Photophysics of Pt(II) 4,6-diphenyl-2,2′-bipyridyl complexes in solution and in LB film

Article abstract of DOI:10.1016/j.jorganchem.2009.05.005

Two amphiphilic platinum(II) 4,6-diphenyl-2,2′-bipyridyl complexes (1 and 2) were synthesized and characterized. LB films of these two complexes were prepared and characterized by AFM technique. Electronic absorption and emission characteristics of these

Full text of DOI:10.1016/j.jorganchem.2009.05.005

Journal of Organometallic Chemistry 694 (2009) 2750–2756
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Photophysics of Pt(II) 4,6-diphenyl-2,20-bipyridyl complexes in solution
and in LB film
*
Iswarya Mathew, Wenfang Sun
Department of Chemistry and Molecular Biology, North Dakota State University, 258 Dunbar Hall, Fargo, ND 58108-6050, United States
a r t i c l e i n f o
a b s t r a c t
Two amphiphilic platinum(II) 4,6-diphenyl-2,20-bipyridyl complexes (1 and 2) were synthesized and
characterized. LB films of these two complexes were prepared and characterized by AFM technique. Elec-
tronic absorption and emission characteristics of these complexes in solutions at room temperature, in
glassy solutions at 77 K, and in LB films were studied. The emission energies of the complexes in LB films
are similar to those in acetonitrile solutions at room temperature. However, the lifetimes of 1 and 2 in LB
films are 4–7 times as long as those in CH3CN solutions. The triplet transient difference absorption spec-
tra of these complexes exhibit broad absorption in the visible region to the near-IR region. Introducing a
hydroxyl substituent on the 4,6-diphenyl-2,20-bipyridyl ligand favors the formation of intermolecular
hydrogen bonding and thus helps the deposition of uniform and stable LB films and increases the self-
quenching rate constant for emission of 2 in CH3CN solution.
Article history:
Received 6 February 2009
Received in revised form 5 May 2009
Accepted 6 May 2009
Available online 15 May 2009
Keywords:
Square-planar platinum (II) complex
Photophysics
LB film
Electronic absorption
Emission
Ó 2009 Elsevier B.V. All rights reserved.
Transient absorption
1. Introduction
extensively by Che and co-workers in solutions [3b,7]. According
to their studies, Pt(C^N^N) complexes exhibit stronger emission
Square-planar Pt(II) complexes have attracted great interests in
recent years because of their potential applications as chemical
sensors [1], molecular probes for biological macromolecules [2],
and in organic light emitting devices [3]. Due to the square-planar
configuration of the Pt(II) complexes, intermolecular metal–metal
in comparison to the platinum(II) terpyridyl complexes due to the
preferred planar geometry by C^N^N ligand, which reduces the
radiationless decay. In addition, it is easier to modify the Pt(C^N^N)
complexes by introducing different substituents on the phenyl ring,
which would in turn tune the emission property of the complexes.
Recently, our group synthesized a series of platinum dphbpy com-
plexes with alkoxyl substituent on the 6-phenyl ring [8]. The intro-
duction of the alkoxyl substituent not only increases the solubility
of the complexes in common organic solvents, such as in CH2Cl2
and CH3CN, but also admixes the 3ILCT (intraligand charge-trans-
and
p–p interactions could occur at high concentration solution
or at solid state. For example, a platinum(II) biphenyl dicarbonyl
complex was reported to form emissive aggregates in solution at
high concentrations [4]. These aggregates are formed due to the
weak interactions between the monomers. The energy level of
the emissive state was affected by such intermolecular interactions
[1b]. Therefore, the emission characteristics of many square-planar
platinum complexes largely depend on the extent of molecular
aggregation [5]. Although solution phase study of these platinum
complexes are crucial and exciting, a significant step towards suc-
cessful application in various devices would be to prepare and
study thin films. It has been reported that solid state emission
properties differ remarkably from those shown in solution because
fer) and 3p p* character with the 3MLCT (metal-to-ligand charge-
,
transfer) character in the emitting state. As a result, the emission
quantum yields of these complexes are significantly increased
and the excited state lifetimes become much longer. However,
the emission of these complexes in solid state has not been studied.
