A superior polypyridine ligand for platinum(II) lumaphors. Methyl substituents with a large stereoelectronic influence

Article abstract of DOI:10.1021/ic025922c

This research deals with the synthesis and characterization of a new series of platinum(II) polypyridine complexes that incorporate a relatively rigid and hydrophobic ligand. The parent complex Pt(php)Cl⁺, where php denotes 2-(2′-pyridyl)-1,10-phenanthroline, resembles Pt(trpy)Cl⁺, where trpy denotes 2,2′:6′,2″-terpyridine, but is photoluminescent in solution. Hence php derivatives should prove to be superior tags and/or spectroscopic probes for biological systems. A theoretical analysis reveals some of the advantages of php over trpy as a platform. Due to a ligand π system with a relatively small HOMO-LUMO gap, the emission from Pt(php)Cl⁺ exhibits significant vibrational structure and a mixed ³π-π^*/³d-π* orbital parentage. In deoxygenated dichloromethane solution the php complex exhibits an emission quantum yield of 3.1 × 10^-3 and an excited-state lifetime of 0.23 μs at room temperature. However, methyl groups have an unusually strong stereoelectronic influence, particularly at the 5,6-positions of the phenanthroline moiety. The platinum(II) complex with 2-(2′-pyridyl)-3,5,6,8-tetramethyl-1,10-phenanthroline is the best emitter with an emission yield of 0.055 and a lifetime of 9.3 μs in dichloromethane. Strongly donating solvents like dimethylformamide are potent quenchers of the emission. The methods of characterization used include absorption and emission spectroscopies, electrochemistry, and, in the case of [Pt- {2-(2′-pyridyl)-4,7-dimethyl-1,10-phenanthroline}Cl] O₃SCF₃, X-ray crystallography. Another intriguing finding is that methyl substituents have preferred orientations with respect to the phenanthroline ligand.

Full text of DOI:10.1021/ic025922c

Inorg. Chem. 2002, 41, 6387−6396
A Superior Polypyridine Ligand for Platinum(II) Lumaphors. Methyl
Substituents with a Large Stereoelectronic Influence
Jeffrey J. Moore, John J. Nash, Phillip E. Fanwick, and David R. McMillin*
Department of Chemistry, Purdue UniVersity, 560 OVal DriVe,
West Lafayette, Indiana 47907-2038
Received August 2, 2002
This research deals with the synthesis and characterization of a new series of platinum(II) polypyridine complexes
that incorporate a relatively rigid and hydrophobic ligand. The parent complex Pt(php)Cl+, where php denotes
2-(2′-pyridyl)-1,10-phenanthroline, resembles Pt(trpy)Cl+, where trpy denotes 2,2′:6′,2′′-terpyridine, but is photolu-
minescent in solution. Hence php derivatives should prove to be superior tags and/or spectroscopic probes for
biological systems. A theoretical analysis reveals some of the advantages of php over trpy as a platform. Due to
a ligand π system with a relatively small HOMO−LUMO gap, the emission from Pt(php)Cl+ exhibits significant
vibrational structure and a mixed 3π−π*/3d−π* orbital parentage. In deoxygenated dichloromethane solution the
php complex exhibits an emission quantum yield of 3.1 × 10-3 and an excited-state lifetime of 0.23 µs at room
temperature. However, methyl groups have an unusually strong stereoelectronic influence, particularly at the 5,6-
positions of the phenanthroline moiety. The platinum(II) complex with 2-(2′-pyridyl)-3,5,6,8-tetramethyl-1,10-
phenanthroline is the best emitter with an emission yield of 0.055 and a lifetime of 9.3 µs in dichloromethane.
Strongly donating solvents like dimethylformamide are potent quenchers of the emission. The methods of
characterization used include absorption and emission spectroscopies, electrochemistry, and, in the case of [Pt-
{2-(2′-pyridyl)-4,7-dimethyl-1,10-phenanthroline}Cl]O SCF , X-ray crystallography. Another intriguing finding is that
3
3
methyl substituents have preferred orientations with respect to the phenanthroline ligand.
Chart 1
Introduction
The goal of this research has been to develop new
luminescent platinum(II) polypyridine complexes incorporat-
ing a rigid ligand framework, substantial hydrophobic
character, and compatibility with a variety of solvents
including water. Interest in the chemistry and spectroscopy
of the archetypical Pt(trpy)Cl+ system, where trpy denotes
2,2′:6′,2′′-terpyridine (Chart 1), continues for a number of
reasons.1 One is that the complex binds covalently to
biological macromolecules by means of substitution of the
relatively labile chloride ligand.2,3 Intercalation into double-
helical DNA is also feasible because of the planar coordina-
tion geometry and the presence of the aromatic polypyridine
ligand.4,5 Metal-to-ligand charge-transfer (MLCT) absorption
bands that occur in the vicinity of 400 nm provide a useful
spectral handle. Unfortunately, even though Pt(trpy)Cl+ is
luminescent in the solid state and in low-temperature
glasses,6,8,9 the complex gives essentially no photolumines-
cence in fluid solution because nonradiative decay via
thermally accessible d-d excited states is quite efficient.7
Luminescent derivatives of Pt(trpy)Cl+ have, however, been
identified. For example, Pt(trpy)(OH)+ is luminescent, but
only in weakly basic organic solvents.7 Emission measure-
ments are useful for DNA-binding studies because the
* To whom all correspondence should be addressed. E-mail: mcmillin@
purdue.edu.
(1) McMillin, D. R.; Moore, J. J. Coord. Chem. ReV. 2002, 229, 113-
121.
(2) Jennette, K. W.; Lippard, S. J.; Vassiliades, G. A.; Bauer, W. R. Proc.
Natl. Acad. Sci. U.S.A. 1974, 71, 3839-3843.
(3) Ratilla, E. M. A.; Brothers, H. M.; Kostic, N. M. J. Am. Chem. Soc.
1987, 109, 4592-4599.
10.1021/ic025922c CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/31/2002
Inorganic Chemistry, Vol. 41, No. 24, 2002 6387

Moore et al.
for 1,10-phenanthroline, 4,7-dimethyl-1,10-phenanthroline, 5,6-
dimethyl-1,10-phenanthroline, 3,4,7,8-tetramethyl-1,10-phenanthro-
line, 2-bromopyridine, 1,5-cyclooctadiene, n-butyllithium (as a 2
M solution in cyclohexane), butyronitrile, and ferrocene. The Na-
[TPFB]‚nH2O10 (TFPB ) tetrakis[3,5-bis(trifluoromethyl)phenyl]-
borate), [Pt(4′-Ph-T)Cl]TFPB12 (4′-Ph-T ) 4′-phenyl-2,2′:6′,2′′-
terpyridine), and Pt(trpy)Cl+ 7 (trpy ) 2,2′:6′,2′′-terpyridine) were
available from previous studies. Exciton supplied the laser dyes.
Reagent-grade chemicals sufficed for synthetic purposes, but the
purification of tetrabutylammonium hexafluorophosphate (TBAH)
electrolyte required two recrystallizations from ethanol. For spectral
studies high-purity-grade acetonitrile (MeCN), dichloromethane
(DCM), and toluene (MePh) came from VWR under the label of
hydroxide complex does not emit in aqueous solution but
becomes luminescent upon intercalating into a DNA host.5
However, in the presence of guanine bases, electron-transfer
quenching can be very efficient.5 Incorporation of electron-
rich substituents, like dimethylamine or 1-pyrene, at the 4′-
position of the trpy ligand also yields derivatives that can
have microsecond-lived emissions in noncoordinating
media.10-12 Even a simple phenyl substituent has an impor-
tant effect. Thus, Pt(4′-Ph-T)Cl+, where 4′-Ph-T denotes
4′-phenyl-2,2′:6′,2′′-terpyridine, exhibits emission with a
lifetime of 85 ns in room-temperature dichloromethane
(DCM).11,12 The presence of the phenyl substituent provides
for an expanded π system, a lower energy MLCT state, and
less efficient deactivation via d-d excited states. However,
it is also possible that the intrinsic excited-state lifetime
increases because the ligand is less prone to distortion.13
The results described below show that 2-(2′-pyridyl)-
1,10-phenanthroline (php in Chart 1) is a superior ligand for
a platinum(II)-based lumaphore. Thus, Pt(php)Cl+ exhibits
an emission signal with a lifetime of 230 ns in DCM solution
at room temperature; more impressive, analogues bearing
only methyl substituents can have lifetimes of many micro-
seconds. The structure of a 4,7-dimethyl complex shows that
there is less dispersion in the Pt-N bonds of a php complex
than in the trpy analogue and that coordinated php is
essentially planar. The rigidity of the php ligand may restrict
excited-state distortions and thereby enhance the emission
relative to that of the trpy analogue. However, an even more
telling observation is that the emission from the php complex
originates in an excited state with not only 3d-π* (CT) but
significant 3π-π* (intraligand) character as well. Introducing
methyl substituents affects the 3π-π*/3d-π* mix much like
the solvent tuning that has been reported for Ir(III) systems.14
The solvent sensitivity of the emission from php-based
systems suggests that the platinum complexes will be useful
as spectroscopic reporter probes and in sensing studies.
