Highly stable 7-N,N-dibutylamino-2-azaphenanthrene and 8-N,N-dibutylamino- 2-azachrysene as a new class of second order NLO-active chromophores

Article abstract of DOI:10.1039/c0cc02781b

7-N,N-Dibutylamino-2-azaphenanthrene (L³), 8-N,N-dibutylamino-2- azachrysene (L⁴) and related Ir(i) complexes or alkylated salts show high second-order NLO responses, as determined by the EFISH technique and DFT calculations. L⁴

Full text of DOI:10.1039/c0cc02781b

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Highly stable 7-N,N-dibutylamino-2-azaphenanthrene and
8-N,N-dibutylamino-2-azachrysene as a new class of second order
NLO-active chromophoresw
Valentina Calabrese,ab Silvio Quici,*ab Ester Rossi,c Elena Cariati,c Claudia Dragonetti,c
Dominique Roberto,*abc Elisa Tordin,c Filippo De Angelisd and Simona Fantacci*de
Received 24th July 2010, Accepted 9th September 2010
DOI: 10.1039/c0cc02781b
7-N,N-Dibutylamino-2-azaphenanthrene (L3), 8-N,N-dibutyl-
amino-2-azachrysene (L4) and related Ir(I) complexes or alkylated
salts show high second-order NLO responses, as determined by the
EFISH technique and DFT calculations. L4 is appealing as
building block for NLO active materials due to its unexpected
large mb1.907value and its very high thermal stability.
The design and synthesis of new non-linear optical (NLO)
chromophores possessing high values of quadratic hyper-
polarizabilities b and characterized by chemical, photochemical
and thermal stability are of great technology-driven interest.1
Stilbene and azobenzene based chromophores, well known NLO
active molecules already incorporated into electro-optic
polymeric systems,2 present several intrinsic drawbacks due to
cis–trans photoisomerism around the CQC or NQN double
bond and photorotamerism of the aromatic rings thus reducing
the chromophore NLO activity.3 Moreover the isolated double
bond may be easily oxidized thus interrupting the p-conjugation
between the donor and the acceptor groups. These problems can
be overcome if the double bond is included in a rigid poly-
aromatic system, which should ensure not only stability, but also
highly electronic delocalization.
Fig. 1 Structure of compounds studied.
evaluate the modification of the chromophores electronic
properties and their effect on the NLO activity.6
Compounds L3 and L4 have been reported as intermediates for
the preparation of anellated hemicyanine dyes.7 Compounds
used in the present study have been prepared in good yields by
careful modification of the known procedure for L3, and using a
different synthetic approach for L4.8 L1 was obtained using a
standard Suzuki coupling and L2 was prepared as reported.5
Complexes IrL3 and IrL4 have been obtained by reaction of L3
and L4 with [Ir(CO)2Cl]2 as previously described for similar
compounds.6a,8 The methyl pyridinium salt of L4 (L4MeI)
was prepared by reaction with MeI following a standard
procedure.6b,8 Details of the synthesis and characterization of
all the compounds are given in ESI.w
In this communication we report the synthesis and the study of
the second order NLO properties, measured by the Electric Field
Induced Second Harmonic generation (EFISH) technique,4
of 7-N,N-dibutylamino-2-azaphenanthrene (L3) and 8-N,N-
dibutylamino-2-azachrysene (L4) (Fig. 1). For comparison
N,N-dimethyl-4-(pyridin-4-yl)aniline (L1) and N-methyl-N-
hexadecylaminostilbazole (L2)5 (Fig. 1), the corresponding non-
anellated chromophores of L3 and L4, respectively, were also
investigated. Finally we synthesized cis-[Ir(CO)2ClL3] (IrL3)
and cis-[Ir(CO)2ClL4] (IrL4) complexes and methylated salts to
The experimental electronic spectra are reported in Table 1,
along with their mb1.907 values measured with the EFISH
technique.4 The theoretical m values (see below) of all
compounds are also given together with the experimental
valuesof L2, L3 and L4.9
As expected L1 is characterized by a low value of mb1.907 in
CHCl3 due to the absence of a complete p-conjugation
between the two aromatic rings. However this value increases
drastically by a factor of 8.9 if the single bond is included into
the polyaromatic scaffold of L3. A further increase of b by a
factor of 1.4 is obtained upon coordination of L3 to the
‘‘Ir(CO)2Cl’’ moiety which acts as an additive electron–
acceptor group producing a red-shift of the ILCT transition
(from 347 to 390 nm, Table 1) dominating the NLO response.
