Photophysical properties of near-infrared phosphorescent π-extended platinum porphyrins

Article abstract of DOI:10.1021/cm202241e

A comprehensive photophysical study is reported on a family of π-extended platinum(II) porphyrin complexes. The platinum(II) complexes are synthesized from the corresponding free base porphyrins by treatment with platinum(II) acetate in hot benzonitrile, affording the complexes in considerably higher yield than by reaction with platinum(II) chloride. A quantitative study of the absorption and luminescence properties of the metalloporphyrins is presented. A series including tetraarylbenzo-, tetraarylnaphtho-, and tetraarylanthroporphyrin exhibits efficient phosphorescence at 773, 890, and 1020 nm in the near-infrared region, with quantum yields of 0.35, 0.15, and 0.08, respectively. The triplet lifetimes and phosphorescence yields decrease with increasing emission wavelength, consistent with energy gap law behavior. A set of six Pt-tetrabenzoporphyrins (TBPs) with different meso-substituents were examined. The Pt-TBPs exhibit efficient phosphorescence with λ_max ～ 770 nm and with a quantum yield ranging from 0.26-0.49, depending on the substitution pattern. The results show that the 5,15-diarylbenzoporphyrins feature 50-60% higher phosphorescence emission yield compared to the 5,10,15,20-tetraarylbenzoporphyrins. The highest phosphorescence quantum efficiency is observed for a platinum(II) 5,15-diarylbenzoporphyin which emits at 770 nm with a quantum yield of 49%.

Full text of DOI:10.1021/cm202241e

Article
pubs.acs.org/cm
Photophysical Properties of Near-Infrared Phosphorescent
π-Extended Platinum Porphyrins
Jonathan R. Sommer, Abigail H. Shelton, Anand Parthasarathy, Ion Ghiviriga, John R. Reynolds,
and Kirk S. Schanze*
Department of Chemistry and Center for Macromolecular Science and Engineering, University of Florida, Gainesville,
Florida 32611-7200, United States
S
* Supporting Information
ABSTRACT: A comprehensive photophysical study is reported on
a family of π-extended platinum(II) porphyrin complexes. The
platinum(II) complexes are synthesized from the corresponding free
base porphyrins by treatment with platinum(II) acetate in hot
benzonitrile, affording the complexes in considerably higher yield
than by reaction with platinum(II) chloride. A quantitative study of
the absorption and luminescence properties of the metalloporphyrins
is presented. A series including tetraarylbenzo-, tetraarylnaphtho-, and
tetraarylanthroporphyrin exhibits efficient phosphorescence at 773,
890, and 1020 nm in the near-infrared region, with quantum yields
of 0.35, 0.15, and 0.08, respectively. The triplet lifetimes and
phosphorescence yields decrease with increasing emission wave-
length, consistent with energy gap law behavior. A set of six
Pt-tetrabenzoporphyrins (TBPs) with different meso-substituents were examined. The Pt-TBPs exhibit efficient
phosphorescence with λmax ∼ 770 nm and with a quantum yield ranging from 0.26−0.49, depending on the substitution
pattern. The results show that the 5,15-diarylbenzoporphyrins feature 50−60% higher phosphorescence emission yield compared
to the 5,10,15,20-tetraarylbenzoporphyrins. The highest phosphorescence quantum efficiency is observed for a platinum(II)
5,15-diarylbenzoporphyin which emits at 770 nm with a quantum yield of 49%.
KEYWORDS: platinum porphyrin, benzoporphyrin, π-conjugated, near-infrared, phosphorescence
One area of growing interest concerns the synthesis and
properties of platinum(II) complexes of π-extended porphyrins.
In general, platinum(II) porphyrins have proven to be
important chromophores for a variety of applications, mainly
because of their propensity to exhibit high quantum yield and
long-lived phosphorescence. Thus, documenting the properties
of platinum(II) complexes of π-extended porphyrins is
important, as these chromophores are expected to feature
large absorptivity in the near-IR region, and exhibit high
quantum efficiency near-IR phosphorescence. Indeed, recent
work by Thompson and co-workers demonstrates that a
platinum(II) tetrabenzoporphyrin (TBP) complex features
efficient near-IR phosphorescence at ∼770 nm, and it has
been used as a phosphor in highly efficient near-IR emitting
organic light emitting diodes.16 Moreover, we recently reported
light emitting diodes based on a platinum(II) tetranaphtho-
porphyrin that emit at ∼900 nm.21
INTRODUCTION
■
Porphyrins with aromatic rings fused to the β-positions of the
pyrrole residues are referred to as π-extended porphyrins,
and they have attracted significant attention in recent years as
optoelectronic materials.1−5 The increased conjugation af-
forded by the fused rings to the porphyrin macrocycle leads
to enhanced light absorption and efficient emission in the near-
infrared (near-IR) region of the spectrum. In particular, the
optical properties of these materials in the near-IR draw interest
for use in numerous applications ranging from biomedical
sensing and imaging to electrooptics.2−6 Consequently
π-extended porphyrins have been desired synthetic targets
dating back to when initial work on the effects of π-extension
on the optical properties of porphyrins was carried out in the
1960s.7−9 Until recently, applications of π-extended porphyrins
were hindered because of the difficulty of synthesizing the
free-base porphyrins in sufficient yields. However, the large
potential of these materials in a broad range of applications has
fueled recent progress toward synthesis of the free-base
porphyrins, along with a few metal complexes.10−14 The
photophysical properties of some π-extended porphyrins have
also been reported,2,3,14−20 although much work still remains
for this interesting class of molecular chromophores.