1
Burdick and Jackson. The H NMR solvent CDCl3 was a product
of Cambridge Isotope Labs.
Syntheses. Literature methods yielded 3,5,6,8-tetramethyl-1,10-
phenanthroline ligand,15 Pt(COD)Cl216 (COD ) 1,5-cyclooctadiene),
and the various Zn(php)Cl2 derivatives.17,18 Preparations of [Pt-
(php)Cl]Cl and complexes with related ligands followed the method
of Annibale et al.,19 with Pt(COD)Cl2 as the starting material. It
was possible to exchange the counterion by simple metathesis
procedures carried out in DCM. A metathesis procedure also yielded
the tetramethylammonium salt of the TFPB anion. Fe(php)22+ and
iron(II) complexes of related ligands were made in situ by
combining Fe(NH4)2(SO4)2‚6H2O with 2 equiv of the appropriate
polypyridine ligand in deionized water.
The following procedure for the preparation of 2-(2′-pyridyl)-
1,10-phenanthroline (php) incorporates several literature methods.20-22
Analogous procedures yield the various methylated forms. Addition
of a stoichiometric amount of n-butyllithium to a solution of
2-bromopyridine in dry tetrahyrdofuran (THF) at -78 °C under
Ar gave a red solution. After stirring for 15 min, a transfer of 1.2
equiv of the resulting lithium reagent, via cannula, to a solution of
1,10-phenathroline in dry THF at -78 °C, also under Ar, produced
another deep red solution. After the mixture was stirred for 2 h,
water was added to quench any unreacted lithium reagent and
organics were extracted into DCM. All organics were combined
and rearomatized by combining with an excess of the oxidant MnO2.
After incubating for 2 h, the solution was filtered, dried with Na2-
SO4, and concentrated to a yellow oil. The crude product was
purified by column chromatography on alumina, with THF/hexanes
(5/1) as the eluent. A white solid was obtained from the column
after evaporation of solvents. Further purification was accomplished
via recrystallization from 1:1 (v/v) DCM/hexanes. Analytical data
for the various php ligands follow.
Experimental Section
Materials. The ZnCl2 and Fe(NH4)2(SO4)2‚6H2O compounds
came from Mallinckrodt, while K2PtCl4 came from Johnson and
Matthey Pharmaceuticals. Aldrich Chemical Co. was the vendor
2-(2′-Pyridyl)-1,10-phenanthroline (php). Anal. Calcd for
C17H11N3‚1/4(CH2Cl2): 74.26 %C, 4.04 %H, 15.02 %N. Found:
74.38 %C, 4.16 %H, 15.08 %N. 1H NMR in CDCl3 (in ppm): 9.25
(dd, 1H), 9.00 (d, 1H), 8.80 (d, 1H), 8.75 (m, 1H), 8.38 (d, 1H),
8.25 (dd, 1H), 7.92 (td, 1H), 7.82 (m, 2H), 7.65 (m, 1H), 7.38 (m,
1H).
(4) Sundquist, W. I.; Lippard, S. J. Coord. Chem. ReV. 1990, 100, 293-
322.
(5) Peyratout, C. S.; Aldridge, T. K.; Crites, D. K.; McMillin, D. R. Inorg.
Chem. 1995, 34, 4484-4489.
(6) Yip, H. K.; Cheng, L. K.; Cheung, K. K.; Che, C. M. J. Chem. Soc.,
Dalton Trans. 1993, 2933-2938.
(7) Aldridge, T. K.; Stacy, E. M.; McMillin, D. R. Inorg. Chem. 1994,
33, 722-727.
(8) Buchner, R.; Field, J. S.; Haines, R. J.; Cunningham, C. T.; McMillin,
D. R. Inorg. Chem. 1997, 36, 3952-3956.
(15) Case, F. H. J. Am. Chem. Soc. 1948, 70, 3994-3996.
(16) McDermott, J. X.; White, J. F.; Whitesides, G. M. J. Am. Chem. Soc.
1976, 98, 6521-6528.
(9) Bailey, J. A.; Hill, M. G.; Marsh, R. E.; Miskowski, V. M.; Schaefer,
W. P.; Gray, H. B. Inorg. Chem. 1995, 34, 4591-4599.
(10) Crites, D. K.; Cunningham, C. T.; McMillin, D. R. Inorg. Chim. Acta
1998, 273, 346-353.
(17) Hill, M. G.; Bailey, J. A.; Miskowski, V. M.; Gray, H. B. Inorg. Chem.
1996, 35, 4585-4590.
(11) Michalec, J. F.; Bejune, S. A.; McMillin, D. R. Inorg. Chem. 2000,
39, 2708-2709.
(18) Douglas, J. E.; Wilkins, C. Inorg. Chim. Acta 1969, 3, 635-638.
(19) Annibale, G.; Brandolisio, M.; Pitteri, B. Polyhedron 1995, 14, 451-
453.
(12) Michalec, J. F.; Bejune, S. A.; Cuttell, D. G.; Summerton, G. C.;
Gertenbach, J. A.; Field, J. S.; Haines, R. J.; McMillin, D. R. Inorg.
Chem. 2001, 40, 2193-2200.
(20) Barigelletti, F.; Ventura, B.; Collin, J. P.; Kayhanian, R.; Gavina, P.;
Sauvage, J. P. Eur. J. Inorg. Chem. 2000, 113-119.
(21) Malmberg, H.; Nilsson, M. Tetrahedron 1986, 42, 3981-3986.
(22) Collin, J. P.; Gavina, P.; Sauvage, J. P.; De Cian, A.; Fischer, J. Aust.
J. Chem. 1997, 50, 951-957.
(13) Strouse, G. F.; Schoonover, J. R.; Duesing, R.; Boyde, S.; Jones, W.
E.; Meyer, T. J. Inorg. Chem. 1995, 34, 473-487.
(14) Crosby, G. A. Acc. Chem. Res. 1975, 8, 231-238.
6388 Inorganic Chemistry, Vol. 41, No. 24, 2002

A Superior Polypyridine Ligand for Pt(II) Lumaphors
2-(2′-Pyridyl)-4,7-dimethyl-1,10-phenanthroline (4,7-Me2-
php). Anal. Calcd for C19H15N3: 79.98 %C, 5.30 %H, 14.73 %N.
Found: 79.74 %C, 5.18 %H, 14.61 %N. H NMR in CDCl3 (in
ppm): 9.12 (d, 1H), 9.05 (d, 1H), 8.75 (m, 1H), 8.68 (s, 1H), 8.09
(m, 2H), 7.92 (dt, 1H), 7.50 (d, 1H), 7.38 (m, 1H), 2.90 (s,3H),
2.80 (s, 3H).
was standard.24 The method of Parker and Rees25 allowed the
estimation of emission quantum yields at 25 °C with Ru(bpy)3
2+
1
in water as the standard (φ ) 0.042).26 For the lifetime studies,
there was a neutral density filter between the sample and the laser
and a 525 nm long-wave-pass filter between the sample and the
detector. The excitation wavelength was usually 420 nm. Residual
plots verified that a single-exponential model was adequate to fit
the emission decay data.
2-(2′-Pyridyl)-5,6-dimethyl-1,10-phenanthroline (5,6-Me2-
php). Anal. Calcd for C19H15N3‚1.25H2O: 74.12 %C, 5.72 %H,
1
13.64 %N. Found: 74.18 %C, 5.47 %H, 13.58 %N. H NMR in
In the electrochemical studies the working electrode was a gold
button, while the auxiliary electrode was a platinum wire. The
experimental setup involved an aqueous Ag/AgCl (3 M NaCl)
reference electrode, but it is more useful to quote potentials versus
ferrocene in the actual working medium. The potentials reported
are the mean of the cathodic (Ep,c) and anodic (Ep,a) peaks in the
CDCl3 (in ppm): 9.20 (dd, 1H), 9.04 (d, 1H), 8.78-8.74 (m, 2H),
8.58 (d, 1H), 8.48 (dd, 1H), 7.92 (td, 1H), 7.66 (m, 1H), 7.38 (m,
1H), 2.75 (s, 3H), 2.70, (s, 3H).
