Photocatalysis and self-catalyzed photobleaching with covalently-linked chromophore-quencher conjugates built around BOPHY

Article abstract of DOI:10.1039/c8pp00162f

Two Chromophore-Quencher Conjugates (CQCs) have been synthesized by covalent attachment of the anti-oxidant dibutylated-hydroxytoluene (BHT) to a pyrrole-BF₂ chromophore (BOPHY) in an effort to protect the latter against photofading. In fluid s

Full text of DOI:10.1039/c8pp00162f

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PhotoPchleeamseicadlo&nPohtoatodbjuiostlomgiacragl iSncsiences
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Photocatalysis and Self-catalyzed Photobleaching with Covalently-
Linked Chromophore-Quencher Conjugates Built Around BOPHY
Dumitru Sirbu, Owen J. Woodford, Andrew C. Benniston and Anthony Harriman*
Received 00th January 20xx,
Accepted 00th January 20xx
Two Chromophore-Quencher Conjugates (CQCs) have been synthesized by covalent attachment of the anti-oxidant
dibutylated-hydroxytoluene (BHT) to a pyrrole-BF2 chromophore (BOPHY) in an effort to protect the latter against
photofading. In fluid solution, light-induced intramolecular charge transfer is favoured in polar solvents and helps to inhibit
photo-bleaching of the chromophore. The rate of photo-fading, which scales with the number of BHT residues, is zero-order
in polar solvents but shows a linear dependence on the number of absorbed photons. The zero-order rate constant shows
an inverse correlation with the fluorescence quantum yield measured in the same solvent. Photo-bleaching in benzonitrile
involves autocatalysis while reaction in cyclohexane shows an unexpected stoichiometry. NMR spectroscopy indicates initial
damage takes place at the BHT unit and allows identification of a reactive hydroperoxide as being the primary product. In
the presence of an adventitious substrate, this hydroperoxide is a photocatalyst for amide formation under mild conditions.
DOI: 10.1039/x0xx00000x
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the early classification of dyes as being light-fast; this refers to
dyes that do not fade under illumination and also cause little or
no photo-tendering. The first real examination of the mechanism
Introduction
Organic dyes tend often to degrade when subjected to prolonged
exposure to white light. A common cause of such photo-fading
is the transient formation of a triplet-excited state that reacts with
molecular oxygen to form the highly reactive species known as
singlet oxygen.1 The latter attacks unsaturated cyclic structures2
and, although short lived in most solvents,3 reacts irreversibly to
bleach the dye. However, inhibiting intersystem-crossing to the
triplet manifold does not always eliminate photo-bleaching4 and
it is clear that degradation of the chromophore can occur through
other types of photochemistry. Some relief can be obtained by
systematic engineering of the sample, including removal of
oxygen, embedding in plastic and/or incorporation of filters.
Addition of stabilizers or anti-oxidants can also be considered as
a means to prolong the duty cycle of the chromophore. A
somewhat similar approach is found in natural photosynthesis
where carotenoids deactivate triplet-excited states5 and remove
oxy-radicals.6 Indeed, in the light-harvesting antennae of green
plants and photosynthetic bacteria, -carotene is employed as an
anti-oxidant at very high concentration but this strategy is less
attractive for artificial analogues.
behind the photochemical damage caused by exposure of organic
dyes to sunlight was reported by Bridge and Porter in 1958.8
Using the newly developed flash photolysis technique, these
authors showed that 9,10-anthraquinone dyes formed the highly
reactive n,* excited triplet state under illumination. This meta-
stable species abstracts a hydrogen atom from any available
source, thereby promoting both loss of dye and destruction of the
host. The simple strategy of adding hydroxyl groups to the 1,4-
positions rendered the n,* triplet state impotent9 because of
intramolecular proton transfer. The reactive n,* triplet state can
also be deactivated by substituting electron donating groups at
positions where intramolecular charge transfer is strongly
favoured.10 Such observation exemplify early attempts to
stabilize the dye against photo-fading but the majority of light-
fast dyes are identified by serendipity.
Recent advances in single-molecule microscopy11 and super-
resolution microscopy12 have fuelled interest in light-fast dyes.
Here, there is a particular need for dyes that absorb and emit in
the far-red region. Cyanine dyes, which are by far the most
popular emitter used for such experiments, are well known to
fade on exposure to visible light.13 This realisation has led to new
strategies for protecting the dye against deleterious
photochemical damage. For example, recent work by Cosa et
al.14 has reported beneficial effects from addition of triplet
quenchers that enter into light-induced electron-transfer
reactions. A key feature here concerns spin conversion in the
resultant geminate radical ion pairs. Related work15 has found an
extended duty cycle for a conjugated polymer used in total
internal reflection microscopy when certain triplet quencher
additives are present. The importance of geminate recombination
as a photoprotective protocol has also been stressed by Tinnefeld
The need for organic dyes that resist fading under
illumination has been recognized for a very long time, as has
been the realization that certain dyes promote the light-induced
degradation of fabrics and other supporting media.7 This led to
a.Molecular Photonics Laboratory, School of Natural and Environmental Sciences
(Chemistry), Bedson Building, Newcastle University, Newcastle upon Tyne, NE1
7RU, United Kingdom
Electronic Supplementary Information (ESI) available: [includes compound
characterisation, photo-polymerisation with P2B, Arrhenius and Lippert-Mataga
plots, characterisation of the amide product and examples of bleaching kinetics in
various solvents]. See DOI: 10.1039/x0xx00000x
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Scheme 1. Illustration of the methodology used to isolate the new Chromophore-Quencher Conjugates P1B and P2B. Reagents and Conditions: (i) hydrazine hydrate,
3,5-dimethylpyrrole-2-carboxaldehyde, glacial AcOH, ethanol, RT, 1.5h; (ii) BF3·Et2O, DIPEA, toluene, 60 0C, 16h; (iii) 3,5-di-tert-butyl-4-hydroxybenzaldehyde, p-TsOH,
piperidine, toluene, reflux to dryness twice; (iv) 1 atm. H2, 10% Pd/C, CH2Cl2, CH3OH, RT, 3h. The percentage yield is provided for each isolated compound.
et al.16 Here, single-molecule fluorescence spectroscopy has This chromophore absorbs at higher energy than the above
indicated improved levels of photostability using combinations cyanine dyes and it might be mentioned that concerns have been
of reagents, such as -mercaptoethanol and Trolox. These raised16 that the geminate recombination strategy might be less
additives quench the triplet-excited state via diffusion-limited, applicable for shorter wavelength emitters. Recent work26 has
electron-transfer reactions. Such strategies can result in greatly shown that simple BOPHY derivatives undergo slow
improved (i.e., >5,000-fold) levels of photochemical protection, photochemical degradation in aerated solution, with the rate of
leading to dyes that can emit in excess of 108 photons per bleaching being dependent on the solvent. The triplet-excited
molecule.17
state is not involved in the bleaching chemistry. Now, the
A logical, but important, extension of the above research into BOPHY dye has been equipped with one or two dibutylated-
the photoprotection of cyanine dyes involves direct attachment hydroxytoluene (BHT) residues. The latter is a lipophilic anti-
of the quencher to the chromophore. This type of covalently- oxidant used to minimize the effects of attack by free
linked conjugate, where the additive is nitrobenzyl alcohol or radicals.28.29 It can be used at low concentrations in food
Trolox, has been developed by Blanchard et al.18-20 and shown to products, where it is considered to be a synthetic analogue of
provide high levels of protection. Somewhat related cyanine dye vitamin E. The new chromophore-quencher conjugates (CQC)
derivatives have been reported21 to give improved (>100-fold) were built by attaching BHT to BOPHY via an ethylene chain
photostability when two or more redox-active quenchers are (Scheme 1). This maintains the two units in close proximity but
attached to the chromophore. These latter conjugates have been the spacer does not promote mutual electronic communication or
described as self-healing and, as above, the primary target is the indeed orbital contact between the terminals.
