Photoluminescence of (quinap)tricarbonylrhenium(I) chloride with quinap = 1-(2-diphenylphosphino-1-naphthyl) isoquinoline

Article abstract of DOI:10.1016/S0009-2614(02)00484-0

The complex (quinap)Re¹(CO)₃Cl, with quinap = 1-(2-diphenylphosphino-1-naphthyl)isoquinoline, shows a luminescence at λ_max = 582 nm with φ = 0.12 and τ = 1.4 μs in solution (e.g., CH₂Cl₂) at r.t. It is suggested that this emission originates from a Re¹ to quinap metal-to-ligand charge transfer triplet.

Full text of DOI:10.1016/S0009-2614(02)00484-0

3 May 2002
Chemical Physics Letters 357 (2002) 143–146
www.elsevier.com/locate/cplett
Photoluminescence of (quinap)tricarbonylrhenium(I)
chloride with quinap ¼ 1-(2-diphenylphosphino-1-naphthyl)
isoquinoline
*
Horst Kunkely, Arnd Vogler
Institut f€ur Anorganische Chemie, Universit€at Regensburg, Universitaetsstrasse 31, D-93040 Regensburg, Germany
Received 25 February 2002; in final form 26 March 2002
Abstract
The complex (quinap)ReIðCOÞ3Cl, with quinap ¼ 1-(2-diphenylphosphino-1-naphthyl)isoquinoline, shows a lumi-
nescence at kmax ¼ 582 nm with / ¼ 0:12 and s ¼ 1:4 ls in solution (e.g., CH2Cl2) at r.t. It is suggested that this
emission originates from a ReI to quinap metal-to-ligand charge transfer triplet. Ó 2002 Published by Elsevier Science
B.V.
1. Introduction
tuned to a certain extent by a variation of the
polypyridine ligands. Unfortunately, the emission
from these states is frequently attenuated by
various effects, including the energy gap law.
Another deactivation path is provided by ligand
field (LF) states located at energies slightly above
the emitting MLCT or IL states. This interference
can be prevented by shifting the LF states to
much higher energies. This, however, requires a
severe modification of the bipyridine. Asuitable
ligand is the deprotonated 2-phenylpyridine which
forms a strong metal–carbon r-bond [2,5–7]
(Scheme 2).
Although the LF states do indeed occur at ra-
ther high energies in this case, the MLCT states are
also shifted up compared to bipy complexes, ow-
ing to the destabilization of the ligand pꢀ orbitals.
Nevertheless, a variety of such ortho-metalated
phenylpyridine complexes has been shown to be
luminescent [2,5–7].
The luminescence of metal complexes is a
rapidly expanding research field [1]. In particular,
potential applications, such as optical sensors or
light emitting diodes, have stimulated the interest
in transition metal compounds which are emissive
under ambient conditions.The majority of such
complexes consists of a reducing metal center
(e.g., RuII; ReI) and polypyridine (or 1,2-diimine)
ligands [2–4] such as 2; 20-bipyridine (bipy)
(Scheme 1). In these cases the luminescence orig-
inates from low-energy metal-to-ligand charge
transfer (MLCT) or intraligand (IL) excited trip-
lets. The energy of the emitting triplet can be
* Corresponding author. Fax: +49-941-943-4488.
E-mail address: arnd.vogler@chemie.uni-regensburg.de (A.
Vogler).
0009-2614/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V.
PII: S0009-2614(02)00484-0

