Synthesis and photophysical studies of back-to-back dinuclear platinum terpyridine complexes with different substituents on the bridging ligand

Article abstract of DOI:10.1016/j.ica.2012.02.031

Four back-to-back dinuclear platinum terpyridine complexes with different substituents on the bridging ligand (1a-1d) were synthesized and characterized. Their electronic absorption, photoluminescence and triplet transient difference absorption were systematically investigated. All complexes exhibit strong ¹MLCT/¹ILCT absorption bands in the visible region, which significantly red-shifts when electron-donating substituents are introduced on the conjugated bridge and blue-shifts when electron-withdrawing substituents are present. Excitation of 1a and 1d in solution at their respective low-energy absorption band at room temperature results in an orange and red luminescence, respectively, which can be tentatively attributed to the ¹MLCT/ ¹ILCT excited state. These complexes exhibit intense broad triplet transient difference absorption in the visible to the near-IR region, which likely arises from the ligand-localized states (³π,π* or ³ILCT). Electron-donating substituent causes a pronounced red-shift, while electron-withdrawing substituents induce a blue-shift of the triplet transient absorption bands.

Full text of DOI:10.1016/j.ica.2012.02.031

Inorganica Chimica Acta 387 (2012) 383–389
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Synthesis and photophysical studies of back-to-back dinuclear platinum
terpyridine complexes with different substituents on the bridging ligand
Rui Liu a,b, Zhongjing Li a, Hongjun Zhu b, Wenfang Sun a,
⇑
a Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58108-6050, United States
b Department of Applied Chemistry, College of Science, Nanjing University of Technology, Nanjing 210009, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Four back-to-back dinuclear platinum terpyridine complexes with different substituents on the bridging
ligand (1a–1d) were synthesized and characterized. Their electronic absorption, photoluminescence and
triplet transient difference absorption were systematically investigated. All complexes exhibit strong
1MLCT/1ILCT absorption bands in the visible region, which significantly red-shifts when electron-donat-
ing substituents are introduced on the conjugated bridge and blue-shifts when electron-withdrawing
substituents are present. Excitation of 1a and 1d in solution at their respective low-energy absorption
band at room temperature results in an orange and red luminescence, respectively, which can be tenta-
tively attributed to the 1MLCT/1ILCT excited state. These complexes exhibit intense broad triplet transient
difference absorption in the visible to the near-IR region, which likely arises from the ligand-localized
Received 23 November 2011
Received in revised form 15 February 2012
Accepted 19 February 2012
Available online 25 February 2012
Keywords:
Dinuclear platinum complex
1,4-Bis(terpyridin-4-yl-vinyl)benzene
Electronic absorption
Emission
Transient absorption
Synthesis
states (3
drawing substituents induce a blue-shift of the triplet transient absorption bands.
Ó 2012 Elsevier B.V. All rights reserved.
p,
p⁄ or 3ILCT). Electron-donating substituent causes a pronounced red-shift, while electron-with-
1. Introduction
to form a face-to-face geometry, which possibly resulted in differ-
ent degrees of metal–metal and interactions [15–18].
p–p
Square-planar platinum(II) complexes have attracted great
attention in the past two decades because of their interesting spec-
troscopic properties and potential applications in optoelectronic
devices [1–5], chemosensors [6,7], photocatalysis [8–10] and non-
linear optical materials [11–14]. The square-planar Pt(II) coordina-
tion of these complexes reduces the D2d distortion that is likely to
result in radiationless decay process [3,5], which enhances the
emission of these complexes. In addition, their photophysical prop-
erties can be modulated by structural modification of the ligands to
meet the requirements for different applications. For example, the
nature of the lowest excited state of these complexes can be tuned
among the metal-to-ligand charge transfer (MLCT) state, ligand-to-
ligand charge transfer (LLCT) state, intraligand charge transfer
However, the studies on the dinuclear platinum terpyridine com-
plexes with a back-to-back geometry are still very limited. Ziessel
group [19] synthesized one back-to-back dinuclear platinum com-
plex using a 1,4-diethynyl-2,5-didodecyloxybenzene as bridging
ligand. Our group has reported the influence of conjugated rigid
bridging ligands on the photophysics of the ‘‘back-to-back’’ dinu-
clear platinum complexes, which exhibit strong reverse saturable
absorption (RSA) in the visible to the near-IR region for nanosec-
ond laser pulses [12]. Meanwhile, to the best of our knowledge,
the influence of the substituents on conjugated bridging ligands
of the back-to-back dinuclear platinum complexes has not been
reported yet.