triplet-excited state. A more general approach, applicable to dyes
Considerable interest has been shown of late in developing
other than cyanines, involves linking the dye to a photostabilizer unusual organic transformations by way of photosensitized
and a biomolecular target via an unnatural amino acid.22 Such catalysis.30,31 One such photosystem32 has used an organic
materials are suitable as fluorescent labels for DNA, antibodies sensitizer in conjunction with a BHT derivative to generate good
and proteins for use with single-molecule microscopy.
yields for amidation of aromatic aldehydes by reaction with
Similar conjugates but with cyclooctatetraene as appendage amines. Reaction relies on bimolecular interception of excited
operate by deactivating the meta-stable triplet-excited state by states and reactive intermediates. As such, it is interesting to
way of intramolecular triplet-triplet energy transfer.23 This latter establish if similar photoreactions can be initiated using the
system has considerable appeal because no charged species are covalently-attached CQC and we have therefore examined the
formed as intermediates and there is less likelihood that toxic photo-driven oxidative amidation of 4-bromobenzaldehyde with
radicals will arise during reaction. Appended Trolox units appear pyrrolidine in solution. The reaction proceeds smoothly under
not to act as triplet quenchers in the protective process18 and their white light illumination to give clean products in yields
precise role is still under investigation.24,25
exceeding 75% conversion. The intermediate believed to be
Here, we describe a related system whereby the cyanine dye responsible for the photo-fading process provides the impetus for
has been replaced with a symmetrical pyrrole-BF2 chromophore photocatalysis.
of the BOPHY class,26 introduced recently by Ziegler et al.27
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Results and Discussion
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Synthesis and Characterisation
A two-step approach was used to synthesize the target CQC
derivatives (Scheme 1), abbreviated hereafter as P1B and P2B.
The starting material, tetramethyl-BOPHY (TMBOPHY,
referred to throughout as the parent dye) was synthesized26,27 in
good yield by acid catalyzed condensation of 3,5-
dimethylpyrrole-2-carboxaldehyde with hydrazine to form the
Schiff base, which was further reacted with BF3·(OEt)2 in the
presence of N,N-diisopropylethylamine (DIPEA), as described
before.33 It might be noted that a 3-fold reduction in the volume
of the reaction filtrate in vacuo allowed isolation of the Schiff
base ligand in higher yields (90% relative to 3,5-
dimethylpyrrole-2-carboxaldehyde) as compared to the literature
procedure (i.e., 65%). Knoevenagel condensation of the parent
dye with 3,5-di-tertiary-butyl-4-hydroxybenzaldehyde, using the
method developed by Ziessel et al.,34 generated a mixture of
mono- and bis-condensed products as indicated by TLC and
mass spectrometry. After twice repeating the evaporation cycle,
any additional reaction time led to increased amounts of side-
products, while employing the piperidine / acetic acid method in
acetonitrile failed to give any significant conversion. The
mixture was not purified at this stage due to the limited stability
of the products during the attempted purification procedure. This
situation was exacerbated by their similar RF values. Instead, the
crude mixture was hydrogenated directly with H2 at 1 atm.,
employing 10% Pd/C as the catalyst. Chromatography on silica
Figure 1. Absorption (black curve), excitation (open circles) and fluorescence (red
curve) spectra recorded for P1B in dichloromethane solution (upper panel) and a
thin (ca. 400 micron) PMMA film cast from toluene (lower panel).
gel provided the desired mono- and di-substituted CQCs in 1). Relative to the parent (i.e., TMBOPHY) compound,
reasonable global yield relative to the parent dye; P1B (31%) and fluorescence from P1B is less pronounced, with the quantum
P2B (14%).
yield (F) showing a general correlation with the polarity of the
The 1H-NMR spectra recorded for P1B and P2B (see solvent as expressed in terms of the static dielectric constant (S).
Supporting Information) show the presence of peaks considered This behaviour is mirrored by changes in the excited-singlet state
characteristic of both BHT and BOPHY units. The integral of the lifetime (S), which shortens with increasing polarity as
tertiary-butyl groups (1.4 ppm), hydroxyl (5.1 ppm), aromatic illustrated by the data listed in Table 1. However, in certain polar
phenyl protons (7 ppm), and the ethylene bridge (two triplets at solvents, such as acetonitrile, the time-resolved emission decay
2.9 ppm and 3.1 ppm) is consistent, respectively, with one and profiles could no longer be well explained35 in terms of a single-
two BHT subunits being attached to the BOPHY module. At the exponential component. In these solvents, the quality of the fit is
same time, the triplet splitting pattern found for protons assigned improved36 on invoking dual-exponential kinetics (see
to the ethylene bridge confirm that the hydrogenation reaction Supporting Information; Table 1). The same situation arises for
was successful. A simple pattern of three singlets (at 6.2 ppm and P2B, where fluorescence quenching is somewhat more
8 ppm, respectively, for protons in β- and meso-positions and at pronounced (Table 1). Again, the time-resolved emission decay
2.3 ppm for the methyl protons) is found for the BOPHY core in profiles are well represented by single-exponential fits35 in non-
the specific case of the symmetrical P2B molecule. In the case of polar solvents but the fit becomes less satisfactory36 in polar
P1B, the β protons are slightly split while the meso- and the solvents. The need to resort to dual-exponential kinetics is far
methyl-protons are subjected to minor broadening. The [M-F]+ from unusual when working with flexibly-linked dyads and is
molecular ion pattern found in ASAP/APCI high-resolution usually interpreted in terms of several families of non-
mass spectra of P1B and P2B further corroborate the assigned interconverting conformers.37,38 These distributions differ in
structures (see Supporting Information for details).
their ability to effect intramolecular fluorescence quenching.
One family gives rise to a short lifetime, presumably indicating
close proximity of the reactants, while the second family is
Photophysical properties of the CQC
The presence of the appended BHT residue has only minor subject to much weaker quenching.
effects on the absorption, excitation and fluorescence spectral
Comparable behaviour has been seen in many other cases and
profiles of the BOPHY unit26,27 in either fluid solution or thin attributed to differences in the level of orbital contact between
plastic films (Figure 1). There is no aggregation under these the reactants.39-41 With a short saturated connecting chain, it is
conditions. The absorption (ABS) and fluorescence (FLU
)
difficult for the BHT group to approach near the BOPHY core, a
maxima are fairly insensitive to the nature of the solvent (Table
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Table 1. Summary of the photophysical properties recorded for the target CQCs in a range of solvents.
P1B
P2B
ΦF
S \ ns (a)
2.8
λABS \ nm
472
λFLU \ nm
489
ΦF
S \ ns (a)
2.7
λABS \ nm
λFLU \ nm
493
Solvent
0.75
0.74
0.73
0.71
475
471
C6H12
2.8
470
490
2.7
493
CH2Cl2
BuCN (b)
BzCN (c)
PC (d)
0.44
0.67
1.9
1.9
465
472
487
495
0.35
0.65
1.9
2.6
468
475
491
497
0.30
0.32
0.5
1.3
78%
1.7
464
464
486
487
0.23
0.23
0.6
1.2
73%
1.4
466
466
491
490
22%
0.9
27%
0.4
Acetone
42%
0.5
58%
1.5
28%
0.4
72%
1.1
0.25
0.17
0.15
0.13
461
466
464
466
485
490
487
491
0.17
0.06
0.08
0.08
463
468
467
468
489
494
490
494
CH3CN
DMF (e)
CH3OH
DMSO (f)
71%
0.5
29%
2.5
63%
0.3
37%
1.6
89%
0.4
11%
2.2
92%
0.3
8%
2.5
89%
0.4
11%
3.0
96%
0.3
4%
2.7
89%
11%
94%
6%
(a) Singlet excited state lifetime derived by fitting the time-resolved emission decay profiles to either mono- or dual-exponential processes. In the latter case, the
fractional amplitudes of the two lifetimes are given below the numerical value. (b) Butyronitrile. (c) Benzonitrile. (d) Propylene carbonate. (e) N,N-Dimethylformamide.
