Acid/base sensitive platinum terpyridyl complex: Switching between metal-to-ligand charge transfer (MLCT), ligand-to-ligand charge transfer (LLCT), and intraligand charge transfer (ILCT) states

Article abstract of DOI:10.1016/j.jorganchem.2009.09.015

A platinum(II) 2,2′:6′,2″-terpyridyl complex (2) with a hydroxylphenyl substituent on the terpyridyl ligand and a dimethylamino substituent on the phenylacetylide ligand was synthesized and characterized. Complex 2 exhibits a metal-to-ligand charge transf

Full text of DOI:10.1016/j.jorganchem.2009.09.015

Journal of Organometallic Chemistry 694 (2009) 4140–4145
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Acid/base sensitive platinum terpyridyl complex: Switching between
metal-to-ligand charge transfer (MLCT), ligand-to-ligand
charge transfer (LLCT), and intraligand charge transfer (ILCT) states
*
Zhiqiang Ji, Yunjing Li, Wenfang Sun
Department of Chemistry and Molecular Biology, North Dakota State University, ND 58108-6050, USA
a r t i c l e i n f o
a b s t r a c t
A platinum(II) 2,20:60,200-terpyridyl complex (2) with a hydroxylphenyl substituent on the terpyridyl
ligand and a dimethylamino substituent on the phenylacetylide ligand was synthesized and character-
ized. Complex 2 exhibits a metal-to-ligand charge transfer (1MLCT) absorption band at ca. 410 nm and
a ligand-to-ligand charge transfer (1LLCT) band at ca. 536 nm. It exhibits dual emission at ca. 450 nm
Article history:
Received 14 July 2009
Received in revised form 5 September 2009
Accepted 7 September 2009
Available online 11 September 2009
and ca. 560 nm at room temperature when excited at 334 nm, which originates from the 1p
,
p
* state
and the 3MLCT/3
p,p* state, respectively. Dramatic color change was observed for 2 with addition of acid
Keywords:
and base. Its emission at 560 nm was enhanced in acidic solution and quenched in basic solution. The
changes in absorption and emission could be attributed to the variation of the nature of the lowest
excited state from LLCT to MLCT in acidic solution and to LLCT/ILCT at basic solution. The drastic color
and emission intensity changes in acidic and basic solutions suggest that 2 could potentially be a color-
imetric and luminescent acid/base sensor.
Platinum terpyridyl complex
Electronic absorption
Emission
Acid/base sensor
Metal-to-ligand charge transfer
Ligand-to-ligand charge transfer
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
intriguing; however, the complexes reported are only responsive
to acids. To date, no platinum complexes that are sensitive to bases
Square-planar platinum(II) complexes have attracted a great
interest in recent years due to their intriguing spectroscopic prop-
erties and potential applications in a variety of photonic devices,
such as in organic light emitting diodes [1–9], photovoltaic cells
[10–13], as nonlinear optical materials [14–19], photosensitizers
[20–23], and chemical and biological sensors [24–30]. Platinum
terpyridyl phenylacetylide complexes are particularly interesting
because of their ease of synthesis and structural modifications.
Upon structural modifications, 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 (ILCT) state, and intrali-
have been reported. To remedy this deficiency, we recently synthe-
sized a new platinum terpyridyl complex (2) with dimethylamino
substituent on the phenylacetylide ligand and a hydroxylphenyl
substituent on the terpyridyl ligand (structure shown in Scheme
1). It is anticipated that upon addition of bases, the hydroxyl group
would be deprotonated, which would influence the electron distri-
bution of the complex and thus affect the nature of the lowest
excited state. Meanwhile, the presence of the dimethylamino sub-
stituent would make this complex sensitive to acids. Such a design
would allow for this complex to be a good candidate for acid/base
sensor. To demonstrate this, the photophysical properties of this
complex, especially the changes of its UV–Vis absorption and emis-
sion in acidic and basic solutions, have been systematically
investigated.
gand (IL)
p,p* state. For example, Wu and co-workers reported
the photophysical properties of platinum terpyridyl phenylacety-
lide complexes bearing amino moieties on phenylacetylide or
terpyridyl ligands [29]. Upon protonation, the lowest excited state
was switched between LLCT, MLCT, and ILCT, which results in dra-
matic color change and luminescence intensity change [29]. Yam
and co-workers has also developed colorimetric and luminescent
pH sensors based on the same mechanism [28]. These results are
2. Experimental section
2.1. Synthesis
K2PtCl4, trifluoromethanesulfonic acid anhydride, p-toluenesul-
fonic acid, tetra-n-butylammonium hydroxide 1.0 M solution and
all the other reagents were purchased from Aldrich and Alfa Aesar
and used without further purification. All solvents are analytical
grade and were purchased from VWR International and used
* Corresponding author. Tel.: +1 701 2316254; fax: +1 701 2318831.
