An investigation on the second-order NLO properties of novel cationic cyclometallated Ir(III) complexes of the type [Ir(2-phenylpyridine) 2(9-R-4,5-diazafluorene)]+ (R = H, fulleridene) and the related neutral complex with the new 9-fulleriden-4-monoazafluorene ligand

Article abstract of DOI:10.1016/j.ica.2011.10.018

Novel cationic cyclometallated Ir(III) complexes bearing diaza- or monoazafluorene substituted or not with a C60-fullerene moiety have been synthesized together with a novel interesting ligand 9-fulleriden-4- monoazafluorene and the related Ir(III) neutral complex [Ir(ppy) ₂(9-fulleriden-4-monoazafluorene)]. All complexes show a negative large value of μβ_1.907 (-600 to -2190 × 10 ^-48 esu), as determined by the Electric Field Induced Second Harmonic generation (EFISH) technique working at 1.907 μm in 10^-3 M CH₂Cl₂ solution. The NLO response increases upon introduction of the fullerene moiety, due to a decrease of the interaction forces between the anion and cation within the ion pair which leads to an increase of the percentage of free ions, as evidenced by diffusion NMR experiments. Besides, it appeared that for the neutral complex [Ir(ppy) ₂(9-fulleriden-4-monoazafluorene)] the EFISH μβ _1.907 value is lower than that of cationic Ir(III) complexes, but comparable or slightly lower to the values of other neutral Ir(III) complexes such as [Ir(ppy)₂(RCOCR′COR)] (ppy = cyclometallated 2-phenylpyridine; R = Me, Ph; R′ = H, 2,4-dinitrophenyl), confirming that the second-order NLO response of these neutral chromophores is dominated by ILCT processes concerning cyclometallated 2-phenylpyridines.

Full text of DOI:10.1016/j.ica.2011.10.018

Inorganica Chimica Acta 382 (2012) 72–78
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
An investigation on the second-order NLO properties of novel cationic
cyclometallated Ir(III) complexes of the type [Ir(2-phenylpyridine)2
(9-R-4,5-diazafluorene)]+ (R = H, fulleridene) and the related neutral complex
with the new 9-fulleriden-4-monoazafluorene ligand
Claudia Dragonetti a,b, , Adriana Valore b,c, Alessia Colombo a,b, Stefania Righetto a,b, Giovanni Rampinini d,
⇑
Francesca Colombo e, Luca Rocchigiani f, Alceo Macchioni f
a Dip. di Chimica Inorganica, Metallorganica e Analitica ‘‘Lamberto Malatesta’’, dell’Università degli Studi di Milano, Italy
b UdR di Milano dell’INSTM, via Venezian 21, I-20133 Milano, Italy
c Istituto di Scienze e Tecnologie Molecolari del CNR, Italy
d Dip. di Chimica Organica e Industriale, dell’Università degli Studi di Milano, via Venezian 21, I-20133 Milano, Italy
e Dip. di Scienze Chimiche ed Ambientali, dell’Università dell’Insubria, Via Valleggio 11, I-22100 Como, Italy
f Dip. di Chimica, Università di Perugia, via Elce di Sotto 8, I-06123 Perugia, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel cationic cyclometallated Ir(III) complexes bearing diaza- or monoazafluorene substituted or not
with a C60-fullerene moiety have been synthesized together with a novel interesting ligand 9-fuller-
iden-4-monoazafluorene and the related Ir(III) neutral complex [Ir(ppy)2(9-fulleriden-4-monoazafluo-
Received 28 June 2011
Received in revised form 29 September
2011
Accepted 5 October 2011
Available online 14 October 2011
rene)]. All complexes show
a
negative large value of
l
b1.907 (ꢀ600 to ꢀ2190 ꢁ 10ꢀ48 esu), as
determined by the Electric Field Induced Second Harmonic generation (EFISH) technique working at
1.907 l
m in 10ꢀ3 M CH2Cl2 solution. The NLO response increases upon introduction of the fullerene moi-
ety, due to a decrease of the interaction forces between the anion and cation within the ion pair which
leads to an increase of the percentage of free ions, as evidenced by diffusion NMR experiments. Besides,
Keywords:
Cyclometallated Iridium(III) complexes
Diazafluorene
it appeared that for the neutral complex [Ir(ppy)2(9-fulleriden-4-monoazafluorene)] the EFISH
lb1.907
value is lower than that of cationic Ir(III) complexes, but comparable or slightly lower to the values of
other neutral Ir(III) complexes such as [Ir(ppy)2(RCOCR0COR)] (ppy = cyclometallated 2-phenylpyridine;
R = Me, Ph; R0 = H, 2,4-dinitrophenyl), confirming that the second-order NLO response of these neutral
chromophores is dominated by ILCT processes concerning cyclometallated 2-phenylpyridines.
Ó 2011 Elsevier B.V. All rights reserved.