(f) Dimethylsulfoxide.
situation well known from exciplex emission studies with other and P2B. These values are in accord with the limited literature
dyads.42,43 It is also possible that hydrogen bonding between data available for simpler BOPHY derivatives.44,45
BHT and an appropriate solvents will affect the conformational
The cyclic voltammograms indicate a two-electron oxidation
distributions. Many factors related to the solvent (e.g., size, wave for each BHT unit at peak potentials lower than found for
structure, polarity, functional groups, viscosity) combine to mask oxidation of BOPHY. Thus, P1B shows an irreversible, two-
any simple correlation between the extent of fluorescence electron wave with a peak at 1.27 V vs Ag/Ag+ while P2B shows
quenching and the nature of the solvent (Table 1).
a four-electron wave peaking at 1.20 V vs Ag/Ag+. Even on fast
We can explain the overall fluorescence quenching in terms scans (i.e., 5 V/s), these oxidation steps are electrochemically
of light-induced electron transfer between BOPHY and BHT. irreversible. Most likely, oxidation of the phenol is followed by
Indeed, cyclic voltammetry carried out in CH2Cl2 indicates that rapid loss of the proton, as is known to occur for tyrosine46 and
one-electron reduction of the BOPHY unit occurs with a half- related phenols.47 The poor reversibility of these electrochemical
wave potential of -1.49 V vs Ag/Ag+ for both P1B and P2B.44 steps precludes proper determination of thermodynamic driving
This reduction process is not fully reversible and at slow scan forces for light-induced, intramolecular electron transfer
rates (i.e., 100 mV/s) about 10% of the CQC (initial occurring within the CQC. Nonetheless, the ordering of the
concentration 2 mM) is lost on each cycle. One-electron potentials indicates electron transfer from BHT to the excited-
oxidation of the BOPHY unit can be recognized44 with half-wave singlet state of BOPHY will be favoured in polar solvents.48
potentials of 1.32 and 1.28 V vs Ag/Ag+, respectively, for P1B Therefore, the solvent effect observed for F and S can be
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attributed, at least in the major part, to thermodynamic effects
associated with intramolecular electron transfer.49 The
significance of this finding is that interaction between the two
subunits has to be taken into account when considering the
mechanism for any subsequent photo-fading of the
chromophore. It might be noted that such solvent effects are not
observed for the parent fluorophore.
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These CQCs differ from the earlier prototypes18,21,24,25 in as
much as protection is aimed at the singlet-excited state, rather
than the meta-stable triplet state. Quenching involves light-
induced charge transfer and is followed by rapid charge
recombination within the geminate radical ion pair16 to restore
the ground state (Scheme 2). Transient absorption spectroscopy
did not resolve the presence of long-lived intermediates, nor
indeed any sign of the triplet-excited state, a situation consistent
with the rate of charge recombination exceeding that of charge
separation. The latter is limited by the modest thermodynamic
driving force and, at least in some conformations, by poor orbital
contact. This circumstance is often observed with flexibly-linked
dyads.38 It is notable that charge recombination does not lead to
formation of the triplet-excited state (Scheme 2).16 Efficient
charge recombination serves to deactivate the excited state and
thereby provide protection against photo-fading. In order to
establish the effectiveness of this mode of operation, kinetic
studies were undertaken to monitor the rate of photo-bleaching
of the CQC in fluid solution.
Figure 2. Progressive photo-bleaching observed for P2B in air-equilibrated (a)
acetonitrile and (b) butyronitrile under broadband illumination. Absorption
spectra were recorded at frequent intervals up to 100 hours continuous exposure.
time, depends on the total light intensity (I0) and on the fraction
of light absorbed by the chromophore (IABS). The latter term
depends on the chromophore concentration as indicated by
Equation 1.51 In polar solvents, notably acetonitrile,
butyronitrile, acetone and DMF, it is found that the reaction rate
is quite insensitive to the chromophore concentration, at least
over most of the bleaching profile (Figure 3). This indicates that
the reaction is essentially zero-order with respect to
chromophore concentration but is linearly dependent on the
number of photons absorbed. Under these conditions, the rate of
bleaching is controlled entirely by the light flux (Equation 2).
This allows estimation of a pseudo-zero-order rate constant (k0),
which is the rate constant under standardised incident light
intensity (i.e., I0 = 1), as illustrated by Figure 3. The derived k0
values are collected in Table 2. It was observed that P1B
followed similar behaviour but with somewhat slower rates of
bleaching.
Scheme 2. Representation of the photo-processes that follow from excitation of
the BOPHY chromophore in the new CQCs: FL = fluorescence, CS = charge
separation to form a geminate ion pair (CTS), CR = charge recombination to restore
the ground state. Intersystem crossing to the triplet manifold is not observed.
[
]
퐼퐴퐵푆 = 1 − 10−휖 푃2퐵
(1)
(2)
Photochemical bleaching in polar solvent
Solutions of P2B in air-equilibrated polar solvents were
illuminated with broadband light to simulate exposure to
sunlight. Bleaching of the absorption profile occurs but there is
no obvious indication for formation of a coloured product.
Increased absorption does appear in the near-UV region and, at
early times, isosbestic points are seen at wavelengths around 370
nm. Two examples of this behaviour are shown as Figure 2 and
further examples are given in the Supporting Information. Global
analysis50 of the spectral data accompanying loss of the
chromophore, using the entire wavelength window from 400 nm
to 480 nm, allows the reaction kinetics to be evaluated. Thus, the
rate of bleaching () at any wavelength was determined as a
function of chromophore concentration. This rate, at any given
0
[
]
휔 = 퐼0 × 퐼퐴퐵푆 × 푘0 × 푃2퐵
An important variation is found in benzonitrile solution
where it will be recalled fluorescence quenching is less
noticeable than for the other polar solvents. Here, the rate of
photo-bleaching () increases slightly at early times before
reaching a plateau. On longer exposure times, the rate begins to
decrease (Figure 4). As above, bleaching of the BOPHY
chromophore does not lead to the appearance of a relatively
stable product that absorbs in the visible region. The rate of
bleaching is consistent across the entire 400-480 nm spectral
window. The shape of the rate vs concentration plot exhibits the
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Table 2. Summary of the derived rate constants for the photo-bleaching of P2B in various
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air-equilibrated solvents.
Solvent
C6H12
S
2
26
20.7
20.7
37.5
36.7
F
Rate constant
0.73
0.65
0.35
0.20
0.17
0.06
BzCN (a)
BuCN (b)
acetone
CH3CN
DMF (c)
1.5 (d, e)
0.78 (d)
0.75 (d)
0.60 (d)
0.60 (d)
(a) Benzonitrile. (b) Butyronitrile. (c) N,N-Dimethylformamide. (d) Refers to the
pseudo-zero-order rate constant having units of M h-1 (see inset to Figure 4). (e)
Fit is improved using the autocatalysis expression given as Equation 3.
Figure 3. Effect of chromophore concentration on the light-intensity corrected
rate of photo-bleaching for P2B in air-equilibrated (a) acetonitrile and (b)
butyronitrile. The horizontal lines drawn through the higher concentration data
correspond to the averaged zero-order rate constants.
Scheme 3. Representation of the processes that follow from excitation of the
BOPHY chromophore in the new CQCs: FL = fluorescence, CS = charge separation
to form a geminate ion pair (CTS), CR = charge recombination, BL = bleaching, SH
= self-healing. The notation B-BHT~ is used to indicate the “stable” product that
promotes further bleaching of the chromophore, later identified as being the
hydroperoxide.
solvent polarity (Table 2), although the variation is modest.