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.2009.09.015

Z. Ji et al. / Journal of Organometallic Chemistry 694 (2009) 4140–4145
4141
PF6
PF6
N
N
N
N
N
Pt
N
N
a
c
b
HO
N
TfO
N
HO
N
N
TfO
N
Pt Cl
N
1
2
a. (OTf) O, Pyridine; b. K PtCl , CH CN/H O; c. N,N'-dimethylphenylacetylene, KOH, CuI, MeOH.
2
2
4
3
2
Scheme 1. Synthetic route of 2.
without further purification unless otherwise stated. 40-(p-Hydrox-
ylphenyl)terpyridine was synthesized according to the literature
procedure [31]. All products were characterized by 1H NMR,
elemental analysis, and high resolution MS. 1H NMR spectra were
obtained on a Varian 400 MHz or 500 MHz Varian NMR spectrom-
eter. Elemental analyses were conducted by NuMega Resonance
Labs, Inc. in San Diego, CA. High resolution MS data were obtained
using a Bruker BioTof III spectrometer.
C, 45.98; H, 3.09; N, 6.92%. HRMS: calcd for [C31H25N4OPt195]+,
664.1673; found, 664.1676 (100%).
2.5. Photophysical studies
The electronic absorption spectra were recorded on a SHIMA-
DZU 2501 PC UV–Vis spectrophotometer. The emission spectra
were recorded on a SPEX Fluorolog-3 fluorometer/phosphorome-
ter. The complex was dissolved in HPLC grade CH3CN. The emission
lifetimes were measured on an Edinburgh LP920 laser flash photol-
ysis spectrometer. Excitation resource was the third-harmonic out-
put (355 nm) of a Quantel Brilliant Q-switched Nd:YAG laser
(FWHM pulsewidth was 4.1 ns and the repetition rate was set at
1 Hz). The emission quantum yield was obtained via a comparative
method [33], in which a degassed aqueous solution of [Ru(b-
2.2. 40-(p-Trifluoromethylsulfonylphenyl)-terpyridine [32]
40-(p-Hydroxyphenyl)-2,20:60,200-terpyridine (0.82 g, 2.52 mmol)
was dissolved in 10 ml dry pyridine. Trifluoromethanesulfonic
anhydride (0.83 ml, 4.93 mmol) was slowly added at 0 °C. The
resultant mixture was stirred at 0 °C for another 30 min, and then
the reaction was warmed to room temperature. The mixture was
kept at room temperature for 2 days. After that, the mixture was
poured into 200 ml cold water, and solid was separated by filtra-
tion. The crude product was purified by recrystallization in ethanol
to afford 1.03 g white solid (yield: 95%). 1H NMR (DMSO-d6,
400 MHz) d 7.54 (2H, ddd, J = 1.2, 8.7, 12.3 Hz), 7.70 (2H, d,
J = 9.0 Hz), 8.05 (2H, dt, J = 1.5, 7.8 Hz), 8.13 (2H, m), 8.67 (2H, d,
J = 7.8 Hz), 8.71 (2H, s), 8.75 (2H, m) ppm.
py)3]Cl2 (Uem = 0.042 at 436 nm excitation) was used as the refer-
ence [34].
3. Results and discussion
3.1. Electronic absorption
The electronic absorption spectrum of 2 is shown in Fig. 1.
Within the concentration range studied (4.8 ꢀ 10ꢁ6–9.6 ꢀ 10ꢁ5
mol/L), the absorption of 2 obeys Beer–Lambert’s law, indicating
no ground-state aggregation. Complex 2 exhibits intense UV
2.3. Complex 1
40-(p-Trifluoromethylsulfonylphenyl)-2,20:60,200-terpyridine
(218 mg, 0.48 mmol) was dissolved in 30 ml hot CH3CN, K2PtCl4
(198 mg, 0.48 mmol) was dissolved in 10 ml hot water. These
two solutions were added together, and the mixture was heated
to reflux for 24 h. After the mixture was cooled to room tempera-
ture, yellow solid was separated by filtration, and washed with
hexane (50 ml) to give white solid 207 mg (yield: 60%). 1H NMR
absorption below 370 nm, which can be attributed to the p,p* tran-
sitions within the terpyridyl and phenylacetylide ligands. In addi-
tion, two broad absorption bands are observed in the visible
spectral region. With reference to the UV–Vis absorption of other
platinum complexes reported in the literature [28], the absorption
band at 380–460 nm can be assigned to the d
p(Pt) ?
