A photochromic platinum(II) bis(alkynyl) complex containing a versatile 5,6-dithienyl-1,10-phenanthroline

Article abstract of DOI:10.1021/om060731t

A luminescent diarylethene-functionalized platinum-(II) diimine bis(alkynyl) complex displays interesting photochromic behavior with successful photosensitization by excitation into the ³MLCT/LLCT excited state of this complex to trigger photocyclization.

Full text of DOI:10.1021/om060731t

12
Organometallics 2007, 26, 12-15
Communications
A Photochromic Platinum(II) Bis(alkynyl) Complex Containing a
Versatile 5,6-Dithienyl-1,10-phenanthroline
Jason Ka-Wai Lee, Chi-Chiu Ko, Keith Man-Chung Wong, Nianyong Zhu, and
Vivian Wing-Wah Yam*
Department of Chemistry and Centre for Carbon-Rich Molecular and Nano-Scale Metal-Based Materials
Research, The UniVersity of Hong Kong, Pokfulam Road, Hong Kong SAR, People’s Republic of China
ReceiVed August 11, 2006
Summary: A luminescent diarylethene-functionalized platinum-
(II) diimine bis(alkynyl) complex displays interesting photo-
chromic behaVior with successful photosensitization by excita-
(I) tricarbonyl complex that was ligated with a photochromic
5,6-dithienyl-1,10-phenanthroline (L).3 We envision that by
altering the identity of the metal center, the photoluminescence
properties of various metal complexes may blend into the
interesting photochromic properties of this ligand, resulting in
multifunctional materials. Such perturbation brought about by
a simple change in the metal center would help to circumvent
tedious synthetic procedures required to tune the photochromic
behavior of this class of compounds in that a diversity of
interesting photochromic properties could be readily realized
in a variety of different metal complexes.4
3
tion into the MLCT/LLCT excited state of this complex to
trigger photocyclization.
The exploration of photochromic materials has aroused a
surge of interest in the field of materials science, mainly due to
their potential applications in optical data storage and optoelec-
tronic devices. Diarylethenes bearing two thiophene rings are
among one of the important classes of photochromic compounds
with regard to their thermal irreversibility and high fatigue
resistance.1a The majority of research work reported has been
devoted to their development and investigative studies of their
fundamental properties. These results have contributed to our
panoramic understanding of the chemistry of diarylethenes.1b-d
A handful of reports have demonstrated the functionalization
of nitrogen ligands, mostly monodentate, through the attachment
of diarylethene units as photochromic pendants or bridges.2
Coordination of these ligands to metal centers such as copper-
(I),2a,o gold(I),2b,c manganese(II),2o platinum(II),2d ruthenium(II),2e-h
silver(I),2i tungsten(I),2g,k and zinc(II)2l,o,p has been reported.
However, functionalization of the photochromic diarylethene
moiety as part of the nitrogen ligand framework followed by
coordination to metal centers is still an issue in its infancy.2m,n
We first reported the sensitized photochromism of a rhenium-
Platinum(II) bis(alkynyl) complexes with the general formula
[Pt(diimine)(CtCR)2] were shown to be strongly emissive in
fluid solution at room temperature.5 With our recent interest in
the luminescence studies of platinum(II) alkynyls,6 it is believed
that this class of compounds, when furnished with this func-
tionalized ligand, may demonstrate striking photochromic
behavior in addition to its rich inherent properties. Herein, we
report the syntheses, crystal structures, sensitized photochromic
properties, and photoluminescence behavior of a platinum(II)
(3) (a) Yam, V. W. W.; Ko, C. C.; Zhu, N. Y. J. Am. Chem. Soc. 2004,
126, 12734. (b) Ko, C. C.; Kwok, W. M.; Yam, V. W. W.; Phillips, D. L.
Chem. Eur. J. 2006, 12, 5840.