To remedy this deficiency, we have designed and synthesized a
series of novel dinuclear platinum(II) complexes with different
substituents on the bridging ligand (1a–1d, Chart 1). Br and CN
were selected as the electron-withdrawing groups and OC12H25
was chosen as electron-donating substituent. The tert-butyl groups
were introduced on the terpyridine ligands in order to increase the
(ILCT) state, and intraligand (IL)
of the ligand.
Among these, the photophysical properties of dinuclear plati-
num terpyridine complexes are more interesting by virtue of
p,
p⁄ state depending on the nature
the possible intramolecular Pt–Pt and
p–p interactions in the
dinuclear complexes, which could tune their electronic absorption
and luminescence properties. Most of the earlier work reported
on the dinuclear platinum complexes used a rigid bridging ligand
solubility of the target complexes and avoid the p–p stack between
the neighboring molecules. The photophysical properties of these
complexes were systematically investigated with the aim of under-
standing the structure–property correlations and developing novel
broadband nonlinear transmission materials. The synthetic routes
are illustrated in Scheme 1.
⇑
Corresponding author. Tel.: +1 701 231 6254; fax: +1 701 231 8831.
E-mail address: Wenfang.Sun@ndsu.edu (W. Sun).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.02.031

384
R. Liu et al. / Inorganica Chimica Acta 387 (2012) 383–389
Chart 1. Molecule structures of dinuclear Pt(II) complexes 1a–1d.
band in the TA spectrum, respectively, and eS is the ground-state
2. Experimental
molar extinction coefficient at the wavelength of the bleaching
band minimum. The triplet excited-state quantum yield (UT) was
obtained by relative actinometry [37], and SiNc in benzene was
2.1. Materials
used as the reference (e , UT = 0.20) [38].
590 = 70000 Mꢃ1 cmꢃ1
All of the chemicals and solvents were purchased from Alfa Aesar
and used as is unless otherwise stated. 4-Tert-butyl-2-acetylpyri-
dine (9) [20], 1-(4-tert-butylpyridin-2-yl)-3-hydroxybutan-1-one
(8) [21], 1-(4-tert-butylpyridin-2-yl)but-2-en-1-one (7) [21], 4,
400-bis-tert-butyl-40-methyl-2,20:60,200-terpyridine (6) [21,22], 4,
400-bis-tert-butyl-40-formyl-2,20:60,200-terpyridine (5) [23,24], 1,4-
bis(bromomethyl)benzene derivatives (4a–4d) [25–28], tetra-
2.3. Synthesis
2.3.1. General procedure for synthesis of complexes 1a–1d
Pt(DMSO)2Cl2 (260 mg, 0.6 mmol) and AgCF3SO3 (160 mg,
0.6 mmol) were added in 5 mL of dimethylsulfoxide (DMSO). The
mixture was stirred at room temperature for 24 h, and the white
solid was filtered out. The filtrate was heated to 80 °C, and terpyr-
idine ligands (2a–2d) (0.3 mmol) was added. The mixture was kept
at 80 °C for 2 h. After cooling to room temperature, excess amount
of saturated NH4PF6 aqueous solution was added. The mixture was
stirred at room temperature for 1 h, and the solid was collected by
centrifugation. Then the solid was purified by recrystallization
from DMF/ether to afford the desired product.
ethyl-[1,4-phenylenbis(methylen)]-bisphosphonate
(3a–3d)
[25,28–30] and terpyridine ligands (2a–2d) [29,31–33] were all syn-
thesized according to the literature procedures. Column chromatog-
raphy was carried out using silica gel (Sorbent Technologies, 60 Å,
230 ꢀ 400 mesh) or neutral aluminum oxide (Sigma–Aldrich, 58 Å,
ꢁ150 mesh).