Moreover, a clear correlation exists between the fluorescence
quantum yield and k0 in polar solvents. When considered in
terms of the solvent dielectric constant (S), benzonitrile gives an
anomalously high fluorescence quantum yield and a surprisingly
high rate of photo-fading. This can be explained in terms of -
stacking between the chromophore and the phenyl ring of the
solvent as was observed previously by NMR spectroscopy with
toluene and the parent BOPHY.26 This transient alignment of
solvent and chromophore would force the appended BHT residue
Figure 4. (a) Progressive photo-bleaching observed for P2B in air-equilibrated
benzonitrile under broadband illumination. (b) Kinetic fit to the autocatalytic away from the chromophore. The resultant “open” conformation
expression given as Equation 3 with the parameters mentioned in the text. The
inset shows how the light-intensity corrected rate of bleaching depends on P2B is less amenable to light-induced electron transfer because of the
concentration in an analogous manner to that illustrated in Figure 3.
increased separation. Hence, intramolecular charge transfer is
switched off and the consequent protection is lost. Indeed, the F
found for benzonitrile approaches that observed in cyclohexane
solution.
We interpret this overall behaviour in terms of Scheme 3,
where excitation of the BOPHY chromophore leads to transient
population of a radical ion pair due to light-induced charge
transfer from BHT to BOPHY.55 Rapid geminate recombination
restores the ground state but, according to the cyclic voltammetry
results, a minor fraction of the oxidized BHT fragment
undergoes irreversible chemical transformation. Most likely, this
general appearance of that expected52,53 for an autocatalytic
process. Indeed, the reaction kinetics, again treated in terms of
global analysis across the 400-480 nm window, fit nicely to
Equation 3.54 Here, k1 (= 0.070 h-1) is a first-order rate constant
attributed to a chemical reaction that forms an autocatalytic
species and kAC (= 0.0084 M-1 h-1) is the corresponding
bimolecular rate constant for autocatalysis (Table 2). At high
conversion, the rate of reaction decreases due to the scarcity of
substrate and the linear fit to Equation 3 becomes lost.
reaction is initiated by loss of the phenoxyl proton from the BHT
-radical cation.56 It is this deprotonation that breaks the
recycling step and leads to formation of an oxy-radical.57 The
corresponding BOPHY -radical anion is rapidly re-oxidized by
휔
(
[
] ) ]
[
= 푘1 + 푘퐴퐶 푃2퐵 − 푘퐴퐶 푃2퐵
(3)
0
[
]
푃2퐵
There is a general trend for the derived rate constant, k0,
attributed to the photo-bleaching step to increase with decreasing
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PhotoPchleeamseicadlo&nPohtoatodbjuiostlomgiacragl iSncsiences
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DOI: 10.1039/C8PP00162F
Journal Name
ARTICLE
Figure 5. Formation of P1B-OOH and associated changes in the 1H-NMR spectrum of P1B in acetone-d6 after illumination with white light. *Peak intensities in the 1–1.5
ppm region are decreased by a factor of 5-fold for a clearer image.
molecular oxygen (vide infra). The fate of the oxy-radical, which shifts on the order of 0.15 to 0.8 ppm. This is in agreement with
leads to formation of one or more products that promote further the formation of a 4-hydroperoxycyclohexa-2,5-dien-1-one ring
bleaching of BOPHY, was explored by NMR spectroscopy (vide due to reaction of the BHT radical and the hydroperoxyl radical
infra). In this proposed scheme, escape from the geminate ion- (Figure 5). Additional proof is provided by the peak at m/z = 611
pair plays a key role in the overall process. This is evident from found in high-resolution nano-ESI mass spectrometry, that
the link between F and k0 and is confirmed by the situation matches well with the [M+Na]+ molecular ion pattern (see
outlined for benzonitrile. Increasing the oxygen concentration Supporting Information). Conversion to the hydroperoxyl P1B-
has no real effect on the reaction kinetics. In the absence of OOH is quite selective, reaching 90% yield at modest conversion
oxygen, bleaching occurs slowly but by a different mechanism.
of starting compound. This falls to 70% at full consumption,
The course of reaction for P1B in acetone-d6 was followed possibly reflecting the high reactivity of the resultant P1B-OOH
1
by H-NMR spectroscopy (Figure 5). Illumination causes clean species. These NMR studies indicate that little damage occurs at
conversion to an intermediate species within a few hours. It is the BOPHY site during the initial phase but continued
clear that most of the chemistry is associated with the BHT illumination leads to the formation of multiple products, as is
residue. Thus, the proton from the hydroxyl group (a) is shifted evidenced by the appearance of many new peaks and broadening
downfield by about 5 ppm to what looks like a hydroperoxide of the entire spectrum.
peak (δ = 10.9 ppm).58 At the same time, protons from phenyl
The quantum yield for the overall bleaching process is kept
(b), ethylene bridge (c) and tertiary-butyl moieties (d) show low by the high level of reversibility of the initial charge-transfer
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bleaching chemistry might be more complex than fVoieuwnAdrtiicnlepOonllianer
solvents. For both CQCs in air-equilDibOrIa: t1e0d.103c9y/Ccl8oPhPe0x0a16n2eF,
illumination with broadband light induces a relatively fast
reaction that causes only a minor change in the absorption
spectral profile. The absorption data, corrected for minor
changes in the fraction of light absorbed by the chromophore,66
fits well to first-order kinetics over the early stage of reaction.
The corresponding first-order rate constants (k1) are 0.015 and
0.008 h-1, respectively, for P1B and P2B. It is noticeable that this
initial phase of the reaction persists far longer for P2B than for
P1B. The implication is that the BHT residue is involved in the
process. Following this initial reaction, the rate of photo-
bleaching increases to a maximum before decreasing as the
course of reaction continues (Figure 7). The parabolic form of
the rate vs conversion profile is as might be expected52-54 for an
autocatalytic process, as already noted for benzonitrile solutions.
Scheme 4. Representation of the effect of added DABCO (DAB) on the photo-
processes that follow from excitation of the BOPHY chromophore in the new
CQCs.
process. Short illumination of P2B in aerated acetonitrile
solution at 390 nm allows estimation of the quantum yield for the
bleaching of the BOPHY chromophore as being on the order of
10-7. Laser flash photolysis of P1B, with excitation at 355 nm,
confirms the high level of reversibility of the primary
photochemical step. Under such conditions, most of the signal
corresponding to ground-state absorption by the BOPHY
chromophore recovers within the excitation pulse. A small
fraction recovers on a much slower timescale with a half-life of
ca. 300 ns in air-equilibrated acetonitrile. This latter process is
attributed to re-oxidation of the BOPHY -radical anion by
molecular oxygen. The activation energy measured for the
overall bleaching in air-equilibrated DMF was found to be 9.2
kJ/mol, which is comparable to the temperature dependence for
the viscosity of this solvent.59
A number of trapping studies were attempted. Thus, the
typical quenchers DMPO (a spin trap60 that reacts with O-centred
radicals) and DABCO (which, among other reactions, is a
quencher61,62 for singlet oxygen) were added at 1 mM
concentration to solutions of P2B in acetonitrile. Addition of
DABCO was seen to exacerbate the bleaching while DMPO had
an initial inhibiting effect on the rate of photo-fading but later
catalysed loss of the chromophore. It is considered that DMPO
intercepts the initially formed phenoxyl radical but this
beneficial effect is soon lost as reactive products emerge. It was
also found that DABCO caused dynamic fluorescence quenching
Figure 6. Progressive photo-bleaching observed for (a) P1B and (b) P2B in air-
equilibrated cyclohexane under broadband illumination. Absorption spectra were
recorded at frequent intervals up to 105 hours continuous exposure.
