Luminescent metal complexes containing naphthalene and anthracene units 3

72





Chapter 3

The Spectroscopy and Photochemistry
of
Pt(II) Complexes
















73

3.1 Spectroscopy of d
8

Metal Complexes
Luminescent coordinatively unsaturated metal complexes are appealing from a
photochemical perspective. Saturated congeners such as [Ru(bpy)
3
]
2+
are restricted to outer-
sphere interactions with substrates. Chromophores that allow inner-sphere electron-transfer
reactions and applications for chemical sensing,
1
solar energy conversion, and photocatalysis
2

have been developed. Investigation into square planar d
8
platinum(II) compounds have been
prominent since this class of molecules can mediate excited-state atom transfer reactions and
bond activation. In particular, the prolific excited-state chemistry of the binuclear derivative
[Pt
2
(µ-P
2
O
5
H
2
)
4
]
4-

has been demonstrated.
3
The triplet (dσ*, pσ) excited state, which is a
manifestation of the d
8
-d
8
interaction between the diplatinum centers is capable of C-H, and C-
halogen bond cleavage and electron-transfer reactions. Understanding the nature of the exited
state of platinum is useful for the future development of photocatalysts for light to chemical
energy conversion as well as other possible practical uses.
Reviewing many of the d
8
complexes, especially those of Pt(II), it can be determined that
there are several types of absorptions observed in the near UV-visible region such as intraligand
π→π* (IL) and metal-to-ligand charge transfer (MLCT) transitions. Each of these types of
absorptions can have luminescence associated with the lowest spin-forbidden state if it is the
lowest in energy. If both metal-metal and ligand-ligand interactions exist in dinuclear platinum(II)
complexes, these interactions could give rise to a number of electronic transitions. A simplified
molecular-orbital diagram is shown in Figure 3.1.
4
The HOMO is a σ*(π) orbital from the ligand,
but the energy of the d
σ*
and the σ*(π) is very close. Sometimes the low-energy absorption is
mixed with the
1
MMLCT (the metal-metal-to-ligand charge transfer) and
1
IL. Most of the

information regarding the excited states of Pt(II) complexes have been obtained from studies of
luminescence from both solid state and solution.
74



Figure 3.1 Simplified schematic molecular-orbital diagram of platinum(II) complexes of
polypyridine ligands with metal-metal and ligand-ligand interaction

The nature and energy of the excited state in platinum complexes varies with the ligands.
In the majority of complexes formed by low-valent transition metal atoms and ligands with low
lying empty orbitals, in particular α-diimine, the lowest excited state is for the metal–to-ligand
charge transfer, however, co-ligands with relatively high lying filled orbitals may participate in
the orbital from which low lying electronic transitions originated (highest occupied molecular
orbital HOMO). For instance, in Pt(α-diimine)X
2
(X = CN), the energy and nature of the emitting
state were dependent on the anionic ligand. The emission was proposed to originate from a state
involving Pt(dz
2
) and diimine (π*) orbitals,
5
and excimer emission has been detected from
concentrated solutions. For the dithiolate and tridentate complexes, the lowest energy excited
state has been attributed to either a triplet metal-to-ligand charge transfer state (
3
MLCT) or a
ligand-based state; either a ligand-ligand charge transfer (LLCT), or single ligand centered triplet
excited state
3

LC.
6
However, in complexes of metals in low oxidation states like Pd(II) and Pt(II),
ligand-to-metal charge transfer transitions cannot be responsible for the observed luminescence.
It is easy to distinguish LC and MLCT excited states: 1) LC luminescence is only slightly red
shifted from the emission of the corresponding free-protonated ligand. The larger the energy-shift,
5dz
2

p
σ*


π
5d
z
2

π
*

p
z

π
*

π
σ
*(

π
)

d
σ

σ
*(
π
)

d
σ*


σ(π*)

p
σ

p
z

σ
*(
π
*)


i ii iii

i) MMLCT
d
σ*
→ σ(π*)
ii) metal centered
d
σ*
→pσ
iii) ligand-ligand
σ*(π)→ σ(π*)


75

the higher is the MLCT character of the emitting excited state; 2) LC luminescence is less
sensitive to the presence of the metal atom than MLCT emission. Thus, the longer the radiative
lifetime, the purer is the LC character of the emitting excited state; and 3) The energy of MLCT
transitions is expected to depend on the solvent polarity because of the change in the dipole
moment of the molecule, whereas the energy of the LC transition is expected to be less
influenced by the solvent.
6

Platinum(II) diimine solids exhibited unusual colors, strong emission, and highly
anisotropic properties as a result of intermolecular stacking interactions. A series of square plane
of Pt(diimine) complexes with anionic ligands such as arylacetylides were synthesized and their
absorption and emission were compared. Their emissions will shift to lower energy with the
increase of electron-withdrawing ability of the diimine substitute and the increase donation of the
arylacetylide ligands. The behavior is consistent with a mainly metal-based HOMO and a π*
diimine