2.2. Measurements
Complex 1a (yield: 44%) as orange solid. 1H NMR (400 MHz,
CDCl3): d 8.98 (s, 4H), 8.79 (d, J = 6.0 Hz, 4H), 8.56 (s, 4H), 8.03–
7.88 (m, 10H), 7.49 (d, J = 16.0 Hz, 2H), 1.42 (s, 36H). HRMS (m/
1H NMR spectra were recorded on either a Bruker AV-400 or
AV-500 spectrometer using DMSO-d6 as the solvent, with tetra-
methylsilane (TMS) as internal standard. Elemental analyses were
conducted by NuMega Resonance Laboratories, Inc., San Diego, CA.
High resolution mass (HRMS) analyses were performed at Bruker
BioTof III mass spectrometer. UV–Vis absorption spectra were ob-
tained by using an Agilent 8453 spectrophotometer. Emission
spectra were carried out on a SPEX fluorolog-3 fluorometer/phos-
phorometer. The emission quantum yields were determined by
the comparative method [34], in which a degassed aqueous solu-
z): calcd for [C56H60N6Cl2Pt2]2+
, 638.6768; found, 638.6743
(100%). Anal. Calc. for C56H60N6Cl2P2F12Pt2ꢄCH2Cl2: C, 41.41; H,
3.78; N, 5.10. Found: C, 41.75; H, 3.90; N, 5.58%.
Complex 1b (yield: 57%) as yellow solid. 1H NMR (400 MHz,
CDCl3): d 8.98 (s, 4H), 8.79 (d, J = 6.0 Hz, 4H), 8.36 (s, 4H), 8.27 (s,
2H), 8.00 (d, J = 16.0 Hz, 2H), 7.95 (dd, J = 6.0 Hz, J = 2.0 Hz, 4H),
7.49 (d, J = 16.0 Hz, 2H), 1.42 (s, 36H). HRMS (m/z): calcd for
[C56H58N6Br2Cl2Pt2]2+, 718.0862; found, 718.0880 (100%). Anal.
Calc. for C56H58N6Br2Cl2P2F12 Pt2ꢄDMF: C, 36.73; H, 3.40; N, 4.96.
Found: C, 36.42; H, 3.40; N, 5.33%.
tion of [Ru(bpy)3]Cl2 (Uem = 0.042, excited at 436 nm) [35] was
used as the reference. An Edinburgh LP920 laser flash photolysis
spectrometer was used to acquire the triplet transient difference
absorption (TA) spectra in degassed solutions. The excitation
source was the third harmonic output (355 nm) of a Nd:YAG laser
(Quantel Brilliant, pulsewidth ꢁ4.1 ns, repetition rate was set at
1 Hz). Each sample was purged with Ar for 30 min prior to
measurement.
Complex 1c (yield: 48%) as yellow solid. 1H NMR (400 MHz,
CDCl3): d 9.00 (s, 4H), 8.85 (d, J = 6.0 Hz, 4H), 8.61 (s, 4H), 8.06–
7.93 (m, 6H), 7.77 (d, J = 16.0 Hz, 2H), 1.42 (s, 36H). HRMS (m/z):
calcd for [C58H58N8Cl2Pt2]2+, 633.6720; found, 633.6734 (100%).
1
Anal. Calc. for C58H58N8Cl2P2F12Pt2ꢄ /3DMFꢄCH2Cl2: C, 41.71; H,
3.67; N, 6.75. Found: C, 41.63; H, 3.73; N, 7.14%.