It is apparent from examination of Figure 6 that the bleaching
due to light-induced electron transfer.63 Transient absorption process is far from complete. Indeed, the rate of photo-bleaching,
spectroscopy was used to show that, at high concentrations (i.e., , reaches an optimum value before decreasing with increasing
0.1 M) of DABCO, the intramolecular electron transfer is reaction time but not ceasing altogether. The extent of bleaching
replaced by an intermolecular reaction (Scheme 4). Now the is significantly more pronounced for P2B than for P1B, again
radical ion products recombine rapidly within the geminate ion fully consistent with the idea that the BHT residue is involved. It
pair without separation.14,16,18,64 This latter reaction provides appears that the bleaching chemistry enters the dormant phase at
stabilisation against photo-fading but is far from being practical precise conversion fractions. For P1B, this corresponds to ca.
because of the high concentrations needed.
50% conversion to the bleached form while the conversion
fraction is ca. 75% for P2B. Such behaviour must be indicative
of the bleaching mechanism and allows definition of the reaction
Photo-bleaching in nonpolar solvents
Due to thermodynamic restrictions,65 it is unlikely that light- stoichiometry as follows:
induced intramolecular electron transfer will be effective in non-
In cyclohexane solution, illumination of P1B leads to the
polar media. Nonetheless, photo-bleaching occurs quickly under relatively fast conversion to a product whereby the BHT residue
these conditions (Figure 6), again without formation of coloured has been chemically modified. It is tempting to raise the
products. There is, however, a partial accumulation of products likelihood that this involves formation of the hydroperoxide but
absorbing in the near-UV region. Global analysis of the kinetic we do not have experimental support for this claim. The modified

data acquired over the 420-490 nm window indicates that the form is designated as B-BHT on Scheme 5, where B is used to
8 | J. Name., 2012, 00, 1-3
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designate BOPHY. It has an absorption profile similar to that of fraction of the CQC is recycled in a form that lacks a reactive
View Article Online
18-21

DOI: 10.1039/C8PP00162F
the starting chromophore. Two B-BHT residues react together BHT residue represents an unusual type of self-healing.
to bleach one of the chromophores and to render the second
chromophore intact but with the BHT residue deactivated
(designated as B-X on Scheme 5). For P2B under the same
conditions, illumination leads to successive modification of the
two BHT residues followed by similar chemistry to that outlined
for P1B (Scheme 5). The net result is that this type of photo-
bleaching will account for 50% of P1B but 75% of P2B. A key

feature of this scheme is that B-BHT does not attack the
chromophore directly but bleaching occurs only after diffusional

approach of two B-BHT residues.
Figure 8. Absorption spectra recorded during the course of photo-bleaching of (a)
P1B and (b) P2B dispersed in a thin film of PMMA. The illumination source was
light white and the films were cast from toluene. Spectra were recorded at regular
intervals up to 60 hours.
Figure 7. Evolution of the reaction rate as a function of the degree of bleaching for
P1B (filled circles) and P2B (open circles) in air-equilibrated cyclohexane under
Photo-bleaching in thin plastic films
broadband illumination.
To further assess the significance of the bimolecular bleaching
step identified above, samples of the CQCs were prepared after
incorporation into thin (ca. 400 micron) PMMA films prepared
by casting from toluene. The dielectric constant68 of these films
is similar to that of cyclohexane although the concentration of
dissolved oxygen and the diffusion coefficients of the solutes are
likely to differ markedly from those for fluid solution. Under
illumination, the plastic-bound CQCs bleach as in solution
(Figure 8). Again, the entire spectral envelope fades under
exposure to white light and there is no clear indication for the
transient formation of a coloured intermediate. Global analysis
of the bleaching data indicates that loss of chromophore is well
described in the form of two successive first-order reactions
(Figure 9). The faster step accounts for about 12% of the
chromophore and occurs with half-lives of 2.0 h and 2.6 h,
respectively, for P1B and P2B. The slower step takes place with
half-lives of 160 h and 270 h, respectively. These reaction half-
Scheme 5. Outline of the reactions proposed to account for the bleaching
lives are corrected for the number of photons absorbed but it
stoichiometry observed for P1B and P2B in cyclohexane solution. The symbol B’ is
intended to describe a bleached chromophore where X refers to a damaged BHT
might not be wise to make a direct comparison of the derived
residue that cannot recycle.
values with those from solution. The important point, however,
is that the bimolecular process is inhibited in the film and
Figure 7 shows how the rate of bleaching evolves as a
replaced by a slow unimolecular step.
function of the extent of bleaching (i.e., the loss of colour). The
reaction corresponds to the simple scheme given as an inset to
Photocatalytic amidation
the Figure. The first-order process, which leads to only a small
Organic hydroperoxides69 can be considered as relatively strong
degree of bleaching, occurs with a rate constant of k1, as
oxidants useful for driving certain organic transformations.70
With this in mind, we examined the possibility to trap the in situ
formed BHT-OOH species in order to promote amide synthesis32
between an aldehyde and an amine through oxidation of the
intermediate α-hydroxyamine. The amide functional group plays
described above. In principle, the full reaction profile can be
evaluated to obtain an estimate for the second-order rate constant
but this is not straightforward and requires specialised kinetics
treatments,67 especially for P2B. The idea that a substantial
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the covalent attachment of a stabilizer to the chroVmieowpAhrtoicrleeO.1n8li-n2e3
We have designed a pair of new chDrOoI:m10o.p10h3o9r/eC-8qPuPe0n0c1h62eFr
conjugates that combine key features of the two approaches.
Thus, illumination of a BOPHY-BHT conjugate, where BHT
refers to a butylated hydroxytoluene anti-oxidant, in polar
solution causes light-induced, intramolecular charge transfer.
The resultant radical ion pair is prone to rapid charge
recombination, in line with many other such systems,75,76 but loss
of a proton from the phenolic -radical cation46,47 provides an
escape route. This deprotonation, which has been recognized for
certain acridinium-based dyads,77 initiates the photo-bleaching
process and, in aerated solution, leads to formation of an organic
hydroperoxide that readily enters into subsequent chemical
reactions at ambient temperature. Without a suitable substrate,
the hydroperoxide promotes bleaching of the chromophore. It is,
therefore, a key intermediate in the cycle and, used properly,
offers an opportunity to eliminate the sensitizer once any useful
work is complete. We have demonstrated its utility by driving an
amidation reaction under exceptionally mild conditions but other
oxidative syntheses might be possible with the same approach.
The reaction kinetics depend on the nature of the local
environment, possibly reflecting different levels of reactivity of
key intermediates. Perhaps the most interesting case from a
mechanistic viewpoint involves nonpolar solutions. Here, the net
stoichiometry clearly indicates that the primary intermediate
formed in a photochemical step undergoes reaction with a second
such species. This reaction destroys one of the chromophores but
converts the second chromophore to a form that has greatly
enhanced photostability. This represents a new type of self-
healing.21 Equally important is the effect of a flat aromatic
solvent, namely benzonitrile, which curtails the initial electron
transfer reactions and thereby promotes autocatalysis.
Figure 9. Kinetic profiles recorded for photo-bleaching of the CQC in a thin PMMA
film where diffusion is severely limited. The solid lines drawn through the data
points correspond to fits to two successive first-order reactions, with the
parameters given in the text.
a critical role in both organic and, more importantly, bioorganic
chemistry and it is recognised71-74 that direct amidation of
aldehydes avoids the additional oxidative step. Thus, when the
above photo-bleaching reaction is carried out in the presence of
4-bromobenzaldehyde (30 mM), pyrrolidine (90 mM) and 1
mol% P2B, formation of (4-bromophenyl) (pyrrolidin-1-
yl)methanone takes place (Scheme 6). Performing the reaction in
acetonitrile gives only 5% conversion before P2B is completely
bleached. Changing the solvent to the less polar dioxane
increases the yield to over 49% relative to the starting aldehyde.