Herein, we report a detailed investigation that characterizes
the photophysical properties of the family of platinum(II)
π-extended porphyrins shown in Figure 1. The study is broken
Received: August 1, 2011
Revised: November 2, 2011
Published: November 3, 2011
© 2011 American Chemical Society
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Figure 1. Structures of free-base and platinum π-extended porphyrins.
down into two sets of chromophores. In the first set (Series-1)
we explore the effects of extending the π-conjugation on the
phosphorescence energy, lifetime, and quantum yield of the
porphyrin chromophores. An energy gap law effect is observed
across this series, which gives rise to a decrease in the emission
quantum yield with decreasing phosphorescence energy. In the
second set (Series 2) we explore the effect of meso-substitution
on a family of platinum(II) tetrabenzoporphyrins. Each of the
TBP complexes exhibits intense phosphorescence at ∼770 nm,
and the quantum yield and lifetime of the emission increase
substantially for the 5,15-disubstituted chromophores relative
to those with 5,10,15,20-tetraryl substitution. Although there
have been a number of previous studies of free-base and
metallo-substituted π-extended porphyrins,1,3,15,16,21 this is the
first comprehensive report which compares the properties of
a broad family of platinum(II) complexes. A companion paper
describes the application of this series of Pt(II) expanded
conjugation porphyrins in organic- and polymer light emitting
diodes.22
in this work are shown in Figure 1 (see Supporting Information
for details on the synthesis of the starting materials and the
free-base porphyrins). The free-base porphyrins were prepared
via the recently developed dihydroisoindole method.2,14,23,24
To date only 4 of the 10 free-base porphyrins in Figure 1 have
been previously reported (H2TPTBP, H2DPTBP, H2Ar2TBP,
and H2TPTNP).14,23,24 While derivatives and some metal
complexes have been reported for two of the other free-base
porphyrins (i.e., H2Ar4TNP(OMe)8 and H2Ar4TAP), there has
not been a previous study which has systematically explored the
relationship between structure and photophysical properties
for this important family of molecular chromophores.14,18 In
particular, most of the known photophysical data for the free-
base π-extended porphyrins in Figure 1 is focused on the
tetrabenzoporphyrin derivatives.17,23,24 Metalloporphyrins have
been reported for specific examples from both Series 1 and 2 in
Figure 1, with Zn(II) and Pd(II) complexes being the best
explored systems.2,14,17,18 A few literature reports have also
described a limited set of photophysical data for free-base and
metallo-tetrabenzoporphyrins and tetranaphthoporphyrins.15,17
However, to date platinum(II) complexes of π-extended
porphyrins have only been reported for a few tetrabenzopor-
phyrin derivatives.1,3,15,16
RESULTS AND DISCUSSION
■
Structures and Synthesis. The structures of the free-base
and platinum(II) complexes of π-extended porphyrins studied
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The classical approach used to prepare Pt(II) porphyrins
involves reacting the free base porphyrin with excess (molar
equivalents) PtCl2 in refluxing benzonitrile (>190 °C). It is
believed that benzonitrile and PtCl2 react to form a Pt(II)-
benzonitrile complex that enhances the solubility of the
platinum reagent in the reaction medium. However, despite
the increase in solubility, metalation often proceeds slowly;
therefore, long reaction times are often required (24 h is
typical). Complexation of the metal to the porphyrin center
increases the symmetry of the porphyrin chromophore, leading
to significant shifts in the predominant absorption bands (Soret
and Q-bands). Therefore, the metalation reaction is often
followed by UV−visible absorption spectroscopy; specifically,
the reaction progress is monitored by the changes in the Soret
and Q-bands characteristic of the free-base porphyrin reactant
and the metalloporphyrin product.
Thompson and co-workers recently reported preparation of
Pt-TPTBP via reaction of a free-base tetrahydrobenzoporphyrin
precursor with PtCl2 in refluxing benzonitrile followed by
oxidation of the metalated porphyrin with DDQ to form the
TBP ring.16 This overall two-step process afforded Pt-TPTBP
in a relatively low yield (30%). In the course of the present
investigation, the initial reaction used to prepare Pt-TPTNP
(Scheme 1) by reacting H2TPTNP with PtCl2 in hot
formation of Pt-TPTNP (<30 min reaction time, Supporting
Information, Figure S-1). After approximately 5 h the absorp-
tion spectrum reveals that the reaction is essentially complete;
the Soret and Q-bands of H2TPTNP have completely
disappeared in favor of the absorptions of the metalated
product. This reaction affords a method for preparation of
π-extended platinum(II) porphyrins at lower temperatures
using shorter reaction times, resulting in improved yields
compared to the traditional route based on PtCl2. Metalation
with platinum(II) acetate has allowed us to prepare platinum-
(II) complexes of the tetranaphthoporphyrin and tetraanthryl-
porphyrin chromophores for the first time, while also providing
improved yields in the synthesis of previously reported
platinum tetrabenzoporphyrins. In this work the target
π-extended platinum(II) porphyrin complexes (Figure 1)
were all prepared by reacting the free-base with platinum(II)
acetate in a manner similar to that used to prepare Pt-TPTNP
(Scheme 1). The materials were subjected to vacuum
distillation to remove the high boiling solvent followed by
chromatography and multiple precipitations to remove trace
amounts of free-base starting material prior to characterization
1
by H and 13C NMR and HRMS.
Photophysics of Free-Base π-Extended Porphyrins
(Series 1). Initially, we describe the effect of increased
π-conjugation on the photophysical properties of the free-
base benzoporphyrins in Series 1; then we will turn to the
platinum(II) complexes. The absorption and fluorescence
spectra obtained in toluene solution for the Series 1 free-base
porphyrins are shown in Figure 2, and Table 1 collects
Scheme 1. Synthetic Scheme for π-Extended Pt(II)
Porphyrins
benzonitrile (200 °C) was monitored by UV−visible absorption
spectroscopy (Supporting Information, Figure S-1). The reaction
was followed for 5 h, and, during this period, the metalated
Pt-TPTNP was not observed in the absorption spectrum. A
small amount of Pt-TPTNP could be detected in the UV−visible
absorption spectrum after the reaction mixture was heated to
higher temperature (>230 °C); however, these conditions are
impractical because of decomposition of H2TPTNP.
