Perylenetetracarboxy-3,4:9,10-diimide derivatives with large two-photon absorption activity

Article abstract of DOI:10.1039/C8NJ03216E

Three new perylenetetracarboxy-3,4:9,10-diimides, bearing 2,6-diisopropylphenyl groups at the imide positions and 4-(R-ethynyl)phenoxy moieties (R = 4,7-di(2-thienyl)benzo[c][1,2,5]thiadiazole (P2), pyrene (P3) or pyrene-CH₂OCH₂ (P4)) at the four bay positions, were prepared, along with the known related derivative (R = phenyl (P1)), and well characterized. They have large two-photon absorption (TPA) cross-sections (σ₂), as determined by the Z-scan technique, the highest values being reached with P2 which bears a planar π-delocalized donor moiety. P3 is characterized by higher σ₂ values than both P1, as expected for the higher π-conjugation of the donor pyrene moiety with respect to phenyl, and P4, due to the presence of the flexible and non-conjugated CH₂OCH₂ bridge between the pyrene and the ethynyl fragment in the latter compound. The molecular geometry of P1-P4 has been optimized by DFT modeling, showing that in P2 and P3 the bay substituents are stacked due to the π-π interactions of both pyrene and thiophene groups. The LUMO of P1-P4 lies at the same energy and is essentially delocalized on the perylene core whereas the HOMO and HOMO?1 of both P2 and P3 are degenerate and do not show contribution from the perylene core contrarily to that of P1 and P4. The HOMO-LUMO gap is therefore essentially influenced by the HOMO which reflects the electronic charge delocalization on the bay substituents, the lower gaps being observed for P2 and P3, which are characterized by the best TPA properties.

Full text of DOI:10.1039/C8NJ03216E

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Perylenetetracarboxy-3,4:9,10-diimide derivatives
with large two-photon absorption activity†
Cite this:DOI: 10.1039/c8nj03216e
Eleonora Garoni,ab Filippo Nisic,a Alessia Colombo, *a Simona Fantacci,*c
d
a
a
Gianmarco Griffini,
Claudia Dragonetti
Kenji Kamada,*b Dominique Roberto
and
Three new perylenetetracarboxy-3,4:9,10-diimides, bearing 2,6-diisopropylphenyl groups at the imide
positions and 4-(R-ethynyl)phenoxy moieties (R = 4,7-di(2-thienyl)benzo[c][1,2,5]thiadiazole (P2), pyrene
(P3) or pyrene-CH2OCH2 (P4)) at the four bay positions, were prepared, along with the known related
derivative (R = phenyl (P1)), and well characterized. They have large two-photon absorption (TPA) cross-
sections (s2), as determined by the Z-scan technique, the highest values being reached with P2 which
bears a planar p-delocalized donor moiety. P3 is characterized by higher s2 values than both P1, as
expected for the higher p-conjugation of the donor pyrene moiety with respect to phenyl, and P4, due
to the presence of the flexible and non-conjugated CH2OCH2 bridge between the pyrene and the
ethynyl fragment in the latter compound. The molecular geometry of P1–P4 has been optimized by
DFT modeling, showing that in P2 and P3 the bay substituents are stacked due to the p–p interactions
of both pyrene and thiophene groups. The LUMO of P1–P4 lies at the same energy and is essentially
delocalized on the perylene core whereas the HOMO and HOMOꢀ1 of both P2 and P3 are degenerate
and do not show contribution from the perylene core contrarily to that of P1 and P4. The HOMO–
LUMO gap is therefore essentially influenced by the HOMO which reflects the electronic charge deloca-
lization on the bay substituents, the lower gaps being observed for P2 and P3, which are characterized
by the best TPA properties.