(4) During the course of this study, the synthesis of a related dithie-
nylethene-containing ruthenium(II) 1,10-phenanthroline complex was re-
ported, which was photochromically inactive. Ku¨hni, J.; Adamo, V.; Belser,
P. Synthesis 2006, 12, 1946.
* To whom correspondence should be addressed. E-mail: wwyam@
hku.hk. Fax: +(852)2857-1586. Tel: +(852)2859-2153.
(1) (a) Irie, M. Chem. ReV. 2000, 100, 1685. (b) Tian, H.; Yang, S. Chem.
Soc. ReV. 2004, 33, 85. (c) Matsuda, K.; Irie, M. J. Photochem. Photobiol.
C 2004, 5, 169. (d) Raymo, F. M.; Tomasulo, M. Chem. Soc. ReV. 2005,
34, 327.
(5) (a) Chan, C. W.; Cheng, L. K.; Che, C. M. Coord. Chem. ReV. 1994,
132, 87. (b) James, S. L.; Younus, M.; Raithby, P. R.; Lewis, J. J.
Organomet. Chem. 1997, 543, 233. (c) Hissler, M.; Connick, W. B.; Geiger,
D. K.; McGarrah, J. E.; Lipa, D.; Lachicotte, R. J.; Eisenberg, R. Inorg.
Chem. 2000, 39, 447. (d) Hissler, M.; McGarrah, J. E.; Connick, W. B.;
Geiger, D. K.; Cummings, S. D.; Eisenberg, R. Coord. Chem. ReV. 2000,
108, 115. (e) Chan, S.-C.; Chan, M. C. W.; Wang, Y.; Che, C.-M.; Cheung,
K.-K.; Zhu, N. Chem. Eur. J. 2001, 7, 4180. (f) Whittle, C. E.; Weinstein,
J. A.; George, M. W.; Schanze, K. S. Inorg. Chem. 2001, 40, 4053. (g)
McGarrah, J. E.; Kim, Y. J.; Hissler, M.; Eisenberg, R. Inorg. Chem. 2001,
40, 4510. (h) Kang, Y.; Lee, J.; Song, D.; Wang, S. Dalton Trans. 2003,
18, 3493. (i) Wadas, T. J.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chem.
2003, 42, 3772. (j) McGarrah, J. E.; Eisenberg, R. Inorg. Chem. 2003, 42,
4344. (k) Hua, F.; Kinayyigit, S.; Cable, J. R.; Castellano, F. N. Inorg.
Chem. 2005, 44, 471. (l) Wadas, T. J.; Chakraborty, S.; Lachicotte, R. J.;
Wang, Q. M.; Eisenberg, R. Inorg. Chem. 2005, 44, 2628. (m) Siemeling,
U.; Bausch, K.; Fink, H.; Bruhn, C.; Baldus, M.; Angerstein, B.; Plessow,
R.; Brockhinke, A. Dalton Trans. 2005, 2365.
(2) (a) Murguly, E.; Norsten, T. B.; Branda, N. R. Angew. Chem., Int.
Ed. 2001, 40, 1752. (b) Matsuda, K.; Ikeda, M.; Irie, M. Chem. Lett. 2004,
33, 456. (c) Sud, D.; McDonald, R.; Branda, N. R. Inorg. Chem. 2005, 44,
5960. (d) Jung, I.; Choi, H. B.; Kim, E. Y.; Lee, C. H.; Kang, S. O.; Ko,
J. J. Tetrahedron 2005, 61, 12256. (e) Fraysse, S.; Coudret, C.; Launay, J.
P. Eur. J. Inorg. Chem. 2000, 1581. (f) Jukes, R. T. F.; Adamo, V.; Hartl,
F.; Belser, P.; de Cola, L. Inorg. Chem. 2004, 43, 2779. (g) Jukes, R. T. F.;
Adamo, V.; Hartl, F.; Belser, P.; de Cola, L. Coord. Chem. ReV. 2005,
249, 1327. (h) Samachetty, H. D.; Branda, N. R. Chem. Commun. 2005,
2840. (i) Konaka, H.; Wu, L. P.; Munakata, M.; Kuroda-Sowa, T.; Maekawa,
M.; Suenaga, Y. Inorg. Chem. 2003, 42, 1928. (j) Ferna´ndez-Acebes, A.;
Lehn, J.-M. AdV. Mater. 1998, 10, 1519. (k) Ferna´ndez-Acebes, A.; Lehn,
J.-M. Chem. Eur. J. 1999, 5, 3285. (l) Matsuda, K.; Takayama, K.; Irie, M.
