Photophysical investigation of neutral and diprotonated free-base bis(arylethynyl)porphyrins

Article abstract of DOI:10.1021/jp205309f

The photophysical properties for a series of free-base arylethynyl porphyrins and the corresponding trans-disubstituted tetraphenylporphyrin (H₂TPP) derivatives lacking arylethynyl functionalities have been studied via electronic absorption and emission spectroscopy in both neutral and diacid forms. Enhanced substituent effects on porphyrin absorption spectra are observed in the arylethynyl porphyrins relative to the H₂TPP derivatives, owing to the presence of the ethynyl spacer that allows for a coplanar geometry between the porphyrin macrocycle and the appended phenyl substituents. Upon protonation, both series of porphyrins exhibit substantially red shifted absorption and emission spectra and enhanced oscillator strengths, with the magnitude of the spectral shifts being more substantial in the presence of the ethynyl functionalities. Spectral features of the arylethynyl porphyrin bearing p-dimethylamino substituents closely resemble those previously classified as "hyperporphyrin spectra" and are indicative of excited-state charge-transfer character. Protonation of both series of porphyrins results in reduced fluorescence lifetimes and enhanced nonradiative decay rates, and the impact of protonation on these parameters is attenuated in the presence of the arylethynyl functionalities. Our results coupled with previous structural data showing that arylethynyl porphyrins exhibit less structural distortion upon diacid formation relative to H₂TPP further substantiate the proposal that significant alteration of porphyrin photophysical properties upon diacid formation can be attributed to nonplanar structural distortions induced by protonation.

Full text of DOI:10.1021/jp205309f

ARTICLE
pubs.acs.org/JPCA
Photophysical Investigation of Neutral and Diprotonated Free-Base
Bis(Arylethynyl)porphyrins
Peter K. Goldberg,† Tom J. Pundsack,‡ and Kathryn E. Splan*,†
†Department of Chemistry, Macalester College, 1600 Grand Avenue, Saint Paul, Minnesota 55105, United States
‡Department of Chemistry, University of Minnesota, 207 Pleasant Street Southeast, Minneapolis, Minnesota 55455, United States
S Supporting Information
b
ABSTRACT: The photophysical properties for a series of
free-base arylethynyl porphyrins and the corresponding trans-
disubstituted tetraphenylporphyrin (H2TPP) derivatives lack-
ing arylethynyl functionalities have been studied via electronic
absorption and emission spectroscopy in both neutral and
diacid forms. Enhanced substituent effects on porphyrin ab-
sorption spectra are observed in the arylethynyl porphyrins
relative to the H2TPP derivatives, owing to the presence of the
ethynyl spacer that allows for a coplanar geometry between the
porphyrin macrocycle and the appended phenyl substituents. Upon protonation, both series of porphyrins exhibit substantially red
shifted absorption and emission spectra and enhanced oscillator strengths, with the magnitude of the spectral shifts being more
substantial in the presence of the ethynyl functionalities. Spectral features of the arylethynyl porphyrin bearing p-dimethylamino
substituents closely resemble those previously classified as “hyperporphyrin spectra” and are indicative of excited-state charge-
transfer character. Protonation of both series of porphyrins results in reduced fluorescence lifetimes and enhanced nonradiative
decay rates, and the impact of protonation on these parameters is attenuated in the presence of the arylethynyl functionalities. Our
results coupled with previous structural data showing that arylethynyl porphyrins exhibit less structural distortion upon diacid
formation relative to H2TPP further substantiate the proposal that significant alteration of porphyrin photophysical properties upon
diacid formation can be attributed to nonplanar structural distortions induced by protonation.