Remarkably, an increase of the p-conjugation of the free
ligand leads to a huge positive effect on the second order
NLO response, with an enhancement factor of 4.4 on going
from L3 to L4, reaching a mb1.907 value much higher than that
of the related stilbazole L2.
a Istituto di Scienze e Tecnologie Molecolari del CNR (ISTM-CNR),
Via C. Golgi 19, I-20133, Milano, Italy
b Polo Scientifico e Tecnologico (PTS), Consiglio Nazionale delle
Ricerche (CNR), via Fantoli 16/15, 20138 Milano, Italy.
E-mail: silvio.quici@istm.cnr.it
c Dipartimento di Chimica Inorganica, Metallorganica e Analitica
`
‘‘Lamberto Malatesta’’, Universita degli Studi di Milano and UdR
INSTM di Milano, Via Venezian 21, I-20133, Milano, Italy.
E-mail: dominique.roberto@unimi.it
d Istituto di Scienze e Tecnologie Molecolari del CNR (ISTM-CNR),
c/o Dipartimento di Chimica, Via Elce di Sotto 8, 06123 Perugia,
Italy. E-mail: simona@thch.unipg.it
e Italian Institute of Technology (IIT), Center for Biomolecular
Nanotechnologies, Via Barsanti, Arnesano, I-73010, Lecce, Italy
w Electronic supplementary information (ESI) available: Synthesis
and characterization of all compounds; full ref. 14. See DOI:
10.1039/c0cc02781b
c
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Table 1 Experimental electronic spectra, EFISH mb1.907 and m values
of L3, L4 and related compounds
from the DSC spectrum, the melting and the crystallization
peaks show similar intensities within the three cycles
thus indicating that there is neither sublimation nor partial
decomposition of the product. The decomposition temperature
of 434 1C was determined by further heating of the sample over
its melting temperature. The very high thermal stability,
combined with its high NLO activity, makes L4 particularly
appealing from an applicative point of view. As we have already
mentioned NLO chromophores should be highly resistant to
thermal degradation for a practical use in electro-optic systems,
where temperatures higher than 300 1C are required to process
and assemble NLO-active materials into devices.12
l/nm
Compound [e/MÀ1 cmÀ1 a
mb1.907
mc (mtheor
)
b1.907
]
(10À48 esu)a,b (10À18 esu) (10À30 esu)
L1
417 [46 500]
506 [9160]
48
2.4
20d
L2
L3
3816
2236
430
3.76
3.5
(7.6)
(16.2)
606
278 [28 626]
347 [14 397]
279 [35 420]
390 [25 670]
305 [41 462]
361 [17 179]f
272 [34 541]
421 [12 737]
272 [39 886]
285 [37 869]
315 [19 045]
364 [14 386]
473 [26 523]
123d
(56)e
(38)e
IrL3
L4
620
1800
À2310
1820
4.2
(8.0)
(16.9)
429d
(224)e
IrL4
L4MeI
(À137)e
In order to get insight on the peculiar second order NLO
properties of the newly synthesized systems, we carried out
DFT (Density Functional Theory) and Time Dependent DFT
(TDDFT) computational studies on L3 and L4 by means of
Gaussian 03 (G03) program package.13
(18.9)
(96)e
The molecular structures of L3, L4, IrL3 and IrL4 have been
optimized in CHCl3 using B3LYP exchange–correlation
functional, a 6-31G* basis set for C, H and N and Lanl2DZ
basis set with related pseudo-potentials for Ir and Cl. The
solvation effects have been included by means of a Polarizable
Continuum Model (PCM) as implemented in G03.14 The
geometry optimization of L3 and L4 provides very similar
molecular structures characterized by planarity of the
azaphenanthrene and azachrysene and similar CC and CN bond
distances (see ESIw). The N(butyl)2 moiety goes out of the plane
of the anellated rings by ca. 61 for both species, with the two
butyl groups oriented in opposite directions. We computed
similar dipole moments both in vacuo, 5.2 and 5.6 D, and in
CHCl3 solution, 7.6 and 8.0 D, for L3 and L4, respectively. The
optimized IrL3 and IrL4 are square planar complexes with the L3
and L4 twisted with respect to the square planar coordination
plane, by 541 and 651, respectively (see ESIw). From the analysis
of the electronic structure of L3 and L4 it emerges that the highest
occupied orbitals (HOMOs) are of p character and show similar
energies and similar charge distribution apart from an increased
delocalization due to the presence of four rings in L4. The L4
lowest unoccupied orbitals (LUMOs) of p* character are, on the
other hand, the result of different carbon and nitrogen p orbital
combinations with respect to L3 (see ESIw), which is reflected by
their different energies. The L4 LUMO is at lower energy with
respect to that of L3 as a consequence of the increased
p-conjugation, leading to a 0.38 eV decrease of the L4 HOMO–
LUMO gap. We then simulated the absorption spectra of both
systems by computing the lowest 20 singlet–singlet excitations,
the comparison between the experimental and computed spectra
of L3 and L4 is reported in Fig. 3. The agreement between theory
and experiment is good, in the L3 case we are able to reproduce
all the features of the experimental absorption spectrum, and
only the lowest absorption band is computed slightly red-shifted
by 0.16 eV. For L4, the calculated intensity distribution does only
qualitatively compare with the experimental absorption bands,
even though also in this case the computed transitions nicely fit
with the main experimental features, see Fig. 3.