Monoazafluorene
Fullerene
Nonlinear optical properties
Diffusion NMR
1. Introduction
and terpyridines bearing a NR2 donor group produces a significant
increase of their NLO response, due to a red-shift of the intraligand
In the last two decades, organometallic and coordination com-
plexes have stood out as new intriguing molecular chromophores
with second-order nonlinear optical (NLO) properties, since they
can offer, compared to traditional organic second-order NLO chro-
mophores, additional electronic properties acting on the NLO
response such as charge-transfer transitions between the metal
and the ligands, usually at low energy and of relatively high inten-
sity, tunable by virtue of the nature, oxidation state and coordina-
tion sphere of the metal center [1].
charge-transfer transition (ILCT) induced by the metal acting as a
Lewis acceptor. On the contrary when the NLO chromophores bear
a strong electron acceptor group, the NLO response is mainly con-
trolled by metal to ligand charge-transfer transitions (MLCT) [1].
Few years ago, various cationic cyclometallated Ir(III) com-
plexes with a phenanthroline ligand [2] have shown interesting
photoemissive properties. Density Functional Theory (DFT) investi-
gations on this class of luminescent Ir(III) complexes [2] have
shown that the HOMO is mainly an antibonding combination of
For instance, coordination to a metal of second order push–pull
NLO chromophores such as stilbazoles, bipyridines,phenanthrolines
Ir (t2g) orbitals and p orbitals of the cyclometallated ligand while
the LUMO is a p⁄ antibonding orbital of the chelated phenanthro-
line. Therefore transitions between HOMO and LUMO orbitals have
a significant charge-transfer character and these Ir(III) complexes
could behave as second order NLO chromophores with the
cyclometallated ligands acting as donors towards the acceptor p⁄
system of he phenanthroline ligand. Some of us have confirmed
⇑
Corresponding author at: Dip. di Chimica Inorganica, Metallorganica e Analitica
‘‘Lamberto Malatesta’’, dell’Università degli Studi di Milano, Italy. Tel.: +39 02
50314358; fax: +39 02 50314405.
E-mail address: claudia.dragonetti@unimi.it (C. Dragonetti).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.10.018

C. Dragonetti et al. / Inorganica Chimica Acta 382 (2012) 72–78
73
in a series of investigations [3–7] that various cationic cyclometal-
lated Ir(III) complexes with a substituted phenanthroline and with
various cyclometallated ligands show a significant second order
NLO response, as determined in CH2Cl2 solution (10ꢀ3 M) by the
Electrical Field Induced Second Harmonic generation (EFISH)
technique [8] and this response is reached without any cost in
transparency [9].
Besides, recently, we have investigated the second-order
nonlinear optical properties of some neutral Ru(II) complexes with
substituted 1,10-phenanthroline or 4,5-diazafluorene. In particular
it was appealing to investigate the effect on the p⁄ acceptor
properties of the 4,5-diazafluorene ligand of a substituent such
as the highly polarizable and non conventional electron withdraw-
ing C60-fullerene system, connected through a cyclopropane
group. Remarkably, for these Ru(II) complexes with the fullerene
ture was cooled to ꢂ0 °C. Green plates of the desired product
formed overnight.
(Yield 80%). 1H NMR (400 MHz, CD2Cl2) d ppm: 8.23 (d, J = 7.6 Hz,
2H), 7.99 (d, J = 8.0 Hz, 2H), 7.79 (m, 6H), 7.71 (dd, J1 = 0.7, J2 = 5.8 Hz,
2H), 7.53 (dd, J1 = 5.3, J2 = 7.8 Hz, 2H), 7.11 (dt, J1 = 1.2, J2 = 7.6,
J3 = 7.8 Hz, 2H), 7.01 (m, 4H), 6.45 (dd, J1 = 0.9, J2 = 7.4 Hz, 2H), 4.52
(s, 2H). Anal. Calc. forIrC33H24N4PF6:C, 48.71;H, 2.97;N, 6.88. Found:
C, 48.78; H, 3.01; N, 6.79%.
2.2.2. Synthesis of 1b: [Ir(ppy)2(4,5-diazafluorene)]+C12H25SOꢀ
3
A solution of [Ir(ppy)2Cl]2 (0.100 g, 0.093 mmol) and 4,5-diaza-
fluorene (0.031 g, 0.187 mmol) in CH2Cl2/MeOH (15 mL, 2/1 v/v)
was heated under reflux. After 5–6 h, the green solution was cooled
to room temperature, and then a 10-fold excess of Sodium dodec-
ansulfonate (0.506 g) was added. The suspension was stirred for
15 min and then filtered to remove insoluble inorganic salts. The
solution was evaporated to dryness under reduced pressure to
obtain a crude green solid. The solid was dissolved in CH2Cl2
(5 mL) and filtered to remove the residual traces of inorganic salts.
Diethyl ether was layered onto the filtrate, and the mixture was
cooled to ꢂ0 °C. Green plates of the desired product formed
overnight.
moiety, the high absolute values of
lb1.907 clearly evidence the
important role of the fullerene group in enhancing the NLO
response. This effect cannot be explained by a particularly strong
electron-accepting ability of the fullerene (there is only a small
red shift in the MLCT band on going from the complex without
to the complex with fullerene) but can be attributed to its large
polarizability [10].
These results prompted us to investigate the NLO properties
of new cationic cyclometallated Ir(III) complexes bearing
diaza- or monoazafluorene substituted or not with a C60-fuller-
ene moiety. In addition we synthesized the novel ligand 9-fuller-
iden-4-monoazafluorene, the related Ir(III) neutral complex
[Ir(ppy)2(9-fulleriden-4-monoazafluorene)] and studied its sec-
ond-order NLO properties.
