Low temperature emission spectra of 2-pyridyl-substituted platinum-1,2-enedithiolates

Article abstract of DOI:10.1016/S0020-1693(03)00211-1

The emissions from the 2-pyridyl- and 2-pyridinium-substituted platinum-1,2-enedithiolate complexes, L₂Pt{S₂C ₂(2-pyridyl(ium))(R)} where L=PPh₃, PPh₂Me, PPhMe₂, P(propyl)₃ and L₂=dppm, dppe, and dppp were studied in DMM (DMF-CH₂Cl₂-MeOH, 1:1:1, v/v/v) glasses at 77 K. All of the pyridyl-substituted complexes have an emission between 18,600 and 17,900 cm^-1 with pronounced 1200 cm^-1 vibronic structure. The lifetimes of the pyridyl-substituted complexes are 216-350 μs. The pyridinium-substituted complexes have emissions in the range of 16,600-16,100 cm^-1 with less resolved vibronic structure than their pyridyl counterparts. The lifetimes of the pyridinium-substituted complexes are 158-290 μs. The emission at 77 K from both the pyridyl- and pyridinium-substituted complexes contrasts with the room temperature studies where only selected pyridinium complexes were emissive.

Full text of DOI:10.1016/S0020-1693(03)00211-1

Inorganica Chimica Acta 353 (2003) 231Á237
/
www.elsevier.com/locate/ica
Low temperature emission spectra of 2-pyridyl-substituted platinum-
1,2-enedithiolates
Kelly A. Van Houten a,1, Neil V. Blough a, Robert S. Pilato b,*
a Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA
b LUMET LLC, 5404 Huntington Parkway, Bethesda, MD 20814, USA
Received 12 December 2002; accepted 13 January 2003
Abstract
The emissions from the 2-pyridyl- and 2-pyridinium-substituted platinum-1,2-enedithiolate complexes, L2Pt{S2C2(2-pyridy-
l(ium))(R)} where Lꢀ
CH2Cl2ÁMeOH, 1:1:1, v/v/v) glasses at 77 K. All of the pyridyl-substituted complexes have an emission between 18,600 and
17,900 cmꢁ1 with pronounced 1200 cmꢁ1 vibronic structure. The lifetimes of the pyridyl-substituted complexes are 216Á
350 ms. The
pyridinium-substituted complexes have emissions in the range of 16,600Á
16,100 cmꢁ1 with less resolved vibronic structure than
their pyridyl counterparts. The lifetimes of the pyridinium-substituted complexes are 158Á290 ms. The emission at 77 K from both
/
PPh3, PPh2Me, PPhMe2, P(propyl)3 and L2ꢀ
/
dppm, dppe, and dppp were studied in DMM (DMFÁ
/
/
/
/
/
the pyridyl- and pyridinium-substituted complexes contrasts with the room temperature studies where only selected pyridinium
complexes were emissive.
# 2003 Elsevier B.V. All rights reserved.
Keywords: Low-temperature; Platinum-1,2-enedithiolate; Dual emitter
1. Introduction
emissions from these complexes have been assigned to
1
a short-lived intraligand charge transfer singlet, ILCT*
and long-lived triplet 3ILCT*. Given that the lifetimes of
the two excited states allows the use of ratiometric
detection methods, these complexes have broad analy-
Platinum-1,2-enedithiolates have emerged as a im-
portant new class of room temperature solution lumi-
phores [1Á
heterocyclic-substituted
complexes of the
/13]. Our group has focused on the study of
tical applications [5Á13]. At present these dual-emitters
/
platinum-1,2-enedithiolate
L2Pt{S2C2(Het)(R)},
are under development as pH probes and luminescent
tags and [L2Pt{S2C2(CH2CH2-N-2-pyridnium)}]ꢂ is
used as the sensor for a commercially available oxygen
type
[L2Pt{S2C2(HetH)(R)}]ꢂ, and [L2Pt{S2C2(CH2CH2-N-
2-pyridnium)}]ꢂ where L is a mono- or L2 a bidentate
detector [5,6,11Á14].