144
H. Kunkely, A. Vogler / Chemical Physics Letters 357 (2002) 143–146
able from Strem and used without further purifi-
cation.
ðQuinapÞReðCOÞ Cl was obtained by the fol-
3
lowing procedure: Asuspension of Re ðCOÞ5Cl
(0.25 g, 0.7 mmol) and quinap (0.31 g, 0.7 mmol)
in argon-saturated toluene (10 ml) was refluxed in
the dark for 20 min. The initially colourless reac-
tion mixture rapidly turned yellow. It was con-
centrated by evaporation to about 2 ml. After
dropwise addition of 20 ml n-hexane while stirring,
the solution was allowed to stand in the dark for
20 min. The yellow crystals that slowly precipi-
tated were collected by filtration, washed with a
small amount of n-hexane, and dried under re-
duced pressure. The crude product was recrystal-
lized twice from acetone/pentane, yielding 0.13 g
(24%).
Scheme 1.
Scheme 2.
Anal. Calc. for C34H22ClNO3PRe (745.18): C,
54.80; H, 2.98; N, 1.88; Cl, 4.76. Found: C, 54.85;
H, 3.02; N, 1.88; Cl, 4.70%.
2.2. Instrumentation
Scheme 3.
Absorption and emission spectra were mea-
sured with a Hewlett Packard 8452Adiode array
spectrophotometer and a Hitachi 850 fluores-
cence spectrophotometer equipped with a Ham-
amatsu 928 photomultiplier for measurements up
to 900 nm. The luminescence spectra were cor-
rected for monochromator and photomultiplier
efficiency variations. Absolute emission quantum
yields were determined by comparison of the in-
tegrated emission intensity with that of Rhod-
amine B under identical conditions, such as
exciting wavelength, optical density, and appa-
ratus parameters. Emission lifetimes were mea-
sured on a luminescence analysis system (model
LS-100-07) from Photon Technology Interna-
tional.
In the present study we explored a related ap-
proach. We selected quinap ¼ 1-(2-diphenyl-
phosphino-1-naphthyl)isoquinoline as a bidentate
ligand. Arhodium(III) complex with this ligand
has been prepared quite recently (Scheme 3) [8].
The isoquinoline moiety of quinap is expected
to provide pꢀ orbitals at relatively low energies
while the coordinating phosphine is a strong-field
ligand [9]. As a suitable complex we chose
(quinap)ReIðCOÞ3Cl. This compound was ex-
pected to be easily accessible by reacting
ReðCOÞ Cl with quinap. Moreover, the
5
ReIðCOÞ Cl fragment is an excellent electron do-
3
nor for probing MLCT states in absorption and
emission [2,10–14].
3. Results
2. Experimental
The electronic spectrum of the free ligand qui-
nap shows absorptions (Fig. 1) at kmax ¼ 324
(sh; e ¼ 6200 Mꢁ1 cmꢁ1), 312 (sh, 7800), and 275
(18 100) nm. Quinap exhibits a weak blue fluores-
cence at kmax ¼ 380 nm (Fig. 1) and a phospho-
2.1. Materials
All solvents used were of spectrograde quality.
ReðCOÞ Cl and quinap were commercially avail-
5

H. Kunkely, A. Vogler / Chemical Physics Letters 357 (2002) 143–146
145
roughly matches the absorption spectrum. The
emission is somewhat quenched by oxygen. In ar-
gon-saturated CH2Cl2 the emission quantum yield
is / ¼ 0:12 at kexc ¼ 350 nm and the lifetime is
s ¼ 1:4 ls ꢃ 0:11. In O2 free CH3CN (quinap)
ReðCOÞ3Cl is essentially insensitive to light, but in
the presence of oxygen it undergoes a slow pho-
todecomposition. This is an indication for the oc-
curence of an energy transfer from the excited
complex to O2, leading to the generation of singlet
oxygen [2,4]. The photodecomposition is then a
result of the irreversible oxidation of the complex
Fig. 1. Electronic absorption (a) and emission (e) spectrum of
5:91 ꢄ 10ꢁ5 M (quinap) in CH2Cl2 under argon at room tem-
perature, 1 cm cell. Emission: kexc ¼ 330 nm, intensity in arbi-
trary units.
1
by O2.
4. Discussion
rescence at kmax ¼ 520 nm which appears only at
low temperatures (77 K).
The absorption spectrum of (quinap)
The electronic spectra of numerous complexes
with the general composition ðL-LÞReðCOÞ Cl
3
have been studied in quite some detail [2,10–14].
Generally, their spectra are dominated by MLCT
and/or IL transitions. MLCT absorptions prevail
when L-L is a polypyridine (or 1,2-diimine) such as
bipy which provides empty pꢀ orbitals at low en-
ergies. The MLCT bands undergo a blue shift with
increasing solvent polarity (negative solvatochro-
mism). While these absorptions appear at rather
long wavelengths, the IL bands occur at shorter
wavelengths and resemble those of the free ligand.
The IL absorptions are almost solvent indepen-
dent. According to these criteria, the longest-
ReðCOÞ Cl (Fig. 2) displays bands at kmax ¼ 400
3
(sh, 1300), 344 (7700), and 292 (sh, 13 300) nm.
The longest-wavelength band is solvent dependent.
It is shifted from 414 nm in toluene to shorter
wavelength with increasing polarity of the solvent
(e.g., 404 nm in ether; ꢂ390 nm in CH3CN). In
more polar solvents this absorption is obscured by
the more intense band at 344 nm, which is solvent
independent. The complex (quinap)ReðCOÞ Cl is
3
strongly luminescent under ambient conditions. It
appears at kmax ¼ 582 nm (Fig. 2) and is only
slightly solvent sensitive. The excitation spectrum
wavelength band of (quinap)ReðCOÞ Cl near 400
3
nm is logically assigned to a MLCT transition
while the next band at kmax ¼ 344 nm is certainly
of the IL (quinap) type. Since both bands are rel-
atively intense they are assigned to spin-allowed
singlet–singlet transitions.
In analogy to many other ðL-LÞReðCOÞ3Cl
complexes with long-wavelength MLCT absorp-
tions [2,10–14] the emission of (quinap)ReðCOÞ Cl
3
is suggested to originate from the lowest-energy
MLCT triplet. This assignment is supported by
various arguments. The luminescence of (quinap)
ReðCOÞ3Cl (kmax ¼ 582 nm) appears at much
longer wavelength than the phosphorescence of
free quinap. Moreover, the lifetime of the MLCT
Fig. 2. Electronic absorption (a) and emission (e) spectrum of
7:49 ꢄ 10ꢁ5 M (quinap)ReðCOÞ Cl in CH2Cl2 under argon at
3
triplet of (quinap)ReðCOÞ Cl ðs ¼ 1:4 ls ꢃ 0:11Þ
room temperature, 1 cm cell. Emission: kexc ¼ 350 nm, intensity
3
in arbitrary units.
is similar to that of many other ðL-LÞReðCOÞ Cl
3