a
b
In CHCl3 at 10À4 M with an incident radiation of 1.907 mm. The
c
error on EFISH measurements is Æ10%. Experimental m obtained
by the Guggenheim method10 in CHCl3; the error is Æ1 Â 10À18
d
e
f
esu. By using experimental m. By using theoretical m. There is a
band tail at ca. 400 nm.
Surprisingly, contrarily to what was observed in the case of
stilbazolium salts,6b,11 methylation of the pyridine ring of L4 does
not affect strongly the second order NLO response although
there is a relevant red shift (Dlmax = 112 nm) of the ILCT
transition. This effect might be related to a different orientation
of the dipole moment in the two salts, which determines the
overall EFISH response. Coordination of L4 to ‘‘Ir(CO)2Cl’’
leads to a slight increase (enhancement factor = 1.2) of the
absolute mb1.907 value which, unexpectedly, becomes negative.
This behaviour is in contrast with that observed upon
coordination of L2 to the same Ir(I) moiety where mb1.907 is
enhanced but remains positive, being dominated by an ILCT
transition.5 It is worth noting that the very high mb1.907 values of
L3 and L4 are due to large b1.907 values since their dipole
moments are quite similar to that previously reported5 for L2.
Furthermore of particular interest is the unexpectedly very
high thermal stability of L4 measured by Differential Scanning
Calorimetry (DSC) (Fig. 2). The sample was subjected to three
reversible cycles of melting and crystallization with a heating
rate of 5 1C minÀ1 under nitrogen atmosphere. As it is evident
The absorption spectrum of L3 is characterized by a
low-energy band computed at 332 nm and one more intense
band at 277 nm, to be compared with the experimental bands
at 347 and 287 nm. The lowest transition is calculated as being
Fig. 2 Differential Scanning Calorimetry (DSC) spectrum of L4.
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Moreover, at higher energy the 314 nm transition is that mainly
responsible for the converged higher b value of L4 compared to
L3, indeed the same state in L3 is characterized by a lower
transition dipole moment.
In conclusion L3 and L4 are particularly appealing because
of their notable values of EFISH mb1.097 that are likely due to
the absence of photorotamerism and photoisomerism pro-
cesses operating in the corresponding compounds L1 and L2
respectively. These properties together with the absence
of isolated double bonds ensure the astonishingly high
thermal stability of L4 that makes it a good candidate for
the preparation of materials characterized by high second
harmonic generation properties. We believe that, because of
their relevant technological interest, the results here reported
can constitute a springboard for the design of other NLO
chromophores of the same family such as 11-N,N-dibutyl-
amino-3-azapicene and 12-N,N-dibutylaminobenzo[m]-3-
azapicene with 5 and 6 rings respectively.
We thank deeply S. Righetto for EFISH measurements and
S. Proutiere for DSC study. This work was supported by
MIUR (FIRB RBNE033KMA) and CNR-INSTM
(PROMO 2006).
Fig. 3 Comparison between theoretical (blue line) and experimental
(red line) spectra of L3 and L4 with related computed hyperpolariz-
abilities through a SOS approach in the insets.
a weak HOMO–LUMO excitation at 361 nm, of p–p* character.
The transition giving the band at 332 nm is essentially from the
HOMO to the LUMO+1 and has charge-transfer character,
going from the donor dibutylamino part of the ligand to the
acceptor pyridine ring. The absorption band computed at 277 nm
is constituted by two main transitions of global p–p* character.
The experimental absorption spectrum of L4 has a quite similar
shape with respect to that of L3, even though it is red-shifted and
it shows the appearance of a band tail at ca.400 nm. We
computed bands at 353 nm and 312 nm to be compared to the
experimental absorption bands at 361 nm and 305 nm.