A careful literature survey suggested that metal(II) acetate
complexes may undergo reaction under milder conditions with
free base porphyrins;25 thus, we decided to attempt to use
platinum(II) acetate as a reagent to metalate the expanded
conjugation porphyrins. Platinum(II) acetate is currently not
commercially available, but it is readily prepared by reacting
PtCl2 with silver acetate in refluxing acetic acid under an inert
atmosphere.26 Platinum(II) acetate provides two main
advantages over other platinum(II) reagents with respect to
porphyrin metalation in that it is highly soluble in good
solvents for porphyrins (CH2Cl2 and CHCl3), and it has a
higher reactivity compared to the halogen salts. UV−visible
absorption spectroscopy shows that reaction of platinum(II)
acetate with H2TPTNP in benzonitrile (180 °C) leads to rapid
Figure 2. Absorption (black solid lines) and fluorescence (red dashed
lines) of Series 1 free-base π-extended porphyrins in air saturated
toluene. (A) H2TPTBP, (B) H2TPTNP, (C) H2Ar4TNP(OMe)8, and
(D) H2Ar4TAP.
pertinent data including the absorption and fluorescence
wavelength maxima, the absorption coefficients, fluorescence
quantum yields (ϕfl), and fluorescence lifetimes (τfl).
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a
Table 1. Photophysical Properties of Series 1 Free-Base π-Extended Porphyrins
free-base porphyrins
H2TPTBP
absorption λmax / nm Soret, Q-band (log εmax/M−1 cm−1
)
fluorescence λmax / nm
ϕfl (τfl / ns)
kr / s−1 (knr / s−1
)
465 (5.48)
633 (4.52)
500 (5.35)
728 (5.02)
502 (5.42)
730 (5.07)
510 (5.32)
824 (5.29)
704, 785
0.031 ± 0.01
(3.1)
9.9 × 106
(3.1 × 108)
3.7 × 107
H2TPTNP
756, 840
755, 837
841, 932
0.14 ± 0.02
(3.9)
(2.2 × 108)
3.8 × 107
H2Ar4TNP(OMe)8
H2Ar4TAP
0.17 ± 0.01
(4.6)
(1.8 × 108)
5.1 × 107
0.051 ± 0.05
(1.0)
(9.5 × 108)
a
Measured in air-saturated toluene solutions. Fluorescence quantum yields were measured relative to ZnTPP in toluene (ϕfl = 0.03).27−29
Fluorescence decays obtained by time-correlated single photon counting, and the decays followed single exponential kinetics.
The addition of fused-aromatic rings to the pyrrole units of
the porphyrin macrocycle across the series from H2TPTBP to
H2Ar4TAP gives rise to an approximately 200 nm red shift of
the Q-band absorption. This results in the transition shifting
from the red region of the visible (H2TPTBP) fully into the
near-IR region of the spectrum (H2Ar4TAP). The Q-band
oscillator strength increases substantially across the series as
evidenced by the nearly 10-fold increase in molar absorptivity
constant from H2TPTBP to H2Ar4TAP (ε = 3.32 × 104−1.95 ×
105 M−1 cm−1). However the Soret band is only red-shifted
50 nm across the series from H2TPTBP to H2Ar4TAP. The
Soret transition is strongly allowed and remains in the
midvisible region (ε ∼(2−3) × 105 M−1 cm−1). Interestingly,
the absorption of anthroporphyrin H2Ar4TAP features a
transition in the near-UV region (340−400 nm) which exhibits
a well-defined vibronic progression. This transition is similar to
that of the lowest energy absorption of anthracene, and it is
thus likely that in H2Ar4TAP the absorption feature arises from
transitions localized on the fused anthryl units.
Photophysics of Platinum(II) π-Extended Porphyrins
(Series-1). The photoluminescence spectra from Series 1
π-extended platinum porphyrins are shown in Figure 3, and
The photoluminescence of the Series 1 free-base porphyrins
appears as a single fluorescence band (split into vibronic sub-
bands) that exhibits a small Stokes shift relative to the lowest
energy Q-band (Figure 2). Across the series H2TPTBP to
H2Ar4TAP the fluorescence maxima red-shift by approximately
140 nm. The quantum yield for H2TPTBP in toluene (∼0.03)
is in good agreement with the reported literature value.17
Interestingly, ϕfl is substantially larger for the naphthoporphy-
rins (H2TPTNP and H2Ar4TNP(OMe)8). This trend results
from a significant increase in the radiative decay rate constant
(kr, Table 1); the increased radiative decay rate is consistent
with the increase in the “allowedness” of the S1−So transition
induced by the π-extended arylene ring systems.
Ono et al. have reported the fluorescence spectra for two
fluorescent zinc(II) tetraanthroporphyrins; however, photo-
physical characterization of a free-base tetraanthroporphyrin
has not been reported previously.18 The fluorescence of
H2Ar4TAP is dominated by a band with λmax = 841 nm with
a vibronic shoulder at 932 nm. Interestingly, this compound
exhibits broad, weak fluorescence throughout the visible region;
this emission is believed to arise from Sn-So radiative decay
arising from the porphyrin ring- and anthryl-based chromo-
phores. Despite having a larger radiative decay rate, the
fluorescence yield for H2Ar4TAP (0.051) is significantly less
than that for the tetranaphthoporphyins. This anomaly arises
because the nonradiative decay (knr) rate is substantially larger
for the anthroporphyrin. Nonetheless, the fluorescence
quantum yield for H2Ar4TAP is sufficiently large to make this
system a notable near-IR fluorescent chromophore.
Figure 3. Absorption (black solid lines) and phosphorescence (red
dashed lines) of Series 1 π-extended platinum porphyrins in toluene.
(A) Pt-TPTBP, (B) Pt-TPTNP, (C) Pt-Ar4TNP(OMe)8, and (D) Pt-
Ar4TAP (Note Y-axis scale is different).
photophysical parameters are summarized in Table 2. The
emission spectra are dominated by a single phosphorescence
band with a weak vibronic shoulder. No fluorescence is seen in
the spectra, indicating that the rate of singlet−triplet
intersystem crossing is rapid, as promoted by the strong
spin−orbit coupling imparted by platinum. In addition, the
efficient phosphorescence seen for all of the Series 1
platinum(II) complexes (Table 2) is also the result of the
strong platinum induced spin−orbit coupling which promotes
radiative decay from the lowest triplet state (T1). For each of
these complexes, the phosphorescence band is shifted
significantly to the red from the lowest Q-band absorption.