The triplet excited-state molar extinction coefficients (eT) at the
Complex 1d (yield: 54%) as black solid. 1H NMR (400 MHz,
CDCl3): d 8.92 (s, 4H), 8.83 (d, J = 6.0 Hz, 4H), 8.63 (d, J = 2.0 Hz,
4H), 8.14 (d, J = 16.0 Hz, 2H), 8.63 (d, J = 2.0 Hz, 4H), 7.53 (dd,
J = 2.0 Hz, J = 6.0 Hz, 2H), 7.50 (s, 2H), 4,17 (t, J = 5.8 Hz, 4H),
1.88–1.85 (m, 2H), 1.52–1.49 (m, 2H), 1.38–1.32 (m, 4H), 1.42 (s,
36H), 1.21–0.98 (m, 32H), 0.72 (t, J = 7.2 Hz, 6H). HRMS (m/z): calcd
for [C80H108N6Cl2O2Pt2]2+, 823.3600; found, 823.3601 (100%). Anal.
TA band maximum were determined by the singlet depletion
method [36], in which the following equation was used to calculate
the eT [36].
eS
D
½
D
ODS
ODTꢂ
;
eT
¼
1
where
D
ODS and
D
ODT are the optical density changes at the min-
Calc. for C80H108N6Cl2O2P2F12Pt2ꢄ /2DMF: C, 48.99; H, 5.70; N, 4.61.
imum of the bleaching band and at the maximum of the positive
Found: C, 48.70; H, 5.32; N, 4.83%.

R. Liu et al. / Inorganica Chimica Acta 387 (2012) 383–389
385
Scheme 1. Synthetic routes for complexes 1a–1d.
from AgCF3SO3 in DMSO. After filtration of AgCl, one-half molar
equivalent of terpyridine ligand (2a–2d) was added to the filtrate
and the reaction solution was heated to 80 °C for 2 h. Pure final
product was obtained in 58–68% yield after recrystallization from
DMF/Et2O. All the synthesized complexes were characterized by
1H NMR, HRMS, and elemental analysis.
3. Results and discussion
3.1. Synthesis
The synthetic routes for 1a–1d are shown in Scheme 1. Four
tert-butyl substituents were introduced to the terpyridine compo-
nents in order to reduce the intermolecular p–p stacking and thus
improve the solubility of the Pt(II) complexes. Ligands 2a–2d were
prepared by the Wittig–Horner reaction between the bis-phospho-
nate (3a–3d) and the formyl-functionalized terpyridine (5), with t-
BuOK as the base [29,31–33].
Coordination reactions of the target complexes (1a–1d) were
carried out via the procedure reported previously [12]. A highly
reactive platinum salt was prepared by exchange of one of the
chloride coligands in Pt(DMSO)2Cl2 with 1 equiv. of ꢃCF3SO3 anion
3.2. UV–Vis absorption
The UV–Vis absorption spectra of complexes 1a–1d in CH3CN
solution are presented in Fig. 1, and the absorption band maxima
and the extinction coefficients are listed in Table 1. The UV–Vis
absorption obeys Lambert–Beer’s law in the concentration range
of 1 ꢀ 10ꢃ6–1 ꢀ 10ꢃ4 mol/L, suggesting that no ground-state
aggregation occurs in this concentration range. The absorption

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R. Liu et al. / Inorganica Chimica Acta 387 (2012) 383–389
1.0x105
8.0x104
6.0x104
4.0x104
1.2
CH2Cl2
1d
1a
1b
1c
1d
DMF
1.0
CH3CN
MTHF
Toluene
0.8
0.6
0.4
0.2
0.0
ε
2.0x104
0.0
300
400
500
600
700
300
400
500
600
Wavelength (nm)
Wavelength (nm)
Fig. 3. Normalized UV–Vis absorption spectra of 1d in different solvents.
Fig. 1. UV–Vis absorption spectra of complexes 1a–1d in CH3CN solution.
1.2
1.2
1.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1d
1a, emission
1a, excitation
1d, emission
1d, excitation
2d + 2 equiv. p-TsOH
1.0
(ClO4)
2
2d + 2 equiv. Zn
2d
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
250 300 350 400 450 500 550 600 650
300
400
500
600
700
800
900
Wavelength (nm)
Wavelength (nm)
Fig. 2. Comparison of UV–Vis spectra of Pt complex 1d, ligand 2d, and 2d with 2
equivalents of p-TsOH or Zn(ClO4)2 in CH2Cl2.