/
phosphine and Het(H)ꢀ
idyl(ium), pyrazyl(ium) and (Rꢀ
CH2CH2OH) [1Á10]. Several of these complexes are
unique inorganic lumiphores since they are dual-emit-
ters at room temperature. The room temperature
/
quinoxalyl(ium), 2- or 4-pyr-
In previous studies of the L2Pt{S2C2(Het)(R)} com-
plexes, it was demonstrated that the 1,2-enedithiolate
appended R-group, ancillary phosphine ligand(s) and
the protonation state of the heterocycle all had a
profound effect upon the solution emission from these
/
H, CH3, and
/
complexes. [1Á/4] This study at low temperature (77 K)
in DMM glass (DMFÁ
extends our fundamental knowledge of this new class of
emitters and compliments the previous room tempera-
/
CH2Cl2ÁMeOH, 1:1:1, v/v/v)
/
* Corresponding author. Tel./fax: ꢂ1-301-654 3768.
E-mail address: rp78@bellatlantic.net (R.S. Pilato).
1
Present address: Sensors for Medicine and Science Inc., 12321
/
Middlebrook Road, Ste. 210 Germantown, MD 20874, USA.
ture and transient absorption studies [1Á10].
/
0020-1693/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0020-1693(03)00211-1

232
K.A. Van Houten et al. / Inorganica Chimica Acta 353 (2003) 231Á237
/
2. Experimental
2.3. Physical measurements
NMR spectra were acquired with a Bruker AF 200,
¨
AM 400, DRX 400 or a DRX 500. IR spectra were
2.1. Materials
collected either with a PerkinÁ
DXL FT-IR Spectrometer. UVÁ
corded on either a PerkinÁElmer Lambda 2S or a
/
Elmer 1600 or a Nicolet 5
/
Vis spectra were re-
The
(dppe)Pt{S2C2(2-pyridine)(CH2CH2OH)},
complexes
(dppe)Pt{S2C2(2-pyridine)(H)},
[(dppe)-
/
Hewlet Packard 8452A spectrometer. EI and FAB mass
spectral data were collected on a Magnetic Sector VG
7070E. Room temperature excitation and emission
spectra were acquired with a SLM AB2 fluorescence
spectrometer.
Pt{S2C2 (CH2CH2-N-2-pyridinium)}][BPh2] were pre-
pared as previously described [1,5,10]. (dppm)Pt(Cl)2
[15Á
(where dpppꢀ
diphenyldiphosphinomethane, and PPh3ꢀ
/
17], (dppp)Pt(Cl)2 [15Á
diphenyldiphosphinopropane, dppmꢀ
triphenyl
/17], and (PPh3)2Pt(SH)2 [18]
/
/
/
phosphine) and 1-pyridin-2-yl-2-bromoethanone [19]
were prepared according to the literature procedures.
All reactions were performed under an atmosphere of
nitrogen using standard Schlenk line techniques. Work-
ups were performed in air unless stated otherwise.
Dichloromethane, acetonitrile, and pentane were dried
over calcium hydride and distilled under nitrogen.
Diethyl ether, tetrahydrofuran, and dioxane were dried
over Na/benzophenone and distilled under nitrogen.
Triethylamine was dried over potassium hydroxide and
vacuum distilled. DMSO and DMF were dried over
calcium hydride and vacuum distilled. Neutral activated
3. Synthesis
3.1. (dppm)Pt(SH)2
A 0.27 M sodium hydrogensulfide solution was
prepared by adding Na (0.09 g, 3.85 mmol) to 15 ml
of absolute EtOH. After the Na dissolved, the solution
was exposed to H2S (1 atm) for 1 h. To this solution was
added (dppm)PtCl2 (1.00 g, 1.54 mmol). The mixture
was stirred under an H2S atmosphere for 24 h. The
resulting solid was collected by filtration and washed
alumina, 80Á325 mesh, was purchased from EM
/
Science, Cherry Hill, NJ and treated with 6% H2O by
weight to generate the Brockmann activity 3 material
used throughout these studies.
with water (2ꢃ
(2ꢃ20 ml). (dppm)Pt(SH)2 was isolated in 70% yield
(0.69 g, 1.08 mmol). H NMR (DMSO-d6): d 7.84 (m,
8H, C6H5P), 7.51 (m, 12H, C6H5 P), 4.90 (t, JPÁP 10
Hz, 2H, PCH2P), ꢁ0.49 (d, second order spectrum with
line spacing of 4 Hz and Pt satellites, JPtÁH 28 Hz, 2H,
SH). 31P NMR (DMSO-d6): d ꢁ
50.9 (s with Pt
satellites, JPtÁP
2430 Hz). IR (thin film, cmꢁ1): 3849
(w), 3049 (w), 2966 (w), 1435 (s), 1261 (m), 1100 (s).