p*(terpyr-
(DMSO-d6, 400 MHz)
J = 6.4 Hz), 8.31 (2H, d, J = 6.8 Hz), 8.55 (2H, dt, J = 8.0, 1.6 Hz),
8.81 (2H, d, J = 7.2 Hz), 8.96 (2H, d, J = 5.6 Hz), 9.02 (2H, s) ppm.
d 7.85 (2H, d, J = 9.6 Hz), 7.98 (2H, t,
7x104
PF6
6x104
N
2.4. Complex 2
HO
N Pt
N
N
5x104
4x104
3x104
2x104
4-Ethynyl-N,N-dimethylaniline (14.5 mg, 0.1 mmol) and KOH
(11.2 mg, 0.2 mmol) were dissolved in MeOH and stirred at r.t.
for 30 min. Complex 1 (61.4 mg, 0.1 mmol) and CuI (1.2 mg,
0.06 mmol) were added. The mixture was purged under Ar for
30 min, and then stirred at r.t. for 18 h. After the reaction, the reac-
tion mixture was concentrated, and ether was added. The black
precipitants were separated by filtration. The solid was dissolved
in 20 ml MeOH, and 20 ml saturated aqueous solution of NH4PF6
was added. The mixture was stirred at r.t. for 3 h, then the dark
solid was separated by filtration. The crude product was purified
by recrystallization in CH3CN/Et2O to give 43.8 mg black solid
(yield: 54%). 1H NMR (DMSO-d6, 400 MHz), 2.97 (6H, s), 6.68 (2H,
d, J = 8.0 Hz), 7.02 (2H, d, J = 8.0 Hz), 7.31 (2H, d, J = 7.5 Hz), 7.90
(2H, m), 8.06 (2H, m), 8.49 (2H, m), 8.80 (4H, m), 9.15 (2H, m).
Anal. Calc. for C31H25F6N4OPPt: C, 45.75; H: 3.22; N: 6.97. Found:
2
ε
1x104
0
300
400
500
600
700
Wavelength (nm)
Fig. 1. UV–Vis absorption spectrum of 2 in CH3CN.

4142
Z. Ji et al. / Journal of Organometallic Chemistry 694 (2009) 4140–4145
idyl) 1MLCT transition; while the lowest absorption band from 460
to 670 nm is attributed to the [N(CH3)2PhC„C] ? *(terpyridyl)
4x105
3x105
2x105
p
p
Emission spectra
1LLCT transition [28,29]. The assignment of the low-energy absorp-
tion bands to charge transfer transitions is supported by the nega-
tive solvatochromic effect of 2 in solvents with different polarities,
i.e. these bands shift to longer wavelengths in less polar solvents
such as CH2Cl2 and THF compared to those in more polar solvents
like CH3OH and CH3CN (shown in Fig. 2). This implies a more polar
ground state than the excited state. The charge transfer nature of
the lowest-energy band is further evident by the dramatic blue-
shift of this band upon protonation by acid, which significantly
decreases the electron-donating ability of the dimethylaminophe-
nylacetylide ligand and moves the 1LLCT to a much higher energy.
This point will be discussed further in Section 3.3.
9.6x10-5
4.8x10-5
9.6x10-6
4.8x10-6
M
M
M
M
1x105
0
3.2. Photoluminescence
400
450
500
550
600
Wavlength (nm)
Distinct from the nonemissive platinum terpyridyl complexes
with dialkylamino substituent reported in the literature [28,29],
2
exhibits dual emission in CH3CN at r.t. when excited at
Excitation spectra
2.0x105
1.5x105
1.0x105
334 nm, and the relative emission intensity is concentration-
dependent. As shown in Fig. 3, at lower concentrations
(4.8 ꢀ 10ꢁ6 and 9.6 ꢀ 10ꢁ6 mol/l), the emission at 445 nm domi-
nates. At higher concentrations (4.8 ꢀ 10ꢁ5 and 9.6 ꢀ 10ꢁ5 mol/l),
the emission at 445 nm diminishes, while the emission at ca.