LUMO. The assignment is consistent with the notion that variation of the diimine affects the
energy of the lowest unoccupied molecular orbital, and that the variation of the arylacetylide
leads to only minor change in the Pt-based HOMO.
5
Similarly, the luminescent properties of
[Pt
II
(tpy)X]
n-
( tpy = terpyridine) derivatives and oligomers have been extensively investigated.
Such processes were found to give low energy triplet emission in the 600-700 nm region
(MMLCT: dσ*-π*) and the excimeric ligand-to-ligand (π-π*) excited state.
7
A series of discrete
d
8
-d
8
complexes, namely, [Pt(tpy)]
2
(µ-L)
n+
( L = bidentate ligand) and [{Pt(CNN)
2
}(µ-L)]
n-

( CNN=6-pheny-2,2’-bipyridine) have been prepared to model the low-energy [dσ(dz
2
)(Pt)-

π*(diimine)] and excimeric ligand-to-ligand emissions.
8

Some other factors also affect the nature of the excited state of platinum(II) complexes as
below:
1) Temperature. The red-shift of the solid-state emission upon cooling can be rationalized by the
shortening of intermolecular Pt-Pt and π- π separations in the crystal lattice, which results in
3
[dσ*, π*] emissions of lower energy. However, the blue shift of the emission is observed in
solution of a Pt complex that exhibits
3
MLCT.
9

76

2) Concentration. The red-shifted emission at higher concentration is assigned to an excimer
emission or oligomer emission.
7, 10

3) Solvent. The solvatochromic shift of the lower-energy absorption indicates the
1
MLCT nature.
Unlike the
1
MLCT absorption band which displays discernible solvatochromic behavior, the
3
MLCT excited state shows minimal variation in different solvents. MMLCT also shows the
solvatochromic shift. The respective emission energy, lifetime, and quantum yield are highly
sensitive to the solvent polarity but insensitive to the complex concentration. The energy

decreases from aqueous solutions to chloroform solutions.
11

4) Solvent acidity, basicity and other reasons. For example, to the ligand of Pt complexes with N,
S atoms which keep uncoordinated proton, changing the pH is attributed to protonation of N or S
and can affect the ability of energy of the π*
diimine
LUMO. The different protonation behaviors
between the two complexes results from the difference in their electronic structure.
12
The energy
of
3
MMLCT is highly dependent on the counterion (PF
6
-
, ClO
4
-
, Cl
-
, CF
3
SO
3
-
) in line with the
different colors of the various salts.
13

There are several ways to clarify Pt(II) complexes. One is to use the Pt-Pt distance in the
solid state crystal structure. A complex is considered to be monomeric if it is in a lattice where
the nearest Pt-Pt distance is larger than 4.5 Å and if the solid state emission spectrum is not
significantly different from that in dilute solution or a glass. A linear chain structure is one in
which the Pt(II) complexes are stacked equidistantly along an axis that is usually but not always
perpendicular to the plane of the complex. Typically, the Pt-Pt distance is from 3.2 to 3.4 Å. A
dimeric structure contains pairs of complexes with an intermolecular distance that is clearly
shorter than the distance of the next pair.
14
However, Pt-Pt distance and π-π stacking of the
ligand are different in solid and solution. In this chapter Pt(II) complexes are separated into two
classes according to the structure and whether a bridge connects the two Pt units.



77

3.1.1 Mononuclear Platinum Complexes
In this section, the case that is presented contains only a single platinum center with
primarily the ligand 6-phenyl-2, 2’-bipyridine (CNN: this indicates C and two N atoms are the
coordination sites).
N
N
R
Pt
C
l
l : R=H
2 :R=Ph
3: R=4-ClPh

4: R=4-MePh
5: R=MeOPh
6: R=3,4,5-(MeO)
3
Ph
F
i
g
u
r
e

3
.
2


Complexes Pt(L
1-6
)Cl, 1-6
15
(Figure 3.2) had similar absorptions in CH
2
Cl
2
. The
absorbance λ < 370 nm, was dominated by
1
IL (π→π*), while the absorbance around 438 nm
was assigned to the

1
MLCT [(5d)Pt→π*(L)] transition and the weak absorption tails at 519 - 525
nm (ε < 60 dm
3
mol
-1
cm
-1
) wais attributed to the
3
MLCT. They also exhibited emission in solution
or in the solid state. They exhibited an emission about 565 nm at 298 K and shifted somewhat at
77 K in CH
2
Cl
2
and were assigned to the
3
MLCT. In the solid state, the emission about 565 nm at
298 K in CH
2
Cl
2
was due to a
3
MLCT and changed a little in a glass at 77 K. The emission
maximum in complexes 1, 5, and 6 were shifted to 700 nm, 716 nm, 722 nm respectively and
assigned to
3
[dσ*, π*] because of the shortening of the intermolecular Pt-Pt and π-π separations

in the crystal lattice which resulted in
3
[dσ*, π*] emissions of low energy. In contrast complexes
2-4 exhibited a blue shift of the
3
MLCT emission at 77 K. Other compounds with a CNN type
ligand also showed similar photophysical properties as complexes 1-6. With the increase of
electrophilicity of the metal center, the emission is expected to be shifted to higher energy.
78