/
20 ml), ethanol (2ꢃ
/
20 ml) and ether
/
1
ꢀ
/
2.2. Physical measurements
/
ꢀ
/
UVÁ
/
Vis spectra were recorded on a PerkinÁElmer
/
/
Lamda 2S spectrometer. Low temperature (77 K)
emission spectra were acquired with a SLM AB2
fluorescence spectrometer. Emission spectra were cor-
rected for instrumental response using factors supplied
by the manufacturer. Low temperature emission spectra
ꢀ
/
High resolution mass spectrum (FAB) calc. m/zꢀ
/
645.04425 for C25H24P2S1295Pt; found 645.04332.
were made on 10ꢁ4Á
/
10ꢁ5 M solutions of all complexes
in DMM glass (DMFÁ
/
CH2Cl2ÁMeOH, 1:1:1, v/v/v) in
/
a 5 mm diameter glass tubes placed in a liquid nitrogen
Dewar with quartz windows.
3.2. (dppp)Pt(SH)2
Low temperature emission lifetimes (t) were deter-
mined using a SLM AB2 fluorescence spectrometer with
a 25 ms gate width and a 10 ms minimum flash period.
The pulse delay was varied in 25 ms intervals from 0 to
350 ms using software provided by the manufacturer.
Prepared and isolated as described for complex
(dppm)Pt(SH)2 using Na (85 mg, 3.68 mmol) and
dpppPtCl2 (1.00 g, 1.46 mmol). (dppp)Pt(SH)2 was
isolated in 65% yield (0.59 g, 0.88 mmol). 1H NMR
(DMSO-d6): d 7.66 (m, 8H, C6H5P), 7.44 (m, 12H,
C6H5 P), 3.32 (m, 4H, C3H6P), 2.71 (m, 2H, C3H6P),
The system was verified using Ru(bpy)3Cl2 in ethanolÁ
/
methanol glass (4:1, v/v) as a standard [20]. Lifetimes
were calculated from the slope of Imax versus pulse delay
ꢁ
/
1.0 (d, second order spectrum with line spacing of 7
Hz and Pt satellites, JPtÁH
26 Hz, 2H, SH). 31P NMR
2754
ꢀ
/
and were fit with a least-squares analysis using tꢀ
/
(DMSO-d6): d ꢁ
/
0.97 (s with Pt satellites, JPtÁP
ꢀ
/
(pulse delay)/[Imax Imax(tꢀ0)]. Lifetimes (t) are reported
ꢁ
/
Hz). IR (thin film, cmꢁ1): 3849 (w), 3049 (w), 2917 (w),
1434 (s), 1262 (m), 1100 (s). High resolution mass
at the 95% confidence limit which was propagated from
the standard error of the slope and the estimated
uncertainties in t.
spectrum
C27H28P2S1295Pt; found 673.07656.
(FAB)
calc.
m/zꢀ/673.07556
for

K.A. Van Houten et al. / Inorganica Chimica Acta 353 (2003) 231Á
/
237
233
3.3. (dppm)Pt{C2S2(2-pyridine)(H)}
none (95 mg, 0.478 mmol). (Ph3P)2Pt{C2S2(2-pyridi-
ne)(H)} was isolated as a yellow crystalline solid in in
1
60% yield (169 mg, 0.19 mmol). H NMR (CDCl3): d
To a DMF (5 ml) solution of (dppm)Pt(SH)2 (250 mg,
0.156 mmol) was added 1-pyridin-2-yl-2-bromoetha-
none (46 mg, 0.233 mmol). The orange solution was
stirred for 2 h becoming purple. The DMF was removed
8.34 (m, 1H, C5H4N), 7.65 (m, 1H, C5H4N), 7.53Á
/7.42
(m, 15H, PC6H5 and 1H, C5H4N), 7.28Á7.26 (m, 8H,
/
PC6H5 and 1H, S2C2H), 7.17Á7.12 (m, 15H, PC6H5 and
/
in vacuo and the solid was chromatographed on a 1ꢃ
/
20
1H, C5H4N). 31P NMR (CDCl3): d 20.2 (q, second
order spectrum with line spacings of 27 and 65 Hz and
cm alumina column where the product eluted with
