Photophysical and electrochemical properties of 1,3-bis(2-pyridylimino) isoindolate platinum(ii) derivatives

Article abstract of DOI:10.1039/c2dt30354j

A series of twelve platinum(ii) complexes of the form (N^N^N)PtX have been synthesized and characterized where N^N^N is 1,3-bis(2-pyridylimino)isoindolate ligands (BPI) or BPI ligands whose aryl moieties are substituted with tert-butyl, nitro, alkoxy, iodo or chloro groups, and X is a chloride, fluoride, cyano, acetate, phenyl or 4-(dimethylamino)phenyl ligand. All complexes display at least one irreversible oxidation and two reversible reduction waves at potentials dependent on the position and the electron donating or withdrawing nature of both X and the substituted N^N^N ligand. Broad room temperature phosphorescence ranging in energy from 594 to 680 nm was observed from the complexes, with quantum efficiencies ranging from 0.01 to 0.05. The efficiency of emission is dictated largely by nonradiative processes since the rate constants for nonradiative deactivation [(1.1-100) × 10⁵ s ^-1] show greater variation than those for radiative decay [(0.57-4.0) × 0⁴ s^-1]. Nonradiative deactivation for compounds with X = Cl follow the energy gap law, i.e. the nonradiative rate constants increase exponentially with decreasing emission energy. Deactivation of the excited state appears to be strongly influenced by a non-planar distortion of the BPI ligand.

Full text of DOI:10.1039/c2dt30354j

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PAPER
Photophysical and electrochemical properties of 1,3-bis(2-pyridylimino)-
isoindolate platinum(II) derivatives†
Kenneth Hanson,a Luke Roskop,b Niral Patel,a Laurent Griffe,a Peter I. Djurovich,a Mark S. Gordon*b and
Mark E. Thompson*a
Received 15th February 2012, Accepted 25th April 2012
DOI: 10.1039/c2dt30354j
A series of twelve platinum(II) complexes of the form (N^N^N)PtX have been synthesized and
characterized where N^N^N is 1,3-bis(2-pyridylimino)isoindolate ligands (BPI) or BPI ligands whose
aryl moieties are substituted with tert-butyl, nitro, alkoxy, iodo or chloro groups, and X is a chloride,
fluoride, cyano, acetate, phenyl or 4-(dimethylamino)phenyl ligand. All complexes display at least one
irreversible oxidation and two reversible reduction waves at potentials dependent on the position and the
electron donating or withdrawing nature of both X and the substituted N^N^N ligand. Broad room
temperature phosphorescence ranging in energy from 594 to 680 nm was observed from the complexes,
with quantum efficiencies ranging from 0.01 to 0.05. The efficiency of emission is dictated largely by
nonradiative processes since the rate constants for nonradiative deactivation [(1.1–100) × 105 s−1] show
greater variation than those for radiative decay [(0.57–4.0) × 04 s−1]. Nonradiative deactivation for
compounds with X = Cl follow the energy gap law, i.e. the nonradiative rate constants increase
exponentially with decreasing emission energy. Deactivation of the excited state appears to be strongly
influenced by a non-planar distortion of the BPI ligand.
to platinum through three imine nitrogen atoms.4,11 Analogs of
terpy have also been investigated in which a phenyl group is sub-
Introduction
Square planar platinum(II) complexes are known to exhibit a rich
array of photophysical properties and many potential appli-
cations have been suggested to exploit their luminescent
characteristics.1–4 Chelating ligands that have been used to
produce these square planar complexes include bidentate, triden-
tate and tetradentate ligands.5 Of these, the tridentate ligands
possess two notable advantages. First, they can inhibit square
planar to tetrahedral (D2d) distortions6 that promote nonradiative
decay. Second, unlike tetradentate ligands, tridentate ligands
provide an available coordination site that permits further modifi-
cation of the complex. This feature enables one to use either
anionic (e.g., Cl, NCS, OH, OMe, or alkyne) or neutral (e.g.,
amines, pyridines) ligands to alter the charge and/or spectro-
scopic properties of the parent structure.7–10
stituted for one or two of the imine moieties to produce mono-
and dianionic ligands of the type (C^N^N)− or (N^C^N)− or
(C^N^C)2− 2–5
However, there are few examples of Pt(II) com-
.
plexes with monoanionic (N^N^N)− ligands,12–15 and even
fewer reports of luminescence from complexes with these kinds
of ligands.12,16,17 One interesting version of a (N^N^N)− ligand
is 1,3-bis(2-pyridylimino)isoindolate (BPI). BPI ligands are used
for a variety of different applications18–22 because they are ther-
mally stable, readily synthesized23 and easily modified. BPI has
been used as a chelate for a number of metals, including plati-
num.15 Reports on these metal–BPI complexes have been
focused primarily on catalytic properties, and only minimal data
has appeared regarding the photophysical characteristics of these
compounds.24
A variety of tridentate ligands have been used to prepare lumi-
nescent Pt(II) complexes. Perhaps the most studied of these is
2,2′:6′,2′′-terpyridine (terpy) and related ligands that coordinate
Recently, a series of (BPI)PtCl derivatives were synthesized
and their photophysical properties investigated.25,26 The parent
(BPI)PtCl was found to display phosphorescent emission at
631 nm with a quantum efficiency of 0.54% in CH2Cl2. Repla-
cing the coordinating chloride anion with a neutral N-hetero-
cyclic ligand has a minimal effect on absorption/emission
energies and efficiencies. Additionally, unlike the significant
changes in photophysical properties of [(terpy)PtCl]+1 when the
chloride ligand is replaced with an acetylene group,27 the (BPI)
Pt acetylene complex has similar absorption/emission spectra to
the parent (BPI)PtCl, with the emission efficiency decreasing to
0.36%. This result is contrary to the expectation that the addition
of a strong field ligand will increase the platinum d–d energy
aDepartment of Chemistry, University of Southern California,
Los Angeles, CA 90089, USA. E-mail: met@usc.edu
bDepartment of Chemistry, Iowa State University, Ames, Iowa 50011,
USA. E-mail: mark@si.msg.chem.iastate.edu
†Electronic supplementary information (ESI) available: Calculated
HOMO/LUMO orbitals for 1–12, emission spectra at 77 K for 1–12, the
calculated transitions of 6, cyclic voltammograms for 1, 6–12, 1H/13
C
NMR spectra of 1–12 and crystallographic data for 6. CCDC reference
number 867303. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt30354j
8648 | Dalton Trans., 2012, 41, 8648–8659
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1,2-dicyanonaphthalene, 4,5-dichlorophthalonitrile, 3-nitrophtha-
lonitrile,
4-nitrophthalonitrile,
4-pentoxyphthalonitrile,
4-iodophthalonitrile (TCI America), calcium chloride (J.T.
Baker), and K2PtCl4 (Pressure Chemical Company) were pur-
chased from the corresponding supplier (in parentheses) and
used without further purification for synthesis. (COD)PtCl2,31
1,3-bis(2-pyridylimino)isoindole,23 4-NO2BPI,26 1,3-bis(2-pyri-
dylimino)isoindolate platinum(II) chloride (1),15 (COD)PtPh2,32
33
and (COD)Pt(Ph-4-NMe2)2 were synthesized according to lit-
erature procedures. All reported NMR spectra were obtained on
a Bruker AC-250 MHz FT NMR or a Varian 400 MHz NMR
with all shifts relative to residual solvent signals. Solid probe
mass spectrometer (MS) spectra were taken with a Hewlett-
Packard MS instrument with electron impact ionization and
model 5973 mass selective detector. Mass spectra were also
recorded using a Voyager-DE STR mass spectrometer using a
positive MALDI-TOF method without matrix. Elemental ana-
lyses (CHN) were performed at the Microanalysis Laboratory at
the University of Illinois, Urbana-Champaign, IL.
(BPI)PtF (2)
400 mg (0.76 mmol) 1 and 491 mg (3.79 mmol) AgF were rigo-
rously stirred in 100 mL CH2Cl2 for 24 hours. The solution was
then passed through a plug of silica eluting with CH2Cl2 and
evaporated to dryness under reduced pressure. The resultant
solid was dissolved in a minimal amount of CH2Cl2 followed by
layered addition of methanol. After gradual diffusion, precipitate
began to form. The precipitate was collected by filtration,
washed with methanol and dried under vacuum to give 0.325 g
Fig. 1 Structure of substituted BPI platinum complexes 1–12.
