Photoinduced charge separation in wide-band capturing, multi-modular bis(donor styryl)BODIPY-fullerene systems

Article abstract of DOI:10.1039/c6cp03479a

A new series of multi-modular donor-acceptor systems capable of exhibiting photoinduced charge separation have been designed, synthesized and characterized using various techniques. In this series, the electron donor was a BF₂-chelated dipyrromethene (BODIPY) appended with two styryl linkers carrying two electron rich triphenylamine or phenothiazine entities. Fulleropyrrolidine linked at the meso-position of the BODIPY ring served as an electron acceptor. As a result of extended conjugation and multiple electroactive chromophore entities, the bis-styryl BODIPY revealed absorbance and emission well-into the near-infrared region covering a 300-850 nm spectral range. Using redox, computational, absorbance and emission data, an energy level diagram was constructed that helped in envisioning the different photochemical events. Spectral evidence for photoinduced charge separation in these systems was established from femtosecond and nanosecond transient absorption studies. The measured rate constants indicated fast charge separation and relatively slow charge recombination revealing their usefulness in light energy harvesting and optoelectronic device building applications. The bis(donor styryl)BODIPY-fullerene systems populated BODIPY triplet excited states during the process of charge recombination.

Full text of DOI:10.1039/c6cp03479a

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Page 1 of 31
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Photoinduced charge separation in wide-band capturing,
multi-modular bis(donor styryl)BODIPY-fullerene
systems
Christopher O. Obondi,a Gary N. Lim,a Paul A. Karr,b Vladimir N. Nesterov,a
and Francis D’Souzaa,*
aDepartment of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton, TX
76203-5017, USA; E-mail: Francis.DSouza@UNT.edu;
bDepartment of Physical Sciences and Mathematics, Wayne State College, 111 Main Street,
Wayne, Nebraska 68787, USA
1

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(Abstract) A new series of multi-modular donor-acceptor systems capable of exhibiting
photoinduced charge separation have been designed, synthesized and characterized using various
techniques. In this series, the electron donor was a BF2-chelated dipyrromethene (BODIPY)
appended with two styryl linkers carrying two electron rich triphenylamine or phenothiazine
entities. Fulleropyrrolidine linked at the meso-position of the BODIPY ring served as an
electron acceptor. As a result of extended conjugation and multiple electroactive chromophore
entities, the bis-styryl BODIPY revealed absorbance and emission well-into the near-infrared
region covering 300-850 nm spectral range. Using redox, computational, absorbance and
emission data, an energy level diagram was constructed that helped in envisioning the different
photochemical events. Spectral evidence for photoinduced charge separation in these systems
was established from femtosecond and nanosecond transient absorption studies. The measured
rate constants indicated fast charge separation and relatively slow charge recombination
revealing their usefulness in light energy harvesting and optoelectronic device building
applications. The bis(donor styryl)BODIPY-fullerene systems populated BODIPY triplet
excited states during the process of charge recombination.
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1. Introduction
Photoactive multi-modular donor-acceptor systems capable of mimicking the primary events
of natural photosynthesis is one of the active research areas in modern chemical science.1-18
Towards this, a number of donor-acceptor systems have been constructed and formation of
charge separated states upon photoexcitation of either the donor or acceptor entity have been
demonstrated. Among the organic photosensitizers, porphyrins and phthalocyanines are the
widely used class of compounds while the role of electron acceptor has been mainly fulfilled by
fullerene, C60.1-18 Wealth of information, largely on the structure-spectral, structure-redox, and
structure-photochemical properties of the donor-acceptor systems have been gathered from these
studies. Systems that performed well have eventually been used in the construction of light-to-
electricity and light-to-fuel conversion devices.19
In recent years, utilization of BODIPYs (= BF2-chealted dipyrromethene, derived from
4,4-difluoro-1,3,5,7-tetra-methyl-4-bora-3a,4a diaaza-s-indacene) and azaBODIPYs (= BF2-
chealted azadipyrromethene) in the construction of donor-acceptor conjugates have gained
appreciable momentum.10 Both are stable compounds allowing one to perform a wide range of
reactions to tune their spectral, electrochemical and photochemical properties, needed in building
novel donor-acceptor architectures. Depending upon the counter molecular entity, these
compounds can fulfill the role of either an energy/electron donor or acceptor. Other applications
of BODIPYs and azaBODIPYs include their utilization in developing chemosensors and imaging
agents for biochemical applications.20
In the present study, we have designed and synthesized a supramolecular conjugate
comprised of BODIPY and fullerene (1 in Fig. 1). Further, in order to capture light beyond 525
nm of pristine BODIPY absorption, the BODIPY in the dyad has been -extended to possess two
styryl groups shown as 2 in Fig. 1.21 Additionally, to build multi-modular donor-acceptor
assemblies, the styryl groups of BODIPY have been terminated with secondary electron donors
viz., triphenylamine (3) and phenothiazine (4).22 The structures of these molecules are shown in
Fig. 1. Upon characterization of these conjugates using spectral, electrochemical and
computational methods, photoinduced charge separation has been systematically investigated
using femtosecond and nanosecond transient spectroscopic techniques.
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Fig. 1. Structure of the BODIPY-C60 (1), bis(phenyl styryl)BODIPY-C60 (2), bis(triphenylamine
styryl)BODIPY-C60 (3), and bis(phenothiazine styryl)BODIPY-C60 (4) multi-modular systems
developed to probe photoinduced electron transfer.
2. Results and Discussion
2.1 Synthesis and X-ray structure of bis(styryl)BODIPY
The synthesis of the multi-modular donor-acceptor systems was accomplished by a multi-
step synthetic procedure as outlined in Scheme 1. Briefly, 2,4-dimethyl pyrrole was reacted with
methyl 4-formylbenzoate in dry dichloromethane and trifluoroacetic acid followed by p-chloranil
reaction and treatment with boron trifluoroborane ethrate in triethylamine. The desired
compound 1a was purified over silica gel column. For syntheses of compounds 2b-4b,
compound 1a was used as the starting material and was treated with benzaldehyde (for 2b), or 4-
(diphenylamino)benzaldehyde (for 3b) or 3-formyl-10-methyl phenothiazine (for 4b) in dry
benzene using Dean-Stark apparatus to separate water from the reaction mixture. Next,
compounds 2b-4b were hydrolized using LiOH in acetic acid to obtain 2c-4c that was followed
by reacting them with 4-hydroxybenzaldehyde to obtain compounds 2d-4d. Finally, 2d-4d were
reacted with C60 and sacosine in toluene to obtain the final multi-modular products, 2-4. For
compound 1, the step involving styryl groups attachment was avoided and rest of the steps were
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followed. The final compounds were purified by column chromatography and purity was
checked by thin-layer chromatography. The structural integrity was established from 1H and 13C
NMR, MALDI-TOF-mass, spectral and electrochemical studies.