Fig. 4. Normalized emission spectra of 1a and 1d in CH3CN solution (1 ꢀ 10ꢃ5 mol/
L). The excitation wavelength was 450 nm for 1a and 488 nm for 1d.
spectra of 1a–1d show strong intraligand (IL) 1p
,
p⁄ transitions in
extinction coefficient of this 1MLCT band is much high than those
of the typical 1MLCT transition in mononuclear Pt complexes. This
should arise from the large transition dipole of the
the UV region, which is in line with our previous work on ‘‘back-
to-back’’ dinuclear platinum(II) terpyridine complexes [12] and
with that of dinuclear ruthenium diimine complexes [39,40]. In
addition, a broad, intense absorption band appears at 400–
600 nm. With reference to that reported for dinuclear ruthenium
complexes with similar bridging ligands, this absorption band
could be tentatively assigned to the 1MLCT (metal-to-bridging
ligand charge transfer) transition. Similar to that observed in
dinuclear Ru complexes with similar bridging ligand, the molar
Pt(dp
) ? tpydvbtpy(p⁄) (dvb refers to divinylbenzene linker and
tpy refers to terpyridine) due to the extended conjugation in the
bridging ligand. On the other hand, this band could have some con-
tributions from the intraligand charge transfer (1ILCT) transition
from the dvb linker to the terpyridine components. This notion is
supported by the appearance of the intramolecular charge transfer
(1ICT) band at ca. 460 nm upon acid or Zn2+ titration of the
Table 1
Photophysical parameters of 1a–1d in CH3CN.a
d
kabsb/nm (
e
/103 L molꢃ1 cmꢃ1
)
kemb/nm (
s
em/ns; Uem
)
kT1–Tn/nm (sTA/ns; e ; UT)
T1–Tn/103 L molꢃ1 cmꢃ1
1a
1b
1c
1d
256 (64.7), 335 (52.4), 450 (75.0)
258 (65.4), 334 (55.9), 424 (50.3)
257 (71.5), 335 (56.3), 418 (38.6)
258 (43.7), 340 (37.7), 488 (36.6)
527 (3.2; (41%), 0.32 (59%); 0.02)
c
c
700 (1550; 123200; 0.15)
685 (190; 54200; 0.17)
660 (160; 175200; 0.09)
745 (1630; 31800; 0.21)
631 (3.7 (5%), 0.23 (95%); 0.003)
a
b
c
Measured at room temperature.
At a concentration of 1 ꢀ 10ꢃ5 mol/L.
Too weak to be measured.
d
Nanosecond TA band maximum, triplet extinction coefficient, triplet excited-state lifetime, and quantum yield. SiNc in C6H6 was used as the reference.
UT = 0.20).
(e
590 = 70000 L molꢃ1 cmꢃ1
,

R. Liu et al. / Inorganica Chimica Acta 387 (2012) 383–389
387
Table 2
oligomers revealed that there are strong couplings between the
-electron system on the phenylenevinylene backbone and orbi-
Emission energy and quantum yields of 1a–1d in different solvents.
p
tals on the OCH3 and CN groups [42]. These interactions would al-
ter the ligand field strength, which in turn influences the Pt-based
HOMO. Electron-donating OC12H25 substituent would increase the
ligand field strength and consequently raises the Pt-based HOMO.
Meanwhile, the bridging ligand based LUMO is also raised. How-
ever, the degree of the HOMO energy increase exceeds that of
the LUMO, leading to a decrease of the HOMO–LUMO energy gap
and the red-shift of the 1MLCT band. In contrast, the electron with-
drawing substituents, Br and CN, stabilize both the HOMO and
LUMO. However, the HOMO is stabilized more than the LUMO,
which causes the blue-shift of the 1MLCT band. On the other hand,
this substituent effect is also consistent with the nature of the
intramolecular charge transfer transition. When electron-donating
ability of the dvb linker increases, the ILCT from dvb to terpyridine
is enhanced, which causes a red-shift of the charge-transfer band.