1
(84%), orange solid. H NMR (400 MHz, CDCl3, δ) 9.91 (dd,
JHH = 6.4 Hz, JHF = 10.8 Hz, 2H), 8.08 (dd, J = 5.6 and 3.2 Hz,
2H), 8.0 (t, J = 7.2 Hz, 2H), 7.68 (d, J = 8 Hz, 2H), 7.61 (dd, J
= 5.2 and 2.8 Hz, 2H), 7.2 (t, J = 6.0 Hz, 2H). 13C NMR
(100 MHz, CDCl3, δ)148.8, 145.9, 145.6, 137.6, 131.1, 127.7,
122.0, 119.62, 119.58. MS m/z (relative intensity): 511.10
(83.16%), 512.05 (100%), 513.00 (78.45%), 514.10 (15.67%),
515.00 (18.28%). UV-vis (CH2Cl2) λmax, nm (ε/104 M−1 cm−1):
250 (4.20), 277 (2.61), 349 (1.88), 387 (0.87), 476 (1.00), 497
(0.91). Elemental analysis for C18H12N5PtF·CH2Cl2: calcd: C
38.20, H 2.36, N 11.72; found: C 38.82, H 2.09, N 11.87.
separation and thus, reduce non-radiative deactivation through
ligand field states.7,28–30 On the other hand, the platinum chlor-
ide complexes with 1,3-bis(2-pyridylimino)isoindolate ligands
appended with electron donating (NH2) and electron withdraw-
ing (NO2) groups display significant variations in electrochemi-
cal properties, emission wavelength and efficiency relative to
(BPI)PtCl.26
To better understand this new class of emitter, (BPI)PtX, it is
important to expand the investigation into how perturbations on
either the BPI or anionic monodentate ligand affect its lumines-
cent properties. Herein we report the synthesis, photophysical
and electrochemical characterization of a series of platinum(II)
complexes of the form (R-BPI)PtX, where R-BPI is an assort-
ment of substituents (R) on the BPI ligand and X represents
various anionic monodentate ligands (Fig. 1). The photophysical
data indicate that structural distortion of the BPI ligand in an
electronic excited state imparts a significant contribution to the
rate of nonradiative decay.
(BPI)PtCN (3)
500 mg (0.95 mmol) 1 and 995 mg (5 mmol) KAg(CN)2 were
rigorously stirred in 250 mL CH2Cl2 for 24 hours. The solution
was then evaporated to dryness under reduced pressure. The
product was extracted using a Soxhlet apparatus with 250 mL
refluxing CH2Cl2. When the extracting solution was no longer
yellow, the solution was cooled to room temperature and then to
−40 °C. After one day of cooling, the precipitate was collected
and the solvent in the remaining solution was reduced and
cooled again. This process was repeated several times to yield
0.210 g (43%) of dark red needles. 1H NMR (400 MHz, CDCl3,
Experimental section
General information
3
2-Aminopyridine, 1,2-dicyanobenzene, 1,5-cyclooctadiene,
4-tert-butylphthalonitrile, 4-iodophthalonitrile, 4-(N,N-dimethyl)-
aniline magnesium bromide, phenylmagnesium bromide, silver(I)
fluoride, potassium silver(I) cyanide, silver acetate (Aldrich),
δ) 10.17 (d, JHH = 6.4 Hz, JPtH = 26 Hz, 2H), 8.02 (dd, J = 5.2
and 3.2 Hz, 2H), 7.93 (t, J = 7.6 Hz, 2H), 7.66 (d, J = 8 Hz,
2H), 7.59 (dd, J = 5.6 and 3.2 Hz, 2H), 6.98 (t, J = 6.8 Hz, 2H).
13C NMR (100 MHz, CDCl3, δ) 157.6, 152.4, 150.8, 138.5,
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 8648–8659 | 8649

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137.6, 131.7, 128.8, 122.6, 120.8. MS m/z (relative intensity):
518.25 (85.76%), 519.20 (100%), 520.20 (72.19%), 521.21
precipitate was collected by filtration and washed with methanol.
The product was then recrystallized from a minimal amount of
CH2Cl2 followed by layered addition of methanol to give
(17.13%), 522.25 (17.19%). UV-vis (CH2Cl2) λmax
, nm
(ε/104 M−1 cm−1): 245 (4.46), 268 (2.21), 342 (2.24), 437
(1.69), 465 (1.96). Elemental analysis for C19H12N6Pt: calcd: C
43.93, H 2.33, N 16.18; found: C 43.97, H 2.05, N 15.33.
0.054 g (49%), black crystals. H NMR (400 MHz, CDCl3, δ
1
ppm) 8.81 (dd, J = 6.8 and 1.6 Hz, JPtH = 26.8 Hz, 2H), 8.12
(dd, J = 5.2 and 2.8 Hz, 2H), 7.82 (ddd, J = 6.8, 5.6 and 1.6 Hz,
2H), 7.68–7.62 (m, 4H), 7.48 (d, J = 8.8 Hz, 2H), 6.79 (d, J =
8.8 Hz, 2H), 6.59 (td, J = 6.8 and 1.6 Hz, 2H), 2.98 (s, 6H).
13C NMR (100 MHz, CDCl3, δ ppm) 154.2, 153.5, 152.2,
147.6, 139.1, 136.6, 136.4, 130.8, 128.6, 122.1, 118.6, 115.3,
110.0. MS m/z (relative intensity): 612.15 (99.94%), 613.15
(100%), 614.15 (84.14%), 615.15 (22.34%), 616.15 (13.26%).
UV-vis (CH2Cl2) λmax, nm (ε/104 M−1 cm−1): 252 (4.91), 287
(2.87), 342 (1.78), 470 (0.94). Elemental analysis for
C26H22N6Pt: calcd: C 50.90, H 3.61, N 13.70; found: C 51.09,
H 3.40, N 13.36.
(BPI)PtOOCCH3 (4)
300 mg (0.57 mmol) 1 and 440 mg (3 mmol) silver(I) acetate
(AgOAc) were rigorously stirred in 50 mL CH2Cl2 for 24 hours.
The solution was then passed through a plug of Celite eluting
with CH2Cl2. The volume was reduced to 15 mL under reduced
pressure followed by slow evaporation at room temperature. The
precipitate was collected by filtration and washed with methanol
to give 0.155 g (49%), orange plates. 1H NMR (400 MHz,
CDCl3, δ) 9.36 (dd, JHH = 6.8 and 1.6 Hz, JPtH = 11.6 Hz, 2H),
8.08 (dd, J = 5.6 and 3.2 Hz, 2H), 7.95 (ddd, J = 8.4, 6.8 and
1.6 Hz, 2H), 7.66 (dd, J = 7.2 and 1.6 Hz, 2H), 7.62 (dd, J = 5.2
and 2.8 Hz, 2H), 7.09 (td, J = 5.2 and 1.6 Hz, 2H), 2.3 (s, 3H).
13C NMR (100 MHz, CDCl3, δ) 177.6, 151.2, 150.0, 147.1,
137.7, 137.6, 131.3, 128.1, 122.2, 119.8, 24.7. MS m/z (relative
intensity): 551.10 (83.66%), 552.10 (100%), 553.10 (78.57%),
554.10 (17.36%), 555.45 (18.56%). UV-vis (CH2Cl2) λmax, nm
(ε/104 M−1 cm−1): 249 (4.86), 278 (2.93), 349 (2.21), 464
(1.34), 488 (1.31). Elemental analysis for C19H12N6Pt: calcd:
C 43.48, H 2.74, N 12.68; found: C 43.44, H 2.52, N 12.17.
4-t-BuBPI
A solution of 1 g (5.43 mmol) 4-tert-butylphthalonitrile, 1.28 g
(13.6 mmol) 2-aminopyridine and 0.12 g (1.0 mmol) CaCl2 in
20 ml 1-butanol was refluxed under N2 for 5 days. Upon cooling
to room temperature, the solution was poured into 50 mL of
water and precipitate began to form. The precipitate was col-
lected by filtration and was then chromatographed on silica gel
eluting with CH2Cl2. A fluorescent blue fraction of 4-tert-
butylphthalonitrile starting material was collected first, followed
by a yellow fraction containing the desired product. The yellow
fraction was evaporated to dryness and used without further
(BPI)PtPh (5)
1
purification. 0.439 g (23%), yellow solid. H NMR (250 MHz,
0.095 g (0.208 mmol) (COD)PtPh2 and 0.072 g (0.192 mmol)
BPI were suspended in 20 ml of methanol. To this solution
0.038 ml (0.208 mmol) triethylamine was added and the solution
was heated to 50 °C under nitrogen overnight. A precipitate
began to form upon cooling to room temperature. The precipitate
was collected by filtration and washed with methanol. The
product was then recrystallized from a minimal amount of
CH2Cl2 followed by a layered addition of methanol to give
CDCl3, δ) 8.61 (d, J = 5 Hz, 2H), 8.1 (d, J = 1 Hz, 1H), 8.0 (d, J
= 8 Hz, 1H), 7.82–7.67 (m, 3H), 7.46 (dd, J = 8, 5 Hz, 2H),
7.11 (t, J = 8.0 Hz, 2H), 1.64 (s, 9H).
