The role of 5-R-1,10-phenanthroline (R = CH3, NO2) on the emission properties and second-order NLO response of cationic Ir(III) organometallic chromophores

Article abstract of DOI:10.1016/j.ica.2008.03.051

[Ir(cyclometallated 4,5-diphenyl-2-methyl-thiazole)₂(5-R-1,10-phenanthroline)][PF₆] (R = CH₃, NO₂) complexes were prepared and fully characterized, the structure of the complex with 5-CH₃-1,10-phenanthroline being also determined by X-ray diffraction. The emission properties of both complexes have been investigated and their second-order nonlinear optical (NLO) response has been determined experimentally by the EFISH technique and found to be similar but slightly lower than that of related [Ir(ppy)₂(5-R-1,10-phenanthroline)][PF₆] (ppy = cyclometallated 2-phenylpyridine), characterized by one of the highest second-order NLO response ever reported for a metal complex. In the complexes, SOS/TDDFT calculations show that the large and negative sign of the measured hyperpolarizability is mainly due to the significant contribution of rather intense MLCT transitions involving the phenanthroline as acceptor ligand.

Full text of DOI:10.1016/j.ica.2008.03.051

Inorganica Chimica Acta 361 (2008) 4070–4076
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The role of 5-R-1,10-phenanthroline (R = CH3, NO2) on the emission properties
and second-order NLO response of cationic Ir(III) organometallic chromophores
a,c
Claudia Dragonetti a,c, Stefania Righetto a,c, Dominique Roberto a,c,d, Renato Ugo a,c,d, , Adriana Valore
,
*
Francesco Demartin b, Filippo De Angelis e, Antonio Sgamellotti e, Simona Fantacci e,
*
a Dipartimento di Chimica Inorganica, Metallorganica e Analitica ‘‘Lamberto Malatesta”, Università di Milano, Via Venezian 21, I-20133, Milano, Italy
b Dipartimento di Chimica Strutturale e Stereochimica Inorganica dell’Università di Milano, Via Venezian 21, I-20133, Milano, Italy
c UdR dell’INSTM di Milano, Via Venezian 21, I-20133, Milano, Italy
d ISTM-CNR, via Venezian 21, Via Venezian 21, I-20133, Milano, Italy
e ISTM-CNR and UdR dell’INSTM di Perugia, Dipartimento di Chimica, 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:
[Ir(cyclometallated 4,5-diphenyl-2-methyl-thiazole)2(5-R-1,10-phenanthroline)][PF6] (R = CH3, NO2)
complexes were prepared and fully characterized, the structure of the complex with 5-CH3-1,10-phenan-
throline being also determined by X-ray diffraction. The emission properties of both complexes have been
investigated and their second-order nonlinear optical (NLO) response has been determined experimentally
by the EFISH technique and found to be similar but slightly lower than that of related [Ir(ppy)2(5-R-1,10-
phenanthroline)][PF6] (ppy = cyclometallated 2-phenylpyridine), characterized by one of the highest sec-
ond-order NLO response ever reported for a metal complex. In the complexes, SOS/TDDFT calculations
show that the large and negative sign of the measured hyperpolarizability is mainly due to the significant
contribution of rather intense MLCT transitions involving the phenanthroline as acceptor ligand.
Ó 2008 Elsevier B.V. All rights reserved.
Received 28 January 2008
Received in revised form 9 March 2008
Accepted 11 March 2008
Available online 16 March 2008
This work is dedicated to Prof. Dante
Gatteschi and to his outstanding
contribution to the modern magnetic
aspects of coordination chemistry.
Keywords:
Emission
Nonlinear optics
Cyclometallated iridium complexes
Phenanthroline complexes
DFT/TDDFT calculations
X-ray diffraction
1. Introduction
metallated 2-phenylpyridine) are characterized by interesting
luminescence properties [4] and large second-order NLO response
Organometallic complexes with luminescent [1] and second-or-
der nonlinear optical (NLO) [2] properties are of interest as molec-
ular building blocks of composite materials for light emitting
systems such as OLED or for electrooptical devices and optical
communications. In particular, they can offer additional flexibility,
when compared to organic NLO-phores, by introducing electronic
charge-transfer transitions between the metal and the ligand, usu-
ally at relatively low energy and of relatively high intensity, tun-
able by virtue of the nature, oxidation state and coordination
sphere of the metal centre [3].
[5], as determined by the electric field induced second harmonic
generation method (EFISH) [6]. Good transparency towards the
second harmonic emission renders these NLO-phores appealing
as building blocks for composite second-order NLO materials [5].