The singlet−triplet splitting value (EST, Table 2) for each
complex was computed based on the energy difference between
the fluorescence (of the free-base) and the phosphorescence,
and the splitting is relatively constant at ∼0.3 eV across the
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a
Table 2. Photophysical Data for Series 1 π-Extended Platinum(II) Porphyrins
b
c
platinum porphyrins
Pt-TPTBP
absorption λmax / nm Soret, Q-band (log εmax/M−1 cm−1
)
emission λmax / nm
ϕp (τT / μs)
kr / s−1 (knr / s−1
)
EST / eV
430 (5.28)
612 (5.13)
436 (4.96)
689 (5.17)
455 (4.92)
690 (5.12)
455 (4.47)
773
0.35 ± 0.05
(29.9)
1.2 × 104
0.33
(2.2 × 104)
1.2 × 104
Pt-TPTNP
891
0.15 ± 0.001
(12.7)
0.34
0.32
0.35
(6.7 × 104)
7.8 × 103
Pt-Ar4TNP(OMe)8
Pt-Ar4TAP
883
0.12 ± 0.005
(15.3)
(5.8 × 104)
2.6 × 104
1022
0.08 ± 0.02
(3.2)
762 (4.66)
(2.9 × 105)
a
b
Measured in vacuum degassed toluene solutions. Emission quantum yields measured relative to ZnTPP (ϕfl = 0.03),27−29 with the exception of
c
Pt-Ar4TAP which was measured relative to H2TPTBP (ϕfl = 0.031) with excitation at 420 nm. Triplet lifetimes (τT) were determined by transient
absorption spectroscopy.
series. Importantly, the phosphorescence emission for all of the
Series 1 porphyrins is well into the near-IR region, ranging from
∼773 nm for the benzoporphyrin to ∼1020 nm for the
anthroporphyrin, making these complexes significant from the
standpoint of near-IR phosphor applications.
(Figure 1). This series was designed to examine the effect of
meso substitution as well as the use of bulky substitutents on the
benzoporphyrin periphery to enhance solubility and minimize
the tendency of the macrocycle to aggregate. The latter effect
may be especially important in organic- and polymer-LED
application where the porphyrin chromophores are present at
high concentration in the solid state.33,34 The ultimate goal of
this work was to optimize the phosphorescence yield for the
Pt(II)-benzoporphyrin system, which would then advance this
system for use in high efficiency near-IR OLED and PLED
application.
Our work on the Series 2 benzoporphyrins was inspired by a
recent report by Vinogradov and co-workers which systemati-
cally examined the relationship between porphyrin substitution
and photophysics for a series of free-base and Pd(II)-
benzoporphyrins.17 Especially pertinent to the present study
was their observation that the lifetime and phosphorescence
quantum yield for 5,15-diphenyl meso-substituted porphyrins
Pd-DPTBP is nearly double that for corresponding 5,10,15,20-
tetraphenyl substituted complex, Pd-TPTBP (0.15 vs 0.08).
The lower emission yield for the tetraphenyl derivative arises
because this porphyrin is more distorted from planarity
(saddled conformation) because of steric interactions induced
by the meso substituents. The decrease in planarity increases the
nonradiative decay rate, shortening the triplet lifetime and
phosphorescence yield.17 Other reports have also shown that
bulky substituents that either block meso-aryl rotation or prevent
aggregation in solution lead to an increase in the emission
quantum efficiency,33,34 and this work was considered
specifically as we developed structures Pt-ArF4TBP and Pt-
TAr2TBP which contain fluorene or t-butylphenyl substituents
on the meso-phenyl groups.
The photophysical properties for the Series 2 free-base
benzoporphyrins are identical to the literature reports, and our
data is summarized in the Supporting Information.17 The
absorption and photoluminescence obtained in deoxygenated
toluene solutions for selected Series 2 platinum benzoporphy-
rins are shown in Figure 4, while photophysical data for the
entire Series 2 of platinum complexes is listed in Table 3. In
each case, the absorption spectra for the Series 2 platinum
benzoporphyrins are blue-shifted relative to the free-base
macrocycles, consistent with porphyrins having local D4h
symmetry (metal center).
Comparison of the absorption spectral data in Table 3
and Figure 4 reveals that the tetraryl- and diaryl- complexes
show slightly different properties. In particular, comparison
of Figures 4a and 4b (Pt-Ar4TBP and Pt-ArF4TBP) with
Figures 4c and 4d (Pt-TAr2TBP and Pt-Ar2OPrTBP) reveals
Phosphorescence quantum yields (ϕp) were measured for
the Series 1 Pt-porphyrins, and the values are listed in Table 2.
The phosphorescence yields are comparatively large, ranging
from 0.35 for Pt-TPTBP to 0.08 for Pt-Ar4TAP. The triplet
lifetimes (τT) were determined by transient absorption spectro-
scopy, and the values range from ∼30 μs for Pt-TPTBP to
3.2 μs for Pt-Ar4TAP. (The triplet−triplet absorption spectra
are included in the Supporting Information, Figure S-4).