560 nm becomes dominant. When excited at 411 nm, only the
560 nm band is observed. The lifetimes of these two emission
bands are quite distinct. The emission band at 445 nm is short-
lived, with a lifetime that is too short to be measured on our spec-
trometer (<5 ns). In contrast, the intrinsic lifetime for the emission
at 560 nm is approximately 830 ns. This implies that the origins of
these two emission bands are different. This notion is confirmed by
the excitation spectra measurement. When monitored at the
445 nm emission band, the excitation band appears at ca. 260–
9.6X10-5
4.8X10-5
9.6X10-6
4.8X10-6
M
M
M
M
5.0x104
0.0
400 nm, which coincides with the 1p
,p* transition in the UV–Vis
300
350
400
450
500
spectrum. When monitored at the 560 nm emission band, the exci-
tation band maximum occurs at ca. 430 nm, which lies in the
1MLCT band region. In view of the different excitation spectra
and different lifetimes of these two emission bands, we conclude
that these two emission bands emanate from different excited
states. The emission at 445 nm likely emanates from the intrali-
Wavelength (nm)
Fig. 3. Concentration-dependent room temperature emission and excitation spec-
tra of 2. For emission spectra measurements, the excitation wavelength was
334 nm.
gand 1p
,p* excited state, while the 560 nm band could originate
emission band is very sensitive to oxygen. In deaerated solution,
the emission intensity is several orders of magnitude higher than
that in aerated solution. The emission quantum yield for the
560 nm band is measured to be 0.0048 in deaerated CH3CN solu-
tion. The dual emission was also reported by Wong and co-workers
for a class of trifunctional Pt(II) cyclometallated platinum com-
plexes [35].
from a triplet excited state due to the much large Stokes shift
(ꢂ130 nm) and the significantly long lifetime. In addition, this
0.9
CH3CN
CH2Cl2
DMF
Considering the nature of the emitting triplet excited state, it
CH3OH
could admix the 3MLCT and 3p
,p* configurations. This notion can
THF
0.6
be supported by the following pieces of evidences: (1) As shown
in Fig. 4, this band exhibits insignificant solvent effect, which is a
characteristic of the
p,p* transition. (2) Although the emission
band shows somewhat structures, it is quite broad. The broad
emission profile indicates the involvement of charge transfer char-
acter in the emitting state, which should be the 3MLCT state. (3)
The presence of oxygen in solution not only quenches the emission
intensity but also changes the emission band apex. As displayed in
Fig. 5, the apex shifts from 558 nm in an aerated solution to
530 nm after degassing. This implies that the broad emission orig-
inates from two configurationally distinct emitting states that lie in
close proximity but show different sensitivity to oxygen. However,
the emission decays symmetrically with time in deaerated solu-
tion, which is evidenced by the time-resolved emission spectra
and the lifetimes measured at 530 nm and 558 nm are almost
0.3
0.0
400
500
600
700
800
Wavelength (nm)
Fig. 2. UV–Vis absorption spectra of 2 in different solvents in 1-cm cuvette (c = 4.8
ꢀ 10ꢁ5 mol/l).

Z. Ji et al. / Journal of Organometallic Chemistry 694 (2009) 4140–4145
4143
0.8
0.16
0.12
0.08
0.04
0.00
CH3CN
1.0
0.8
0.6
0.4
0.2
0.0
536 nm
CH2Cl2
DMF
CH3OH
0.6
0.4
0.2
0.0
0.0 0.4 0.8 1.2 1.6 2.0 2.4
C
p-TsOH/Ccomplex 2
500
550
600
650
700
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 4. Normalized emission spectra of 2 in different solvents (c = 4.8 ꢀ 10ꢁ5 mol/l).
kex = 411 nm.
600000
450000
300000
150000
0
750000
600000
450000
300000
150000
0
Aerated
1.0
0.8
0.6
0.4
0.2
0.0
558 nm
Deaerated
0.0
0.4
0.8
1.2
1.6
C
p-TsOH/Ccomplex 2
500
550
600
Wavelength (nm)
650
700
Fig. 6. UV–Vis absorption spectra and emission spectra at room temperature of 2 in
CH3CN (5.0 ꢀ 10ꢁ5 mol/l) with addition of p-TsOH (1.5 ꢀ 10ꢁ2 mol/l).
500
550
600
650
700
Wavelength (nm)
Fig. 5. Normalized emission spectra of 2 in aerated and deaerated CH3CN solution.
kex = 411 nm.