(Yield 71%).1H NMR (400 MHz, CD2Cl2) d ppm: 8.21 (d, J = 7.6 Hz,
2H), 7.97 (d, J = 8.0 Hz, 2H), 7.8 (m, 6H), 7.73 (dd, J1 = 0.7, J2 = 5.8 Hz,
2H), 7.51 (dd, J1 = 5.3, J2 = 7.8 Hz, 2H), 7.12 (dt, J1 = 1.2, J2 = 7.6,
J3 = 7.8 Hz, 2H), 7.02 (m, 4H), 6.48 (dd, J1 = 0.9, J2 = 7.4 Hz, 2H),
4.51 (s, 2H), 2.66 (m, 2H), 1.49 (m, 4H), 1.29 (m, 16H), 0.91 (t,
J = 6.8, 3H).
Anal. Calc. for IrC45H49N4SO3: C, 58.86; H, 5.38; N, 6.10. Found:
C, 58.44; H, 5.28; N, 6.00%.
All complexes [Ir(ppy)2(9-fulleriden-4,5-diazafluorene)]+Yꢀ
were prepared with the same procedure described for [Ir(p-
py)2(4,5-diazafluorene)]+ Yꢀ, but using a mixture of CS2/CH2Cl2/
MeOH (2/1/0.5 v/v/v) instead of CH2Cl2/MeOH (2/1 v/v).
2. Experimental
2.1. General comments
2.2.3. Synthesis of 2a: [Ir(ppy)2(9-fulleriden-4,5-diazafluorene)]+ PF6ꢀ
A solution of [Ir(ppy)2Cl]2 (0.018 g, 0.017 mmol) and 9-fuller-
iden-4,5-diazafluorene (0.030 g, 0.034 mmol) in CS2/CH2Cl2/MeOH
(15 mL, 2/1/0.5 v/v/v) was heated under reflux. After 5–6 h, the
brown solution was cooled to room temperature, and then a
10-fold excess of ammonium hexafluorophosphate (0.056 g) was
added. The suspension was stirred for 15 min and then filtered to
remove insoluble inorganic salts. The solution was evaporated to
dryness under reduced pressure to obtain a crude brown solid.
The solid was dissolved in CH2Cl2 (5 mL) and filtered to remove
the residual traces of inorganic salts. Diethyl ether was layered
onto the dark red filtrate, and the mixture was cooled to ꢂ0 °C.
Brown plates of the desired product formed overnight.
All reagents and solvents were purchased from Sigma–Aldrich,
while IrCl3 hydrate was purchased from Engelhard. The cyclomet-
allated Ir(III) chloro-bridge dimer [Ir(ppy)2Cl]2 was prepared
according to the literature [11]. The 4,5-diazafluorene was com-
mercially available whereas 9-fulleriden-4,5-diazafluorene was
synthesized as previously reported [12].
All reactions were carried out under nitrogen. 1H NMR spectra
were obtained on a Bruker Avance 400 MHz instrument. Elemental
analyzes were carried out in the Dipartimento di Chimica Inorga-
nica, Metallorganica e Analitica ‘‘Lamberto Malatesta’’ of Milan
University.
2.2. Synthesis of cationic Ir(III) complexes
(Yield 75%). 1H NMR (400 MHz, CD2Cl2/CS2) d ppm: 9.15 (d,
J = 8.1 Hz, 2H), 8.06 (t, J1,2 = 7.3 Hz, 4H), 7.91 (m, 4H), 7.83 (m,4H),
7.17 (m, 4H), 7.06 (dt, J1 = 1.3, J2 = 7.5 Hz, J3 = 7.5 Hz, 2H), 6.53
(dd, J1 = 0.6, J2 = 7.6 Hz, 2H).
All [Ir(ppy)2L][PF6] cationic complexes were prepared with the
same procedure, slightly modified with respect to the literature
[2], described below in the case of [Ir(ppy)2(4,5-diazafluo-
Anal. Calc. for IrC93H22N4PF6: C, 72.89; H,1.45; N, 3.66. Found: C,
72.67; H, 1.46; N, 3.70%.
rene)]+PF6 (1a).
-
2.2.1. Synthesis of 1a: [Ir(ppy)2(4,5-diazafluorene)]+ PF6ꢀ
2.2.4. Synthesis of 2b: [Ir(ppy)2(9-fulleriden-4,5-diazafluorene)]+
A solution of [Ir(ppy)2Cl]2 (0.100 g, 0.093 mmol) and 4,5-diaza-
fluorene (0.031 g, 0.187 mmol) in CH2Cl2/MeOH (15 mL, 2/1 v/v)
was heated under reflux. After 5–6 h, the green solution was cooled
to room temperature, and then a 10-fold excess of ammonium
hexafluorophosphate (0.304 g) was added. The suspension was
stirred for 15 min and then filtered to remove insoluble inorganic
salts. The solution was evaporated to dryness under reduced pres-
sure to obtain a crude green solid. The solid was dissolved in
CH2Cl2 (5 mL) and filtered to remove the residual traces of inor-
ganic salts. Diethyl ether was layered onto the filtrate, and the mix-
C12H25SOꢀ
3
A solution of [Ir(ppy)2Cl]2 (0.018 g, 0.017 mmol) and 9-fuller-
iden-4,5-diazafluorene (0.030 g, 0.034 mmol) in CS2/CH2Cl2/MeOH
(15 mL, 2/1/0.5 v/v/v) was heated under reflux. After 5–6 h, the
brown solution was cooled to room temperature, and then a
10-fold excess of Sodium dodecansulfonate (0.092 g) was added.
