Observation and Interpretation of Annulated Porphyrins: Studies on the Photophysical Properties of meso-Tetraphenylmetalloporphyrins

Article abstract of DOI:10.1021/jp0354705

We present results of a joint computational and experimental study for a series of annulated metalloporphyrins in order to establish structure-property relationships. Specifically, we have examined the effects of substitution by meso-tetraphenylation, tetrabenzo and tetranaphtho annulation, and effects of changing the central metal from zinc (Zn) to palladium (Pd). Utilizing absorption and emission spectroscopy and laser flash photolysis techniques, the photophysical properties of these porphyrins have been determined. Upon the addition of benzo or naphtho groups, we observed an overall red shift in the ground state absorption spectra of both the B-bands and the Q-bands with increased conjugation and an increase in the Q-band to B-band intensity ratios. Time-dependent density functional theory calculations were performed on both series of porphyrins to identify the effects of phenyl, benzo, and naphtho substituents on the spectra. The benzo and naphtho adducts provide a larger contribution (typically 40-90%) to the observed red shifts due to increased π-conjugation, while there is a smaller contribution (typically 0-25%) from distortion of the porphyrin. Similarly, a red shift for the T₁-T _n absorption spectrum and an overall general broadening in the spectrum were found with increased conjugation. An increase in the triplet molar extinction coefficient through the near-infrared region with annulation was also found. Varying the metal has an effect on the overall absorption spectra; i.e., the ground state spectra of the Zn porphyrins are red-shifted relative to the Pd porphyrins. For the triplet excited state spectra there were small effects in the spectra by changing the metal with a significant contribution to the kinetic properties by the heavy atom effect of the Pd.

Full text of DOI:10.1021/jp0354705

J. Phys. Chem. A 2003, 107, 11331-11339
11331
Observation and Interpretation of Annulated Porphyrins: Studies on the Photophysical
Properties of meso-Tetraphenylmetalloporphyrins
Joy E. Rogers,*,†,‡ Kiet A. Nguyen,*,†,‡ David C. Hufnagle,†,§ Daniel G. McLean,†, Weijie Su,†,§
Kristi M. Gossett,†,§ Aaron R. Burke,†,§ Sergei A. Vinogradov,# Ruth Pachter,† and
Paul A. Fleitz†
Materials and Manufacturing Directorate, Air Force Research Laboratory, MLPJ, 3005 Hobson Way Bldg 651,
Wright Patterson Air Force Base, Ohio 45433; UES, Inc., Dayton, Ohio 45432; Anteon Corporation,
Dayton, Ohio 45431; Science Applications International Corporation, Dayton, Ohio 45434; and Department of
Biochemistry and Biophysics, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104
ReceiVed: May 27, 2003; In Final Form: August 4, 2003
We present results of a joint computational and experimental study for a series of annulated metalloporphyrins
in order to establish structure-property relationships. Specifically, we have examined the effects of substitution
by meso-tetraphenylation, tetrabenzo and tetranaphtho annulation, and effects of changing the central metal
from zinc (Zn) to palladium (Pd). Utilizing absorption and emission spectroscopy and laser flash photolysis
techniques, the photophysical properties of these porphyrins have been determined. Upon the addition of
benzo or naphtho groups, we observed an overall red shift in the ground state absorption spectra of both the
B-bands and the Q-bands with increased conjugation and an increase in the Q-band to B-band intensity ratios.
Time-dependent density functional theory calculations were performed on both series of porphyrins to identify
the effects of phenyl, benzo, and naphtho substituents on the spectra. The benzo and naphtho adducts provide
a larger contribution (typically 40-90%) to the observed red shifts due to increased π-conjugation, while
there is a smaller contribution (typically 0-25%) from distortion of the porphyrin. Similarly, a red shift for
the T1-Tn absorption spectrum and an overall general broadening in the spectrum were found with increased
conjugation. An increase in the triplet molar extinction coefficient through the near-infrared region with
annulation was also found. Varying the metal has an effect on the overall absorption spectra; i.e., the ground
state spectra of the Zn porphyrins are red-shifted relative to the Pd porphyrins. For the triplet excited state
spectra there were small effects in the spectra by changing the metal with a significant contribution to the
kinetic properties by the heavy atom effect of the Pd.
Introduction
MTPTNP are studied and compared with similar systems that
do not contain the meso-phenyl groups, i.e., tetrabenzoporphyrin
Porphyrins form an important class of chromophores that are
being developed and used for many different applications such
as photodynamic therapy, commercial dye industry, nonlinear
absorption, artificial photosynthesis, oxygen transport, and solar
energy conversion.1 Much research has been done to develop
new porphyrin chromophores that display certain characteristics
dependent on the application of use.2 The understanding of
structure-property relationships is crucial for the development
of novel porphyrins targeted for specific applications. In the
work presented here we focus on the structure-property
relationships in a series of meso-tetraphenylporphyrins that have
been modified by annulation of the pyrrole ring with either
benzo or naphtho substituents.
(MTBP) and tetranaphthoporphyrin (MTNP). The qualitative
observations, e.g., red shifts in the ground and excited state
spectra with annulation, are discussed, but no attempt to quantify
the data was made.11-16 We synthesized the same series of
chromophores and report a detailed quantitative study of
photophysical parameters. In addition, calculations using time-
dependent density functional theory (TDDFT) were carried out
to interpret the observed spectra and to shed light on the origin
of the spectral shifts.
Experimental Section
Synthesis. ZnP and PdP were both purchased from Frontier
Scientific, Inc., with 97% purity and used as received.
ZnTPP. Chlorin-free ZnTPP was purchased from Frontier
Scientific, Inc., with 97% purity and used as received.
