Two-photon absorption and polymerization ability of intramolecular energy transfer based photoinitiating systems

Article abstract of DOI:10.1039/b808457b

We design a new photoinitiating system where the two-photon absorption of a 2,7 bisaminofluorene moiety leads to the photoactivation of a camphorquinone subunit through a Foerster-type intramolecular energy transfer: the application to a two-photon polymerization reaction is demonstrated. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b808457b

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Two-photon absorption and polymerization ability of intramolecular
energy transfer based photoinitiating systemsw
Ming Jin,a Jean-Pierre Malval,*a Davy-Louis Versace,a Fabrice Morlet-Savary,a
b
b
a
a
´
Helene Chaumeil, Albert Defoin, Xavier Allonas and Jean-Pierre Fouassier
Received (in Cambridge, UK) 19th May 2008, Accepted 6th October 2008
First published as an Advance Article on the web 12th November 2008
DOI: 10.1039/b808457b
We design a new photoinitiating system where the two-photon
absorption of a 2,7 bisaminofluorene moiety leads to the photo-
¨
activation of a camphorquinone subunit through a Forster-type
intramolecular energy transfer: the application to a two-photon
polymerization reaction is demonstrated.
The potential and promising applications1–6 of multi-photon
absorption spectroscopy in high density optical data storage,
3D micro-nanofabrication, power limiting, fluorescence
microscopy, photodynamic therapy has strongly stimulated
the research for the elaboration of modular architectural
macrosystems combining both large two-photon absorption
2PA cross sections (d) and desired photo activated properties.
In the case of applications involving a two-photon initiated
polymerization reaction, the introduction of an efficient
photoinitiator PI is necessary.1,7 Commercial PIs, however,
exhibit very poor d and cannot be efficiently excited according
to a 2PA process.8 In the present paper, we propose a new
approach based on the design of a PS-PI linked photo-
initiating system where PS is a photosensitizer that must
exhibit a large d and lead to an excited state of PI through
an intramolecular energy transfer process. This is illustrated
by the following PS-PI system (Scheme 1).
Scheme 1
The absorption spectra of compounds 0–2 (Fig. 1) show the
typical features14–16 of the FL moiety with maximum at 375 nm in
the acetonitrile and similar extinction coefficients (emax = 3.6 ꢀ
104 molꢁ1 L cmꢁ1). They exhibit a small shoulder in the
420–475 nm wavelength range corresponding to the strongly
forbidden n–p* electronic transition of CQ (e475 = 36.8 molꢁ1
l cmꢁ1).17 The latter band is twice higher for 2 than for 1 as a
consequence of the number of camphorquinone units.
The fluorescence emission spectra, located in the 370–560 nm
region, are slightly red shifted when going from cyclohexane to
acetonitrile (Fig. 1 and Table 1): this suggests a low dipole
moment for the emitting excited states consistent with the
specular symmetry of the FL unit.7 However, both the emission
quantum yields and the fluorescence lifetimes of FL strongly
decrease from 0 to 1 or 2. For example, this leads to a
quenching efficiency of 93% in cyclohexane and 95% in
acetonitrile for 1. In contrast with the strong quenching
observed for the emission of 1 or 2 with respect to fluorescence
It consists in a 2,7-bis(diphenylamino)fluorene FL moiety as
an energy donor (PS) and camphorquinone CQ (PI) as an energy
acceptor. CQ is a UV-visible absorbing molecule that is largely
used, when combined with tertiary amines, in radical photo-
polymerization.8–11 Moreover, its efficiency is weakly affected by
oxygen9,12,13 which constitutes a relevant asset for 3-D stereo-
lithography. Fluorene derivatives exhibit relatively large 2PA
cross-sections and can efficiently absorb NIR light.14–16
Furthermore, FL fluoresces within the absorption spectral
range of CQ. As a result, the introduction of FL should
generate the excited state CQ* upon 2PA.
Herein, we report on the photophysical and photochemical
properties of this new generation of 2PA working photo-
initiators. Both the energy transfer efficiency and the two-
photon absorption properties of 1 and 2 will be discussed in
comparison with the model molecules 0 and CQ.
a
´
´ ´
Departement de Photochimie Generale, CNRS UMR 7525,
Universite´ de Haute Alsace, ENSCMu, 3 rue Alfred Werner, 68093
Mulhouse Cedex, France. E-mail: jean-pierre.malval@uha.fr
b Laboratoire de Chimie Organique et Bioorganique, UMR CNRS
Fig. 1 Normalized steady-state absorption and fluorescence spectra of
compounds 0, 1 and 2 in acetonitrile at room temperature. Inset: spectra in
the 410–540 nm region showing the absorption of the camphorquinone
chromophores in compounds 1 and 2 (c = 2.8 ꢀ 10ꢁ6 mol. Lꢁ1).
´
7015, Universite de Haute Alsace, ENSCMu, 3 rue Alfred Werner,
68093 Mulhouse Cedex, France
w Electronic supplementary information (ESI) available: Details of
experimental procedures. See DOI: 10.1039/b808457b