The suspension was stirred for 15 min and then filtered to remove
insoluble inorganic salts. The solution was evaporated to dryness
under reduced pressure to obtain a crude brown solid. The solid
was dissolved in CH2Cl2 (5 mL) and filtered to remove the residual

74
C. Dragonetti et al. / Inorganica Chimica Acta 382 (2012) 72–78
traces of inorganic salts. Diethyl ether was layered onto the dark
red filtrate, and the mixture was cooled to ꢂ0 °C. Brown plates of
the desired product formed overnight.
chromatography (silica gel, hexane/AcOEt 6/4) to give the product
as a brown oil.
(Yield 33%).1H NMR (200 MHz; CDCl3): d ppm 8.42 (1H, d,
J = 2.2 Hz) 8.31 (1H, dd, J1 = 4.8 Hz, J2 = 1.6 Hz) 7.93 (1H, dd,
J1 = 8.1 Hz, J2 = 1.3 Hz) 7.85 (1H, dd, J1 = 7.8 Hz, J2 = 1.3 Hz) 7.66
(2H, m) 7.45 (1H, ddd, = 6.7 Hz, J2 = 4.8 Hz, J3 = 2.2 Hz) 7.22 (1H,
m) 6.44 (1H,s).
(Yield 72%).1H NMR (400 MHz, CD2Cl2/CS2) d ppm: 9.17 (d,
J = 8.2 Hz, 2H), 8.06 (dd, J1 = 7.1 Hz, J2 = 8.1 Hz, 4H), 7.92 (m, 4H),
7.83 (m, 4H), 7.18 (m, 4H), 7.06 (dt, J1 = 1.2, J2 = 7.5 Hz, J3 = 7.5 Hz,
2H), 6.53 (d, J1 = 7.3 Hz, 2H), 2.64 (m, 2H), 1.58 (m, 4H), 1.30 (m,
16H), 0.91 (t, J = 6.8, 3H). Anal. Calc. for IrC105H47N4SO3: C, 77.00;
H, 2.89; N, 3.42. Found: C, 76.89; H, 3.00; N, 3.48%.
2.3.2. Synthesis of o-nitro-benzo-3-pyridone (b)
Activated MnO2 (45 g) was added, under vigorous stirring, to a
solution of the 3-pyridine-o-nitrophenyl-carbinol (29 g) in CH2Cl2
(180 ml). After 6 days MnO2 was removed from the reaction mix-
ture by filtration on Celite. The filtrate were collected and washed
several times with water. The organic phase was dried over Na2SO4
and the solvent removed at reduced pressure to give the desired
product as a yellow solid.
2.3. Synthesis of the 9-fulleriden-4-azafluorene ligand
The 9-fulleriden-4-azafluorene ligand was synthesized in two
steps, isolating all the reaction intermediates: (1) synthesis of
4-aza-fluoren-9-one, following
a procedure slightly modified
respect to that reported in literature [14], see Scheme 1A; (2)
synthesis of 9-fulleriden-4-azafluorene, following a procedure
already published for a similar ligand [15], see Scheme 1B.
(Yield 80%).1H NMR (200 MHz; CDCl3): d ppm 8.78 (2H, m) 8.25
(1H, d, J = 5.5 Hz) 8,13 (1H, d, J = 5.3 Hz) 7.81 (1H, m) 7.74 (1H, m)
7.47 (2H, m).
2.3.1. Synthesis of 3-pyridine-o-nitrophenyl-carbinol (a)
2.3.3. Synthesis of o-amino-benzo-3-pyridone (c)
A solution of 3-bromopyridine (9.15 ml) in dry diethylether
(70 ml) was dropped at -70 °C, under nitrogen atmosphere, into a
stirred solution of BuLi in hexane (1.6 M, 80 ml). After 15 min a
solution of o-nitrobenzaldehyde (14.5 g) in diethylether (100 ml)
was added and the mixture was stirred for 2 h at ꢀ70 °C, then
12 h at room temperature. The reaction mixture was poured into
cold water and extracted from with ethyl acetate. The organic
phase was separated, dried over Na2SO4 and the solvent removed
at reduced pressure. The crude residue was purified by
A suspension of o-nitro-benzo-3-pyridone (2.5 g) in ethanol
(90 ml) was hydrogenated at atmosphere pressure with 5% Pd/C
(600 mg) as catalyst. The mixture was filtered to remove the cata-
lyst and the solvent removed at reduced pressure to give a solid
which was purified by chromatography (silica gel, toluene/MeOH
9/1); the product was obtained as a yellow solid.