ZnTPTBP. In the preparation of ZnTPTBP the template
method, developed by Kopranenkov et al.11 and improved by
Ichimura et al.,17 was utilized. A mixture of phthalimide, zinc
benzoate, and phenylacetic acid (with molar ratio 1:1:2) was
well ground together and put into a 25 mL flask and heated at
360-380 °C for 1 h under a N2 atmosphere. After the reaction
mixture was cooled and washed with hot water several times,
the reaction product was extracted with chloroform and purified
The metalated meso-tetraphenyltetrabenzoporphyrin (MTPT-
BP) and meso-tetraphenyl[2,3]tetranaphthoporphyrin (MTPTNP)
shown in Figure 1 were previously synthesized,3-13 but relatively
few thorough and comprehensive studies were carried out.
Several papers have been published in which MTPTBP and
† Air Force Research Laboratory.
‡ UES, Inc.
§ Anteon Corporation.
Science Applications International Corporation.
# University of Pennsylvania.
* Corresponding author: E-mail: Joy.Rogers@wpafb.af.mil (exp.),
Kiet.Nguyen@wpafb.af.mil (theoret.).
10.1021/jp0354705 CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/02/2003

11332 J. Phys. Chem. A, Vol. 107, No. 51, 2003
Rogers et al.
phosphorescence measurements by either bubbling with argon
or by three successive freeze-pump-thaw cycles. Nanosecond
transient absorption measurements were carried out in deoxy-
genated pyridine (by three successive freeze-pump-thaw
cycles) using the third harmonic (355 nm) of a Q-switched Nd:
YAG laser (Continuum YG661, pulse width ca. 15 ns). Pulse
fluences of up to 8 mJ cm-2 at the excitation wavelength were
typically used. A detailed description of the laser flash photolysis
apparatus has been given earlier.20
Fluorescence quantum yields were determined using the
actinometry method previously described.20 Zinc meso-tetra-
phenylporphyrin was used as an actinometer with a known
fluorescence quantum yield of 0.033 in deaerated benzene.21
All samples were deoxygenated using the freeze-pump-thaw
method and excited at 500 nm with a matched optical density
of 0.1. Phosphorescence quantum yields and time-resolved
measurements were done as previously described.19
The molar absorption coefficients of the porphyrin triplet
states were determined in deoxygenated pyridine using the
method of singlet depletion, which has been described previ-
ously.20 Quantum yields for intersystem crossing of the Zn series
were determined using the method of relative actinometry as
previously described utilizing a benzophenone actinometer.20
Matched optical densities of the porphyrins (deoxygenated
pyridine) and benzophenone (air-saturated benzene) at 355 nm
were utilized in each determination. For the Pd chromophores
photoacoustic calorimetry was utilized to measure intersystem
crossing quantum yields as previously reported.22
Computational Studies. The calculations were carried out
using TDDFT23-25 with Becke’s three-parameter hybrid exchange-
correlation functional (B3LYP)26-28 and the 6-31G(d)29,30 basis
set for carbon, hydrogen, and nitrogen atoms and the effective
core potentials (ECP) and basis sets of Stevens et al.31,32 for
the central metals. The structures used in the TDDFT calcula-
tions have been predicted using the Kohn-Sham (KS)33 density
functional theory with the same ECP and basis sets as those
used in the excited state calculations. These structures were
verified to be minima with a smaller 6-31G basis set. All
electronic structure calculations were carried out using the
Gaussian 98 program.34
Figure 1. Shown are the structures of the porphyrin series MTPP
(meso-tetraphenylporphyrin), MTPTBP (meso-tetraphenyltetrabenzo-
porphyrin), and MTPTNP (meso-tetraphenyltetranaphthoporphyrin). The
metal (M) is either Pd or Zn.
by repeated chromatography on a column of aluminum oxide
using benzene/hexanes/THF (10:10:1). Thin-layer chromatog-
raphy, UV/vis absorption, and NMR were used to check the
1
purity of the sample. H NMR (CDCl3, 300 MHz): 8.30 (d,
8H, ortho phenyl), 7.95 (t, 4H, para phenyl), 7.87 (t, 8H, meta
phenyl), 7.28 (dd, 8H, B position on benzo ring), 7.17 (dd, 8H,
A position on benzo ring). The NMR spectrum matches that
obtained by Ichimura et al.,3 Cheng et al.,4 and Ito et al.9
ZnTPTNP. This synthesis followed the template method by
Kopranenkov et al.13 A mixture of 2,3-naphthalenecarboxyimide,
zinc benzoate, and phenylacetic acid (with molar ratio 1:1:2)
was used. The reaction product was purified by repeated
chromatography with chloroform first and then with chloroform/
hexanes/THF (30:10:1) on a column of neutral aluminum oxide.
Thin-layer chromatography, UV/vis absorption, and NMR were
used to check the purity of the sample. Because of the instability
of the compound, the sample may contain contaminants of
degraded or otherwise changed material. 1H NMR (CDCl3, 300
MHz): 8.41 (d, 8H, ortho phenyl), 8.10 (t, 4H, para phenyl),
7.97 (t, 8H, meta phenyl), 7.69 (m, 16H, B and C positions on
naphtho rings), 7.45 (m, 8H, A position on naphtho rings). The
NMR data matched that obtained by Ito et al.10
Results
In this work we are interested in establishing structure-
property relationships by systematically studying a series of
structurally related porphyrins to understand how chemical
changes affect the photophysical properties. A series of por-
phyrins were synthesized with increased degree of conjugation
of the macrocycle by addition of fused benzo or naphtho groups
to the pyrrole ring. Shown in Figure 1 are the structures of the
porphyrins (MTPP, MTPTBP, and MTPTNP); M denotes a
metal, which is either Zn or Pd.
