Organometallic emitting dyes: Palladium(II) nile red complexes

Article abstract of DOI:10.1016/j.jorganchem.2004.10.028

The Nile Red dye, H(NR), forms cyclometalated R₂-disubstituted- acetylacetonato square planar Pd(II) complexes (1-3; R = CH₃, CF ₃, C₆H₅ respectively) whose photophysical properties were tested in cyclohexane, dichloromethane or methanol solutions. In cyclohexane 1-3 emit in the range 580-650 nm with a quantum yield ranging from 0.12 (R = CH₃) to 0.50 (R = CF₃) and lifetimes between 0.88 and 4.46 ns. These complexes form a new family of notably efficient red emitting organometallic dyes which could be of interest for practical applications.

Full text of DOI:10.1016/j.jorganchem.2004.10.028

Journal of Organometallic Chemistry 690 (2005) 857–861
www.elsevier.com/locate/jorganchem
Organometallic emitting dyes: Palladium(II) nile red complexes
a
a,
a
a
Massimo La Deda , Mauro Ghedini *, Iolinda Aiello , Teresa Pugliese ,
b
Francesco Barigelletti , Gianluca Accorsi
b
a
` `
Centro di Eccellenza CEMIF.CAL – LASCAMM, Unita INSTM della Calabria, Dipartimento di Chimica, Universita della Calabria,
I-87030 Arcavacata (CS), Italy
`
Istituto di Sintesi Organica e Fotoreattivita (ISOF – CNR), Via P. Gobetti, 101, I-40129 Bologna, Italy
b
Received 30 July 2004; accepted 15 October 2004
Abstract
The Nile Red dye, H(NR), forms cyclometalated R2-disubstituted-acetylacetonato square planar Pd(II) complexes (1–3;
R = CH3, CF3, C6H5 respectively) whose photophysical properties were tested in cyclohexane, dichloromethane or methanol solu-
tions. In cyclohexane 1–3 emit in the range 580–650 nm with a quantum yield ranging from 0.12 (R = CH3) to 0.50 (R = CF3) and
lifetimes between 0.88 and 4.46 ns. These complexes form a new family of notably efficient red emitting organometallic dyes which
could be of interest for practical applications.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Cyclopalladated complexes; Nile red complexes; Substituted acetylacetonato complexes; Red emitter; Luminescence
1. Introduction
tions [3]. Some cases of Pd(II) complexes investigated in
the solid state have also been reported; however, lumi-
nescence was found to originate from intermolecular
states produced by pairing interactions in the crystal
packing [4].
Luminescent palladium complexes have been recently
proposed for different purposes [1]. Nevertheless, studies
concerning the luminescence of Pd(II) complexes at
room temperature are not common because most of
the palladium compounds suffer from inherent difficulty
which hampers an appreciable light emission. This is due
to the fact that the lowest-lying excited states of such
complexes are usually depleted by thermal population
of higher-lying yet thermally accessible metal-centred
states (MC) which, geometrically distorted with respect
to the ground state, favour a rapid non-radiative deacti-
vation [2]. On the other hand, studies in a rigid matrix at
low temperatures allow observation of the luminescence
coming from metal-to-ligand charge transfer (MLCT)
or ligand-localized (LC) states; this is because thermal
population of the MC states is inhibited in these condi-
Recently, we have reported a series of mixed 2-phe-
nylpyridine and 5-substituted-8-hydroxyquinolines
Pd(II) complexes which emit in solution at room tem-
perature [5]. In these compounds luminescence arises
from an intra-ligand charge-transfer (ILCT) state, which
is located at a substantially lower energy with respect to
the competing non-emissive MC states (that are there-
fore not populated). These complexes showed a bluish-
green emission and a photoluminescence quantum yield
lower than 1%. We have subsequently extended our
studies to include other photoluminescent chromoph-
ores, and have synthesized a number of organopalla-
dium complexes [6]; among them, the Nile Red
derivative, (NR)Pd(acac), 1 in [H(NR) = 9-diethyl-
amino-5H-benzo[a]phenoxazine-5-one, usually reported
as Nile Red; H(acac) = acetylacetone]. Complex 1,
*
Corresponding author. Tel.: +39 984 492062; fax: +39 984 492066.