(Yield 70%).1H NMR (200 MHz; CDCl3): d ppm 8.80 (2H, m) 7.94
(1H, dd, J1 = 4.8 Hz, J2 = 1.6 Hz) 7.35 (3H, m) 6.74 (1H, d, J = 5.6 Hz)
6.61 (1H, t, J = 5.4 Hz) 6.19 (2H, s).
(A)
CHO
H
OH
Br
nBuLi
Et2O
NO2
(a)
O2N
N
N
Et2O
33 %
CH2Cl2
MnO2
O
O
O
NaNO2
Cu
H
2 Pd/C 5%
EtOH
(b)
N (d)
(c)
H2N
N
O2N
N
80 %
40 %
70 %
N
N
NH2
O
N
+
(B)
N2H4
H+ cat.
EtOH
MnO2
-
KOH Et2O
N
N
N
(d)
(e)
(f)
76 %
84 %
C60
toluene
(g)
30 %
N
Scheme 1. (A) Synthesis of 4-azafluore-9-one; (B) synthesis of 9-fulleriden-4-azafluorene.

C. Dragonetti et al. / Inorganica Chimica Acta 382 (2012) 72–78
75
2.3.4. Synthesis of 4-aza-fluoren-9-one (d)
(Yield 66%). 1H NMR (400 MHz, CD2Cl2) d ppm: 9.35 (ddd,
J1 = 0.68, J2 = 1.45, J3 = 5.81 Hz, 2H), 7.96 (d, J = 7.79 Hz, 3H), 7.86
(ddd, J1 = 1.56, J2 = 7.43, J3 = 8.15 Hz, 3H), 7.66 (dd, J1 = 1.19,
J2 = 7.75 Hz, 3H), 7.30 (ddd, J1 = 1.50, J2 = 5.85, J3 = 7.36 Hz, 3H),
6.90 (ddd, J1 = 1.21, J2 = 7.51, J3 = 8.43 Hz, 3H), 6.77 (dt, J1 = 1.35,
J2 = 7.48 Hz, J3 = 7.38 Hz, 3H), 6.35 (dd, J1 = 0.77, J2 = 7.65 Hz, 2H).
Anal. Calc. for IrC94H22N3: C, 81.49; H,1.60; N,3.03. Found: C,
81.45; H, 1.59; N, 3.09%.
A solution of NaNO2 (1 g) in H2O (8 ml) was added at 0 °C to a
solution of o-amino-benzo-3-pyridone (2.5 g) in a 10 % H2SO4 solu-
tion (100 mL). After 15 min, urea (1 g) and Cu (2 g) were added in
one portion and the mixture stirred for 1 h at 0 °C. The salts were
removed by filtration and the filtrate was made alkaline with a
NH3 concentrated solution, then extracted with CH2Cl2 (500 mL).
The organic phase was washed with a 5% NaOH solution, then
dried over Na2SO4 and the solvent removed at reduced pressure
to give a residue which was purified by column chromatography
(silica gel, 95/5 toluene/MeOH) to give the desired product.
(Yield 40%). 1H NMR (200 MHz; CDCl3): d ppm 8.61 (1H, dd,
J1 = 4.2 Hz, J2 = 1.6 Hz) 7.87 (2H, m) 7.72 (1H, d, J = 7.3 Hz) 7.60
(1H, t, J = 7.5 Hz) 7.43 (1H, t, J = 7.4 Hz) 7.21 (1H, m).
2.5. PGSE (Pulsed field Gradient Spin Echo) NMR measurements
1H and 19F PGSE NMR measurements were performed by using
the standard stimulated echo sequence on a Bruker AVANCE DRX
400 spectrometer equipped with a GREAT 1/10 gradient unit and
a QNP probe with a Z-gradient coil, at 296 K without spinning.
The dependence of the resonance intensity (I) on a constant wait-
ing time and on a varied gradient strength (G) is described by the
following equation:
2.3.5. Synthesis of 4-aza-9-fluorenylhydrazone (e)
Hydrazine hydrate (70 ll) and two drops of acetic acid were
added to a stirred solution of 4-aza-fluoren-9-one (130 mg) in
EtOH (30 mL). The mixture was refluxed for 10 h. The mixture
was cooled at room temperature and poured into CH2Cl2
(100 mL), then washed with a saturated NaHCO3 solution until
neutral pH. The organic phase was dried over Na2SO4 and the sol-
vent removed at reduced pressure to give the desired product as a
yellow solid.
ꢀ
ꢁ
I
I0
d
2
ln ¼ ꢀð
c
dÞ Dt
D
ꢀ
G2
3
where I = intensity of the observed spin echo, I0 = intensity of the
spin echo in absence of gradient, = delay between the midpoints
of the gradients, d = length of the gradient pulse, and = magneto-
D
(Yield 76%). 1H NMR (200 MHz; CDCl3): d ppm 8.58 (1H, d;
J = 2.9 Hz) 8.13 (1H, d, J = 2.9 Hz) 7.97 (1H, d, J = 2.7 Hz) 7.85 (1H,
d) 7.76 (1H, d) 7.40 (5H, m) 7.20 (1H, m).