’ INTRODUCTION
photosynthetic systems,6,11,15 nonlinear optical materials,10,12,13
and biological imaging.14
The remarkable photophysical properties of porphyrins
render these compounds popular components in a range of
biomimetic and optoelectronic systems.1ꢀ4 However, the nar-
row absorption profiles exhibited by porphyrins limit their use
in many photophysical applications. Common synthetic por-
phyrins such as meso-tetraphenylporphyrin (H2TPP) exhibit an
intense absorption ca. 400ꢀ450 nm (B-band region) but
weaker absorption in the red region of the spectrum from
600 to 800 nm (Q-band region). Addition of arylethynyl
moieties to the porphyrin macrocycle constitutes an effective
strategy with which to enhance light-harvesting properties of
porphyrins across the visible region.5ꢀ15 The ethynyl function-
alities elongate the conjugated π system of the porphyrin and
remove the steric barrier between the phenyl groups and the
porphyrin macrocycle, expanding conjugation along one or
both axes of the molecule. Such modification results in con-
siderable red shifts of the porphyrin B- and Q-band regions and
enhanced oscillator strengths of the Q-bands.9 Furthermore,
the enhanced conjugation facilitates electronic communication
between the porphyrin and appended functional groups, allow-
ing for synthetic modulation of electronic properties.5,8,9
Owing to their unique optical properties, arylethynyl porphyr-
ins are used in a wide range of applications including artificial
While zinc,5ꢀ11 copper,16 nickel,17,18 and iron19 arylethynyl
porphyrins have been reported, studies of free-base arylethynyl
porphyrins are limited to only a few examples,9,20ꢀ23 despite the
unique functionality of metal-free porphyrins. Free-base por-
phyrins typically exhibit enhanced fluorescence quantum yields
and excited state lifetimes relative to metalated derivatives.24 Por-
phyrin diacids may be obtained upon protonation of the pyrrolic
nitrogen atoms and exhibit perturbed photophysical properties
relative to the neutral derivatives.25ꢀ28 Under appropriate con-
ditions, some free-base porphyrins exhibit “hyperporphyrin”
spectra that feature spectral patterns that cannot be described
by the porphyrin four-orbital model29 but instead arise from a
charge-transfer transition between the appended substituent
and the porphyrin ring.30ꢀ34 Moreover, these transitions
occur at lower energy and with higher intensity than regular
porphyrin Q-band transitions, allowing for enhanced light-
harvesting properties in the red region of the spectrum.
Surprisingly, these aspects of free-base porphyrin chemistry have
Received: June 6, 2011
Revised:
July 25, 2011
Published: July 27, 2011
r
2011 American Chemical Society
10452
dx.doi.org/10.1021/jp205309f J. Phys. Chem. A 2011, 115, 10452–10460
|

The Journal of Physical Chemistry A
ARTICLE
(99%+) and trifluoroacetic acid (Reagent Plus) were purchased
from Sigma-Aldrich. The synthesis and characterization of 1aꢀe
and 2bꢀe are included in the Supporting Information.
Spectroscopic Methods. UVꢀvis absorption spectra were
recorded on a Cary 50 spectrophotometer (Varian, Inc.) or an
Agilent 8453 diode array spectrophotometer. Steady-state emis-
sion spectra to determine wavelength maxima and Stokes shifts
were collected on a Cary Eclipse fluorimeter (Varian, Inc.) and
corrected for instrument response. Excitation wavelengths were
in the Soret or near-UV region, and spectral line shapes were
found to be independent of excitation from 375 to 450 nm. All
measurements were performed on porphyrin solutions that were
<6 μM. All photophysical measurements were carried out in
toluene or chloroform solutions containing oxygen from air at
ambient temperature.
Extinction coefficients for the free-base porphyrins were mea-
sured in chloroform owing to the enhanced solubility of the
compounds in this solvent relative to toluene. Spectra for free-
base porphyrins were recorded in the presence of 0.5% triethyla-
mine to neutralize trace amounts of HCl released from chloro-
form. Porphyrin dications were generated upon addition of 5 μL
neat TFA to a 3.0 mL porphyrin solution or upon titration of the
free-base derivative with dilute TFA (∼0.01%) until no further
spectral shifts were observed. Specifically for 1e and 2e, dilute TFA
was added to ensure protonation occurred only at the central
nitrogens and not the peripheral functional groups. Oscillator
Figure 1. Molecular structures of porphyrins studied herein.
not been extended to arylethynyl porphyrins nor have any funda-
mental photophysical studies of free-base arylethynyl porphyrins
been reported.
Protonation at the central nitrogen atoms of H2TPP results
in nonplanar distortions of the porphyrin macrocycle27,28 and
induces significant alteration of porphyrin photophysical proper-
ties, including red-shifted absorption spectra, larger Stokes shifts,
broadened fluorescence line shapes, and shortened excited state
lifetimes.25,26,32,35 Moreover, the degree to which these photo-
physical properties are modulated in H2TPP and other free-base
porphyrins correlates to the degree of structural distortion upon
protonation.25 For example, substituent effects on porphyrin
spectra are enhanced in porphyrin diacids of H2TPP derivatives,
in part owing to structural distortions that allow for the phenyl
groups to assume a coplanar geometry with the porphyrin
macrocycle.32 However, crystal data reveal that arylethynyl
porphyrins show less structural distortion upon diacid formation
relative to H2TPP,19 suggesting that the impact of protonation
on porphyrin photophysics may be modulated in free-base
arylethynyl porphyrins.