Chem. Commun. 2001, 363. (m) Chen, B. Z.; Wang, M. Z.; Wu, Y. Q.;
Tian, H. Chem. Commun. 2002, 1060. (n) Tian, H.; Chen, B. Z.; Tu, H. Y.;
Mullen, K. AdV. Mater. 2002, 14, 918. (o) Matsuda, K.; Takayama, K.;
Irie, M. Inorg. Chem. 2004, 43, 482. (p) Qin, B.; Yao, R. X.; Tian, H.
Inorg. Chim. Acta 2004, 357, 3382.
(6) (a) Yam, V. W. W.; Wong, K. M.-C.; Zhu, N. Y. J. Am. Chem. Soc.
2002, 124, 6506. (b) Yam, V. W. W.; Wong, K. M.-C.; Zhu, N. Y. Angew.
Chem., Int. Ed. 2003, 42, 1400. (c) Tao, C.-H.; Zhu, N. Y.; Yam, V. W.
W. Chem. Eur. J. 2005, 11, 1647. (d) Yam, V. W. W. Acc. Chem. Res.
2002, 35, 555.
10.1021/om060731t CCC: $37.00 © 2007 American Chemical Society
Publication on Web 12/06/2006

Communications
Scheme 1. Photochromism of Complex 1
Organometallics, Vol. 26, No. 1, 2007 13
Figure 2. UV-vis absorption spectral changes of (a) L in
dichloromethane solution (3.78 × 10-5 M) upon IL excitation at
313 nm and (b) 1 in dichloromethane solution (2.19 × 10-5 M)
upon MLCT/LLCT excitation at 420 nm.
Å). The Pt-C(alkynyl) bond distance of 1 is 1.994(1) Å, which
is comparable to the close analogues of this class of platinum-
(II) diimine bis(alkynyl) complexes.5c The closest Pt‚‚‚Pt contact
is 4.477 Å, which indicates the absence of any interaction
between the metal centers in the crystalline solid state.
Figure 1. Perspective drawings of 1 as the parallel (a) and
antiparallel (b) conformers with atomic numbering scheme. Hy-
drogen atoms have been omitted for clarity. Thermal ellipsoids are
shown at the 30% probability level.
The absorption properties of L in benzene solution were
previously reported.3 The open form of L (Lo) dissolves in
dichloromethane to give a colorless solution, with the absorption
maxima at ca. 270 nm (ꢀ ) 23 220 dm3 mol-1 cm-1) and 316
nm (ꢀ ) 4970 dm3 mol-1 cm-1), ascribed to the intraligand
(IL) π f π* and n f π* transitions of the phenanthroline
backbone with some mixing of the π f π* and n f π*
transitions of the two thiophene rings. Upon UV irradiation at
λ ) 313 nm, the colorless solution of Lo in dichloromethane
becomes red, with emergence of an intense absorption band at
ca. 366 nm (ꢀ ) 19 160 dm3 mol-1 cm-1) and a lower energy
band at ca. 518 nm (ꢀ ) 3100 dm3 mol-1 cm-1), ascribed to
the absorptions of the closed form of L (Lc) via photocyclization
of its antiparallel open form (Figure 2a).