Ground State Absorbance. Shown in Figure 2 are the
ground state spectra of both series of porphyrins in air-saturated
pyridine. The molar extinction coefficients are given in Table
1 in both pyridine and benzene. These agree with literature data
for various solvents.3,7,10,35,36 ZnTPTBP and ZnTPTNP are not
stable in benzene when left in the room light so therefore we
switched our solvent to pyridine where these chromophores are
reasonably stable. We included the data in benzene because they
show that there is a large red shift of the Zn series in pyridine
with no shift in the Pd series. It is known that pyridine
coordinates with the Zn by axial ligation,14,37,38 therefore
stabilizing the chromophore by lowering the energy. It was
observed in both solvents that annulation with the benzo or
PdTPP. PdTPP (97%) was purchased from Frontier Scien-
tific, Inc., and used as received.
PdTPTBP. The synthesis of PdTPTBP was done according
to the previously published procedure.7 1H NMR (CDCl3, 300
MHz): 8.27 (d, 8H, ortho phenyl), 7.91 (d, 4H, para phenyl),
7.86 (d, 8H, meta phenyl), 7.21 (m, 8H, B position on benzo
ring), 7.11 (m, 8H, A position on benzo ring).
PdTPTNP. This compound was synthesized as previously
reported.18,19 1H NMR (DMF-d7, 500 MHz): 8.38 (8H, ortho
phenyl), 8.22 (4H, para phenyl), 8.09 (8H, meta phenyl), 7.66
(16H, B and C positions on naphtho rings), 7.57 (8H, A position
on naphtho rings).
General Techniques. Ground state UV/vis absorption spectra
were measured on a Cary 500 spectrophotometer in air-saturated
pyridine. Emission spectra were measured using a Perkin-Elmer
model LS 50B fluorometer or a Spex Fluorolog-2 using an IR-
sensitive PMT (Hamamatsu R2658P). Oxygen was removed
from the sample for the room temperature fluorescence and

Annulated Porphyrins
J. Phys. Chem. A, Vol. 107, No. 51, 2003 11333
TABLE 1: Ground State Absorption Data
transition/solvent
ZnTPP
ZnTPTBP
ZnTPTNP
PdTPP
PdTPTBP
PdTPTNP
B-band
benzene
423 nm (2.93 eV) 463 nm (2.68 eV) 482 nm (2.57 eV) 418 nm (2.97 eV) 444 nm (2.79 eV) 463 nm (2.68 eV)
4.35 ( 0.03 2.81 ( 0.18 2.64 ( 0.03 1.44 ( 0.05
431 nm (2.88 eV) 473 nm (2.62 eV) 502 nm (2.47 eV) 418 nm (2.97 eV) 444 nm (2.79 eV) 463 nm (2.68 eV)
5.33 ( 0.51 2.21 ( 0.14 2.42 ( 0.09 2.90 ( 0.14 1.97 ( 0.24 1.55 ( 0.04
382 nm (3.25 eV) 422 nm (2.94 eV) 431 nm (2.88 eV) 382 nm (3.25 eV) 415 nm (2.99 eV) 423 nm (2.93 eV)
1.19 1.25 1.20 1.10 1.01 0.97
ꢀ (×10-5) (M-1 cm-1
pyridine
)
ꢀ (×10-5) (M-1 cm-1
)
TDDFT
oscillator strength (f)
Q-band
benzene
588 nm (2.11 eV) 653 nm (1.90 eV) 725 nm (1.71 eV) 551 nm (2.24 eV) 629 nm (1.97 eV) 705 nm (1.76 eV)
0.03 ( 0.01 0.02 ( 0.01 1.05 ( 0.07 1.68 ( 0.08
602 nm (2.06 eV) 660 nm (1.88 eV) 725 nm (1.71 eV) 554 nm (2.24 eV) 629 nm (1.97 eV) 709 nm (1.75 eV)
0.10 ( 0.01 0.31 ( 0.02 1.57 (0.05 0.02 ( 0.01 0.80 ( 0.11 1.71 ( 0.05
539 nm (2.30 eV) 605 nm (2.05 eV) 681 nm (1.82 eV) 517 nm (2.40 eV) 579 nm (2.14 eV) 653 nm (1.90 eV)
0.02 0.09 0.28 0.01 0.12 0.31
ꢀ (×10-5) (M-1 cm-1
pyridine
)
ꢀ (×10-5) (M-1 cm-1
)
TDDFT
oscillator strength (f)
Figure 3. Q-band absorbance and emission of MTPP, MTPTBP, and
MTPTNP (M ) Pd or Zn). Extinction coefficients are given for
absorbance data (left axis); all others are in intensity (arbitrary units,
right axis). All data were obtained at room temperature (25 °C) in
pyridine. Samples were excited at 500 nm.
Figure 2. Ground state absorption of MTPP, MTPTBP, and MTPTNP
(M ) Pd or Zn). Extinction coefficients are given for absorbance data.
All data were obtained at room temperature (25 °C) in pyridine.
temperature are reported in Table 2. Our work is consistent with
other measurements of room temperature phosphorescence
quantum yields for PdTPTBP,7 PdTPTNP,19 and ZnTPTNP19
in DMF.
T1-Tn Properties. Figure 4 shows the transient observed
upon 355 nm excitation of each of the porphyrins in deoxy-
genated pyridine. The identities of the transients were confirmed
by molecular oxygen quenching of the excited state. The T1-
Tn molar extinction coefficients were determined using a singlet
naphtho group results in red shifts of both the B-bands and the
Q-bands, where the Q-bands gain intensity while the B-bands
lose intensity (Table 1).
