-linear optical applications, such as reverse saturation absorption materials
optimal limiting materials).[14]
(
12

ACCEPTED MANUSCRIPT
0
0
.01
.00
0.005
.000
0
.00
-0.05
0.10
0
-
-
-
-
0.01
0.02
0.03
0.04
8
7
07.4
22.4
µ
µ
s
s
τT = 172
µ
s
-0.005
-0.010
-0.015
τT = 31 µs
...
-
4
0
2.4
µ
s
µ
s
a
b
c
-0.15
4
00 500 600 700
Wavelength / nm
0
200 400 600 800 1000
Time /
2
00 400 600 800 1000
Time /
µ
s
µs
0
0
.05
.00
0.01
0.00
0.05
0.00
-0.01
-0.02
-0.03
-0.04
-0.05
-
0.05
0.10
0.15
0.20
0.25
2
5
.
16.6
2.0
µ
s
-
τT = 208 µs
µ
s
τT = 126 µs
-
-
-
-
..
8.6
µs
-0.10
0.15
0 µs
d
f
e
-
4
00 500 600 700
0
500 1000 1500 2000
Time /
0
500 1000 1500 2000
Time /
Wavelength / nm
µ
s
µs
Fig. 6. Nanosecond transient absorption of the compounds. (a) Transient absorption spectra of
BDP-SQ, λex = 550 nm. (b) Decay trace of BDP-SQ at 563 nm in toluene, λex = 532 nm. (c)
Decay trace of BDP-SQ at 563 nm in acetonitrile,
λex = 532 nm. (d) Transient absorption spectra
of SQ, λex = 532 nm. (e) Decay trace of SQ at 563 nm in toluene, λex = 532 nm. (f) Decay trace
−6
of SQ at 563 nm in acetonitrile, λex = 532 nm. (a), (b), (c) and (d) c = 5.0 × 10 M. (e) and (f) c
−5
=
1.0 × 10 M. 20 °C.
The triplet state lifetime of SQ was determined as 126 µs in toluene (Figure 6e). Interestingly,
this lifetime was increased to 208 µs in a polar solvent such as acetonitrile (Figure 6f). At the
same time the optical density (O.D.) values decreased in acetonitrile. These informations indicate
that the triplet state quantum yield decreased in acetonitrile as compared to that in toluene
(Triplet state quantum yield in toluene is 51.0%). Previously the triplet state properties of the
13

ACCEPTED MANUSCRIPT
1
2
closely-related squaraine dyes were probed only with formation of singlet oxygen ( O ).[33]
Normally the triplet state of squaraine was observed with the heavy atom effect.[50] Without
heavy atom-effect, the ISC ability of SQ is poor. It was reported that squaraine dyes with Bis[4-
(dimethylamino) phenyl]squaraine structure show much lower triplet state quantum yield (<
5
1
0
.1%), and the nanosecond transient absorption spectra were obtained with sensitizing method.
The triplet state lifetime was reported as ca. 35 µs.[51] Bis(3,5-dibromo-2,4,6-
trihydroxyphenyl)squaraine was reported to show much shorter triplet state lifetime, ca. 1.7
µs.[52]
The nanosecond transient absorption spectra of BDP-SQ were also studied (Fig. 6a). The
bleaching band and the ESA band of BDP-SQ are literally identical to that of SQ, leading us to
propose that the T state of BDP-SQ is localized on the SQ unit, not on the BDP unit. The ESA
1
bands of BDP-SQ are also similar to that of SQ. This conclusion is supported unambiguously by
the position of the bleaching band of this triad.[38,53,54] The triplet state lifetime of BDP-SQ
was determined as 172.2 µs, which decreased to 31 µs in acetonitrile (Fig. 6a and 6c). The O.D.
values also decreased sharply in acetonitrile. These results indicate that the triplet state quantum
yield decreased in acetonitrile as compared with that in toluene. We propose that a charge
transfer state may be responsible for the yield of the triplet state of BDP-SQ in polar solvents.
This postulation was confirmed by the calculation of the ∆GCS values (Table 3) and the results
obtained by measuring femtosecond transient absorption spectra of the compounds.
No nanosecond transient absorption signal can be recorded for Styryl-BDP-SQ. Femtosecond
TA spectra indicated ultrafast FRET for Styryl-BDP-SQ, and CS in toluene. The lack of triplet
state in Styryl-BDP-SQ can thus be justified by the fast FRET and the formation of the CS state
(see later section).
14