CH2Cl2. The elutent was evaporated to dryness to give
(dppm)Pt{C2S2(2-pyridine)(H)} as a yellow crystalline
solid in 52% yield (60 mg, 0.080 mmol). 1H NMR
Pt satellites, JPtÁP
ꢀ
2868 Hz). IR (thin film, cmꢁ1):
3060 (w), 2955 (w), 1672 (m), 1578 (m), 1525 (m), 1455
(w), 1435 (s), 1261 (s), 1094 (s), 1020 (m). High
/
(DMF-d7): d 8.53 (d, 1H, C5H4N, JHÁH
with 195Pt satellites, 1H, JPÁH, 6 Hz, S2C2H), JPtÁH
Hz), 7.91Á7.81 (m, 8H, PC6H5 and 1H, C5H4N), 7.70
(m, 1H, C5H4N), 7.55 (d, 1H, C5H4N, JHÁH 6 Hz),
7.38Á7.36 (m, 12H, PC6H5), 7.08 (m, 1H, C5H4N), 6.97
(d, 1H, C5H4N, JHÁH 6 Hz), 4.70 (t, 2H, PCH2,P
JPÁH 45.7 (q,
11 Hz). 31P NMR (DMF-d7): d ꢁ
second order spectrum with line spacings of 74 and 67
ꢀ
/
6 Hz), 8.21 (d
resolution mass spectrum (FAB) calc. m/zꢀ
for C43H36NP2S1295Pt; found 887.14452. UVÁ
(nm), o, CH2Cl2): (Ph3P)2Pt{C2S2(2-pyridine)(H)}, 359
/
887.14124
ꢀ
/
98
/
Vis (l
/
ꢀ
/
(6100),
415
(1400);
[(Ph3P)2Pt{C2S2(2-
/
pyridinium)(H)}]ꢂ, 335 (6800), 472 (5400).
ꢀ
/
ꢀ
/
/
3.6. ((Propyl)3P)2Pt{C2S2(2-pyridine)(H)}
Hz and Pt satellites, JPtÁP
ꢀ
/
2314 Hz). IR (thin film,
To a DMF (1 ml) solution of (Ph3P)2Pt{C2S2(2-
pyridine)(H)} (20 mg, 0.02 mmol) was added tri-n-
propylphosphine (23 ml, 0.11 mmol). The solution was
heated at 80 8C for 1 h. The DMF was removed in vacuo
cmꢁ1): 3049 (w), 2959 (w), 1579 (m), 1461 (s), 1436 (s),
1276 (m), 1101 (s). High resolution mass spectrum
(FAB) calc. m/zꢀ
/
746.07080 for C32H27NP2S1295Pt;
Vis (l (nm), o, CH2Cl2):
found 746.07334. UVÁ
/
and the solid was chromatographed on a 1ꢃ5 cm
/
(dppm)Pt{C2S2(2-pyridine)(H)}, 355 (5000), 415 (900);
[(dppm)Pt{C2S2(2-pyridinium)(H)]ꢂ 334 (6500), 449
(5100).
alumina column where the product eluted with CH2Cl2.
The elutent was evaporated to dryness to give ((pro-
pyl)3P)2Pt{C2S2(2-pyridine)(H)} as a crystalline solid in
52% yield (7 mg, 0.01 mmol). 1H NMR (CDCl3): d 8.53
3.4. (dppp)Pt{C2S2(2-pyridine)(H)}
(m, 1H, C5H4N), 7.76Á
S2C2H), 7.35 (m, 1H, C5H4N), 7.04 (m, 1H, C5H4N),
2.11Á1.94 (m, 12H, CH2P), 1.63Á1.53 (12H, CH2),
1.08Á
1.01 (m, 18H, CH3). 31P NMR (CDCl3): d
2.85 (q, second order spectrum with line spacings of
/
7.45 (m, 2H, C5H4N and
Prepared
and
isolated
as
described
for
/
/
(dppm)Pt{C2S2(2-pyridine)(H)} using (dppp)Pt(SH)2
(100 mg, 0.149 mmol) and 1-pyridin-2-yl-2-bromoetha-
none (44 mg, 0.223 mmol). (dppp)Pt{C2S2(2-pyridi-
ne)(H)} was isolated as a yellow crystalline solid in in
/
ꢁ
/
33 and 27 Hz and Pt satellites, JPtÁP
ꢀ2714 Hz). IR
/
(thin film, cmꢁ1): 2960 (w), 2929 (w), 2870 (w), 1698
(m), 1651 (m), 1575 (m), 1520 (m), 1455 (w), 1458 (s),
1076 (s). High resolution mass spectrum (FAB) calc. m/
1
81% yield (93 mg, 0.12 mmol). H NMR (CDCl3): d
8.34 (m, 1H, C5H4N), 8.17 (d with 195Pt satellites,
S2C2H, JPÁH
ꢀ
/
5, JPtÁH
ꢀ
/
88 Hz), 7.61Á
/7.51 (m, 8H,
zꢀ
/
683.23511 for C25H48NP2S1295Pt; found 683.23566.