To delineate the impact of the ethynyl linkers on free-base
arylethynyl porphyrin photophysics, herein we report the synthesis
and photophysical characterization of arylethynyl porphyrins 1aꢀe
and the trans-disubstituted H2TPP derivatives lacking arylethynyl
functionalities 2aꢀe (Figure 1). We chose to focus on 5,15-bis-
(arylethynyl)-10,20-diphenylporphyrins owing to the noted insolu-
bility of metal-free meso-tetra(arylethynyl)porphyrins,21,22 which has
hindered their use in optical applications. Furthermore, expansion
of π conjugation along only one axis of the molecule imparts a
dominant transition dipole moment oriented along this axis,9 which
may result in anisotropic charge and energy transport properties
throughout aggregates or other self-assembled structures of the
molecules. Compounds 1aꢀe and 2aꢀe comprise a series of
porphyrins that feature both electron-withdrawing and electron-
donating functional groups, allowing for evaluation of substituent
effects on porphyrin photophysics. Moreover, the presence of the
ethynyl spacers in 1aꢀe reduces the structural distortion upon
protonation of the porphyrin, allowing for observed photophysical
properties to be correlated with the degree of structural distortion.
Taken together, this work constitutes the first thorough photophy-
sical study of free-base arylethynylporphyrins and will expand the
utility of these chromophores in photoactive applications.
strength (f) for a given transition was calculated as f = (4.319 ꢁ
R
10ꢀ9/n) ε dv, where n is the refractive index (1.45 for CHCl3),
ε is the extinction coefficient (Mꢀ1 cmꢀ1), and ν is the frequency
in wavenumbers (cmꢀ1). Values were calculated over an appro-
priate wavelength domain specific to fully account for each
transition as described in the text.
To determine relative acid-dissociation constants of the pyrrolic
nitrogens, 1a and 2a were titrated with 0.2 and 0.02 M TFA in
toluene, respectively. Five microliter aliquots were added to a
3.0 mL porphyrin solution (6 μM or less), and the spectrum was
recorded. Relative acid-dissociation constants were calculated
from a plot of ꢀlog [TFA] vs log ([H2P]/[H4P2+] as described
in the text. [H2P]/[H4P2+] was calculated from the Soret band
absorbance for the neutral species as (A ꢀ Af)/(Ai ꢀ A), where A
is the observed absorbance of the solution and Ai and Af are the
initial and final absorbance values at the beginning and end of the
titration, respectively.
Quantum yield measurements were collected in toluene
using a SPEX Fluorolog 1680 0.2 m double spectrometer with
an excitation wavelength of 375 nm. The optical density of each
sample at 375 nm was between 0.29 and 0.31 in a 1 cm quartz
cell (measured on a Cary 14 spectrophotometer). All spectra
were corrected for instrument sensitivity. After each porphyrin
fluorescence measurement, the fluorescence spectrum of
[2-[2-[4-(dimethylamino)phenyl]ethenyl]-6-methyl-4H-pyran-
4-ylidene]-propanedinitrile (Exciton) was also collected to ac-
count for any shift in the fluorimeter response over time.
Quantum yields for the entire data set were then calculated
and normalized such that the quantum yield of H2TPP (2a) was
in agreement with the literature value of 0.11.36 The 95%
confidence limit of each quantum yield measurement was less
than 4% of the quantum yield value.
’ EXPERIMENTAL SECTION
Fluorescence lifetimes were measured in toluene using time-
correlated single-photon counting. Samples were excited with a
375 nm diode laser with a repetition rate of 40 MHz (PicoQuant
PDL 800-B laser driver with a PicoQuant LDH-P-C-375 laser
General. All solvents were ACS reagent grade and used as
received. Chloroform was from Sigma-Aldrich, and toluene was from
either Sigma-Aldrich or Pharmco. 5,10,15,20-Tetraphenylporphyrin
10453
dx.doi.org/10.1021/jp205309f |J. Phys. Chem. A 2011, 115, 10452–10460

The Journal of Physical Chemistry A
ARTICLE
Scheme 1
head). Emission from the sample was sent through a double
monochromator (Jobin-Yvon DH10) and detected with an
avalanche photodiode (Micro Photon Devices PDM detector).