ACCEPTED MANUSCRIPT
2
.4. Ultrafast Transient Absorption Spectroscopy: FRET, Charge Separation and Intersystem
Crossing
In order to study the FRET and the charge separation processes of the triads upon
photoexcitation, femtosecond ultrafast transient absorption spectra of the triads were measured
(Fig. 7−10) upon different excitation conditions both in toluene and acetonitrile. All the data
were analyzed by a global analysis procedure imposing a linear kinetic scheme with increasing
lifetimes.
For BDP-SQ, excitation of the BDP moiety at 485 nm induces electronic energy transfer
towards the SQ unit, indicated by the diminishment of the ground state bleaching band at 500 nm
and the concomitant growth of the bleaching band at 560 nm (Fig. 7a). The former ground state
bleaching band is due to the BDP unit while the bleaching band at 560 nm is due to the SQ unit.
Similar results were observed in acetonitrile (Fig. 7b). Therefore the FRET process occurs in
both toluene and acetonitrile, however, the kinetics of the process are different in two solvents. In
both cases the process appears to be bi-exponential, which may be connected with the presence
of different molecular conformers in solution, in which the relative distance between the BDP
and SQ molecules can vary, as recently observed in analogous systems.[55] In toluene, energy
transfer occurs on a 1.6 ps and 14 ps timescale. The system then recovers on a 1.3 ns timescale,
possibly forming triplet states. The occurrence of FRET is well evidenced by the analysis of
kinetic traces presented in Fig. 7c, showing that signals due to BDP decay on the same timescale
at which signals belonging to SQ increase. By considering the residual BDP signal peaking at
5
00 nm observed in the last spectral component it can be inferred that the efficiency of energy
transfer is quite high (almost 90%).
15

ACCEPTED MANUSCRIPT
0.00
0.00
-0.02
-0.04
-0.06
-0.08
-0.10
1
1
1
.6 ps
4.9 ps
.3 ns
0
5
.66 ps
.5 ps
-
-
-
-
0.02
0.04
0.06
0.08
232 ps
a
b
5
00
600
700
5
00
600
700
Wavelength / nm
Wavelength / nm
0
.00
0
.00
-
-
-
-
-
0.02
0.04
0.06
0.08
0.10
-0.02
-0.04
-0.06
-0.08
-0.10
5
05 nm
570 nm
FIT
5
5
FIT
00 nm
60 nm
c
d
0
50 100 150 200
0
50 100 150 200
Time / ps
Time / ps
Fig. 7. The femtosecond time-resolved transient absorption spectra of BDP-SQ in (a) toluene and
(b) acetonitrile. λex = 485 nm. The decay curves at a few selected wavelengths were presented in
–
5
(
c) and (d), respectively. c = 1.0 × 10 M. 20 °C.
Energy transfer was also observed in acetonitrile, and again it proceeds with a bi-exponential
kinetics (Fig. 7d). The process appears to be faster in this solvent, the time constants retrieved
from global analysis result 0.6 and 5.5 ps. Differently from what observed in toluene, most of the
signal recovers on a relatively fast 232 ps timescale in acetonitrile. It is thus unlikely that ISC can
occur very efficiently in this solvent, however since the SQ signal goes not completely to zero on
the ns timescale, a certain amount of triplet can still be produced in this solvent, as
16