In addition, although it has been reported that Pt(II) complexes
with alkyl chain can be fabricated as LB films [9], no study on the
preparation and investigation of LB films of the Pt(II) dphbpy com-
plexes has been carried out. Because the highly ordered nature of
the LB films, the intermolecular interaction in LB films could be dif-
ferent from that in solutions or amorphous solid. The incorporation
of the alkoxyl substituent on the phenyl ring of the dphbpy ligand
makes these complexes amphiphilic; therefore, it is feasible to pre-
pare LB films of these complexes.
of the different degree of metal–metal and p–p interactions [5–7].
However, the emission properties of platinum(II) complex in thin
films have not been well understood.
The photophysics of platinum(II) 4,6-diphenyl-2,20-bipyridyl
(dphbph or C^N^N) complexes (Pt(C^N^N)) have been studied
In this work, two amphiphilic platinum dphbpy complexes (1
and 2 in Fig. 1) were synthesized for the purpose of preparing LB
* Corresponding author.
E-mail address: Wenfang.Sun@ndsu.edu (W. Sun).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.05.005

I. Mathew, W. Sun / Journal of Organometallic Chemistry 694 (2009) 2750–2756
2751
R
the alkyl chain sticking out from the air/water interface (illustrated
in Fig. 3). The limiting molecular area is found to be ꢀ80 Å2 for the
monolayer of 1 mixed with SA. The estimated molecular areas of 1
and 2, obtained by geometry optimization through density func-
tional theory (DFT) calculation in vacuum using DMOL3 program
implemented in Material Studio 4.3, are 201 Å2 (17.8 ꢁ 11.3 Å)
and 226 Å2 (17.8 ꢁ 12.7 Å), respectively. The tilt angle (h) (see
Fig. 3) with respect to the interface normal can be calculated using
the equation sin h = limiting molecular area/calculated molecular
area. The tilt angles thus calculated are 13.2° and 14.3° for 1 and
2, respectively. The slightly higher limiting area and higher tilt an-
gle of 2 could be due to hydrogen bonding of –OH group with the
water subphase, which would allow the (C^N^N)Pt part of 2 to lie
more flat on the sub phase compared to 1.
N
N
Pt
Cl
O
1. R = H; 2. R = OH
Fig. 1. Chemical structures of 1 and 2.
films. To evaluate the effect of other intermolecular interactions,
such as hydrogen bonding on the preparation and the emission
properties of the LB films of the platinum complexes, a hydroxyl
group was introduced on the dphbpy ligand for 2. The photophys-
ical properties of these two complexes in LB films have been sys-
2.2. AFM images of the LB films
To understand the surface morphology of the LB films, the AFM
images of the 5- and 11-layer LB films of 1 and 2 were taken. Fig. 4
shows the AFM height images of the 11-layer films of 1 and 2. For
both complexes, the height images show a uniform grain-like
structure. The remarkable difference between the two 11-layer
films is that the film for 1 shows larger grain than that for 2; and
the film of 1 is more rough than that of 2, which is reflected by
the brightness of the grain-like structure on each respective film.
This phenomenon could be ascribed to the larger aggregates
formed in 1 compared to 2. Due to the presence of intermolecular
hydrogen bonding in 2, each layer deposited could be more uni-
form and stable compared to that for 1, which is indicated by the
decreased size of the grain-like aggregates. The stabilizing effect
of hydrogen bonding in 2 accounts for the well spread film on
the subphase, which is then transferred to the glass substrate.
The absence of intermolecular hydrogen bonding in 1 results in
easier and larger aggregate formation.
tematically investigated.
To
compare the degree of
intermolecular interactions in amorphous solid form to that in LB
films, the emission properties of 1 and 2 in amorphous solid form
were also investigated.
2. Results and discussions
2.1. Surface pressure–mean molecular area isotherm measurement
Fig. 2 shows the surface pressure–mean molecular area iso-
therms of 1 and 2 with and without stearic acid (SA) added. The
isotherms indicate that the monolayer without SA starts to lift
around the same molecular area of ca. 60 Å2 for these two com-
plexes, after which the surface pressure increase sharply. 1 and 2
collapses at a surface pressure of ca. 44 mN/m and 58 mN/m,
respectively. The observed higher collapse pressure for 2 implies
that it forms a more stable film compared to 1. This could be due
to the stabilizing hydrogen bonding formed by the hydroxyl group,
which in turn results in expanded monolayer on water. The steep-
ness of the curve for 2 compared to 1 indicates a more likely liquid
condensed state for 2. For the monolayer of 1 mixed with SA, the
isotherm starts to lift at a mean molecular area of 90 Å2. The iso-
therm collapses at ꢀ54 mN/m.