Fluorescence and Phosphorescence. Typical S1-S0 emis-
sion from both series of porphyrins is shown in Figure 3. The
fluorescence quantum yields (Φf) were determined using zinc
meso-tetraphenylporphyrin (ZnTPP) as a standard with a known
quantum yield of 0.033 in deaerated benzene.21 These are given
in Table 2 along with the singlet state energy (Es1). Fluorescence
quantum yields in deoxygenated pyridine for ZnTPP and PdTPP
are consistent with measurements in other solvents.39
depletion method. The values determined are ZnTPP ꢀ475nm
76 000 ( 4000 M-1 cm-1, ZnTPTBP ꢀ550nm ) 46 000 ( 2000
M-1 cm-1, ZnTPTNP ꢀ475nm ) 58 000 ( 3000 M-1 cm-1
ꢀ595nm ) 57 000 ( 3000 M-1 cm-1, PdTPP ꢀ475nm ) 54 000 (
3000 M-1 cm-1, PdTPTBP ꢀ530nm ) 36 000 ( 2000 M-1 cm-1
)
;
For the Pd series, room temperature phosphorescence was
easily observed in the absence of oxygen, and it is shown in
Figure 3. Room temperature phosphorescence was observed for
ZnTPP and ZnTPTNP under argon in pyridine. Phosphorescence
of ZnTPTBP at room temperature in deoxygenated pyridine was
not observed. The phosphorescence quantum yields (ΦP) at room
,
)
and PdTPTNP ꢀ480nm ) 45 000 ( 2000 M-1 cm-1; ꢀ610nm
33 000 ( 2000 M-1 cm-1. Our data are consistent with values
reported in the literature for ZnTPP,21,40 PdTPP,35 and ZnTPT-
BP.41,42

11334 J. Phys. Chem. A, Vol. 107, No. 51, 2003
Rogers et al.
PdTPTNP
TABLE 2: Photophysical Properties of Porphyrins in Pyridine
ZnTPP
ZnTPTBP
ZnTPTNP
PdTPP
565 nm
PdTPTBP
669 nm
Fl Q (0,0)
620 nm
0.029 ( 0.003
2.03
676 nm
0.007 ( 0.001
1.86
736 nm
0.052 ( 0.016
1.70
741 nm
a
Φfl
∼0.0004
∼0.0003
∼0.0003
E
S1 (eV)b
2.22
1.92
1.72
Ph λmax
804 nm
0.0005 ( 0.0001
1.58
972 nm
0.0002 ( 0.0001
1.29
705 nm
0.0040 ( 0.0003
1.82
797 nm
0.167 ( 0.005
1.59
937 nm
0.065 ( 0.002
1.36
Φp
< 0.00001
E
T1 (eV)c
∆ES1T1 (eV)
T1-Tn λmax
ΦISC
0.45
0.41
0.40
0.33
0.36
475 nm
0.84 ( 0.07
> 406
550 nm
0.94 ( 0.08
148 ( 3
167
475, 595 nm
0.62 ( 0.13
> 225
475 nm
0.96 ( 0.04
65 ( 3
11
530 nm
0.97 ( 0.06
143 ( 20
195
480, 610 nm
0.77 ( 0.07
65 ( 10
60
τT (µs)d
τT (µs)e
3680
440
a Oxygen removed from sample by three freeze-pump-thaw cycles. b Value obtained from cross section of red edge of absorbance and blue
edge of fluorescence. c Value obtained from blue edge of phosphorescence. d Oxygen removed from sample by three freeze-pump-thaw cycles.
Measured by laser flash photolysis. e Oxygen removed by bubbling with argon. Measured by phosphorescence decay at room temperature.
fields of science and engineering.1 The ability to simply change
the central metal allows for variations in the overall properties
and also provides some fine-tuning capabilities. In general,
metalloporphyrins contain two types of metals: regular metals
or irregular metals.44 Regular metals, like Zn, have only a small
effect on the optical absorption and emission spectra, which is
understood as a small perturbation on the π electrons of the
porphyrin ring. Irregular metals, e.g. Pd, have much stronger
effects on absorption and emission of porphyrins, either through
stronger mixing or through the introduction of new low-energy
optical transitions. Varying the metal centers as well as changing
the ring structure alters the photophysical properties of metal-
loporphyrins. It is interesting and important to understand how
these changes affect the overall photophysical properties of the
chromophores. We will discuss the effects of increasing the
conjugation due to annulation of the porphyrin ring and also
the effects of changing the central metal from Zn to Pd. We
begin with the spectral changes observed and then discuss the
kinetic properties.
Spectral Changes. Effect of Extension of Porphyrin Ring.
Extension of the porphyrin ring by benzo and naphtho annulation
results in red shifts in both the ground (Q- and B-bands) and
triplet excited state (T1-Tn) absorption spectra. It is known that
red shifts of the ground state spectra may be attributed to some
extent to saddling/ruffling of the porphyrin as well as electronic
effects due to increased π-conjugation, a notion that is continu-
Figure 4. Triplet excited state absorbance data (T1-Tn) observed after
ously being addressed with a range of substitutions.45 To assess
nanosecond pulsed 355 nm excitation of MTPP, MTPTBP, and
the relative contributions of distortion and increased π-conjuga-
tion, we compiled the experimental and theoretical results from
model compounds not containing meso-phenyl substituents that
directly compare the data from our series of chromophores. The
model compounds are: unsubstituted Zn porphyrin (ZnP), Pd
porphyrin (PdP), Zn tetrabenzoporphyrin (ZnTBP),46 Pd tetra-
benzoporphyrin (PdTBP),46 Zn tetranaphthoporphyrin (Zn-
TNP),13 and Pd tetranaphthoporphyrin (PdTNP).18 These com-
pounds are all planar in the ground state, which make them
useful for understanding spectral red shifts that are due to purely
electronic effects.