ACCEPTED MANUSCRIPT
0
.00
0.00
-
-
-
-
0.05
0.10
0.15
0.20
3
.1 ps
0
1
1
.5 ps
55 ps
.2 ns
612.2 ps
-
-
0.05
0.10
3.1 ns
a
b
5
00
600
700
500
600
700
Wavelength / nm
Wavelength / nm
0
.00
0
.00
-
-
-
0.05
0.10
0.15
5
00 nm
-0.05
-0.10
4
5
FIT
95 nm
62 nm
570 nm
FIT
d
c
0
200 400 600 800100 01 200
Time / ps
0 200 400 600 800100 01 200
Time / ps
Fig. 8. The femtosecond time-resolved transient absorption spectra of BDP-SQ in (a) toluene and
(b) acetonitrile. λex = 580 nm. The decay curves at a few selected wavelengths were presented in
–
5
(
c) and (d), respectively. c = 1.0 × 10 M. 20 °C.
previously shown from the ns TA measurements. The reason for the high fraction of fast
recovery is not totally clear: it could be connected with a higher interaction between the BDP and
SQ moiety in this solvent, possibly favored by the presence of conformations in which the two
molecules are very close. Indeed the absorption spectrum of the triads is slightly different from
the sum of the absorptions of the individual components, possibly indicating some kind of
interaction among them. The presence of conformations in which the chromophores are very
close to each other could determine the possibility of additional non-radiative decay pathways or
alternatively lead to the formation of excited state charge transfer states, whose lifetime may be
17

ACCEPTED MANUSCRIPT
short even in polar solvents. Energy transfer seems slightly less efficient in acetonitrile, since in
the last spectral component the intensity of the BDP signal is ca 30% of its initial value.
When the system is excited at 580 nm the transient spectra (Fig. 8) in toluene mostly resemble
that of the isolated SQ molecule. However a small signal peaked at 500 nm poorly visible on a
short timescale, gains some intensity on a 155 ps timescale. This could be indicative of the
occurrence of electron transfer in the system. In acetonitrile, BDP spectral features can be already
observed in the initial EADS (evolution associated difference spectra), possibly indicating the
occurrence of fast electron transfer. On a 3 ps timescale the BDP signal increases in intensity,
while the SQ signal decreases in intensity, mostly in its red-most side. The overall signal mostly
decreases on a 100 ps timescale, the remaining fraction decays on a 612.2 ps timescale.
For Styryl-BDP-SQ, excitation of the SQ moiety at 525 nm induces FRET in toluene (Fig. 9a).
As already observed for BDP-SQ, the kinetics of energy transfer is multi-exponential. Spectral
signatures of the Styryl-BDP unit can be observed already in the first EADS, indicating that in a
fraction of the molecules energy transfer is very fast. The intensity of the Styryl-BDP signal
increases bi-exponentially on a 0.3 and 17 ps timescale. The signal recovers only at a minimum
amount in the last spectral component indicating a long ns lifetime and possibly the occurrence
of ISC (not evidenced on the timescale of ps-transient absorption measurements). The estimated
efficiency of energy transfer is almost 60% (from the residual SQ signal at the long timescale).
The fast FRET may compete efficiently with the ISC of the SQ unit (the ISC of this unit takes ca.
1
ns, confirmed with separate experiment, see Supporting Information, Fig. S32). Thus the lack
of formation of triplet state with Styryl-BDP-SQ (upon excitation into the SQ unit) can be
interpreted as due to the fast FRET.
18