The solution of 1 in its open form (1o) in dichloromethane
appears as a bright yellow solution. It shows an intense
absorption band at ca. 268 nm, attributed to the L-based and
alkynyl-based IL transitions. In addition, it shows a compara-
tively moderate absorption band at ca. 390 nm, originating from
the metal-to-ligand charge transfer (MLCT; dπ(Pt) f π*(Lo))
transition,5 probably with some mixing of a ligand-to-ligand
charge transfer (LLCT; π(CtC) f π*(Lo)) transition as
suggested from DFT calculations by Castellano et al.11 The
values of ꢀ of these two bands are on the order of 104 dm3
mol-1 cm-1. Upon UV irradiation at λ ) 313 nm, an intense
absorption band at ca. 390 nm and a broad band at ca. 560 nm
emerged as the closed form of 1 (1c) was generated. The slight
red shift of these bands in 1c relative to those of Lc suggests
that they are predominantly metal-perturbed IL π f π* and n
f π* transitions of the 8a,8b-dimethyl-1,8-dithia-as-indacene
moiety. In addition to excitation at 313 nm that triggers
photocyclization originated from the IL transitions, excitation
into the MLCT/LLCT absorption bands with λ e 480 nm of
bis(alkynyl) complex containing the 5,6-dithienyl-1,10-phenan-
throline moiety (Scheme 1).
The target complex Pt(L)(CtCPh)2 (1) was prepared from
Pt(L)Cl2, which was prepared according to modification of a
literature procedure for Pt(phen)Cl2,7 under Sonogashira condi-
tions.8 The 1H NMR spectrum showed two sets of signals
corresponding to the two conformers: antiparallel conformer
(photochromic active) and parallel conformer (photochromic
inactive). This observation agrees with the possible steric
hindrance to the free rotation of the two thiophene rings exerted
by the protons at the 4- and 7-positions of the phenanthroline
ring.3
The two conformers were distinguished from each other in
terms of the orientations of the thiophene rings. The antiparallel
conformer has two thiophene rings in C2 symmetry, while those
of the parallel conformer are in Cs symmetry.9 The crystal
structures of the two conformers of 1 were determined by X-ray
crystallography, as depicted in Figure 1. The ratio of the
population of parallel to antiparallel conformers in the cocrystal
was found to be approximately 2:1, as revealed from the disorder
of one of the thiophene rings in 1. This is the first example of
X-ray crystal structures in which both antiparallel and parallel
conformers of a single photochromic diarylethene molecule have
been resolved. The interplanar angles between the two thiophene
rings are 65.9 and 55.3° in the antiparallel and parallel
conformers, respectively. The mean Pt-N bond distance of 1
is found to be 2.059(8) Å, which agrees with the trans influence
from the σ-bonded alkynyl group in comparison with those from
the chlorides in [Pt(phen)Cl2]10 (Pt-N bond distance 2.033(6)
(7) (a) Palocsay, F. A.; Rund, J. V. Inorg. Chem. 1969, 8, 524. (b)
Palocsay, F. A.; Rund, J. V. Inorg. Chem. 1969, 8, 2242.
(8) Sonogashira, K.; Fujikura, Y.; Yatake, T.; Toyoshima, N.; Takahashi,
S.; Hagihara, N. J. Organomet. Chem. 1978, 145, 101.
(9) Woodward, R. B.; Hoffmann, R. In The ConserVation of Orbital
Symmetry Verlag Chemie: Weinheim, Germany, 1970; p 38.
(10) Hazell, A.; Mukhopadhyay, A. Acta Crystallogr. 1980, B36, 1647.
(11) Castellano, F. N.; Pomestchenko, I. E.; Shikhova, E.; Hua, F.; Muro,
M. L.; Rajapakse, N. Coord. Chem. ReV. 2005, 250, 1819.

14 Organometallics, Vol. 26, No. 1, 2007
Communications
Table 1. Photochemical Quantum Yields of L and 1 in
Dichloromethane Solution at 298 Ka
photocyclization
photocycloreversion
φ313
φ420
φ366
φ510
L
1
0.272
0.035
0.029
0.014
0.012
0.030
a Values reported are corrected for the ratio of the photochromic active
conformation.