c
gyric ratio. The shape of the gradients was rectangular, their dura-
tion (d) was 4–5 ms and their strength (G) was varied during the
experiments. All the spectra were acquired using 32 K points and
a spectral width of 5000 (1H) and 18000 (19F) and were processed
with a line broadening of 1.0 (1H) and 1.5 (19F). The semilogarithmic
plots of ln (I/I0) versus G2 were fitted using a standard linear regres-
sion algorithm, and an R factor better than 0.99 was always ob-
2.3.6. Synthesis of 4-aza-9-diazofluorene (f)
Na2SO4 (165 mg), MnO2 supported on silica (800 mg) and a sat-
urated KOH solution in EtOH (300 lL) were added to a stirred solu-
tion of 4-aza-9-fluorenylhydrazone (105 mg) in Et2O (20 ml). After
17 h, the MnO2 was removed by filtration and the solvent removed
at reduced pressure to give the desired product as a bright orange
solid.
tained. Different values of
D, number of transients and number of
different gradient strengths (G) were used for different samples.
The self-diffusion coefficient Dt, that is directly proportional to
the slope of the regression line obtained by plotting ln(I/I0) vs.
G2, was estimated by measuring the proportionality constant using
a sample of HDO (5%) in D2O (known diffusion coefficients in the
range of 274–318 K) [17 in the same exact conditions as the sam-
ple of interest using the solvent (CHDCl2) as internal standard. Dt
data were treated as described in the literature [18]. The measure-
ment uncertainty was estimated by determining the standard
(Yield 84%). 1H NMR (200 MHz; CDCl3) d ppm 8.552 (1H, d) 7.28
(1H, d) 7.85 (1H, d) 7.71 (1H, m) 7.50 (2H, m) 7.38 (1H, m). IR
(CHCl3): 2260 cmꢀ1 stretching N2.
2.3.7. Synthesis of 9-fulleriden-4-azafluorene (g)
A
mixture of 9-diazo-4-azafluorene (84 mg) and fullerene
(630 mg) in toluene (450 mL) was stirred vigorously for 4 days.
The solvent was removed at reduced pressure to give a residue
which was purified by column chromatography (silica gel, 9/1 tol-
uene/methanol).
deviation of m by performing experiments with different
D values.
Standard propagation of error analysis yielded a standard deviation
of approximately 3–4% in the hydrodynamic radius.
(Yield 30%). 1H NMR (200 MHz; CDCl3/CS2): d ppm 9.05 (1H, d,
J = 8.1 Hz) 8.81 (2H, m) 8.39 (1H, d, J = 9 Hz) 7.65 (2H, m) 7.41 (1H,
m) 7.22 (2H, m).
3. Results and discussion
3.1. Synthesis of ligands and complexes
2.4. Synthesis of 3: [Ir(ppy)2 (9-fulleriden-4-monoazafluorene)]
The synthesis of the new complex 3 followed a procedure pre-
All complexes are easily prepared starting from cyclometallated
Ir(III) chloro-bridge dimer [Ir(ppy)2Cl]2 synthesized according to
the literature [11]. The [Ir(ppy)2L][Y] (Y = PF6, C12H25SO3) cationic
complexes (1a, 1b, 2a and 2b) are obtained by bridge splitting
reaction [13,2]. The ligand L = 9-fulleriden-4,5-diazafluorene can
be prepared as previously reported [12], whereas the novel
interesting ligand 9-fulleriden-4-monoazafluorene is obtained in
two steps, isolating all the reaction intermediates: (1) synthesis
of 4-aza-fluoren-9-one, following a procedure slightly modified
with respect to that reported in literature [2], see Scheme 1A; we
have used milder conditions in particular during the alcohol
oxidation and the reduction of the nitro group. (2) Synthesis of
9-fulleriden-4-azafluorene, following a procedure already pub-
lished for a similar ligand (cyclopentadithiophene) [15] (see
Scheme 1B and Section 2). Then the related Ir(III) neutral complex
viously reported for
phenyl)pyridine)] [17].
a similar complex [Ir(ppy)2(2-(4-nitro-
A mixture of [Ir(ppy)2Cl]2 (0.036 g, 0.033 mmol) and 9-fuller-
iden-4-monoazafluorene (0.0594 g, 0.067 mmol), and AgO2CCF3
(0.0148 g, 0.067 mmol) in 30 mL of 2-methoxyethanol was stirred
and refluxed in the dark for 6 h under nitrogen. The mixture was
cooled to room temperature and the precipitated AgCl was sepa-
rated from the resulting dark brown solution by filtration. The sol-
vent was completely removed in vacuo. The remaining residue was
dissolved in 5 mL of CH2Cl2 and the crude product crystallized by
layering with ethanol overnight. The solid was separated from
the solution, dissolved in 10 mL of CH2Cl2 and chromatographed
on alumina column with CH2Cl2 as eluent.

76
C. Dragonetti et al. / Inorganica Chimica Acta 382 (2012) 72–78
Table 1
systems like [Ir(ppy)2(1,10-Phen)][Y] [7]. The dissociation degree
) for 1a is easily obtained from the equation:
UV spectra in CH2Cl2 solution.