Notes and references
1 For example: Molecular Nonlinear Optics: Materials, Physics,
Devices, ed. J. Zyss, Academic Press, Boston, 1994.
2 (a) S. K. Yesodha, C. K. S. Pillai and N. Tsutsumi, Prog. Polym.
Sci., 2004, 29, 45; (b) L. R. Dalton, A. W. Harper, R. Ghosn,
W. H. Steier, M. Ziari, H. Fetterman, Y. Shi, R. V. Mustacich, A.
K.-Y. Jen and K. J. Shea, Chem. Mater., 1995, 7, 1060;
(c) K. D. Singer, E. Sohn, L. A. King, H. M. Gordon,
H. E. Katz and P. W. Dirk, J. Opt. Soc. Am. B, 1989, 6,
1339.
3 Photoreactive Organic Thin Films, ed. Z. Sekkat and W. Knoll,
Accademic Press, San Diego, California.
4 I. Ledoux and J. Zyss, Chem. Phys., 1982, 73, 203.
The band at 353 nm is a charge-transfer transition essentially of
HOMO–LUMO+1 character. We computed a weak lower
energy transition at 392 nm of charge transfer character,
corresponding to the HOMO–LUMO transition, related to the
low energy experimental feature and more intense than that of the
L3 ligand, consistently with the experimental spectra. To gain
insight into the electronic transitions responsible for the observed
quadratic hyperpolarizability b of L3 and L4, we performed a
Sum Over States (SOS) analysis.15 The results, reported as insets
of Fig. 3, show in both cases a converged positive value in
agreement with the experimental evidence. The ratio of the
computed b values of L3 and L4 is 2.2, which is in line with the
experimental increase of the EFISH b value observed passing
from L3 to L4. This increase is mainly due to the red-shifted
absorption spectrum of L4, related to the HOMO–LUMO gap
decrease. Indeed, according to the two-level model, b increases
quadratically with decreasing the energy of the electronic
transitions. From an in depth analysis of calculated SOS b values,
we noticed that the largest contributions to b are provided for
both L3 and L4 by ILCT transitions. Visualization of the charge
transfer processes originating the NLO response is reported in the
ESI.w We found that for L4 the lowest energy transition
considerably contributes to the calculated b, for L3 such excited
state has almost vanishing contribution to b, due to its
higher energy and considerably lower transition dipole moment.
5 D. Locatelli, S. Quici, S. Righetto, D. Roberto, F. Tessore,
G. J. Ashwell and M. Amiri, Prog. Solid State Chem., 2005, 33, 223.
6 (a) D. Roberto, R. Ugo, S. Bruni, E. Cariati, F. Cariati,
P. C. Fantucci, I. Invernizzi, S. Quici, I. Ledoux and J. Zyss,
Organometallics, 2000, 19, 1775; (b) F. Tessore, E. Cariati,
F. Cariati, D. Roberto, R. Ugo, P. Mussini, C. Zuccaccia and
A. Macchioni, ChemPhysChem, 2010, 11, 495.
7 G. Hubener, A. Lambacher and P. Fromherz, J. Phys. Chem. B,
2003, 107, 7896.
8 V. Calabrese, PhD thesis, University of Milan, Italy, 2008.
9 The absolute mb1.907 value decreases by increasing the concentra-
tion from 10À4 M to 10À3 M due to aggregation effects at higher
concentrations. Due to aggregation effects,
determined experimentally.
m could not be
10 E. A. Guggenheim, Trans. Faraday Soc., 1949, 45, 714.
11 V. Alain, M. Blanchard-Desce, I. Ledoux-Rak and J. Zyss, Chem.
Commun., 2000, 353.
12 D. M. Burland, R. D. Miller and C. A. Walsh, Chem. Rev., 1994,
94, 31.
13 M. J. Frisch, et al., Gaussian 03, Revision C.02, ESIw.
14 M. Cossi and V. Barone, J. Chem. Phys., 2001, 115, 4708.
15 This approach requires in principle the calculation of dipole matrix
elements between all possible couples of excited states (three level
terms), in addition to ground to excited states transition dipole
moments (two level terms).16 However two- and three-level terms
show approximately the same scaling with the number of excited
states as two-level terms, so that the latter can be used for a
semi-quantitative assessment of the contributions to the quadratic
hyperpolarizability.
16 D. R. Kanis, P. G. Lacroix, M. A. Ratner and T. J. Marks, J. Am.
Chem. Soc., 1994, 116, 10089.
c
8376 Chem. Commun., 2010, 46, 8374–8376
This journal is The Royal Society of Chemistry 2010