Scheme 2. Qualitative Energy State Diagram of Lo and 1o
Figure 3. Overlaid normalized corrected emission spectra of (a)
the open form 1o (A) and the closed form 1c (B) and the species
at the photostationary state (C) in dichloromethane solution at 298
K and (b) the open form 1o (D) and the closed form 1c (E) in
EtOH-MeOH glass (4:1 v/v) at 77 K.
1o also gives rise to photocyclization.2f,3 Figure 2b shows the
absorption spectral changes upon MLCT/LLCT excitation at λ
) 420 nm. The quantum yield for the photocyclization reaction
of 1o when compared to that of Lo at λ ) 313 nm was found
to be significantly reduced (0.035 vs 0.272) (Table 1). In view
of the similar quantum yields (0.035 vs 0.030) obtained upon
excitations into the IL and the MLCT/LLCT bands of 1o, it is
likely that an efficient energy transfer from the IL state to the
MLCT/LLCT state has occurred upon excitation into the IL
Table 2. Photophysical Properties of Lo, 1o,
[Pt(phen)(CtCPh)2], and [Pt(5,6-dmphen)(CtCPh)2]a
λ
abs/nm
λ
em/nm
(τo/µs)
b
(ꢀ/dm3 mol-1 cm-1
)
φem
Lo
1o
270 (23 220), 316 (4970)
268 (68 000), 390 (11 480) 580 (1.6)c
405 (<0.1)c 0.006d
0.18e
0.42
0.6
1
band. Once the MLCT/LLCT state is populated, intersystem
[Pt(phen)(CtCPh)2]f 268 (58 400), 396 (7900)
561 (1.9)
554 (2.3)
3
crossing to the MLCT/LLCT state would occur that leads to
[Pt(5,6-dmphen)-
279 (55 700), 377 (7000),
404 (7900)
(CtCPh)2]f
3
the sensitization of the photochromic reactive IL state, which
may be very close in energy and in equilibrium with the 3MLCT/
LLCT state, going through the same common pathway as the
MLCT/LLCT-sensitized photochromic reaction (Scheme 2).2f,5f
The extension of the excitation wavelengths to the visible region
up to λ e 480 nm to trigger the photochromism of 1 favors the
use of less destructive visible light sources and is desirable.
Upon excitation into the absorption bands of 1c in the visible
regions with λ g 500 nm, the absorbance at 560 nm drops
accordingly. This indicates that photocycloreversion has taken
place, regenerating the open form of 1. The quantum yield for
the photocycloreversion reaction of 1c upon excitation at
510 nm is found to be similar to that of Lc.
Upon excitation at λ g 300 nm, Lo in dichloromethane
solution produced luminescence at 405 nm (τo < 0.1 µs). With
reference to our previous works,3 this emission is tentatively
assigned as IL (π f π*) fluorescence in origin, most probably
derived from the phenanthroline moiety. On the other hand, upon
excitation into the MLCT/LLCT band at λ g 390 nm, 1o in
dichloromethane solution displayed luminescence at 580 nm (τo
) 1.6 µs, φem ) 0.18), in the neighborhood of the emission
maximum of the close analogue [Pt(phen)(CtCPh)2] observed
at 561 nm (τo ) 1.9 µs, φem ) 0.42).5e It is therefore assigned
as 3MLCT (5d(Pt) f π*(Lo))/LLCT (π(CtC) f π*(Lo))
phosphorescence (Figure 3a). The lower luminescence quantum
yield of 1o in compared to the yields of [Pt(diimine)(CtCR)2]
analogues reported in the literature5e may be ascribed to the
presence of an intramolecular energy transfer pathway from the
3MLCT/LLCT state to the photochromic reactive 3IL state, i.e.,
the possible quenching by the sensitized photocyclization,
rendering the photoluminescence of 1o from the 3MLCT/LLCT
state less efficient (Table 2). The emission spectrum of 1o in
EtOH-MeOH glass (4:1 v/v) at 77 K displayed well-resolved
a 5,6-dmphen ) 5,6-dimethyl-1,10-phenanthroline. b Absorption maxima
measured in dichloromethane solution. c Emission maxima measured in
degassed dichloromethane solution at room temperature. d Data from ref
3a. e Values reported are relative to [Ru(bpy)3](PF6)2 in degassed acetonitrile
solution (φ ) 0.062). f Data from ref 5e.