The electronic origin of the second-order NLO response has been
attributed by a theoretical SOS-TDDFT investigation mainly to me-
tal to ligand charge transfer (MLCT) processes from the orbitals of
the ppy-Ir based moiety acting as donor system to p* orbitals of the
coordinated phenanthroline acting as acceptor system. The sec-
ond-order NLO response may be tuned by an adequate choice of
the R substituent on the phenanthroline ligand, the best response
being obtained with an electron-withdrawing substituent such as
NO2 (see complexes 1a–1b, with R = Me and NO2, respectively;
Table 1) [5]. Substitution of cyclometallated 2-phenylpyridine
with the more p-delocalized cyclometallated 2-phenylquinoline
(pq) does not affect significantly both the luminescence proper-
ties and second-order NLO response (complexes 2a–2b, with
R = Me and NO2, respectively; Table 1) whereas a slightly lower
We previously reported that some cationic Ir(III) complexes of
the type [Ir(ppy)2(5-R-1,10-phenanthroline)][PF6] (ppy = cyclo-
* Corresponding authors. Addresses: Dipartimento di Chimica Inorganica, Met-
allorganica e Analitica ‘‘Lamberto Malatesta”, Via Venezian 21, I-20133, Milano,
Italy (R. Ugo). ISTM-CNR, Via Elce di Sotto 8, I-06123, Perugia, Italy (S. Fantacci). Fax:
+39 02 50314405.
E-mail addresses: dominique.roberto@unimi.it (R. Ugo), simona@thch.unipg.it
(S. Fantacci).
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.03.051

C. Dragonetti et al. / Inorganica Chimica Acta 361 (2008) 4070–4076
4071
Table 1
Absorption and emission spectra, u and EFISH lb1907 in CH2Cl2 for complexes 1–4
a,b
Compound
Absorption max (nm) [e (Mꢀ1 cmꢀ1)]
Emission Quantum
EFISH lb1907
(10ꢀ30 D cm5 esuꢀ1
)
max
yield r
(nm)
R = Me,
1a
R = NO2,
1b
255(sh), 268[60300], 333(sh), 377[8070],
410(sh)c,d
559c
38%c
ꢀ1565e
ꢀ2230e
+
-
254(sh), 264[86900], 378[12400]c,d
<0.1%c
R
PF6
N
N
Ir
N
2
R = Me,
2a
R = NO2,
2b
273[70400], 329[20700], 347(sh),
431[4750]c,d
556c
34%c
ꢀ2090f
ꢀ1720f
+
-
268[81100], 324[27600], 430[6930]c,d
<0.1%c
R
PF6
N
N
Ir
N
2
+
R = Me,
3a
R = NO2,
3b
260 [44500], 319(sh), 396[17378]d,f
257[62300], 315[28100], 397(sh)d,f
563f
1.3%f
ꢀ1320f
ꢀ1640f
-
N
PF
6
<0.1%f
N
N
R
S
Ir
S
S
2
+
R = Me,
4a
R = NO2,
4b
261[48000], 272(sh), 358[9400]d
262[67200], 357[15200]d
586
12%
ꢀ1414
ꢀ1780
-
PF6
N
R
<0.1%
Ir
N
N
S
2
a
EFISH measurements are carried out in CH2Cl2 at 10ꢀ3 M.
the error of EFISH measurements is 10%.
Ref. [4].
There is a band tail above 400 nm up to about 500–550 nm.
Ref. [5]
Ref. [7].
b
c
d
e
f
second-order NLO response and a much poorer luminescence was
reported for the related Ir(III) complexes with the more p delocal-
ized donor system of the cyclometallated 30-(2-pyridil)-2,20:50,200-
terthiophene (ttpy) (complexes 3a–3b, with R = Me and NO2,
respectively; Table 1) [7]. In the present paper we extend our
investigation to the emission properties and second-order NLO re-
sponse of the series [Ir(cyclometallated 4,5-diphenyl-2-methyl-
thiazole)2(5-R-1,10-phenanthroline)][PF6] (R = CH3, NO2; com-
plexes 4a–4b).
chloro-bridge dimer [Ir(cyclometallated 4,5-diphenyl-2-methyl-
thiazole)2Cl2] was prepared following the procedure reported for
[Ir(cyclometallated 2-phenylpyridine)2Cl]2 [8] whereas the com-
plexes [Ir(cyclometallated 4,5-diphenyl-2-methyl-thiazole)2(5-R-
1,10-phenanthroline)][PF6] (R = Me, NO2) were prepared with a
procedure similar to that reported for this kind of cationic com-
plexes [4,7]. The detailed synthetic procedure are reported below.