Synthesis of control
compounds, 5-8 (see Fig. S1 for structures) were carried out according Scheme S1 in ESI. The
compounds were stored in dark prior performing spectral and transient absorption measurements.
Scheme 1. Synthetic methodology adopted for bis(donor styryl)BODIPY-C60 multi-modular
donor-acceptor systems.
The X-ray structure and packing diagram of bis(phenyl styryl)BODIPY, 6 is shown in Fig. 2a
(see Scheme S1 in the ESI for structure). The molecule crystallized in the triclinic crystal system
with space group P-1.23 The boron of BODIPY macrocycle assumed a tetrahedral geometry with
the two ring nitrogens and two fluorines. In agreement with previous structural reports on
BODIPYs,24 the core of the BODIPY macrocycle was flat while the meso-phenyl ring was
almost orthogonal with a torsion angle of 76o. The phenyl ring attached to the styryl groups were
slightly tilted with torsion angles of 23.7o and 29.8o, respectively, and were spatially separated by
over 8.0 Å causing no steric hindrance. The crystal packing diagram shown in Fig. 2b revealed
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no significant intermolecular interactions. The atomic coordinates, bond lengths and angles,
anisotropic displacement parameters, hydrogen coordinates (Tables S1-S5) are given in the ESI.
Fig. 2. Projection and crystal packing diagrams of bis(styryl)BODIPY with 50% thermal
ellipsoids.
2.2 Absorbance and fluorescence studies
Normalized absorption spectra of the investigated donor-acceptor systems is shown in Fig. 3
while Fig. S2 in ESI shows the spectra of the control compounds, viz., the BODIPY derivatives
lacking electron acceptor, C60 (compounds 5-8). The two styryl entities in the case of 2 and 6
caused bathochromic shift of the absorption bands by about 125 nm compared with pristine
BODIPY, 1 or 5. That is, the 505 nm peak observed in the case of 1 and 5 appeared at 634 and
628 nm, respectively, for 2 and 6 primarily due to extended conjugation effect of the two styryl
tails. Appending C60 caused an additional 6 nm red-shift in the case of 2 as compared with 6. In
all of the donor-acceptor conjugates, 1-4, a characteristic sharp peak of fulleropyrrolidine at 432
nm was also observed. Interestingly, in the case of 3 and 4, and their respective control
compounds 7 and 8 having either two triphenylamine or phenothiazine entities, additional
bathochromic shifts were noticed. In the case of 3 the main peak was at 676 while for 4 this
peak was at 702 nm. For control 7 the main peak was at 685 nm and for 8 this
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Fig. 3. Absorption spectra (normalized to the most intense peak maxima) of the indicated
compounds in benzonitrile.
Table 1. Absorption, fluorescence and singlet excited state lifetime data of the investigated
compounds.
Compounda
Absorption peak
Fluorescence Fluorescence Lifetime,
peak maxima,
quantum
yield, f
0.04
ns
maxima,  nm
 nm
516
313, 506
0.2
1
2
3
4
352, 581, 634
334, 418, 619, 676
330, 440, 509, 650,
702
652
0.114
0.3
763
0.07
1.0
709
<0.01
<0.1
314, 505
516
0.577
0.571
0.203
<0.01
3.33
3.74
2.73
0.05
5
6
7
8
578, 628
641, 699(sh)
754, 820(sh)
708
445, 685
636, 696
a-see Fig. 1 and Fig. S1 for structures
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Fig. 4. Fluorescence spectra of (a) 1 (black) and 5 (red), (b) 2 (black) and 6 (red), (c) 3 (black)
and 7 (red), and (d) 4 (black) and 8 (red) in benzonitrile. The samples were excited at the most
intense normalized peak maxima shown in Fig. 2.
was at 696 nm. These results indicate strong interactions between the triphenylamine or
phenothiazine with the BODIPY via the styryl linkers.22 Table 1 lists the peak maxima for all
the studied compounds. It is important to note that in the case of 3 and 4, absorption peaks
covering the entire 300-780 nm were observed, signifying the importance of the present
approach towards wide-band capturing multi-modular systems.
Fig. 4 displays the fluorescence spectra of the investigated donor-acceptor systems along
with their control compounds while the data are summarized in Table 1. Trends similar to that
observed in the case of absorption spectra were observed. That is, bathochromically shifted peak
maxima with significant degree of spectral broadening. Appending the triphenylamine and
phenothiazine entities caused additional red-shift accompanied by diminished intensities. While
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the fluorescence intensities of 5 and 6 were comparable, about 65% diminished intensity was
observed for 7 and 8 due to the presence of triphenylamine and phenothiazine entities. These
observations suggest that the triphenylamine and phenothiazine entities in 7 and 8 are involved in
excited state events. Presence of C60 in the donor-acceptor conjugates caused additional
quenching. The percent quenching due to C60 was 93%, 80%, 77% and 99%, respectively, for 1,
2, 3, 4 as compared to their control compounds, 5-8. Such quenching could mainly arise from
either energy or electron transfer from the singlet excited BODIPY derivatives.25 At this point,
additional electrochemical and computational studies were warranted in search for an answer for
the quenching mechanism.
2.3
Computational and electrochemical studies
Computational studies were performed to visualize the geometry and electronic structure
of the supramolecular donor-acceptor assemblies.
In these studies, the geometry of
supramolecular structures 1-4 were optimized to a stationary point on the Born-Oppenheimer
surface using the B3LYP method and the 6-31G(d,p) split valence basis set, to form the
B3LYP/6-31G(d,p) model chemistry as parameterized in the Gaussian-09 software.26 The
structures and orbitals (HOMO and LUMO were visualized with the GaussView software. As
shown in Fig. 5, in the optimized structures the HOMO was delocalized over the entire molecule
including BODIPY, styryl tails and the donor entities while the LUMO was on C60. These
results suggest that the donor entities of 3 or 4 acting as a single entity, although some
intramolecular push-pull type interactions between the entities could be envisioned from the
location of the LUMO orbitals. Such charge transfer type interactions could be responsible for
the reduced fluorescence as compared to that of 2. The distance between the centers of fullerene
to boron of BODIPY was found to be 18.37 Å in 1 and 18.28 Å in 2, respectively. While the
distances between the center of fullerene to the nitrogens of the triphenylamine entities in 3 were
26.27 Å and 25.39 Å. The distances between the center of fullerene to the nitrogens of the
phenothiazine entities in 4 were found to be 26.46 Å and 23.83 Å. As pointed out earlier, no
noticeable interactions between fullerene and BODIPY -structure were observed in any one of
the conjugates.