In Table 3 both the theoretical and experimental data are
shown for the excitation energies of the B and the Q-bands of
both the Zn and Pd series. The total red shift for a given band,
relative to ZnP or PdP, is due to the additive effects of (a) the
meso-phenyl groups, (b) the benzo groups (for MTPTBP), (c)
the naphtho groups (for MTPTNP), (d) nonplanar distortion,
(e) combined effects of nonplanar distortion, meso-tetraphen-
ylation and tetrabenzoannulation (for MTPTBP), and (f) com-
bined effects of nonplanar distortion, meso-tetraphenylation, and
tetranaphthoannulation (for MTPTNP). The difference in energy
relative to the planar Zn or Pd porphyrin is given in Table 3
MTPTNP (M ) Pd or Zn) in deoxygenated pyridine. Triplet excited
state molar absorption coefficients were obtained using the method of
singlet depletion as described in the text.
Shown in Table 2 are the triplet excited state lifetimes (τT)
in deoxygenated pyridine. They were determined and are
reported by both laser flash photolysis and time-resolved
phosphorescence measurements. Using the laser flash photolysis
experiment for PdTPP in pyridine, the data were fit to a first-
order exponential decay with a lifetime of 65 ( 3 µs. However,
the transient does not return to the baseline, indicating that a
long-lived transient is formed. We did not observe this in
benzene. This suggests that either the singlet excited state or
the triplet excited state of PdTPP is reacting with the pyridine.
Also reported in Table 2 are the intersystem crossing quantum
yields (ΦISC). We observe that ΦISC is smaller for the MTPTNP
chromophores than the others. The ΦISC for ZnTPP has been
reported as 0.88 in benzene21 and for PdTPP 1.00 in ethanol.43
Our values are within the error margins of these numbers.
Discussion
Porphyrins are an important class of materials and have
attractive characteristics that make them widely used in various

Annulated Porphyrins
J. Phys. Chem. A, Vol. 107, No. 51, 2003 11335
TABLE 3: TD-B3LYP/6-31G(d) Excitation Energies (E in eV) for Zinc and Palladium Porphyrins (Experimental Data
Measured in Pyridine)
system/transition
E (eV) theo (exp)
∆E (eV)
system/transition
E (eV) theo (exp)
∆E (eV)
ZnP (D4h)
11Eu (Q)
PdP (D4h)
11Eu (Q)
2.44 (2.19)
3.54 (3.05)
0.00
0.00
2.54 (2.32)
3.49 (3.16)
0.00
0.00
21Eu (B)
21Eu (B)
ZnTPP (D4h)a
11Eu (Q)
PdTPP (D4h)a
11Eu(Q)
2.35
3.34
0.09
0.20
2.45
3.31
0.09
0.18
21Eu (B)
21Eu (B)
ZnTBP (D4h)
PdTBP (D4h)
11Eu (Q)
2.18 (1.97)b
3.28 (2.88)b
0.26 (0.22)
0.26 (0.17)
11Eu (Q)
2.26 (2.04)b
3.29 (3.05)b
0.28 (0.28)
0.20 (0.11)
31Eu (B)
21Eu (B)
ZnTNP (D4h)
PdTNP (D4h)
11Eu(Q)
1.90 (1.77)c
3.13 (2.82)c
0.54 (0.42)
0.41 (0.23)
11Eu (Q)
1.97 (1.77)d
3.17 (2.81)d
0.57 (0.55)
0.32 (0.35)
31Eu(B)
31Eu (B)
ZnTPP (D2d)
PdTPP (D2d)
11E (Q)
2.30 (2.06)
3.25 (2.88)
0.14 (0.13)
0.29 (0.17)
11E (Q)
2.40 (2.24)
3.25 (2.97)
0.14 (0.08)
0.24 (0.19)
21E (B)
41E (B)
ZnTPTBP (D2d)
PdTPTBP (D2d)
11E (Q)
2.05 (1.88)
2.94 (2.62)
0.39 (0.31)
0.60 (0.43)
11E (Q)
2.14 (1.97)
2.99 (2.79)
0.40 (0.35)
0.50 (0.37)
21E (B)
21E (B)
ZnTPTNP (D2d)
PdTPTNP (D2d)
11E (Q)
1.82 (1.72)
2.88 (2.47)
0.62 (0.47)
0.66 (0.58)
11E (Q)
1.90 (1.75)
2.93 (2.68)
0.64 (0.57)
0.56 (0.48)
21E (B)
31E (B)
a The meso-phenyl groups constrained to a planar geometry. b Opt. Spectrosc. 1973, 34, 635. c In DMF; J. Gen. Chem. USSR 1985, 55, 803. d In
DMF; AdV. Exp. Med. Biol. 1997, 411, 597.
TABLE 4: Contributions (All Values in %) to Red Shift in Ground State Absorption Spectrum (Experimental Data Measured
in Pyridine)
effect of
phenylation
effect of benzo
annulationc
effect of naphtho
annulationd
total substituent
effect
distortione and
other effectsf-h
ZnTPTBP
Q
B
23a (42)b
33 (40)
67 (71)
43 (40)
90 (113)
76 (80)
10 (0)
24 (20)
ZnTPTNP
Q
B
15 (28)
30 (29)
87 (89)
62 (40)
102 (117)
92 (69)
0 (0)
8 (31)
PdTPTBP
Q
B
23 (23)
36 (51)
70 (80)
40 (30)
93 (103)
76 (81)
7 (0)
24 (19)
PdTPTNP
Q
B
14 (14)
32 (40)
89 (96)
57 (73)
103 (110)
89 (113)
0 (0)
11 (0)
a The meso-phenyl groups constrained to a planar conformation (theoretical data only). b The meso-phenyl groups (experimental data only).
c The benzo groups. d The naphtho groups. e Nonplanar distortion. f Combined effects of nonplanar distortion, meso-tetraphenylation, and tetrabenzo
annulation. g Combined effects of nonplanar distortion, meso-tetraphenylation, and tetranaphtho annulation. h Determined by subtracting the total
substituent effect from 100.