(4-t-BuBPI)PtCl (7)
1
0.044 g (37%), orange needles. H NMR (400 MHz, CDCl3, δ)
0.2 g (0.54 mmol) (COD)PtCl2 and 0.173 g (0.486 mmol) 4-t-
BuBPI were suspended in 15 ml of methanol. To this solution
0.074 ml (0.54 mmol) triethylamine was added and the solution
was heated overnight to 50 °C under nitrogen. A bright orange
precipitate began to form upon cooling to room temperature. The
precipitate was collected by filtration and washed with methanol.
The solid was recrystallized from a minimal amount of CH2Cl2
followed by layered addition of hexane to give 0.171 g (70%),
orange plates. 1H NMR (400 MHz, CDCl3, δ) 10.4–10.2 (m,
2H), 8.13 (dd, J = 1.6, 0.4 Hz, 1H), 7.99 (dd, J = 8.0, 0.4 Hz,
1H), 7.97–7.90 (m, 2H), 7.69 (dd, J = 8.0, 1.6 Hz, 1H),
7.68–7.61 (m, 2H), 7.07–7.01 (m, 2H), 1.46 (s, 9H). 13C NMR
(100 MHz, CDCl3, δ) 155.7, 152.4, 151.5, 151.3, 150.1, 150.0,
138.1, 137.5, 134.8, 128.8, 127.5, 127.4, 122.1, 119.7, 119.6,
119.3, 35.6, 31.4. MS m/z (relative intensity): 583.15 (86.21%),
584.15 (100%), 585.10 (96.98%), 586.15 (48.63%), 587.10
(41.17%). UV-vis (CH2Cl2) λmax, nm (ε/104 M−1 cm−1): 250
(5.36), 278 (3.86), 352 (2.49), 472 (1.33), 486 (1.32). Elemental
analysis for C22H20N5PtCl: calcd: C 45.17, H 3.45, N 11.97;
found: C 45.16, H 3.27, N 11.65.
8.57 (d, JHH = 6.4 Hz, JPtH = 28 Hz, 2H), 8.08–8.05 (m, 2H),
7.76 (td, J = 7.6 and 2 Hz, 2H), 7.65–7.55 (m, 6H), 7.13 (t, J =
7.6 Hz, 2H), 7.03 (t, J = 7.2 Hz, 1H), 6.49 (td, J = 6.8 and 2 Hz,
2H). 13C NMR (100 MHz, CDCl3, δ) 156.9, 154.0, 153.6,
152.2, 139.0, 136.9, 136.8, 130.9, 129.3, 128.7, 123.5, 122.2,
118.7. MS m/z (relative intensity): 568.15 (75.44%), 569.10
(100%), 570.15 (95.29%), 571.20 (48.40%), 572.15 (39.80%).
UV-vis (CH2Cl2) λmax, nm (ε/104 M−1 cm−1): 249 (4.35), 289
(2.37), 352 (1.99), 472 (1.17), 497 (1.13). Elemental analysis for
C24H17N5Pt: calcd: C 50.53, H 3.00, N 12.28; found: C 50.83,
H 2.81, N 11.98.
(BPI)Pt(Ph-4-NMe2) (6)
0.100 g (0.18 mmol) (COD)Pt(Ph-4-NMe2)2 and 0.049 g
(0.165 mmol) BPI were suspended in 5 ml of methanol. To this
solution 0.030 ml (0.18 mmol) triethylamine was added and the
solution was heated to 60 °C under nitrogen for 4 days. Precipi-
tate began to form upon cooling to room temperature. The
8650 | Dalton Trans., 2012, 41, 8648–8659
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3-NO2BPI
cooling to room temperature, the solution was poured into
50 mL of water and precipitate began to form. The precipitate
was collected by filtration, washed with water and used without
further purification. 1.56 g (87%), yellow powder. 1H NMR
(250 MHz, CDCl3, δ) 8.59 (t, J = 4.75 Hz, 2H), 7.94 (d, J = 8.5
Hz, 1H), 7.80–7.70 (m, 2H), 8.23 (d, J = 2.25 Hz, 1H), 7.42
(dd, J = 8, 4.75 Hz, 2H), 7.20–7.05 (m, 3H), 4.12 (t, J = 10.8
Hz, 2H), 1.94–1.77 (m, 4H), 1.58–1.32 (m, 2H), 0.95 (t,
J = 10.8 Hz, 3H).
A solution of 0.86 g (4.79 mmol) 3-nitrophthalonitrile, 0.98 g
(10.4 mmol) 2-aminopyridine and 0.10 g (0.89 mmol) CaCl2 in
15 ml 1-butanol was refluxed under N2 for 4 days. Upon cooling
to room temperature, precipitate began to form. The precipitate
was collected by filtration and the crude product was dissolved
in ethylacetate and passed through a plug of silica. The yellow
fraction was evaporated to dryness and used without further
1
purification. 0.703 g (41%), yellow solid. H NMR (250 MHz,
CDCl3, δ) 8.92 (d, J = 2 Hz, 2H), 8.29 (d, J = 7 Hz, 1H),
7.85–7.72 (m, 4H), 7.45 (t, J = 8 Hz, 2H), 7.20–7.10 (m, 2H).
(4-OC5H11BPI)PtCl (10)
0.1 g (0.268 mmol) (COD)PtCl2 and 0.094 g (0.243 mmol)
4-OC5H11BPI were suspended in 10 ml of methanol. To this
solution 0.037 ml (0.268 mmol) triethylamine was added and
the solution was heated to 50 °C under nitrogen overnight. Upon
cooling to room temperature, precipitate began to form. The pre-
cipitate was collected by filtration and washed with water and
methanol. The orange solid was then recrystallized by dissolving
it in a minimal amount of CH2Cl2 followed by layered addition
of hexane to give 0.078 g (52%), red powder. 1H NMR
(400 MHz, CDCl3, δ) 10.3 (dd, J = 6.4, 2 Hz, 1H), 10.27 (dd,
J = 6.4, 2 Hz, 1H), 7.95–7.86 (m, 3H), 7.61–7.55 (m, 2H), 7.54
(d, J = 2.4 Hz, 1H), 7.12 (dd, J = 8.0, 2.4 Hz, 1H), 7.04–6.98
(m, 2H), 4.13 (t, J = 6.4 Hz, 2H), 1.91–1.82 (m, 2H), 1.55–1.37
(m, 4H), 0.96 (t, J = 7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3,
δ) 162.7, 152.4, 151.1, 151.0, 150.1, 150.0, 139.6, 138.0, 129.2,
127.4, 127.3, 123.7, 119.6, 119.4, 118.7, 107.0, 68.7, 28.9, 28.2,
22.4, 14.0. MS m/z (relative intensity): 613.15 (81.44%), 614.10
(100%), 615.05 (95.62%), 616.10 (47.02%), 617.10 (41.05%).
UV-vis (CH2Cl2) λmax, nm (ε/104 M−1 cm−1): 261 (4.13), 280
(3.69), 351 (1.74), 386 (1.63), 472 (0.90), 492 (0.83). Elemental
analysis for C23H22N5OPtCl: calcd: C 44.92, H 3.61, N 11.39;
found: C 45.06, H 3.49, N 10.82.
(3-NO2BPI)PtCl (8)
0.1 g (0.268 mmol) (COD)PtCl2 and 0.084 g (0.243 mmol)
3-NO2BPI were suspended in 15 ml of methanol. To this solution
0.037 ml (0.268 mmol) triethylamine was added and the solution
was heated to 50 °C under nitrogen overnight. Upon cooling to
room temperature, precipitate began to form. The precipitate was
collected by filtration and washed with water and methanol. The
orange solid was then recrystallized by dissolving hot CH2Cl2
and cooled to −40 °C overnight. The precipitate was collected
by filtration and washed with methanol to give 0.097 g (64%),
1
maroon powder. H NMR (400 MHz, CDCl3, δ) 10.33 (t, J =
6.4 Hz, 2H), 8.31 (d, J = 6.8 Hz, 1H), 7.99–7.93 (m, 2H),
7.81–7.74 (m, 2H), 7.64 (d, J = 8 Hz, 1H), 7.58 (d, J = 8 Hz,
1H), 7.13–7.06 (m, 2H). MS (MALDI-TOF): m/z = 574.11. UV-
vis (CH2Cl2) λmax, nm (ε/104 M−1 cm−1): 235 (3.23), 278
(2.75), 338 (1.53), 483 (1.05), 505 (1.05). Elemental analysis for
C18H11N6O2PtCl: calcd: C 37.67, H 1.93, N 14.64; found: C
37.44, H 1.78, N 13.70.