Timing between the laser excitation and emission detection was
monitored using a Timeharp 200 PicoQuant PCI Board. For
each sample, the monochromator was set to the wavelength at
the maximum in the fluorescence spectrum. Lifetimes were
determined by nonlinear regression fitting of the data to a
single-exponential decay convoluted with the instrument
response, which was 600 ps (Gaussian, fwhm). The 95% con-
fidence limits of the lifetime fits were all less than 5% of the
lifetime.
’ RESULTS AND DISCUSSION
Synthesis of Compounds. Porphyrins 1aꢀe were synthe-
sized upon demetalation of the corresponding Zn(II) precursors
6aꢀe, whose syntheses have been previously reported with the
exception of 6c.9 Zn(II) porphyrins 6aꢀd were synthesized via
an alternative route to that reported previously, which is pre-
sented in Scheme 1. Compound 3 was synthesized via condensa-
tion of 5-phenyldipyrromethane37 with trimethylsilylpropynal
according to methods developed by Anderson and co-workers,38
followed by zinc metalation to prevent insertion of the metal
catalysts employed in the subsequent coupling step. Upon TMS
deprotection, Pd-catalyzed coupling with suitable aryl iodides
afforded the Zn(II) precursors 6aꢀd. Compound 6e was pre-
pared as described by LeCours and co-workers.9 The metalated
products were treated with hydrochloric acid to remove the
central Zn(II), yielding 1aꢀe. The reversibility of the demetala-
tion step was demonstrated upon addition of zinc acetate to a
solution of the isolated free-base porphyrin in chloroform.
Remetalation was confirmed via UVꢀvis spectral data. We note
that preparation of 1a, 1b, 1d, and 1e was described by Therien
and co-workers, but no NMR, mass spectral, elemental analysis,
nor photophysical data were presented.9 To allow for direct
comparison between trans-disubstituted H2TPP derivatives
lacking arylethynyl functionalities, porphyrins 2bꢀe were
synthesized via condensation of 5-phenyldipyrromethane and
Figure 2. Absorption spectra of 4 μM solutions of H2TPP, 1a, and 3
recorded in chloroform. (Inset) Magnified view of the Q-band region.
the corresponding para-substituted aldehyde via a procedure
devised by Lindsey and co-workers.39 Full characterization of
1aꢀe and 2bꢀe are presented in the Supporting Information.
Absorption Spectroscopy of Neutral and Diacid Porphyr-
ins. Porphyrin 1a exhibits distinctive electronic spectral char-
acteristics relative to H2TPP (Figure 2 and Table 1). Upon
functionalization of the meso positions with ethynyl linkages,
both the B and Q bands are substantially red shifted owing to the
expanded conjugation of the porphyrin macrocycle. Moreover,
the spectrum of 1a is considerably more red shifted than that of 3,
revealing that orbital delocalization expands over the appended
phenyl groups. The Q-band pattern of 1a is markedly different
from that of H2TPP. The intensities of the Q x(0,0) and Q y(0,0)
bands are significantly enhanced, while the Q(1,0) bands are
nearly unaffected. This enhanced oscillator strength in the
Q-band region results from breaking the degeneracy of the a1u
and a2u orbitals via meso-carbon substitution with the arylethynyl
groups, allowing for a more allowed transition. The enhanced
light-harvesting efficiency in the red region of the spectrum is of
10454
dx.doi.org/10.1021/jp205309f |J. Phys. Chem. A 2011, 115, 10452–10460

The Journal of Physical Chemistry A
Table 1. Selected Electronic Absorption Data Recorded in CHCl3 for Compounds 1aꢀe and 2aꢀe (n.a. = not applicable)
ARTICLE
Soret band
Q-(0,0)a
λmax,b nm
(ε/103 cmꢀ1 Mꢀ1) neut./cat.
λmax,
b nm
(ε/103 cmꢀ1 Mꢀ1) neut./cat.
subst
shift, nmc
fBd neut./cat.
shift, nmc
fQe neut./cat.