Received 28th June 2018,
Accepted 27th December 2018
DOI: 10.1039/c8nj03216e
rsc.li/njc
attracted growing interest for a wide range of applications,1–4
including 3D optical data storage5–8 up-converted stimulated
Introduction
Molecules may undergo two-photon absorption (TPA) when emission,9,10 microscopy,11–13 microfabrication,14,15 photodynamic
exposed to high-intensity light sources, reaching a final state therapy,16–19 localized release of bio-active species,20 and cell
by the simultaneous absorption of two photons, each one with imaging,21–23 to name just a few examples. The requirements
half of the energy required for the electronic transition. One of for maximizing the TPA cross-section of a chromophore include
the advantages of this process is the quadratic dependence with (i) long p-conjugated systems with enforced co-planarity that
the irradiance that makes it possible to confine the laser excita- ensure large conjugation lengths to have high transition dipole
tion in a minute spatial volume.1,2 In addition, TPA processes moments, (ii) donor (D) and acceptor (A) groups at the center
allow high depth penetration and the use of low-energy photons. and ends of the molecule, and (iii) narrow one-photon and two-
Thanks to these features, molecular two-photon absorption has photon absorption bands.2 A wide range of D and A groups have
been investigated and it is generally considered that D–p–D and
D–p–A–p–D structures are often more effective than A–p–A and
a
A–p–D–p–A systems.2 In recent years, molecular engineering
directed towards TPA optimization has become very active,
leading to a range of molecules of various symmetries including
dipoles, quadrupoles, octupoles, and branched structures,2,24–31
in addition to exploring novel electronic structures such as
singlet diradicaloid.32
`
Department of Chemistry, Universita degli Studi di Milano, UdR dell’INSTM,
Via Golgi 19, 20133 Milano, Italy. E-mail: alessia.colombo@unimi.it
b IFMRI, National Institute of Advanced Industrial Science and Technology (AIST),
Ikeda, Osaka 563-8577, Japan. E-mail: k.kamada@aist.go.jp
c Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), CNR-ISTM,
Via Elce di Sotto 8, I-06213, Perugia, Italy. E-mail: simona@thch.unipg.it
d Department of Chemistry, Materials and Chemical Engineering ‘‘Giulio Natta’’,
Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy
† Electronic supplementary information (ESI) available: Tabulated TPA cross-
sections values obtained with the power-scan at fix wavelength and typical Z-scan
traces, NMR spectra of new compounds and isodensity plots of the frontier
molecular orbitals of P1–P4. See DOI: 10.1039/c8nj03216e
Among the investigated molecules, there is a growing interest
in the study of the TPA properties of perylenetetracarboxy-
3,4:9,10-diimide (PTCDI) derivatives, already known for their
thermal stability, chemical inertness, outstanding electrical
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and optical properties which led to various applications such
Another way of bay substitution involves the use of an ether
as photoelectrochemical solar fuel generators33–36 and lumi- linker between the perylene core and the p-delocalized groups.
nescent devices.37 The reason for the suitability of PTCDIs for For example, substitution with phenoxy groups, connected via
such a large and diverse range of applications is encoded in an ether group instead of the direct connection with the
its molecular structure: a rigid polycyclic aromatic scaffold aromatic ring, at the four bay positions of PTCDI bearing
(perylene) substituted with two dicarboxylic acid imide groups 2,6-diisopropylphenyl groups at the imide positions was reported.
at the 3,4- and 9,10-peri-positions; the strong conjugation Thus, by using p-(sulphonic acid)phenoxy groups as bay sub-
between the electron-rich perylene and the electron-withdrawing stituents, a water soluble perylenetetracarboxy-3,4:9,10-diimide
imide groups affords an interesting acceptor–donor–acceptor was realized with s2 values between 10 and 50 GM in the range
system.38 Mendonça et al. put in evidence the interesting TPA 840–980 nm, as determined by the two-photon-excited fluores-
properties of a series of PTCDI derivatives, bearing various cence (TPEF) technique, of interest for biology applications.47
substituents at the imide positions.39–42 By using the open- By using the same ether-mediated bay substitution method,
aperture white-light continuum Z-scan technique with femto- 2,6-diisopropylphenyl PTCDI with thiophene-based dendron
second laser pulses, higher TPA cross-sections s2 were observed substituted phenoxy groups were studied; their cross-sections
for benzyl and phenylethyl substituents (ca. 1500 GM at 654 nm) are around 100–300 GM and 20–50 GM at 720 nm and 890 nm,
in comparison to butyl (ca. 1000 GM), probably related to the respectively, with the TPEF technique.48 Interestingly, the s2
higher planarity of the perylene moiety in these molecules.42 values don’t increase by increasing the number of thiophene
When the excitation wavelength approached the linear absorp- units in the dendrons because the charge transfer character
tion band (below 630 nm), a significant increase in s2 was decreases in highly branched structures, due to the twisting
observed, in agreement with the resonant enhancement of of the perylene core which causes interruption of the charge
TPA42 based on the four-state model with two TPA states.43 correlation between the perylene itself and the dendrons.48
Although the functional substitution was mostly made on the
These interesting TPA properties of perylenetetracarboxy-
imide moieties of PTCDI in their studies,39–42 ‘‘bay’’-positions 3,4:9,10-diimides, bearing 2,6-diisopropylphenyl groups at the
(i.e., positions 1,6,7 or 12) are also useful for structural modifica- imide positions and tetrasubstituted at all bay positions via a
tion of PTCDI. Such a modification at the bay positions is flexible ether linker prompted us to prepare other members of
interesting because it can directly influence the electronic prop- this family (Fig. 1), in order to investigate their TPA cross-
erties and geometric structure of the perylene core.38 Marder sections. As substituents for the bay positions, we choose
et al. showed that a perylenetetracarboxy-3,4:9,10-diimide bear- 4-(R-ethynyl)phenoxy moieties, where
R
is phenyl49 (P1),
ing 2,6-diisopropylphenyl groups at the imide positions and 4,7-di(2-thienyl)benzo[c][1,2,5]thiadiazole (P2), pyrene (P3) or
pyrrolidin-1-yl substituents in 1 and 7 bay positions has TPA pyrene-CH2OCH2 (P4). Thus, the known compound P1 could be
cross-sections up to 2800 GM at ca. 800 nm, close to the one- taken as reference to measure the effect of the more p-extended
photon absorption band, along with s2 values up to ca. 800 GM structures of pyrene and di(thienyl)benzothiadiazole. Also,
at 1000 nm.44 Similar values were obtained by substitution of comparison of P3 and P4 would allow to study the effect of a
pyrrolidin-1-yl with p-(dibutylaminophenyl)ethynyl, whereas non-conjugated linker between the lateral pyrenes and the
lower values were observed with the poorer electron donor (ethynyl)phenoxy moieties of the PTCDI core. Remarkably, the
p-(butoxyphenyl)ethynyl as expected for a donor–acceptor–donor novel compound P2 shows excellent s2 values (3340 ꢁ 270 GM
structure where perylene acts as the electron-acceptor moiety.44 at 639 nm and 1530 ꢁ 120 GM at 720 nm) much higher than
Last year, Cao et al. reported that a perylenetetracarboxy-3,4:9,10- those previously reported for PTCDI bearing an ether linker
diimide bearing cyclohexyl groups at the imide positions and a between the perylene core and the p-delocalized groups at the
p-electron-rich triphenylamine unit in 1 bay position is charac- bay positions.