Excess pyrrolidine leads to faster bleaching of the
photocatalyst and thus lower overall reaction yields for amide
synthesis. Absorption and fluorescence spectroscopy indicates
that it is depletion of the photocatalyst that causes the reaction to
stop prematurely. Consequently, increasing the amount of
photocatalyst to 2 mol% leads to complete consumption of the
starting material within 24 h and with a selectivity of 75% for the
formation of (4-bromophenyl)(pyrrolidin-1-yl)methanone as
determined by 1H- and 13C-NMR spectroscopy.29 These reaction
conditions are mild and require no metal catalyst or hazardous
reagent.
This work serves to indicate how a damaging reaction,
namely the photodegradation of a chromophore, can be put to
good use by, for example, the synthesis of a valuable reagent
under mild conditions.30,31 Other applications might be imagined
for the hydroperoxide intermediate. For example, we have shown
that the CQCs function as photo-initiators for polymerization78
of methyl methacrylate at room temperature to leave a colorless
plastic (see Supporting Information). Under visible light, P2B is
well able to bleach other dyes (notably Chlorophenol Red),
offering possibilities to design photosystems for antimicrobial
disinfectation79 and/or removal of toxic wastes.80 Work is
underway to test the capacity of these CQCs for the sustainable
epoxidation81,82 of alkenes under green conditions.
Scheme 6. Outline of the oxidative amidation of 4-bromobenzaldehyde with
pyrrolidine promoted by illumination of P2B.
There are no conflicts to declare.
Conclusion
There is tremendous current interest in the development of
strategies that aid protection of organic chromophores against
light-induced degradation. This research is driven by the ever-
expanding range of applications being proposed for fluorescent
dyes and by the emerging fields of single-molecule microscopy
and super-resolution imaging microscopy. Of the promising
approaches being advocated, we draw attention to two systems
that have been shown to bring about major improvements in
photoprotection; namely, the rapid charge-recombination (with
or without spin conversion) in geminate radical ion pairs14-16 and
Acknowledgements
We thank EPSRC for the award of an Industrial CASE award
(OJW) and Newcastle University for financial support of this
work. We also thank the EPSRC Mass Spectrometry service at
Swansea for access to facilities.
References
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Page 11 of 13
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Please do not adjust margins
Journal Name
ARTICLE
1
M. C. De Rosa and R. J. Crutchley, Photosensitized singlet 20 Q. Zheng, M. F. Juette, S. Jockusch, M. R. WasserVmieawnA,rtZic.leZOhnoliune,
oxygen and its applications, Coord. Chem. Rev., 2002, 233
,
R. B. Altman and S. C. Blanchard,DOUIl:t1r0a.-1s0t3a9b/Cle8PPo0rg0a16n2iFc
fluorophores for single-molecule research, Chem. Soc. Rev.,
2014, 43, 1044-1056.
351-371.
2
3
A. A. Gorman and M. A. J. Rodgers, Singlet molecular-oxygen,
Chem. Soc. Rev., 1981, 10, 205-231.
21 J. H. M. van der Velde, J. Oelerich, J. Y. Huang, J. H. Smit. M.
Hiermaier, E. Ploetz, A. Herrmann, G. Roelfes and T. Cordes,
The power of two: Covalent coupling of photostabilizers for
P. R. Ogilby and C. S. Foote, Chemistry of singlet oxygen. 42.
Effect of solvent, solvent isotopic-substitution, and
temperature on the lifetime of singlet molecular-oxygen (1-
delta-g), J. Am. Chem. Soc., 1983, 105, 3423-3430.
fluorescence applications, J. Phys. Chem. Lett., 2014, 5, 3792-
3798.
M. A. H. Alamiry, A. Harriman, A. Haefele and R. Ziessel, 22 J. H. M. Van der Velde, J. Oelerich, J. Y. Huang, J. Smits, A. A.
4
5
6
7
8
9
Photochemical bleaching of an elaborate artificial light-
harvesting antenna, ChemPhysChem, 2015, 16, 1867-1872.
A. Telfer, Too much light? How beta-carotene protects the
photosystem II reaction centre, Photochem. Photobiol. Sci.,
Jazi, K. Galiani, K. Kolmakov, S. Guoridis, C. Eggeling, A.
Hermann, G. Roelfes, and T. Cordes, A simple and versatile
design concept for fluorophore derivatives with
intramolecular photostabilization, Nature Commun., 2016, 7,
2005,
4
, 950-956.
10144.
R. Edge, D. J. McGarvey and T. G. Truscott, The carotenoids as 23 Q. Zheng, S. Jockusch, G. G. Rodriguez-Calero, Z. Zhou, H.
anti-oxidants - a review, J. Photochem. Photobiol. B Biol.,
1997, 41, 189-200.
H. Zöllinger, Color Chemistry. Syntheses, Properties and
Applications of Organic Dyes and Pigments. VHCA Publishers;
Zurich. 3rd Edition, Revised. 2003.
N. K. Bridge and G. Porter, Primary photoprocesses in
quinones and dyes. Part 1. Spectroscopic detection of
intermediates, Proc. Roy. Soc. A, 1958, 244, 259-275.
D. K. Palit, H. Pal, T. Mukherjee and J. P. Mittal, Triplet excited-
states and semiquinone radicals of 1,4-disubstituted
Zhao, R. B. Altman, H. D. Abruñab and S. C. Blanchard, Intra-
molecular triplet energy transfer is a general approach to
improve organic fluorophore photostability, Photochem.
Photosbiol. Sci., 2016, 15, 196-203.
24 P. Tinnefeld and T. Cordes, 'Self-healing' dyes: Intramolecular
stabilization of organic fluorophores, Nature Methods, 2012,
9
, 426-427.
25 Q. Zheng, S. Jockusch, Z. Zhou, R. B. Altman, J. D. Warren, N.
J. Turro and S. C. Blanchard, On the mechanisms of cyanine
fluorophore photostabilization, J. Phys. Chem. Lett., 2012, 3,
2200-2203.
26 O. Woodford, A. Harriman, W. McFarlane and C. Wills,
Dramatic effect of solvent on the rate of photobleaching of
anthraquinones, J. Photochem. Photobiol. A Chem., 1990, 52
375-390.
,
10 B. Armitage, C. J. Yu, C. Devadoss and G. B. Schuster, Cationic
anthraquinone derivatives as catalytic DNA photonucleases –
mechanisms for DNA-damage and quinone recycling, J. Am.
Chem. Soc., 1994, 116, 9847-9859.
11 J. L. Alejo, S. C. Blanchard and O. S. Andersen, Small-molecule
photostabilizing agents are modifiers of lipid bilayer
properties, Biophys. J., 2013, 104, 2410-2418.
organic pyrrole-BF2 (BOPHY) dyes, ChemPhotoChem, 2017, 1,
317-325.
27 I. S. Tamgho, A. Hasheminasab, J. T. Engle, V. N. Nemykin and
C. J. Ziegler, A new highly fluorescent and symmetric pyrrole-
BF2 chromophore: BOPHY, J. Am. Chem. Soc., 2014, 136, 5623-
5626.
12 S. van der Linde, S. Wolter, M. Heilemann and M. Sauer, The 28 O. Brede, H. Orthner, V. Zubarev and R. Hermann, Radical
effect of photoswitching kinetics and labelling densities on
super-resolution fluorescence imaging, J. Biotechnol., 2010,
149, 260-266.
cations of sterically hindered phenols as intermediates in
radiation-induced electron transfer processes, J. Phys. Chem.,
1996, 100, 7097-7105.
13 M. Levitus and S. Ranjit, Cyanine dyes in biophysical research: 29 M. Foti, K. U. Ingold and J. Lusztyk, The surprisingly high
The photophysics of polymethine fluorescent dyes in
reactivity of phenoxyl radicals, J. Am. Chem. Soc., 1994, 116,
9440-9447.
biomolecular environments, Quart. Rev. Biophys., 2011, 44
123-151.