1a
1b
1c
1d
1e
2a
2b
2c
2d
2e
H
441 (347)/459 (278)
441 (324)/459 (262)
443 (292)/464 (228)
447 (309)/473 (216)
464 (112)/433 (110), 500 (34)
418 (414)/437 (391)
418 (424)/437 (423)
419 (351)/439 (342)
420 (395)/447 (311)
425 (150)/418(133), 496 (81)
18
18
21
26
n.a.
19
19
20
27
n.a.
1.44/1.52
1.28/1.51
1.29/1.32
1.24/1.48
1.04f/1.07g
1.42/1.48
1.38/1.70
1.12/1.41
1.27/1.40
1.28/1.29j
688 (23)/708 (62)
689 (21)/706 (59)
692 (18)/712 (55)
695 (20)/726 (65)
715 (21)/802 (49)
646 (4.5)/653 (39)
646 (3.5)/653 (44)
647 (4.0)/660 (40)
648 (3.4)/669 (51)
656 (5.5)/723 (60)
20
17
20
31
87
7
0.200/0.247
0.196/0.251
0.194/0.272
0.214/0.332
0.212h/0.297i
0.114/0.175
0.0916/0.196
0.100/0.198
0.0561/0.174
0.117/0.286k
F
Me
OMe
NMe2
H
F
7
Me
13
21
67
OMe
NMe2
a Data corresponds to the lowest energy Q-band transition. Full spectral data is included in the Supporting Information. b (1 nm. c Observed red shift
upon protonation of the free-base porphyrin. d Oscillator strengths calculated from 350 to 500 nm and 350 to 530 nm for the neutral and cation species,
respectively, except where noted. e Oscillator strengths calculated from 500 to 800 nm and 530 to 800 nm for the neutral and cation species, respectively,
except where noted. f Calculated from 350 to 530 nm. g Calculated from 350 to 610 nm. h Calculated from 530 to 800 nm. i Calculated from 610 to
900 nm. j Calculated from 350 to 600 nm. k Calculated from 600 to 800 nm.
Figure 3. Titration spectra of 1a (top) and H2TPP (bottom) with
trifluoroacetic acid in toluene. The fully formed dication is represented
Figure 4. Plot of ΔE (kcal/mol) for the Q(0,0) transition vs 2σ
in red. (Inset) Plot of log ([H2P]/[H4P2+]) as a function of ꢀlog[TFA].
for 1aꢀe (top) and 2aꢀe (bottom). Hammett values for σ: 0.062
(F), ꢀ0.14 (Me), ꢀ0.28 (OMe), ꢀ0.63 (NMe2) were taken from ref 41.
The values for F given in parentheses represent the slopes for lines fit
only to the data from 1aꢀd and 2aꢀd.
particular interest for applications in solar energy conversion and
photodynamic therapy, wherein the narrow spectral response of
porphyrins poses significant limitations. Moreover, the trans-
arylethynyl substitution pattern imparts a dominant transition
dipole moment on the molecules9 and when assembled into
aggregate structure may result in facilitated charge and energy
transport.
Protonation of 1a with trifluoroacetic acid (TFA) (Figure 3)
results in significant electronic absorption spectral changes that
are consistent with the spectral features exhibited by H4TPP2+.
Owing to an increase in symmetry, the Q-band region collapses
10455
dx.doi.org/10.1021/jp205309f |J. Phys. Chem. A 2011, 115, 10452–10460

The Journal of Physical Chemistry A
ARTICLE
to one major peak, analogous to spectral changes observed upon
metalation of the porphyrin macrocycle. Furthermore, proton-
ation results in substantial red shifts in the B- and Q-band regions
of the spectrum and a significant increase in intensity of the
lowest energy transition.
The relative basicity of the central nitrogens of both 1a and
H2TPP can be quantified upon titration of the porphyrins with
TFA. Titration spectra of both 1a and H2TPP feature stable
isosbestic points, implying that the monocation species is not
observed during titration and that the two protonation steps are
nearly indistinguishable under the experimental conditions. An
equilibrium constant can then be defined for the two-proton
2
process given in eq 1 as K = (Ka,obs
)
H4P2þ f H2P þ 2Hþ
ð1Þ
where Ka,obs represents the observed acid dissociation constant
for the protonated pyrrolic nitrogen. Therefore, pKa,obs = ꢀlog-
[TFA]1/2, wherein log[TFA]1/2 is the midpoint concentration of
trifluoroacetic acid as obtained from titration data (Figure 3,
inset). A plot of log ([H2P]/[H4P2+]) as a function of ꢀlog-
[TFA] yields x intercepts that correspond to pKa,obs = 2.9 for 1a
and pKa,obs = 4.1 for H2TPP. It should be noted that pKa,obs
represents only a relative acidity constant, since dissociation of
TFA is not taken into account and is only proportional to, but not
equal to, the proton concentration in solution. The higher
concentration of TFA required to protonate 1a relative to
H2TPP is attributed to the enhanced delocalization of the central
nitrogen electron density over the elongated porphyrin macro-
cycle. This attenuated basicity further highlights the impact of the
ethynyl spacers on free-base porphyrin chemistry.