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Fig. 5. Frontier HOMO (left) and LUMO (right) of (a) 1, (b) 2, (c) 3, and (d) 4 generated using
GaussView on their respective B3LYP/6-31G** optimized structure.
Electrochemical studies using differential pulse (DPV) and cyclic voltammetry (CV)
were performed to evaluate the redox potentials of compounds 1-8 and to calculate the energetics
of the charge separation processes. Fig. 6 shows the DPV curves for compounds 1-4 in
benzonitrile, 0.1 M (n-Bu4N)ClO4 while the CVs are shown in Fig. S4 in ESI. Multiple
oxidation processes were observed. The redox potentials of the BODIPY segment in 1 were very
similar to pristine BODIPY, 527 while having two styryl groups in compound 2 affected both
oxidation and reduction potentials. That is, lower oxidation and reduction potentials affecting
the overall HOMO-LUMO gap was noticed (see potentials corresponding to first oxidation and
second reduction in Table 2). Further, appending triphenylamine or phenothiazine entities had
additional effect on the oxidation potentials compared to that over the reduction potentials. That
is, lowered oxidation potential resulting in smaller HOMO-LUMO gap in the case of 3 and 4
compared to that in 2 was observed. In all these donor-acceptor systems, the first reduction
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Fig. 6. Differential pulse voltammograms (DPVs) corresponding to both oxidation and reduction
processes of the indicated compounds in benzonitrile containing 0.1 M (t-Bu4N)ClO4 as
supporting electrolyte. Scan rate = 5 mV/s, pulse width = 0.25 s, pulse height = 0.025 V.
Table 2. Redox potentials, free-energy change for charge separation (GCS) and recombination
(GCR) for the donor-acceptor conjugates in benzonitrile.
Compound
1st Oxa
1st Redb
-1.01
-1.00
-1.01
-1.00
2nd Redc
-1.59d
-1.27
-GCR, eVe -GCS, eVe
0.76
1.72
1.46
1.16
1.32
0.71
0.48
0.57
0.43
1
2
3
4
0.52
0.20
-1.36
0.39
-1.31
a-bis(donor styryl)BODIPY centered
b-C60 centered
c- bis(donor styryl)BODIPY centered
d-third reduction wave
e-see text for relevant equations
potential corresponding to C60 revealed no significant changes meaning absence of significant
intramolecular interactions with the BODIPY -system, as predicted by computational studies.
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Table 2 lists the first oxidation and first two reduction potentials needed to calculated the free-
energy changes of these investigated compounds.
The free energy change for charge separation (ΔGCS) from the singlet excited state of the
modified BODIPY within the donor-acceptor system was calculated using spectroscopic,
computational and electrochemical data following equations 1-3.28
-GCR = Eox – Ered + GS
-GCS = GCR
(1)
(2)
where and GCS correspond to the energy of excited singlet state being, 2.43 eV for 1, 1.94
eV for 2, 1.73 eV for 3, and 1.75 eV for 4,22 and electrostatic energy, respectively. The Eox and
Ered represent the oxidation potential of the electron donor (BODIPY) and the reduction potential
of the electron acceptor (C60), respectively. The term Gs refers to the static Coulombic energy,
calculated by using the “dielectric continuum model” according to equation (3):
e2


1



 
1

1
1

GS 




(3)
 




4 0
2R
2R
R
 
 RCCR
The symbols 0, and R represent vacuum permittivity and dielectric constant of the
solvent used for photochemical and electrochemical studies, respectively. RCC is the center-to-
center distance between donor and acceptor entities. The symbols R+ and R- refer to radii of the
cation and anion species, respectively. The calculated GS was found to be 0.07+0.02 eV for the
present donor-acceptor systems.
Fig. 7 shows an energy level diagram constructed using spectral and free energy
calculations data depicting different photophysical events in the donor-acceptor systems. In all
of these dyads, formation of D•+-C60•- charge separated state from 1D* (where D = BODIPYs) is
1
1
evident. Singlet-singlet energy transfer from D* to C60 to produce C60* is also possible.
•-
Formation of D•+-C60 from 1C60* is also a possibility, however, at the excitation wavelength of
400 nm, most of the light was absorbed by the BODIPY sensitizers, hence this could be
considered to be a minor process. Once formed, the D•+-C60•- charge separated state could relax
directly to the ground state or alternatively populate the triplet excited state of either BODIPY or
fullerene entities. Energetically, charge separated state formed by 1 (energy level shown in blue)
could populate either 3BODIPY* or 3C60* states, while for radical ion-pairs involving conjugates
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3
2-4, population of only BODIPY* is energetically possible. In order to unravel these
mechanistic routes and to secure evidence for charge separation, transient absorption studies
using femtosecond and nanosecond transient techniques were systematically performed and the
outcome of these results are discussed below.
Fig. 7. Energy level diagram showing the different photochemical events in donor-acceptor
conjugates, 1-4 in benzonitrile. Energies of different states were evaluated from spectral and
electrochemical studies. Energy of 3[bis(styryl)BODIPY]* was taken from Ref. 21a. Solid arrow
indicates major photo processes, dashed arrow indicates minor photo processes. Abbreviations:
CS = charge separation, CR = charge recombination, EnT = energy transfer, T = triplet state.
2.4
Femtosecond and nanosecond transient absorption studies
To aid-in spectral interpretation, the one-electron oxidation products of compounds 5-8
by chemical oxidation using nitrosonium hexafluoroborate as an oxidizing agent in benzonitrile
were carried out, and spectral data is shown in Fig. S3. For 5, during oxidation, a new peak at
497 nm close to the intense peak of the neutral compound at 505 nm was observed (Fig. S3a).
For compounds 6-8 with two -extended styryl chains, the spectral identity of radical cation
peaks was distinct from the peaks of the neutral compound. As shown in Fig. S3b, for 6, new
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peaks at 529 and 567 nm corresponding to [bis(phenyl styryl)BODIPY]•+ (6•+) was observed.
For 7, peaks at 520 and 601 nm, and for 8 peaks at 466, and 625 nm, corresponding to their
respective radical cation were observed (Fig. S3c and 3d). Peaks corresponding to C60•- at 1020
nm for fulleropyrrolidine derivatives is well-known.7 Presence of transient absorption bands at
these locations would provide evidence of charge separation in these dyads.