146
H. Kunkely, A. Vogler / Chemical Physics Letters 357 (2002) 143–146
Acknowledgements
complexes with emitting MLCT triplets, indicating
a considerable singlet–triplet mixing. However,
while a MLCT assignment for the emitting state
seems to be reasonable a contribution by IL exci-
tation is certainly conceivable.
Support of this research by the Fonds der
Chemischen Industrie is gratefully acknowledged.
Generally, the emission from MLCT triplets of
(1,2-diimine)ReðCOÞ Cl complexes takes place
References
3
with rather low quantum yields under ambient
conditions [15], e.g., / ¼ 0:005 for (bipy)
[1] A. Vogler, H. Kunkely, Top. Curr. Chem. 213 (2001) 143.
[2] K. Kalyanasundaram, Photochemistry of Polypyridine and
Porphyrin Complexes, Academic Press, London, 1992.
[3] A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P.
Belser, A. von Zelewsky, Coord. Chem. Rev. 84 (1988) 85.
[4] D.M. Roundhill, Photochemistry and Photophysics of
Metal Complexes, Plenum Press, New York, 1994.
ReðCOÞ Cl [16]. This stands in strong contrast to
3
(quinap)ReðCOÞ Cl, which emits with / ¼ 0:12.
3
While this difference may be partially caused by
the energy gap law, other effects could be also
important [16]. Emitting MLCT or IL triplets may
be deactivated by the thermal population of LF
states which are located at slightly higher energies.
€
[5] M.G. Colombo, A. Hauser, H.U. Gudel, Top. Curr.
Chem. 171 (1994) 143.
[6] M. Maestri, V. Balzani, C. Deuschel-Cornioley, A. von
Zelewsky, Adv. Photochem. 17 (1992) 1.
[7] H. Kunkely, A. Vogler, Chem. Phys. Lett. 319 (2000)
486.
In the case of (quinap)ReðCOÞ Cl this quenching
3
process should be attenuated, because phosphine
coordination shifts LF states to higher energies.
However, it must be admitted that the compa-
rison between (bipy)ReðCOÞ3Cl and (quinap)
ReðCOÞ3Cl suffers from the rather different struc-
tural and electronic features of the two ligands. In
this context it should be noted that the complex
[8] H. Doucet, E. Fernandez, T.P. Layzell, J.M. Brown,
Chem. Eur. J. 5 (1999) 1320.
[9] A. Vogler, H. Kunkely, Coord. Chem. Rev., in press.
[10] G.L. Geoffroy, M.S. Wrighton, Organometallic Photo-
chemistry, Academic Press, New York, 1979.
[11] D.J. Stufkens, Comments Inorg. Chem. 13 (1992) 359.
[12] A. Lees, Chem. Rev. 87 (1987) 711.
[13] K.S. Schanze, D.B. MacQueen, T.A. Perkins, L.A. Ca-
bana, Coord. Chem. Rev. 122 (1993) 63.
[14] A. Vogler, H. Kunkely, Coord. Chem. Rev. 200–202 (2000)
991.
(binap)ReðCOÞ Cl, with binap ¼ 2; 20-bis(diph-
3
enylphosphino)-1; 10-binaphthyl, has also been
observed to emit from a MLCT triplet at r.t., but
in this case no coordinating pyridine moiety is left
at the binap ligand [17].
[15] M. Wrighton, D.L. Morse, J. Am. Chem. Soc. 96 (1974)
998.
In summary, the complex (quinap)ReðCOÞ3Cl
shows a phosphorescence with high quantum yield
(/ ¼ 0:12) under ambient conditions. This com-
pound is thus an attractive candidate for an ap-
plication in light emitting diodes [18].
[16] J.V. Caspar, T.J. Meyer, J. Phys. Chem. 87 (1983) 952.
[17] H. Kunkely, A. Vogler, Inorg. Chem. Commun. 2 (1999)
533.
[18] M.A. Baldo, M.E. Thompson, S.R. Forrest, Pure Appl.
Chem. 71 (1999) 2095.