PC6H5), 7.72 (m, 1H, C5H4N), 7.43Á
/
7.26 (m, 12H,
UVÁ
/
Vis (l (nm), o, CH2Cl2): ((Propyl)3P)2Pt{C2S2(2-
PC6H5 and 1H, C5H4N), 6.84 (m, 1H, C5H4N), 2.69 (m,
4H, PC2H2), 2.57 (m, 2H, PC2H2). 31P NMR (DMF-d7):
pyridine)(H)}, 356 (4000), 412 (5600); [((Pro-
pyl)3P)2Pt{C2S2(2-pyridinium)(H)}]ꢂ, 339 (1200), 464
(4500).
d ꢁ3.85 (q, second order spectrum with line spacings of
/
7 and 37 Hz and Pt satellites, JPtÁP
film, cmꢁ1): 3049 (w), 2943 (m), 1644 (s), 1579 (m), 1520
(w), 1460 (w), 1435 (s), 1264 (m), 1102 (s). High
ꢀ2642 Hz). IR (thin
/
3.7. (Ph2MeP)2Pt{C2S2(2-pyridine)(H)}
resolution mass spectrum (FAB) calc. m/zꢀ
/
775.10992
Vis (l
Prepared
(Pr3P)2Pt{C2S2(2-pyridine)(H)}
{C2S2(2-pyridine)(H), (20 mg, 0.02 mmol) and diphe-
nylmethylphosphine (21 0.11 mmol).
and
isolated
as
using
described
for
for C34H32NP2S1295Pt; found 775.10790. UVÁ
/
(Ph3P)2Pt-
(nm), o, CH2Cl2): (dppp)Pt{C2S2(2-pyridine)(H)}, 355
(6400), 410 (800); [(dppp)Pt{C2S2(2-pyridinium)(H)}]ꢂ,
334 (7000), 464 (5300).
ml,
(Ph2MeP)2Pt{C2S2(2-pyridine)(H)} was isolated as a
1
crystalline solid in 47% yield (7 mg, 0.009 mmol). H
NMR (CDCl3): d 8.61 (m, 1H, C5H4N), 7.96 (m, 1H,
3.5. (Ph3P)2Pt{C2S2(2-pyridine)(H)}
C5H4N), 7.76Á
7.57Á7.28 (m, 12H, PC6H5 and 1H, C5H4N), 7.09 (m,
1H, C5H4N), 1.85Á
1.69 (6H, CH3P). 31P NMR
(CDCl3): d ꢁ1.21 (q, second order spectrum with line
/
7.60 (m, 8H, PC6H5 and 1H, S2C2H),
Prepared
and
isolated
as
described
for
/
(dppm)Pt{C2S2(2-pyridine)(H)} using (Ph3P)2Pt(SH)2
(250 mg, 0.318 mmol) and 1-pyridin-2-yl-2-bromoetha-
/
/

234
K.A. Van Houten et al. / Inorganica Chimica Acta 353 (2003) 231Á237
/
spacings of 30 and 24 Hz and Pt satellites, JPtÁP
ꢀ
/
2813
PPh2Me, PPhMe2, P(propyl)3 were prepared by phos-
phine substitution of (PPh3)2Pt{S2C2(2-pyridine)(H)}
(Scheme 2).
Hz). IR (thin film, cmꢁ1): 3051 (w), 2962 (w), 1596 (m),
1508 (m), 1437 (m), 1380 (m), 1262 (m), 1028 (m), 825
(m). 747 (m). High resolution mass spectrum (FAB)
calc. m/zꢀ
/
763.10992 for C33H32NP2S1295Pt; found
4.2. UVÁVisible spectra
/
763.10729.