E-mail address: m.ghedini@unical.it (M. Ghedini).
0022-328X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.10.028

858
M.L. Deda et al. / Journal of Organometallic Chemistry 690 (2005) 857–861
partly characterized in dichloromethane solution at
room temperature, showed red emission (kem = 660
nm) and a very high fluorescence quantum yield,
U = 0.23.
In this paper we report the results of an extended
investigation carried on 1 and the photophysical proper-
ties of two newly synthesized complexes, 2, (NR)Pd(hfa-
cac) [H(hfacac) = 1,1,1,5,5,5-hexafluoroacetyl acetone]
and 3, (NR)Pd(dpacac) [H(dpacac) = 1,5-diphenylace-
tylacetone], Chart 1. Remarkably, the obtained results
show that the b-diketonato Nile Red cyclometalated
Pd(II) complexes form a new family of notably efficient
red emitters.
The photophysical characterization of 1–3 and of the
parent ligand H(NR) was performed in solution at room
temperature (Table 1).
In the absorption spectrum of 2 (Fig. 1), based on
comparison with the spectrum for H(NR), we attribute
the intense band at 614 nm (e = 15,000 MÀ1 cmÀ1) and
the 263, 296 and 340 nm features to metal-perturbed
LC transitions. It is to notice that the solvatochromism
exhibited by the lowest-energy band (kmax = 588 nm in
cyclohexane, 614 nm in dichloromethane, 618 nm in
methanol) is consistent with some degree of CT charac-
ter for the concerned pp* transition (likely involving
filled and empty orbitals localized on different aromatic
rings) [6]. As for the low-intensity band at 428 nm
(e = 1770 MÀ1 cmÀ1), absent in the ligand spectrum,
the bathochromic shift (from 428 to 452 nm) observed
when the 1,1,1,5,5,5-hexafluoroacetylacetonato ancillary
ligand is replaced by the weaker electron withdrawing
acetylacetonato group suggests a MLCT assignment [7].
A similar attribution scheme might be proposed for
the bands in the electronic spectrum of 3 (Fig. 1). Thus,
LC transitions should be responsible for the bands at
258, 325, 364 and 465 nm. For the band at 610 nm, an
ILCT character seems likely, based on its solvatochro-
mic behaviour (kmax = 576 nm in cyclohexane, 610 nm
in dichloromethane, 614 nm in methanol), quite similar
to that evidenced by 2. Finally, the band at 447 nm is as-
cribed to a MLCT transition (its position is bathochro-
mically shifted with respect to the position of the related
band of 2, but at higher energy with respect to the ana-
logue band of 1). Interestingly, the absorption band
intensities are enhanced by the presence of 1,5-diphenyl-
acetylacetonate ancillary ligand.
2. Results and discussion
The newly synthesized complexes 2 and 3 were pre-
pared by reacting the cyclopalladated Nile Red ace-
tato-bridged dimer [(NR)Pd(l-OAc)]2 with the
appropriate b-diketonate ligand and the expected molec-
ular structure (Chart 1) confirmed by mass spectra, IR
1
and H NMR spectroscopies and by elemental analysis
(Experimental).
CH CH
2
3
CH CH
N
3
2
10
11
8
R
R
1: R = CH
2: R = CF
O
O
3
3
O
N
a
H
Pd
3: R = C H
6
5
6
At room temperature, 2 and 3, in dichloromethane
solution, showed high-yield emission in the red portion
of the spectrum (Fig. 2), with an emission maximum
at 670 and 660 nm, respectively (Table 1). The solvato-
2
3
O
4
Chart 1.