(4-NO2BPI)PtCl (9)
0.1 g (0.268 mmol) (COD)PtCl2 and 0.084 g (0.243 mmol)
4-NO2BPI were suspended in 20 ml of methanol. To this solu-
tion 0.037 ml (0.268 mmol) triethylamine was added and the
solution was heated to 50 °C under nitrogen overnight. Upon
cooling to room temperature, precipitate began to form. The pre-
cipitate was collected by filtration and washed with methanol.
The orange solid was then recrystallized by dissolving the solid
in hot CH2Cl2 and then cooling the solution to −40 °C over-
night. The precipitate was collected by filtration and washed
4-IBPI
A solution of 1 g (3.94 mmol) 4-iodophthalonitrile, 0.927 g
(9.85 mmol) 2-aminopyridine and 0.085 g (0.75 mmol) CaCl2
in 12 ml 1-butanol was refluxed under N2 for 2 days. Upon
cooling to room temperature precipitate began to form. The pre-
cipitate was collected by filtration, washed with water and used
without further purification. 1.42 g (85%), green powder.
1H NMR (250 MHz, CDCl3, δ) 8.63 (dd, J = 3.2, 1.2 Hz, 1H),
8.61 (dd, J = 3.2, 1.2 Hz, 1H), 8.44 (dd, J = 2.4, 1.2 Hz, 1H),
7.98 (dd, J = 12.8, 2.4 Hz, 1H), 7.84–7.73 (m, 3H), 7.45 (d,
J = 12.8 Hz, 2H), 7.17–7.10 (m, 2H).
1
with methanol to give 0.092 g (60%), maroon powder. H NMR
(400 MHz, CDCl3, δ) 10.5–10.3 (m, 2H), 8.92 (d, J = 2.0 Hz,
1H), 8.53 (dd, J = 8.0, 2.0 Hz, 1H), 8.26 (d, J = 8.0 Hz, 1H),
8.04–7.98 (m, 2H), 7.70 (t, J = 6.8 Hz, 2H). MS (MALDI-TOF):
m/z = 574.01. UV-vis (CH2Cl2) λmax, nm (ε/104 M−1 cm−1): 232
(3.23), 277 (3.41), 346 (2.14), 492 (1.09), 511 (1.08). Elemental
analysis for C18H11N6O2PtCl: calcd: C 37.67, H 1.93, N 14.64;
found: C 37.58, H 1.74, N 14.04.
(4-IBPI)PtCl (11)
0.1 g (0.268 mmol) (COD)PtCl2 and 0.103 g (0.243 mmol)
4-IBPI were suspended in 10 ml of methanol. To this solution
0.037 ml (0.268 mmol) triethylamine was added and the solution
was heated to 50 °C under nitrogen overnight. Upon cooling to
room temperature, precipitate began to form. The precipitate was
collected by filtration and washed with water and methanol. The
orange solid was then recrystallized by dissolving hot CH2Cl2
and cooling the solution to −40 °C overnight. The precipitate
4-OC5H11BPI
A solution of 1.0 g (4.67 mmol) 4-pentoxyphthalonitrile, 1.10 g
(11.68 mmol) 2-aminopyridine and 0.10 g (0.89 mmol) CaCl2 in
10 ml 1-butanol was refluxed under N2 for 11 days. Upon
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Table 1 Crystal and structure refinement parameters for complex 6,
(BPI)Pt(Ph-4-NMe2)
was collected by filtration and washed with methanol to give
0.085 g (53%), orange powder. H NMR (400 MHz, CDCl3, δ)
1
10.33 (d, J = 6.8 Hz, 2H), 8.64 (dd, J = 1.6, 0.8 Hz, 1H), 8.00
(dd, J = 8.0, 1.6 Hz, 1H), 7.98–7.93 (m, 2H), 7.83 (dd, J = 8.0,
0.8 Hz, 1H), 7.64 (d, J = 8.0 Hz, 2H), 7.07 (t, J = 6.8 Hz, 2H).
MS m/z (relative intensity): 653.00 (79.09%), 653.95 (100%),
654.95 (94.33%), 655.95 (45.88%), 657.00 (31.88%). UV-vis
(CH2Cl2) λmax, nm (ε/104 M−1 cm−1): 257 (3.80), 279 (3.57),
353 (2.04), 478 (1.00), 497 (0.97). Elemental analysis for
C18H11N5IPtCl: calcd: C 33.02, H 1.69, N 10.70; found:
C 32.79, H 1.54, N 10.06.
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
C26H22N6Pt
613.59
296(2) K
0.71073 Å
Monoclinic
C 2/c
Unit cell dimensions
a = 11.1436(7) Å
b = 23.4283(13) Å
c = 9.3583(6) Å
2214.1(2) Å3
4
α = 90°
β = 115.0110(10)°
γ = 90°
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
Crystal size
θ range for data collection
Index ranges
1.841 Mg m−3
6.365 mm−1
1192
4,5-diClBPI
0.29 × 0.17 × 0.10 mm3
2.20 to 30.53°
A solution of 1.0 g (5.1 mmol) 4,5-dichlorophthalonitrile, 1.2 g
(12.75 mmol) 2-aminopyridine and 0.12 g (1 mmol) CaCl2 in
15 ml 1-butanol was refluxed under N2 for 2 days. Upon cooling
to room temperature, precipitate began to form. The precipitate
was collected by filtration and washed with water to give 1.80 g
(96%), light green solid. 1H NMR (250 MHz, CDCl3, δ) 8.60 (d,
J = 5.0, 2H), 8.17 (s, 1H), 7.78 (td, J = 8.0, 1.5 Hz, 2H), 7.46
(d, J = 8.0 Hz, 2H), 7.14 (t, J = 5.0 Hz, 2H).
−15 ≤ h ≤ 15, −32 ≤ k ≤ 33,
−13 ≤ l ≤ 13
Reflections collected
Independent reflections
Completeness to theta =
30.53°
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
26 464
3363 [R(int) = 0.0251]
99.2%
Semi-empirical from equivalents
0.54 and 0.46
Full-matrix least-squares on F2
3363/0/153
1.143
R1 = 0.0186, wR2 = 0.0427
R1 = 0.0221, wR2 = 0.0452
1.301 and −1.003 e Å−3
(4,5-diClBPI)PtCl (12)
Largest diff. peak and hole
0.1 g (0.268 mmol) (COD)PtCl2 and 0.106 g (0.29 mmol) 4,5-
diClBPI were suspended in 10 ml of methanol. To this solution
0.043 ml (0.29 mmol) triethylamine was added and the solution
was heated to 50 °C under nitrogen overnight. Upon cooling to
room temperature, precipitate began to form. The precipitate was
collected by filtration and washed with water and methanol to
give 129 mg crude product (81%). A 40 mg sample of this crude
product was sublimed at 315 °C (∼10−4 torr) to give 26 mg (sub-
transmission was 0.815. The calculated minimum and maximum
transmission coefficients (based on crystal size) are 0.2562 and
0.5629. The structure was solved and refined using the Bruker
SHELXTL V6.12 software package.57 Additional refinement
details and the resulting factors for 6 are given in Table 1 and the
ESI.†
1
limation yield: 65%), dark orange needles. H NMR (400 MHz,
CDCl3, δ) 10.35 (dd, J = 6.4, 1.6 Hz, 2H), 8.20 (s, 2H), 7.97 (td,
J = 7.2, 1.6 Hz, 2H), 7.64 (dd, J = 8.4, 2.0 Hz, 2H), 7.09 (td,
J = 6.4, 1.6 Hz, 2H). MS m/z (relative intensity): 594.95
(53.66%), 595.95 (72.475%), 596.95 (100%), 597.95 (69.59%),
598.95 (71.85%), 599.90 (30.89%), 600.95 (26.56%). UV-vis
(CH2Cl2) λmax, nm (ε/104 M−1 cm−1): 257 (4.60), 279 (3.62),
351 (1.90), 479 (1.06), 498 (1.05). Elemental analysis for
C18H10N5PtCl3: calcd: C 36.17, H 1.69, N 11.72; found:
C 36.12, H 1.50, N 11.42.