the same (the intrinsic lifetime obtained at 530 nm is 815 ns and
833 ns at 558 nm). (4) The mixture of the 3MLCT and 3
p,p* config-
urations is further evidenced by base titration that will be dis-
cussed in Section 3.3. As shown in Fig. 7, upon base titration, the
emission at 558 nm gradually decreases, while the emission at
530 nm is much less affected by base. As will be discussed in detail
later, upon addition of base, the low-lying excited states switch
from MLCT/LLCT to LLCT/ILCT (intraligand charge transfer), which
consequently decreases the 3MLCT emission with the 3p
sion largely unaffected. In addition to the aforementioned
,p* emis-
Fig. 7. Complex
2
in CH3CN (5.0 ꢀ 10ꢁ5 mol/l, middle) with various p-TsOH
evidences, the mixture of 3MLCT with 3
p,p* characters in the emit-
concentration (left 1 to left 3: 5.6 ꢀ 10ꢁ5, 3.9 ꢀ 10ꢁ5, 3.4 ꢀ 10ꢁ5 mol/l); and various
n-Bu4NOH concentration (left 4 to right): 0.6 ꢀ 10ꢁ5, 1.3 ꢀ 10ꢁ5, 2.3 ꢀ 10ꢁ5 mol/l.
ting state has been proposed by McMillin and co-workers for 40-
substituted Pt(trpy)Cl complexes bearing aryl groups [36] and by
Castellano and co-workers for [Pt(40-C„CR-tpy)Cl]+ complexes
[37].
Another point worthy of mention is that the concentration-
dependent behaviors of these two emission bands are different.
The emission band at 445 nm is completely quenched when the
concentration is increased to 4.8 ꢀ 10ꢁ5 mol/l, while the emission
at 560 nm is still apparent. The rapid quenching of the 445 nm
band could possibly be due to the formation of nonemissive
ground-state aggregates, the inner-filter effect, or the self-quench-
ing effect. The formation of ground-state aggregates could be easily
excluded due to the fact that the UV–Vis absorption obeys Beer–
Lambert’s law from 4.8 ꢀ 10ꢁ6 to 9.6 ꢀ 10ꢁ4 mol/l. In view of the
larger extinction coefficients of the electronic absorption at
560 nm than that at 445 nm, the inner-filter effect could not be
the dominant contributor, either. This is because if the inner-filter
effect dominates, the emission at 560 nm should be quenched
more quickly than the band at 445 nm. Therefore, the rapid

4144
Z. Ji et al. / Journal of Organometallic Chemistry 694 (2009) 4140–4145
quenching of the 445 nm band should be mainly attributed to the
self-quenching effect, although the inner-filter effect could also
contribute. Unfortunately, confirmation of the self-quenching ef-
fect could not be conducted due to the limitation of our spectrom-
eter’s resolution (<5 ns). Nevertheless, the more rapid quenching of
the 445 nm band than the 560 nm band suggests that the self-
ation. The shortened lifetime after protonation suggests more con-
tribution from the charge transfer state. This further confirms that
the emissive 3MLCT state becomes the lowest excited state after
protonation of 2, which is consistent with what have been reported
by Yam and co-workers [28] and Wu and co-workers [29]. The
same change of the UV–Vis spectrum and the emission intensity
was observed upon acidification using HCl or other common
organic acids such as HBF4.
quenching constant of the 1p
,p* state is larger than that of the
3MLCT/3
p,p
* state.
For the 560 nm emission band, the emission intensity also de-
creases with increased concentration. The lifetime measurement
of different-concentration solutions revealed that the emission
lifetime becomes shorter at higher concentrations. The lifetime
measured at 5 ꢀ 10ꢁ6, 1 ꢀ 10ꢁ5, 3 ꢀ 10ꢁ5 and 6 ꢀ 10ꢁ5 mol/l is
979, 597, 481 and 390 ns, respectively. Extrapolation of the inter-
cept and slope from the plot of decay rate constant (kobs.) vs. con-
centration gives rise to the intrinsic lifetime and self-quenching
constant of 833 ns and 2.4 ꢀ 1010 l molꢁ1 sꢁ1, respectively. The life-
times monitored at 530 nm for different concentrations are similar
to those obtained at 560 nm, and a similar self-quenching constant
is observed (the results are shown in Table 1).
Complex 2 is also responsive to bases. With addition of (n-Bu)4-
NOH to CH3CN solution of 2, both the electronic absorption spec-
trum and the emission intensity of 2 change (shown in Fig. 8).
The change of the UV–Vis spectrum of 2 is stepwise: with addition
of 0–0.5 equiv. (n-Bu)4NOH, the lowest-energy absorption band
maximum bathochromically shifts from 536 to 594 nm, accompa-
nied by the decrease of the absorption around 425 nm, and an
isosbestic point appears at 439 nm; with addition of 0.6–1.0 equiv.
(n-Bu)4NOH, the absorption at 594 nm continuously increases and
a new isosbestic point occurs at 494 nm. No further change was
observed upon addition of excess amount of (n-Bu)4NOH. Corre-
sponding to this spectral change, the color of the solution changes
from purple to blue (illustrated in Fig. 7). On the other hand,
addition of (n-Bu)4NOH gradually quenches the emission at
558 nm, while the emission at 530 nm is essentially unaffected.
