Influence of bromine substitution pattern on the singlet oxygen generation efficiency of two-photon absorbing chromophores

Article abstract of DOI:10.1039/c2ob25536g

A molecular engineering strategy based on rational variations of the bromine substitution pattern in two-photon absorbing singlet oxygen sensitizers allows studying the relations that exist between the positioning of an inter-system crossing promoter on the charge-transfer chromophore and its ability to generate singlet oxygen. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2ob25536g

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Chemistry
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COMMUNICATION
Influence of bromine substitution pattern on the singlet oxygen generation
efficiency of two-photon absorbing chromophores†
Pierre-Henri Lanoë,a Thibault Gallavardin,a Aurore Dupin,a Olivier Maury,a Patrice L. Baldeck,a,b
Mikael Lindgren,a,c Cyrille Monnereau*a and Chantal Andraud*a
Received 12th March 2012, Accepted 21st June 2012
DOI: 10.1039/c2ob25536g
various studies during the last two decades, and the associated
rules of molecular engineering and the structure–property
relationship are now well established.20–23 In particular, it
has been established that chromophores that feature electronic
transition of the internal charge-transfer (ICT) type are optimal
candidates for TPA, especially those with quadrupolar sym-
metry.7 Comparatively, in spite of the fact that many studies have
been focused on improving ISC efficiency and ultimately one-
photon PDT activity in chromophores with localized excited
states (porphyrin24 and rhodamine25 derivatives, mostly) only a
few authors have systematically studied the relationship
that exists between structural parameters and 1O2 generation
efficiency in the context of ICT chromophores. In most cases,
1O2 generation is achieved through substitution of the chromo-
phore with a group responsible for the ISC. This approach,
which we have also recently explored,26 has been successful in
designing efficient chromophores for two-photon PDT;13–15 yet,
the relations between ISC efficiency (that is, the mechanism by
which the delocalized ICT excited singlet state evolves into a
localized triplet state) and the position and/or number of ISC
promoters remain poorly understood in the context of ICT
chromophores. With the aim to more particularly address that
point, we initiated a systematic study based on an original
engineering approach: it markedly differs from previously
published works on the topic, since the chemical nature of the
ISC promoter (two bromine substituents in the 2,5-positions on a
phenyl moiety) is maintained in all molecules, and the variations
we operate concern exclusively its number and positioning along
the ICT axis (“p + 1” and/or “p + 2” position, Fig. 1). This
A
molecular engineering strategy based on rational
variations of the bromine substitution pattern in two-photon
absorbing singlet oxygen sensitizers allows studying the
relations that exist between the positioning of an inter-system
crossing promoter on the charge-transfer chromophore and
its ability to generate singlet oxygen.
Photodynamic therapy (PDT) is a therapeutic protocol which is
currently limited to the treatment of several surface diseases, like
skin or mucosal cancers.1–3 It requires a photosensitizer,4,5 i.e. a
molecule possessing an accessible triplet excited state able to
generate singlet oxygen (1O2) by an energy transfer to ground
state oxygen, ultimately leading to cell death.6–8 Two-photon
sensitized PDT is an emerging approach, which allows a
deeper penetration of light into tissues (since irradiation operates
in the biological transparency window, 700–1100 nm) and
a better control on the irradiation volume than its one-photon
counterpart.9–18 Thus, it is possible to envision more in-depth
treatment of tumors, while minimizing detrimental side-effects
on the neighboring healthy tissues. Ideal two-photon absorbing
(TPA) chromophores for PDT must possess two essential spec-
troscopic characteristics: (i) a large two-photon absorption cross
section (σTPA) at the excitation wavelength and (ii) a high
quantum yield of singlet oxygen generation (ϕΔ), which is
relevant to the efficiency of singlet-to-triplet intersystem crossing
(ISC) of the chromophore in its excited state.19
These two parameters can be used to define a “figure of
merit” at 800 nm‡ (σTPA800 × ϕΔ) for the photosensitizer.12 Two-
photon absorbing chromophores have been the subject of
allows us to vary exclusively the ISC kinetic constant (kISC
)
while keeping all other kinetic constants almost unaffected (vide
infra). In this preliminary study, we show that this systematic
approach can be used to tune the O2 quantum yield generation
aLaboratoire de Chimie CNRS UMR 5182, Université Lyon 1, Ecole
Normale Supérieure de Lyon, 46 allée d’Italie, 69364 Lyon Cedex 07,
France. E-mail: chantal.andraud@ens-lyon.fr,
1
cyrille.monnereau@ens-lyon.fr
along with other spectroscopic features of interest, to a rather
unexpected extent.