Electrochemical and photophysical characterization
All cyclic voltammetry (CV) and differential pulse voltammetry
(DPV) were performed using an EG&G Potentiostat/Galvanostat
model 283. DMF (purchased from VWR) was used as the
solvent under inert atmosphere with 0.1 M tetra(n-butyl)
ammonium hexafluorophosphate (Aldrich) as the supporting
electrolyte or a solution of anhydrous 0.1 M NBu4ClO4 in
CH2Cl2 (VWR) as indicated. A glassy carbon rod, a platinum
wire and a silver wire were used as the working electrode, the
counter electrode, and the pseudo reference electrode, respect-
ively. CV was used to establish electrochemical reversibility
using the peak separation between the forward and reverse scans,
while all redox potentials were determined using DPV and
reported relative to a decamethylferrocenium/ferrocene or ferro-
cenium/ferrocene (Fc+/Fc) redox couple used as an internal
standard.34
X-ray crystallography
A clear, intense red prism-like specimen of 6 obtained from
CH2Cl2–MeOH was used for the X-ray crystallographic analysis.
The X-ray intensity data were measured on a Bruker APEX II
CCD system equipped with a TRIUMPH curved-crystal mono-
chromator and a MoKα fine-focus tube source (λ = 0.71073 Å).
The data collection was performed at room temperature due to a
crystal phase transition that occurred at 100 K. A total of 2520
frames were collected over a total exposure time of 14.0 hours.
The frames were integrated with the Bruker SAINT V7.68A
software package56 using a narrow-frame algorithm. Data were
corrected for absorption effects using the multi-scan method
(SADABS). The ratio of minimum to maximum apparent
The UV-visible spectra were recorded on a Hewlett-Packard
4853 diode array spectrophotometer. Steady state emission
experiments at room temperature and 77 K were performed on a
Photon Technology International QuantaMaster Model C-60SE
8652 | Dalton Trans., 2012, 41, 8648–8659
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spectrofluorimeter. Quantum efficiency measurements were
carried out using a Hamamatsu C9920 system equipped with a
xenon lamp, calibrated integrating sphere and model C10027
photonic multichannel analyzer.
Samples for transient luminescent decay measurements were
prepared in 2-MeTHF solution. All phosphorescent lifetimes
were measured by time-correlated single-photon counting using
an IBH Fluorocube instrument equipped with a 405 nm LED
excitation source.
Computational methods
All theoretical properties reported here are determined from first
principles electronic structure methods using the GAMESS
(General Atomic and Molecular Electronic Structure System)
electronic structure code.35 As previously reported, density func-
tional theory (DFT) was shown to reliably predict X-ray crystal-
lographic, electrochemical and photophysical properties of
several (BPI)PtCl derivatives including complex 1,36 thus similar
treatments are applied to complexes 2–12.
Fig. 2 ORTEP diagram of 6 from single crystal X-ray analysis with
thermal ellipsoids at the 50% probability level. Selected hydrogen atoms
omitted for clarity.
Platinum is described using the small core model core poten-
tial MCPtzp (triple zeta + polarization basis set) while chlorine
and iodine use MCPdzp (double zeta + polarization basis set).37
All remaining atoms are treated with the all-electron cc-pVDZ
basis set.38 Geometry optimizations for 1–12 are performed
with the DFT hybrid B3LYP functional.39–43 All stationary
points are confirmed as minima on the ground state potential
energy surface by calculating and diagonalizing the
Hessian (matrix of energy second derivatives). Vertical excitation
energies are computed with time dependent density functional
theory (TDDFT) using the B3LYP functional. The effects
of solvent on the geometry and vertical excitation energies
are studied using the Conductor-like Polarizable Continuum
Model (CPCM).44
the corresponding aryl platinum precursor (COD)Pt(Ph–R)2.13,47
Complexes 1–6 are soluble in common organic solvents, with
solubility decreasing along the trend 6 > 5–4 > 2–1 > 3.
The BPI substituted ligand precursors for 7–12 were syn-
thesized using the method developed by Siegl, metal-ion-cata-
lyzed nucleophilic addition of aromatic amines to dicyano
aromatics.23 Reaction progress was monitored by thin layer
chromatography (eluting with CH2Cl2) for disappearance of the
fluorescent blue dicyanobenzene starting material. The reaction
time is affected by both steric and electronic effects of the dicya-
nobenzene substituent. The addition of electron withdrawing
groups (NO2, I, Cl) at the 4- and 5-positions results in short (<2
days) reaction times, whereas electron donating groups (t-Bu,
OC5H11) at the 4-position increases reaction times (5–10 days).
The reaction rates are presumably altered due to changes in the
electron density at the site of 2-aminopyridine addition. A nitro
substituent at the 3-position (8) increases the reaction time, pre-
sumably due to steric hindrance, since starting material is still
present after 4 days. The substituted ligands were metalated
using (COD)PtCl2 to give complexes 7–12 in 52–81% yield. All
complexes are air-stable and were obtained as yellow to red
solids, except for complex 6, which would range from a red
powder to a black crystalline solid. The complexes were fully
characterized by NMR spectroscopy, mass spectrometry and
elemental analysis.
A single crystal X-ray analysis was performed on complex
6 (Fig. 2). The complex has a near ideal square planar geometry
(N1–Pt–N3 = 89.3°, N1–Pt–C10 = 90.6°, N1–Pt–N5 = 180.88°,
N3–Pt–C10 = 180.00°) with the aryl ring oriented perpendicular
to the N1–N3–N1′–C10 coordination plane (89.12°). Steric con-
gestion between the aryl ligand and the ipso-hydrogens
(C10⋯H1 = 2.276 Å) causes the pyridyl groups in the BPI to
warp 8.4° away from the coordination plane. The Pt–C10
(2.022 Å) and Pt–N1 (2.053 Å) distances are unexceptional
whereas the Pt–N3 distance (2.038 Å) is ca. 0.05 Å longer than
values reported for other (BPI)PtX (X = Cl,15,25 ethynylaryl25
and pyridyl26) derivatives due to the strong trans influence of the
p-dimethylaminophenyl group.
The
Tamm–Dancoff
approximation
to
TDDFT
(TDA-TDDFT) is used here for the purposes of conveniently
analyzing the excitation energies.45 The TDA-TDDFT provides
an analog to the simple configuration interaction with single
excitations (CIS) method. This is a crucial feature since an
excited state dominated by a single CIS configuration can be
approximately characterized using simple orbital arguments.36
Similar use of an orbital argument is justified here if the
TDA-TDDFT method predicts an excited state dominated by a
single excitation (i.e. HOMO → LUMO).
Results and discussion
Synthesis
The BPI ligand was synthesized according to literature pro-
cedures,23 followed by platination with (COD)PtCl2 to give 1.15
Complexes 2–4 were synthesized in 43–84% yield by the
addition of AgF, KAg(CN)2 and AgOAc, respectively, to the
parent complex 1. Unlike in the preparation of several [Pt(terpy)-
X]+ complexes from Pt(terpy)Cl]+, where exchange of the chlor-
ide ligand is facile,8,46 attempts to synthesize (BPI)PtX deriva-
tives 2–4 by simple nucleophilic substitution of 1 were found to
be unsuccessful. The (BPI)Pt(Ph–R) complexes 5 (R = H) and
6 (R = NMe2) were synthesized in 37 and 49% yield from
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Table 2 HOMO–LUMO energies, TDA-TDDFT and TDDFT vertical excitation energies, experimental absorption peaks, and the square of the
HOMO → LUMO TDA-TDDFT coefficient for the CPCM solvated complexes
HOMO
LUMO
TDA-TDDFT vertical
Exp. lowest energy
absorption peak (nm) xcitation energy (nm)
TDDFT vertical
Square of HOMO → LUMO
TDA-TDDFT coefficient
Complex energy (eV) energy (eV) excitation energy (nm)
1
2
3
4
5
6
7
8
−5.77
−5.76
−6.00
−5.84
−5.68
−4.83
−5.75
−5.86
−5.89
−5.72
−5.81
−5.90
−2.64
−2.63
−2.70
−2.64
−2.52
−2.48
−2.61
−2.89
−3.07
−2.54
−2.73
−2.81
478
480
430
458
462
660
481
503
524
480
487
481
487
497
465
488
497
470
486
505
511
492
497
498
489
492
444
471
475
661
491
515
537
490
498
494
0.84
0.91
0.91
0.91
0.91
0.99
0.92
0.94
0.95
0.91
0.92
0.92
9
10
11
12
Computational results
In accordance with our previous report,36 excitation energies
incorporating solvation effects using the CPCM are in better
agreement with experiment than are the gas-phase calculations.