2-(2′-Pyridyl)-3,4,7,8-tetramethyl-1,10-phenanthroline (3,4,7,8-
Me4-php). Anal. Calcd for C21H19N3‚1/4(H2O): 79.33 %C, 6.18
%H, 13.22 %N. Found: 79.53 %C, 6.13 %H, 13.24 %N. 1H NMR
in CDCl3 (in ppm): 8.97 (s, 1H), 8.70 (m, 1H), 8.0-8.12 (m, 3H),
7.90 (td, 1H), 7.35 (m, 1H), 2.78 (s, 3H), 2.7 (s, 3H), 2.58 (s, 3H),
2.52 (s, 3H).
cyclic voltammogram. The scan rate for all scans was 50 mV s-1
,
and the electrolyte solution was 0.1 M TBAH in DMF. A simple
purge of dinitrogen gas sufficed to deaerate the sample.
Instrumentation. The absorption spectrometer was a Varian
Cary 100 Bio instrument, and the spectrofluorometer was an SLM-
Aminco SPF-500C. The cryostat was an Oxford Instruments model
DN1704 liquid-nitrogen-cooled system complete with an Oxford
Instruments model ITC4 temperature controller. A description of
the laser excitation system and photomultiplier detector appears in
the literature.27 The cyclic voltammetry unit was a model CV-27
from Bioanalytical Systems, Inc., connected to a Hewlett-Packard
7015B XY chart recorder. The 1H NMR spectrometer was a Varian
Gemini 200 MHz. The Nonius KappaCCD package served for the
preliminary examination of the crystals and data collection with
Mo KR radiation (λ ) 0.71073 Å). The computer for all structure
refinements was an AlphaServer 2100 using SHELXL97.28
2-(2′-Pyridyl)-3,5,6,8-tetramethyl-1,10-phenanthroline (3,5,6,8-
Me4-php). Anal. Calcd for C19H15N3‚1/4(H2O): 79.34 %C, 6.18
%H, 13.21 %N. Found: 79.06 %C, 6.18 %H, 12.98 %N. 1H NMR
in CDCl3 (in ppm): 8.95 (s, 1H), 8.72 (d, 1H), 8.32 (s, 1H), 8.22
(m, 2H) 7.90 (dt, 1H), 7.34 (m, 1H), 2.78 (s, 3H), 2.74 (s, 3H),
2.70 (s, 3H), 2.6 (s, 3H).
Preparation of [Pt(php)Cl]TFPB. To obtain the php complex,
one combines 100 mg of the php ligand (dissolved in a small
volume of acetone) with a suspension of 1 molar equiv of Pt(COD)-
Cl2 in aqueous 0.05 M 2-(N-morpholine)ethanesulfonic acid. The
solution clarifies and turns yellow upon heating. After filtration
and the addition of NaCl(aq), Pt(php)Cl+ precipitates as a red
chloride salt. A solution of the TFPB salt forms with the gentle
heating of the aforementioned solid in DCM containing a substo-
ichiometric amount (ca. 0.9 equiv) of Na[TFPB]‚nH2O. After
removal of the insoluble materials, evaporation of the solvent yields
the desired yellow solid. The final purification step involves
recrystallization from 1:1 DCM/hexane. Analogous procedures yield
the related compounds involving methylated forms of php. See
below for analytical data.
Crystal Structure Determination. Slow evaporation of a 10:1
benzene/MeCN (v/v) solution of [Pt(4,7-Me2-php)Cl]CF3SO3 pro-
duced a pale orange needle of dimensions 0.40 × 0.23 × 0.05 mm
that was suitable for diffraction studies. Least-squares refinement
of the setting angles of 21386 reflections in the range 2° < θ <
27° provided cell constants and an orientation matrix for the crystal
mounted on a glass fiber. The maximum value of 2θ was 55.9°.
There were 4767 unique reflections in the data set, and the average
transmission coefficient was 0.614. The input for the refinement
was the average of all equivalent reflections (agreement factor of
[Pt(php)Cl]TFPB. Anal. Calcd (C49H23N3ClF24BPt): 43.56 %C,
1.72 %H, 3.11 %N. Found: 43.73 %C, 1.99 %H, 2.88 %N.
[Pt(4,7-Me2-php)Cl]TFPB‚0.5(CH2Cl2). Anal. Calcd (C51.5H28N3-
Cl2F24BPt): 43.48 %C, 1.98 %H, 2.95 %N. Found: 43.54 %C,
2.02 %H, 2.76 %N.
10.6%). The refinement took account of each of the unique
2
reflections, but only data points that satisfied the criterion Fo
>
2
2σ(Fo ) were used in calculating the R value. Solution of the
structure was done using the program PATTY in DIRDIF92.29
Hydrogen atoms were included in the refinement with the constraint
that they ride on the bonded atoms. Table 1 contains the key
crystallographic data.
[Pt(5,6-Me2-php)Cl]TFPB. Anal. Calcd (C53H31N3ClF24BPt):
44.42 %C, 1.97 %H, 3.05 %N. Found: 44.24 %C, 2.17 %H, 2.76
%N.
[Pt(3,4,7,8-Me4-php)Cl]TFPB. Anal. Calcd (C53H31N3ClF24-
BPt): 45.24 %C, 2.22 %H, 2.99 %N. Found: 45.21 %C, 2.24 %H,
2.93 %N.
[Pt(3,5,6,8-Me4-php)Cl]TFPB‚1/2(CH2Cl2). Anal. Calcd
(C53H31N3ClF24BPt‚1/2(CH2Cl2)): 44.50 %C, 2.22 %H, 2.76 %N.
Found: 44.65 %C, 2.05 %H, 2.48 %N.
Computational Methodology. Geometries for php (Cs), 3,8-
Me2-php (Cs), 4,7-Me2-php (Cs), 5,6-Me2-php (Cs), trpy (C2V), 1,-
10-phenanthroline (C2V), and pyridine (C2V) were optimized (frozen-
core approximation) at the Moller-Plesset (MP2) level of theory
Methods. A series of freeze-pump-thaw cycles removed
dioxygen prior to luminescence measurements. For steady-state
emission studies, the slit settings were 10 nm for both the excitation
and emission beams. The excitation wavelength was 420 nm, and
a 440 nm long-wave-pass filter removed the scattered light. The
source of the instrumental correction factors has been described,23
and the conversion from wavelength to an emission-energy scale
(24) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd ed.;
Kluwer Academic: Plenum: New York, 1999; p 52.
(25) Parker, C. A.; Rees, W. T. Analyst (London) 1960, 85, 587-600.
(26) Van Houten, J.; Watts, R. J. J. Am. Chem. Soc. 1975, 97, 3843-
3844.
(27) Cunningham, K. L.; Hecker, C. R.; McMillin, D. R. Inorg. Chim. Acta
1996, 242, 143-147.
(28) Sheldrick, G. M. SHELX97. A Program for Crystal Structure Refine-
ment; University of Go¨ttingen: Go¨ttingen, 1997.
(29) Beurskens, P. T.; Beurskens, G.; Garcia-Granda, R.; Gould, R. O.;
Israel, R.; Smits, J. M. M. The DIRDIF-99 Program System;
Crystallography Laboratory, University of Nijmegen: Nijmegen, 1999.
(23) Eggleston, M. K.; McMillin, D. R.; Koenig, K. S.; Pallenberg, A. J.
Inorg. Chem. 1997, 36, 172-176.
Inorganic Chemistry, Vol. 41, No. 24, 2002 6389

Moore et al.