N
N
Pt
C
N
R
N
N
Pt
C
O
R
=
i
B
u
,

7

;

n
B
u
,

8

;

i
P
r
,

9
;

C
y
,

10
;

2
,
6
-

M
e
2
P
h
,

11
12
F
i
g
u
r
e

3
.
3


In CH
2
Cl
2
or CH
3
CN solutions (concentration between 10
-6
- 10

-3
M), in the higher-
energy region, the vibronically structured absorption of complexes 7 - 11 (Figure 3.3) centered
at 340 nm (ε ≈ 10
4
M
-1
cm
-1
) were assigned to an interligand
1
IL (π- π*) of the CNN group. The
absorptions at 390-430 nm (ε ≈ 10
2
M
-1
cm
-1
) were attributed to
1
MLCT and the 529 nm band was
assigned to a
3
MLCT, which can be self quenching when the concentration was increased from
10
-5
M to 10
-3
M. When the C > 10
-3

M, the band at λ
max
= 510 nm (ε ≈ 120 M
-1
cm
-1
) was
assigned to
1
(dσ*→π*) (
1
MMLCT) transitions. The emissions below 600 nm were assigned to
3
MLCT whatever in solution or solid phrase and emissions above 700 nm were assigned to
3
[dσ*-
π*] (MMLCT). At higher concentration at 77 K, the complexes 8 and 10 exhibited peaks at 500
nm and 710 nm respectively which were attributed to
3
MLCT (8) and
3
MMLCT (10). Peaks were
observed at 600 - 625 nm and were tentatively ascribed to the excimeric intraligand excited state
arising from a weak π-π stacking interactions of the CNN ligand. The emissions below 600 nm in
the solid were attribute to
3
MLCT with excimeric character due to weak CNN ligand π-π
interactions. The orange cyclohexyl isocyanide complex 9 exhibited a broad structureless
emission at λ
max

= 625 nm which was an excimeric
3
IL transition resulting from π-stacking of the
ligand and emissions above 700 nm were assigned to the
3
[dσ*-π*]. For complexes 7 and 11 at
77 K, a dramatic change occurred in the band shape when the concentration was increased. The
λ
max
shifted from 505 nm to 627 nm for complex 7 and 519 nm to 730 nm for complex 11. The
former was tentatively ascribed to an excimeric intraligand emission arising from weak π-
79

stacking of the CNN ligand and the latter can be attributed to a MMLCT excited state resulting
from oligomerization of the Pt(II) centers in a glassy matrix.
14,

16
Emissions below 500 nm were
assigned to a
3
IL transition.
14, 17
The low-temperature emission spectra (5:5:1
ethanol:methanol:DMF) of [Pt(tpy)Cl]
+
revealed a 740 nm band indicative of M-M
oligomerization (
3
MMLCT), a 650 nm band attributable to tpy π-π interactions (

3
MLCT) and a
470 nm band characteristic of mononuclear [Pt(tpy)Cl]
+
π-π* emission at the higher
concentration. At low temperature in the solid state only a
3
MMLCT emission was observed.
From these results, it can be concluded that the platinum photoluminescent properties
can be systematically tuned through modification of the diimine or cyclometalated ligand. Their
square-planar geometry confers different photophysical and photochemical properties from those
of the octahedral [Ru
II
(bpy)
3
]
2+
complexes. The red shift of the MLCT transition of Pt(II)-
derivative is through metal-metal and ligand-ligand interactions. By recording the excitation
spectrum, a well-resolved absorption band at 500 nm is substantially red-shifted from the
absorption spectrum of monomeric derivatives.
Che
18
also attempted to change the σ –donating strength of the alkynyl ligand of
Pt(CNN)(C≡CR), which destabilized the dπ(Pt)→HOMO of the relatively low-energy MLCT
5d(Pt)→π*(CNN) transitions. Electron-withdrawing groups stabilized the Pt-based HOMO to
yield blue-shifted emissions. Other Pt complexes with an α-diimine ligand and an acetylene
ligand
19
had the same nature of the excited state. Because of the electron-withdrawing nature of

the substituted bipyridine and phenanthroline ligands, the emissions exhibit a red shift, although
Pt(4,4’-dtbpy)(C≡C-C≡CPh) (13, Figure 3.4) displayed a strong solid-state and solution
phosphorescence at 77 K and 298 K. The associated excited state was proposed to arise from a
triple intraligand (
3
ππ*) translation from the (C≡C-C≡CPh) unit and a
3
MLCT [Pt→ π*(diimine)]
transition from excimer emission. In the UV spectrum, the
1
MLCT (5d(Pt)- π*) overlapped with
the
1
IL for the low lying HOMO in the 250-350 nm region.
80

N
N
Pt
C
C
C
C
F
i
g
u
r
e

3
.
4
13

3.1.2 Platinum Complexes Linked by a Bridge
When two platinum(II) units are in close proximity so as to allow a metal-metal and a
ligand-ligand (π-π) contact, a low-energy photoluminescence which was red shifted from the
3
MLCT emission of the mononuclear species was typically observed. The electronic excited-state
associated with this emission was denoted as
3
[dσ*, π*] as discussed in the literature.
3, 4, 15-17, 20-22