Substituent Effects on Porphyrin Spectra. The substituent
effects on the spectra of the neutral and diacid porphyrins 1aꢀe
and corresponding series of substituted tetraphenylporphyrin
derivatives 2aꢀe can be described by the Hammett linear free-
energy equation, ΔE = 2σF, where σ is the substituent constant
that takes into account both inductive and resonance effects of a
given substituent and F is the “reaction” constant.40,41 The
Hammett relation has successfully been used to correlate the
nature of the substituent to multiple porphyrin properties includ-
ing transition energies,32,42 redox potentials,8,43 and axial ligation
reactions at porphyrin metal centers.44 As shown in Figure 4 and
summarized in Table 1, the Q-band regions of the spectra for the
neutral porphyrins 1aꢀe and 2aꢀe reveal a modest decrease in
energy of the transitions upon greater electron-donating ability of
the substituent. The larger F values for 1aꢀe relative to 2aꢀe
indicate that the impact of the phenyl substituent is enhanced by
the presence of the arylethynyl moieties, as previously noted for
Zn(II) (arylethynyl)porphyrins.8,9 Similar but less substantial
trends are observed in the Soret region.
Figure 5. (Top) Absorption spectra of 1e upon titration with dilute
trifluoroacetic acid to form the porphyrin dictation, depicted in red.
(Bottom) Absorption spectra upon further addition of concentrated
TFA to H41e2+, resulting in a spectrum similar to that of H41a2+ and
shown in blue.
observed for porphyrins that exhibit “hyperporphyrin” spectra.
This point is further expanded upon below.
The red-shifted absorption spectrum exhibited by H4TPP2+
relative to that for H2TPP has been attributed, in part, to the
distortion of the porphyrin ring upon protonation. Ruffling of the
macrocycle allows the phenyl rings to become coplanar with the
porphyrin ring and extends the aromatic system.25 Restricting
our analysis to compounds 1aꢀd and 2aꢀd, wherein the charge-
transfer character alluded to above is small or nonexistent, the
enhanced substituent effects on Q-band transition energy seen
for 2aꢀd upon protonation can reasonably be attributed to
increased porphyrin/phenyl group conjugation upon nonplanar
distortion. In contrast, the appended phenyl groups in 1aꢀd can
assume a coplanar geometry with the porphyrin core in both the
neutral and the dicationic forms, implying that dication forma-
tion should not enhance substituent effects to the degree
observed for 2aꢀd. Analysis of the Hammett plots reflect this
notion, as F values (given in parentheses in Figure 4) increase
∼6-fold upon protonation of 2aꢀd but only ∼3-fold for 1aꢀd.
While Hammett correlations for the two series of porphyrin
diacids display similar F values and therefore similar substituent
effects, it is interesting to note that the absolute magnitude of the
Q-band spectral shifts upon protonation are considerably larger
for 1aꢀe vs 2aꢀe (Table 1). As stated above, the H2TPP Q-band
shifts to lower energy upon protonation owing to conjugation of
the phenyl groups induced by structural distortion. In contrast,
the Q-band absorption for the octaethylporphyrin dication
(H4OEP2+) (functionalized at the β positions) is blue shifted
Upon protonation, the Q-band transition energies for both
series of porphyrins become considerably more sensitive to the
nature of the para substituent, demonstrated by a steeper slope in
the Hammett plots. For both series of porphyrins, the plots for
the dications seem to hint at a slight nonlinear decrease in
transition energy. The nonlinearity persists when transition
energies are plotted against σ+ constants (data not shown),
which take into account a substituent’s propensity to delocalize a
positive charge.40 The observed curvature of the dication plots
may be indicative of an excited state that assumes an increasing
degree of charge-transfer character as the substituent becomes
more strongly electron donating, which has previously been
10456
dx.doi.org/10.1021/jp205309f |J. Phys. Chem. A 2011, 115, 10452–10460

The Journal of Physical Chemistry A
ARTICLE
from that for H2OEP, enforcing the rationale that protonation
induces enhanced conjugation in H4TPP2+.25 By analogy, it is
reasonable to expect 1aꢀe to exhibit attenuated shifts upon
protonation relative to 2aꢀe, again owing to phenyl/porphyrin
coplanarity in both the neutral and the diacid species. For a given
substituent however the Q-band transition energy shifts con-
siderably more in the presence of the ethynyl groups than in the
absence. One possibility is that protonation at the central
nitrogens lowers the energy of the porphyrin π/π* orbitals to
better match those of the appended phenyl group and the
presence of the ethynyl functionality allows for greater electron
delocalization.