(a
Complexes
Abs max (nm) [e
(Mꢀ1 cmꢀ1)]
VHꢀ
¼
a
V0Hꢀ þ ð1 ꢀ
a
ÞV0HIP
1a [Ir(ppy)2(4,5-diazafluorene)]+PF6ꢀ
256 [78323]
265 [136801]
307 [58678] sh
366 [14855] vbr vw
246 [67517]
261 [66051]
305 [38275] sh
376 [10565] vbr vw
256 [607570] 316
[209360]
256 [257871] 321
[73601]
where VꢀH is the measured hydrodynamic volume and V0HIP is the
volume of the ion pair. V0Hꢀ of PF6ꢀ is also known (74 Å3 [22])
consequently the percentage of free ions was determined. As far
as the fulleridene complexes are concerned, free ions are present
1b [Ir(ppy)2(4,5-diazafluorene)]+C12H25SO3ꢀ
even at the highest concentration both with PFꢀ6 and C12H25SOꢀ
3
anions, so it is not possible to directly extrapolate V0HIP. By the
way, since for 2a VH0þ>>VH0ꢀ, VH0IP was approximated to be equal to
2a [Ir(ppy)2(9-fulleriden-4,5-
diazafluorene)]+PF6ꢀ
V0Hþ. For the complex 2b VH0IPꢀ was calculated by adding the V0Hꢀ of
2b [Ir(ppy)2(9-fulleriden-4,5-
diazafluorene)]+C12H25SO3ꢀ
+
C
12H25SOꢀ to the approximate value of VH derived for 2a.
3
3
[Ir(ppy)2(9-fulleriden-4-monoazafluorene)] 247 [313120]
277 [272990]
Hydrodynamic data reported in Table 2 show that
a is strongly
influenced by the nature of both the anion and cation. In the case of
complex 1a at 0.8 mM, the ion pairs amount to ca. half of the total
[Ir(ppy)2(9-fulleriden-4-monoazafluorene)] 3 is formed following a
procedure described in literature [16] for a similar complex.
The relevant bands of the electronic absorption spectra of the
various complexes are shown in Table 1.
ionic species, while for 1b at 2.3 mM
from 1 to 2 the dissociation degree increases: for 2a at 0.8 mM
a is lower than 5%. Passing
a
is 0.91 while for 2b its value is 0.74. It can be noted that the
percentage of free ions is higher when that least ‘‘sticky’’ anion
(PFꢀ6 ) is used [7a]. The introduction of the fullerene weakens the
interaction forces between the anion and cation within the ion pair
and, consequently, leads to an increase of the percentage of free
ions. This can be explained considering, on one hand, that the
methylene moiety of the unsubstituted diazafluorene is quite
acidic and easily approachable by the anion; on the other hand,
the introduction of the bulky fullerene inhibits to the anion to
closely approach the positive charge on the cation.
All complexes display strong absorption bands at about
k = 250 nm, that can be attributed to
transitions of the systems of the N,N ligand and of the cyclomet-
p–
p⁄ ligand-centered (LC)
p
allated ligands; weaker absorption bands at longer wavelengths
(above k = 300 nm) can be attributed to metal-to-ligand charge-
transfer transitions (MLCT), mainly from the Ir(III) metal center
to vacant p⁄ orbitals of the N,N ligand, making a comparison with
similar complexes [2].
When a C60 fullerene moiety is linked to the 4,5-diazafluorene
ligand (2a and 2b), the bands, in particular those at about 250 nm,
become strong due to the presence of the fullerene moiety.[19].
3.3. Study of the second-order NLO properties
The second-order NLO properties were determined by the EFISH
technique [8] working with a non-resonant 1.907
lm wavelength
in CH2Cl2 solution (10ꢀ3 M). The EFISH
l
b1907 values of the synthe-
3.2. Diffusion NMR studies
sized complexes are shown in Table 3, together with the percent-
age of free ions obtained as described above using 1H and 19F
pulsed field gradient spin echo (PGSE) diffusion NMR techniques
[23].
Diffusion NMR measurements [20] were performed for 1 and 2
complexes with both PFꢀ6 and C12H25SOꢀ anions in CD2Cl2 (used as
3
internal standard, see Section 2) and they allowed to obtain the
translational self-diffusion coefficient (Dt) for the cation (Dþ) and
t
All complexes show a negative large value of
l
b1907 (ꢀ600 to
the anion (Dꢀt ) in an independent way (Table 2). The average
hydrodynamic radius (rH) and, consequently, the hydrodynamic
volume of the diffusing species (VH) were derived through the
Stokes–Einstein equation [18] in the assumption that they had a
spherical shape. If only the equilibrium between free ions and
ion pairs is present in solution, the dissociation degree can be
derived from a single PGSE measurement [21].
ꢀ2190 ꢁ 10ꢀ48 esu), higher than that reported for Disperse Red
One, which has already been widely used as NLO chromophores.
The negative sign of
the dipole moment in the excited state due to MLCT transitions,
which dominate the second order NLO response [2]. The b1907
lb1907 is in agreement with a reduction of
l
EFISH values are in perfect agreement with those of similar cyclo-
metallated cationic Ir(III) complexes with substituted phenanthro-
lines [7a] or bipyridines [7b].