Table 1. Crystal Data and Collection Parameters for
Table 2. Electrochemical Data from DMF Solutions at Room
[Pt(4,7-Me2-php)Cl]CF3O3S
Temperature
formula
fw
space group
a, Å
b, Å
c, Å
C20H15ClF3N3O3PtS
664.96
P21/n (No. 14)
7.4316(3)
13.6633(7)
20.1448(12)
96.5965(19)
2032.0(3)
Pt(NNN)Cl+
E1, V vs Fc+/0 E2, V vs Fc+/0
Zn(NNN)Cl2
NNN
E0, V vs Fc+/0
php
-1.09 (90, 1.1)a -1.69 (60, 1.0)a -1.64 (80, 1.2)a
-1.17 (160, 1.3) -1.76 (60, 1.2) -1.68 (70, 1.3)
4,7-Me2-php
3,4,7,8-Me4-php -1.18 (210, 1.2) -1.77 (60, 1.2) -1.77 (90, 1.2)
5,6-Me2-php
-1.13 (170, 1.2) -1.75 (90, 1.1)
b
b
â, deg
V, Å3
Z
3,5,6,8-Me4-php -1.14 (190, 1.2) -1.77 (50, 1.1)
4
a Ep,a - Ep,c separation in mV and ic/ia current ratios in parentheses. b Not
measured due to poor solubility.
d
calc, g cm-3
2.174
cryst dimens, mm
temp, K
0.40 × 0.23 × 0.05
150
radiation (wavelength)
monochromator
linear abs coeff, mm-1
abs correction applied
transm factors: min, max
diffractometer
h, k, l ranges
Mo KR (0.71073 Å)
graphite
7.263
empiricala
0.32, 0.70
Nonius KappaCCD
0 to 9, 0 to 17, -26 to 26
5.05-55.86
1.05
2θ range, deg
mosaicity, deg
programs used
F000
SHELXL-97
1272.0
2
weighting
1/[σ2(Fo ) + (0.0552P)2 + 0.0000P]
where P ) (Fo2 + 2Fc )/3
2
data collected
unique data
Rint
21386
4767
0.106
4761
data used in refinement
R-factor cutoff
data with I > 2.0σ(I)
refined extinction coeff
no. of variables
largest shift/esd in final cycle
R(Fo)
2
Fo2 > 2.0σ(Fo )
Figure 1. ORTEP32 representation and crystallographic-numbering scheme
for Pt(4,7-Me2-php)Cl+. The probability level for the thermal ellipsoids is
50%.
2690
0.0021
292
<0.00
0.053
0.113
0.950
Table 3. Selected Structural Data for [Pt(4,7-Me2-php)Cl]CF3SO3
2
Rw(Fo )
Bond Distances (Å)
GOF
Pt-N1
2.013(8)
1.949(8)
2.025(8)
2.302(2)
1.426(16)
1.457(13)
1.357(14)
1.424(13)
1.405(13)
C5-C14
1.395(12)
1.410(13)
1.490(13)
1.480(13)
1.407(12)
1.324(11)
1.387(13)
1.378(14)
1.396(13)
a Otwinowski, Z.; Minor, W. Methods Enzymol. 1996, 276, 307.
Pt-N12
Pt-N112
Pt-Cl
C13-C14
C9-C91
C4-C41
using the 6-31G(d,p) basis set.30 All calculations were carried out
C11-C111
C5-C6
C6-C7
C7-C8
C8-C13
C111-N112
N112-C113
C113-C114
C114-C115
C115-C116
with the Gaussian 98 electronic structure program suite.31
Results
Electrochemistry. Like platinum(II) terpyridines,10,17 the
Pt(php)Cl+ systems exhibit two, ostensibly ligand-based
reduction waves about 0.6 V apart in DMF (Table 2). Ligand
reduction occurs at a more cathodic potential for the Zn-
(php)Cl2 analogues, again in keeping with results for
complexes involving terpyridine ligands.17 For example, the
potential for the reduction of Zn(php)Cl2 is 0.55 V more
negative than that of Pt(php)Cl+. The reduction potential is
ca. 0.2 V more positive for coordinated php compared to
coordinated trpy. In accordance with an electron-donating
Angles (deg)
N1-Pt-N12
N12-Pt-N112
N1-Pt-N112
N1-Pt-Cl
82.4(3)
80.3(3)
162.7(3)
98.1(2)
N112-Pt-Cl
99.2(2)
178.8(2)
115.1(9)
114.8(12)
N12-Pt-Cl
N12-C13-C14
N12-C11-C111
effect, however, adding methyl substituents to the php ligand
induces small negative shifts in the reduction potential (Table
2). The effect is larger for ring substitution at the 4,7-
positions in comparison with the 3,8- or the 5,6-positions.
Structure of [Pt(4,7-Me2-php)Cl]O3SCF3. The ORTEP32
drawing in Figure 1 reveals the crystallographic-numbering
scheme for the 4,7-Me2-php ligand as well as the planar
coordination geometry about platinum. Table 3 contains a
listing of important bond angles and bond distances. In the
crystal the cations stack in head-to-tail fashion in zigzag
columns that angle off the crystallographic a axis. Alternating
between 4.331(1) and 4.397(1) Å, the Pt-Pt separations are
too long for any significant metal-metal interactions.
Uncoordinated anions distribute in spaces between columns.
(30) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213-222.
(31) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. J. A.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,
V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;
Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts;
R. Martin, R. L.; Fox, D.; J. Keith, T.; Al-Laham, M. A.; Peng, C.
Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98; Gaussian, Inc.:
Pittsburgh, PA, 1998.
(32) Johnson, C. K. ORTEP II; Oak Ridge National Laboratory: Oak Ridge,
TN, 1976.
6390 Inorganic Chemistry, Vol. 41, No. 24, 2002

A Superior Polypyridine Ligand for Pt(II) Lumaphors
Table 4. Absorbance Data for Complexes of php and Related Ligands with Pt(II), Zn(II), and Fe(II) in Room-Temperature Solution
λ
max, nm (absorbance)
Zn(NNN)Cl2
Pt(NNN)Cl+ a
Fe(NNN)2
a
2+ b
NNN
php
4,7-Me2-php
3,4,7,8-Me4-php
5,6-Me2-php
3,5,6,8-Me4-php
264, 305, 325, 345sh, 405sh, 421 [2600]c
263, 302, 320, 338s, 345sh, 395sh, 415 [3600]
269, 290sh, 301, 326, 330sh, 405, 423 [3600]
265, 302, 318sh, 329, 362, 415 [3900], 437sh
264, 304, 330, 357sh, 416 [3800], 437sh
305, 328, 343, 358
305, 328, 342, 358
305, 330, 344, 361
314, 342, 360, 379sh
315, 341, 356, 372
440sh, 480, 535sh, 580
440sh, 485, 540, 590
440sh, 480, 545sh, 595
440sh, 480, 530sh, 575
440sh, 475, 540sh, 585
a In methylene chloride solution. b In water. c Molar absorptivity (M-1 cm-1) in brackets.
Figure 2. Absorption spectra of Pt(4′-Ph-T)Cl+ (-O-), Pt(php)Cl+ (s),
Figure 3. Absorption spectra of Zn(4′-Ph-T)Cl2 (s) and Zn(php)Cl2
(s, thick) in DCM solution at room temperature.
and Pt(5,6-Me2-php)Cl+ (s, thick) in DCM solution at room temperature.
Inset: Expansion of long-wavelength region.
The 4,7-Me2-php ligand itself is essentially planar, and,
within experimental error, the mean planes of the C111-
C116 pyridine moiety and the N1-C14 phenanthroline core
are coplanar. The average Pt-N bond distance is very similar
in Pt(4,7-Me2-php)Cl+ and Pt(trpy)Cl+; however, the spread
is greater in the trpy complex where the Pt-N distances are
1.93, 2.108, and 2.030 Å.6 In contrast, the two Pt-Cl bond
distances are virtually identical. Geometric constraints
prevent tridentate ligands like trpy from achieving ideal
bonding with a metal center,7,33 and bond angles within the
coordinated 4,7-Me2-php ligand reveal the strain that occurs,
especially within the pyridine moiety. Compare, for example,
bond angles at C111 and C14: (C11-C111-C116) )
127.0(9)° vs (C13-C14-C5) ) 120.3(9)°, and (C116-
C111-N112) ) 118.2(9)° vs (C5-C14-N1) ) 123.3-
(9)°. In the 4,7-Me2-php complex, the N-Pt-N angle
relating the terminal nitrogen donors is 162.7 (3)° compared
with 161.8 (2)° in the trpy complex.
Absorption Spectra. The UV spectrum of Pt(php)Cl+
exhibits two groups of bands analogous to those observed
for the well-studied Pt(trpy)Cl+ system (Figure 2).7 First, a
set of relatively intense, π-π* intraligand absorptions occur
between 250 and 300 nm. A second, more highly structured
series of intraligand π-π* absorptions occur in the 300-
350 nm range. Figure 3 shows that analogous intraligand
π-π* absorption bands appear in the spectra of Zn(4′-Ph-
T)Cl2 and Zn(php)Cl2, but at distinctly longer wavelengths
for the php complex. Data in Table 4 show that incorporating
methyl substituents typically has little effect on the energy
of the intraligand absorptions except for substitution at the
5,6-positions. More specifically, the π-π* absorptions of
Zn(5,6-Me2-php)Cl2 and Zn(3,5,6,8-Me4-php)Cl2 shift to
longer wavelengths, and there is evidence of band broaden-
ing.