,
30 E. R. Welin, C. Le, D. M. Arias-Rotondo, J. K. McCusker and D.
W. C. MacMillan, Photosensitized, energy transfer-mediated
organometallic catalysis through electronically excited
nickel(II), Science, 2017, 355, 380-385.
14 V. Glembockyte and G. Cosa, Redox-based photostabilizing
agents in fluorescence imaging: The hidden role of
intersystem crossing in geminate radical ion pairs, J. Am.
Chem. Soc., 2017, 139, 13227-13233.
31 M. Parasram and V. Gevorgyan, Visible light-induced
15 C. F. Calver, B. A. Lago, K. S. Schanze and G. Cosa, Enhancing
the photostability of poly(phenylene ethynylene) for single
particle studies, Photochem. Photobiol. Sci., 2017, 16, 1821-
1831.
transition
metal-catalyzed
transformations:
Beyond
conventional photosensitizers, Chem. Soc. Rev., 2017, 46
,
6227-6240.
32 X.-F. Wang, S.-S. Yu, C. Wang, D. Xue and J. B. Xiao, BODIPY
catalyzed amide synthesis promoted by BHT and air under
visible light, Org. Biomol. Chem., 2016, 14, 7028-7037.
16 P. Holzmeister, A. Gietl and P. Tinnefeld, Geminate
recombination as
a
photoprotection mechanism for
fluorescent dyes, Angew. Chem., Int. Ed., 2014, 53, 5685- 33 A. Mirloup, Q. Huaulmé, N. Leclerc, P. Lévêque, T. Heiser, P.
5688.
Retailleau and R. Ziessel, Thienyl-BOPHY dyes as promising
templates for bulk heterojunction solar cells, Chem.
Commun., 2015, 51, 14742-14745.
17 J. Vogelsang, C. Steinhauser, C. Forthmann, I. H. Stein, B.
Person-Skegro, T. Cordes and P. Tinnefeld, Make them blink:
Probes for super-resolution microscopy, ChemPhysChem, 34 R. Ziessel, T. Bura and J.-H. Olivier, Elaborating boron
2010, 11, 2475-2490.
dipyrromethene dyes with conjugated polyaromatic
frameworks, Synlett, 2010, 15, 2304-2310.
18 R. B. Altman, Q. Zheng, Z. Zhou, D. S. Terry, J. D. Warren and
S. C. Blanchard, Enhanced photostability of cyanine 35 D. V. O’Connor and D. Phillips, Time-Correlated Single Photon
fluorophores across the visible spectrum, Nature Methods,
2012, , 68.
Counting, Academic Press, London, 1984.
9
36 H. E. Grecco, P. Roda-Navarro and P. J. Verveer, Global
analysis of time correlated single photon counting FRET-FLIM
data, Opt. Express, 2009, 17, 6493-6508.
19 Q. Zheng, S. Jockusch, Z. Zhou and S. C. Blanchard, The
contribution of reactive oxygen species to the photobleaching
of organic fluorophores, Photochem. Photobiol., 2014, 90
448-454.
,
37 W. Schroeyers, R. Vallee, D. Patra, J. Hofkens, S. Habuchi, T.
Vosch, M. Cotlet, K. Mullen, J. Enderlein and F. C. De Schryver,
Fluorescence lifetimes and emission patterns probe the 3D
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 11
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Please do not adjust margins
Photochemical & Photobiological Sciences
Page 12 of 13
ARTICLE
Journal Name
orientation of the emitting chromophore in
a
53 J. F. Perez-Benito, C. Arais and E. Amat, A kinetic study of the
View Article Online
multichromophoric system, J. Am. Chem. Soc., 2004, 126
,
reduction of colloidal manganese dioxide by oxalic acid, J.
14310-14311.
Colloid. Interface Sci., 1996, 177, 288-2D97O.I: 10.1039/C8PP00162F
38 A. Harriman, K. J. Elliott, M. A. H. Alamiry, L. Le Pleux, M. 54 A. K. Horvath, I. Nagypal and I. R. Epstein, Three autocatalysts
Severac, Y. Pellegrin, E. Blart, C. Fosse, C. Cannizzo, C. R. Mayer
and F. Odobel, Intramolecular electron transfer reactions
observed for Dawson-type polyoxometalates covalently
and self-inhibition in a single reaction: A detailed mechanism
of the chlorite-tetrathionate reaction, Inorg. Chem., 2006, 45
,
9877-9883.
linked to porphyrin residues, J. Phys. Chem. C, 2009, 113
,
55 D.
M.
Guldi,
Fullerene-porphyrin
architectures;
5834-5842.
photosynthetic antenna and reaction center models, Chem.
Soc. Rev., 2002, 31, 22-36.
39 F. D. Lewis, J. M. Wagner-Brennan and J. M. Denari,
Intramolecular electron transfer in donor-spacer-acceptor 56 C. Costentin, M. Robert and J.-M. Saveant, Concerted proton-
systems with alkylamide spacers, J. Phys. Chem. A, 1998, 102
519-525.
,
electron transfers in the oxidation of phenols, Phys. Chem.
Chem. Phys., 2010, 12, 11179-11190.
40 A. F. Morales, G. Accorsi, N. Armaroli, F. Barigelletti, S. J. A. 57 W. H. Koppenol, D. M. Stanbury and P. L. Bounds, Electrode
Pope and M. D. Ward, Interplay of light antenna and
excitation "energy reservoir" effects in a bichromophoric
potentials of partially reduced oxygen species, from dioxygen
to water, Free Rad. Biol. Med., 2010, 49, 317-322.
system based on ruthenium-polypyridine and pyrene units 58 N. A. Stephenson and A. T. Bell, Quantitative analysis of
linked by a long and flexible poly(ethylene glycol) chain, Inorg.
Chem., 2002, 41, 6711-6719.
hydrogen peroxide by H-1 NMR spectroscopy, Anal. Bioanal.
Chem., 2005, 381, 1289-1293.
41 S. Mula, K. J. Elliott, A. Harriman and R. Ziessel, Energy transfer 59 Viscosity of Pure Organic Liquids and Binary Liquid Mixtures.
by way of an exciplex intermediate in flexible boron
dipyrromethene-based allosteric architectures, J. Phys. Chem.
A, 2010, 114, 10515-10522.
Supplement to IV/18. Lechner, M. D. Editor: Landolt-Börnstein
– Group IV Physical Chemistry, Vol. 25, 2009.
60 I. Yamazaki and L. H. Piette, EPR spin-trapping study on the
oxidizing species formed in the reaction of the ferrous ion
with hydrogen-peroxide, J. Am. Chem. Soc., 1991, 113, 7588-
7593.
42 A. Onkelinx, F. C. De Schryver, L. Viaene, M. Van der Auweraer,
K. Iwai, M. Yamamoto, M. Ichikawa, H. Masuhara, M. Maus
and W. Rettig, Radiative depopulation of the excited
intramolecular charge-transfer state of 9-(4-(N,N- 61 E. Baciocchi, T. Del Giacco, F. Elisei, M. F. Gerini, M. Guerra, A.
dimethylamino)phenyl)phenanthrene, J. Am. Chem. Soc.,
1996, 118, 2892-2902.
Lapi and P. Liberali, Electron transfer and singlet oxygen
mechanisms in the photooxygenation of dibutyl sulfide and
thioanisole in MeCN sensitized by N-methylquinolinium
tetrafluoborate and 9,10-dicyanoanthracene. The probable
involvement of a thiadioxirane intermediate in electron
43 Y. V. Il’ichev, W. Kuhnle and K. A. Zachariasse, Intramolecular
charge
transfer
in
dual
fluorescent
4-
(dialkylamino)benzonitriles. Reaction efficiency enhancement
by increasing the size of the amino and benzonitrile subunits
by alkyl substituents, J. Phys. Chem. A, 1998, 102, 5670-5680.
transfer photooxygenations, J. Am. Chem. Soc., 2003, 125
,
16444-16454.