Next, femtosecond transient spectra of the functionalized BODIPY derivatives 6-8 along
with pristine BODIPY, 5 was investigated in benzonitrile. For the large part, spectral features
were similar to the earlier reported on crown ether appended BODIPY derivatives.22 As shown
in Fig. 8a for compound 5, the instantaneously formed singlet BODIPY revealed a negative peak
at 508 nm having contributions from ground state absorption and stimulated emission of
Fig. 8. Femtosecond transient spectra (100 fs pulse width at 400 nm) of the indicated
compounds in Ar-saturated benzonitrile at the indicated delay times. The time profiles of the
peak corresponding to ground state bleaching are shown as figure insets.
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BODIPY. Recovery of the 508 nm peak (Fig. 8a inset) was slow as predicted by its relatively
long fluorescence lifetime (Table 1). The transient spectral features of bis(phenyl
styryl)BODIPY in benzonitrile is shown in Fig. 8b. Positive peaks at 544, 594, 667, 830, 944
and 1016 nm, and negative peaks at 576, 631 and 704 nm were observed. The positive peaks are
mainly attributed to the instantaneously formed singlet excited state transitions. The first two
negative peaks corresponded mainly to the ground state bleaching as the absorption peak
maxima matched well with these peaks while the 704 nm peak was due to stimulated emission of
the styryl BODIPY entity. Recovery of the 634 nm peak is shown in Fig. 8b inset where
relatively slow recovery was observed.
The femtosecond transient spectral features of compounds 7 and 8 having triphenylamine and
phenothiazine, respectively, were considerably different from that observed for compounds 5 and
6 due to the interactions of these entities with the bis-styryl BODIPY -system.22 Compound 7
revealed positive peaks at 458, 556, 649, 732, and 1292 nm and negative peaks at 631, 694 and
852 nm (Fig. 8c). The negative peaks corresponded to the ground state bleaching and stimulated
emission. Time constant for the recovery of the 694 nm peak was about 3180 ps (Fig. 8b inset).
The broad near-IR peak is attributed to the singlet-singlet transition of the singlet excited 7. The
reduced fluorescence lifetime and quantum yield of 7 compared to either 5 and 6 is supportive of
intramolecular charge transfer type interactions involving the triphenylamine entities. The
spectral features observed for 8 having two phenothiazine entities linked to the bis-styryl
BODIPY unit were comparatively short lived (Fig. 8d). Positive peaks at 467, 570, 751, and
1400 nm and negative signal at 675 nm were observed. The strong negative peak at 675 nm
corresponding to ground state bleaching revealed faster recovery (Fig. 8d inset), in agreement
with its lower fluorescence lifetime. However, no strong negative signal in the 720 nm region
revealing for stimulated emission was observed supporting occurrence of rapid intramolecular
events. Similar to compound 7, the near-IR peak at 1400 nm of 8 revealed initial growth
followed by rapid decay. The time constants for growth and decay of this signal was found to be
14.9 and 105.5 ps, respectively, revealing occurrence of rapid intramolecular charge transfer type
interactions involving the phenothiazine and styryl BODIPY entities.
Fig. 9a shows the femtosecond transient spectra at different delay times of 1 in benzonitrile.
1
Evidence for charge separation from the BODIPY* was evident from the spectral observations.
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That is, a transient peak at 500 nm range nm range corresponding to BODIPY•+ and a near-IR
band at 1020 nm characteristic of C60•- was obtained. Note the weak BODIPY•+ peak in the 500
nm range developing at the latter time scale due to the presence of strong negative peak at 506
1
nm corresponding to ground state depletion and stimulated emission of BODIPY*. The
recovery of the 506 nm peak was faster than that observed in pristine 5, due to additional charge
separation process. The rate constant of charge separation, kCS and charge recombination, kCR,
evaluated by monitoring rise and decay of the C60•- (see Fig. 9a inset for decay time profile) and
were found to be 1.8 x 1010 s-1 and 7.5 x 108 s-1, respectively. Faster charge separation and
•-
relatively slow charge recombination were observed. The decay of the C60 peak representing
charge recombination was accompanied by a new peak in the 700 nm range corresponding to the
Fig. 9. Femtosecond transient spectra (100 fs pulse width at 400 nm) of the indicated
compounds in Ar-saturated benzonitrile at the indicated delay times. The time profiles of the
peak corresponding to C60•- monitored at 1020 nm is shown in figure inset.
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3
3
formation of C60*. These results indicate that the charge separated state populates the C60*
prior returning to the ground state.
Femtosecond transient absorption spectra of 2 in benzonitrile is shown in Fig. 9b. The
decay of peaks corresponding to singlet excited state and recovery of the 636 nm peak were
much faster than that observed for control compound 6 lacking the C60 entity. This was
accompanied by new peaks in the 570 corresponding to the formation of BODIPY•+ and 1020
nm range corresponding to C60•-. The kCS and kCR evaluated by monitoring rise and decay of the
•-
C60 (see Fig. 9b inset) were found to be 2.2 x 1011 s-1 and 8.2 x 108 s-1, respectively.
Femtosecond transient spectra corresponding to 3 and 4 were much more complex to establish
charge separation due to intense transient peaks of triphenylamine and phenothiazine appended
styryl BODIPYs as shown in Fig. 9c and 9d. In the case of 3, the radical cation peak appeared as
a shoulder peak at 610 nm to the main peak at 560 nm while the radical anion peak appeared as a
shoulder at peak 1020 nm to the main peak at 1300 nm. Fig. 10a shows spectrum at 50 ps for 3
and the control compound 7 that clearly shows transient peaks of the radical ion-pair
demonstrating occurrence of charge separation in 3. The strong overlap of the radical ion-pair
and the broad singlet excited state of bis(triphenylamine styryl)BODIPY at 1295 nm made
almost impossible to evaluate the kCS and kCR from the radical-ion pair. Hence, it was then
evaluated from the changes in the kinetics of the singlet excited state at 1295 nm. The estimated
kCS and kCR were 3.1 x 1011 s-1 and 2.3 x 108 s-1, respectively. A similar case was also observed
Fig. 10. Femtosecond transient spectrum (100 fs pulse width at 400 nm) of (a) 3 (dark yellow)
and 7 (blue), and (b) 4 (dark yellow) and 8 (blue) in Ar-saturated benzonitrile at a delay time of
50 ps.
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for 4 where the peak corresponding to the radical ion-pair were shadowed by intense peaks of
parent donor entity. As shown in Fig. 10b, the spectrum recorded at 50 ps clearly showed cation
•-
radical peak at 610 nm and a shoulder peak of C60 at 1020 nm. The kCS and kCR evaluated by
monitoring rise and decay of the C60•- (see Fig. 9d inset) were found to be 4.1 x 1010 s-1 and 3.3 x
108 s-1, respectively. However, a major part of decay signal (biexponential) persisted beyond 3
ns time window of our instrument setup revealing charge stabilization.