(∆E) for both the theoretical and experimental data. Analysis
of the shifts suggests that geometry of distortion, which
facilitates the interactions between the porphyrin extended
macrocycle and meso-phenyls, plays a role in the observed red
shift (Table 4). By comparing red shifts observed for the model
systems with those of our porphyrins, it is possible to discern
the overall contribution of each type of substitution in MTPTBP
and MTPTNP. The percentage is determined by dividing the
change in the energy (∆E) of either the effect of phenylation,
benzo annulation, or naphtho annulation by the change in the
energy (∆E) of all effects MTPTBP and MTPTNP. These
percentages are given in Table 4 for the corresponding functional
group contribution for both theoretical and experimental results.
The data show that a large percentage of the red shifts
observed for MTPTBP and MTPTNP is due to electronic effects,
which result from increased π-conjugation from the added benzo
or naphtho groups. Comparing MTPTBP and MTPTNP, for both
Zn and Pd, the effect of the addition of the four meso-phenyl
groups (theoretical data) is approximately one-quarter of the
total for the B-band and one-third of the total for the Q-band.
A large contribution, more than 70% of the total for the Q-band
and more than 40% of the total for the B-band, is attributed to
benzo and naphtho annulation. Thus, only about 25%-0% is
attributed to geometrical distortion and any other contributing
effects depending on the band and compound. There is a larger
effect observed for the Q-band than the B-band with annulation.
This is consistent with the origins of these transitions shown in
Figures 5 and 6. Note that the HOMO (b2) coefficients are
largely localized on the pyrrole nitrogen and meso-carbon atoms
while the HOMO-1 (b1) receives a major contribution from the
R- and â-carbon. Therefore, to selectively red shift the B-band,
an addition of π-conjugated groups at the meso position should
be made. Similarly to selectively red-shift the Q-band, the
addition of π-conjugated groups to the â position results in
lowering of the energy.
Shown in Table 3 are calculated data for both D4h and D2d
symmetry of ZnTPP and PdTPP. The D4h molecule is planar
whereas the D2d structure is slightly saddled. In the theoretical
calculations we are able to hold the geometry of the MTPP
chromophore planar (a) to calculate only the percentage of the
red shift that is attributed to electronic effects and eliminate
that due to distortion. The experimental data include distortion
effects (a*) from saddling of the porphyrin. It is known from
X-ray crystallography data47 that the MTPP ring of the metalated

11336 J. Phys. Chem. A, Vol. 107, No. 51, 2003
Rogers et al.
Figure 5. Molecular orbital diagrams for the Q- and B-transitions of zinc meso-tetraphenylporphyrin and its annulated derivatives. Solid and
dotted arrows represent the major and minor contributions, respectively.
chromophore is fairly planar with a slight saddle formation.
However, the phenyl groups are twisted out of the plane of the
porphyrin by as much as 70-80°. The twisting and slight saddle
formation is partly responsible for the red shift from metal-
loporphyrin (MP) to metalated meso-tetraphenylporphyrin (MTPP)
as well as for the π-conjugation effects of the phenyl group to
the porphyrin ring. The experimental data take this distortion
into account when calculating the percentages due to each effect.
Looking at the totals given in Table 4, the percentages are
sometimes greater than 100%. For the experimental data this is
due to slight saddle formation and also solvent effects. The
theoretical data for the Q-band of ZnTPTNP and PdTPTNP also
total to slightly more than 100%, suggesting that the substituents
effects are not strictly additive.
Theoretical interpretations of the spectra of the parent
compounds ZnP, ZnTBP, and ZnTPP have been presented
previously.48,49 Detailed comparisons of the computed spectra
with experiments have been given and will not be repeated here.
The results for the parent systems, computed using the same
level of theory as in the present work, are included in Table 3.
They will serve as references for analyzing the cumulative
effects of the benzo and phenyl groups on the absorption spectra.
TDDFT results for the other porphyrins have not been reported.
The average deviation between the computed excitation energies
and the spectral maxima taken in benzene for the Q-band is
about 0.2 eV. Slightly larger deviations are observed for the
B-band. Figures 5 and 6 display principal molecular orbitals
with major contributions to the excited state Q and B wave
functions. For MTPP and MTPTBP, the origins of the Q- and
B-bands can be traced to the two highest occupied (HOMO and
HOMO-1) and lowest unoccupied (LUMO and LUMO+1)
MOs. Note that the HOMO and HOMO-1 are similar in energy
in MTPP while they are well separated in MTPTBP (see Figures
5 and 6). However, these orbitals and the degenerate LUMOs
are well separated from the rest of the occupied and virtual MOs.
This type of MO arrangement provides the basis for the “four-
orbital model” to interpret the Q- and B-bands.50 In MTPP, the
HOMO (b2) coefficients are largely localized on the pyrrole
nitrogen and meso-carbon atoms, while the HOMO-1 (b1)
receives a major contribution from the R- and â-carbons. Thus,
tetrabenzo and tetranaphtho annulations significantly destabilize
the HOMO-1 of MTPP but leave the energy of the HOMOs
and LUMOs largely unchanged. The destabilization of the
HOMO-1 in MTPTBP and MTPTNP removes the quasi-
degeneracy of two highest occupied MOs found in MTPP. This,
by itself, does not cause a serious breakdown in the four-orbital
approximation. However, in MTPTNP, HOMO-1 and LUMO
are not well separated from other occupied and virtual MOs.