vibronic structures at 512 nm (τo ) 11.0 µs) with vibrational
progressions of around 1250 cm-1, typical of the ring-breathing
mode of the phenanthroline ligand. On the basis of the similarity
of its band shape to the 3IL (π f π*) emission of [Pt(phen)2]-
Cl2 in EtOH-MeOH glass (1:3 v/v) at 77 K and the red shift
of emission energy relative to [Pt(phen)2]Cl2 (λem ∼ 460 nm),12
the emission band of 1o in 77 K glass is assigned as derived
from states of a metal-perturbed 3IL (Lo) origin, probably with
3
some mixing of a MLCT/LLCT character.
Upon conversion of Lo to Lc at the photostationary state, the
emission maximum was found to show a red shift to 600 nm
(τo < 0.1 µs) in dichloromethane at 298 K and to 577 nm (τo
) 5.2 µs)3a in EtOH-MeOH glass (4:1 v/v) at 77 K upon
excitation at λ g 400 nm. The red shift in emission maximum
is probably attributed to the increase in the extent of π
conjugation in the closed form. In dichloromethane solution,
1c emits with a maximum at 650 nm, while in EtOH-MeOH
glass (4:1 v/v) at 77 K its emission spectrum exhibits fairly
well-resolved vibronic-structured bands at 630 nm. The close
resemblance of the emission spectral band shapes of 1c
and those of Lc phosphorescence as well as the relatively
small red shift of emission energy from Lc to 1c (Figure 3b)
in EtOH-MeOH glass (4:1 v/v) at 77 K suggest that the
origin of the emission of 1c is metal-perturbed 3IL in
character, probably with some mixing of a 3MLCT/LLCT
(12) Miskowski, V. M.; Houlding, V. H. Inorg. Chem. 1989, 28, 1529.

Communications
Organometallics, Vol. 26, No. 1, 2007 15
character. The emission lifetimes of 1c were not obtained, due
to the inherently weak emission intensities at 77 K. A similar
weakly emissive IL state was reported for the closed form of
diarylethenes, in which the distinct emission band was measured
to be at 630 nm.13 We therefore speculate that the IL excited
state becomes lower lying in energy in comparison to the
MLCT/LLCT state in its closed form, which renders the IL
excited state as the predominant emissive state, probably with
some mixing of a MLCT/LLCT state into the emission origin;
in contrast, the MLCT/LLCT state of 1o is the predominant
emissive state, as it is lower lying than the IL excited state.
Acknowledgment. We acknowledge support from the
University Development Fund and the Faculty Development
Fund of The University of Hong Kong, the URC Seed Funding
for Strategic Research Theme on Organic Optoelectronics, the
HKU Foundation for Educational Development and Research
Limited, and the RGC Central Allocation Vote (HKU 2/05C).
J.K.-W.L. acknowledges the receipt of a postgraduate student-
ship and C.-C.K. the receipt of a University Postdoctoral
Fellowship. This work has been supported by the Research
Grants Council of the Hong Kong SAR, People’s Republic of
China (Project No. HKU 7050/04P).
To conclude, we have successfully incorporated a photochro-
mic diimine ligand to a platinum(II) center demonstrating
3MLCT/LLCT excited-state photosensitized photochromism
while maintaining inherent photoluminescence behavior.
Supporting Information Available: Text giving details of the
characterization of 1 and tables and a CIF file giving full
crystallographic descriptions of two conformers of 1. This material
is available free of charge via the Internet at http://pubs.acs.org.
(13) Shim, S.; Joo, T.; Bae, S. C.; Kim, K. S.; Kim, E. J. Phys. Chem.
A 2003, 107, 8106.
OM060731T