3
The energy level diagram shown in Fig. 7 predicted formation of C60* in the case of 1
3
and BODIPY* in the case of 2-4 conjugates during the process of charge recombination due
high energy of the radical ion-pairs compared to the triplet energy of sensitizers. It is known that
directly excited BODIPY derivatives do not populate the triplet excited states with high quantum
efficiency via intersystem crossing.29 This seems to be also true in the case of bis-styryl
BODIPY derivatives where weak transient bands were observed. However, if charge
3
recombination could populate the BODIPY* as in the case of 2-4, then one could expect well
3
developed transient peaks of BODIPY*. Nanosecond transient absorption studies were
performed to unravel such recombination path of the radical ion-pairs.
Fig. 11. Nanosecond transient spectrum at the indicated delay time of (a) 1 (505 excitation), (b)
2 (630 nm excitation), (c) 3 (685 nm excitation), and (d) 4 (355 nm excitation) in Ar-saturated
benzonitrile.
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Fig. 11 illustrates the nanosecond transient spectra of the donor-acceptor systems, 1-4 in
benzonitrile. For 1, the transient spectrum revealed a strong band at 700 nm and a shoulder band
at 824 nm, characteristic of 3C60* formation (Fig. 11a).30 This indicates that the radical ion-pair
3
3
populates the C60*, as predicted for 1 in Fig. 6, prior returning to the ground state. The C60*
thus formed relaxed to the ground state at a rate constant of 3.2 x 105 s-1. Well-defined transient
3
spectra revealing peaks of 3BODIPY* (compared to their respective pristine BODIPY*) and not
3
that of C60* were observed in the case of 2-4. For 2, the peak positions were close to that
reported in literature for bis styryl BODIPY derivatives (Fig. 11b)21h,22 and appeared at 511 and
713 nm. The 3BODIPY* thus formed relaxed to the ground state at a rate constant of 2.5 x 104 s-
1. For 3 and 4, the transient peaks were red-shifted (Fig. 11c and 11d). For 3, the transient peaks
were at 590, 870 and 1408 nm which decayed at a rate constant of 4.2 x 104 s-1. Transient peaks
of 4 were located at 450, 543, 801 and 1264 nm which decayed at a rate constant of 3.5 x 103 s-1.
A comparison between the presently investigated bis(donor styryl)BODIPY-C60 conjugates
to the earlier reported BODIPY-C60 type dyads and triads was made. First, in the case of self-
assembled bis(donor styryl)BODIPY-C60 conjugates (C60 was assembled via alkyl cation-crown
ether complexation)22 having similar secondary electron donors, although the values of kCS were
close to that observed in the present study, the kCR values were slightly lower due to the
increased distance between BODIPY and C60 entities. In the case of donor-BODIPY-C60 triads
in which the secondary donor and acceptor were directly attached to the BODIPY, as a result of
close proximity between the entities, ultrafast charge separation and charge recombination was
observed.24a,31a Recently, a pyrrolyl-bridged BODIPY-C60 was reported by us.31c Due to -
carbon substitution and extended conjugation offered by the pyrrole ring, efficient charge
separation and recombination was observed. These studies bring out the importance of geometry
and electronic structure of donor-acceptor systems in govering the kinetics of charge separation
and charge recombination processes.
3. Summary
The multi-modular donor-acceptor systems built using bis-styryl BODIPY as the central unit
and C60 and secondary electron donors revealed several interesting features. First, as a result of
extended p-conjugation and strong electronic interactions between the secondary donors and
styryl BODIPY -system, these systems offered better spectral capture covering the visible and
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near-infrared portions of the electromagnetic spectrum. Electrochemical studies helped in
evaluating redox potentials of different entities of the multi-modular systems. The established
energy level diagram, using spectral, electrochemical and computational results, helped in
visualizing the mechanistic details of photophysical processes.
Femtosecond transient
absorbance studies provided evidence for the occurrence of charge separation in these donor-
acceptor systems. Ultrafast charge separation and relatively slow charge recombination were
3
witnessed. The charge recombination populated the C60* in the case of donor-acceptor system
3
lacking styryl groups, 1 while BODIPY* was formed in the case of systems having bis-styryl
entities, 2-4 as evidenced by nanosecond transient absorption studies. The charge separation
processes witnessed in these systems brought out their importance as broad band capturing,
energy harvesting materials for solar energy harvesting and optoelectronic device building
applications.
4.
Experimental Section
4.1 Chemicals. Buckminsterfullerene, C60 (+99.95%) was from SES Research, (Houston,
TX). All the reagents were from Aldrich Chemicals (Milwaukee, WI) while the bulk solvents
utilized in the syntheses were from Fischer Chemicals. Tetra-n-butylammonium perchlorate, (n-
Bu4N)ClO4, used in electrochemical studies was from Fluka Chemicals. Synthetic details of
control compounds, 5-8 is given in the ESI.
4.2 Syntheses
Synthesis of 1a: The aryl-difluoroboron dipyrrin compounds were synthesized according to the
procedure of Imahori and coworkers.32 To a mixture of methyl 4-formylbenzoate (12.4 mmol)
and 2, 4-dimethylpyrrole (2.16 mL, 21.1 mmol) in 800 mL of dry dichloromethane,
trifluoroacetic acid (0.19 mL, 2.47 mmol) was added. The reaction mixture was stirred for 2
hours at room temperature under nitrogen. After which the solution mixture was washed with
0.1M NaOH (200 mL) followed by water (200 mL). The organic layer was dried over anhydrous
Na2SO4 and evaporated under reduced pressure. The residue was dissolved in toluene (50 mL)
and p-chloranil (2.73 g, 11.1 mmol) was added. After 10 minutes, Et3N (8 mL) was added
followed by BF3·Et2O (7 mL) and the mixture was stirred for 3 hours. To quench the reaction,
the mixture was poured into water. The organic layer was extracted and dried over anhydrous
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Na2SO4 and evaporated under reduced pressure. The crude product was purified using column
chromatography using mixtures of CH2Cl2 and hexane as eluent.
Yield: 10.50 %. (1H NMR in CDCl3, 400 MHz, 25 °C), δ (ppm): 8.19 (d, 2 H, Ar-H), 7.41 (d, 2
H, Ar-H), 6.00 (s, 2H, pyrrole-H), 3.98 (s, 3 H, OCH3-H), 2.57 (s, 6 H, CH3-H), 1.37 (s, 6 H.
CH3-H).