All reactions were carried out under nitrogen atmosphere. 1H NMR
spectra were obtained on a Bruker Avance DRX 300 MHz instru-
ment. Elemental analyses were carried out in the Dipartimento di
Chimica Inorganica, Metallorganica e Analitica ‘‘Lamberto Malates-
ta” of the Milan University. EFISH measurements [6] were
carried out in the same Department working in CH2Cl2 (10ꢀ3 M)
at a non-resonant incident wavelength of 1.907 lm, using a
Q-switched, mode-locked Nd3+:YAG laser manufactured by
Atalaser equipped with a Raman shifter [5,7].
2. Experimental
2.1. General information
All reagents and solvents were purchased from Sigma–Aldrich,
while IrCl3 tri-hydrate was purchased from Engelhard. The Ir(III)

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C. Dragonetti et al. / Inorganica Chimica Acta 361 (2008) 4070–4076
2.2. [Ir(cyclometallated 4,5-diphenyl-2-methyl-thiazole)2Cl2]
2.5. Photophysical measurements
Iridium trichloride tri-hydrate (0.500 g) was combined with
4,5-diphenyl-2-methyl-thiazole (0.848 g; 3.35 mmol), dissolved
in a mixture of 2-methoxyethanol (30 mL) and water (10 mL),
and refluxed for 24 h. The solution was cooled to room tempera-
ture, and the yellow precipitate was filtered under nitrogen atmo-
sphere. The precipitate was washed with 95% ethanol (60 mL) and
acetone (60 mL) and then dissolved in dichloromethane (75 mL)
and filtered. The desired product was obtained evaporating the
solution to dryness (0.982 g, 80% yield). 1H NMR (300 MHz, DMSO)
7.58 (s, 10H), 6.77 (d, J = 6.55, 1H) 6.59 (t, J = 8.57, 1H), 6.47 (t,
J = 7.64, 1H), 6.17 (d, J = 7.69, 1H), 3.31 (s, 3H). Anal. Calc. for
Ir2C64H48N4S4Cl2: C, 52.77; H, 3.32; N, 3.85. Found: C, 52.80; H,
3.33; N, 3.80%.
UV spectra were recorded at room temperature in a 1 cm quartz
cuvette using a Jasco V-570 spectrometer. Emission spectra were
recorded using a Jobin-Yvon Fluorolog-3 spectrometer equipped
with double monochromators and Hamamatsu-928 photomulti-
plier tube (PMT) as the detector. All complexes were excited at
260 nm. Emission quantum yields have been determined, using
the method of Demas and Crosby [9a] by comparison with the
emission of Coumarine 540, employed as a standard [9b]. All solu-
tions were de-aerated by nitrogen bubbling for 30 min before
measurements.
2.6. X-ray determination of the structure of [Ir(cyclometallated 4,5-
diphenyl-2-methyl-thiazole)2(5-Me-1,10-phenanthroline)][PF6] (4a)
Crystals of [Ir(cyclometallated 4,5-diphenyl-2-methyl-thia-
zole)2(5-Me-1,10-phenanthroline)][PF6], suitable for X-ray diffrac-
tion studies, were obtained by slow addition at room temperature
of pentane to its solution in CH2Cl2.
2.3. Synthesis of [Ir(cyclometallated 4,5-diphenyl-2-methyl-
thiazole)2(5-CH3-1,10-phenanthroline)][PF6] (4a)
A solution of [Ir(4,5-diphenyl-2-methyl-thiazole)2Cl2] (0.100 g,
0.0683 mmol) and 5-methyl-1,10-phenanthroline (0.0291 g,
0.1498 mmol) in CH2Cl2-MeOH (50 ml, 2:1 v/v) was heated to
reflux. After 5–6 h, the orange solution was cooled to room tem-
perature, and then a 10-fold excess of ammonium hexafluoro-
phosphate 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, affording an
orange crude solid. The solid was redissolved in CH2Cl2 and fil-
tered to remove the last traces of inorganic salts. Diethyl ether
was layered onto the orange filtrate, and the mixture was cooled
to ꢁ0 °C. Orange-plates of the desired product were formed over-
night and recovered by filtration (0.1057 g, 75% yield). 1H NMR
(300 MHz, CD2Cl2) d 8.79 (dd, J1 = 8.45, J2 = 1.35, 1H), 8.60 (dd,
J1 = 8.25, J2 = 1.35, 1H), 8.54 (dd, J1 = 5.03, J2 = 1.33, 1H), 8.46 (dd,
J1 = 8.37, J2 = 5.05, 1H), 8.06 (s, 1H), 7.95 (dd, J1 = 8.44, J2 = 5.04,
1H), 7.88 (dd, J1 = 8.25, J2 = 5.06, 1H), 7.58 (s, 10 H), 7.22 (d,
J = 7.86, 2H), 6.96 (m, 2H), 6.85 (m, 2H), 6.54 (dd, J1 = 7.56,
J2 = 3.68, 2H), 2.98 (s, 3H), 1.54 (s, 6H). Anal. Calc. for
Ir1C77H34N4S2PF6: C, 52.37; H, 3.32; N, 5.43. Found: C, 52.30; H,
3.29; N, 5.45%.