bLaboratoire Interdisciplinaire de Physique CNRS UMR 5588,
Université Grenoble 1, Grenoble, F-38402, France
cDepartment of Physics, Norwegian University of Science and
Technology (NTNU), NO-7491 Trondheim, Norway
†Electronic supplementary information (ESI) available: Synthesis,
analytical characterization, discussion on the excited state kinetics, 1H
and 13C NMR spectra, HPLC traces, solvatochromism study, 77 K time-
gated luminescence, fluorescence and 1O2 phosphorescence plots,
including those of compound 2Br′. See DOI: 10.1039/c2ob25536g
Following the aforementioned strategy, and using previously
reported compound 115,18 (Fig. 1) as a benchmark and model, a
series of four quadrupolar chromophores was designed and syn-
thesized. For convenience, those chromophores will be referred
to as 0Br, 2Br, 4Br and 6Br in the following. The series was
obtained through a convergent “building-blocks” synthesis,
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based on successive Sonogashira–Hagihara reactions from the
electron donating group to the molecule’s core. For experimental
details and characterizations, see ESI.†
Absorption and emission spectra were obtained using diluted
CHCl3 solutions of the compounds at 298 K (Fig. 2 and
Table 1). In all cases, a broad low-energy absorption band is
observed, centered at 389, 395, 410 and 435 nm, for 0Br, 2Br,
4Br and 6Br, respectively. As expected, this band can be
unambiguously ascribed to an ICT transition, regarding its
characteristic shape, large Stokes shift and positive solvatochro-
mism (Fig. S4†). The bathochromic shift along the series is in
agreement with the electro-deficiency induced by the increased
number of dibromophenyl moieties along the conjugated skel-
eton. All compounds are luminescent under irradiation in their
ICT band, with a broad, solvatochromic ICT emission band
centered at 476, 490, 500 and 560 nm, for 0Br, 2Br, 4Br and
6Br in chloroform, respectively. The decay times (Table 1,
Fig. 4) are in the ns range, which indicates a fluorescence emis-
sion process.
Attempts to observe triplet excited state emission of the com-
pounds at room temperature have to date remained unsuccessful.
However, at low temperature in glassy solution, structured
phosphorescence emissions were observed, except for 0Br
(Table 1, Fig. S5†).
1O2 generation quantum yield (ϕΔ) quantifies the chromophore
efficiency as a potential PDT sensitizer. It was determined from
the characteristic Δ1O2 → Σ3O2 phosphorescence at 1270 nm.
All brominated compounds (2Br, 4Br and 6Br) generate singlet-
oxygen, with quantum yields of 0.10, 0.50 and 0.35, respectively
(Table 1, Fig. S6†). Interestingly, fluorescence quantum yields
and lifetimes evolve opposite to (and almost mirror) the 1O2
generation quantum yield (Fig. 3). This tendency is in excellent
Fig. 1 Compounds of the study and benchmark 1 [ref. 15 and 18].
p + n (n = 1, 2) refers to the phenyl ring position relative to the terminal
aniline.
1
agreement with the fact that O2 quantum yield can be directly
Fig. 3 Evolution of the photophysical parameters (ϕf, ϕΔ, τf) among
the chromophore’s series (298 K, CHCl3).
Fig. 2 Absorption (full) and emission (dash) spectra (CHCl3, 298 K).
Table 1 Relevant spectroscopic data for the study
0Br
2Br
4Br
6Br
1d
λmax (ε)
λfluao (τf)
ϕf
389 (79 × 103)
476 (1.83)
0.80
—
—
750 (460)
0
395 (46.6 × 103)
490 (1.68)
0.70
530, 569
0.10
410 (49.9 × 103)
500 (0.96)
0.30
594, 637
0.50
435 (53.8 × 103)
560 (1.27)
0.53
600, 674
0.35
421 (91.5 × 103)
458 (0.26)
0.25
586
0.53
b
λphos
c
ϕΔ
λTPA max (σTPA
σTPA800 × ϕΔ
)
760 (450)
30
830 (380)
175
880 (260)
40
550 (730)
37
λ in nm, ε in L mol−1 cm−1, τfluo in ns and σTPA in Goeppert-Mayer (GM). λexc were 390 nm, 395 nm, 410 nm and 435 nm.a Coumarin 153 in MeOH
as a standard, ϕref = 0.45. b Emission at 77 K (MeOH–EtOH, 1 : 4, v/v, time delay of 65 μs). c Naphthalenone in CHCl3 as a standard, ϕΔ = 1. d From
ref. 18.