2.3. UV–Vis absorption spectra
Fig. 5 shows the UV–Vis spectra of 1 and 2 in acetonitrile solu-
tions and in LB films. In acetonitrile solutions, both 1 and 2 obey
Beer–Lambert’s law in the concentration range of 1 ꢁ 10ꢂ6 mol/L
to 1 ꢁ 10ꢂ4 mol/L, indicating that no ground-state aggregation oc-
curs in this concentration range. With reference to the reported
platinum terpyridyl or C^N^N complexes [5,6,7c], the absorption
bands observed below 400 nm can be assigned to the ligand
The limiting molecular areas calculated by extrapolating the
plot to zero surface pressure are 46 and 56 Å2 for pure monolayers
of 1 and 2, respectively, indicating that the molecules take up an
‘‘edge-on” orientation rather than a ‘‘flat-on” arrangement [9b] with
1p p* transitions [7c]; while the low-energy absorption band be-
,
tween 400 and 450 nm is ascribed to charge-transfer transitions.
According to our previous time-dependent density functional the-
ory (TDDFT) calculation for 1, the HOMO of 1 is predominantly on
the C^N component of the ligand (95.5%), with little contribution
(4.5%) from the Pt; whereas the LUMO is dominated by the bipyr-
idine (N^N) component [8]. Therefore, the lowest excited state of 1
70
1 with SA
60
50
40
30
20
10
0
1
2
is attributed to a mixture of 1ILCT/1 */1MLCT. In view of the iden-
p,p
tical low-energy charge-transfer band of 2 to that of 1, we believe
that 2 has the similar origin of the lowest excited state as that of 1.
In contrast, the molar extinction coefficients and the shape of the
1p * bands between 250 and 325 nm for 2 are quite different from
,p
those of 1. This could be due to the influence of the –OH group.
In LB films without SA added, with increasing number of layers,
the absorbance was found to increase proportionally and the vib-
ronic structures in the UV region disappear. A broad, structureless
absorption band appears at ca. 300 nm for 1 and 330 nm for 2, with
the tail extended to approximately 525 nm. In contrast, the low-
energy charge-transfer band is still clearly evident in the LB film
of 1 mixed with SA. Nevertheless, no new absorption band appears
at longer wavelength for the LB films, implying that no observable
20
40
60
80
100
120
Mean Molecular Area (Å2)
Fig. 2. The surface pressure–mean molecular area isotherms of 1 and 2.
p–p and Pt–Pt interactions occur in the LB films even though the

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I. Mathew, W. Sun / Journal of Organometallic Chemistry 694 (2009) 2750–2756
_
_
_
_
_
_
θ
_
_
_
_
O
O
O
O
O
O
_
_
_
_
_
O
_
_
_
_
_
R
Air
_
R
_
_
_
N
_
------------------------------------------------------
Pt
_
Cl
_
_
_
N
_
Water
_
N
Pt
Cl
_
_
_
=
N
Fig. 3. Proposed molecular orientation and packing at the air-water interface for 1 and 2.
Fig. 4. AFM height images of 11-layer LB films of 1 and 2. The scan area was 1
lm ꢁ 1
lm, and the Z-range was 25 nm.
1.2
b
6x104
11 Layers
1
2
a
5 Layers
1.0
0.8
0.6
0.4
0.2
0.0
in CH3CN
11 Layers with SA
5x104
4x104
3x104
2x104
ε
1x104
0
300
400
500
600
300
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
1.2
c
11 Layers
5 Layers
in CH3CN
1.0
0.8
0.6
0.4
0.2
0.0
350
400
450
500
550
600
Wavelength (nm)
Fig. 5. UV–Vis absorption spectra of 1 and 2 in acetonitrile (a), 1 in LB films (b), and 2 in LB films (c).