The mixture of 1ILCT transition into the low-energy absorption
band is further supported by the acid and Zn2+ titration experi-
ments of 2d. As shown in Fig. 2 and Supporting information
Fig. S4, upon addition of acids or Zn2+, the lowest-energy absorp-
tion band in 2d red-shifts from 400 nm to ca. 460 nm, which is
in close proximity to the charge-transfer band in 1d. Due to the
lack of metal center in acidified ligand and the absence of MLCT
transition in the Zn complex, the new absorption band at ca.
460 nm should be attributed to the intramolecular charge transfer
transition. After ligand complexation with Pt, electron density on
the terpyridine components is reduced, which increases the elec-
tron-withdrawing ability of the terpyridine components, similar
to the effect of protonation of the terpyridine component or com-
plexation with the Zn2+ ion. Therefore, intraligand charge transfer
from the dvb component to the terpyridine components contrib-
utes to the low-energy charge transfer band in 1a–1d. However,
the ILCT contribution is strongest in 1d that contains electron-
donating –OC12H25 substituent on the dvb linker, while the ILCT
contribution is reduced in 1b and 1c that bear electron-withdraw-
ing substituents on dvb. A similar assignment was reported for
kem/nm (Uem
)
CH3CN
DMF
CH2Cl2
MTHF
a
Toluene
a
1a
527 (0.02)
1d 618 (0.003)
540 (0.0096) 530 (0.012)
a
621 (0.027) 631 (0.003) 638 (0.006)
a
Signal too weak to be detected.
1.2
CH2Cl2
CH3CN
1d
1.0
MTHF
0.8
0.6
0.4
0.2
Toluene
0.0
600
700
800
Wavelength (nm)
Fig. 5. Normalized emission spectra of 1d in different solvents. The excitation
wavelength was 436 nm.
respective 1,4-bis(terpyridin-4-yl-vinyl)benzene ligand, as exem-
plified in Fig. 2 for ligand 2d, which is buried in the 1MLCT band
in 1d and will be discussed in more detail later. Compared to the
dinuclear Pt complex with 1,4-diethynyl-2,5-didodecyloxybenzene
bridging ligand (complex 16 in reference [19]), the low-energy
absorption band (1MLCT/1ILCT) of complex 1d is red-shifted and
the molar extinction coefficient is significantly increased. This phe-
nomenon can be attributed to the better conjugation provided by
the vinylene linker in the ligand than the ethynylene linker used
dinuclear Pt(II) complexes with
ing group [12].
p-donating fluorene as the bridg-
for complex 16 due to the better
p–p –
and p⁄ p⁄ energy match at
3.3. Emission
the C(sp2)–C(sp2) connections and the smaller bond length alterna-
tion for the double bond connection [41].
The emission of complexes 1a–1d in different solvents at room
temperature was investigated. Only 1a and 1d exhibit structureless
emission upon excitation at the low-energy absorption band. 1b
The charge transfer nature of the low-energy absorption band is
clearly evident from the solvent-dependency UV–Vis study of
these complexes. As exemplified in Fig. 3 for 1d, the low-energy
absorption band exhibits a negative solvatochromic effect (namely,
in low polarity solvents, such as toluene, this band shows a batho-
chromic shift; whereas in more polar solvents, such as CH3CN, the
band exhibits a hypsochromic shift), which is indicative of a
charge-transfer transition. This phenomenon is in accordance with
that observed in many of the platinum bipyridine or terpyridine
complexes reported in the literature [11–13]. The same solvato-
chromic effect is observed for the other three complexes in this
work, and the results are provided in Supporting information
Figs. S1–S3.
0.04
0.03
0.02
0.01
0.00
Δ
In contrast to the intraligand 1 p⁄ transition band, the low-en-
p,
-0.01
1a
ergy charge-transfer band is influenced significantly by the nature
of the substituents on the bridging ligand. Electron-donating sub-
stituent, OC12H25, causes a pronounced red-shift of this transition,
while electron-withdrawing substituents like Br and CN induce a
blue-shift compared to that in complex 1a. On one hand, this
change is related to the effect of the substituent on the relative en-
ergy level of the Pt-based HOMO and bridging ligand based LUMO.