2.6.1. Crystal data
C
45H34F6IrN4PS20 ꢂ 5CH2Cl2,
M=1074.51,
monoclinic,
a =
15.083(2), b = 14.223(2), c = 21.301(3) Å, b = 99.09(1)°, U =
4512.2(11) Å3, T = 292(2) K, space group P21/n (No. 14), Z = 4,
l = (Mo Ka) 3.208 mmꢀ1
.
21036 reflections (5594 unique,
Rint = 0.086) were collected at room temperature in the range
2.00 6 2h 6 52.06°, employing
a
0.08 ꢃ 0.03 ꢃ 0.02 mm crystal
fragment mounted on a Bruker Apex II area-detector diffractome-
ter. Graphite monochromatized Mo Ka radiation (k = 0.71073 Å)
was used with the generator working at 45 kV and 40 mA.
Intensities were corrected for Lorentz-polarisation effects and
empirical absorption correction (SADABS [10]; minimum transmis-
sion factor 0.798).The structure was solved by direct methods
(
(
SIR-97 [11]) and refined on Fo2 with the SHELXL-97 [12] program
WINGX suite [13]) All non-hydrogen atoms were refined with aniso-
tropic thermal parameters, while hydrogen atoms, located on the
DF maps, were allowed to ride on their carbon atom. Final R1
[wR2] values of 0.0479 [0.1233] on 3434 reflections with I > 2r(I)
and 557 parameters and S = 1.001. The crystal examined contains
clathrated CH2Cl2 molecules showing different orientations: only
one majority model with occupancy close to 0.5 could be refined:
the maximum residual electron density on the final DF map is
0.95 e Åꢀ3 and is close to the position of the disordered solvent
molecule. The minimum residual of ꢀ0.60 e Åꢀ3 is instead close
to the iridium atom. Selected interatomic distances are reported
in Table 2.
2.4. Synthesis of [Ir(cyclometallated 4,5-diphenyl-2-methyl-
thiazole)2(5-NO2-1,10-phenanthroline)][PF6] (4b)
A solution of [Ir(cyclometallated 4,5-diphenyl-2-methyl-thia-
zole)2Cl2] (0.0697 g, 0.0476 mmol) and 5-nitro-1,10-phenanthro-
line (0.0214 g, 0.095 mmol) in CH2Cl2–MeOH (50 ml, 2:1 v/v)
was heated to reflux. After 5–6 h, the orange solution was cooled
to room temperature, and then a 10-fold excess of ammonium
hexafluorophosphate 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,
affording an orange crude solid which was redissolved in CH2Cl2
and filtered to remove the last traces of inorganic salts. Diethyl
ether was layered onto the orange filtrate, and the mixture was
cooled to ꢁ0 °C. Orange-plates of the desired product were formed
overnight and recovered by filtration (0.0778 g, 77% yield). 1H
NMR (400 MHz, CD2Cl2) 9.38 (dd, J1 = 1.25, J2 = 8.72, 1H), 9.18 (s,
1H), 8.93 (dd, J1 = 1.00, J2 = 8.15, 1H), 8.69 (ddd, J1 = 1.17,
J2 = 5.03, J3 = 16.35, 2H), 8.09 (dd J1 = 5.04, J2 = 8.75, 2H), 7.59 (d,
J1 = 1.50, 10H), 7.23 (dd, J1 = 1.10, J2 = 7.90, 2H), 6.98 (m, 2H),
6.87 (m, 2H), 6.51 (m, 2H), 2.98 (s, 6H). Anal. Calc. for Ir1C44H31N5-
S2O2PF6: C, 49.71; H, 2.94; N, 6.59. Found: C, 49.67; H, 3.18; N,
6.21%.