I. Mathew, W. Sun / Journal of Organometallic Chemistry 694 (2009) 2750–2756
2753
3.0x104
2.5x104
2.0x104
1.5x104
1.0x104
of the alkoxyl substituent when excited at the low-energy
charge-transfer band [8]. Similar to 1, 2 is also emissive at room
temperature in acetonitrile and exhibits dual emission upon exci-
tation at different wavelengths. When excited at the UV region that
1 x 10-4
5 x 10-5
1 x 10-5
5 x 10-6
1 x 10-6
M
M
M
M
M
(a)
corresponds to the intraligand 1p p* absorption, e.g. 279 nm, 2
,
exhibits a short-lived (<5 ns) emission band at 395 nm (Fig. 6a).
The energy of this emission band coincides with the emission en-
ergy of the corresponding C^N^N ligand, thus this emission band
is attributed to the 1p p* emission. When the concentration of
,
the solution is increased to 5 ꢁ 10ꢂ5 mol/L, the emission intensity
of this band decreases and the band maximum is red-shifted. Con-
sidering the significant absorption at 395 nm, the aforementioned
changes of emission at higher concentrations can be attributed to
inner-filter effect. In contrast, when excited at the low-energy
charge-transfer band, e.g. 420 nm, 2 exhibits a broad, structureless
emission band at ca. 589 nm as shown in Fig. 6b. Compared to its
excitation spectrum, the Stokes shift is approximately 170 nm
(ꢀ7000 cmꢂ1). The intrinsic lifetime of this low-energy emitting
state at infinite dilute concentration is deduced to be 625 ns, and
the emission quantum yield is approximately 1.2% for this emis-
sion band. Considering the large Stokes shift and the long lifetime,
we believe that the emission at 589 nm originates from a triplet
excited state. In view of the similar structural feature and the same
emission energy between 1 and 2, the emitting state of 2 at 589 nm
5.0x103
0.0
350
400
450
500
Wavelength (nm)
(b)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
Em
Ex
0.8
0.6
0.4
0.2
0.0
could also be attributed to a mixture of 3MLCT/3ILCT/3 * charac-
p,p
ters [8,10,11]. The emission intensity of 2 at 589 nm keeps increas-
ing when the concentration increases from 1 ꢁ 10ꢂ6 mol/L to
1 ꢁ 10ꢂ4 mol/L, however, the measured lifetimes decrease with
the increased concentration. This clearly indicates the occurrence
of self-quenching in the concentration range used in our experi-
ment. The self-quenching constant was calculated to be 1.24 ꢁ
1010 Mꢂ1 sꢂ1, which is almost one order of magnitude higher than
that measured for 1 in CH2Cl2 (Table 1) [8]. This is probably related
to the hydroxyl substituent in 2. The intermolecular hydrogen
bonding could favor the formation of excimers, which expedites
self-quenching of the excited state.
350 400 450 500 550 600 650 700
Wavelength (nm)
Fig. 6. (a) Concentration-dependent emission spectra of 2 when excited at 279 nm
at room temperature. (b) Normalized emission and excitation spectra of 2 at room
temperature at a concentration of 1 ꢁ 10ꢂ5 mol/L in acetonitrile. The excitation
wavelength was 420 nm for the emission spectrum measurement, and the
excitation spectrum was monitored at kem = 589 nm.
In acetonitrile glassy solution at 77 K, the emission of 1 and 2
appears at ca. 580 nm with a slight shoulder at ꢀ670 nm at a con-
centration of 1 ꢁ 10ꢂ4 mol/L when excited at 355 nm. The 0–0
emission is somewhat blue-shifted compared to that at room tem-
AFM images indicate some degrees of aggregation. This notion is
supported by the similar feature in the visible region of the UV–
Vis absorption spectra for the LB films with and without SA mixed.
perature, with a thermally induced Stokes shift (
DEs) of approxi-
mately 380 cmꢂ1. Such a small
D
Es suggest that the emitting
Alternatively, if observable
p–p and Pt–Pt interactions present in
state at 77 K could not be a pure charge-transfer state. Considering
the similar emission energy at room temperature and at 77 K, we
the LB films without SA, the visible spectral region would be differ-
ent from that mixed with SA because the dilution from SA would
prevent the intermolecular interactions. The absence of intermo-
lecular interactions in LB films is presumably attributed to the
‘‘side-on” arrangement of the molecules in the LB films, which is
not close enough for the intermolecular interactions to occur.