44 Q. Huaulmé, A. Mirloup, P. Retailleau and R. Ziessel, Synthesis 62 B. M. Aveline, S. Matsugo and R. W. Redmond, Photochemical
of highly functionalized BOPHY chromophores displaying
large Stokes shifts, Org. Lett., 2015, 17, 2246-2249.
mechanisms responsible for the versatile application of
naphthalimides and naphthaldiimides in biological systems, J.
Am. Chem. Soc., 1997, 119, 11785-11795.
45 Y. V. Zatsikha, D. B. Nemez, R. L. Davis, S. Singh, D. E. Herbert,
A. J. King, C. J. Ziegler and V. N. Nemykin, Testing the limits of 63 D. M. Guldi, G. Torres-Garcia and J. Mattay, Intramolecular
the BOPHY platform: Preparation, characterization, and
theoretical modeling of BOPHYs and organometallic BOPHYs
energy transfer in fullerene pyrazine dyads, J. Phys. Chem. A,
1998, 102, 9679-9685.
with electron-withdrawing groups at beta-pyrrolic and 64 J. R. Darwent, P. Douglas, A. Harriman, G. Porter and M.-C.
bridging positions, Chem. Eur. J., 2017, 23, 14786-14796.
46 A. Harriman, Further comments on the redox potentials of
tryptophan and tyrosine, J. Phys. Chem., 1987, 91, 6102-6104.
Richoux, Metal phthalocyanines and porphyrins as
photosensitisers for the reduction of water to hydrogen,
Coord. Chem. Rev., 1982, 44, 83-126.
47 J. Bonin, C. Costentin, C. Louault, M. Robert, M. Routier and J. 65 M. R. Wasielewski, Photoinduced electron-transfer reactions
M. Saveant. Intrinsic reactivity and driving force dependence for artificial photosynthesis, Chem. Rev., 1992, 92, 435-461.
in concerted proton-electron transfers to water illustrated by 66 M. Hippler, Photochemical kinetics: Reaction orders and
phenol oxidation, Proc. Natl. Acad. Sci. USA, 2010, 107, 3367-
analogies with molecular beam scattering and cavity ring-
3372.
down experiments, J. Chem. Educ., 2003, 80, 1074-1077.
48 H. Oevering, M. N. Paddon-Row, M. Heppener, A. M. Oliver, 67 D. Lavabre, V. Pimienta, G. Levy and J. C. Micheau, Reversible,
E. Cotsaris, J. W. Verhoeven and N. S. Hush, Long-range
photoinduced through-bond electron-transfer and radiative
recombination via rigid nonconjugated bridges – distance and
mixed 1st and 2nd order and autocatalytic reactions as
particular cases of a single kinetic rate law, J. Phys. Chem.,
1993, 97, 5321-5326.
solvent dependence, J. Am. Chem. Soc., 1987, 109, 3258-3269. 68 L. N. Ismail, H. Zulkefle, S. H. Herman and M. R. Mahmood,
49 A. Harriman, G. Porter and N. Searle, Reversible
photooxidation of zinc tetraphenylporphyrin by benzo-1,4-
quinone, J. Chem. Soc., Faraday Trans. II, 1979, 75, 1515-1521.
Influence of doping concentration on dielectric, optical, and
morphological properties of PMMA thin films, Adv. Mater. Sci.
Eng., 2012, 2012, 605673.
50 P. Kuzmič, Program DYNAFIT for the analysis of enzyme kinetic 69 W. Liu, Y. Li, K. Liu and Z. Li, Iron-catalyzed carbonylation-
data: Application to HIV proteinas, Anal. Biochem., 1996, 237
260-273.
,
peroxidation of alkenes with aldehydes and hydroperoxides,
J. Am. Chem. Soc., 2011, 133, 10756-10759.
51 J. K. G. Karlsson, O. J. Woodford, R. Al-Aqar and A. Harriman, 70 W.-J. Yoo and C.-J. Li, Highly efficient oxidative amidation of
Effects of temperature and concentration on the rate of
photobleaching of Erythrosine in water. J. Phys. Chem. A,
2017, 121, 8569-8576.
aldehydes with amine hydrochloride salts, J. Am. Chem. Soc.,
2006, 128, 13064-13065.
71 J. W. W. Chang, T. M. U. Ton, S. Tania, P. C. Taylor and P. W.
52 F. Mata-Perez and J. F. Perez-Benito, The kinetic rate law for
autocatalytic reactions, J. Chem. Educ., 1987, 64, 925-927.
H.
Chan, Practical copper(I)-catalysed amidation of
aldehydes, Chem. Commun., 2010, 46, 922-924.
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72 R. E. Rodríguez-Lugo, M. Trincado and H. Grützmacher, Direct
amidation of aldehydes with primary amines under mild
conditions catalyzed by diolefin-amine-Rh-I complexes, Chem.
View Article Online
DOI: 10.1039/C8PP00162F
Cat. Chem., 2013, 5, 1079-1083.
73 Z. Yun-Fei, B. Wu and Z.-J. Shi, Ir-catalyzed C-H amidation of
aldehydes with stoichiometric/catalytic directing group,
Chem. Eur. J., 2016, 22, 17808-17812.
74 S. Debbarma and M. Sudan Maji, Cp*Rh-III-catalyzed directed
amidation of aldehydes with anthranils, Eur. J. Org. Chem.,
2017, 3699-3706.
75 N. Mataga, T. Asahi, Y. Kanda, T. Okada and T. Kakitani,
Mechanisms of the strongly exothermic charge separation
reaction in the excited singlet-state – picosecond laser
photolysis studies on the aromatic hydrocarbon
tetracyanoethylene and aromatic hydrocarbon pyromellitic
dianhydride systems in polar solutions, Chem. Phys., 1988,
127, 249-261.
76 J. Mattay and M. Von den Hof, Contact and solvent-separated
radical ion-pairs in organic photochemistry, Topics Curr.
Chem., 1991, 159, 219-255.
77 A. C. Benniston, A. Harriman, P. Li, J. P. Rostron, H. J. van
Ramesdonk, M. M. Groeneveld, H. Zhang, H. and J. W.
Verhoeven, Charge shift and triplet state formation in the 9-
mesityl-10-methylacridinium cation, J. Am. Chem. Soc., 2005,
127, 16054-16064.
78 W. A. Braunecker and K. Matyjaszewski, Controlled/living
radical polymerization: Features, developments, and
perspectives, Prog. Polym. Sci., 2007, 32, 93-146.
79 T. G. St Denis, T. Dai, L. Izikson, C. Astrakas, R. R. Anderson, M.
R. Hamblin and G. P. Tegos, All you need is light. Antimicrobial
photoinactivation as an evolving and emerging discovery
strategy against infectious disease, Virulence, 2011, 2, 509-
520.
80 S. R. Lewis, S. Datta, M. H. Gui, E. L. Coker, F. E. Huggins, S.
Daunert, L. Bachas and D. Bhattacharyya, Reactive
nanostructured membranes for water purification, Proc. Natl.
Acad. Sci. USA, 2011, 108, 8577-8582.
81 S. Funyu, T. Isobe, S. Takagi, D. A. Tryk and H. Inoue, Highly
efficient and selective epoxidation of alkenes by
photochemical oxygenation sensitized by a ruthenium(II)
porphyrin with water as both electron and oxygen donor, J.
Am. Chem. Soc., 2003, 125, 5734-5740.
82 D. Tatsumi, T. Tsukamoto, R. Honna, S. Hoshino, T. Shimada
and S. Takagi, Highly selective photochemical epoxidation of
cyclohexene sensitized by Ru(II) porphyrin-clay hybrid
catalyst, Chem. Lett., 2017, 46, 1311-1314.
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