Consequently, only CPCM-B3LYP excitation energies computed
with TDDFT and TDA-TDDFT are reported for structures 1–12
(Table 2). The platinum chloride complexes, 1 and 7–12, have
relatively small variations (489–537 nm) in the lowest energy
transitions compared to the chloride substituted complexes 2–6
(444–661 nm).
Since the squares of the HOMO → LUMO TDA-TDDFT
coefficients for 1–12 are close to unity (0.84–0.99), it is clear
that the HOMO → LUMO excitation dominates the lowest
energy transitions within the TDA-TDDFT approximation
(Table 2). In addition, the TDA-TDDFT excitation energies are
in good qualitative agreement with the trends established by
TDDFT and experiment (except for complex 6). Thus it is
reasonable to use Kohn–Sham orbitals as a means to understand
the observed trends in the lowest energy excitations for com-
plexes 1–5 and 7–12.
The HOMO and LUMO energy values for the ground state
structures of 1–12 can be seen in Table 2 and schematics of the
associated Kohn–Sham orbitals can be found in the ESI
(Fig. S1†). The HOMO of 1 is largely localized on the [(2-pyri-
dylimino)3,4-pyrrolate]PtCl portion of the molecule and the
LUMO is localized over the entire BPI ligand, with decreased
orbital character on the platinum center and chlorine atom
(Fig. 3). Similar HOMO and LUMO characteristics are seen for
all complexes reported with the exception of 6. For structure 6,
the HOMO resides on the p-(dimethylmino)phenyl moiety.
Fig. 3 HOMO (solid, left) and LUMO (transparent, right) of 1. With
carbon, chlorine, hydrogen and nitrogen indicated by black, green, white
and blue spheres respectively.
displays an irreversible oxidation (Epa = 0.79 V) and two revers-
ible reduction waves (E1/2 = −1.44 V, −1.87 V). The initial oxi-
dation and reduction potentials of a series of benzannulated
(BPI)PtCl derivatives have been shown to display good corre-
lation with both the location and the relative energies of the cal-
culated HOMO and LUMO, respectively.36 With respect to the
spatial features of the HOMO and LUMO of 1, replacing the
chloride with a different ligand would be expected to have a
larger effect on the oxidation potentials than the reduction poten-
tials, which it does. Conversely, substitution at the 3–5-positions
of the BPI ligand is expected to have a larger effect on the
reduction potentials than the oxidation potentials, as observed.
Complexes 2–5 all display a single irreversible oxidation wave
between 0.69 and 0.97 V. The most pronounced changes in oxi-
dation potential were found in complexes with cyano (3, Epa
=
0.97 V) and phenyl (5, Epa = 0.69 V) ligands. The shifts in oxi-
dation potential are due to the electron withdrawing (CN) or
donating (Ph) nature of the ligands. The addition of a
p-(dimethylamino)phenyl donor in 6 shifts the irreversible oxi-
dation peak by −0.11 V relative to 5. An additional reversible
oxidation wave (E1/2 = 0.08 V) in 6 is assigned to a process cen-
tered on the p-(dimethylamino)phenyl moiety (HOMO of 6).
The first reduction potential of 2–6 is between −1.42 to
−1.67 V. Again the most pronounced changes in reduction
potential are found in complexes with cyano (3, E1/2 = −1.42 V)
and p-(dimethylamino)phenyl (6, E1/2 = −1.67 V) ligands.
Electrochemistry
The electrochemical properties of 1–12 were investigated using
cyclic voltammetry (CV) and differential pulse voltammetry
(DPV). The results of these measurements are shown in Table 3
(all values are reported relative to the ferrocenium/ferrocene
couple). All of the complexes reported here display at least one
irreversible oxidation from 0.58 to 0.97 Vand at least two revers-
ible reduction peaks from −1.15 to −2.39 V. In good agreement
with values reported by Wen et al.,25 the parent complex 1
8654 | Dalton Trans., 2012, 41, 8648–8659
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Table 3 Electrochemical potentials for 1–12 reported in volts (V)
relative to Fc+/Fc.a Electrochemical oxidations are irreversible and
reductions are reversible, unless otherwise noted
Complex Epaox (V)b E1/2red1 (V)
E1/2red (V)
ΔE1/2 (V)c
1
2
3
4
5
6e
7
8
0.79
0.73
0.97
0.88
0.69
−1.44 (69)d
−1.51
−1.87 (70)d
−2.01
2.23
2.24
2.39
2.39
−1.42
−1.86
−1.51
−1.96
−1.62
−2.09
2.31
0.08,f 0.58 −1.67 (63)d
−2.12 (78)d
−1.90 (79)d
−1.93 (101)d,
−1.81(90)d,
−1.60 (65)d
1.75,g 2.25
2.30
0.83
0.95
−1.47 (80)d
−1.24 (66)d
2.19
9
0.93
−1.15 (102)d −2.39 (94)d,
−1.42 (135)d
2.08
10
11
0.84
0.84
−1.48 (89)d
−1.36 (62)d
−1.92 (88)d
−1.89 (100)d,
−1.75h
2.32
2.20
12
0.84
−1.30 (56)d
−1.69 (81)d
2.14
a Measurements were performed in an anhydrous solution of 0.1 M
ox
NBu4PF6 in DMF. b Peak anodic potential. c ΔE1/2 = Epa − E1/2
red1
.
d The Epa − Epc separation (in mV) are in parentheses. e Measured using
decamethylferrocene as the internal standard and corrected for −0.57 V
vs. Fc+/Fc. f Reversible oxidation. g ΔE1/2
=
E1/2
−
E1/2
.
ox1
red1
h Irreversible reduction.
The complexes with substituted BPI ligands (7–12) all display
a single irreversible oxidation, and at least two reversible
reduction waves. The irreversible oxidation potentials of 7–12
range from 0.83 to 0.95 V (Table 3). On the other hand, the first
reduction shows greater variation, ranging from −1.15 to
−1.48 V. The addition of electron donating (C(CH3)3, OC5H11)
or electron withdrawing (NO2, Cl, I) groups to 1 result in either
an anodic (7 and 10) or a cathodic shift (8, 9, 11 and 12) in the
first reduction potential (Table 3). Similar shifts in redox poten-
tials upon substitution of the ligand have been observed with
(4-R-terpy)PtCl48 and (4-R-BPI)PtCl complexes.26 Although 8
and 9 both have NO2 substituents on the BPI ligand, attachment
at the 4-position leads to a reduction potential that is ∼0.1 V
more negative than substitution at the 3-position. If one were to
consider only inductive effects, it might be expected that substi-
tution with NO2 at either position would lead to similar
reduction potentials. However, a calculated structure of 8 shows
that steric hindrance at the 3-position forces a 45° out-of-plane
rotation of the nitro group. This out-of-plane rotation reduces the
π-electron withdrawing ability of NO2 and, as a result, complex
8 is more difficult to reduce than 9.
Fig. 4 Absorption spectra of complexes 1, 3, 5 (top) and 7, 9, 10
(bottom) at room temperature in CH2Cl2.
lowest energy transition in BPI (calculated: 391 nm) is domi-
nated by transitions from the HOMO to the LUMO (84%). The
HOMO is a π orbital localized on the (2-pyridylimino)3,4-
pyrrole portion of the molecule, with minimal character on the
benzene ring, whereas the LUMO is delocalized over the entire
molecule. The lowest energy absorption in BPI is thus qualitat-
ively assigned as a π–π* transition.