terized by a s2 value of 130 GM at 900 nm, as determined by the
Z-scan technique, a slightly higher value (140 GM) being reached
by introduction of a second triphenylamine in position 7.45 In
these systems, the aromatic substituents were directly connected
to the bay positions. In the same way, the TPA properties of
PTCDI tetrasubstituted in the four bay positions (1,6,7,12) with
directly-connected phenyl groups bearing electron donor/
acceptor moieties have also been investigated. These molecules
have 2,6-diisopropylphenyl groups at the imide positions and
the derivative with four p-methoxyphenyl as bay substituents
has a cross section of 180 GM at 850 nm, one order of
magnitude greater respect to the reference derivative with no
substituents in the bay positions.46 It was also shown that
tetrasubstitution causes twisting of the perylene core because
of the steric hindrance between the phenyl rings connected at
the bay positions.
Fig. 1 Structure of the investigated perylenetetracarboxy-3,4:9,10-
diimide derivatives (P1–P4).
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Results and discussion
Synthesis of perylenetetracarboxy-3,4:9,10-diimide derivatives
The known perylenetetracarboxy-3,4:9,10-diimide derivative P1
and the related compounds P2–P4 are readily prepared follow-
ing the synthetic procedure reported in Scheme 1.
The synthesis starts with the chlorination of 3,4:9,10-
perylene tetracarboxylic dianhydride followed by reaction with
2,6-diisopropylaniline to give I1. Subsequently, the tetrakis-
(iodophenoxy)perylene-derivative (I2) is readily prepared from
I1 by reaction with p-iodophenol in the presence of anhydrous
K2CO3 in 1-methyl-2-pyrrolidone (NMP). The iodo group is
optimal to attach an alkynyl moiety. The four iodo substituents
are easily replaced with the suitable alkyne (A–D), by fourfold
Sonogashira coupling, affording derivatives P1–P4 in good
yields (65–80%). This reaction is carried out in THF/NEt3 at
room temperature with a [PdCl2(PPh3)2]/CuI catalyst system
(see Experimental).
Fig. 2 Absorption spectra of P1–P4 in dichloromethane.
substituted with 4-(R-ethynyl)phenoxy moieties we observe a blurring
of the vibronic structure and a bathochromic shift of the lowest-
energy intense band, at ca. 570–590 nm (Fig. 2), as expected for
the presence of tetraphenoxy bay-substituted compounds,
because of the loss of rigidity due to core twisting and slight
electron-donating nature of phenoxy substituents.53,54 How-
ever, the peak position was almost the same for all substituents
at bay position. In the case of P2, the absorption bands of the
PTCDI core and the di(thienyl)benzothiadiazole moiety55–57 are
partially superimposed. At shorter wavelengths, structured
bands ascribed to absorption of the aromatic moieties (pyrene,
phenyl, di(thienyl)benzothiadiazole) can be noticed. It is important
pointing out that the four compounds P1–P4 are transparent at
wavelengths longer than 600 nm, which makes them suitable for
the investigation of two-photon absorption in the near-IR spectral
range, the most interesting from an application point of view.