ACCEPTED MANUSCRIPT
0
0
.02
.00
0.05
0
.00
-
-
-
0.02
0.04
0.06
-
0.05
0.3 ps
17 ps
3 ns
0.5 ps
92 ps
4.8 ps
a
b
-0.10
5
00
600
700
500
600
700
Wavelength / nm
Wavelength / nm
0
.02
.00
c
d
0
.00
560 nm
30 nm
0
6
FIT
-0.02
-
0.02
-
-
-
0.04
0.06
0.08
5
6
55 nm
30 nm
-0.04
FIT
0
5
10 15 20 25
Time / ps
0
50 100 150 200
Time / ps
Fig. 9. The femtosecond time-resolved transient absorption spectra of Styryl-BDP-SQ in (a)
toluene and (b) acetonitrile. ex = 525 nm. The decay curves at a few selected wavelengths were
λ
–
5
presented in (c) and (d), respectively. c = 1.0 × 10 M. 20 °C.
In acetonitrile once again the dynamics of the system appears to be faster (Fig. 9b and 9d). A
significant fraction of Styryl-BDP signal is observed in the initial component, indicating fast
energy transfer. In the next EADS the SQ signal slightly decreases, while positive bands peaked
at ca 600 and 660 nm develops. The transient signal recovers almost completely on a 90 ps
timescale. The fast recovery could indicate that electron transfer is also occurring in this case.
+
Possible signature of the Styryl-BDP formation could be the increased absorption at 660 nm
(see spectroelectrochemical studies, Supporting Information, Fig. S30g). In a recent work we
19

ACCEPTED MANUSCRIPT
found that upon oxidation of a bodipy system an absorption band on the red side of its stimulated
emission develops.[56]
0
.05
.00
0
.05
.00
0
0
-
-
-
0.05
0.10
0.15
-
-
-
0.05
0.10
0.15
1
6
2
.3 ps
6 ps
.5 ns
1
.9 ps
85 ps
925 ps
a
b
-0.20
5
00
600
700
500
600
700
Wavelength / nm
Wavelength / nm
0
.02
.00
0.00
0
-0.02
-0.04
-0.06
-0.08
-
-
0.05
0.10
634 nm
50 nm
FIT
5
6
FIT
70 nm
40 nm
5
c
d
0
200 400 600 800100 01 200
Time / ps
0
50 100 150 200
Time / ps
Fig. 10. The femtosecond time-resolved transient absorption spectra of Styryl-BDP-SQ in (a)
toluene and (b) acetonitrile. λex = 620 nm. The decay curves at a few selected wavelengths were
–
5
presented in (c) and (d), respectively. c = 1.0 × 10 M. 20 °C.
The photophysical processes of Styryl-BDP-SQ upon excitation at 620 nm were also studied
(Fig. 10). The initial EADS obtained upon excitation at 620 nm in toluene closely resemble the
spectral signatures of the isolated Styryl-BDP dye. However at later times the Styryl-BDP
bleaching/stimulated emission (SE) signal partially recovers, while at the same time the signal
peaking at 570 nm, which can be assigned to the SQ chromophore increases in intensity (see
kinetic traces in Fig. 10c). This observation indicates the occurrence of electron transfer in the
20

ACCEPTED MANUSCRIPT
system. An estimation of the yield of charge separation is not straightforward, but it does not
appear extremely high. However the lifetime of the CS state looks quite long, since signatures of
SQ are still visible at the long ns timescale.
In acetonitrile once again the first EADS closely resembles the spectra of the isolated Styryl-
BDP. Upon ca 2 ps the system evolves towards the second spectral component: here the
appearance of SQ bleaching/SE signal and of the excited state absorption at 660 nm, possibly
+
attributable to the increased Styryl-BDP , indicate the occurrence of electron transfer. The
+
absorption of Styryl-BDP at 660 nm was confirmed with spectroelectrochemical studies (see
Supporting Information, Fig. S30g). A significant part of the signal recovers on a 90 ps timescale,
the SQ signals completely disappear, indicating fast charge recombination, while a residual
Styryl-BDP signal remains, further decaying on a 900 ps timescale.
All the kinetic constants retrieved from ps-transient absorption measurements are summarized
in Table S1 in SI
2
.5 Electrochemical Studies: Cyclic Voltammetry and the Gibbs Free Energy Changes of the
Photo-induced Charge Separation (CS)
In order to study the probability of photo-induced charge separation, i.e. photo-induced
electron transfer, the electrochemical properties of the compounds were analyzed (Fig. 11).
For the reference compound BDP, a reversible reduction wave was observed at −1.47 V and a
quasi-reversible oxidation wave was recorded at +0.97 V. For SQ, an irreversible reduction wave
at −1.05 V was observed and a quasi-reversible oxidation wave was observed at +0.75 V. For
BDP-SQ triad, the reduction wave at −1.47 V is similar to that of BDP, but the irreversible
reduction wave at −1.16 V is slightly cathodically shifted. In the anode region, a quasi-reversible
oxidation wave was observed at +0.72 V, which is similar to that of SQ. A quasi-reversible
21