A time-dependent density functional theory study of the molecular
structure on the excited state polarizability of phenylenevinylene
1b
1c
-0.02
1d
-0.03
400
500
600
700
800
Wavelength (nm)
Fig. 6. Triplet transient difference absorption spectra of 1a–1d at zero-delay after
laser excitation in CH3CN solution. kex = 355 nm.

388
R. Liu et al. / Inorganica Chimica Acta 387 (2012) 383–389
and 1c are nonemissive at room temperature in solution. The nor-
malized emission spectra of 1a and 1d in CH3CN (1 ꢀ 10ꢃ5 mol/L)
along with their respective excitation spectra are illustrated in
Fig. 4, and the emission quantum yields of these two complexes
in different solvents are summarized in Table 2. The emission band
maxima of 1a and 1d in CH3CN solution (kmax) appear at 527 and
631 nm, respectively; and the emission for both complexes exhib-
its biexponential decay, with a longer lifetime of ꢁ3 ns and a short-
er one of hundreds of ps (listed in Table 1). In addition, the
emission is insensitive to oxygen quenching. Both the emission
intensity and lifetime remain the same in air-saturated solution
and in deaerated solution (see Supporting information Fig. S5 for
1d). The short lifetime along with the insensitivity to oxygen
quenching imply that the observed emission is likely fluorescence
from the 1MLCT/1ILCT states. The charge-transfer nature of the
emitting states is evident from the negative solvatochromic effect
as exemplified in Fig. 5 for 1d and in Supporting information
Fig. S6 for 1a. Compared to their respective ligands 2a and 2d (Sup-
porting information Figs. S7, S8 and S11), the emission of the Pt(II)
complexes exhibit significants red-shifts from 434 nm for 2a to
527 nm for 1a, and from 484 nm for 2d to 631 nm for 1d. This sig-
nificant red-shift also reflects the charge-transfer nature of the
emitting states in 1a and 1d, while the emitting states in the li-
CH3CN solution; and the triplet excited-state absorption coeffi-
cients, lifetimes and quantum yields are listed in Table 1.
The spectral features for 1a–1d are similar, with a negative
band below 550 nm and broad, structureless absorption bands in
the visible to the NIR region. The absorption band is red-shifted
for the complex with electron-donating substituents (1d) and
blue-shifted for complexes with electron-withdrawing substitu-
ents (1b and 1c) compared to 1a, which is consistent with that ob-
served in the UV–Vis absorption spectra. The negative bands occur
at the similar positions as the charge-transfer absorption bands in
their respective UV–Vis absorption spectra; thus, they should be
ascribed to the bleaching of the ground state. The lifetimes moni-
tored from the decay of the transient absorption at the band max-
ima for 1a–1d vary from 160 ns to 1.6 ls, which are drastically
longer than the lifetimes obtained from the decay of emission. This
indicates that the observed transient absorption originates from
the triplet excited state. Considering the similar red to near-IR
absorption band in the TA spectra of 1a–1d to those reported in
the literature for dinuclear Ru(II) or Re(I) complexes with same
or similar divinylbenzene (dvb) bridging linker, we attribute the
broad red to near-IR absorption band to bridging ligand localized
triplet excited states (3 or 3ILCT) [39,40,43,44]. This assignment
p,p
is also supported by the substituent effect at the dvb linker, which
gands are 1p p⁄ state in nature. Our attribution of the emitting
,
couples with the bridging ligand backbone and alters the electron
states in 1a and 1d to mixed 1MLCT/1ILCT rather than a pure 1ILCT
or 1MLCT state is based on the following three facts: first, as shown
in Fig. 4, the emission bands of 1a and 1d exhibit mirror images to
their low-energy 1MLCT/1ILCT bands in UV–Vis spectra; secondly,
acid and zinc titration study of ligand 2d emission demonstrates
that the emission band of ligand 2d red-shifts from 475 to
528 nm upon addition of acids (Fig. S13) and to 562 nm upon addi-
tion of Zn(ClO4)2 salts (Fig. S14), which are ascribed to the 1ILCT
state and are buried in the broad emission band of Pt complex
1d at 631 nm. Considering the red-shift of the emission band in
1d compared to the 1ILCT emission in the acidified ligand 2d or
Zn-complexed 2d, we believe that the observed emission at
631 nm for Pt complex 1d is dominated by the 1MLCT emission,
but admixing with some 1ILCT character. Thirdly, the emission of
1a and 1d exhibits biexponential decay, with the longer lifetime
attributed to 1ILCT and the shorter one to 1MLCT. Furthermore,
confirmation of the emission being from the singlet excited states
rather than triplet excited states arises from the fact that the emis-
sion lifetimes of 1d are shorter than those measured for the acidi-
fied 2d (1.95 ns (22%) and 5.15 ns (78%)) and Zn-complexed 2d
(2.62 ns (55%) and 4.78 ns (45%)).