One-photon absorption
The UV-Vis spectral shapes of P1–P4 are typical of the family
of perylenetetracarboxy-3,4:9,10-diimide compounds with large
e values, showing high one-photon absorption.38,44–46,50–52 The
parent perylenetetracarboxy-3,4:9,10-diimide bearing 2,6-diisopropyl-
phenyl groups at the imide positions but without substituents
at bay positions shows, in CHCl3, an absorption band peak at
527 nm with characteristic vibronic fine structure which is attributed
to the perylene core S0 - S1 p–p* transition.52 For the derivatives
Two-photon absorption
Nonlinear absorption measurements were carried out on
compounds P1–P4, dissolved in dichloromethane, by means
of the Z-scan technique (see Experimental). The related two-
photon absorption spectra, recorded from 638 to 800 nm, are
shown in Fig. 3 whereas the tabulated TPA cross-section values,
obtained by confirming the incident-power dependence (power
scan), are reported in the ESI,† (Table S1) along with the typical
Z-scan traces (Fig. S1–S4, ESI†). The TPA properties have also
been tested between 1100 and 1200 nm, but in this range of
wavelengths the compounds response was under the detection
limit of 3 GM.
The TPA spectra of the four compounds (Fig. 3) show a
similar feature to each other: the maximum s2 value is at the
investigated shortest wavelength (639 nm), close to the one-
photon absorption (OPA) band. At longer wavelengths, the s2
intensity decreases with a shoulder (P2, P3) or small peak
(P1, P4) at 720 nm.
The TPA magnitude differs considerably depending on the
substituents at bay positions.
P2 shows the highest s2 values (3340 ꢁ 270 GM at 639 nm
and 1530 ꢁ 120 GM at 720 nm, Table S1, ESI†) thanks to the
p-delocalized electron-donor di(thienyl)benzothiadiazole moieties
that contribute to the planarity of the branches, as confirmed by
DFT modelling (see later).
Scheme 1 Synthetic pathway of the PTCDI derivatives.
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the frequency term (in the denominator) in which the detuning
energy is involved.43
In addition to the s2 magnitude in the resonance enhance-
ment part, the substituents influence the shape and intensity of
the shoulder at lower wavelength. This shows that the transi-
tion probability of TPA is higher for the compound depending
on the p-conjugation of the bay substituent. The TPA transition
probability is proportional to the product of the transition
dipole moment, |mfi|2|mig|2, where f, i, g means the final,
intermediate, and ground states, respectively. Clearly the final
state is higher than S1 for the shoulder band and the TPA
transition related to the resonance enhancement from their
transition energies. On the other hand, S1 (i = 1) is the most
probable candidate for the intermediate state because it is the
excited state closest to the incident photon energy. Moreover,
the lowest-energy one-photon absorption peak is insensitive to
the bay substituents (Fig. 1), showing that the bay substitution
does not perturb the S1, which is supported by the confined MO
in the perylene core. Thus, the bay substitution has no influence
on m1g but largely affects mf1, resulting in the change of s2 by the
bay substitution. Namely, the TPA properties are governed by the
properties of the excited states. This type of behaviour, i.e.
the one-photon absorption is insensitive but the TPA magnitude
Fig. 3 TPA spectra of PTCDI derivatives: the full markers are the values
obtained with the wavelength scan at a fix power, while the empty markers
with error bars are the ones obtained with the power scan at a fixed
wavelength.
P3 has lower but still high s2 values (2390 ꢁ 200 GM at is sensitive to the substituents, was observed for systems where
639 nm and 1060 ꢁ 120 GM at 720 nm, Table S1, ESI†) thanks to rotation of the p-extension arms was restricted to the p-core.58
the planar pyrene substituents. P1 has the same p-conjugated
Remarkably, the s2 values of P1–P4 are quite high (in the range
backbone as P3 but bears phenyl instead of the more 630–1530 GM) even at 720 nm where the one-photon absorption
p-delocalized pyrene, which is the cause of its lower s2 values resonance enhancement is negligible, putting in evidence the
(1280 ꢁ 100 GM at 639 nm and 770 ꢁ 60 GM at 720 nm, Table S1, goodness of 4-(R-ethynyl)phenoxy groups as substituents for the
ESI†). P3 can be discussed in comparison to P4 since both of four bay positions in order to design highly TPA-active PTCDI
these compounds possess pyrene moieties. While in the former derivatives. The use of an adequate p-conjugated donor R fragment
the pyrene is linked directly to the alkyne, in the latter the may allow optimization of the s2 values.
conjugation path is interrupted by a dimethylenether bridge.
This bridge not only breaks the p-conjugation of the branches,
DFT modeling
but also reduce their planarity. As a consequence, the s2 values The molecular geometry of P1–P4 has been optimized in
for P4 (1000 ꢁ 80 GM at 639 nm and 630 ꢁ 50 GM at 720 nm, CH2Cl2 solution and the optimized molecular structures are
Table S1, ESI†) are lower than for the parent P3, which has shown in Fig. 4.
better p-conjugated branches.