ACCEPTED MANUSCRIPT
oxidation band was observed at +0.91 V, which is slightly anode shifted as compared with that of
BDP. These shifts indicate that there is a dipole-dipole interaction between the chromophores in
the triad BDP-SQ. This postulation is in agreement with the UV−vis absorption of the
compounds. The redox potentials were summarized in Table 2.
+
a
Fc / Fc
b
+
Fc / Fc
BDP
BDP-SQ
SQ
Styryl-BDP
Styryl-BDP-SQ
SQ
1
0
-1
-2
1
0
-1
-2
Potential / V
Potential / V
Fig. 11. Cyclic voltammogram of (a) BDP, SQ and BDP-SQ (b) Styryl-BDP, SQ and Styryl-
BDP-SQ. Deaerated DCM with 0.10 M Bu NPF was used as thesupporting electrode and
4
6
Ag/AgNO was used as the reference electrode. Scan rates are 50 mV/s. Ferrocene (Fc) was used
3
−3
as an internal reference. c = 1.0 × 10 M. 20 °C.
a
Table 2. Redox Potentials of the Compounds.
E(ox) / V
E(red) / V
−1.05
SQ
+0.75
BDP
+0.97
−1.47
Styryl-BDP
BDP-SQ
+0.70
−1.20
+0.91, +0.72
−1.47, −1.16
−1.06, −1.20
Styryl-BDP-SQ +0.68, +0.63
Cyclic voltammetry in N
a
2
6
saturated DCM 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.
3
22

ACCEPTED MANUSCRIPT
In Fig. 11(b), the reversible reduction wave of the reference compound Styryl-BDP was
observed at −1.20 V, and the reversible oxidation wave was obtained at +0.70 V. For Styryl-
BDP-SQ triad, the reduction waves at −1.05 V and −1.20 V are similar to the position of
reduction waves of SQ and Styryl-BDP, respectively. In the anode region, a pseudo-reversible
oxidation wave at +0.63 V and an irreversible oxidation wave at +0.68 V were observed. The
wave at +0.63 V and +0.68 V can be assigned to the Styryl-BDP moieties and SQ moiety
respectively. Possibly, these shifts could be attributed to dipole-dipole interaction between the
Styryl-BDP part and the SQ part in the triad Styryl-BDP-SQ.
0
∆
G
CS = e[E − E ] − E + ∆G
(1)
OX
RED
00
S
2
2
e
e  1
1  1
1 
∆
G = −
−


+


−


(2)
S
4
πεSε0 RCC
8
πε0 RD RA εREF εS



∆
G = −
(
∆G + E
)
(3)
CR
CS
00
In order to study the PET in BDP-SQ and Styryl-BDP-SQ, the Gibbs free energy changes
(
∆G ) of the electron transfer processes were calculated with the Rehm-Weller equation.
CS
Here ∆G is the static Columbic energy, which is described by eq. 2; e is electronic charge, E
S
OX
is half-wave potential for mono-electron oxidation of the electron-donor unit, ERED is half-wave
potential for one-electron reduction of the electron-acceptor unit. It should be noted that the
formal potential E cannot be derived from the cyclic voltammogram profile as the oxidation is
1
/2
irreversible. In this case, the anodic and cathodic peak potentials were used for calculation; E00 is
energy level approximated with the fluorescence emission wavelength (for the singlet excited
state). εS = static dielectric constant of the solvent, RCC (the values are 13.7 and 17.1 Å for BDP-
23