After addition of 0.5 equiv. (n-Bu)4NOH, no further change in the
emission spectrum is observed.
3.3. Acid and base titration
Yam and co-workers reported that platinum terpyridyl com-
plexes containing dimethylamino substituent(s) exhibit drastic
color change and the emission was ‘‘turned on” upon protonation
[28,29]. These changes were attributed to the switching of the ex-
cited state from LLCT/ILCT to MLCT after protonation. Because
complex 2 also possess a dimethylamino substituent on the phen-
ylacetylide ligand and the lowest-energy absorption band is as-
signed to the 1LLCT transition, it is expected that a similar
change of the nature of the lowest-energy excited state from LLCT
to MLCT would occur. To demonstrate this, the electronic absorp-
tion and emission spectra of 2 was monitored upon addition of
p-TsOH in CH3CN. The acid titration was performed in CH3CN solu-
tion because 2 is not water-soluble. As shown in Fig. 6, the 1LLCT
band at ca. 536 nm gradually decreases after addition of acid,
accompanied by the increase of the 1MLCT absorption band at ca.
425 nm. An isosbestic point at 472 nm is formed during the titra-
tion, indicating that only two interconverting species are present
in the solution. In accordance with this spectroscopic change, the
color of the solution changes from the original purple color to yel-
low (Fig. 7). The drastic change of the UV–Vis spectrum of 2 with
addition of p-TsOH should be attributed to the protonation of the
dimethylamino substituent, which dramatically lowers the phen-
ylacetylide-based molecular orbital and moves the Pt-based orbital
as the HOMO. Consequently, the lowest excited state is switched
from 1LLCT to 1MLCT. The change of the nature of the lowest
excited state also influences the emission intensity drastically. As
shown in Fig. 6, upon addition of p-TsOH, the emission intensity
of 2 at 558 nm gradually increases. When the acid concentration
is increased to 1 equiv., the emission intensity reaches the maxi-
mum point (20 times higher than the original intensity). The life-
time of the emission monitored at 558 nm in the presence of
excess amount of p-TsOH is 260 ns, which is shorter than that of
2.5
1.0
0.8
2.0
0.6
0.4
598 nm
1.5
0.2
0.0
0.3
0.6
0.9
1.2
1.5
1.8
C
/Ccomplex 2
Bu4NOH
1.0
0.5
0.0
300
400
500
600
700
800
Wavelength (nm)
75000
60000
45000
30000
558 nm
90000
75000
60000
45000
30000
15000
0.0 0.2 0.4 0.6 0.8 1.0
C
/ Ccomplex 2
Bu4NOH
the original 3MLCT/3
p,p* emission from 2 as discussed in the pre-
vious section. The lifetime variation reflects the mixing of different
amount of 3MLCT and 3p
,p* characters before and after proton-
Table 1
Photophysical parameters of 2 at room temperature in CH3CN.
0
/Mꢁ1 cmꢁ1 em/ns; Uem; kqa/Mꢁ1 sꢁ1
)
s
500
550
600
650
700
kabs/nm ( kem/nm ( )
e
445, 530 (815; -; 3.25 ꢀ 1010),
Wavelength (nm)
289 (62 520), 334 (24 500),
407 (8460), 536 (6440)
558 (833; 0.0048; 2.42 ꢀ 1010
)
Fig. 8. UV–Vis absorption and emission spectra of 2 in CH3CN (5.0 ꢀ 10ꢁ5 mol/l)
with addition of n-Bu4NOH (1.5 ꢀ 10ꢁ2 mol/l).
a
Self-quenching constant, obtained from the slope of the plot kobs = 1/s0 + kq[C].

Z. Ji et al. / Journal of Organometallic Chemistry 694 (2009) 4140–4145
4145
in acidic solution and deprotonation of the hydroxyl group in basic
solution, which in turn changes the nature of the lowest excited
state. In acidic solution, the lowest excited state is switched from
LLCT to MLCT; while in basic solution, it is altered from LLCT to
ILCT. The dramatic color change and emission intensity change
make 2 a promising candidate as a chromogenic and luminescent
acid/base sensor in organic solvent. With further modification of
2 to become water-soluble, 2 could potentially be used as a pH
sensor.
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
2
2 + 1 eq. n-Bu NOH
2 + 1 eq. n-Bu4NOH + 1 eq. p-TsOH
2 + 1 eq. n-Bu44NOH + 2 eq. p-TsOH
2 + 1 eq. p-TsOH
Acknowledgments
This research was supported by the National Science Founda-
tion (CAREER CHE-0449598). We are grateful to North Dakota State
EPSCoR (ND EPSCoR Instrumentation Award) for support.