The binuclear Pt-Pt complexes can result in an increase in the metal-metal and ligand-ligand (π-π)
interaction. They also can be spectroscopically characterized by model intermolecular
interactions in Pt(II) polypyridine species and to investigate the photophysics and solid-state
structures of cyclometalated Pt(II) oligomers.
A series of binuclear cyclometalated Pt(II) complexes,
15
Pt
2
(L)
2
(µ-dppm)(ClO
4
)
2
14-19,

Pt
2
(L
1
)(pz)(ClO
4
)
2
20, Pt
2
(L
1
)(dppC
3
/C
5
)(ClO
4
)
2
21 and 22 were synthesized in order to examine
solution and solid-state oligomeric d
8
-d
8
and ligand-ligand interactions (Figure 3.5). The
intramolecular Pt-Pt bond was 3.245 Å and 3.612 Å for complexes 17 and 20 respectively as
determined by X-ray crystal structures. The UV-absorption spectra for complexes 14-20
exhibited λ
max

> 400 nm were due to
1
[dσ*-π*] transition. Complexes 14-19 also exhibited
structureless emissions with peak maxima at 654 - 662 nm in CH
3
CN at room temperature which
were blue shifted to 633 - 644 nm at 77 K. The solid state also showed emission at 298 K but at
77 K the energy was red-shifted. This may be attributed to the shortening of the Pt-Pt distances
due to lattice contraction. Though a bridge existed in 21 and 22, the Pt-Pt distance was too long
for the length of the bridge. The absorption tail at λ > 500 nm represented the transition between
81

1
MLCT and
1
[dσ*-π*]. Complexes 21 and 22 had an emission band at 570 nm due to a
3
MLCT.
However, at 77 K in CH
3
CN, three peaks at 555, 590, 651 nm appeared in the spectrum of 22 and
were indicative of weak π-π interaction, assigned to
3
MLCT and
3
[dσ*-π*].

N
N
Pt

N
N
Pt
N
N
20
14: R=H
15: R=Ph
16: R=4-ClPh
17: R=4-MePh
18: R=MeoPh
19: R=3,4,5-(MeO)
3
Ph
N
N
R
Pt
Ph
2
P
N
N
R
Pt
PPh
2
F
i
g

u
r
e

3
.
5
2+
N
N
Pt
N
N
Pt
Ph
2
P
PPh
2
n
n
=
3
,
21
;

n
=
4

,

22
2+
2+


82

N
N
Pt
N
N
Pt
C
C
N
N
2
+
23
F
i
g
u
r
e

3

.
6


The complex [(CNN)Pt]
2
(C≡N(CH
2
)
3
N≡C)](PF
6
)
2
23
20
(Figure 3.6) also, has a bridge
that brings the two Pt centers closer. The 340 nm (ε = 10
4
M
-1
cm
-1
) absorption (in CH
2
Cl
2
) was
assigned to the intraligand
1

IL (π-π*) transition and intense

low-energy bands with λ
max
in the
range 390-430 nm (ε = 102 M
-1
cm
-1
) were assigned to
1
MLCT [(5d)Pt→π*(CNN)] transitions.
Though compound 23 had absorption at 511 nm, it was not similar to the
1
[5d
σ
*→6p
σ
*] transition
in Pt
2
(µ-P
2
O
5
H
2
)
4
.

2a
In solution, the emission spectrum of 23 had two peaks at 555 nm for a
3
MLCT and 630 nm for
3
IL. However when reducing the temperature to 77 K, the 600 nm was
observed which was assigned to
3
MMLCT in the glass matrix. In the solid state, a band at 711
nm was observed at 298 K and had red shift to 744 nm at 77 K. From these data, the nature of the
exited state was determined by the metal-metal bond and the distance between the two ligands. If
the bridge is quite long, the emission and absorption spectra are similar to those of the
mononuclear system; if the alkyl bridge between the Pt centers is reduced, the [CNN]Pt groups
would exhibit dinulcear properties. (Figure 3.7)

N
N
Pt
N
N
Pt
N
N
Pt
N
N
Pt
F
i
g

u
r
e

3
.
7

83

The electronic absorption spectrum of [Pt
2
(µ-dppm)
2
(C≡CC
5
H
4
N)
4
] (24) in ethanol-
dichoromethane (1: 4 v/v) showed a low-energy absorption at ca. 368 nm with low-energy tails
that extended to ca. 500 nm.
21
According to the previous study,
22
the absorption band were
assigned to spin-allowed and spin-forbidden metal-metal-to-ligand-transfer (MMLCT)
[d
σ