tentatively assign the emitting state at 77 K as 3MLCT/3ILCT/3 p*
p,
as well. The lack of well defined vibronic structure at 77 K is prob-
ably due to the poor quality of the CH3CN glassy solution and the
very weak emission. The lifetime measured at ca. 580 nm is
ꢀ6.4
ls for 1 and 2, which is the typical lifetime for platinum poly-
pyridine complexes at 77 K [5–7,12]. In contrast, the lifetime mon-
itored at 670 nm is much shorter than that monitored at 580 nm,
indicating that the emission at ca. 580 nm and 670 nm could orig-
inate from different emitting states. With reference to that ob-
served in platinum terpyridyl complexes at 77 K glassy solutions,
the lower-energy band could be attributed to the 3MMLCT emis-
2.4. Emission studies
Our previous study on 1 revealed that this complex is emissive
at r.t. in acetonitrile solution, and the emitting state admixes
3MLCT/3ILCT/3 p* characters due to the electron-donating effect
p,
Table 1
Emission characteristics of 1 and 2 in CH3CN solution and in solid forms.
kem/nm (
s
0/ns; Uem; kQ/Mꢂ1 sꢂ1
)
kem/nm (sem/ls)
In CH3CN at r.t.
77 Kb
LB films
Powders
1
2
590 (460; 0.006; 1.70 ꢁ 109)a
577 (6.4), 670 (sh., 3.6)
576 (6.3), 670 (sh., 2.8)
580 (2.1)
580 (4.5)
583, 622 (sh.)
558 (sh.), 600
589 (625; 0.012; 1.24 ꢁ 1010
)
a
From Ref. [8]. The lifetime was measured at a concentration of 2 ꢁ 10ꢂ5 mol/L. kQ was measured in CH2Cl2 for 1.
Measured in 1 ꢁ 10ꢂ4 mol/L CH3CN glassy solutions. kex = 355 nm.
b

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I. Mathew, W. Sun / Journal of Organometallic Chemistry 694 (2009) 2750–2756
0.04
11-L film
1.0
0.8
0.6
0.4
0.2
0.0
0 ns
77 K glassy solution
11-L film with SA
200 ns
400 ns
600 ns
800 ns
0.03
0.02
0.01
400
500
600
700
800
900
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 7. Emission spectra of 1 in acetonitrile glassy solutions at 77 K and in LB films
at room temperature when excited at 355 nm.
Fig. 9. Time-resolved triplet transient difference absorption of 2 in CH3CN. The
concentration used is 1.8 ꢁ 10ꢂ5 mol/L. kex = 355 nm. The time indicated in the
legend is the delay time after excitation.
sion due to ground-state aggregation at higher concentrations
[5a,12]. The occurrence of this band at 77 K but not at room tem-
perature suggests that the association constant for the aggregation
could be too small to be observed at room temperature.
The vibronic spacing is ca. 1254 cmꢂ1, corresponding to the aro-
matic vibration mode of the C^N^N ligand. The same emission fea-
ture was observed when excited at 355 nm. When the excitation
spectrum was monitored at 560 nm and 600 nm, respectively,
identical spectra were observed. This indicates that the two emis-
sion bands originate from the same emitting state. The similar phe-
nomenon was observed for 1, with the dominant emission peak
appearing at 583 nm and a shoulder at ꢀ622 nm, and the vibronic
spacing being of ca. 1075 cmꢂ1. With reference to the similar emis-
sion energy to those observed in solution and in LB film at room
temperature, and in glassy solution at 77 K, the emitting state of
1 and 2 in amorphous solid (powders) is tentatively assigned to
Emission was also observed from the LB films and powders of 1
and 2, respectively, at room temperature. As shown in Fig. 7, emis-
sion at ca. 580 nm was observed from the LB film of 1, which ap-
pears at the similar energy as those observed in acetonitrile
solutions at room temperature and in glassy solution at 77 K. The
emission from the 11-layer LB film of 1 mixed with stearic acid also
appears at the same energy of 580 nm (shown in Fig. 7). These fea-
tures clearly indicate that no intermolecular interactions occur in
the LB films, which is consistent with the results observed from
the UV–Vis absorption study and could be attributed to the ‘side-
on’ arrangement in the LB film. The same emission energy and fea-
ture were observed in the LB film of 2 (see Table 1). In addition, the
observed emission lifetimes for the LB films of 1 and 2 are of the
order of microseconds, which are comparable to those observed
in acetonitrile glassy solutions at 77 K for these two complexes.