We found that the perilene core of the four compounds is
The monotonic increase of the TPA values at shorter wave- not planar and the dihedral angle (f) which measures the
lengths (ca. 640 nm), which gave the maximum value of each twisting torsion of the perylene core due to the bay substituents
compound, is caused by the resonance enhancement of TPA has been computed in between 29.7–31.71 (see Fig. 4). It is
through decreasing detuning energy of the one-photon absorp- interesting to notice that in P2 and P3, which are characterized
tion transition43 like other reported PTCDIs.42,44 Observing the by the best TPA properties, the bay substituents are stacked due
spectra, the spectral shape in the resonance enhancement to the p–p interactions of both pyrene and thiophene groups.
region at the shorter wavelength is almost the same and it isn’t This geometrical constraint is not present in P1, while in P4 the
much influenced by the nature of the investigated substituents outer pyrenes are prevented to be stacked and orient each other
at the bay positions. This is consistent with the lowest energy almost in a perpendicular configuration due to CH–p interac-
one-photon absorption peaks of P1–P4 (Fig. 1) being at wave- tions between the pyrene hydrogen and the conjugated p
lengths close to each other. On the other hand, the substituents system of the other pyrene. The twisting torsion seems to be
influence the s2 magnitude in the resonance enhanced region only marginally affected by the substituents (the maximum
and a better p-conjugated substituent gives a larger magnitude. variation being computed 21), nevertheless the larger dihedral
These results suggest that the inherent transition probability of f angles are computed for P2 and P3.
TPA is enhanced by the bay substituent with better p-conjugation
Besides, we compared the frontier molecular orbitals of
because the magnitude of the TPA spectral is defined by the P1–P4 to analyze the relative energy alignment and the electronic
transition dipole moments products (in the numerator in the charge distribution; in Fig. 5 we report the energy levels of the
theoretical expression of s2) while its spectral shape is defined by highest occupied molecular orbitals (HOMOs) and of the lowest
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P3 are stabilized by ca. 0.5 eV with respect to the P1 HOMO. The
HOMO and HOMOꢀ1 of P4 are degenerate and are delocalized
on the two opposite pyrene groups in the same orientation, while
at lower energy by 0.17 eV, the degenerate couple HOMOꢀ2/
HOMOꢀ3 lyes showing an electronic charge distribution on the
two pyrene groups with a different orientation. The LUMO of
compounds P1–P4 lyes at the same energy and is essentially
delocalized on the same molecule region: the perylene core. The
HOMO–LUMO gap is therefore essentially influenced by the
HOMO which reflects the electronic charge delocalization on
the bay substituents. The HOMO–LUMO gap of P2 and P3 are
similar and the increasing order is P2 o P3 o P4 o P1.
Experimental
Synthesis of perylenetetracarboxy-3,4:9,10-diimide derivatives
Fig. 4 Optimized molecular structure of the investigated compounds
P1–P4 together with the dihedral angle f which measures the twisting
torsion of the perylene core.
Reagents and alkyne derivatives (A–C). All reagents, includ-
ing phenylacetylene A, were obtained from commercial sources
and used without further purification. 1-Ethynylpyrene (C) was
readily prepared, as previously reported,59 from 1-bromopyrene
by Sonogashira coupling reaction with trimethylsilyl acetylene
followed by TMS removal in the presence of tetrabutylammo-
nium fluoride. The same synthetic pathway was used to obtain
alkyne B.56
N,N0-Bis(2,6-diisopropylphenyl)-1,6,7,12-tetrachloro-3,4:9,10-
perylenetetracarboxydiimide (I1) was prepared as reported in
Scheme 1 following a well-known synthetic procedure.53,60,61
Synthesis of the alkyne D. First, the 1-hydroxymethyl pyrene
intermediate was prepared as follows: to
a solution of
1-pyrenecarboxaldehyde (1.0 g, 4.34 mmol) in dry MeOH/CH2Cl2
(1/1, 17 mL), under argon atmosphere at 0 1C, was added NaBH4
(181.2 mg, 4.8 mmol) in three portions; the reaction mixture was
stirred at 0 1C for 30 min and at room temperature for 1 h; the
mixture was diluted with water (50 mL) and extracted with
CH2Cl2 (3 ꢂ 50 mL); the organic layer was dried on Na2SO4
and the solvent was removed at reduced pressure to give the
solid product (yield = 97%).
Fig. 5 Schematic diagram of the energy levels of P1–P4 compounds.
Surface isodensity plots (isodensity contour: 0.02) of HOMO (bottom) and
LUMO (up) are also shown.
1H NMR (400 MHz; CDCl3) d ppm: 5.44 (2H, s), 8.02–8.11
unoccupied molecular orbitals (LUMOs) together with the iso- (4H, m), 8.17–8.24 (4H, m), 8.41 (1H, d, J = 9.21 Hz).