Table 1
Photophysical dataa of H(NR) and 1–3
kabsb(nm), (e, MÀ1 cmÀ1
)
kem (nm) Uc (%) sc (ns)
c
Compound Solvent
H(NR)
Cyclohexane 257, 290, 300, 490, 510
Dichloromethane 262(39,400), 304(9100), 538(42,700)
570,620*
602
638
60
44
38
12
23
3
2.95
4.85
2.62
0.88
4.6
Methanol 264, 290*, 554
Cyclohexane 256, 281*, 313, 443*, 465, 534, 578
Dichloromethane 260(34,900), 296(14,500), 320(12,900), 452(5600), 570*,608(30,000)
1
2
3
a
585, 638
660
712
Methanol 260, 296, 322, 455, 590
Cyclohexane 258, 300*, 360*, 440*, 545, 588
Dichloromethane 263(12,500), 296(4700), 340(4700), 428(1770), 570*, 614(15,000)
1.36
4.46
5.95
1.49
1.05
4.78
1.52
595, 650
670
705
50
35
2
Methanol
Cyclohexane
264, 305, 340*, 450*, 618
253, 300, 306, 367, 440*, 465, 537, 576
587, 636
14
31
3.5
Dichloromethane 258(67,200), 325(26,700), 364(22,900), 447(9410), 465(9530), 570*, 610(50,100) 660
Methanol 258, 308*, 348, 460, 614 709
Asterisks indicate shoulders or weak bands.
Low solubility in some solvents prevents the accurate determination of e.
Excitation performed at 550 nm (for emission spectra) or at 560 nm (for lifetimes); the absorbance at 550 nm of the employed solutions was
b
c
<0.15.

M.L. Deda et al. / Journal of Organometallic Chemistry 690 (2005) 857–861
859
5x104
4x104
3x104
2x104
properties (band shift and quantum yields) to solvent
polarity could be explained taking into account the CT
character of the emitting state. Actually, CT excited lev-
els are expected to undergo energetic stabilization in po-
lar solvents, which leads to ‘‘energy gap law’’ effects
enhancing non-radiative transitions [9].
Regarding the sensitivity of H(NR) to the solvent
polarity, some authors have proposed the formation of
twisted intramolecular charge transfer (TICT) states
[10], because the diethylamino group could acts as the
electron donor group and the quinoid part of the mole-
cule could behave as the electron acceptor [8]. Thus,
TICT formation might be involved in the presently
investigated compounds even if we could not provide
any evidence supporting this hypothesis.
H(NR)
1
2
3
1x104
0
300
400
500
λ /nm
600
700
Fig. 1. Absorption spectra of 1–3 and H(NR) recorded in dichloro-
methane solution.
In conclusion, we have shown that the properties
exhibited by H(NR), such as red emission, high quan-
tum yield and responsiveness to environmental polar-
ity, are imparted to its square planar Pd(II)
complexes 1–3. This study further indicates that with
respect to the CH3 (1) or C6H5 (3) substituted com-
plexes, in a low-polar solvent as cyclohexane, the pres-
ence, on the b-diketonato skeleton, of CF3 groups (2),
enhances considerably the fluorescence quantum yield
(Table 1). In particular, the value measured for 2
was 50%, which to the best of our knowledge, is the
highest reported to date for a Pd(II) complex [11].
Thus, this study proves that complexes 1–3 actually
form a new family of efficient red emitting organome-
tallic dyes. Moreover, in spite of the different elec-
tronic effects exerted by the groups featuring the
ancillary ligand (e.g.: CH3 in 1; CF3 in 2; C6H5 in 3)
the remarkable peculiar features carried by the
‘‘(NR)Pd’’ fragment are preserved. On this basis, this
cyclometalated luminophore can be proposed as a ver-
satile moiety that, appended to molecules or macro-
molecules, confers to them valuable photophysical
properties.