The absorption spectra for complexes 1–12 were recorded in
CH2Cl2. Representative spectra are shown in Fig. 4, and the
remaining spectra are available in the ESI.† Upon platination of
the BPI derivatives, the ligand centered (LC) π–π* transitions
undergo a red shift to ∼350 nm (ε = 1.5–2.5 × 104 M−1 cm−1
)
Additional reduction waves, not present in 1, are observed for
complexes 8, 9 and 11. These additional reduction waves are pre-
sumably due to reversible reduction of the nitro groups in 8, 9
and the irreversible reductive elimination of the iodo substituent
of 11 at −1.75 V.49
while a new, low energy transition appears between 425–550 nm
(ε ≈ 1.4 × 104 M−1 cm−1). DFT calculations predict that the low
energy absorption (calculated for 1 at 489 nm) is predominately
a HOMO to LUMO transition (84%). Upon platination, the
HOMO is destabilized and now includes contributions from the
platinum and ancillary ligand portions of the molecule, whereas
the LUMO is stabilized and remains localized on the BPI ligand
with minimal metal character (Fig. 3). The nature of the frontier
orbitals suggests that the low energy absorption band is best
described as a combined metal-to-ligand/intraligand charge-
transfer (CT) transition. TDDFT calculations are in good agree-
ment with experiment, as deviations are less than 30 nm
Absorption spectroscopy
The photophysical properties of BPI, (BPI)PtCl (1) and several
(BPI)Pt(4-X-pyridine)+ complexes have recently been
described.25,26 The BPI ligand displays several absorption bands
between 300–425 nm. According to TDDFT calculations, the
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10 nm, whereas the acetate ligand causes little change in 4. The
addition of electron donating groups at the 4-position of the BPI
ligand in 7 (R = C(CH3)3) and 10 (R = OC5H11) has a minimal
effect on the absorption onset and peak positions relative to 1.
Electron withdrawing groups, on the other hand, red shift the
absorption onset according to the trend 9 (R4 = NO2) > 8 (R3 =
NO2) > 12 (R4,5 = Cl) > 11 (R4 = I). As already noted based on
the TDA-TDDFT coefficients, the lowest energy vertical exci-
tations of these complexes are dominated by a HOMO →
LUMO transition. The relatively small variability in the HOMO
of 8, 9, 11 and 12 (<0.1 eV) indicates that stabilization of the
LUMO in the absolute value order 9 (−3.07 eV) > 8 (−2.89 eV)
> 12 (−2.81 eV) > 11 (−2.73 eV) is likely responsible for the
observed trends in this absorption band.
In complex 6, with a p-(dimethylamino)phenyl ligand, the CT
band appears as a single structureless feature from 400 to
550 nm (Fig. 5a). The twenty lowest energy transitions of 6
identified by TDDFT (Table S1†) are indicated by vertical bars
in Fig. 5a with heights equal to the oscillator strength. Shown in
Fig. 5b are the valence orbitals associated with lowest CT states.
As mentioned earlier in the computation section, the HOMO for
this complex resides primarily on the p-(dimethylamino)phenyl
ligand and a low energy HOMO → LUMO transition at 661 nm
is predicted from the calculation. However, due to the minimal
orbital overlap between the HOMO and LUMO, the oscillator
strength of this transition (f = 7.3 × 10−4) is three orders of mag-
nitude lower than the oscillator strength at 476 nm (f = 0.23). On
the other hand, since the orbitals that dominate the latter tran-
sition (81% HOMO-1 → LUMO) closely resemble the HOMO
and LUMO of the parent complex (Fig. 5b), a molar absorptivity
similar to 1 is observed for this transition.
Fig. 5 (a) Absorption spectra of complex 6 at room temperature in
CH2Cl2. Calculated transitions are shown as vertical bars with heights
equal to the oscillator strength. (b) The relative intensities and orbitals
involved in the two lowest energy transitions of 6 found by TDDFT.
Emission spectroscopy
(Table 4). To confirm the nature of the absorption transitions, the
highly soluble (4-tert-butylBPI)PtCl complex (7) was examined
in solvents of varying polarity. While the energy of the LC
absorption band remains constant, the low energy CT band exhi-
bits a negative solvatochromic effect, shifting from 505 nm in
hexane to 485 nm in acetonitrile (Fig. S4†). Hypsochromic shifts
in polar solvents are attributed to a decrease in the molecular
dipole moment upon excitation.50 Since the calculated dipole
moment in the ground state (μ = 3.5 D) is oriented along the
Pt–Cl bond, and the LUMO has electron density that is shifted
away from this portion of the complex, the solvatochromic shifts
support the assigned HOMO → LUMO charge transfer descrip-
tion for this transition.
The energy of the CT band in complexes 1–12 is altered by
both the type of anionic ligand (X) and substituents (R) on the
BPI ligand. Relative to 1, the greatest perturbation is observed in
3 (X = CN) as the CT band energy is blue shifted by ∼50 nm
and becomes highly structured with a vibronic spacing of
∼1400 cm−1 (Fig. 4(top)). The blue shift is primarily due to a
0.23 eV stabilization of the HOMO by the cyano ligand that
diminishes the metal character of the CT band and shifts the
Franck–Condon factors in favor of the 0–0 transition. Other
ligands have a smaller effect on the energy of the CT band. Flu-
oride (2) and phenyl (5) ligands red shift the transition by
Complexes 1–5 and 7–12 display broad emission spectra at
room temperature in CH2Cl2 with λmax ranging from 594 to
680 nm. Spectra for 1, 3, 5, 7, 9 and 10 are shown in Fig. 6; data
for the emissive properties of complexes 1–12 are listed in
Table 4. Emission spectra for 2, 4, 8, 11 and 12 recorded at room
temperature and for complexes 1–12 at low temperature (77 K)
in glassy solvent (2-MeTHF) are given in the ESI (Fig. S7†).
Spectra recorded at 77 K undergo rigidochromic blue-shifts of
30–50 nm and display a structured manifold with a vibronic pro-
gression of ∼1500 cm−1. The luminescent decay lifetimes at
77 K are in the microsecond regime, and therefore assigned to
emission from a metal-perturbed, ligand-centered triplet state.
The emission energies for complexes 1–5 and 7–12 at room
temperature follow the same general trends as found in the CT
absorption bands. Relative to 1 (λmax = 641 nm), emission is
blue-shifted by the electron withdrawing cyano ligand in 3 (λmax
= 594 nm) and red-shifted by the electron donating phenyl
ligand in 5 (λmax = 656 nm). Conversely, addition of electron
donating groups at the 4-position of the BPI ligand leads to a
small blue shift in 7 (λmax = 638 nm) and 10 (λmax = 635 nm),
whereas a red shift in emission maxima is observed for deriva-
tives substituted with electron withdrawing groups. The red shift
follows a trend 9 (680 nm) > 8 (678 nm) > 12 (657 nm) > 11
8656 | Dalton Trans., 2012, 41, 8648–8659
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Table 4 TDDFT vertical excitation energies and photophysical properties of complexes 1–12
Emission at
Emission at rtc
77 Kd
TDDFT vertical
λmax
(nm)
τ
kr
knr
λmax
(nm)
τ
Absorbance λ (nm) (ε, ×104 M−1 cm−1 a
)
excitation energy (nm)b
(μs)
ΦPL
(104 s−1 e
)
(105 s−1 f
)
(μs)
1
2
3
4
5
250(5.37), 277(3.51), 348(2.24),
473(1.36), 487(1.35)
250(4.20), 277(2.61), 349(1.88),
387(0.87), 476(1.00), 497(0.91)
245(4.46), 268(2.21), 342(2.24),
437(1.69), 465(1.96)
249(4.86), 278(2.93), 349(2.21),
464(1.34), 488(1.31)
249(4.35), 289(2.37), 352(1.99),
472(1.17), 497(1.13)
252(4.91), 287(2.87), 342(1.78), 470(0.94)
250(5.36), 278(3.86), 352(2.49),
472(1.33), 486(1.32)
235(3.23), 278(2.75), 338(1.53),
483(1.05), 505(1.05)
232(3.23), 277(3.41), 346(2.14),
492(1.09), 511(1.08)
489(2)
641
620
594
620
0.99 0.014
0.84 0.011
1.4
10
12
1.1
17
595
592
567
584
603
6.8
6.6
35
492(5)
1.3
444(21)
471(17)
475(22)
8.4
0.60 0.011
0.1 0.004
0.048
0.57
1.8
11.5
3.5
656
[615]
—
638
[610]
678
4.0
100
[3.8]
—
8.9
[4.1]
31
[2.6] [0.018] [0.69]
6
7
476(6)g
491(5)
—
1.1
—
0.018
—
1.6
594
593
7.6
7.3
[2.4] [0.029] [1.2]
8
9
515(10)
537(26)
490(2)
498(1)
494(4)
0.32 0.003
0.30 0.004
0.93 0.014
0.73 0.01
0.63 0.007
0.90
1.3
1.5
1.4
1.1
625
632
592
607
615
2.9
2.4
6.9
5.2
3.5
680
635
650
657
33
11
14
16
10 261(4.13), 280(3.69), 351(1.74),
386(1.63), 472(0.90), 492(0.83)
11 257(3.80), 279(3.57), 353(2.04),
478(1.00), 497(0.97)
12 257(4.60), 279(3.62), 351(1.90),
479(1.06), 498(1.05)
a In CH2Cl2. b The absolute difference between calculated HOMO → LUMO and experimental transitions, reported in nm, is given in parentheses.
c In toluene deaerated with N2. d In 2-MeTHF. e kr = Φ/τ. f knr = (1 − Φ)/τ. 2% complex by weight doped in [PMMA]. g HOMO-1 → LUMO transition.