ACCEPTED MANUSCRIPT
SQ and Styryl-BDP-SQ, respectively) is the center-to-center separation distance determined by
DFT optimization of the geometry, R is the radius of the electron donor, R is the radius of the
D
A
electron acceptor, εREF is the static dielectric constant of the solvent used for the electrochemical
studies, ε permittivity of free space. The solvents used in the calculation of free energy of the
electron transfer are CH CN ( = 37.5), CH Cl ( = 8.93), toluene ( = 2.4, 20 °C). The redox
0
ε
ε
2
ε
3
2
potentials and the free energy changes of the electron transfer are reported in Table 3.
The results show that, the photoinduced intramolecular electron transfer is thermodynamically
prohibited for BDP-SQ in toluene due to the positive value of ∆G (+0.01 eV). In polar solvents
CS
2 2 3
such as CH Cl and CH CN, the values are −0.46 eV and −0.58 eV, respectively. Thus the PET
process is thermodynamically allowed for BDP-SQ in polar solvents. The fluorescence
quenching of BDP-SQ in polar solvents (Fig. 2) is in accordance with this postulation (after
FRET).
Table 3. Gibbs Free Energy Changes (∆G ) of the PET Processes and the Energy Level of the
CS
Charge Transfer States of BDP-SQ and Styryl-BDP-SQ in Different Solvents.
∆
G
CS[eV]
∆GCR(eV)
∆GCS[eV]
∆GCR(eV)
∆GCS[eV]
∆GCR(eV)
(
Toluene)
(Toluene)
(DCM)
(DCM)
(MeCN)
(MeCN)
a
a
a
BDP→SQ
+0.01
−2.41
−2.41
−0.46
−1.95
−1.95
−1.60
−1.60
−0.58
−1.83
−1.83
−1.52
−1.52
c
c
c
+
−
−
0.27
−0.18
−0.30
b
b
b
Styryl-
BDP→SQ
0.005
−1.90
−0.35
−0.43
c
c
c
0.25
−1.90
−0.55
−0.63
a
1
b
1
c
1
Via [BDP]*. Via [Styryl-BDP]*. Via [SQ]*.
The Gibbs free energy changes of the possible PET in Styryl-BDP-SQ were also calculated.
GCS are negative not only in polar solvents (∆G = −0.35 eV in DCM and ∆G = −0.43 eV in
∆
CS
CS
MeCN), but also in toluene (∆G = −0.005 eV). These data suggest that fluorescence of Styryl-
CS
24

ACCEPTED MANUSCRIPT
BDP-SQ may be quenched in all solvents by PET effect, which is in agreement with the
experimental results presented above (Fig. 3).
2
.6. DFT Computations: Rationalization of the Photophysics Properties of the Triads
The photophysical properties of the triads were rationalized with the DFT computations. Firstly,
the confinement of the triplet excited states of the compounds were studied (Fig. 12). As reported
in the previous sections, nanosecond transient absorption spectra indicated that for BDP-SQ, the
triplet state is exclusively localized on the SQ unit (supported by the ground state bleaching
band). For Styryl-BDP-SQ, however, the T triplet state is exclusively localized on the Styryl-
1
BDP unit. The computed spin density surfaces reported in Fig. 12 confirm the experimental
results.
BDP-SQ
SQ
Styryl-BDP-SQ
Fig. 12. Isosurfaces of spin density of BDP-SQ, Styryl-BDP-SQ and SQ at the optimized triplet
state geometries. Calculation was performed at B3LYP/6-31G(d) level with Gaussian 09W.
25