2.7. Computational details
The geometries of complexes 4a and 4b were optimized with-
out symmetry constraints by density functional theory (DFT) cal-
culations using the BP86 exchange–correlation function [14,15],
together with a TZP (DZP) basis set for Ir, S (N, C, O, H), including
scalar-relativistic corrections as implemented in the ADF program
[16]. On the optimized geometries, we performed single-point and
Time Dependent DFT (TDDFT) calculations at the B3LYP/LANL2DZ
level of theory [17,18] in dichloromethane solutions by means of
the polarizable continuum model (PCM) solvation model [19], as
implemented in the GAUSSIAN 03 (G03) program package [20]. Calcu-
lation of the lowest 100 singlet–singlet excitations at the ground
state optimized geometries allowed us to simulate a large (up to
below 230 nm) portion of the absorption spectrum. We computed
also the lowest 10 singlet–triplet excitations at the ground state
geometry; the oscillator strength of singlet–triplet transitions is
set to zero, due to the neglect of the spin–orbit coupling. The sim-

C. Dragonetti et al. / Inorganica Chimica Acta 361 (2008) 4070–4076
4073
Table 2
molar ratio 1:1:0.5, respectively. A perspective view of the cation
is reported in Fig. 1. The iridium atom displays a distorted octahe-
dral coordination geometry being surrounded by two cyclometal-
lated 4,5-diphenyl-2-methyl-thiazolato ligands coordinated
through the nitrogen atom of the thiazole moiety and one ortho
carbon atom of the phenyl substituent at position 5 and by the che-
lated 5-Me-1,10-phenanthroline ligand. Within the thiazolato li-
gands one of the two phenyl substituents (that at position 5) lies
essentially in the plane of the thiazole ring, whereas the plane of
the phenyl at position 4 is almost normal to the average plane of
the thiazole ring [S1–C16–C18–C19 torsion angle is ꢀ100.7(16)°
and S2–C32–C34–C35 is 89.3(16)°]. The thiazole ring in both li-
gands does not deviate significantly from planarity. The two Ir–
N1 and Ir–N2 distances of the phenanthroline ligand are signifi-
cantly larger than the Ir–N distances of the two thiazolato moieties
being trans to the C29 and C45 atoms r-linked to Ir. The average P–
F distance in the PFꢀ6 anion is 1.527 Å; P–F distances up to 1.73 Å
have been reported only when this anion strongly interacts with
its surrounding through hydrogen bonds (for instance in HPF66H2O
[23]). Here, the value of 1.527 Å is in line with the shortest dis-
tances found in well isolated PFꢀ6 anions.
The main optimized geometrical parameters of complex 4a ob-
tained by theoretical DFT calculations are reported in Table 2, to-
gether with the related data obtained by the X-ray investigations,
showing an excellent agreement between the experimental and
computed data. The computed optimized molecular structure of
complex 4b is quite similar to that computed for complex 4a, both
in terms of bond lengths and angles.
Both complexes 4a and 4b show two intense absorption bands
around 260–270 and 360 nm, respectively. This is a typical trend of
this kind of cationic Ir(III) complexes such as 1a–1b and 3a–3b
[4,5,7]. These latter show a small red-shift of the absorption at low-
er energy (from around 360 to around 395–400 nm), while, for 2a–
2b the band at lower energy shows a more pronounced red shift at
around 430 nm (see Table 1). Complex 4a shows, as all the other
Ir(III) complexes 1a, 2a, 3a containing 5-CH3-1,10-phenanthroline
[4,7], a emission around 586 nm with a quantum yield of 12%,
lower than that of 1a and 2a (see Table 1) but high enough for
the preparation of a single-layer electroluminescent device [24].
Complex 4b, as all the other Ir(III) complexes 1b, 2b, 3b
containing 5-NO2-1,10-phenanthroline [4,7], is not emissive
(Table 1).
The second-order NLO response of 4a and 4b was measured in
CH2Cl2 by the EFISH technique [6] working with a non-resonant
1907 nm incident wavelength. Comparison of the lb values of
4a–4b with those of related cationic NLO-phores 1a, 2a, 3a–1b,
2b, 3b (Table 1) evidences that the cyclometallated 4,5-diphenyl-
2-methyl-thiazole leads to a second-order NLO response slightly
higher than that reported for the related complexes with the
cyclometallated 30-(2-pyridil)-2,20:50,200-terthiophene (complexes
3a–3b) but still slightly lower than those reported in the case of
cyclometallated 2-phenylpyridine (complexes 1a–1b).