UVÁVis (l (nm), o, CH2Cl2):
/
(Ph2MeP)2Pt{C2S2(2-pyridine)(H)}, 355 (4700), 415
(1100); [(Ph2MeP)2Pt{C2S2(2-pyridinium)(H)}]ꢂ, 334
(4400), 462 (4000).
All of the complexes have a UV or visible band
assignable to a 1,2-enedithiolate p to pyridyl(ium) p*
intraligand charge-transfer transition (ILCT). The ILCT
band in the neutral complexes is found between 28,100
and 26,900 cmꢁ1, while in the cationic complexes the
band is found between 23,200 and 21,500 cmꢁ1, (Table
1, DMM Glass). The ILCT assignment was established
in previous studies and was based upon the similarity in
the energy of the bands in the corresponding Ni, Pd, and
Pt complexes, the solvent dependence of the energy of
these bands, and the red shift of these bands upon
protonation or alkylation of the appended heterocycle
3.8. (Me2PhP)2Pt{C2S2(2-pyridine)(H)}
Prepared
(Pr3P)2Pt{C2S2(2-pyridine)(H)}
and
isolated
as
using
described
for
(Ph3P)2Pt-
{C2S2(2-pyridine)(H) (20 mg, 0.02 mmol) and dimethyl-
phenylphosphine (16 mg, 0.011 mmol). (Me2PhP)2-
Pt{C2S2(2-pyridine)(H)} was isolated as a crystalline
solid in 56% yield (8 mg, 0.01 mmol). 1H NMR (CDCl3):
d 8.61 (m, 1H, C5H4N), 8.05 (m, 1H, C5H4N), 7.76Á
7.28 (m, 10H, PC6H5, 1H, C5H4N, and 1H S2C2H), 7.07
(m, 1H, C5H4N), 1.90Á
1.61 (12H, CH3P). 31P NMR
(CDCl3): d ꢁ17.4 (q, second order spectrum with line
spacings of 31 and 29 Hz and Pt satellites, JPtÁP 2762
/
[1,3,5,21Á
In the platinum complexes, a low energy shoulder at
410 nm of the ILCT band was assigned to a d to d (d
/
24].
/
ꢀ
/
/
to s*) transition. Given the low extinction coefficient of
this band and its proximity to the ILCT band, the
energy of the d to d transition is difficult to accurately
determine. In the corresponding Ni and Pd analogs of
(dppe)Pt{S2C2(Het)(H)}, where the d to d (d to s*) is
substantially red shifted from the transition in the Pt
complexes and easily assigned, it was shown that the d
to d transition was essentially unaffected by varying or
ꢀ
/
Hz). IR (thin film, cmꢁ1): 3049 (w), 2966 (w), 1578 (m),
1522 (s), 1437 (s), 1381 (s), 1262 (m), 1105 (m), 1028 (m),
911 (m). 743 (m). High resolution mass spectrum (FAB)
calc. m/zꢀ
639.08279. UVÁ
/
639.07861 for C23H28NP2S1295Pt; found
Vis (l (nm), o, CH2Cl2): (Me2-
/
PhP)2Pt{C2S2(2-pyridine)(H)}, 351 (1700), 409 (506);
[(Me2PhP)2Pt{C2S2(2-pyridinium)(H)}]ꢂ, 355 (1600),
457 (1600).
protonating the appended heterocycle [1Á
/5].
4.3. Low-temperature emission spectra
4. Results and discussion
At room temperature, the luminescent pyridinium
complexes have two emissive states, assigned to the
ILCT* singlet and triplet (Fig. 1). The solution singlet
quantum yields are 10ꢁ5, while the triplet quantum
yields range from 10ꢁ4 to 10ꢁ3. None of the pyridyl-
substitued complexes are emissive. However at 77 K,
both the pyridyl and pyridinium complexes are emissive
but only the triplet emission is easily observed. The
pyridyl substituted complexes have a strong emission
between 18,600 and 17,900 cmꢁ1 while the pyridinium-
substituted complexes have emissions between 16,600
4.1. Complex studies
The 2-pyridyl substituted platinum-1,2-enedithiolates
used in this study are shown in Scheme 1. The complexes
L2Pt{S2C2(2-pyridine)(H)}, L2ꢀ
(dppe)Pt{S2C2(2-pyridine)(CH2CH2OH)},
Pt{S2C2 (CH2CH2-N-2-pyridinium)}][BPh2] were pre-
/
dppm, dppe, dppp,
[(dppe)-
pared according to the literature procedure [1,5,10]. The
complexes
L2Pt{S2C2(2-pyridine)(H)},
LꢀPPh3,
/
Scheme 1.