*(Pt
2
)→p
σ
(Pt
2
)/π*(C≡CR)] transitions.
Complexes M
2
(dcpm)
2
(CN)
4
(M=Pt, 25; Pd, 26) in CH
2
Cl
2
showed absorption bands at
337 nm (ε = 2.41x10
4
M
-1
cm
-1
) and 328 nm (ε = 2.43x10
4
dm
3
mol
-1

cm
-1
) which were assigned to
1
(5d
σ
*→6p
σ
) electrons transitions originating from a Pt(II)-Pt(II) interaction. There was also a
weak, long-wavelength shoulder at 388 nm (ε = 300 M
-1
cm
-1
) and 375 nm (ε = 50 M
-1
cm
-1
) which
was assigned to spin-forbidden analogues of the intense bands.
23

3.2 Photochemistry of Pt(II) Complexes.
The previous study of Pt(II) complexes showed that there were rich photophysical and
photochemical properties. Some of them even had emission in solution at room temperature.
There were have different excited states achieved by tuning different ligand coordinated with
Pt(II). This type of complexes has potential use as chromophores for the conversion of light-to-
chemical energy. Efforts are now focused on the use of the Pt(diimine)X
2
chromophores in dyads
and triads with the goal of constructing a molecular photochemical device for light-to-chemical

energy conversion.
24
Connection of the Pt diimine chromophores to both a donor or reductive
quencher and an acceptor is envisioned through new ligand bridges currently being synthesized
using Pd-catalyzed coupling reactions and carbonyl condensations. Three fields are discussed
next for Pt(II) complexes .
3.2.1 Induced Electron Transfer in Pt(II) Complexes
Photoluminescent transition metal complexes are playing an increasingly useful role as
probes of electron and energy transfer involving DNA and proteins
.
25, 26
Photoinduced electron
transfer can be regarded as a process where absorbed light energy is transformed into chemical
energy. After formation of the equilibrated excited data, the subsequent event is the actual
transfer of the electron. The property of some excited states acts is similar to such process as
photosynthesis. Multicomponent molecules have been designed and constructed containing an
84

electron donor, an electron acceptor, and a metal complex-based charge transfer chromophore.
Most commonly examined transition metal chromophores are octahedral d
6
diimine complexes
such as Ru(diimine)
3
2+
and Re(diimine)(CO)
3
(py)
+

27
which have a
3
MLCT excited state.
Recently a relatively new transition metal chromophore, containing platinum complexes, also
possesses a long-lived
3
MLCT excited state. The nature of this excited state is similar to the
charge transfer excited states of d
6
diimine chromophores.
The Eisenberg
28
group used it to understand the factors influencing the longevity of
charge separation in a multicomponent system containing a novel platinum diimine chromophore.
The luminescent complex [Pt(terpy)OH]BF
4
(2,2’:6’,2”-terpyridine)
29
underwent photoinduced
electron transfer reactions with phenyl amine electron donors and nitrophenyl electron acceptors.
Stern-Volmer analysis of the quenching of metal-to-ligand charge transfer phosphorescence
(
3
MLCT) was used to calculate bimolecular rate constants for electron transfer. Rate constants
varied from 10
8
to 10
10
M

-1
s
-1
, depending on the thermodynamic driving force of the electron
transfer reaction. The rate constants indicated that [Pt(terpy)OH]BF
4
is a powerful photo-oxidant.
Aromatic triplet energy acceptors can also quench the
3
MLCT emission. When this complex
bound to a guanine residue, the
3
MLCT emission was completely quenched; an effect attributed
to photoinduced electron transferred from guanine to the platinum complex excited state.
26a, d

A bridge connecting both the electron donor and the electron acceptor is also useful to
transferring an electron. Mixed valence compounds have attracted considerable attention because
of their capability for photoinduced electron transfer, which has potential applications in energy
conversion and photocatalysis. In such applications, the ability to transfer multiple electrons with
a single photon is much desirable. Platinum seems to be suited ideally as the basis for such a
complex capable of carrying photoinduced multielectron charge transfer because of the stability
of Pt(II) and Pt(IV) complexes and the short-lived and unstable nature of Pt(III). Pt(II) and Pt(IV)
prefer different geometry, thus charge transfer can also induce the formation and breaking of
coordinate covalent bonds. Thus, it is possible to design species that would be capable of two-
electron charge transfer upon excitation with a single photon.
31
Based on this theory,
[L(NC)
4

Fe(II)-CN-Pt(IV)(NH
3
)
4
-NC-Fe(II)(CN)
4
L]
4-
(L is a CN
-
or a S-donor ligand) was
85

designed and provided such photoinduced multielectron charge transfer processes. These
complexes exhibited intense metal–metal charge transfer (MMCT) bands in the blue portion of
the spectrum (350–450 nm). Irradiation the MMCT band centered at 425 nm produced a new two
electron charge transfer with a quantum yield of 0.01. Well defined oligomers and polymers of
the iron based system which can be synthesized either as soluble materials or as adherent films
on electrode surfaces. The photochemical reactivity and photophysics of these species were
found to be a function of molecular geometry. In the case of the polymeric systems, one-
dimensional, two-dimensional, and network materials can be synthesized, using electrochemical
techniques, to control the polymer reactivity sites. Polymer modified electrodes exhibit a
photocurrent response which is diagnostic for the photochemistry occurring within the film.
Correctly selected polymer morphologies lead to primary photoproducts on the electrode surface
which is capable of oxidizing chloride to chlorine. This chemistry can be used to produce a
photochemical energy conversion cycle in which visible light induces the oxidation of halides to
energy rich halogens.
The ion pair complex, {A
2+
[ML