2+
2+
Figure 4. Visible spectra of Fe(4′-Ph-T)2 (thin line) and Fe(php)2
(thick line) in water at room temperature.
Lower energy bands, centered around 420 nm in the
spectra of the platinum(II) analogues, are attributable to
metal-to-ligand charge-transfer (MLCT) absorptions (Figure
2). The CT absorption of Pt(php)Cl+ shows two poorly
resolved components, separated by ca. 1100 cm-1, which
may correspond to vibronic structure. The longer wavelength
component exhibits greater absorption intensity in the case
of the php and 3,4,7,8-Me4-php complexes, but the pattern
reverses when there are methyl groups in the 5,6-positions
(Figure 2). From complex to complex, the energy shifts are
fairly small in the CT region, but introduction of methyl
groups at the 5,6-positions of the phenanthroline core induces
a shift toward longer wavelength. Related CT absorptions
2+
occur in the spectrum of Fe(php)2 but at lower energies
due to the ease of oxidation of Fe(II) (Table 4). A second
group of CT transitions is also relatively prominent in the
region of 440-480 nm in the spectrum of Fe(php)2+ (Figure
4). The principal CT maximum occurs at 576 nm, and a
shoulder at ca. 530 nm may represent vibrational structure
and/or a splitting of the dπ orbitals of the metal center.34,35
Figure 4 also reveals that similar structure appears in the
absorption spectrum of Fe(4′-Ph-T)22+. The CT absorption
2+
of Fe(php)2 shifts to lower energy upon the introduction
(34) Ceulemans, A.; Vanquickenborne, L. G. J. Am. Chem. Soc. 1981, 103,
2238-2241.
(35) Seneviratne, D. S.; Uddin, J.; Swayambunathan, V.; Schlegel, H. B.;
Endicott, J. F. Inorg. Chem. 2002, 41, 1502-1517.
(33) Figgis, B. N.; Kucharski, E. S.; White, A. H. Aust. J. Chem. 1983,
36, 1563-1571.
Inorganic Chemistry, Vol. 41, No. 24, 2002 6391

Moore et al.
Figure 5. Corrected emission spectra of Pt(php)Cl+ complexes in DCM
solution at room temperature. The absorbance at the excitation wavelength
is the same for (A) Pt(php)Cl+, (B) Pt(4,7-Me2-php)Cl+, (C) Pt(3,4,7,8-
Me4-php)Cl+, and (D) Pt(3,5,6,8-Me4-php)Cl+.
Figure 7. Corrected emission spectra of (A) Zn(4′-Ph-T)Cl2 and (B) Zn-
(php)Cl2 in ethylene glycol at 77 K.
Figure 8. Temperature-dependent decay rates measured in DCM for Pt-
(php)Cl+ (0), Pt(4,7-Me2-php)Cl+ (4), and Pt(5,6-Me2-php)Cl+ (O). The
curves are the best fits of the data to eq 1.
Figure 6. Corrected emission spectrum of Pt(5,6-Me2-php)Cl+ in buty-
ronitrile at 77 K. The platinum concentrations are 1.7 µM (thin line) and
17 µM (thick line).
Table 6. Data from Corrected Emission Spectra of php and Related
Complexes of Pt(II) and Zn(II) in Rigid Glasses at 77 K
Table 5. Data from Corrected Emission Spectra of Pt(php)Cl+ and
Related Systems in Deoxygenated Dichloromethane Solution at Room
Temperature
Pt(NNN)Cl+ in
butyronitrile
Zn(NNN)Cl2 in ethylene glycol
NNN
λmax, nm
λ
max, nm
τ, s
Ligand
λ
max, nm
τ, µs
102φ
kr, s-1
php
4,7-Me2-php
3,4,7,8-Me4-php
5,6-Me2-php
3,5,6,8-Me4-php
533, 576, 631
540, 587, 640
545, 591, 643
558, 608
489, 528, 572
498, 538, 582
500, 539, 583
523, 563, 612
522, 563, 612
0.93
0.89
1.0
0.76
0.87
php
547, 591, 650
553, 599, 653
554, 601, 655
563, 609, 673
567, 611, 675
0.23
1.5
4.0
5.0
9.3
0.31
1.6
3.8
2.6
5.5
1.2 × 104
1.1 × 104
9.4 × 103
5.3 × 103
6.0 × 103
4,7-Me2-php
3,4,7,8-Me4-php
5,6-Me2-php
561, 610, 665
3,5,6,8-Me4-php
6 the corrected emission spectrum from the dilute glass is
probably representative of a monomeric platinum(II) system.
The band shape of the emission from the monomeric
component is qualitatively similar to that observed in fluid
solution, except that there is a small shift toward shorter
wavelength in the low-temperature matrix. For reference,
Figure 7 includes the frozen solution emission spectrum of
the Zn(php)Cl2 complex, and Table 6 includes data for
analogous methyl-substituted php derivatives. The emissions
from the zinc systems occur at higher energies but show the
same kind of vibrational structure. Here, too, introduction
of methyl substituents induces a bathochromic shift in the
emission, but substituents have comparatively little influence
on the emission lifetime within the zinc series, at least at 77
K (Table 6).
of methyl substituents at the 3,8- or 4,7-positions, while
substitution at the 5,6-positions shifts the absorption in the
opposite direction (Table 4).
Emission Data. The Pt(php)Cl+ systems exhibit relatively
intense, structured emission spectra in room-temperature
DCM solution (Figure 5). The short wavelength maximum
at 545 nm in the corrected emission spectrum of the parent
php complex is the 0-0 transition that connects the ground
vibrational levels of the two participating electronic states.
Under a nitrogen atmosphere, the php complex exhibits an
emission quantum yield of 3.1 × 10-3 and an excited-state
lifetime of 0.23 µs at room temperature. For comparison,
the analogous Pt(4′-Ph-T)Cl+ system has a 0-0 emission
band at 535 nm and an excited-state lifetime of 85 ns.11,12
Incorporating methyl substituents into the Pt(php)Cl+ system
enhances the lifetime and the quantum yield despite the fact
that the emission shifts to lower energy (Table 5). Substitu-
tion at the 5,6-positions is most effective, and the 3,5,6,8-
Me4-php system exhibits the highest emission yield (0.055)
and the longest excited-state lifetime (9.3 µs) in the series.
The data in Figure 8 reveal that the decay rate has a weak
temperature dependence in DCM solution for Pt(php)Cl+
derivatives when the excited-state lifetime is of the order of
a microsecond at room temperature. However, the decay rate
falls off rapidly for the Pt(php)Cl+ system at lower temper-
atures. A fit of the data to the equation
k ) k1 exp(∆E/RT) + k0
(1)
As is the case with trpy analogues,8,9 the emission from
low-temperature glasses containing Pt(php)Cl+ is very
concentration dependent. The broad, intense emission evident
at longer wavelengths in concentrated solutions is attributable
to aggregated forms of the luminophore; see ref 9. In Figure
gives a ∆E of 1840 cm-1 and a preexponential factor of 2.3
× 1010 s-1 for the php system. The ∆E is a reasonable value
for thermal population of a higher energy d-d state. It is
probably coincidental that the fits for the 4,7-Me2-php and
6392 Inorganic Chemistry, Vol. 41, No. 24, 2002

A Superior Polypyridine Ligand for Pt(II) Lumaphors
Figure 10. Frontier orbitals of php. The radius of the circle indicates the
relative participation of a particular p(π) atomic orbital. Shaded circles depict
negative amplitudes.
For php, the MO calculations also show that the p(π)
atomic orbital of the central nitrogen atom makes a signifi-
cant contribution to the LUMO+1. In contrast, this same
orbital coefficient is equal to zero in the LUMO+1 for trpy.
This is a significant difference between the two ligands
because the involvement of the p(π) orbital of nitrogen is
essential for the development of oscillator strength for CT
absorption.34,36,37 Indeed, the CT transitions that occur
Figure 9. Calculated energies for the frontier orbitals of 4′-Ph-T, trpy,
pyridine, php, and 1,10-phenanthroline. The LUMO and LUMO+1 orbitals
all have positive energies while the energies are negative for the HOMO
and HOMO-1 orbitals.