Hyperporphyrin Spectra. Some H4TPP2+ derivatives bearing
highly electron-donating substituents exhibit “hyperporphyrin”
spectra, which feature absorption bands that are not described by
the four-orbital model.29 The decrease in energy of the HOMO
upon protonation coupled with the destabilized orbital of the
phenyl substituent in the presence of a strong electron donor allows
for the π(phenyl) orbital or the substituent lone-pair orbital to
appear as the HOMO. The lowest-energy electronic transition
is of charge-transfer character and strongly allowed, resulting in
a highly red-shifted and intense absorption.31,32,34,45 Previous
studies show that protonated meso-(p-(dimethylamino)phenyl
porphyrins that contain one to four ꢀNMe2 substituents exhib-
it substantially broadened and red-shifted spectra relative to
H4TPP2+.31,32 Electronic structure calculations that revealed
the HOMO was localized on the aminophenyl ring confirmed
the charge-transfer nature of the lowest energy transition but
could not distinguish the HOMO as purely phenyl or sub-
stituent based.45 Wamser and co-workers employed porphyrin
spectral shifts combined with measured redox potentials to
predict HOMO and LUMO energy levels for H2TPP dication
derivatives. On the basis of this analysis, hyperporphyrin effects
become relevant for dications bearing any electron-donating
substituent (relative to H2TPP).32
Analogous to these observations, formation of H41e2+
(Figure 5, top) produces distinctive spectral changes relative to
H41a2+ that are indicative of hyperporphyrin character. Specifi-
cally, the porphyrin B band is split while the Q-band is drama-
tically shifted by 87 nm relative to that of the neutral 1e. Further
addition of TFA (Figure 5, bottom) protonates the peripheral
dimethylamine functionalities, resulting in a “normal” acidic
porphyrin spectrum with single B (450 nm) and Q (697 nm)
bands. Significantly, the putative hyperporphyrin transition of
H41e2+ is of substantially lower energy than that of H42e2+
(Table 1), which lacks the ethynyl spacers, establishing arylethynyl
hyperporphyrins as an elegant strategy to extend the light-harvesting
properties of porphyrins well beyond that of tetraphenylporphyrin
derivatives. The nonlinear decrease in transition energy detectable in
Figure 4 is consistent with an excited state that assumes increasingly
more charge-transfer character as the substituent becomes more
strongly electron donating and would support the trend predicted
Figure 6. Fluorescence spectra for neutral and diacid forms of 1a (top)
and H2TPP (bottom) recorded in toluene. For all solutions, λex
431 nm and A431 = 0.06 au.
=
Table 2. Selected Fluorescence Data Recorded in Toluene for Compounds 1aꢀe and 2aꢀe (n.a. = not applicable)a
stokes shift,d nm,
neut./cat.
τ (ns)f
kr (107 sꢀ1
)
knr (107 sꢀ1
)
subst
λmax,b nm neut./cat.
shift,c nm
Φfle neut./cat.
neut./cat.
neut./cat.
neut./cat.
knr(cat.)/knr(neut.)
1a
H
695/743
695/744
697/753
702/768
726/n.a.g
652/692
650/695
652/701
655/716
671/n.a. g
48
49
56
66
n.a.
40
45
49
61
n.a.
4/36
4/39
3/40
5/41
11/n.a.
4/39
3/41
4/43
5/49
14/n.a.
0.204/0.144
0.205/0.133
0.227/0.138
0.228/0.088
0.249/n.a.