Interestingly, it is evident that τT systematically decreases with
phosphorescence energy, suggesting that the emission decay
of this series follows an energy gap law dependence.30−32 By
using the ϕp and τT values we have computed the radiative and
nonradiative decay rates for the triplet states (kr and knr,
respectively, Table 2). Here, it is seen that the radiative rates
are relatively constant across the series; however, the
nonradiative decay rate clearly increases as the emission energy
decreases. An analysis of the energy gap law dependence of the
triplet decay rates is shown in Supporting Information, Figure
S-7 (plot of ln(knr) vs Eem), and it is clear from this correlation
that the triplet lifetime (and consequently the phosphorescence
yield) for this family of phosphors is ultimately limited by the
triplet-ground state energy gap.30−32
Comparison of our results with previous work in the
literature reveals some interesting and useful points. First, the
phosphorescence quantum yield and triplet lifetime values for
Pt-TPTBP reported by Thompson et al. (53 μs, 0.70) differ
from our measurements (29.9 μs, 0.35), but our results are in
line with those reported for the same complex by Kilmant
et al.3,16 We note that the quantum yield reported by Thompson
was based on an extrapolation from a low temperature
measurement where it was assumed that the quantum yield
was unity. Second, on the basis of a literature report for a
palladium(II) octamethoxy substituted tetranaphthoporphrin,14
we expected that the phosphorescence for Pt-Ar4TNP(OMe)8
might be red-shifted relative to Pt-TPTNP. However, it is
evident that these two complexes exhibit virtually identical
photophysical behavior, perhaps not surprisingly as the methoxy
groups are not expected to interact strongly with the porphyrin
chromophore.
Photophysics of Platinum(II) Benzoporphyrins (Series
2). To explore the effect of ring substitution on the photo-
physics of platinum(II) π-extended conjugation porphyrins
we examined the Series 2 Pt(II)-benzoporphyrin complexes
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These results show that the phosphorescence yields are 50−60%
larger for the diaryl benzoporphyrins (Pt-Ar2TBP, Pt-TAr2TBP,
and Pt-Ar2OPrTBP) relative to the tetraaryl complexes (Pt-
TPTBP, Pt-Ar4TBP, Pt-ArF4TBP). The exception to this trend is
Pt-DPTBP, which exhibits a phosphorescence yield similar to
that of the tetraaryl complexes. The highest quantum yield is
observed for the diaryl complex Pt-Ar2TBP with a value of 0.49.
Examination of the triplet lifetimes for the series shows that there
is good correlation between the phosphorescence yields and the
triplet lifetimes. The diaryl substituted complexes, which exhibit
more efficient phosphorescence, have lifetimes in the range of
∼50 μs, whereas the tetraaryl complexes have lifetimes ∼30 μs.
This correlation indicates that the triplet decay rate (and
consequently the phosphorescence quantum yield) is determined
by the nonradiative decay rate (knr). This fact is underscored by
the significant variation in knr across the series, while the radiative
rate (kr) is relatively constant at ∼1.0 × 104 s−1.
The anomalously low phosphorescence yield and lifetime for
the parent diaryl complex Pt-DPTBP may be due to the fact
that rotation of the meso-phenyl substituents enhances the rate
of nonradiative decay.34,35 This mode of nonradiative decay is
blocked by the bulky substituents in the other diaryl
porphyrins, affording them with considerably higher triplet
lifetimes and phosphorescence yields.
Taken together, the results on the Series 1 Pt-benzoporphy-
rins show that all of these chromophores exhibit efficient,
relatively long-lived phosphorescence in the near-infrared region
near 770 nm. As seen in previous studies, the diaryl substitution
pattern gives rise to chromophores with substantially higher
phosphorescence yield and lifetime compared to cogeners with
tetraaryl substitution. The most efficient phosphor of the set is
Pt-Ar2TBP which exhibits a phosphorescence quantum yield
of ∼49%. This is believed to be the highest phosphorescence
efficiency observed to date for phosphorescence from a near-
infrared emitting phosphor.
Figure 4. Absorption (black) and photoluminescence (red) of select
Series 2 π-extended platinum porphyrins in deoxygenated toluene. (A)
Pt-Ar4TBP, (B) Pt-ArF4TBP, (C) Pt-TAr2TBP, (D) Pt-Ar2OPrTBP.
that the Soret and Q-bands are blue-shifted for the diaryl-
systems. This trend was reported previously for a series of
Pd-tetrabenzoporphyrins.17 More interestingly, the Q-band for
the diaryl-porphyrins (Figure 4c and 4d) is split, likely because
of the decreased symmetry of the macrocycle systems.
Comparison of the photoluminescence data in Table 3 and
Figure 4 shows that all of the Pt-benzoporphyrins exhibit strong
room temperature phosphorescence. In general the phospho-
rescence of the diaryl- complexes is slightly blue-shifted relative
to that of the tetraaryl- derivatives (∼770 vs 772 nm), with the
exception of Pt-Ar2OPrTBP which exhibits a distinct red-
shifted phosphorescence band at 786 nm. The triplet lifetimes
for the entire set of Series 2 complexes were measured by
transient absorption spectroscopy, and the values are listed in
Table 3, along with the phosphorescence quantum yields.
SUMMARY AND CONCLUSIONS
■
This report describes a complete investigation of the photo-
physical properties of a series of expanded π-conjugation
platinum(II) porphyrin chromophores that exhibit efficient
phosphorescence in the near-infrared region. The series of free-
base porphyrins were prepared by using techniques developed
over the past several years, and metalation with platinum(II)
Table 3. Photophysical Data for Series 2 Platinum TBPs in Deoxygenated Toluene
a
b
complex
absorption λmax / nm Soret, Q-band (log εmax / M−1 cm−1
)
emission λmax / nm
ϕp (τT/ μs)
kr / s−1 (knr / s−1
)
Pt-TPTBP
430 (5.28)
612 (5.13)
432 (5.26)
610 (5.02)
438 (5.35)
617 (5.05)
409 (4.43)
604 (4.33)
410 (5.24)
604 (5.12)
411 (5.33)
605 (5.22)
411 (5.31)
614 (5.17)
773
0.35 ± 0.05
(29.9)
1.2 × 104
(2.2 × 104)
1.0 × 104
Pt-Ar4TBP
772
772
770
770
769
786
0.33 ± 0.003
(32.0)
(2.1 × 104)
1.3 × 104
Pt-ArF4TBP
Pt-DPTBP
0.26 ± 0.01
(20.1)
(3.7 × 104)
1.1 × 104
0.3 ± 0.02
(28.0)
(2.5 × 104)
9.2 × 103
Pt-Ar2TBP
0.49 ± 0.08
(53.0)
(9.7 × 103)
8.4 × 103
Pt-TAr2TBP
Pt-Ar2OPrTBP
0.44 ± 0.03
(51.7)
(1.1 × 104)
8.7 × 103
0.45 ± 0.01
(51.8)
(1.1 × 104)
a
b
Phosphorescence quantum yields were measured relative to ZnTPP (0.03)27−29 by excitation at 420 nm in toluene. Triplet lifetimes were obtained
by transient absorption spectroscopy.