To provide insights into the effects of the different nature of the
cyclometallated ligands on the electronic transitions controlling
the second-order NLO properties of this kind of complexes and in
particular of 4a–4b, we performed DFT/TDDFT calculations. The
orbital energy levels of complexes 1a–1b, 2a–2b, 3a–3b, 4a–4b,
obtained at a consistent level of theory, are reported in Fig. 2, to-
gether with the density plots of selected molecular orbitals of 4a
and 4b. The highest occupied molecular orbital (HOMO) of both
4a and 4b is, as previously found for 1a, 2a, 3a and 1b, 2b, 3b
[4,5,7] and related complexes [25–27], a t2g(Ir) orbital mixed with
a p orbital of the cyclometallated ligand (4,5-diphenyl-2-methyl–
thiazole in the present case). In particular, assuming that the phe-
nanthroline ligand is lying in the xy plane, with the x axis pointing
from the metal to the ligand, the HOMO is originated from the dxy
Selected interatomic distances (Å) and angles (°) for compound 4a, together the
corresponding optimized values
X-ray
Theor.
X-ray
Theor.
Ir–N1
Ir–N2
Ir–N3
Ir–N4
Ir–C29
Ir–C45
N3–C15
N3–C17
C17–C24
C24–C29
C15–S1
S1–C16
C16–C17
P–F1
2.161(9)
2.149(10)
2.049(10)
2.050(9)
2.010(11)
2.012(11)
1.322(14)
1.398(14)
1.468(16)
1.425(16)
1.682(13)
1.728(13)
1.365(16)
1.542(10)
1.545(10)
1.539(13)
89.7(4)
81.3(5)
93.8(4)
94.8(4)
85.4(4)
172.1(4)
97.9(4)
76.5(4)
116.2(12)
113.3(8)
114.0(11)
114.5(9)
114.7(12)
91.3(7)
114.3(8)
114.0(11)
112.1(10)
110.1(9)
113.1(9)
2.173
2.170
2.071
2.072
2.017
2.018
1.327
1.401
1.453
1.418
1.725
1.742
1.374
N1–C5
C5–C9
N2–C9
1.348(14)
1.412(16)
1.341(14)
1.341
1.412
1.348
N4–C31
N4–C33
C33–C40
C40–C45
C31–S2
S2–C32
C32–C33
P–F4
1.329(14)
1.410(14)
1.430(16)
1.421(15)
1.684(13)
1.721(12)
1.360(16)
1.514(10)
1.552(10)
1.471(12)
79.7(4)
172.6(4)
96.0(4)
174.0(4)
101.4(4)
100.2(4)
84.4(4)
114.4(8)
119.5(12)
114.9(8)
115.2(11)
110.8(11)
108.8(10)
114.4(10)
115.0(11)
115.8(9)
113.0(11)
91.6(6)
1.327
1.401
1.452
1.418
1.724
1.742
1.374
P–F2
P–F3
P–F5
P–F6
C29–Ir–C45
C29–Ir–N3
C45–Ir–N3
C29–Ir–N4
N3–Ir–N2
C29–Ir–N1
C45–Ir–N1
N2–Ir–N1
N1–C5–C9
C9–N2–Ir
N3–C17–C24
C24–C29–Ir
N3–C17–C16
C16–S1–C15
Ir–N4–C33
C33–C40–C45
C31–N4–C33
C33–C32–S2
S2–C31–N4
91.9
80.0
92.6
92.9
86.7
171.8
96.1
C45–Ir–N4
N3–Ir–N4
C29–Ir–N2
C45–Ir–N2
N4–Ir–N2
N3–Ir–N1
N4–Ir–N1
Ir–N1–C5
C5–C9–N2
Ir–N3–C17
C17–C24–C29
C15–N3–C17
C17–C16–S1
S1–C15–N3
N4–C33–C40
C40–C45–Ir
N4–C33–C32
C32–S2–C31
79.9
169.6
95.7
172.2
101.4
101.3
87.1
114.3
117.0
114.7
114.5
113.3
109.7
112.4
114.7
116.0
113.5
91.1
76.3
117.8
114.5
114.8
115.9
113.5
91.1
114.8
114.5
113.3
109.7
112.4
ulation of the absorption spectrum of complex 4a has been per-
formed by a Gaussian convolution with FWHM = 0.4 eV; the re-
lated experimental spectrum have been rescaled so that the
intensity of the experimental and theoretical main bands in the
absorption spectrum matches. The quadratic hyperpolarizability
(b) can be evaluated using a Sum Over States (SOS)-TDDFT ap-
proach [21]. This approach requires in principle the calculation of
dipole matrix elements between all possible couples of excited
states (three level terms), in addition to the ground to excited
states transition dipole moments (two level terms) [22]. It turns
out, however, that 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 assess-
ment of the major contributions to the quadratic hyperpolarizabil-
ity. In the present study, we calculated the lowest excited states
and ground state dipole moments for 4a and 4b. Ground and ex-
cited states dipole moments were computed from the SCF and Rho-
CI density, respectively.