(650 nm) that is similar to the observed variation in ΔE1/2
values.
dynamic bimolecular interactions do not contribute to nonradia-
tive decay. On the other hand, variations in solvent polarity,
while only having a minimal effect on the emission energies,
significantly alter the quantum efficiency (Table 5). The highest
The luminescent quantum yields (Φ) measured at room temp-
erature for the complexes vary between 0.003–0.048, with 6
being non-emissive. The non-emissive nature of 6 at room temp-
erature may be due to the formation of a transient charge separ-
ated state, involving a ligand-to-ligand electron transfer from the
p-(dimethylamino)phenyl moiety to the electron deficient BPI
portion of the complex. On the basis of the electrochemical
potentials, the free energy for the formation of such a charge-
separated species in fluid solution (Table 3, ΔE1/2 = 1.75 eV,
709 nm) is favored over emission (E0–0 = 2.09 eV, 594 nm at
77 K). The other complexes have radiative rate constants (kr) that
vary between 0.57–4.0 × 104 s−1, with most being near 1.5 ×
104 s−1. The ancillary ligand exerts the greatest effect on kr since
the complexes with the lowest and highest values are those with
electron withdrawing cyano (3, kr = 0.47 × 104 s−1) or electron
donating phenyl (5, kr = 4.0 × 104 s−1) ligands. The difference
most likely reflects a respective lower and higher degree of metal
character in the triplet state. However, since the overall values of
kr deviate by less than an order of magnitude, the variations in
photoluminescent efficiency of 1–5 and 7–12 are largely due to
quantum yields are observed in non-polar solvents (Φhexane
=
0.022, Φcyclohexane = 0.018), gradually decrease with increased
solvent polarity (Φ2-MeTHF = 0.011, Φacetone = 0.010, Φacetonitrile
= 0.006) until finally, there is no observed emission in the most
polar solvent, DMSO. The decrease in the quantum efficiency of
7 in coordinating solvents, (2-MeTHF and CH3CN) relative to
hydrocarbon solvents (hexane and toluene) suggests that for-
mation of a 7–solvent exciplex is a contributing process for non-
radiative decay.51–54 However, the low photoluminescent (PL)
efficiency in alkane solvents indicates that exciplex formation is
not the predominant deactivation pathway. Moreover, when 7 is
doped in PMMA (2%, w/w) the PL efficiency (Φ = 0.029) and
excited state lifetime (τ = 2.4 μs) only double relative to fluid
solution (Table 3). Since the radiative rate constant of 7 is nearly
invariant in solid matrix (kr(PMMA) = 1.2 × 104 s−1) and solution
(kr(toluene) = 1.6 × 104 s−1), the increased efficiency in PMMA is
due to a decrease in the rate of nonradiative decay (knr(PMMA)
=
4.1 × 105 s−1, knr(toluene) = 8.9 × 105 s−1). Finally, temperature-
dependent decay mechanisms are typically associated with
thermal population of a ligand field (d–d) type of excited
state. However, the lifetime of the complex in 2-MeTHF at 77 K
(τ = 7.3 μs) is only three times longer than the value in
PMMA at room temperature. Consequently, the estimated
quantum yield at 77 K is still far below unity, which implies
that nonradiative decay of the excited state is not a thermally
activated process.
much greater variations in the nonradiative rate constants (knr
=
1.1–100 × 105 s−1).
To better understand the nonradiative decay channels of the
(BPI)PtX complexes, the highly soluble tert-butyl substituted
derivative 7 was investigated with respect to concentration,
solvent media and temperature. The quantum yield of 7 in
toluene remains constant (Φ = 0.018) over concentrations
ranging from 3 to 80 μM, and indicates that excimer-type
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Fig. 7 Plot of emission energy vs. ln(knr) for 1–12 in toluene. 5′ and 7′
are ln(knr) values in PMMA (open symbols).
monodentate anionic ligands results in different vibrational coup-
ling paths from the excited state to the ground state. For
example, the role played by molecular distortion is evidenced in
the pronounced change in the nonradiative rate constant of 5
depending on the solvent viscosity, decreasing from 1.0 ×
107 s−1 in toluene to 3.8 × 105 s−1 in PMMA (5′ in Fig. 7). The
large decrease in knr for this species (and to a lesser extent in 7)
in rigid media suggests a large amplitude vibrational mode may
be responsible for the higher knr value in fluid solution. In par-
ticular, various degrees of out-of-plane deformations of the BPI
ligand are found in all of the reported X-ray structures of (BPI)Pt
derivatives.15,25,26 These distortions, similar to those observed in
the X-ray structure of 6, are caused by steric repulsion with the
monodentate ligand and are likely present in 5 as well. Libration
of the phenyl ring around the Pt–Ph bond axis would further
exacerbate the warp in the BPI ligand and thus, enhance the rate
of surface crossing back to the ground state.
Fig. 6 Emission spectra of complexes 1, 3, 5 (top) and 7, 9, 10
(bottom) at room temperature in CH2Cl2.
Table 5 Emission properties of
7 in various solvents at room
temperature
Solvent
Hexane
λmax (nm) ΦPL
τ (μs) kr (104 s−1
)
knr (105 s−1
)
Conclusion
632
0.022 1.1
0.018 0.98
2.1
9.1
10
Cyclohexane 635
1.8
1.6
1.6
1.6
1.4
1.3
1.4
1.1
0.94
In this report we describe the synthesis and the photophysical
and electrochemical characterization of a series of platinum(II)
complexes of the form (N^N^N)PtX where N^N^N is an assort-
ment of substituted 1,3-bis(2-pyridylimino)isoindolate ligands
and X represents various anionic monodentate ligands. Relative
to the parent (BPI)PtCl, variation in X and substitution at the
BPI ligand are found to have the most pronounced effect on the
oxidation and reduction potentials, respectively. The shift in
potential is dependent on the electron donating or withdrawing
nature of the substituent. The photophysical properties of the
complexes are also dictated by the nature of the substituent. The
most blue- and red-shifted absorption–emission spectra are
found for the (BPI)PtCN and the (4-NO2BPI)PtCl complexes
respectively.
Toluene
Benzene
Heptane
CH2Cl2
Diethyl ether 634
2-MeTHF
Acetone
638
639
630
636
0.018 1.1
0.017 1.1
0.017 1.1
0.015 1.1
0.014 1.1
0.011 0.77
0.010 0.92
0.006 0.64
8.9
8.9
8.9
9.0
9.4
13
637
639
638
11
16
Acetonitrile
The energy gap law (EGL) states that the nonradiative decay
rate will decrease exponentially with increasing energy of emis-
sion for molecules with similar vibrational coupling between the
excited and ground states.55 The plot of the natural log of the
nonradiative rate constant versus emission energy (Eem) for 1–12
can be seen in Fig. 7. Complexes 1 and 7–12, all with a Pt–Cl
bond, exhibit a linear relationship with a slope of −1.2 and
y-intercept of 33.2. However, (BPI)Pt–X complexes 1–5 do not
follow the same linear relationship found for the Pt–Cl deriva-
tives. These deviations imply that substitution of the
As with our previous report, molecular orbital based argu-
ments are used to rationalize the experimental observations. The
use of molecular orbitals for this purpose is valid since the
square of the HOMO → LUMO TDA-TDDFT coefficients are
close to unity (0.84–0.99) and the TDA-TDDFT excitation
8658 | Dalton Trans., 2012, 41, 8648–8659
This journal is © The Royal Society of Chemistry 2012

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Acknowledgements
K. H., L. G., N. P., P. I. D. and M. E. T. thank Universal Display
Corporation and the Department of Energy for their financial
support of this work. L. R. and M. S. G. are grateful for the
support of the Air Force Office of Scientific Research. We would
like to thank Dr Ralf Haiges for assistance with the X-ray struc-
ture analysis. The X-ray diffractometer was purchased with
support from NSF CRIF grant 1048807.
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Dalton Trans., 2012, 41, 8648–8659 | 8659