300
400
500
600
700
800
Wavelength (nm)
References
Fig. 9. UV–Vis absorption spectra of 2 in CH3CN (5.0 ꢀ 10ꢁ5 mol/l) with addition of
n-Bu4NOH (1.5 ꢀ 10ꢁ2 mol/l) and p-TsOH (1.5 ꢀ 10ꢁ2 mol/l).
[1] W. Lu, B.-X. Ma, M.C.W. Chan, Z. Hui, C.-M. Che, N. Zhu, S.-T. Lee, J. Am. Chem.
Soc. 126 (2004) 4958.
[2] H. Yersin, D. Donges, W. Humbs, J. Strasser, R. Sitters, M. Glasbeek, Inorg. Chem.
41 (2002) 4915.
The bathochromic shift of the lowest-energy absorption band in
the UV–Vis spectrum and the quench of the emission at 558 nm
can be rationalized as following: upon addition of (n-Bu)4NOH,
the OH substituent at the terpyridyl ligand is deprotonated, which
increases the electron density on the phenyl ring. This would
destabilize the molecular orbital of the phenoxide and move it
up as the HOMO. In such a case, the intraligand charge transfer
state (1ILCT) between the phenoxide-based HOMO and the terpyr-
idine-based LUMO becomes the lowest excited state, which
appears at 594 nm and partially overlaps with the 1LLCT absorp-
tion at ca. 536 nm. Thus, the broad band from 450 to 750 nm after
deprotonation can be assigned as a mixture of 1ILCT and 1LLCT. The
appearance of the low-lying nonemissive ILCT state adds an addi-
tional decay path for the emitting MLCT state and thus quenches
the 3MLCT emission at 558 nm more rapidly. In contrast, the
[3] J. Brooks, Y. Babayan, S. Lamansky, P.I. Djurovich, I. Tsyba, R. Bau, M.E.
Thompson, Inorg. Chem. 41 (2002) 3055.
[4] J.C. Shi, H.Y. Chao, W.F. Fu, S.M. Peng, C.M. Che, J. Chem. Soc., Dalton Trans. 18
(2000) 3128.
[5] L. Chassot, A. von Zelewsky, D. Sandrini, M. Maestri, V. Balzani, J. Am. Chem.
Soc. 108 (1986) 6084.
[6] M. Maestri, D. Sandrini, V. Balzani, L. Chassot, P. Jolliet, A. von Zelewsky, Chem.
Phys. Lett. 122 (1985) 375.
[7] W.-Y. Wong, Z. He, S.-K. So, K.-L. Tong, Z. Lin, Organometallics 24 (2005) 4079.
[8] G.-J. Zhou, X.-Z. Wang, W.-Y. Wong, X.-M. Yu, H.-S. Kwok, Z. Lin, J. Organomet.
Chem. 692 (2007) 3461.
[9] C.-L. Ho, W.-Y. Wong, B. Yao, Z. Xie, L. Wang, Z. Lin, J. Organomet. Chem. 694
(2009) 2735.
[10] S. Chakraborty, T.J. Wadas, H. Hester, C. Flaschenreim, R. Schmehl, R. Eisenberg,
Inorg. Chem. 44 (2005) 6284.
[11] S. Chakraborty, T.J. Wadas, H. Hester, C.R. Schmehl, R. Eisenberg, Inorg. Chem.
44 (2005) 6865.
[12] W.-Y. Wong, X.Z. Wang, Z. He, A.B. Djurisic, C.-T. Yip, K.-Y. Cheung, H. Wang,
C.S.K. Mak, W.-K. Chan, Nat. Mater. 6 (2007) 521.
[13] W.-Y. Wong, Macromol. Chem. Phys. 209 (2008) 14.
[14] W. Sun, Z.-X. Wu, Q.-Z. Yang, L.-Z. Wu, C.-H. Tung, Appl. Phys. Lett. 82 (2003)
850.
3p
,p* state remains intact energetically as evidenced in Fig. 8,
where the emission at 530 nm has essentially no change with
[15] W. Sun, F. Guo, Chin. Opt. Lett. S3 (2005) S34.
[16] F. Guo, W. Sun, J. Phys. Chem. B 110 (2006) 15029.
[17] T.M. Pritchett, W. Sun, F. Guo, B. Zhang, M.J. Ferry, J.E. Rogers-Haley, W.
Shensky, A.G. Mott, Opt. Lett. 33 (2008) 1053.