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Platinum(II) Tetra(3,5-di-tert-butylphenyl)tetrabenzo-
porphyrin (Pt-Ar4TBP). The title compound was prepared from
H2Ar4TBP (80 mg, 63.3 μmol) and platinum acetate (87 mg,
63.3 μmol) in benzonitrile (30 mL) using the procedure for
Pt-TPTBP. The crude material was purified via column
chromatography eluting with 30% CH2Cl2 in hexanes yielding 32
mg of the title compound (35%). 1H NMR (pyridine- d5, 500
MHz): δ = 8.31 (s, 8H), 8.17 (s, 8H), 7.51 (m, 8H), 7.44 (m, 8H),
1.46 (s, 72H); 13C NMR (pyridine- d5, 125 MHz) δ = 153.3, 138.9,
136.6, 129.0, 126.4, 125.6, 122.9, 120.5, 35.6, 32.0; DART-MS
[M+H]+ 1457.7687, calcd 1457.7694.
acetate afforded the metallo-pophyrins in moderate to high
yield. Comparison of the photophysical properties of the
expanded conjugation series reveals that the tetrabenzo-,
tetranaphtho-, and tetraanthroporphyrins exhibit phosphores-
cence at wavelengths ranging from ∼770−1020 nm. Analysis of
the quantum yield and lifetime data shows that the radiative
decay rates from the triplet states are relatively constant across
the series; however, the nonradiative decay rate increases
substantially with decreasing T1−So energy gap. An energy gap
law correlation is observed, which indicates there is a limited
prospect for extending the emission wavelength to longer
wavelengths because the phosphorescence yield will become
very low. A study of an extensive set of variously substituted
platinum(II) benzoporphyrins was also completed, and the
results show that the 5,15-diaryl-substituted chromophores
exhibit 50−60% longer triplet lifetimes and substantially
enhanced phosphorescence yields compared to the 5,10,15,20
tetraryl-substitued chromophores. All of the complexes studied
give rise to sufficiently efficient phosphorescence such that they
show considerable promise as the phosphors for application
in efficient near-infrared organic- and polymer-light emitting
diodes. These applications are explored thoroughly in the
accompanying paper.22
Platinum(II) Tetra(4-(9,9-dihexyl-fluorenyl)-phenyl)-
tetrabenzoporphyrin (Pt-ArF4TBP). The title compound was
prepared from H2ArF4TBP (76 mg, 35.4 μmol) and platinum acetate
(49 mg, 35.4 μmol) in benzonitrile (25 mL) using the procedure for
Pt-TPTBP. The crude material was purified by chromatography
eluting with 20% hexanes in CH2Cl2. The fractions were combined
and concentrated in CH2Cl2 then diluted with excess MeOH to
1
precipitate 78 mg of the title compound (94%). H NMR (pyridine-
d5, 500 MHz): δ = 8.50 (m, 12H), 8.21 (d, 4H), 8.18 (d, 4H), 8.04 (d,
4H), 7.69 (d, 8H), 7.66 (d, 4H), 7.53 (t, 8H), 7.30 (m, 8H), 2.37 (t,
4H), 2.27 (t, 4H), 1.14 (m, 16H), 1.11 (q, 16H), 0.99 (m, 16H), 0.95
(m, 16H), 0.77 (t, 24H); 13C NMR (pyridine- d5, 125 MHz) δ =
152.9, 152.0, 142.8, 142.2, 141.7, 141.2, 140.0, 138.5, 137.2, 135.1,
128.7, 128.4, 127.9, 127.4, 126.6, 125.1, 124.0, 121.4, 121.0, 119.3,
41.0, 32.1, 30.4, 24.8, 23.2, 14.5; MALDI-TOF MS [M+H]+
2338.2735, calcd 2338.2709.
EXPERIMENTAL SECTION
Platinum Tetraphenyltetranaphthoporphyrin (Pt-TPTNP). The
title compound was prepared from H2TPTNP (112 mg, 110.3 μmol)
and platinum acetate (150 mg, 109.3 μmol) in benzonitrile (25 mL)
using a procedure similar tor Pt-TPTBP with heating at 180 °C. The
crude material was passed a column of neutral silica gel eluting with
CH2Cl2:THF (9:1). The first dark green band was collected, and the
solvent removed. The material was precipitated from a mixture of
CH2Cl2 and acetonitrile. The precipitate was collected and washed
repeatedly with cold MeOH yielding 70 mg of the title compound
(53%). The 1H and gHMBC spectrum of compound Pt-TPTNP were
taken on a Varian Inova 500 NMR spectrometer, equipped with a
5 mm indirect detection probe and with z-axis gradients, and operating
■
Optical Characterization. Absorption spectra for all free-base
and platinum π-extended porphyrins were measured using a
PerkinElmer Lambda 25 UV−vis spectrometer. The emission spectra
were obtained by excitation at the Soret band absorption maximum for
each compound. Emission spectra over the range 500−1000 nm were
recorded on a home-built spectrometer consisting of a 75 W Xe
source, an ISA H-20 excitation monochromator, and an ISA Spex
Triax 180 emission spectrometer coupled to a Spectrum-1 liquid
nitrogen-cooled silicon charge coupled device detector. The emission
spectra for Pt-Ar4TAP were measured on a PTI fluorimeter equipped
with InGaAs near-IR detector, or a Spex Fluorolog II equipped with
InGaAs near-IR detector. All emission spectra were corrected for the
spectrometer response by using correction factors generated with a
primary standard lamp. The emission fluorescence and phospho-
rescence quantum yields were calculated relative to ZnTPP in toluene
(ϕ = 0.03)27−29 unless otherwise noted according to a previously
described method.36 The sample and actinometer solutions had
matched optical density at a shared excitation wavelength. Time-
resolved transient absorption spectra for π-extended platinum
porphyrins in toluene were collected by using previously described
laser systems for the visible and near-IR regions.37
1
at 500 MHz for H and 125 MHz for 13C. The chemical shifts were
referenced to the residual solvent signals, 7.22 ppm in 1H and 123.9 in
13C. Because of the limited solubility of Pt-TPTNP in the NMR
solvent, 13C chemical shifts were measured by indirect detection, in a
gHMBC spectrum. 1H NMR (pyridine-d5, 500 MHz) δ 8.47−8.44 (m,
8H), 8.22−8.14 (m, 4H), 8.10−8.03 (m, 8H), 7.93 (s, 8H), 7.90−7.84
(m, 8H), 7.61 (8H, overlap with solvent); 13C NMR (pyridine-d5, 125
MHz) δ 142.4, 135.9, 134.2, 131.6, 130.5, 130.0, 129.7, 126.7, 124.1,
117.9; ESI-TOF m/z 1206.3188, calcd 1206.3157.