Synthesis of 2b. Knoevenagel condensation reaction for the synthesis of bis(styryl)BODIPY
compounds was accomplished according to the procedure of Rurack et. al.33 BODIPY, 1a (0.102
g, 0.268 mmoles) and benzaldehyde (0.134 g, 1.07 mmoles), piperidine (0.27 mL), and AcOH
(0.17 mL) in benzene (15 mL) were refluxed and a reaction monitored using UV. Using Dean–
Stark apparatus, the water formed during the reaction was removed azeotropically. When all the
starting material had been consumed, the mixture was cooled to room temperature and washed
with water. The organic phase was dried over Na2SO4 and solvent was evaporated under reduced
pressure. The residue was purified by silica gel column chromatography with CH2Cl2/ hexane
(2/3) eluting solvent system to afford the desired the compound 2b.
1
Yield (3.2 %). H NMR in CDCl3, 400 MHz, 25 °C, δ (ppm): ), 8.12-8.16 (d,2H, Ar-H), 7.66–
7.70 (d,2ꢀH, Ar-H, ) 7.56–7.60 (d, 2H, Ar-H), 7.42–7.46 (m, 2ꢀH, Ar-H), 7.32–7.38 (t, 4ꢀH,Ar-H),
7.24–7.29 (m,2ꢀH, Ar-H), 6.90-7.21 (m 4H) 6.60 (s, 2ꢀH, pyrrole-H), 3.9 ( s, 3H, H-OCH3), 1.40
(s, 6ꢀH, CH3-H)
Synthesis of 3b. In a similar manner as 2b, compound 3b was synthesized, BODIPY 1a (0.30 g.
0.78 mmoles) and diphenyl amino benzaldehyde(0.856 g, 3.13 mmoles) in a mixture of benzene
(15 mL), piperidine (0.1 mL) and acetic acid (0.1 mL) were refluxed and reaction monitored by
UV. The crude obtained by the reaction was purified on silica gel column using solvent system
hexane and methylene chloride (1/1) to obtain the desired compound:
Yield (81.9%)UV/Vis 1H NMR in CDCl3, 400 MHz, 25 °C:1H NMR (400 MHz, CDCl3) δ =
8.16 (d, 2H, Ar-H), 7.58 (d,2H, Ar-H), 7.42-7.8 (m, 6H, Ar-H), 7.26-7.29 (m, 8H, Ar-H), 7.16 (d,
2H, Ar-H), 7.08-7.15 (m, 8H-Ar-H), 7.00-7.08 (m, 8H), 6.56(s, 2H), 3.59 (s, 3H), 1.50(s, 6H)
Synthesis of 4b.
Compound 1a (0.037g.0.09 mmoles) and 3-formyl-10-methyl-4aH-
phenothiazin-10-ium (0.064 g, 0.261 mmoles) in a mixture of benzene (15 mL), piperidine (0.1
mL) and acetic acid (0.1 mL) were refluxed and reaction monitored by UV-vis. The crude
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obtained by the reaction was purified on silica gel column using solvent system hexane and
methylene chloride (1/1) to obtain the desired compound:
Yield (21.5%). 1H NMR in CDCl3, 400 MHz, 25 °C: δ= 8.10 ( d, 2H) 7.50–7.56 ppm (d,2ꢀH Ar-
H), 7.36–7.44 (m, 6ꢀH, Ar-H), 7.34 (d, 2ꢀH, Ar-H), 7.06-7.14 (m, 4H, Ar-H) 6.88-6.92 (m, 2H,
Ar-H), 6.76–6.83 (m, 4ꢀH), 6.54 (s, 2ꢀH, pyrrole-H),3.85 (s,3H, -OCH3-H) 3.38 (s, 6ꢀH, CH3-H),
1.50 (s, 6ꢀH, CH3-H).
Synthesis of 2c. The ester on bis(styryl)BODIPY were hydrolyzed according to a procedure
reported by Rampazzo and coworkers.34 In a round-bottomed flask, compound 2b (0.149 g,
0.264 mmoles) were dissolved in 2.5 mL of THF. To this solution, lithium hydroxide
monohydrate (0.066 g, 1.322 mmoles) dissolved in 1.0 mL of water were rapidly added drop
wise. The reaction mixture was stirred for 5 hours at room temperature. The mixture was then
diluted with a 0.1 M solution of hydrochloric acid and extracted with dichloromethane and the
organic phases were dried on sodium sulfate and evaporated under reduced pressure. The
resulting mixture was purified by means of flash chromatography on silica gel using a methylene
chloride/hexane (98:2 %) to afford compounds 2c.
1
Yield (45.5 %). H NMR in CDCl3, 400 MHz, 25 °C), δ (ppm): ), 8.12-8.16 (d, 2H, Ar-H), 7.66–
7.70 (d, 2ꢀH, Ar-H, ) 7.56–7.60 (d, 2H, Ar-H), 7.42–7.46 (m, 5 ꢀH, Ar-H), 7.38–7.42 (d, 2ꢀH, Ar-
H), 7.32–7.37 (t, 4ꢀH, Ar-H), 7.24–7.29 (m, 2ꢀH, Ar-H), 6.60 (s, 2ꢀH, pyrrole-H), 1.40 (s, 6ꢀH,
CH3-H)
Synthesis of 3c. In a round-bottomed flask, compound 3b (0.140 g, 0.157 mmoles) were
dissolved in 2.5 mL of THF. To this solution, lithium hydroxide monohydrate (0.033 g, 0.784
mmoles) dissolved in 1.0 mL of water were rapidly added drop wise. The reaction mixture was
stirred for 5 hours at room temperature. The mixture was then diluted with a 0.1 M solution of
hydrochloric acid and extracted with dichloromethane and the organic phases were dried on
sodium sulfate and evaporated under reduced pressure. The resulting mixture was purified by
means of flash chromatography on silica gel using a methylene chloride/hexane (98:2 %) to
afford compounds 3c.
1
Yield (29.0%). H NMR (400 MHz, CDCl3): δ = 8.26 (d, 2H, Ar-H), 7.60 (d, 2H, Ar-H), 7.44-
7.52 (m, 6H, Ar-H), 7.28-7.32 (m, 5H, Ar-H), 7.2 (d, 2H, Ar-H), 7.12-7.16 (m, 8H, Ar-H), 7.05-
7.1(m, 8H, Ar-H), 7.00-7.04 (m, 8H, Ar-H),6.6(s ,2H) 1.50(s, 6H)
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Synthesis of 4c. In a round-bottomed flask, compound 4b (0.228 g, 0.0273 mmoles) were
dissolved in 2.5 mL of THF. To this solution, lithium hydroxide monohydrate (0.0136 g, 0.273
mmoles) dissolved in 1.0 mL of water were rapidly added drop wise. The reaction mixture was
stirred for 5 hours at room temperature. The mixture was then diluted with a 0.1 M solution of
hydrochloric acid and extracted with dichloromethane and the organic phases were dried on
sodium sulfate and evaporated under reduced pressure. The resulting mixture was purified by
means of flash chromatography on silica gel using a methylene chloride/hexane (98:2 %) to
afford compounds 4c.