K.A. Van Houten et al. / Inorganica Chimica Acta 353 (2003) 231Á
/
237
235
Scheme 2.
which in room temperature solutions is generally found
in the 16,400Á
14,800 cmꢁ1
/
.
The triplet emissions for all of the complexes have a
vibrational progression of 1200 cmꢁ1 (DMM 1:1:1 v/v/
v). This vibronic progression has been assigned to the
CÄ
and asymmetric CÄ
L2Pt{S2C2(2-pyridyl(ium))(R)} are found at ꢀ
and ꢀ
1430 cmꢁ1 in the IR spectrum, respectively.
/
C stretch of the excited state given that the symmetric
C stretches of the ground state
1460
/
/
/
This assignment is in agreement with that for the
Fig.
1. Emission
spectra
of
[(dppe)Pt{S2C2(CH2CH2-N-2-
pyridnium)}]ꢂ in DMSO at 298 K prior to instrument correction:
solid line air free; dashed line, in air.
structurally related Pt(diimine)(1,2-enedithiolate) and
[Pt(L)(mnt)]ꢁ1 Lꢀ
phite, COD [25Á
of Pt(diimine)(1,2-enedithiolate) and [Pt(L)(mnt)]ꢁ1
Lꢀdiimine, tertiary phosphine or phosphite, COD
complexes are found at 1450Á
1500 cmꢁ1 while the
low temperature emission spectra had vibronic structure
of 1200Á 27].
1300 cmꢁ1 [25Á
/diimine, tertiary phosphine or phos-
/
27]. In the IR the localized CÄC stretch
/
and 16,100 cmꢁ1 (Table 1). There is pronounced 1200
cmꢁ1 vibronic structure in the pyridyl-substituted com-
plexes that is somewhat less resolved in the pyridinium-
substituted complexes (Figs. 2 and 3). The 3ILCT*
/
/
/
/
lifetime of the pyridinium complexes are 136Á
while the pyridyl complexes have lifetimes of 216Á
(Fig. 4). These lifetimes are 20Á30 times the values of the
room temperature lifetimes [3,4,7,8]. As is generally the
case, thermal modes of deactivation are eliminated at 77
K and the 3ILCT* lifetimes and quantum yields
/
290 ms
Contrary to the fluid emissions, low temperature
emissions from glasses are not particularly sensitive to
the steric bulk of either the 1,2-enedithiolate appended
R-group, the ancillary ligands (L2) or the protonation
state of the heterocycle. None of the pyridyl complexes
are emissive at room temperature. In addition,
/
350 ms
/
3
increase. The large increase in ILCT* quantum yield
leads to an inability to easily observe the singlet emission
[L2Pt{S2C2(2-pyridinium)(H)}]ꢂ, L2ꢀ
PPh3, PPh2Me,
/
Table 1
2-Pyridyl substituted platinum 1,2-enedithiolate complexes emission maxima and lifetimes in DMM a glass at 77 K
Compound
Neutral
Protonated
Exmax (nm) Emmax (nm) t77 K (ms) b Exmax (nm) Emmax (nm) t77 K (ms) b t298 K (ms) c
L2Pt{S2C2(2-pyridyl)(H)}
L2ꢀ
dppe
dppp
/
dppm
362
360
358
371
365
365
360
355
538, 579, 640
433
430
433
435
435
436
435
440
464
608, 655
612, 652
612, 656
616, 660
617, 660
608, 657
619, 661
0.71
2.01
550, 586, 640 3119
550, 588, 640
/
15
1589
/
9
d
d
d
d
d
d
(PPh3)
(PPh2Me)
558, 588, 640 2169
547, 585, 640
/9
1369
/8
(PPhMe2)
(propyl3P)
546, 586, 640 2499
548, 591, 640 2489
555 (broad) 3509
/9
1449
/5
/6
13897
/
dppePt{S2C2(2-pyridyl)-(CH2CH2OH)}
[dppePt{S2C2(CH2CH2-N-2-pyridinium)}][BPh4]
/12
601 (broad) 1669
618, 660 2909
/9
/9
8.66
a
DMM glass is 1:1:1 methylene chloride; methanol; DMF.
The uncertainties are reported at the 95% confidence limits.