1000
H(NR)
1
2
3
800
600
400
200
0
600
650
700
/nm
750
800
λ
Fig. 2. Emission spectra of 1–3 and H(NR) recorded in dichlorometh-
ane solution; excitation performed at 550 nm.
chromic behaviour of the emission band (2: kem = 595
nm in cyclohexane, 670 nm in dichloromethane, 705 nm
in methanol; 3: kem = 587 nm in cyclohexane, 660 nm
in dichloromethane, 710 nm in methanol), the lumines-
cence lifetime (2: s = 5.95 ns; 3: s = 4.78 ns, in dichloro-
methane solution) and the comparison with the H(NR)
luminescence features (Table 1) indicate that emission
arises from the partially CT pp* ligand-localized singlet
state. The emission band for 1–3 is bathochromically
shifted with respect to the ligand, which is due to the
presence of the metal centre; furthermore, this shift in-
creases when electronegative fluorine atoms are present
in the b-diketonato chelating ligand.
The particularly high value of fluorescence quantum
yield that 1–3 exhibited in solution (Table 1) is related
to the photophysical characteristic of the parent Nile
Red ligand. The emission spectrum of this dye exhibits
shifts to lower energies, upon increasing solvent polarity
[8], and in non-polar solvent a notable increase in quan-
tum yield is observed for 2 (U = 0.50 in cyclohexane).
For the examined complexes, the sensitivity of emission
3. Experimental
3.1. General procedures
Commercially available chemicals were purchased
from Aldrich Chemical Co. and were used without fur-
1
ther purification. H NMR spectra were recorded on a
Bruker WM-300 (CDCl3 solutions, Me4Si internal
standard) and IR spectra with a Perkin–Elmer 2000
FT-IR (KBr pellets). Elemental analyses were per-
formed with a Perkin–Elmer 2400 analyzer. The MAL-
DI mass spectra were acquired on a 4700 proteomics
analyzer mass spectrometer from Applied Biosystems
(Foster City, CA) equipped with a 200 Hz Nd:YAG la-
ser at 335 nm wavelength using 2,7-dimethoxynaphta-
lene as matrix [12]. The melting points were

860
M.L. Deda et al. / Journal of Organometallic Chemistry 690 (2005) 857–861
determined with
equipped with a Linkam CO 600 heating stage.
a
microscope (Zeiss Axioscope)
Acknowledgements
Photophysical characterization of the examined com-
pounds was performed at room temperature using spec-
trofluorimetric grade solvents. Absorption spectra were
recorded with a Perkin–Elmer Lambda 900 spectropho-
tometer; uncorrected emission spectra and excitation
spectra (that were found to match the absorption pro-
file) were obtained with a Perkin–Elmer LS 50B spectro-
This research was supported by the Italian Ministero
`
dellÕIstruzione, dellÕUniversita e della Ricerca (MIUR)
through the INSTM-FIRB 2001 and Centro di Eccell-
enza CEMIF.CAL grants.
References
fluorimeter, equipped with
a
Hamamatsu R928
photomultiplier tube. Emission quantum yields were
determined using the optically dilute method [13] on aer-
ated solutions and cresyl violet perchlorate in methanol
(U = 0.54) [14] was the standard. Luminescence lifetimes
were obtained with an IBH single-photon counting
apparatus equipped with nanoled excitation sources
(kexc was selected 560 nm). The time resolution of the
single-photon spectrometer was 0.2 ns. The experimen-
tal uncertainty for the band maximum of absorption
and luminescence spectra was 2 nm, which for the
extinction coefficient was within 10%; those for the lumi-
nescent intensities and lifetimes were 15% and 8%,
respectively.
[1] (a) Q. Wu, A. Hook, S. Wang, Angew. Chem. Int. Ed. 39 (2000)
3933;
(b) P. de Silva, H.Q.N. Gunaratne, T. Gunnalaugsson, A.J.M.