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Table 1 Absorption and fluorescence properties
Cyclohexane
Acetonitrile
abs/nm
labs/nm
lfluo/nm
Ffa
tfb/ns
l
lfluo/nm
Ffa
tfb/ns
0
1
2
380
380
380
397
397
397
0.31
0.02
0.01
0.98
0.10
0.20
375
375
374
408
409
409
0.46
0.03
0.02
1.42
0.11
0.12
a
b
max
Fluorescence quantum yields. Fluorescence lifetimes measured by time correlated single photon counting (TCSPC) at lfluo
.
of 0, addition of CQ to a solution of 0 hardly affects the
fluorescence intensity of 0, the quenching coefficient has a value
of 90 Mꢁ1 in cyclohexane. The absence of solvent polarity effect
on fluorescence of 1 or 2 excludes any excited charge transfer
mechanism. A through-bond Dexter-type energy transfer is also
ruled out due to the presence of the methylene spacer
between the subunits, as considered in other systems.3,18
Thus, the fluorescence quenching in 1 and 2 is attributed to a
Energy transfer is also supported by nanosecond laser flash
photolysis. Laser excitation of 1 and 2 in acetonitrile at 266 nm
leads to a transient absorption around 800 nm (see ESI,w
Fig. S3) similar to that observed under the direct excitation of
3
CQ already ascribed to CQ.9,12,13 In the same experimental
conditions, the 3CQ moiety absorbance in 1 and 2 is noticeably
lower than that measured in CQ itself, consistent with a lower
intersystem crossing quantum yield.
Forster-type energy transfer from the absorbing FL moiety to
¨
the CQ chromophore. The corresponding Forster radius19
The 2PA cross-sections were determined by the two-photon-
induced fluorescence method,20,21 relatively to coumarin
480 in methanol.22 The quadratic relationship between the
fluorescence intensity and the excitation power confirms
the two-photon absorption regime. The insert in Fig. 3 shows
the 2PA spectra of 0–2 in acetonitrile. Each compound
exhibits a similar spectrum with an intense band below 690 nm
(4300 GM) and a shoulder at 740 nm in a very good
agreement with the very recent 2PA spectra obtained from
an open aperture Z-scan method.23 Moreover, the quenching
efficiency of the 2PA induced fluorescence of the FL moiety in
1 and 2 is 95% as observed in the one-photon absorption
measurements: this clearly demonstrates that the two-photon
absorption is also followed by an efficient intramolecular
energy transfer process.
¨
calculated from the overlap integral between the absorption
and the fluorescence bands of CQ and compound 0 leads to a
value of 15 A (see ESI,w Fig. S1). Such a distance appears as an
upper limit when compared to the values (D13 A) calculated by
AM1 geometric optimization of 1 and 2. The very weak
fluorescence of the CQ moiety (ff = 0.0029)13 whose maximum
emission wavelength is located at 530 nm cannot be collected in
the steady state experiments because of the overlap with the
residual emission of FL. TCSPC experiments allows one to
detect a new long component (16 ns) in the fluorescence decays
of 1 and 2 at 530 nm (see ESI,w Fig. S2) as the one observed in
the case of CQ which well confirms the subsequent generation
1
of CQ*.
The time-gated structured phosphorescence spectra of CQ, 1
and 2 in EPA (diethyl ether–isopentane–ethanol (4 : 4 : 1 v/v))
glassy matrix at 77 K are perfectly similar (Fig. 2). The
corresponding excitation spectra taken at the maximum emis-
sion wavelength, however, are totally different. The charac-
teristic absorption bands at 450 nm and 375 nm arising from
CQ and 2, respectively, are observed. This definitively supports
the energy transfer from the FL moiety and confirms the
formation of a low lying triplet state centred on CQ. The
intersystem crossing quantum yields of 1 and 2 (using CQ as
an actinometer)13 are evaluated as ca. 0.35.
Finally, the fabrication of 20 mm width lines (Fig. 3) was
achieved by focusing a femtosecond pulsed laser beam
(Ti:Sapphire laser; lexc = 750 nm; average power: 1.2 mW)
into a diacrylate monomer (Sartomer SR344) containing 2 as a
photoinitiator (0.5% wt.) and N-methyl(N,N0-diethanol)-
amine (MDEA) as co-initiator (3% wt.). The presence of
MDEA (as the interaction of the two-photon excited
Fig. 3 Lines produced by 2PA polymerization of a diacrylate matrix.
Scale bar: 20 mm. The low resolution is due to the available exposure
and analysis device. Inset: 2PA spectra of 0–2 in acetonitrile.
Fig. 2 Phosphorescence excitation and emission spectra of CQ and
compound 2 in an EPA glassy matrix at 77 K.
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In conclusion, the design of covalently bonded PS-PI struc-
ture working through energy transfer seems promising for the
development of efficient two-photon photoinitiating systems
for free radical photopolymerization. The main interest lies on
the fact that the initiation mechanism will be obviously the
same as under the usual one-photon absorption. The finding
of other suitable combinations of a molecular skeleton posses-
sing ever higher d with an efficient PI is a fascinating challenge.
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This journal is The Royal Society of Chemistry 2008
6542 | Chem. Commun., 2008, 6540–6542