Apparently, the assumption that the degenerate LUMOs are well
separated from the rest of the virtual MOs, which provides the
basis for Gouterman’s four-orbital model, does not hold for
MTPTNP. This type of MO arrangement (also found in the
nonphenylated MTNP series) gives rise to an additional allowed
(and nonallowed) transition in the B region. On the basis of
their PPP calculations, Kuz’mitskii et al. have predicted the
complication of the ZnTNP absorption spectrum in the B region
due to the appearance of additional transitions.51 Our TDDFT
calculations support their conclusions.

Annulated Porphyrins
J. Phys. Chem. A, Vol. 107, No. 51, 2003 11337
Figure 6. Molecular orbital diagrams for the Q- and B-transitions of palladium meso-tetraphenylporphyrin and its annulated derivatives. Solid and
dotted arrows represent the major and minor contributions, respectively.
In unsubstituted metalloporphyrin (MP) and metalated meso-
tetraphenylporphyrin (MTPP), the two main configurations,
1u f eg and a2u f eg and b2 f e and b1 f e, respectively, are
for MTPTBP, and a 0.21 (0.11) eV shift for MTPTNP. For the
Q-band the opposite is observed with a 0.18 (0.13) eV shift for
MTPP, a 0.09 (0.07) eV shift for MTPTBP, and a 0.04 (0.05)
eV shift for MTPTNP, in pyridine (benzene). Note that for the
B-band a larger shift is observed in the MTPTNP macrocycle,
whereas for the Q-band there is a larger shift for MTPP. Gas-
phase TDDFT calculations reveal only modest effects on
changing the central metal. Therefore, we can assume that the
metal contributes differently in the annulated systems than in
MTPP.
The triplet excited state spectra are similar for both Zn and
Pd porphyrins. There is a small effect on the triplet excited state
spectrum by the metal used. Comparing the data in Figure 4,
we note that the shapes of the transient absorption spectra are
similar for ZnTPP and PdTPP with a sharp peak around 475
nm and a small absorption at lower energies. For ZnTPTBP
and PdTPTBP the spectra contain two larger absorptions in the
region from 400 to 600 nm. For ZnTPTNP and PdTPTNP the
spectra contain more different features. While both are broad,
i.e., from 400 to 800 nm, there are several peaks that differ
with metal. To better understand these features, calculations on
the triplet excited states are being carried out.
a
almost degenerate (to a lesser extent for MTPP), and their
transition moments essentially cancel each other. The incomplete
cancellation of transition moments for porphyrin derivatives was
identified by Weiss et al.52 In the annulated porphyrins, the
absence of quasi-degeneracy between HOMO and HOMO-1
leads to an increase in the intensity of the Q-band due to the
minor contribution from the b2 f e configuration. Thus, the
origin of the increase in the intensity of the Q-band is connected
to the reduction in b1-e orbital energy gaps that can be
attributed to the destabilization of the HOMO-1 of MTPP.
A red shift and a broadening in the triplet-excited state spectra
were observed with benzo or naphtho annulation (Figure 4).
For both the Pd and Zn series similar spectral changes were
observed. The MTPP chromophores have very intense peaks
directly to the red of the B-bands with only small absorptions
to the red of the Q-bands. For MTPTBP we see a broader
absorption from 400 to 600 nm that consists of two peaks. For
MTPTNP the broadest absorption spectrum is found that
contains several peaks. The origin of this broadening with
annulation is not understood.
Kinetic Changes. Effect of Extension of Porphyrin Ring. The
effect of annulation on the kinetic properties for a series of
porphyrins is varied. Comparing the quantum yields and
lifetimes (Table 2), no clear trend is observed as a function of
annulation, indicating a complex interaction of causes. Compar-
ing the intersystem crossing quantum yields given in Table 2,
Effect of Metal. Comparing the ground state spectra of the
Zn and Pd chromophores, we note that the bands in the Zn series
are further red-shifted than for the Pd porphyrins. The B-band
shift observed in pyridine and benzene (shown in parentheses)
is about 0.09 (0.04) eV shift for MTPP, 0.17 (0.11) eV shift

11338 J. Phys. Chem. A, Vol. 107, No. 51, 2003
Rogers et al.
PdTPTNP
TABLE 5: Rate Constants of Singlet and Triplet State Relaxation
ZnTPP
ZnTPTBP
ZnTPTNP
PdTPP
PdTPTBP
ks (s-1
kf (s-1
)
)
3.7 × 108 a
1.1 × 107
3.1 × 108
4.9 × 107
2.7 × 102
1.6 × 10-1
2.7 × 102
1.5 × 109 b
1.1 × 107
1.5 × 109
2.7 × 107
6.0 × 103
6.0 × 10-2
6.0 × 103
5.0 × 1010 c
2.0 × 107
4.8 × 1010
2.0 × 109
9.1 × 105
3.8 × 102
9.1 × 104
k
k
ISC (s-1
)
)
ICS (s-1
kT (s-1
)
)
2.3 × 103
7.3 × 10-1
2.3 × 103
5.1 × 103
8.8 × 102
4.2 × 103
1.7 × 104
1.4 × 103
1.5 × 104
d
kp (s-1
k
ICT (s-1
)
a Measured in methylcyclohexane; J. Chem. Soc., Faraday Trans. 1 1980, 76, 1978. b Measured in benzene; J. Phys. Chem. 1996, 100, 17507.
c Measured in methylcyclohexane; J. Chem. Soc., Faraday Trans. 2 1981, 77, 1281. d Values from Table 2 (phosphorescence decay); kT ) 1/τT.