Huxley, C.P. McCoy, J.T. Rademacher, T.E. Rice, Chem. Rev.
97 (1997) 1515;
(c) D. Song, Q. Wu, A. Hook, I. Kozin, S. Wang, Organometallics
20 (2001) 4683;
(d) S.W. Lai, T.C. Cheung, M.C.W. Chan, K.K. Cheung, S.M.
Peng, C.M. Che, Inorg. Chem. 39 (2000) 255.
[2] (a) G.R. Crosby, Acc. Chem. Res. 8 (1975) 231;
(b) F. Barigelletti, D. Sandrini, M. Maestri, V. Balzani, A. von
Zelewsky, L. Chassot, P. Jolliet, U. Maeder, Inorg. Chem. 27
(1988) 3644;
(c) M. Maestri, V. Balzani, C. Deuschel-Cornioley, A. von
Zelewsky, in: D. Volman, G. Hammond, D. Neckers (Eds.),
Advances in Photochemistry, John Wiley & Sons, New York,
1992, pp. 1–68.
3.2. Synthesis
[3] (a) M. Maestri, D. Sandrini, V. Balzani, A. von Zelewsky, P.
Jolliet, Helv. Chim. Acta 71 (1988) 135;
(b) C.A. Craig, R.J. Watts, Inorg. Chem. 28 (1989) 309;
(c) H. Yersin, D. Donges, J.K. Nagle, R. Sitters, M. Glasbeek,
Inorg. Chem. 39 (2000) 770;
Complex 1 was prepared as previously reported [6],
similarly 2 and 3 were prepared in MeOH by reacting,
in a 1:5 molar ratio, [(NR)Pd(l-OAc)]2 [6] with
1,1,1,5,5,5-hexafluoroacetylacetone or 1,5-diphenylace-
tylacetone at room temperature for six days. Selected
analytical data concerning complexes 2 and 3 are:
(d) V.W.W. Yam, L. Zhang, C.H. Tao, Keith M.C. Wong, K.K.
Cheung, J. Chem. Soc., Dalton Trans. (2001) 1111.
[4] (a) B.C. Tzeng, S.C. Chan, M.C.W. Chan, C.M. Che, K.K.
Cheung, S.M. Peng, Inorg. Chem. 40 (2001) 6699;
(b) F. Neve, A. Crispini, C. Di Pietro, S. Campagna, Organome-
tallics 21 (2002) 3511;
3.2.1. (NR)Pd(hfacac) (2)
(c) D. Pucci, G. Barberio, A. Crispini, O. Francescangeli, M.
Ghedini, M. La Deda, Eur. J. Inorg. Chem. (2003) 3649.
[5] M. Ghedini, I. Aiello, M. La Deda, A. Grisolia, Chem. Commun.
(2003) 2198.
Dark blue solid (CHCl3/MeOH). 61% yield.
M.p. > 300 °C. IR (KBr): m 1020, 1099, 1145, 1210,
1356, 1415, 1472 (sym. C–O acetate), 1590 (asym. C–O
acetate), 1637 (C@O ketone), 2985. 1H NMR (300
MHz, CDCl3, 298 K, standard TMS):d 8.34 (d, 1H;
H11 J = 9.25 Hz,), 7.68 (d, 1H, H4 J = 8.15 Hz), 7.34
(d, 1H, H2, J = 7.39 Hz), 6.45 (t, 1H, H3, J1 = 7.68,
J2 = 3.44 Hz), 6.43 (d, 1H, H10, J = 8.88 Hz), 6.36 (d,
1H; H8, J = 3.02 Hz), 6.16 (s, 1H, H6), 6.14 (s, 1H,
Ha), 3.47 (q, 4H, N–CH2–CH3), 1.26 (t, 6H, N–CH2–
CH3). MALDI/TOFMS m/z 630. Anal. Calc. for
C25H18N2O4F6Pd: C, 47.60; H, 2.88; N, 4.44. Found:
C, 47.56; H, 2.69; N, 4.40%.