The hydrodynamic volume of 1+ was determined from PGSE
measurement of 1b at 2.3 mM concentration, where mainly ion
Analyzing in detail the cationic complexes lb1907 EFISH values,
pairs are present, by subtracting the known volume C12H25SOꢀ
3
it can be pointed out that, as expected taking into account the
(V0Hꢀ = 346 Å3 [7]) to the measured value of VH+. The obtained value
(V0Hꢀ = 890 Å3) is in agreement with the results obtained for similar
percentage of free ions [7a], complex 1a with the PFꢀ6 counter anion
shows a rather higher
complex 1b with
lb1907 absolute value with respect to
C
12H25SOꢀ (ꢀ1640 and ꢀ1035 ꢁ 10ꢀ48 esu,
3
respectively), and the same trend, is found for complexes 2a and
2b (ꢀ2190 and ꢀ1900 ꢁ 10ꢀ48 esu, respectively). It should be
noted that for complexes [Ir(ppy)2(5-Me-1,10-phen)][PF6] and
Table 2
Diffusion coefficients (1010 Dt, m2 sꢀ1), hydrodynamic radius (rH, Å), hydrodynamic
volume (VH, Å3) and fraction of dissociated ion-pairs (
2b as a function of concentration (C, mM).
a) in CD2Cl2 for 1a, 1b, 2a and
[Ir(ppy)2(5-Me-1,10-phen)][ C12H25SOꢀ] the
lb1907 values
3
measured are ꢀ1565 and ꢀ1350 ꢁ 10ꢀ48 esu, respectively, in
Dtþ
VHþ
918
913
rHþ
C
Dtꢀ
rHꢀ
VHꢀ
454
a
analogy with the values of 1a and 1b.
1a
0.8
9.2
9.3
9.3
12.4
11.2
6.03
6.02
4.77
5.17
0.55
0.37
As evidenced in Table 3, the absolute EFISHlb1907 values of
579
complexes with diazafluorene (1a and 1b) are lower than that of
the related complexes with the fullerene moiety (2a and 2b). This
increase of the second-order NLO response in the presence of a
fullerene moiety can be reasonably attributed to the increase of
the percentage of free ions. In fact, in this kind of cationic
cyclometallated phenylpyridine Ir(III) complexes [7a], it is known
1b
2a
2.3
8.2
7.7
6.65
7.06
1236
1473
60.05
0.8
6.8
6.5
6.7
15.5
10.8
8.25
7.95
4.05
5.34
2351
2104
278
637
0.91
0.72
2b
1.0
7.0
6.8
6.8
9.4
7.6
7.97
7.87
6.03
7.14
2195
2041
918
1524
0.74
0.44

C. Dragonetti et al. / Inorganica Chimica Acta 382 (2012) 72–78
77
Table 3
EFISH
4. Conclusions
l
b1907 values in CH2Cl2 (10ꢀ3 M) of the synthesized complexes.
Complex
l
b
% Of
free
ions
We prepared novel cationic cyclometallated Ir(III) complexes
bearing diaza- or monoazafluorene, substituted or not with a
C60-fullerene moiety, and the related Ir(III) neutral complex with
the new ligand 9-fulleriden-4-monoazafluorene.
(ꢁ10ꢀ48 esu
in CH2Cl2)
+
ꢀ1640
57
All complexes show significant second-order NLO properties, as
-
PF6
determined in 10ꢀ3 M CH2Cl2 solution with the EFISH technique,
N
N
1a
with a negative large absolute value of
l
b1.907 (ꢀ600 to ꢀ2190 ꢁ
Ir
10ꢀ48 esu). In cationic complexes, a key role is played by the fuller-
ene moiety which weakens the interaction forces between the an-
ion and cation within the ion pair and, consequently, leads to an
increase of NLO response. In fact, diffusion NMR measurements
showed that the dissociation degree is critically affected by the
nature of both the anion and cation; it increases (i) passing from
N
2
+
ꢀ1035
ꢀ2190
<5
-
C12H25SO3
N
N
12H25SOꢀ to PFꢀ6 and (ii) with the introduction of a fullerene moi-
1b
C
Ir
3
ety in the diazafluorene ligand.
It appeared that for the neutral complex 3, [Ir(ppy)2(9-fuller-
iden-4-monoazafluorene)], the EFISHlb1907 value is lower than
that of cationic Ir(III) complexes, but comparable to the values of
other neutral Ir(III) complexes such as [Ir(ppy)2(RCOCR0COR)]
(ppy = cyclometallated 2-phenylpyridine; R = Me, Ph; R0 = H, 2,4-
dinitrophenyl), confirming that the second-order NLO response of
these neutral chromophores is dominated by ILCT processes con-
cerning the cyclometallated 2-phenylpyridines [24].
N
2
91
+
-
PF6
N
N
Ir
N
2
2a
Acknowledgments
ꢀ1901
ꢀ601
70
+
-
N
N
C12H25SO3
N
We deeply thank Professors Renato Ugo, Dominique Roberto
and Francesco Sannicolò (Università degli Studi di Milano) and
Tiziana Benincori (Università dell’Insubria) for helpful and fruitful
discussions This work was supported by MIUR (FIRB 2004:
RBPR05JH2P and PRIN 2008: 2008FZK5AC_002).
Ir
2b
2
—
N
N
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