The elongated lifetimes in LB films compared to those in CH3CN
solutions at room temperature should be attributed to the absence
of solvent quenching effect in solid films and to the reduced non-
radiative decay rates in solid phase.
3MLCT/3ILCT/3 * as well. The emission from the amorphous pow-
p,p
ders is repeatable even when measured at different times.
2.5. Triplet transient difference absorption (TA)
The time-resolved triplet transient difference absorption spec-
trum of 1 was reported previously by our group in Ref. [8], the
spectrum for 2 is presented in Fig. 9. The spectrum exhibits broad
positive absorption bands from 370 nm to 810 nm, implying a
stronger triplet excited-state absorption than that of the ground-
state in this region. The feature of this spectrum resembles that
for 1 reported in Ref. [8]. The triplet excited-state lifetime mea-
sured from the decay of the transient absorption at 450 nm is
Fig. 8 shows the emission spectra of 1 and 2 in solid state as
powders. When excited at 430 nm, structured emission spectra
with band maxima at 558 nm and 600 nm was observed for 2.
1.2
b
a
at 560 nm
at 600 nm
1
2
1.8x106
1.5x106
1.2x106
9.0x105
6.0x105
0.9
0.6
0.3
0.0
500
600
700
800
350
400
450
500
Wavelength (nm)
Wavelength (nm)
Fig. 8. (a) Normalized emission spectra of 1 and 2 in solid state as powders when excited at 430 nm. (b) Excitation spectra of 2 when monitored at emission wavelength of
560 and 600 nm.

I. Mathew, W. Sun / Journal of Organometallic Chemistry 694 (2009) 2750–2756
2755
555 ns, which is in line with the lifetime measured from the decay
of the emission. This indicates that the transient absorption likely
arises from the same excited state that emits or from a state that is
in equilibrium with the emitting state. Transient absorption of the
LB films could not be measured due to the easy damage of the films
upon laser excitation.
Compound 3. In 150 mL acetone, 40-hydroxyacetophenone
(5.44 g, 39.95 mmol), 2-ethylhexyl bromide (8.00 g, 41.42 mmol),
potassium carbonate (13.80 g, 99.85 mmol), and catalytical
amount of KI were added, and the mixture was refluxed for 36 h.
After cooling down, the solution was filtered. Acetone in the filtra-
tion was removed and the residue was extracted with dichloro-
methane. The dichloromethane layer was washed with 50 mL
aqueous Na2CO3 (10%) and dried with anhydrous Na2SO4. Pale yel-
low oil was obtained as the product (8.80 g, yield: 89%). 1H NMR
(CDCl3, 400 MHz) d: 7.91 (d, J = 8.7 Hz, 2H), 6.91 (d, J = 8.7 Hz,
2H), 3.89 (d, J = 5.7 Hz, 2H), 2.54 (s, 3H), 1.73 (m, 1H), 1.45 (m,
4H), 1.32 (m, 4H), 0.92 (t, J = 7.5 Hz, 6H) ppm.
Compound 4. 4-Hydroxybenzaldehyde (1.66 g, 13.59 mmol) was
dissolved in 100 mL acetic acid, 3 (3.37 g, 13.58 mmol) and sulfuric
acid (98%, 3 mL) were added to the solution. The reaction mixture
was stirred at room temperature for 24 h, and then neutralized
with 5 M NaOH. The crude product was extracted with ethyl ace-
tate and was purified using column chromatography on silica gel.
Ethyl acetate in hexane (v/v = 1/3) was used as the eluent. Pale yel-
low oil was obtained as the product (3.63 g, yield: 76%). 1H NMR
(CDCl3, 400 MHz): d 8.03 (d, J = 9.3 Hz, 2H), 7.79 (d, J = 15.3 Hz,
1H), 7.54 (d, J = 9 Hz, 2H), 7.43 (d, J = 15.3 Hz, 1H), 6.95 (m, 4H),
3.92 (d, J = 5.7 Hz, 2H), 1.75 (m, 1H), 1.47 (m, 4H), 1.34 (m, 4H),
0.93 (t, J = 7.2 Hz, 6H) ppm.