ACCEPTED MANUSCRIPT
TDDFT computations, collected in Table 4, show that for BDP-SQ molecule the first excited
singlet state is responsible of the most intense absorption, with oscillator strength greater than 1.
As experimentally observed, in BDP-SQ a second absorption band appears blue-shifted of about
6
0 nm. It causes the population of two degenerate singlet states, S and S , due to the presence of
3 4
the symmetric BODIPY portions in the molecule.
Table 4. Vertical absorption and emission (ꢀE) in eV, wavelengths (λ) in nm, MO contribution
%) and oscillator strengths (f) from the low-lying singlet state calculated at TD−B3LYP/6-
(
3
1+G* levels of theory in toluene.
a
b
Excitation energy
Excited states
MO contribution
λ
f
∆E
SQ
S1
T1
T2
H→L (100%)
H→L (100%)
H−1→L (99%)
2.35
2.28
1.23
526
1.293
0.000
0.000
BDP-SQ
S
T
T
T3
S3
1
H→L (100%)
H→L (100%)
H−2→L+1 (100%)
H−1→L+2 (100%)
H−2→L+1 (96%)
H−1→L+2 (96%)
2.37
1.17
1.49
1.50
2.80
2.80
524
1.273
0.000
0.000
0.000
0.662
0.599
1
2
444
443
S
4
Styryl-BDP-SQ
S1
H−1→L (99%)
H→L+1 (99%)
H−1→L (100%)
H→L+1 (100%)
H−2→L+2 (100%)
2.03
2.04
1.05
1.06
1.23
2.37
610
608
1.085
1.052
0.000
0.000
0.000
1.277
S
2
T
1
T2
T3
S4
H−2→L+2 (100%)
524
a
b
Only selected excited states were reported; H and L stand for HOMO and LUMO, respectively.
Only the main configurations are presented.
For the triad Styryl-BDP-SQ, instead, the absorption at highest wavelengths is due to two
transitions, S →S and S →S , having approximately the same oscillator strength.
0
1
0
2
26

ACCEPTED MANUSCRIPT
For the most promising triad, BDP-SQ, the electronic excited states were studied in detail (Fig.
S36). From TDDFT computations it results the S →S transition attributable to the SQ unit. It is
0
1
featured as a π−π* transition with an oscillator strength of 1.273. The calculated absorption
wavelength isoverestimated of about 40 nm, as expected.[57,58] The subsequent bright singlet
LUMO
f=0.000
2.28
HOMO−1
S
1
T
2
LUMO
.37 eV
f=1.293
LUMO
f=0.000
T
1
2
1
.23
S
0
HOMO
HOMO
Fig. 13 Selected frontier molecular orbitals involved in the singlet and triplet excited states of
SQ. The calculations were performed at the TD−B3LYP/6-31+G* level using Gaussian 09W.
Toluene was used as solvent in the calculations.
27

ACCEPTED MANUSCRIPT
−1
Table 5. SOC (cm ) between low-lying singlet and triplet excited states at B3LYP/cc-
pVDZ//B3LYP/6-31+G* level of theory obtained in the framework of atomic mean field on the
optimized geometry of the ground state of SQ and BDP-SQ.
〈
ψS1Ĥ ψT1〉
〈ψS1Ĥ ψT2〉
〈ψS1Ĥ ψT3〉
SO
SO
SO
BDP-SQ
SQ
0.93
0.97
0.0
0.0
0.03
−
states, S and S , are generated by transitions involving the BODIPY portions of the triad, with
3
4
excitation energies of 444 and 443 nm, against 504 nm experimentally found. However, it is
known that TDDFT calculations performed on BODIPY chromophores often give absorption
wavelengths different from the experimental counterparts.[59−63]
Since the nanosecond transient absorption spectra indicated the existence of ISC for BDP-SQ,
the ISC channel of BDP-SQ was preliminarily studied with the TDDFT computations (Fig. S36).
Although it is known that ISC normally occurs between two states having similar energy, looking
1
at data reported in Table 5, the SOC values computed for the coupling of S with the triplet states
−1
lying below the bright one are different from zero only for the S →T radiationless (0.93 cm )
1
1
transition. This analysis is in agreement with the experimental results.
It is noteworthy that ISC of the SQ unit is of particular interest, since there are no heavy atoms,
such as bromine or iodine, or transition metal centre. The ISC process is attributed to S →T
1
1
−1
transition with SOC values 0.97 cm (Fig. 13 and Table 5). Heavy atom-free triplet
photosensitizers are of fundamental importance in photochemistry, photocatalysis as well as in
photodynamic therapy (PDT).
2
.7. Summary of the Photophysical Processes of the Triads
28