3. Results and discussion
Complexes 4a and 4b were prepared according to the synthetic
method previously reported [4] in two steps: (i) preparation of the
chloro-bridge dimer [Ir(cyclometallated 4,5-diphenyl-2-methyl-
thiazole)2Cl]2 following the procedure reported for [Ir(ppy)2Cl]2
[8]; (ii) bridge splitting reaction with a 5-R-1,10-phenanthroline
(R = CH3, NO2; see Section 2). The structure of complex 4a was
determined by X-ray diffraction of suitable crystals which contain
[Ir(cyclometallated 4,5-diphenyl-2-methyl-thiazole)2(5-Me-1,10-
phenanthroline)]+ cations, PF6ꢀ anions and disordered CH2Cl2 mol-
ecules, separated by normal van der Waals interactions, in the

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C. Dragonetti et al. / Inorganica Chimica Acta 361 (2008) 4070–4076
Fig. 1. ORTEP view of the [Ir(cyclometallated 4,5-diphenyl-2-methyl-thiazole)2(5-Me-1,10-phenanthroline)]+ cation with displacement ellipsoids at 25% probability.
Fig. 2. Schematic representation of the frontier molecular orbitals of complexes 1–4. For complexes 4a isodensity plots of the HOMO ꢀ 3, HOMO, LUMO, and LUMO + 2 are
reported. For complex 4b isodensity plot of LUMO is shown.

C. Dragonetti et al. / Inorganica Chimica Acta 361 (2008) 4070–4076
4075
metal orbital, with the almost degenerate dxz–dyz orbitals lying ca.
1.0 eV below, being the HOMO ꢀ 3/HOMO ꢀ 4 (see Fig. 2). The
HOMO ꢀ 1/HOMO ꢀ 2 are, on the other hand, p orbitals of the
cyclometallated ligand (not shown in Fig. 2). The lowest unoccu-
pied molecular orbital (LUMO) and LUMO+1 of 4a, separated by
0.18 eV, are, as previously observed [4,5,7], phenanthroline p* orbi-
tals, with the LUMO showing some metal character reflecting p
backdonation (see Fig. 2). The almost degenerate LUMO + 2/
LUMO + 3 couple are p* orbitals of the cyclometallated ligands,
mainly localized on the thiazole ring (see Fig. 2). For 4b the pres-
ence of the NO2 phenanthroline substituent leads, as previously
observed [4,5,7] to a substantial stabilization of the LUMO which
translates into a significant reduction of the HOMO–LUMO gap
for 4b compared to 4a (1.92 versus 2.95 eV).
A comparison between the experimental and TDDFT calculated
absorption spectrum in CH2Cl2 for 4a is reported in Fig. 3. The cal-
culated spectrum (absorption bands at 372 and 253 nm) is in
excellent agreement with the experimental one (absorption bands
at 358 and 272 nm). As in the experimental spectrum, the band
calculated at 253 nm is almost five times more intense than that
at 372 nm. The 230 nm band found in the experimental spectrum
is not reproduced by TDDFT due to the limited number of calcu-
lated excited states, which allowed us to reach only transitions be-
low 230 nm. Inspection of the TDDFT eigenvectors allows the
assignment of the character of the absorption bands. The two
lowest calculated transitions of singlet and triplet character are
at 548–556 and 489–493 nm, respectively, and correspond to
HOMO ? LUMO and HOMO ? LUMO + 1 transitions. The singlet
transitions show negligible oscillator strengths (those for the trip-
let transitions are set to zero due to the neglect of spin–orbit cou-
pling) so that they are possibly related to the band tail extending
into the visible region observed experimentally. The 372 nm band
is originated essentially by an intense HOMO ꢀ3 ? LUMO transi-
tion (f = 0.124) of metal to ligand charge transfer (MLCT) character
since it involves a transition from the Ir dxz orbital to the lowest
phenanthroline p* orbital. The 272 nm experimental band appears
to be composed by an envelop of intense p ꢀ p* transitions, in the
range 294–246 nm, located on the cyclometallated ring and on
the phenanthroline ligand.