TABLE 6: Calculated 6-31G(d) Energies of Triplet Excited State Levelsa
ZnTPP
ZnTPTBP
ZnTPTNP
PdTPP
PdTPTBP
PdTPTNP
T1 (eV)
T2 (eV)
T3 (eV)
T4 (eV)
S1 (eV)b
1.61
1.92
2.21
3.18
2.30
1.44
1.80
2.01
3.10
2.05
1.19
1.51
2.10
2.27
1.82
1.75
2.03
2.25
2.67
2.40
1.54
1.86
2.15
2.54
2.14
1.32
1.60
2.17
2.25
1.90
a T1 energies are the computed adiabatic values while T1+n are computed with the T1 geometries. b Energies from calculated maximum of Q-band
from Table 3.
it is interesting to note that for both metals we see a decrease
in the triplet quantum yield for MTPTNP and an increase for
MTPTBP relative to MTPP.
not the case for the Zn series. However, for the Zn series we
observe an increase in internal conversion (kICT) from the triplet
state as a function of increased annulation. We do not see this
for the Pd series. Since no clear trend is observed, this indicates
that no single process is dominating, but rather there is a
complex interaction of several processes.
Listed in Table 5 are the calculated rates for decay from both
the singlet (using published singlet state lifetimes of ZnTPP,
PdTPP, and ZnTPTBP)39,42 and triplet excited state. Comparing
the rate constants for decay from the singlet excited state
(fluorescence (kf), intersystem crossing (kISC), and internal
conversion (kICS)), we note that intersystem crossing is the
predominant pathway of relaxation. We know that the inter-
system crossing quantum yield of MTPTNP is smaller than these
in the other chromophores, and we do not see a large increase
in its fluorescence quantum yield, so we expect that internal
conversion from the singlet state is more competitive for this
chromophore. This might be expected solely on the basis of
the singlet state energies (ES1, given in Table 2). With extension
of the π-system we observe a remarkable red shift of the ground
state spectrum with a lowering of the singlet excited state energy,
thus providing an easier pathway of relaxation. We hope to
measure the singlet state lifetimes in the near future.
In an attempt to better understand the process of intersystem
crossing from the singlet excited state to the triplet excited state,
the triplet excited state levels were calculated using TDDFT
and are shown in Table 6. Assuming that intersystem crossing
is occurring exclusively from the lowest energy singlet state
(S1), comparing the triplet excited state levels with the first
singlet excited state level (S1), the data show that some of the
energy levels overlap. Given in Table 6 are the S1 energies and
the various T1 levels. For ZnTPP, ZnTPTBP, PdTPP, and
PdTPTBP the energy of the S1 state overlaps fairly well with
the T3 level with only small gaps in energy (<0.15 eV). For
both ZnTPTNP and PdTPTNP there is a larger gap (0.31 and
0.30 eV, respectively) between S1 with the T2 level, and the T3
state is much higher in energy and unattainable by the S1 state.
Therefore, the singlet-triplet coupling for ZnTPP, ZnTPTBP,
PdTPP, and PdTPTBP is larger than for ZnTPTNP and
PdTPTNP comparatively, and thus intersystem crossing is faster
resulting in larger quantum yields.
Effect of Metal. An effect of varying the central metal from
Zn to Pd is observed in the kinetic parameters we measured.
Pd is a heavier atom, so it influences the production of the triplet
state, resulting in faster intersystem crossing through enhanced
spin-orbit coupling. It has been reported53 that an increase in
spin-orbit coupling alters the photophysical processes in such
a way that (a) an increase in phosphorescence quantum yield is
observed, (b) an increase in the radiative rate constant of
phosphorescence, (c) a decrease in fluorescence quantum yield,
and (d) an increase in the rate constant of intersystem crossing.
Comparing the data from Table 2 and Table 5, all of the above
effects on the photophysical processes are met. For the Pd series
relative to the Zn series there is an increase in the phosphores-
cence quantum yield, an increase in the radiative rate constant
of phosphorescence is observed, and a decrease in the fluores-
cence quantum yield was determined. Comparing the limited
data of intersystem crossing, we observe a large increase in the
rate constant for PdTPP relative to ZnTPP. We assume that the
same trend would be true for the annulated porphyrins solely
on the basis of the heavy atom effect. Heavy atoms change
spin-orbit coupling, with little relative change in energy levels
and spectra of the annulated forms, giving an independent
control of intersystem crossing.
Conclusions
The trends observed for Zn and Pd series of structurally
related porphyrins provide the basis for understanding the
photophysical properties inherent for these types of chro-
mophores. Extension of the porphyrin ring leads to dramatic
red shifts in the ground state spectra that are mainly due to
increased conjugation effects with contributions from distortion.
The red shifts observed for these annulated porphyrins can be
traced to the destabilization of the HOMO-1 of the tetraphen-
ylporphyrins. A red shift in the ground state spectra was
observed when changing the central metal from Pd to Zn, thus
indicating involvement of the metal in the optical transitions.
The kinetic parameters of these chromophores are more de-
In Table 5 a comparison is also made for the decay of the
triplet excited state. For both metal series internal conversion
from the triplet excited state is the predominant pathway for
relaxation. For the Pd series there is a trend that the rate of
phosphorescence decay (kp) increases with annulation. This is

Annulated Porphyrins
J. Phys. Chem. A, Vol. 107, No. 51, 2003 11339
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porphyrins. This may be attributed to a difference in the number
of favorable S1-Tn channels, as predicted by the TDDFT
calculations. Future directions of this research include modifica-
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various functional groups to the benzo and/or naphtho region
for maintaining solubility in the absence of the meso-phenyl
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Acknowledgment. We are thankful for the support of this
work by AFRL/ML Contracts F33615-99-C-5415 for D.G.M.
and F33615-97-D-5403 for J.E.R., K.A.N., D.C.H., W.S., and
K.M.G. The calculations were performed on computers at the
Aeronautical Systems Center (ASC) Major Shared Resource
Center (MSRC). The support provided by the ASC MSRC
Service Center is greatly appreciated. We also thank Prof. Kirk
Schanze at the University of Florida for the photoacoustic
calorimetry data.
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