[6] I. Aiello, M. Ghedini, M. La Deda, J. Lumin. 96 (2002) 249.
[7] (a) P.M. Gidney, R.D. Gillard, B.T. Haeton, J. Chem. Soc.,
Dalton Trans. (1973) 132;
(b) V.M. Miskowski, V.H. Houlding, Inorg. Chem. 28 (1989)
1529;
(c) V.M. Houlding, V.M. Miskowski, Coord. Chem. Rev. 111
(1991) 145;
(d) W.B. Connick, V.M. Miskowski, V.M. Houlding, H.B. Gray,
Inorg. Chem. 39 (2000) 2585.
[8] (a) N. Sarkar, K. Das, D.N. Nath, K. Bhattacharyya, Langmuir
10 (1994) 326;
(b) N. Ghoneim, Spectrochim. Acta A 56 (2000) 1003.
[9] (a) W. Siebrand, J. Chem. Phys. 55 (1971) 5843;
(b) P. Avouris, W.M. Gelbart, M.A. El-Sayed, Chem. Rev. 77
(1977) 793.
3.2.2. (NR)Pd(dpacac) (3)
Dark blue solid. 86% yield. M.p. > 300 °C. IR (KBr):
m 1021, 1096, 1121, 1251, 1383, 1451, 1486 (sym. C–O
acetate), 1585 (asym. C–O acetate), 1637 (C@O ketone),
2969. MALDI/TOFMS m/z 646. Anal. Calc. for
C35H28N2O4Pd: C, 64.97; H, 4.36; N, 4.33. Found: C,
65.07; H, 4.52; N, 4.37%. The NMR data are not avail-
able because of the low solubility.
[10] (a) K. Rotkiewicz, K.H. Grellman, Z.R. Grabowski, Chem. Phys.
Lett. 19 (1973) 315;
(b) K. Bhattacharyya, M. Chowdhury, Chem. Rev. 93 (1993) 507;
(c) W. Rettig, Angew. Chem., Int. Ed. Engl. 25 (1986) 971;
(d) J.M. Hicks, M. Vandersall, Z. Babarogic, K.B. Eisenthal,
Chem. Phys. Lett. 116 (1985) 18;
(e) C.M. Golini, B.W. Williams, J.B. Foresman, J. Fluoresc. 8
(1998) 395;

M.L. Deda et al. / Journal of Organometallic Chemistry 690 (2005) 857–861
861
´
(c) G. Garcıa-Herbosa, A. Mun˜oz, M. Maestri, J. Photochem.
Photobiol. A: Chem. 83 (1994) 165;
(f) A.K. Dutta, K. Kamada, K. Ohta, J. Photochem. Photobiol.
A: Chem. 93 (1996) 57;
(g) P. Hazra, D. Chakrabarty, A. Chakraborty, N. Sarkar, Chem.
Phys. Lett. 388 (2004) 150.
(d) M. Ghedini, D. Pucci, G. Calogero, F. Barigelletti, Chem.
Phys. Lett. 267 (1997) 341.
[11] (a) Y. Wakatsuki, H. Yamazaki, P.A. Grutsch, M. Santhanam,
C. Kutal, J. Am. Chem. Soc. 107 (1985) 8153;
(b) A. Giannetto, G. Guglielmo, V. Ricevuto, A. Giuffrida,
S. Campagna, J. Photochem. Photobiol. A: Chem. 53 (1990)
23;
[12] I. Aiello, L. Di Donna, M. Ghedini, M. La Deda, A. Napoli, G.
Sindona, Anal. Chem., 76 (2004) 5985.
[13] J.N. Demas, G.A. Crosby, J. Phys. Chem. 75 (1971) 991.
[14] D. Magde, J.H. Brannon, T.L. Cremers, J. Olmsted, J. Phys.
Chem. 83 (1979) 696.