5,6-Me2-php systems give similar ∆E values of 1380 and
1580 cm-1, respectively. For these systems the thermally
assisted decay process is much less efficient and probably
does not relate to population of a d-d state. According to
-1
the fits, the limiting lifetimes (k0 values) are 3, 5, and 11
2+
between 400 and 500 nm in the spectrum of Fe(php)2
probably involve excitation into the LUMO+1 orbital of php,
µs for the php, 4,7-Me2-php, and 5,6-Me2-php systems,
respectively.
and it is not surprising that the analogous transitions of Fe-
The emission results establish two distinct solvent effects.
The first is that donating solvents (Lewis bases) quench the
emission. In every case, quenching is essentially complete
in DMF at room temperature. In the less basic solvent
acetonitrile, the emission from photoexcited Pt(5,6-Me2-php)-
Cl+ persists with a lifetime of 98 ns while the residual
emission from Pt(3,5,6,8-Me4-php)Cl+ has a slightly longer
decay time of 164 ns. The more surprising observation is
that the addition of noncoordinating toluene also attenuates
the emission signal. As described above, the photolumines-
cence of Pt(5,6-Me2-php)Cl+ decays with a lifetime of 5.0
µs in deoxygenated DCM solution. When the solution
contains 10 vol % toluene, the lifetime drops to 1.25 µs,
independent of the platinum concentration in the range 0.46-
46 µM. Moreover, in the same solvent mixture the addition
of up to a 100-fold excess of the tetramethylammonium salt
of the TFPB anion has no effect on the lifetime. In 50 vol
% toluene, the lifetime is even shorter at 420 ns, again
independent of the platinum concentration in the range 10-
100 µM. In 75% toluene the lifetime is only 330 ns.
MO Calculations. Figure 9 shows how the frontier orbitals
of php correlate with those of the parent compounds, pyridine
and 1,10-phenanthroline. The mixing of the LUMO of
pyridine with the LUMO+1 of 1,10-phenanthroline results
in a relatively low-energy LUMO for php. In addition, php
has a relatively high energy HOMO due to the mixing of
the HOMO-1 of pyridine with the HOMO of 1,10-
phenanthroline. As a result, php has the narrowest HOMO-
LUMO gap of the five ligands in Figure 9. Figure 10 shows
the relative magnitudes of the p(π) atomic orbitals on the
carbon and nitrogen atoms for the HOMO and LUMO of
php. In accordance with the frontier orbital energies of
pyridine and 1,10-phenanthroline (Figure 9), it is not
surprising that most of the amplitude of the HOMO and
LUMO orbitals of php resides on the 1,10-phenanthroline
end of the molecule.
2+
(4′-Ph-T)2 are much weaker (Figure 4). Finally, the two
types of CT transitions are likely to be less resolved for Fe-
(4′-Ph-T)2 because the difference in energy between the
LUMO and LUMO+1 is smaller in the case of the 4′-Ph-T
ligand.
2+
Because the X-ray structure of Pt(4,7-Me2-php)Cl+ indi-
cates that the 4,7-Me2-php ligand is essentially planar, the
MP2 calculations for this ligand, as well as the 3,8-Me2-
php and 5,6-Me2-php ligands, were carried out using Cs
symmetry constraints. Of course, there are four possible Cs
structures (rotamers) for each ligand. For the 4,7-Me2-php
ligand, the lowest energy rotamer has the two in-plane,
methyl C-H bonds oriented toward C3 and C8 of the 1,-
10-phenanthroline group. The X-ray data are also consistent
with this conformation. The calculated distances between the
in-plane, methyl hydrogens and the hydrogen atoms at C3
and C8 are 2.37 and 2.32 Å, respectively. At 0.17 eV higher
energy, the least stable rotamer has the two in-plane, methyl
C-H bonds oriented toward C5 and C6 of the 1,10-
phenanthroline group. For this structure, the closest (MP2
calculated) distances between the in-plane, methyl hydrogens
and the C5 and C6 hydrogens are 1.97 and 1.98 Å. The total
energies of the other two Cs rotamers of 4,7-Me2-php fall
almost exactly halfway between these two extremes. In the
case of the 5,6-Me2-php ligand, MP2 calculations indicate
that the “head-to-head” arrangement of the in-plane, methyl
hydrogens has the lowest energy even though the hydrogen
atoms are calculated to be only 1.86 Å apart. The “tail-to-
tail” rotamer has the highest energy and a calculated
separation of 1.91 Å between the in-plane hydrogen of the
C6 methyl group and the neighboring C7 hydrogen.
In most cases, the presence of methyl substituents affects
the HOMO and LUMO energies. The HOMO shifts from
(36) Day, P.; Sanders, N. J. Chem. Soc. A 1967, 1536-1541.
(37) Phifer, C. C.; McMillin, D. R. Inorg. Chem. 1986, 25, 1329-1333.
Inorganic Chemistry, Vol. 41, No. 24, 2002 6393

Moore et al.
-7.68 eV for php to -7.59, -7.53, and -7.52 eV for the
most stable rotamers of 3,8-Me2-php, 4,7-Me2-php, and 5,6-
Me2-php, respectively. There is less of a trend in the LUMO
energies which are 1.79, 1.86, 1.79, and 1.88 eV for php,
3,8-Me2-php, 4,7-Me2-php, and 5,6-Me2-php, respectively.
Methyl substituents also appear to destabilize the nitrogen-
based n(σ) orbital that has the appropriate symmetry to
interact with the d(σ) orbital of platinum. For example, this
orbital shifts from -10.58 eV in php to -10.31 eV in 3,8-
Me2-php.
Contrary to expectations based on the energy-gap law,41-43
data in Table 5 show that the lifetime in the series of php
complexes increases as the emission energy decreases. Why
does the addition of methyl substituents have such an effect?
Finally, why is the vibrational structure in the emission
spectrum of Pt(php)Cl+ so much more pronounced than that
observed for trpy-based systems like Pt(4′-CN-T)Cl+,10 where
4′-CN-T denotes 4′-cyano-2,2′:6′,2′′-terpyridine?
Other Proximate States. The answers to all these
questions revolve around variations in orbital parentage. In
principle, several types of excitation can cooperate, including
d-p excitation. Thus, Gray and co-workers have pointed out
that the accessibility of the empty platinum 6pz orbital may
account for the anomalously easy reduction of platinum(II)
polypyridine complexes by comparison with zinc(II) ana-
logues.17 However, d-p excitation typically falls at around
250 nm in the absorption spectrum,44 so it probably has little
bearing on the much lower energy emitting states. Intraligand
charge-transfer excited states can also occur in Pt(trpy)Cl+
derivatives, but this requires the presence of electron-rich
substituents like dialkylamines or large, fused-ring aryl
substituents, such as 1-pyrenyl.10,12
Discussion
Photophysics of Pt(php)Cl+. The observation of emission
from Pt(php)Cl+ at room temperature in DCM solution is
quite intriguing because radiationless decay via low-lying
d-d states is often extremely efficient in platinum(II)
complexes.38 Indeed, this process explains why the analogous
Pt(trpy)Cl+ system is nonluminescent.1,7 However, the Pt-
(4′-Ph-T)Cl+ complex provides an interesting contrast in that
the complex is emissive and exhibits an excited-state lifetime
of 85 ns in deoxygenated DCM solution.12 One possible
explanation is that extended conjugation within the 4′-Ph-T
ligand reduces the degree of distortion in the excited state
and thereby enhances the intrinsic lifetime.39 More likely,
the presence of the phenyl substituent leads to a lower energy
CT excited state and a higher barrier to deactivation via d-d
excited states.1,12 The Pt(php)Cl+ system also appears to have
a relatively low energy excited state because it has a CT
absorption maximum of 421 nm in DCM solution versus
405 nm for Pt(trpy)Cl+.7 MO calculations help explain the
difference because the π* LUMO is about 0.5 V lower in
energy for php compared with trpy (Figure 9). Electrochemi-
cal data are also consistent with the expected trend in LUMO
energies. The most straightforward comparison of ligand
reduction involves a comparison of zinc(II) analogues. While
What turns out to be a comparatively low energy process
in the php system is intraligand π-π* excitation. Thus, the
first absorption in the electronic spectrum of Zn(php)Cl2
occurs at 358 nm compared with 335 nm for Zn(4′-Ph-T)-
Cl2.12 Furthermore, data in Table 4 establish that the π-π*
absorption shifts to even longer wavelength with the addition
of methyl substituents. Although the lowest energy electronic
transitions in polycyclic aromatic compounds are not always
well described as HOMO to LUMO excitations,45 this gap
is still a fundamental determinant of the π-π* transition
energies. The comparison of the HOMO-LUMO gaps for
php and 4′-Ph-T in Figure 9 is quite revealing. The greater
HOMO-LUMO separation for 4′-Ph-T may be a result of
the fact that the meta linkages in 4′-Ph-T prevent conjugation
across the central ring.46 MP2 calculations also indicate that
the 4′-phenyl substitutent has little impact on the HOMO or
the LUMO orbital of the trpy core because each of these
orbitals has a node at the C4′ carbon.
reduction of Zn(trpy)Cl2 takes place at -1.86 V vs Fc+/0
,
reduction of the php complex occurs at a less cathodic
potential of -1.64 V (versus the same reference). The data
in Figure 8 indicate that thermally activated deactivation
occurs in Pt(php)Cl+; the process is just more efficient in
Pt(trpy)Cl+.