density plots of HOMO/LUMO of all investigated compounds. In 1-((2-Propynyloxy)methyl)pyrene (D) was then prepared as
addition the isodensity plots of HOMOꢀ1 and LUMO+1 of P1–P4 follows: 1-hydroxymethyl pyrene (601 mg, 2.6 mmol) was added
are collected in the Electronic Supplementary Information. The under argon to a solution at 0 1C of NaH (68.6 mg, 2.86 mmol) in
HOMO of P1 is the result of a combination of p orbitals of the dry DMF (10.4 mL); the reaction mixture was stirred at 0 1C for
central perylene carbons and of the carbons of the two opposite 10 min and then at room temperature for 1 h; propargyl chloride
bay substituents plus the p orbitals of the four oxygen atoms (206 mL, 2.86 mmol) was added at 0 1C and the mixture was stirred
which act as linkers between the substituents and the perylene at room temperature overnight; the reaction mixture was quenched
core. The HOMOꢀ1/HOMOꢀ4 in a range of 0.2 eV are different p with water (50 mL) and extracted with ethylacetate (3 ꢂ 50 mL); the
orbitals combinations on the carbons and oxygens of the bay organic layer was dried on Na2SO4 and the solvent was removed at
substituents, with no contribution from the perylene. The reduced pressure to give the crude product that was purified by
HOMO and HOMOꢀ1 of both P2 and P3 are degenerate and flash chromatography using hexane/dichloromethane 1/1 as eluent;
does not show contribution from the perylene core. In P2 both the pure product was obtained in 90% yield.
HOMO and HOMOꢀ1 are the mixing of p orbitals of the carbon
1H NMR (400 MHz; CDCl3) d ppm: 2.61 (1H, t, J = 2.36 Hz),
atoms of all the four chains with different coefficients, while in 4.32 (2H, d, J = 2.36 Hz), 5.35 (2H, s), 8.02–8.10 (4H, m), 8.16–
P3 they are mainly delocalized on the triple CC bond and pyrene 8.23 (4H, m), 8.43 (1H, d, J = 9.22 Hz). Anal. calcd (%) for
groups of the four bay substituents. Both the HOMOs of P2 and
C20H14O: C, 88.86; H, 5.22. Found: C 88.90; H 5.23.
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Synthesis of 1,6,7,12-tetrakis(4-iodophenoxy)-N,N0-(2,6-diiso- then hexane/dichloromethane 3/7. The pure product was
propylphenyl)perylenetetracarboxy-3,4:9,10-diimide (I2). In a obtained in 80% yield. 1H NMR (400 MHz; CDCl3) d ppm:
100 mL flask, under nitrogen atmosphere, a solution of 8.63 (4H, d, J = 9.2 Hz), 8.46 (4H, s), 8.16 (12H, d, J = 8.4 Hz),
tetrachloroperylene-tetracarboxdiimide I1 (400 mg, 0.46 mmol) 8.06–7.94 (20H, m), 7.73 (8H, d, J = 8.8 Hz), 7.34 (4H, d, J =
4-iodophenol (616 mg, 2.8 mmol) and anhydrous K2CO3 (193.4 mg, 7.6 Hz), 7.13 (8H, d, J = 8.4 Hz), 2.83–2.76 (4H, m), 1.20 (24H, d,
1.4 mmol) in 1-methyl-2-pyrrolidone (23 mL) was stirred overnight J = 6.8 Hz). 13C-NMR (100 MHz; CD2Cl2) d ppm: 145.85, 133.56,
at 70 1C. The reaction mixture was cooled to room temperature and 129.44, 128.28, 127.08, 126.19, 125.54, 124.46, 119.98, 94.62,
poured into 2 N HCl (64 mL). The product was extracted with 29.68,29.13, 23.72. MS (FAB+): m/z 1978. (calc. 1976.7) Anal.
ethylacetate (100 mL) and the organic phase washed three times calcd (%) for C144H90N2O8: C 87.52, H 4.59, N 1.42. Found: C
with water (100 mL), and dried over Na2SO4. The crude product was 87.68, H 4.60, N 1.43. Emission lmax = 602 nm (toluene);
purified by flash chromatography (eluent: Hexane : CH2Cl2 1:4) to fluorescence quantum yield = 0.03.
give I2 as red solid in 80% yield.
1,6,7,12-Tetrakis[4-(1-((2-propynyloxy)methyl)pyrene)phenoxy-
1H NMR (400 MHz; CDCl3) d ppm: 8.26 (4H, s), 7.61 (2H, N,N0-(2,6-diisopropylphenyl)perylenetetracarboxy-3,4:9,10-diimide
d, J = 8.74 Hz), 7.47 (16 H, t, J = 7.68 Hz), 7.30 (4H, t, J = 7.76 Hz), (P4) was prepared starting from 308 mg of alkyne D, the eluent for
2.70 (4H, m), 1.14–1.24 (24H, m). MS(FAB+): m/z 1585 (calc. 1584.0) flash chromatography being hexane/dichloromethane 1/1 and
General procedure for the syntheses of compounds P1–P4. then ethylacetate/dichloromethane 25/1. The pure product
Compound I2 (300 mg, 0.19 mmol) was dissolved in a degassed was obtained in 77% yield. 1H NMR (400 MHz; CD2Cl2) d
mixture of triethylamine (50 mL) and dry THF (14 mL) under ppm: 8.44–8.41 (4H, d, J = 9.2 Hz), 8.33 (4H, s), 8.23–8.20 (8H,
argon. Then [PdCl2(PPh3)2] (21.1 mg, 0.03 mmol), CuI (11.42 mg, m), 8.16–8.12 (8H, m), 8.09–8.00 (16H, m), 7.52–7.50 (10H, m),
0.06 mol), PPh3 (15.7 mg, 0.06 mmol) and the suitable alkyne 7.35 (4H, d, J = 8 Hz), 7.02 (8H, d, J = 8.8 Hz), 5.35 (8H, s), 4.47
A–D (1.14 mmol) were added. The reaction mixture was stirred (8H, s), 2.79–2.71 (4H, m), 1.16 (24H, d, J = 6.8 Hz). 13C-NMR
at 70 1C overnight, and then the solvent was removed under (100 MHz; CD2Cl2) d ppm: 163.07, 155.61, 155.29, 145.94,
reduced pressure. The crude product was purified by column 133.65, 131.40, 131.17, 130.84, 130.75, 129.53, 127.65, 127.46,
chromatography on silica gel.