density on the ligand. Consequently, the 3p or 3ILCT energy is
,p
changed by the electron-donating or withdrawing nature of the
substituent and the TA absorption band maximum is red- or
blue-shifted.
4. Conclusions
Four dinuclear platinum complexes with substituted 1,4-
bis(terpyridin-4-yl-vinyl)benzene ligand were synthesized and
their photophysical properties were systematically investigated.
The dinuclear platinum complexes show intense absorption in
the visible region, which can be assigned as the 1MLCT/1ILCT tran-
sitions. The 1MLCT/1ILCT transition is influenced by the nature of
the substituents on divinylbenzene linker. Electron-donating sub-
stituent causes a pronounced red-shift of this transition, while
electron-withdrawing substituents (Br, CN) induce a blue-shift.
The electron-withdrawing substituents also cause a quench of
the emission. All the complexes exhibit broad absorption bands
in their triplet transient difference absorption spectra, which are
attributed to the bridging ligand localized triplet excited states
(
3p or 3ILCT). The transient absorption band maxima are also
,p
Similar to that observed in the UV–Vis absorption spectra, elec-
tron-donating substituent (OC12H25) causes a significant red-shift
in emission. The lack of emission upon excitation at the charge
transfer band in complexes 1b and 1c could be attributed to the in-
creased energy of the 1MLCT band due to the electron-withdrawing
nature of the Br and CN substituents, which decreases the energy
gap between the 1MLCT state and the other high-lying but ther-
mally accessible nonemissive state(s). As a result, no emission is
observed from 1b and 1c. The lack of emission in 1b and 1c is also
a reflection of weak ILCT in these two complexes due to the pres-
ence of electron-withdrawing substituent on the dvb linker, which
is verified by the acid-titration experiment for 2c (Supporting
information S12).
influenced by the substituents on the dvb linker, with a bathochro-
mic shift for the complex with electron-donating substituents and
a hypsochromic shift for the complexes with electron-withdrawing
substituents. Therefore, the excited-state properties of the mono-
metallic back-to-back dinuclear platinum complexes can be tuned
via the alternation of the substituents on the bridging ligand.
Acknowledgments
We would like to thank the National Science Foundation (CA-
REER CHE-0449598) for financial support. We are grateful to Barry
Pemberton at NDSU for measuring the emission lifetimes of 1a and
1d, and to Professor Sivaguru Jayaraman for valuable discussion.
3.4. Transient difference absorption
Appendix A. Supplementary material
In order to understand the triplet excited-state properties of
1a–1d, the triplet transient absorption (TA) of these complexes in
CH3CN was investigated. Fig. 6 displays the triplet transient differ-
ence absorption spectra of 1a–1d at zero-time delay in degassed
The UV–Vis absorption spectra of 1a–1c and the emission spec-
tra of 1a at room temperature in different solvents, the emission
spectra of 1d in air-saturated and degassed CH3CN solutions, the
UV–Vis absorption and emission spectra of ligand 2d in CH2Cl2

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