Unfortunately, the MP2 calculations pertain to the ground
state and do not provide much insight into the photophysics.
In general, the impact a methyl substituent has on any
particular energy level depends upon the orbital amplitude
at the point of substitution.47-49 This may explain why the
The results discussed so far clearly establish roles for d-d
and CT excited states in shaping the basic photophysical
properties of Pt(php)Cl+ and related systems; however,
several observations remain unexplained. One is that the
rigidity of the php ligand can also lead to low-energy d-d
excited states as is the case with the Ru(php)22+ system which
is nonluminescent in solution.40 Why should the platinum
analogue behave differently? Methyl substituents influence
(41) Englman, R.; Jortner, J. Mol. Phys. 1970, 18, 145-164.
(42) Caspar, J. V.; Kober, E. M.; Sullivan, B. P.; Meyer, T. J. J. Am. Chem.
Soc. 1982, 104, 630-632.
(43) Stufkens, D. J.; Vlcek, A., Jr. Coord. Chem. ReV. 1988, 177, 127-
179.
2+
the CT absorption maxima of Fe(php)2 derivatives in a
(44) Connick, W. B.; Miskowski, V. M.; Houlding, V. H.; Gray, H. B.
Inorg. Chem. 2000, 39, 2585-2592.
positionally dependent way (Table 4). Why do the emission
maxima of the Pt(php)Cl+ not follow the same pattern?
(45) Dewar, M. J. S.; Longuet-Higgins, H. C. Proc. Phys. Soc. (London)
1954, A67, 795-804.
(38) Barigelletti, F.; Sandrini, D.; Maestri, M.; Balzani, V.; von Zelewsky,
A.; Chassot, L.; Jolliet, P.; Maeder, U. Inorg. Chem. 1988, 27, 3644-
3647.
(39) Damrauer, N. H.; Boussie, T. R.; Devenney, M.; McCusker, J. K. J.
Am. Chem. Soc. 1997, 119, 8253-8268.
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Thummel, R. P. Inorg. Chem. 1996, 35, 5953-5956.
(46) Jaffe, H. H.; Orchin, M. Theory and Applications of UltraViolet
Spectroscopy; Wiley and Sons: New York, 1962; pp 273-276.
(47) Farrell, I. R.; Hartl, F.; Zalis, S.; Mahabiersing, T.; Vlcek, A. J. Chem.
Soc., Dalton Trans. 2000, 4323-4331.
(48) Klein, A.; Kaim, W.; Waldhor, E.; Hausen, H. D. J. Chem. Soc., Perkin
Trans. 2 1995, 2121-2126.
(49) Gouterman, M. J. Chem. Phys. 1959, 30, 1139-1161.
6394 Inorganic Chemistry, Vol. 41, No. 24, 2002

A Superior Polypyridine Ligand for Pt(II) Lumaphors
calculated decrease in the HOMO-LUMO gap is only 0.02
eV (9.47 eV to 9.45 eV) when substitution occurs at the C3
and C8 positions of the phenanthroline where the orbital
coefficients are vanishingly small for the two frontier orbitals
of php. On the basis of the spectral data from the zinc
compounds, one might expect the HOMO-LUMO gap to
be smallest for the 5,6-Me2-php derivative; however, theory
predicts this ligand to have a HOMO-LUMO gap of 9.40
eV versus 9.32 eV for 4,7-Me2-php.
occurs at 560 nm in the corrected emission spectrum versus
550 nm in butyronitrile at 77 K. Finally, systematic variations
in the amount of 3d-π* participation account for otherwise
perplexing substituent effects. Consider the lifetimes. They
vary dramatically within the platinum complexes, but there
3
is very little change among the π-π* lifetimes of the Zn-
(php)Cl2 series at 77 K. For the platinum complexes, the
efficiency of thermal deactivation from higher energy d-d
excited states varies somewhat at room temperature, but the
trend in lifetimes actually remains the same down to 200 K
for the Pt(5,6-Me2-php)Cl+, Pt(4,7-Me2-php)Cl+, Pt(php)-
Cl+ series. The obvious explanation is that the addition of
Orbital Parentage of the Emission. There is no question
about the CT character of the lowest energy absorption bands
of Pt(php)Cl+, but the same state assignment need not apply
to the emissions. The multiplicity is clear because the
lifetimes and the calculated radiative rate constants in Table
5 are only consistent with emission from a triplet excited
state, albeit one involving a heavy metal center. However,
the lowest energy triplet state could have intraligand excited-
state character because the singlet-triplet splitting is typically
much larger for π-π* as compared with CT excitation.50,51
Indeed, the vibrational structure in the emission spectra in
Figure 5 provides evidence of substantial 3π-π* character.
Vibrational spacings of ca. 1400 cm-1 implicate CdC and/
or CdN double-bond stretching modes, and the fact that there
are relatively long progressions establishes that the ligand
framework has a different geometry in the excited state. In
principle, either d-π* or π-π* excitation would give rise
to an expansion of the ligand framework due to the
population of an intraligand antibonding orbital. However,
the 0-0 transition would be the most intense component in
emission from a d-π* excited state. Dome-shaped intensity
distributions, like those in Figure 5, are indicative of a
relatively large shift in equilibrium geometry. This is more
consistent with π-π* excitation because the depopulation
of an intraligand bonding orbital is also a factor.52,53
3
methyl substituents induces a drop in the energy of π-π*
excitation and thereby enhances its contribution to the
excited-state character. In line with this explanation, the kr
values generally decrease as the emission energy decreases
(Table 5). Once again, substitution at the 5,6-positions of
the phenanthroline is most effective, and that explains why
the 5,6-Me2-php and 3,5,6,8-Me4-php complexes have similar
radiative rate constants. The Watts and Crosby groups have
found that a related kind of excited-state tuning occurs in
mixed-ligand iridium complexes.57,58 In their systems, which
3
also exhibit π-π/3d-π* emissions, it is possible to alter
the orbital parentage by changing the solvent which affects
3
the energy of the polar d-π* component.
There are at least two mechanisms through which formally
intraligand π-π* excited states can take on d-orbital
character in Pt(php)Cl+ systems. One way is via covalent
bonding. Due to favorable overlap of dπ orbitals of the metal
with π and π* orbitals of the ligand, the molecular orbitals
of the complex naturally delocalize over both metal and
ligand centers.1,52 Because there are no σ-bonding interactions
with axial ligands, the platinum 6pz orbital is also available
to participate in the mix.17,44 Two-electron terms also help
shape the electron distribution because incorporating multiple
configurations generally provides for a decreased electron-
electron repulsion energy.59 When different configurations
interact, the rule of thumb is that the extent of mixing varies
inversely with the energy separation.14 Configuration interac-
tion is especially important among excited states because
multiple excitations often have rather similar energies. The
energetic analysis becomes even more complex when the
states involved have different equilibrium geometries.60 For
the Pt(php)Cl+ series, there are at least two distortion
coordinates to consider. Thus, in addition to displacement
along the intraligand breathing mode discussed above, one
expects a d-π* state to distort along the metal-ligand
stretching coordinate. Another complication is that the
strength of coupling can also vary with the nature of the
d-π* configuration involved.44 The reason is that the
interaction energy depends upon the exchange integral which
can be small if the excitation originates in a dσ orbital which
has no net overlap with the π* orbital of the ligand. In short,
Nevertheless, d-π* excitation must play a role. A radiative
rate constant of the order of 104 s-1 is simply not feasible
3
for a pure π-π* excited state when spin remains a good
quantum number. An admixture of d-π* (CT) character
eases this restriction due to participation of orbitals of the
heavy metal center.14,54,55 A mixed orbital parentage can also
account for the fact that the emission spectra of the Pt(php)-
Cl+ derivatives have lower 0-0 transitions than the Zn(php)-
Cl2 analogues. Pure 3π-π* emission would occur at essen-
tially the same energy.14 The fact that the emission from
Pt(5,6-Me2-php)Cl+ shows modest rigidochromism, i.e., a
blue shift in frozen solution, is also consistent with the
emission having some degree of d-π* character.56 Thus, at
room temperature in acetonitrile solution, the 0-0 transition
(50) Michl, J.; Bonacic-Koutecky, V. Electronic Aspects of Organic
Photochemistry; Wiley-Interscience: New York, 1990; pp 29-43.