1
Yield (68.50%). H NMR (400 MHz, CDCl3): δ= 8.10 ( d, 2H) 7.46–7.56 ppm (d, 2ꢀH CH-H),
7.36–7.44 (m, 4ꢀH, Ar-H), 7.32 (d, 2ꢀH, Ar-H),7.06–7.16 (m, 5ꢀH, Ar-H), 6.92–6.96 (m, 3ꢀH,
Ar-H), 6.76–6.83 (m, 6ꢀH, Ar-H), 6.54 (s, 2ꢀH, pyrrole-H), 3.38 (s, 6ꢀH, CH3-H), 1.50 (s, 6ꢀH,
CH3-H).).
Synthesis of 2d. In a round-bottom flask, 2c (0.034 g, 0.062 mmoles) and 4-hydoxy
benzaldehyde (0.038 g, 0.31 mmoles) were dissolved dry methylene chloride (25 ml) to this, 4-
dimethylaminopyridine, DMAP (0.038 g, 0.31 mmoles) was added The resulting solution was
cooled to 0 °C, followed by addition of dicyclohexylcarbodiimide, DCC (0.064 g, 0.31 mmoles).
The reaction mixture was stirred under nitrogen for 6 hours at room temperature. Excess solvent
was removed under vacuum and the crude compound was washed with water several times and
extracted with CHCl3. Purification of the crude compound was carried out on a silica gel column
with hexane/ dichloromethane (1/4) as eluent giving the compound 2d.
1
Yield (47.5 %). H NMR in CDCl3, 400 MHz, 25 °C), δ (ppm): ),10.1( s, 1H, CHO), 8.12-8.16
(d, 2H, Ar-H), 7.66–7.70 (d, 2ꢀH, Ar-H, ) 7.56–7.60 (d, 2H, Ar-H), 7.38–7.42 (d, 2ꢀH, Ar-H),
7.32–7.37 (t, 4ꢀH,Ar-H), 7.24–7.29 (m, 2ꢀH, Ar-H), 6.60 (s, 2ꢀH, pyrrole-H), 3.9 ( s, 3H, H-
OCH3), 1.40 (s, 6ꢀH, CH3-H)
Synthesis of 3d. In a round-bottom flask, 3c (0.040 g, 0.045 mmoles) and 4-hydroxy
benzaldehyde (0.028 g, 0.22 mmoles) were dissolved dry methylene chloride (25 mL). To this,
DMAP (0.028 g, 0.220 mmoles) was added. The resulting solution was cooled to 0 °C, followed
by addition of DCC (0.046 g, 0.220 mmoles). The reaction mixture was stirred under nitrogen
for 6 hours at room temperature. Excess solvent was removed under vacuum and the crude
compound was washed with water several times and extracted with dichloromethane.
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Purification of the crude compound was carried out on a silica gel column with hexane/
dichloromethane (1/4) as eluent giving the compound 3d.
1
Yield (50.7 %). H NMR (400 MHz, CDCl3) δ = 10.0 (s, 1H), 8.26 (d, 2H), 7.90 (d, 2H), 7.44-
7.52 (m, 10H), 7.28-7.32 (m, 6H), 7.2 (d, 2H), 7.12-7.16 (m, 10H), 7.00-7.1 (m, 9H), 6.6(s, 2H)
1.50(s, 6H)
Synthesis of 4d. In a round-bottom flask, 3d (0.021 g, 0.026 mmoles) and 4-hydoxy
benzaldehyde (0.0157 g, 0.129 mmoles) were dissolved dry methylene chloride (25 mL), and to
this, DMAP (0.016 g, 0.129 mmoles) was added. The resulting solution was cooled to 0 °C,
followed by addition of DCC( 0.027 g, 0.129 mmoles). The reaction mixture was stirred under
nitrogen for 6 hours at room temperature. Excess solvent was removed under vacuum and the
crude compound was washed with water several times and extracted with dichloromethane.
Purification of the crude compound was carried out on a silica gel column with hexane/
dichloromethane (1/9) as eluent giving the compound 4d.
1
Yield (87.2 %). H NMR in CDCl3, 400 MHz, 25 °C), δ=10.05 (s, 1-CHO) 8.35 ( d, 2H), 8.05
(d, 2H), 7.65 (d, 2ꢀH , Ar-H), 7.55 (d, 2H Ar-H), 7.50 (d, 4H, Ar-H ) 7.45 (d, 2H Ar-H) 7.15–
7.25 (m, 5ꢀH, Ar-H), 7.32 (d, 2ꢀH, Ar-H), 6.95–6.96 (m, 3ꢀH, Ar-H), 6.85–6.90 (d, 2H, Ar-H), ,
6.55 (s, 2ꢀH, pyrrole-H), 3.45 (s, 6ꢀH, CH3-H), 1.50 (s, 6ꢀH, CH3-H).
Synthesis of 2. Using Prato’s35 method, compound 2 was synthesized as follows. To dry toluene
(25 mL) in a round-bottomed flask, compound 2d (0.0194 g, 0.0299 mmoles.), C60 (0.0645 g,
0.0896 mmoles), and sarcosine (0.0133 g, 0.149 mmoles) were refluxed for 12 hours. After
cooling to room temperature, the solvent was evaporated and the crude was purified on silica by
flash chromatography using toluene/hexane 9.5/0.5 v/v) solvent system affording the final
compounds 2.
1
Yield (60.3%). H NMR (250 MHz, CDCl3 ) ) δ = 8.30 (d, 2H), 7.64 (d, 2H) 7.58 (d, 4H), 7.50
(d, 2H), 7.26-7.38 (m, 10H), 6.90 (d, 2 H), 6.6 ( s, 2), 5.0 (s, 2H), 4.25 (dd, 1H), 2.85 ( s, 3H),
1.45 (s, 6H). MS (MALDI): Calcd, 1395.29 [M+]; found 1395.8. 13C NMR (400 MHz,
CDCl3):163.41, 152.114, 149.81, 146.36, 145.68, 145.46, 145.35, 145.26, 145.15, 144.99,
144.78, 144.56, 144.47, 144.34, 144.27, 144.20, 143.74,143.45, 142.19, 142.04, 141.74, 141.27,
147.20, 141.13, 141.08, 140.80, 140.59, 139.84, 139.24, 138.95, 138.57, 135.75, 135.46, 129.88,
129.17, 128.16, 127.83, 126.64, 118.14, 117.17, 81.95, 76.28, 76.03, 75.76, 39.13, 30.95, 29.06,
28.72, 28.38, 21.71, 13.93.