Platinum(II) 1,4,10,13,19,22,28,31-Octamethoxy-7,16,25,34-
tetrakis(3,5-di-tert-butylphenyl)-tetranaphthoporphyrin (Pt-
Ar4TNP(OMe)8). The title compound was prepared from
H2Ar4TNP(OMe)8 (115 mg, 67.5 μmol) and platinum acetate (92
mg, 67 μmol) in benzonitrile (30 mL) using the procedure for Pt-
TPTBP. The crude material was passed through a column of neutral
silica gel eluting with CH2Cl2. The first dark green band was collected,
and the solvent removed. The material was precipitated from warm
CHCl3 and MeOH. The precipitate was collected and washed
repeatedly with cold MeOH yielding 110 mg of the title compound
Synthesis and Spectral Characterization of Platinum(II)
Porphyrins. A complete description of the synthesis of all free-base
porphyrins, including starting compounds, synthetic schemes, and
characterization data (including NMR spectra) is provided in the
Supporting Information. In the following section we describe the
metallation of the free-base porphyrins and spectral characterization of
the platinum(II) complexes.
Platinum(II) Tetraphenyltetrabenzoporphyrin (Pt-TPTBP). A
solution of H2TPTBP (49 mg, 60.1 μmol) and platinum acetate (83
mg, 60.1 μmol) in benzonitrile (25 mL) was thoroughly purged with
argon. The reaction was then submerged in a preheated oil bath at
200 °C and refluxed until the Q-band of H2TPTBP disappeared from
the UV−vis spectrum. The solvent was removed under reduced
pressure, and the crude material dissolved in DCM and filtered
through a plug of Celite. The material was then loaded on silica gel
and purified via column chromatography eluting with 30% CH2Cl2 in
hexanes yielding 45 mg of the title compound (74%). 1H NMR
(pyridine-d5, 500 MHz): δ = 8.35 (d, 8H), 7.96 (t, 4H), 7.92 (t, 8H),
7.42 (m, 8H), 7.37 (m, 8H); 13C NMR (pyridine- d5, 125 MHz) δ =
142.1, 138.2, 136.7, 134.2, 129.9, 129.9, 126.3, 124.8; ESI-TOF m/z
1006.2549, calcd 1006.2562.
1
(85%). H NMR (pyridine-d5, 500 MHz): δ = 8.56 (s, 8H), 8.30 (s,
8H), 8.29 (s, 4H), 6.82 (d, 8H), 3.93 (s, 24H), 1.46 (s, 72H); 13C
NMR (pyridine-d5, 125 MHz) δ = 153.6, 151.1, 136.4, 127.5, 125.0,
123.5, 119.7, 119.5, 103.3, 55.8, 35.7, 32.0; MALDI-TOF MS [M]+
1896.9161, calcd 1896.9090.
Platinum(II) Tetra(3,5-di-tert-butylphenyl)tetraanthro-
porphyrin (Pt-Ar4TAP). Because of the sensitivity with oxygen in
the presence of room light for the title compound and H2Ar4TAP, all
manipulations were performed under an inert atmosphere and in a
flask protected from light with foil. A Schlenk flask charged with
benzonitrile (6 mL), H2Ar4TAP (59 mg, 35.5 μmol), and platinum
acetate (48 mg, 35.5 μmol) was degassed by three freeze pump thaw
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Chemistry of Materials
Article
cycles. The mixture was heated at 180 °C under a positive pressure of
argon until the disappearance of the Q-band of H2Ar4TAP
(approximately 3 h). The solvent was removed by vacuum distillation,
and the crude was purified by passing through a short plug of neutral
silica under an argon atmosphere with CH2Cl2. The solvent was
removed under reduced pressure to give 25 mg of the title compound
ASSOCIATED CONTENT
■
S
* Supporting Information
Complete synthesis details, and full spectroscopic character-
ization of all intermediates and target free-base porphyrins
including, absorption, fluorescence, and photophysical data for
Series 2 free base porphyrins, transient absorption spectra of all
Pt-porphyrin derivatives, energy gap law plot for Series 1 Pt-
porphyrins, 1H and 13C NMR spectra for important
intermediates, free-base and Pt-porphyrins, and peak assign-
ment for 1H and 13C chemical shifts for Pt-porphyrins
determined by 2D NMR (1 Table, 25 figures, 9 schemes, and
28 pages of text). This material is available free of charge via the
Internet at http://pubs.acs.org.
1
(38%). H NMR (pyridine- d5, 500 MHz): δ = 8.71 (s, 8H), 8.53 (s,
4H), 8.53 (overlap s, 8H), 8.30 (s, 8H), 8.11 (d, 8H), 7.52 (t, 8H),
1.58 (s, 72H); 13C NMR (pyridine- d5, 125 MHz) δ = 154.5, 136.6,
132.8, 130.4, 129.1, 128.7, 128.2, 126.3, 125.2, 123.4, 118.5, 35.8, 32.1;
MALDI-TOF MS [M]+ 1856.8951, calcd 1856.8872.