2
]
2-
}, A
2+
is a bipyridinium acceptor and [ML
2
]
2-
, M=Ni,
Pd, Pt, contains a planar dithiolene ligand, is redox active and light-sensitive. They possess the
same mean reorganization energy for electron transfer from the dianion to the dication. Proper
selection of the acceptor reduction potential and the central metal allows the tailoring of optical
electron transfer induced activation of dioxygen in solution. The primary electron transfer step
affords the oxidized donor [ML
2
]
-
and the reduced acceptor, A
•+
, which reduces dioxygen to
superoxide. The latter reaction can compete with back-electron transfer step when the reduction
potential of A
2+
is more negative than -0.6 V. The overall reaction proceeds only in the case of M
= Pt, suggesting that the photoreactive state has ion pair charge transfer character.
32

3.2.2 Reaction with Oxygen
In photooxidation, singlet oxygen is a reactive intermediate, produced from molecular

oxygen. The molecular orbital diagram shows that the electronic configuration of oxygen is as
follows: (1σ
g
)
2
(1σ
u
)
2
(2σ
g
)
2
(2σ
u
)
2
(3σ
g
)
2
(3σ
g
)
2
(1π
u
)
4
(1π

g
)
2
, so the ground state of the oxygen is thus
a triplet state, denoted
3
Σ
g
-
utilizing following spectroscopic state notation
1
∆
g
. ( Figure 3.8)
86


Figure 3.8 Simplified representation of the electron occupation of the 1π
g
(π
px,y
*) orbital in
the ground and in the first two excited states of molecular oxygen
33


If the triplet ground state of oxygen absorbs energy two kinds of excited state
1
∆
g

and
1
Σ
g
+
may form. However the electronic transitions between them are spin-forbidden transitions.
Singlet oxygen can be produced by photosensitization or by thermal processes (Equation 3.1).
Sens(S
0
)
hv
Sens*(S
1
)
I
S
I
Sens*(T
1
)
Sens*(T
1
)
+
3
O
2
Sens(S
0
)

1
O
2
+
Equation 3.1


Platinum(II) and Palladium(II) diimine complexes have strong absorptions assigned as
3
MLCT,
3
LC, etc.
34
They can sensitize the formation of singlet oxygen (
1
O
2
) forming ground-
state oxygen as a chromophore. Several mixed ligand Pt(II) and Pd(II) generate
1
O
2
on irradiation
at the LLCT band.
34a
Their ability to photosensitize depends on the metal ion, the number of
metal ions in the complex, the nature of the diimine ligand and distortion from planar geometry
of the metal complex.
There are two mechanisms proposed for the photooxidation of metal complexes. One is
through electron transfer to form a radical cation complex and superoxide (O

2
-
) and the other
involves a ground state complex and a singlet state oxygen,
1
O
2
(exited state) through energy
transfer.
0
3
Σ
g
-

94.2
1
∆
g

E

[kJ•mol
-1
]


156.9

1

Σ
g
+

(1π
g
)
(1π
g
)
(1π
g
)
87

S
S
Pt
Ph
Ph
+
O
2
hv
S
S
Pt
Ph
Ph
+

H
2
O
2
S
S
H
H
+
O
2
hv
S
S
H
H
O
O
S
S
H
H
O
O
O
O
S
S
H
H

O
O
F
i
g
u
r
e

3
.
9
N
N
t-Bu
t-Bu
N
N
t-Bu
t-Bu
Pt
N
N
t-Bu
t-Bu
Pt
N
N
t-Bu
t-Bu

Pt
N
N
t-Bu
t-Bu
Pt
N
N
t-Bu
t
-
B
u
(dbbpy)Pt
II
(edt)
(dbbpy)Pt
II
(dpdt)


Complexes (dbbpy)Pt
II
(edt) and (dbbpy)Pt(dpdt) (dbbpy = 4,4’-di-tert-butyl-
2,2’bipyridine; dpdt = meso-1,2-diphenyl-1,2-ethanedithiolate; edt = 1,2-ethanedithiolate) were
photostable in deoxygenated solution. However, photolysis in the visible charge transfer band in
air-saturated solutions induced moderately efficient photooxidation. Photooxidation of
(dbbpy)Pt
II
(dpdt) produced the dehydrogenation product (dbbpy)Pt

II
, In contrast, photooxidation
of (dbbpy)Pt(dpdt) produced S-oxygenated complexes in which one or two thiolated ligands
were converted to a sulfinated(-SO
2
R) ligand. Mechanistic photochemical studies and transient
absorption spectroscopy revealed that photooxidation occured: 1) energy transfer from the charge
88

transfer from a diimine excited state of (dbbpy)Pt
II
(dpdt) to
3
O
2
to produce
1
O
2
; and 2) reaction
between
1
O
2
and the ground state of (dbbpy)Pt
II
(dpdt). Kinetic data indicated that the excited
state (dbbpy)Pt
II
(dpdt) produced