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Synthesis of 3. To dry toluene (20 mL) in a round-bottomed flask, compound 3d (0.023 g, 0.0232
mmoles), C60 (0.051 g, 0.0695 mmoles), and sarcosine (0.010g, 0.116 mmoles) were refluxed for
12 hours. After cooling to room temperature, the solvent was evaporated and the crude was
purified on silica by flash chromatography using toluene/hexane 9.5/0.5 v/v) solvent system
affording the final compounds 3.
Yield (55.8%). 1H NMR (400 MHz, CDCl3) δ = 8.26 (d, 2H), 7.54 (d, 2H), 7.46( d, 2H) 7.40 (d,
4H) 7.30(d, 2H) 7.44-7.52 (m, 6H), 7.28-7.32 (m, 7H), 7.2 (d, 2H), 7.12-7.16 (m, 9H), 7.00-7.1
(m, 8H),6.6(s ,2H), 4.95 (d, 2H,fulleropyrrolidine-H), 4.45 (d, 1H, fulleropyrrolidine-H) 1.50(s,
6H). 13C NMR (250 MHz, CDCl3):148.7, 147.3, 147.1, 146.6, 146.3, 146.2, 146.1, 146.9, 146.5,
145.3, 145.2, 145.1, 144.7, 144.6, 144.3, 142.7, 142.6, 142.2, 142.1, 141.6, 140.2, 139.9, 135.9,
130.7, 130.2, 129.4, 128.6, 126.3, 125.1, 123.0, 122.4, 82.9,77.3, 77.0, 76.7, 75.5, 70.0, 69.5,
40.1, 29.7, 14.8. MS (MALDI): Calcd, 1729.29 [M+]; found 1729.6.
Synthesis of 4. To dry toluene (20 mL) in a round-bottomed flask, compound 4d (0.021 g, 0.0266
mmoles.), C60 (0.0575 g, 0.0798 mmoles), and sarcosine (0.119 g, 0.133 mmoles) were refluxed
for 12 hours. After cooling to room temperature, the solvent was evaporated and the crude was
purified on silica by flash chromatography using toluene solvent system affording the final
compounds 4.
1
Yield (63.3%). H NMR in CDCl3, 400 MHz, 25 °C), δ= 8.35 ( d, 2H), 8.10 (d, 2H), 7.88- 8.0
(m, 4 H, Ar-H), 7.65 (d, 2ꢀH , Ar-H), 7.41-7.50 (, 6H, Ar-H), 7.32-7.40 (m, 8ꢀH, Ar-H), 6.85–
6.90 (d, 2H, Ar-H), 6.55 (s, 2ꢀH, pyrrole-H) ), 5.0 (d, 2H, fulleropyrrolidine-H), 4.42 (d, 1H,
fulleropyrrolidine-H), 3.49 (s, 6ꢀH, CH3-H), 2.87 (s, 3H, CH3-H), 1.50 (s, 6ꢀH, CH3-H). ), MS
(MALDI): Calcd, 1665.32 [M+]; found 1665.2.
4.3
Spectral measurements
The UV-visible spectral measurements were carried out with a Shimadzu Model 2550 double
monochromator UV-visible spectrophotometer. The fluorescence emission was monitored by
1
using a Varian Eclipse spectrometer. A right angle detection method was used. The H NMR
studies were carried out on a Varian 400 MHz spectrometer. Tetramethylsilane (TMS) was used
as an internal standard. Differential pulse voltammograms were recorded on an EG&G
PARSTAT electrochemical analyzer using a three electrode system. A platinum button electrode
was used as the working electrode. A platinum wire served as the counter electrode and an
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Ag/AgCl electrode was used as the reference electrode. Ferrocene/ferrocenium redox couple
was used as an internal standard. All the solutions were purged prior to electrochemical and
spectral measurements using nitrogen gas.
4.4 Femtosecond pump-probe transient spectroscopy. Femtosecond transient absorption
spectroscopy experiments were performed using an Ultrafast Femtosecond Laser Source (Libra)
by Coherent incorporating diode-pumped, mode locked Ti:Sapphire laser (Vitesse) and diode-
pumped intra cavity doubled Nd:YLF laser (Evolution) to generate a compressed laser output of
1.45 W. For optical detection, a Helios transient absorption spectrometer coupled with
femtosecond harmonics generator both provided by Ultrafast Systems LLC was used. The source
for the pump and probe pulses were derived from the fundamental output of Libra (Compressed
output 1.45 W, pulse width 100 fs) at a repetition rate of 1 kHz. 95% of the fundamental output
of the laser was introduced into harmonic generator which produces second and third harmonics
of 400 and 267 nm besides the fundamental 800 nm for excitation, while the rest of the output
was used for generation of white light continuum. In the present study, the second harmonic 400
nm excitation pump was used in all the experiments. Kinetic traces at appropriate wavelengths
were assembled from the time-resolved spectral data. Data analysis was performed using Surface
Xplorer software supplied by Ultrafast Systems. All measurements were conducted in degassed
solutions at 298 K. The estimated error in the reported rate constants is +10%.
4.5 Nanosecond laser flash photolysis. The studied compounds were excited by a Opolette
HE 355 LD pumped by a high energy Nd:YAG laser with second and third harmonics OPO
(tuning range 410-2200 nm, pulse repetition rate 20 Hz, pulse length 7 ns) with the powers of 1.0
to 3 mJ per pulse. The transient absorption measurements were performed using a Proteus UV-
Vis-NIR flash photolysis spectrometer (Ultrafast Systems, Sarasota, FL) with a fibre optic
delivered white probe light and either a fast rise Si photodiode detector covering the 200-1000
nm range or a InGaAs photodiode detector covering 900-1600 nm range. The output from the
photodiodes and a photomultiplier tube was recorded with a digitizing Tektronix oscilloscope.
Acknowledgments
Support by the National Science Foundation (Grant No. 1401188) is acknowledged. The
computational work was performed at the Holland Computing Centre of the University of
Nebraska.
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Notes and references
‡ Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre
with CCDC reference number CCDC 1480541 (for 6). Copies of the data can be obtained, free
of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
http://www.ccdc.cam.ac.uk/perl/catreq/catreq.cgi, e-mail: data_request@ ccdc.cam.ac.uk, or fax:
+44 1223 336033.
Electronic Supplementary Information (ESI) available: [structure, absorption spectra and
synthesis of control compounds 5-8, spectra of oxidized probes, X-ray structural details of 6, 1H,
13C and MALDI-mass spectra of newly synthesized compounds]. See DOI: 10.1039/x0xx00000x
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