127.42, 127.30, 125.97, 125.24, 125.21, 124.44, 124.05, 123.46,
1,6,7,12-Tetrakis[4-(phenylethynyl)phenoxy-N,N0-(2,6-diiso- 123.23, 120.94, 119.68, 119.01, 85.55, 70.01, 57.98, 29.09, 23.68.
propylphenyl)perylenetetracarboxy-3,4:9,10-diimide (P1) was MS(FAB+): m/z 2153 (calc. 2152.5) Anal. calcd (%) for C152H106N2O12:
prepared following the above reported general procedure using C 84.82, H 4.96, N 1.30. Found: C 84.86, H 4.98, N 1.30. Emission
160 mL (d = 0.95 g mLꢀ1) of phenylacetylene (A), the eluent for
flash chromatography being hexane/dichloromethane 1/1. The
pure product was obtained as dark solid in 70% yield.
l
max = 602 nm (toluene); fluorescence quantum yield = 0.40.
One-photon absorption
1H NMR (400 MHz; CDCl3) d ppm: 8.32 (4H, s), 7.54–7.49 The UV-Vis spectra of the samples solution in dichloromethane
(20H, m), 7.36–7.30 (14H, m), 6.96 (8H, d, J = 8.76 Hz), 2.71 (4H, m), were recorded by a Shimadzu UV-3150 spectrometer using
1.18–1.15 (24H, m). MS(FAB+): m/z 1481 (calc. 1479.5). Anal. calcd quartz cells.
(%) for C104H74N2O8: C 84.42, H 5.04, N 1.89. Found: C 84.46, H
5.03, N 1.90.
Photoluminescence
1,6,7,12-Tetrakis[4-(4-(5-ethynylthiophen-2-yl)-7-(thiophen-2- Photoluminescence quantum yields were measured using a
yl)benzo[c][1,2,5]thiadiazole)phenoxy-N,N0-(2,6-diisopropyl- C11347 Quantaurus – QY Absolute Photoluminescence Quantum
phenyl)perylenetetracarboxy-3,4:9,10-diimide (P2) was prepared Yield Spectrometer (Hamamatsu Photonics K. K.), with a 150 W
starting from 369 mg of alkyne B, the eluent for flash chromato- Xenon lamp, an integrating sphere and a multichannel detector,
graphy being hexane/dichloromethane 1/1 and then hexane/ in a toluene solution.
dichloromethane 1/5. The pure product was obtained as dark
red solid in 65% yield. H NMR (400 MHz; CDCl3) d ppm: 8.39
1
Two-photon absorption activity measurements
(4H, s), 8.10 (4H, dd, J = 3.7 Hz, J = 1.0 Hz), 7.98 (4H, d, J = All open-aperture Z-scan measurements62 were performed at
3.7 Hz), 7.91 (4H, d, J = 7.6 Hz), 7.88 (4H, d, J = 7.6 Hz), 7.51–7.45 AIST in Osaka. The setup for the Z-scan measurements were
(16H, m) 7.36–7.31 (6H, m), 7.24 (4H, dd, J = 1.0 Hz, J = 3.7 Hz), reported elsewhere.63 A femtosecond, wavelength-tunable laser
6.86 (8H, d, J = 8.6 Hz), 2.75 (4H, m), 1.19–1.17 (24H, m). (optical parametric amplifier, SpectraPhysics TOPAS Prime)
13C-NMR (100 MHz; CD2Cl2) d ppm: 145.85, 133.56, 129.44, was employed as light source to obtain the spectrum. Typical
128.28, 127.08, 126.19, 125.54, 124.46, 119.98, 94.62, pulsewidth was 120 fs and the repetition rate was 1 kHz. First,
29.68,29.13, 23.72. MS(FAB+): m/z 2369 (calc. 2368.9). Anal. calcd the outline of the TPA spectrum was measured at a single
(%) for C136H82N10O8S12: C 68.95, H 3.49, N 5.91. Found: C 68.99, excitation power (0.4 mW, corresponding to the on-axis optical
H3.53, N 5.92. Emission lmax = 602 nm (toluene); fluorescence intensity at the focal point of 99–150 GW cmꢀ2) by assuming
quantum yield = 0.05.