3. Conclusion
Emissive LB films are fabricated using amphiphilic platinum(II)
4,6-diphenyl-2,20-bipyridyl complexes. The emission energy in LB
film is similar to that observed in CH3CN solutions at room temper-
ature; however, the lifetime is much longer in LB film. The emitting
state is tentatively attributed to 3MLCT/3ILCT/3 *. No observable
p,p
intermolecular interactions are evident in LB films. The presence of
the hydroxyl substituent on the C^N^N ligand of 2 could favor the
formation of intermolecular hydrogen bonding, which helps the
deposition of uniform and stable LB films and increases the self-
quenching rate constant for emission of 2 in CH3CN solution.
4. Experimental
4.1. Synthesis
Compound 5. The synthesis of 5 was carried out following the
method described in the literature [13]. Yield: 56%. 1H NMR (CDCl3,
400 MHz): d 8.74 (m, 1H), 8.68 (d, J = 7.8 Hz, 1H), 8.41 (d, J = 1.5 Hz,
1H), 8.13 (d, J = 9 Hz, 2H), 7.91 (td, J = 7.8 & 1.5 Hz, 1H), 7.86 (d,
J = 1.5 Hz, 1H), 7.59 (d, J = 8.4 Hz, 2H), 7.39 (m, 1H), 7.04 (d,
J = 9.0 Hz, 2H), 6.95 (d, J = 9.0 Hz, 2H), 6.84 (s, 1H), 3.93 (d,
J = 6.0 Hz, 2H), 1.77 (m, 1H), 1.47 (m, 4H), 1.36 (m, 4H), 0.95 (t,
J = 7.8 Hz, 6H) ppm.
All the chemicals and solvents were purchased from Aldrich and
VWR Scientific Company at analytical grade and used without
purification unless otherwise stated. The synthesized products
were characterized by 1H NMR (Varian, 400 MHz), and ESI-HRMS
(Bruker Daltonics BioTOF III mass spectrometer). Elemental analy-
ses were carried out by NuMega Resonance Laboratories, Inc. in
San Diego, California.
The synthesis of 1 has been reported by our group previously
[8]. The synthesis of 2 follows a similar synthetic route as illus-
trated in Scheme 1.
Complex 2. Complex 2 was synthesized from 5 according to the
reported procedure for similar platinum complex [14]. Yield: 65%.
1H NMR (d6-DMSO, 500 MHz): d 8.91 (s,1H), 8.74 (d, J = 7.7 Hz, 1H),
O
O
K2CO3, KI
O
Br
+
HO
Acetone
3
O
H2SO4, AcOH
24 hrs, R.T.
HO
H
O
O
OH
4
O
I-
NH4OAc / MeOH
Reflux 6 hrs
N
N
+
N
N
N
K2PtCl4, CH3CO2H
Refluxed, 24 hrs
Pt
N
HO
HO
Cl
OC8H17
OC8H17
5
2
Scheme 1. Synthetic route for complex 2.

2756
I. Mathew, W. Sun / Journal of Organometallic Chemistry 694 (2009) 2750–2756
8.37 (s, 2H), 8.09 (s, 1H), 8.02 (d, J = 8.3 Hz, 2H), 7.91 (m, 1H), 7.77
(d, J = 8.5 Hz, 1H), 6.96 (d, J = 8.3 Hz, 3H), 6.68 (d, J = 8.3 Hz, 1H),
3.87 (d, 2H), 1.69 (m, 1H) 1.49–1.32 (m, 8H), 0.95 (t, J = 7.7 Hz,
6H) ppm. ESI-MS m/z calcd for [C30H31N2O2Pt+CH3CN]+:
687.2296. Found: 687.2318 (53%). Anal. Calc. for C30H31ClN2O2Pt:
C, 52.82; H, 4.58; N, 4.11. Found: C, 52.98; H, 4.94; N, 4.11%.
tation Award) for support. We would like to thank Dr. Hongshan
He at the South Dakota State University for helping us to estimate
the molecular areas for 1 and 2.
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Acknowledgments
Acknowledgment is made to the USDA-CSREES program (2005-
34475-15788 and 2006-34475-17127) and the National Science
Foundation (CAREER CHE-0449598) for financial support. We are
also grateful to North Dakota State EPSCoR (ND EPSCoR Instrumen-
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