(51) DePriest, J.; Zheng, G. Y.; Goswami, N.; Eichhorn, D. M.; Woods,
C.; Rillema, D. P. Inorg. Chem. 2000, 39, 1955-1963.
(52) DeArmond, M. K.; Hillis, J. E. J. Chem. Phys. 1971, 54, 2247-2253.
(53) Indelli, M. T.; Bignozzi, C. A.; Marconi, A.; Scandola, F. J. Am. Chem.
Soc. 1988, 110, 7381-7386.
(54) Ayala, N. P.; Flynn, C. M.; Sacksteder, L.; Demas, J. N.; Degraff, B.
A. J. Am. Chem. Soc. 1990, 112, 3837-3844.
(57) Watts, R. J.; Crosby, G. A.; Sansregret, J. L. Inorg. Chem. 1972, 11,
1474-1483.
(58) Cremers, T. L.; Crosby, G. A. Chem. Phys. Lett. 1980, 73, 541-544.
(59) Michl, J.; Bonacic-Koutecky, V. Electronic Aspects of Organic
Photochemistry; Wiley-Interscience: New York, 1990; pp 442-462.
(60) Watts, R. J. Inorg. Chem. 1981, 20, 2302-2306.
(55) Pierloot, K.; Ceulemans, A.; Merchan, M.; Serrano-Andres, L. J. Phys.
Chem. A 2000, 104, 4374-4382.
(56) Wrighton, M. S.; Morse, D. L. J. Organomet. Chem. 1975, 97, 405-
419.
Inorganic Chemistry, Vol. 41, No. 24, 2002 6395

Moore et al.
Chart 2
quenching has been shown to result in a concentration-
dependent excited-state lifetime in DCM solution.65 In
contrast, the excited-state lifetime of the Pt(5,6-Me2-php)-
Cl+ system is independent of concentration in the accessible
range of concentrations. If, however, the cations and anions
largely exist as outer-sphere ion pairs in the low-dielectric
medium, the concentration effect would be small. Despite
outer-sphere complex formation, the combined influence of
the d8 configuration and the weakly nucleophilic anion could
permit the excited state to persist for hundreds of nanosec-
onds. An alternative explanation for the quenching could be
that electron transfer from the TFPB anion becomes less
endoergic in a very nonpolar environment.
it is clear that π-π/3d-π* configurational mixing occurs
in the php systems, although a quantitative analysis is
difficult. Moreover, the introduction of methyl substituents
enhances intraligand character.
3
Methyl Orientation. Chart 2 depicts the favored orienta-
tion of the methyl rotors in the lowest energy structures
calculated for 4,7-Me2-php and 5,6-Me2-php. Similar rela-
tionships occur in 1,5-dimethylnaphthalene. Wilson has
shown that the out-of-plane hydrogens of the methyl groups
of 1,5-dimethylnaphthalene strongly prefer to straddle the
Cγ-H bond in the solid state and has rationalized the result
as a steric effect.61 Wimmer and Mu¨ller invoke an ortho-
substitution effect to explain an analogous rotational barrier.62
For 5,6-Me2-php, however, a “ball-and-stick” picture simply
cannot account for the preferred conformation indicated from
MP2 calculations. In these molecules electronic effects
appear to be important. For example, hyperconjugative
interactions between the out-of-plane methyl hydrogens and
the p(π) orbital at Cγ probably play some role in determining
the energetic ordering for the rotamers.
External Quenching. Previous work has established that
associative attack by Lewis bases (exciplex quenching) is
slow for a photoexcited Pt(trpy)Cl+-like system when the
lowest energy excited state has mainly intraligand orbital
parentage.1,63 As a consequence, Pt(5,6-Me2-php)Cl+ and Pt-
(3,5,6,8-Me4-php)Cl+ retain modest excited-state lifetimes
in a weakly donating solvent like acetonitrile; however,
neither complex exhibits emission in fluid solutions involving
more basic solvents like DMF. The results are somewhat
different with Cu(TPP), where TPP denotes the deprotonated
form of 5,10,15,20-tetraphenylporphyrin.64 Like Pt(5,6-Me2-
php)Cl+, Cu(TPP) is a planar complex with a reactive excited
state that involves intraligand excitation. However, the d9
electronic configuration of Cu(II) presents a smaller barrier
to ligand addition than a d8, heavy-metal center with a large
ligand-field splitting.63 As a consequence, acetonitrile is more
effective at quenching photoexcited Cu(TPP).
Conclusions and Comparisons
While Pt(trpy)Cl+ has proven to be a convenient binding
agent and spectral probe, Pt(php)Cl+ promises to be more
versatile because it exhibits similar CT absorption and is
photoluminescent as well. Theory reVeals some fundamental
limitations of the trpy framework. One problem is restricted
conjugation due to the meta relationship of the three pyridine
rings. Another is that the HOMO and LUMO orbitals of the
trpy ligand each have a nodal plane that bisects the central
pyridine ring. Since available synthetic routes mostly yield
4′-substituted ligands, only strongly perturbing substituents
can significantly impact the excited-state manifold. In
contrast, the php ligand incorporates a fused-ring structure
that gives rise to a π system with a relatively small HOMO-
LUMO gap. As a consequence, the emissions from Pt(php)-
Cl+ and related derivatives exhibit pronounced vibrational
3
structure and a mixed π-π*/3d-π* orbital parentage. An
equally remarkable finding is the stereoelectronic influence
that simple methyl substituents haVe on the photophysics.
Through substituent addition, one can tune the orbital
parentage of the emitting state with dramatic impact. Whereas
the parent complex Pt(php)Cl+ has an excited-state lifetime
of 0.23 µs in DCM solution, the lifetime increases 40-fold
for the Pt(3,5,6,8-Me4-php)Cl+ analogue. The intraligand
character of the excitation is important for the lifetime, while
the d-π* CT character provides for a favorable radiative
rate constant and the solvent sensitivity of the emission.
Strong donor solvents like DMF are potent emission quench-
ers.
Introducing toluene into DCM solutions shortens the
lifetime of photoexcited Pt(5,6-Me2-php)Cl+, but toluene
itself is almost certainly not basic enough to induce exciplex
quenching. However, counterion-induced quenching may be
a solvent-dependent process. The fluorine atoms on the
periphery of the TFPB anion are not very good donor centers,
but they would become more reactive as the average
hydrogen-bonding strength of the solvent decreases. For the
charge-transfer excited state of the bis(2,9-dimethyl-1,10-
phenanthroline)copper(I) ion, counterion-induced exciplex
Acknowledgment. The National Science Foundation
helped fund this research through Grant No. CHE 01-08902.
Kurstan L. Cunningham first synthesized the php ligand in
this laboratory. The authors thank J. B. Grutzner for helpful
discussions. Michael H. Wilson carried out the quenching
studies with the tetramethylammonium salt of the TFPB
anion.
Supporting Information Available: Crystallographic data in
CIF format. This material is available free of charge via the Internet
at http://pubs.acs.org.
(61) Wilson, C. C. Chem. Commun. 1997, 1281-1282.
(62) Wimmer, R.; Mu¨ller, N. J. Magn. Reson. 1997, 129, 1-9.
(63) Crites Tears, D. K.; McMillin, D. R. Coord. Chem. ReV. 2001, 211,
195-205.
(64) Liu, F.; Cunningham, K. L.; Uphues, W.; Fink, G. W.; Schmolt, J.;
McMillin, D. R. Inorg. Chem. 1995, 34, 2015-2018.
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6396 Inorganic Chemistry, Vol. 41, No. 24, 2002