that the observed open-aperture Z-scan signal originates from
1,6,7,12-Tetrakis[4-(pyrenethynyl)phenoxy-N,N0-(2,6-diiso- the TPA process (filled dots in Fig. 3). Then the assumption was
propylphenyl)perylenetetracarboxy-3,4:9,10-diimide (P3) was confirmed and the precise value of the TPA cross-sections (s2)
prepared starting from 258 mg of alkyne C, the eluent for were obtained by changing the excitation power (open dots with
flash chromatography being hexane/dichloromethane 1/1 and experimental errors in Fig. 3). Finally, the relative TPA spectrum
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(filled dots) was superimposed on the absolute TPA spectrum most TPA active compounds, the bay substituents are stacked
(open dots). For all measurements, a in-house standard com- due to the p–p interactions of pyrene and thiophene groups,
pound (MPPBT in DMSO)64 was measured under the same respectively. The LUMO of P1–P4 lyes at the same energy and is
conditions and used for the spectral correction of s2.
essentially delocalized on the perylene core whereas the HOMO
and HOMOꢀ1 of both P2 and P3 are degenerate and do not
show contribution from the perylene core contrarily to that of P1
and P4. The HOMO–LUMO gap is therefore essentially influenced
by the HOMO which reflects the electronic charge delocalization
on the bay substituents, the lower gaps being observed for P2 and
P3, which are characterized by the best TPA properties.
These results clearly show that, in this kind of PTCDI
derivatives, the use of an adequate p-conjugated donor R
fragment allows optimization of the s2 values. Functionalising
the perylene core at the bay positions offers a high synthetic
flexibility and a big choice of substituents and p-conjugated
patterns that will surely lead to novel TPA-active PTCDI com-
pounds for various applications in the near future.
DFT calculations
All the calculations were performed with Gaussian09 (G09),65
without any symmetry constraints. The molecular geometry of
all compounds has been optimized by a density functional
theory (DFT) approach using a 6-31G* basis set66 the B3LYP67
exchange–correlation functional integrated with the D3-BJ
model to include the dispersion interactions.68 We optimized
the geometries in CH2Cl2 solution including solvation effects
by means of the conductor-like polarizable continuum model
(C-PCM)69,70 as implemented in G09.
Conclusions
Conflicts of interest
In summary we prepared and well characterized perylenetetra-
carboxy-3,4:9,10-diimides, bearing 2,6-diisopropylphenyl groups
at the imide positions. Three of them have highly p-delocalized
There are no conflicts to declare.
4-(R-ethynyl)phenoxy moieties (R
= phenyl (P1), 4,7-di(2-
thienyl)benzo[c][1,2,5]thiadiazole (P2) or pyrene (P3)) at the Acknowledgements
four bay positions whereas one (R = pyrene-CH2OCH2 (P4)) has
This work was supported by MIUR and CNR, in Italy, and by
poor delocalization due to the presence of the non-conjugated
CH2OCH2 moiety. They have high TPA cross-sections, as deter-
mined by the Z-scan technique, similar to that obtained for
PTCDIs bearing pyrrolidin-1-yl substituents in 1 and 7 bay
positions44 and much higher than that previously reported
for PTCDIs having an ether linker between the perylene core
and the p-delocalized groups.47,48 While approaching the one-
photon absorption band, at ca. 640 nm, there is a monotonic
increase of the TPA values caused by the resonance enhance-
ment of nonlinearity via the one-photon resonance process.
At longer wavelength (720 nm), the s2 values of P1–P4 are still
quite high (in the range 630–1530 GM), putting in evidence the
goodness of 4-(R-ethynyl)phenoxy groups as substituents for
the bay positions in order to design highly TPA-active PTCDI
derivatives. Besides, remarkably, the bay substituent strongly
affects the s2 magnitude while it does not influence the one-
photon absorption peak to S1 excited state. These facts suggest
that the influence of the bay substitution is mainly caused by
the higher excited state which is the final state of the TPA
transition (Sf) through the transition dipole moment between
the excited states mf1 (i.e. of the transition of S1 - Sf).
a Grant-in-Aid for Scientific Research on Innovative Areas
‘‘Photosynergetics’’ (no. JP26107004, K. K.) from MEXT and a
Grant-in-Aid for Scientific Research (no. JP25248007, K. K.)
from JSPS, Japan. We deeply thank Dr Stefania Righetto and
Dr Daniele Marinotto for photoluminescence measurements.
Regione Lombardia and Fondazione Cariplo are also acknowl-
edged for the use of instrumentation purchased through the
SmartMatLab Centre project (2014).
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