1
O
2
efficiently and that the reaction between the ground state
(dbbpy)Pt
II
(dpdt) and
1
O
2
occured with k = 3x10
8
M
-1
s
-1
.
35
(Figure 3.9)
The violet color of Pt(bpy)(bdt)(bpy=2,2’-bipyridine; bdt=1,2-benzenedithiolate) was
due to a Pt/S→diimine charge-tranfer transition; the emission originated from the corresponding
triplet state (τ = 460 ns). Photochemical oxidation of Pt(bpy)(bdt) occurred in the presence of
oxygen in N,N-dimethylformamide, acetonitrile, or dimethyl sulfoxide solution; the reaction has
been investigated by
1
H NMR and UV-visible absorption spectroscopy. Singlet oxygen produced
by energy transfer of the excited complex was implicated as the active oxygen species. There is
sequential formation of sulfinate, Pt(bpy)(bdtO
2
), and disulfinate, Pt(bpy)(bdtO

4
) products, both
of which have been characterized by X-ray crystallography, The rate of photooxygenation was
strongly dependent on water concentration and transient absorption spectra were consistent with
the formation of at least one intermediate. On the whole, the data suggest that the photooxidation
chemistry of platinum(II) diimine dithiolates was similar to that of organic sulfides.
36
(Figure
3.10)

N
N
Pt
S
S
N
N
Pt
S
S
N
N
Pt
S
S
O
O
O
hv,O
2

O
O O
Figure 3.10
Pt(bpy)(bdt)


89

3.2.3 Reaction with Halocarbon Reactant.
The lowest exited state of Pt(II) is readily to be oxidized to Pt(IV). Irridating a mixture of
the Pt(II) complex and a halocarbon reactant leads to ligand substitution or an oxidation reaction.
The complexes, Pt(CO)(PR
3
)X
2
(X=Cl, Br, I; PR
3
=PEt
3
, PMePh
2
and PPh
3
) exhibited photo-
induced cis-trans isomerization in chlorocarbon solvent such as CHCl
3
and CH
2
Cl
2

.
37

When (TBA)
2
[Pt(Ecda)
2
] in dilute chloroform solution was photolyzed with λ < 360
nm, changes in the absorption spectrum occurred. Four isobestic points between 240 nm and 640
nm were observed during the photoreaction. Similar changes were also observed for the
(TBA)
2
[Pt(i-mnt)
2
] and (PPN)
2
[Pt(Ecda)
2
] in CHCl
3
(TBA=tetra-n-butylamomnium, Ecda=1-
(ethoxycarbonyl)-1-cyanoethylene-2-dithiolate, i-mnt=1,1-dicyanoethylene-2,2-dithiolate, and
PPN=bis(triphenylhosphoranylidene)ammonium). The rate of photolysis was different in CHCl
3
,
and CH
2
Cl
2
, PhCl and PhBr, and followed the rate order: CHCl

3
> CH
2
Cl
2
and PhBr > PhCl.
These results indicated that the process involves photoreduction of the halocarbon solvent and
the rate was dependent on the reduction potential of the halocarbon. If excited by λ > 360 nm, no
photoreaction occurred from out of the lowest-energy excited state.
38

The tetrakis(µ-pyrophosphito)diplatinum(II) tetraanion, Pt
2
(µ-P
2
O
2
H
2
)
4
4-
has a long-lived
phosphorescence at ambient temperature in aqueous solution. This triplet excited state is both a
strong reductant and oxidant. Under photochemical conditions ( λ
max
> 350 nm), the exited state
reacted with alkyl and aryl bromides. The first detectable photo product is Pt
2
(µ-P

2
O
2
H
2
)
4
Br
4-
.
The proposed mechanism followed a S
RN
1
pathway (Equation 3.2).
39

Pt
2
(µ-P
2
O
2
H
2
)
4
4-*
+
RBr
2hv

Pt
2
(µ-P
2
O
2
H
2
)
4
Br
4-
+
R
.
2Pt
2
(µ-P
2
O
2
H
2
)
4
Br
4-
k
2
P

t
2
(
µ
-
P
2
O
2
H
2
)
4
4-
+
Pt
2
(µ-P
2
O
2
H
2
)
4
Br
2
4-
E
q

u
a
t
i
o
n

3
.
2

Another case is Pt(thpy)
2
. thpy
-
is the ortho-C-deprotonated form of 2-(2-
thienyl)pyridine.
40
In CH
2
Cl
2
, CHCl
3
or CH
3
CN/CH
2
Cl
2

solvents, the complex maintained its
luminescent properties and exhibited a photo-oxidative addition reaction with formation of
90