The experimental absorption spectrum of 4b is quite similar to
that of 4a, apart from the tail decaying into the visible region
which is more intense for the former. A HOMO ꢀ 3 ? LUMO tran-
sition at 511 nm (f = 0.025), missing in the spectrum of 4a, was cal-
culated in addition to the same pattern of MLCT and p ꢀ p*
transitions calculated for 4a, even though in the case of 4b the
experimental band at 357 nm was related to two overlapping
bands calculated at ca. 380 and 320 nm, respectively.
To compute the contribution of singlet excited states to the qua-
dratic hyperpolarizability (b), we used the SOS method in the case
of related cationic Ir(III) complexes [5], which showed how the
ILCT and MLCT transitions contribute with a different sign to b.
In analogy with these previous SOS-TDDFT calculations carried
out on 1b [5], the second-order NLO response of 4a and 4b must
be still controlled by the rather intense and low energy MLCT tran-
sitions involving HOMO–LUMO charge transfer processes from the
cyclometallated moiety to the phenanthroline ligand. Therefore in
the present study we limited our investigation to the lowest ex-
cited states which give rise to the MLCT absorption band of 4a
and 4b spectra. Such approximate approach allows us to estimate
the contribution of the lowest MLCT excitations to b and thus to
visualize the charge transfer flow accompanying these electronic
transitions. This seems a good approximation for 4a and 4b, where
no competing ILCT/MLCT processes are computed, which on the
contrary were present in the case of [Ir(cyclometallated 2-phenyl-
pyridine)2(5-NMe2-1,10-phenanthroline)][PF6] due to the presence
of the NMe2 substituent [5].
The increased value of the second-order NLO response of 4b
compared to 4a must be thus related to the decreased HOMO–
LUMO gap of 4b, which provides an increased NLO response
according to the SOS approach. To visualize the charge transfer
process giving rise to the second-order NLO response, in Fig. 4
we report isodensity plots of the electron density difference be-
tween selected singlet excited states involved in the MLCT pro-
cesses and the ground state of complexes 4a and 4b. A blue (red)
color indicates a decrease (increase) of the electron density upon
excitation, thus allowing visual inspection of the charge density
flow accompanying electron excitations. Also in Fig. 4 are reported
dipole moment differences between the excited and ground state,
Fig. 3. Comparison between the experimental (red line) and computed (blue line) absorption spectra of 4a complex. The experimental spectrum has been rescaled so that the
intensity of the 272 nm band of the experimental spectrum matches the theoretical main feature.

4076
C. Dragonetti et al. / Inorganica Chimica Acta 361 (2008) 4070–4076
Fig. 4. Isodensity plots of the electron density difference between selected singlet excited states and the ground state of complexes 4a (top) and 4b (bottom). A blue (red)
color indicates a decrease (increase) of the electron density upon excitation. The dominating x component of the dipole moment differences between the excited and ground
state, excitation wavelengths, oscillator strengths and the character of the considered excitations are also reported.
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whose negative sign, characteristic of MLCT excitations, gives rise
to the negative sign of the second-order NLO response, according
to a negative quadratic hyperpolarizability.
In summary, this work confirms that the second-order NLO
response of these cationic cyclometallated Ir(III) complexes with
5-R-1,10-phenanthrolines is not strongly affected by the nature
of the cyclometallated moiety whereas it is controlled and tuned
by the nature of the R substituent on the phenanthroline ring.
We also confirmed that the quantum yield of the emission is also
controlled mainly by the nature of the R substituent on the
phenanthroline ring.
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(2007) 8533.
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4. Supplementary material
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Guagliardi, A.G.G. Moliterni, G. Polidori, R. Spagna, J. Appl. Crystallogr. 32
(1999) 115.
CCDC 675304 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
[12] G.M. Sheldrick, SHELX97 – Programs for Crystal Structure Analysis (Release 97-
2), University of Göttingen, Germany, 1998.
Acknowledgements
[13] L.J. Farrugia, J. Appl. Crystallogr. 32 (1999) 837.
[14] A.D. Becke, Phys. Rev. A 38 (1988) 3098.
[15] J.P. Perdew, Phys. Rev. B 33 (1986) 8822.
We thank for financial support the Ministero dell’Istruzione,
dell’Università e della Ricerca (Programma di ricerca FIRB, year
2003, Research Title: Molecular compounds and hybrid nanostruc-
tured materials with resonant and non-resonant optical properties
for photonic devices) and CNR (PROMO 2006).
[16] G. te Velde, F.M. Bickelhaupt, E.J. Baerends, C. Fonseca-Guerra, S.J.A. van
Gisbergen, J.G. Snijders, T. Ziegler, J. Comput. Chem. 22 (2001) 931.
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