6276 | Org. Biomol. Chem., 2012, 10, 6275–6278
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connected to ISC efficiency, since all kinetic constants other than
ISC are shown to be unchanged as a result of the structural simi-
k
larity of all chromophores within the series (see ESI† for a full
1
demonstration). 4Br exhibits a O2 quantum yield (ϕΔ = 0.50)
that is five times higher than its dibrominated analogue 2Br
(ϕΔ = 0.10) with a concomitant drop in both fluorescence
quantum yields and lifetime values (Fig. 3, Table 1). This trend
could potentially be ascribed either to a synergistic enhancement
of the ISC efficiency related to the increased number of bromines
on the chromophore, or to a positional effect, with lateral
(“p + 1”) substitution providing higher ISC efficiency than the
central (“p + 2”) one in this particular series. Examination of the
results obtained with 6Br clearly rules out the first hypothesis:
indeed, in spite of the increased number of heavy atom substitu-
ents, 6Br displays a loss of 1O2 generation efficiency (ϕΔ = 0.35)
and a concomitant increase of the fluorescent quantum yield and
lifetime compared to 4Br, which altogether can be ascribed to a
lower ISC efficiency. To make these comparisons more conclus-
ive, an additional compound bearing two bromines on one
“p + 1” position only (referred to as 2Br′, ESI†) was synthesized
and its spectroscopic properties were investigated. Because of
the significantly less modular and straightforward synthetic
approach required for the obtention of the latter, and also
because of complicated symmetry issues that would extend the
discussion on its nonlinear properties out of the scope of this
communication, this compound will not be extensively featured
in the present article. Its synthesis and detailed properties will be
reported elsewhere in the near future, its structure and main
photophysical data are given as ESI.† From the aforementioned
spectroscopic study, it was possible to establish that 2Br′ had
fluorescence (ϕf = 0.32) and singlet oxygen quantum efficiencies
(ϕΔ = 0.50), but also fluorescence lifetime (τf = 0.93 ns), that
were, within experimental error, undistinguishable from those
of 4Br.
Fig. 4 Two-photon absorption spectra (CH2Cl2, 298 K).
moderate cross section value (<200 GM), whereas 4Br shows a
broad TPA peak, with a cross section larger than 300 GM from
770 to 860 nm. This shows that, besides having a strong
influence on 1O2 generation, the bromine substitution pattern can
be used to tune TPA properties (λTPAmax) of the chromophore.
Taken together, this leads 4Br to display the best figure of merit
in the biological window (σTPA800 × ϕΔ, Table 1) of all quadru-
polar compounds in the series (including 1), and makes it a rel-
evant structure for future PDT applications, provided that some
modifications are introduced to render it biocompatible, as pre-
viously illustrated with related chromophore 1.18
In summary, this study allowed us, owing to an original and
straightforward molecular engineering approach, to progress sig-
nificantly in our understanding of some of the relations that
exist, in ICT chromophores, between the position and number of
1
a given ISC promoter (2,5-dibromophenyl) and the O2 gener-
ation efficiency. This is of particular significance in the context
of molecular engineering for two-photon PDT applications, since
the position of the heavy atom has only scarcely been discussed
in previous published works relying on a similar “heavy atom”
strategy for 1O2 sensitization in ICT chromophores,27,28 and
never investigated in such a systematic way. Besides, we have
also shown that varying the bromine substitution pattern could
be used to tune the two-photon absorption properties of the
molecule, as a result of the electron withdrawing effect of the
bromine atoms. Altogether, the results of this study suggest that,
for works dealing with ISC improvement in ICT chromophores,
caution should be taken not only with the choice of the ISC
generating moiety, but also with its number and – most
importantly – positioning along the chromophore ICT transition
axis. Theoretical and experimental investigations are underway
to gain better understanding on the structure-to-property relation-
ships in this class of brominated two-photon photosensitizer.
These results constitute unambiguous evidence that, rather
than the number of O2 generating moieties, their detailed posi-
1
tion on the carbon backbone is the determining parameter that
drives ISC efficiency within our series: an ideal positioning of
the heavy atoms seems to be on the peripheral phenyls next to
the aniline ring (“p + 1” position, as in 4Br and 2Br′), whereas
bromine substitution on the “p + 2” (2Br) ring provides in com-
parison a much lower efficiency. For a given position, the
number of bromine substituted phenyls does not seem to play a
significant role (if any) in the chromophore efficiency (as seen
by the similarity for the results obtained with 4Br and 2Br′).
Interestingly, and quite unexpectedly, the presence of bromine
atoms in both “p + 1” and “p + 2” positions has a detrimental
effect on the ISC, as seen when comparing ϕΔ of 4Br and 6Br
(Fig. 3).
Finally, TPA cross sections were determined by two-photon
induced fluorescence measurements on the quadrupolar chromo-
phore series (ESI†). In the biological transparency window
(750–900 nm), all compounds show significant TPA cross
sections, with values ranging from 250 GM (6Br) to 460 GM
(0Br) (Fig. 4, Table 1). As for their linear absorption spectra, a
significant bathochromic shift is observed throughout the series.
0Br and 2Br give the best results in absolute values, but their
absorption maxima are slightly below the optimal transparency
window. 6Br shows a maximal TPA in the NIR but with a
Notes and references
‡800 nm is taken as a reference, since it is the most classically used
wavelength for in vivo applications based on Ti-sapphire LASER.
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