Platinum(II) 5,15-Diphenyltetrabenzoporphyrin (Pt-DPTBP). The
title compound was prepared from H2DPTBP (50 mg, 75.4 μmol)
and platinum acetate (104 mg, 75.7 μmol) in benzonitrile (25 mL)
using the procedure for Pt-TPTBP. The material was loaded on silica
gel, eluting with CH2Cl2 removing the green band (H2DPTBP). The
solvent was changed to an increasing gradient with a THF:CH2Cl2
mixture (30:70). The dark blue band was collected yielding 16 mg of
the title compound (25%). Analogous to the previously reported
palladium complex, NMR analysis was not possible because of low
solubility of Pt-DPTBP in common NMR solvents.17 The material was
characterized by UV−vis and mass spectrometry. UV−vis, toluene,
λmax: 409 nm, 547 nm, 595 nm, 604 nm; MALDI-TOF MS [M]+
852.4518, 854.1971, 855.2001, 856.2019, 857.2050, 858.2057,
859.2082, 860.2094, 861.4855, calcd 852.1919, 854.1935, 855.1960,
856.1974, 857.1999, 858.2004, 859.2026, 860.2054.
Platinum(II) 5,15-Di(3,5-di-tert-butylphenyl)tetrabenzo-
porphyrin (Pt-Ar2TBP). The title compound was prepared from
H2Ar2TBP (65 mg, 73.3 μmol) and platinum acetate (100 mg,
73.3 μmol) in benzonitrile (30 mL) using the same procedure
described above for Pt-TPTBP. The solvent was removed, and the
crude material loaded on silica gel eluting with 15% CH2Cl2 in hexane
collecting the blue band. The material was further purified by multiple
reprecipitation from boiling CHCl3 and MeOH. The precipitate was
collected and repeatedly washed with MeOH yielding 25 mg of the title
compound (32%). 1H NMR (pyridine- d5, 500 MHz): δ = 11.55
(s, 2H), 9.80 (d, 4H), 8.35 (s, 4H), 8.31 (s, 4H), 8.08 (t, 4H), 7.78 (t,
4H), 7.54 (d, 4H); 13C NMR (pyridine- d5, 125 MHz) δ = 153.1, 141.5,
139.0, 138.2, 136.6, 136.2, 128.1, 127.7, 126.4, 122.8, 121.5, 121.0, 97.5,
35.9, 32.0; ESI-TOF [M]+ 1080.4543, calcd 1080.4481.
Platinum(II) 5,15-Di((3,5-di-tert-butylphenyl)-phenyl)-
tetrabenzoporphyrin (Pt-TAr2TBP). The title compound was
prepared from H2TAr2TBP (110 mg, 92.3 μmol) and platinum
acetate (127 mg, 92.3 μmol) in benzonitrile (25 mL) using the
procedure as described for Pt-TPTBP. Separation with column
chromatography failed because of solubility and small differences in Rf
values. The title compound was purified by multiple reprecipitation
from CH2Cl2 and MeOH to give 70 mg (55%). 1H NMR (pyridine-d5,
500 MHz): δ = 11.60 (s, 2H), 9.86 (d, 4H), 8.93 (s, 2H), 8.84 (s, 4H),
8.12 (d, 8H), 8.08 (t, 4H), 7.99 (d, 4H), 7.81 (t, 4H), 7.63 (d, 8H),
1.34 (s, 36H); 13C NMR (pyridine-d5, 125 MHz) δ = 151.8, 143.5,
139.1, 138.3, 138.2, 136.8, 130.5, 128.2, 128.0, 127.8, 126.9, 126.9,
126.3, 121.8, 120.0, 35.0, 31.7; DART-MS [M+H]+ 1383.5772, calcd
1383.5770.
Platinum(II) 5,15-Di(3,5-di-tert-butylphenyl)octapropyltetra-
benzoporphyrin (Pt-Ar2OPrTBP). The title compound was pre-
pared from H2Ar2OPrTBP (33 mg, 23.2 μmol) and platinum acetate
(32 mg, 23.2 μmol) in benzonitrile (20 mL) using the procedure
described for Pt-TPTBP. After vacuum distillation to remove
benzonitrile, the crude material was loaded on silica gel and eluted
with a hexane:CH2Cl2 mixture (85:15) collecting the first blue band.
The fractions were concentrated, and the material was precipitated by
the addition of excess MeOH. The precipitate was collected to give
22 mg of the title compound (58%). 1H NMR (pyridine- d5, 500 MHz):
δ = 11.86 (s, 2H), 9.83 (s, 4H), 8.36 (s, 2H), 8.33 (s, 4H), 7.42 (s,
4H), 3.11 (t, 8H), 2.95 (t, 8H), 1.90 (sex, 8H), 1.82 (sex, 8H), 1.63
(s, 36H), 1.18 (t, 12H), 1.12 (t, 12H); 13C NMR (pyridine- d5, 125
MHz) δ = 153.0, 140.7, 140.4, 137.6, 136.8, 127.7, 126.8, 123.5,
121.7, 120.3, 97.0, 36.4, 35.9, 32.2, 25.6, 15.0; ESI-TOF m/z
1416.8265, calcd 1416.8301.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: kschanze@chem.ufl.edu. Phone: 352-392-9133.
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from the U.S.
Army Aviation and Missile Research, Development, and
Engineering Center (AMRDEC) (Project No. W31P4Q-08-1-
0003).38
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the authors and should not be interpreted as representing the official
policies, either expressed or implied, of the Defense Advanced Research
Projects Agency; the U.S. Army Aviation and Missile Research,
Development, and Engineering Center; or the U.S. Government.
5304
dx.doi.org/10.1021/cm202241e | Chem. Mater. 2011, 23, 5296−5304