[18] G.-J. Zhou, W.-Y. Wong, Z. Lin, C. Ye, Angew. Chem., Int. Ed. Engl. 45 (2006)
6189.
[19] G. Zhou, Funct. Mater. 19 (2009) 531.
[20] D. Zhang, L.-Z. Wu, Q.-Z. Yang, X.-H. Li, L.-P. Zhang, C.-H. Tung, Org. Lett. 5
(2003) 3221.
[21] Y. Yang, D. Zhang, L.-Z. Wu, B. Chen, L.-P. Zhang, C.-H. Tung, J. Org. Chem. 69
(2004) 4788.
[22] X.-H. Li, L.-Z. Wu, L.-P. Zhang, C.-H. Tung, C.M. Che, Chem. Commun. 21 (2001)
2280.
[23] K. Feng, R.-Y. Zhang, L.-Z. Wu, B. Tu, M.-L. Peng, L.-P. Zhang, D. Zhao, C.-H. Tung,
J. Am. Chem. Soc. 128 (2006) 14685.
[24] Q.-Z. Yang, L.-Z. Wu, H. Zhang, B. Chen, Z.-X. Wu, L.-P. Zhang, Z.-H. Tung, Inorg.
Chem. 43 (2004) 5195.
deprotonation.
A point worthy of mention is that the acid-induced changes in
UV–Vis and emission spectra are essentially reversible. After addi-
tion of 1 equiv. of n-Bu4NOH to the acidified CH3CN solution of 2
with 1 equiv. of p-TsOH, both the UV–Vis and emission spectra
essentially recovered to their original forms. However, as shown
in Fig. 9, the base induced change is not completely reversible upon
addition of an equal amount of acid to the basicified solution. Only
when one more equivalent of p-TsOH was added, the UV–Vis and
emission spectra became almost identical to the acidified solution
from the original form of 2 with 1 equiv. of p-TsOH. This phenom-
enon could be due to the competition between the phenoxide
anionic component and the dimethylamino substituent when
1 equiv. of acid was added. After addition of 2 equiv. of acid, both
the phenoxide anion and the dimethylamino substituent are pro-
tonated, which gives rise to the similar UV–Vis and emission spec-
tra as those for the acidified solution from the original form of 2.
[25] L.Z. Wu, T.C. Cheung, C.M. Che, K.K. Cheung, M.H. Lam, Chem. Commun. 10
(1998) 1127.
[26] K.-H. Wong, M.C.-W. Chan, C.M. Che, Chem. Eur. J. 5 (1999) 2845.
[27] S.C.F. Kui, S.S.-Y. Chui, C.-M. Che, N. Zhu, J. Am. Chem. Soc. 128 (2006) 8297.
[28] K.M. Wong, W.-S. Tang, X.-X. Lu, N. Zhu, V.W.-W. Yam, Inorg. Chem. 44 (2005)
1492.
[29] X. Han, L.-Z. Wu, G. Si, J. Pan, Q.-Z. Yang, L.-P. Zhang, C.-H. Tung, Chem. Eur. J.
13 (2007) 1231.
[30] K.M.-C. Wong, W.-S. Tang, B.W.-K. Chu, N. Zhu, V.W.-W. Yam, Organometallics
23 (2004) 3459.
4. Conclusions
The platinum complex (2) with both dimethylamino substitu-
ent and hydroxylphenyl substituent on its ligands exhibits a dras-
tic color change from yellow in acidic solution to blue in basic
CH3CN solutions. Its emission at 558 nm is significantly enhanced
in acidic solution, and is quenched in basic solution. These dra-
matic changes are caused by the change of the electron density
on the ligands upon protonation of the dimethylamino substituent
[31] D.W. Yoo, S.K. Yoo, C. Kim, J.K. Lee, J. Chem. Soc., Dalton Trans. (2002) 3931.
[32] K.T. Potts, D. Konwar, J. Org. Chem. 56 (1991) 4815.
[33] J.N. Demas, G.A. Crosby, J. Phys. Chem. 75 (1971) 991.
[34] J. Van Houten, R.J. Watts, J. Am. Chem. Soc. 98 (1976) 4853.
[35] Z. He, W.-Y. Wong, X. Yu, H.-S. Kwok, Z. Lin, Inorg. Chem. 45 (2006) 10922.
[36] J.F. Michalec, S.A. Bejune, D.G. Cuttell, G.C. Summerton, J.A. Gertenbach, J.S.
Field, R.J. Haines, D.R. McMillin, Inorg. Chem. 40 (2001) 2193.
[37] M.L. Muro, F.N. Castellano, Dalton Trans. (2007) 4659.