DMSO, from Ref. [10].
b
c
d
Not emissive at room temperature.

236
K.A. Van Houten et al. / Inorganica Chimica Acta 353 (2003) 231Á237
/
Fig. 4. A plot of the log of the emission intensity versus pulse delay
(ms). Lifetimes were calculated from the inverse of the slope. (a)
(propyl3P)2Pt{S2C2(2-pyridine)(H)}, with a lifetime of 248 ms. (b)
[(propyl3P)2Pt{S2C2(2-pyridinium)(H)}]ꢂ, with a lifetime of 138 ms.
Fig. 2. Emission decay profile of (propyl3P)2Pt{S2C2(2-pyridi-
ne)(H)}at 77 K in DMM, 360 nm excitation. The pulse delay was
varied from 50 to 275 ms in 25 ms intervals.
5. Conclusion
In this study we have investigated the low temperature
emissive properties of 2-pyridyl substituted platinum
1,2-enedithiolates, L2Pt{S2C2(2-pyridyl(ium))(R)} and
[L2Pt{S2C2(CH2CH2-N-2-pyridnium)}]ꢂ in DMM
glass. All of the complexes studied, both neutral and
cationic, were emissive at 77 K independent of the R-
group bulk or appended phosphine. This is in stark
contrast to the room temperature emissions where
selected pyridinium complexes were emissive. The life-
time at 77 K of the pyridinium complexes is approxi-
mately half that of the corresponding pyridyl complexes.
A finding that was counter-intuitive from solution
studies but expected from the Energy Gap Law given
that pyridinium emissions are red shifted from the
pyridyl complexes [28]. The room temperature solution
emissions of the cationic complexes, [L2Pt{S2C2(2-
pyridinium)(R)}]ꢂ, and [L2Pt{S2C2(CH2CH2-N-2-
pyridnium)}]ꢂ are controlled by the steric bulk of the
enedithiolate R-groups and the ancillary phosphine
ligands. In the ILCT* excited state, the charge transfer
formally oxidizes a 1,2-enedithiolate sulfur and reduces
the pyridinium nitrogen. Stabilization of this excited
state occurs with the generation of an exocylcic double
bond between the dithiolate and the heterocycle
(Scheme 3). Such a configuration requires a coplanar
1,2-enedithiolate and pyridine. Such resonance forms
have also been evoked to justify the 3 pKa unit increase
in the ground state basicity of the 2- and 4-pyridyl
substituted 1,2-enedithiolate complexes relative to pyr-
idine and the large increase in the excited state basicity
of (dppe)Pt{S2C2(2-quinoxaline)(H)} relative to the
ground state. This likely accounts for the sensitivity of
Fig. 3. Emission decay profile of [(propyl3P)2Pt{S2C2(2-
pyridinium)(H)}]ꢂ at 77 K in DMM, 435 nm excitation. The pulse
delay was varied from 50 to 200 ms in 25 ms intervals.
PPhMe2,
(Ppropyl3)2,
and
[(dppe)Pt{S2C2(2-
pyridinium)(CH2CH2OH)}]ꢂ are not emissive at room
temperature [4,14]. The presence of non-emissive excited
states at room temperature for these complexes was
precluded by transient absorption spectroscopy [10].
The lack of emission from rigid polymer films (such as
cellulose acetate) would suggest that emissions at 77 K
can be attributed to the population of lower lying
vibronic and rotational states within the excited state
manifold. This would suggest that thermal repopulation
of a non-emitting state occurs at room temperature.
Given that subtle changes in the ancillary ligands lead to
an annihilation of the room temperature emission, it is
likely that metal-based atomic orbitals contribute
strongly to this deactivation as is the case in many
metal containing emitters [7,8,25Á27].
/

K.A. Van Houten et al. / Inorganica Chimica Acta 353 (2003) 231Á
/
237
237
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Given that the neutral complexes are emissive at 77 K
but not in room temperature solutions it would appear
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/
10]. This is clearly a temperature effect and not a solvent
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neutral complexes in rigid polymeric media [5,7,8]. At 77
K such thermal repopulation is far less likely and hence
a strong emission is observed. In contrast, emission from
the protonated complexes is sufficiently lower in energy
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likely even in room temperature solution. Even though
an emission is observed for the pyridinium complex at
room temperature, substantial depopulation of the
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/
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6. Supplementary material
The material is available from the authors on request.
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