Pt(thpy)
2
(Cl)(R) (R=CH
2
Cl or CHCl
2
) as the sole observed product. In the mixed solvent, the
quantum yield of the photoreaction increased with increasing CH
2
Cl
2
concentration. In neat
CH
2
Cl
2
the quantum yield of the photoreaction was 0.30 and 0.10 for 313 nm and 430 nm
excitation respectively. In CH
2
Cl
2
, complete quenching of the luminescent
3
MLCT excited state
by anthracene via an energy-transfer mechanism was accompanied by only partial quenching of
the photoreaction. On contrast, oxygen was a better quencher for the photoreaction than for the

luminescent emission. In both cases the fraction of quenched reaction depended on the excitation
wavelength. These and other results were interpreted on the basis of a mechanism involving
generation of Pt(thpy)
2
Cl and CH
2
Cl radicals via 1) a charge transfer to the solvent (CTTS)
exited state populated from the intraligand (IL) and metal–to-ligand charge-transfer (MLCT)
state obtained by light absorption, and 2) the thermally relaxed
3
MLCT luminescent level.
Through conversion to CTTS or a bimolecular reaction with CH
2
Cl
2
the primary radicals were
involved in a chain mechanism of the type discussed for other oxidative addition reactions, with
an average chain length of about 40.
41

Photo-oxidizing [(bpy)Pt(tdt)] in CHCl
3
led to unstable [(bpy)Pt(tdt)]
+
upon irradiation with
577 nm light. The dominant photoinduced reaction of Pt(acac)
2
in weakly coordinating solvents
was redox decomposition ( other products arise from the oxidation of a ligand and/or a molecule
of solvent).

42
(Equation 3.3)

Pt(acac)
2
hv
CH
2
Cl
2
Pt
+
Hacac
other products
+
E
q
u
a
t
i
o
n

3
.
3


In CHCl

3
, the [Pt
II
(bpy)Cl
2
] was smoothly oxidized to [Pt
IV
(bpy)Cl
2
] when short
wavelength light (280 nm < λ < 300 nm) was used for irradiation. The mechanism proposed was
shown in Equation 3.4.
43

91

[Pt
II
(bpy)Cl
2
]
+
hv
[Pt
III
(bpy)Cl
3
]
+
.

CHCl
2
[Pt
III
(bpy)Cl
2
]
+
.
CHCl
2
+
-
CHCl
2
[Pt
IV
(bpy)Cl
3
]
+
+
-
C
H
C
l
2
[Pt
IV

(bpy)Cl
4
]
+
:
C
H
C
l
[Pt
IV
(bpy)Cl
3
]
+
CHCl
3
E
q
u
a
t
i
o
n

3
.
4

Photolysis of a chloroform solution of [Pt(bpy)(dmt)] ( bpy = 2,2’-bipyridine; dmt =
dianion of 3,4-toluenedithiol) at 577 nm in the ligand-to-ligand charge-transfer (LLCT) transition
region resulted in electron transfer from the complex to chloroform. A ligand-centered radical
(probably the easily oxidized dmt) [Pt(bpy)(dmt)]
+
decomposed to unidentified products.
A prominent characteristic of square-planar platinum(II) complexes is their ability to
undergo oxidative addition reactions which are markedly dependent on the nature of ancillary
ligands. A photochemical reaction was observed when a degassed acetonitrile solution containing
24 and iodomethane was exposed to UV radiation for 12h at r.t., and a new Pt(IV) cyanide
complex trans-[(CNN)Pt(CN)I
2
] (25) was obtained, the structure of which was determined by X-
ray crystallography. Hence oxidation of the metal center and fragmentation of the diamino-
carbene ligand was apparent.
44
(Figure 3.11)

N
N
Pt
N
Bu
t
H
H
hv
N
N

Pt
N
I
I
CH
3
I
F
i
g
u
r
e

3
.
1
1
24
25
+
ClO
4




92

3.3.3 Objective of Study

DAP (1,8-bis(diphenylphosphino)anthracene) (Figure 3.12) has two PPh
2
groups
connected at the 1,8 position of anthracene and the intramolecular phosphorus distance is about 5
Å.
45
It has several merits as a ligand: 1) The DAP can act as a tridentate PCP ligand; the metal
ion can insert into a C-H aryl bond facilitated by the adjacent phosphines, thus providing
structurally well-defined metal complexes with many interesting properties; 2) Cyclometalated
complexes are known to be strongly emissive and have long luminescent lifetimes (microseconds)
in solution indicative of emission from the triplet excited state. Because of its rigid coordination
and a restricted number of specific coordination sites, the DAP ligand can form cyclometalated
complexes with interesting photochemical and photophysical properties; 3) The extended π
system with the ligand and the strong σ-donating power of the deprotonated carbon donor would
increase the energy difference between the metal-centered, d-d states and the metal-to–ligand
charge-transfer (MLCT) states. The cyclometallated platinum(II) complexes are known to have
low-lying MLCT states with useful photochemical and photophysical properties.
46
Therefore a
series of mononuclear cyclometalated platinum and palladium complexes were prepared. They
exhibited interesting spectroscopic and luminescent behavior and the results will be discussed in
detail in Chapter 4.
.
P P
F
i
g
u
r
e

3
.
1
2




93

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