6.21/1.59
6.26/1.57
6.28/1.53
5.98/1.39
4.42/n.a.
9.77/1.83
9.37/1.82
9.60/1.88
9.06/1.62
8.44/n.a.
3.29/9.06
3.27/8.47
3.61/9.02
3.81/6.33
5.63/n.a.
1.12/8.03
0.99/8.02
0.94/5.37
1.72/7.65
1.94/n.a.
12.8/53.8
12.7/55.2
12.3/56.3
12.9/65.6
17.0/n.a.
9.11/46.6
9.68/46.9
9.47/47.8
9.32/54.1
9.91/n.a.
4.20
4.35
4.57
5.08
n.a.
1b
1c
F
Me
1d
1e
OMe
NMe2
H
TPP
2b
2c
0.11/0.147
0.093/0.146
0.090/0.101
0.156/0.124
0.164/n.a.
5.12
4.84
5.05
5.80
n.a.
F
Me
2d
OMe
2e
NMe2
a All data was corrected for instrument response. b (1 nm. Peak wavelength corresponds to the most intense Q(0,0) transition (Q(0,1) transition not
listed). c Represents the observed red shift in the Q(0,0) emission peak maximum upon dication formation. d Determined as the difference in nanometers
between the lowest energy absorption and the highest energy fluorescence peak maximum. e Relative to H2TPP (Φfl = 0.11). Error < (4% at the 95%
confidence limit. f Error < (5% at the 95% confidence limit. g No fluorescence detected.
10457
dx.doi.org/10.1021/jp205309f |J. Phys. Chem. A 2011, 115, 10452–10460

The Journal of Physical Chemistry A
ARTICLE
by Wamser and co-workers. However, further experimental and/or
computational studies are needed to clarify this notion.
and H2OEP in both the neutral and the cationic forms. While
triplet yields for both porphyrins decreased upon protonation,
internal conversion yields substantially increased, more so for
H4TPP2+ than for H4OEP2+. Accordingly, the nonradiative
decay rate (calculated from eq 3) for H2TPP increased 5.8-fold
upon protonation, while that for H2OEP, which displays a lower
degree of structural distortion upon protonation, exhibited only a
3.4-fold increase.25
By analogy, we can examine the photophysical parameters of
porphyrins 1aꢀd and 2aꢀd in light of the degree of structural
distortion upon protonation. The crystal structure of the 5,10,15,20-
tetra(phenylethynyl)porphyrin dication (H4TPEP2+) exhibits a
significantly saddle-distorted macrocycle conformation. However,
the nonplanar distortions induced upon protonation are consider-
ably less for H4TPEP2+ than for H4TPP2+.19 The average deviation
of the Cβ atoms from the mean porphyrin plane is 0.574 Å for
H4TPEP2+, while that for H4TPP2+ varies from 0.9 to 1.2 Å.19 The
deviations from planarity exhibited by H4TPEP2+ are similar to
those for H4OEP2+, implying that the lack of steric constraint at the
meso positions aids to alleviate dication structural distortion. It
would therefore be reasonable to anticipate that the nonradiative
decay rates of 1aꢀd, which should exhibit a degree of macrocycle
distortion intermediate to H4TPEP2+ and H4TPP2+, would be less
severely impacted by protonation relative to 2aꢀd.
Emission Spectroscopy. Steady-state emission spectra for 1a
and H2TPP in neutral and dication form are shown in Figure 6,
and fluorescence data for 1aꢀe and 2aꢀe are summarized in
Table 2. The presence of the ethynyl groups results in a
significant red shift in the fluorescence peak maxima relative to
H2TPP derivatives, resulting in emission well into the red region
of the spectrum. Emission spectral peak maxima are impacted by
the nature of the substituent to an extent that correlates well with
the absorption data. Similar to H2TPP derivatives, the arylethy-
nyl porphyrin spectra for the neutral compounds are character-
ized by well-resolved vibronic structure and small Stokes shifts
(∼4 nm). Fluorescence quantum yield values are significantly
higher for the arylethynyl derivatives (ϕfl > 0.20) and are among
the highest values observed for porphyrinic species. The fluor-
escence lifetimes observed for 1aꢀd (∼6 ns) are somewhat
smaller than those observed for 2aꢀd (∼9 ns), likely owing to
additional degrees of freedom that enhance rotational and
vibrational motion and therefore internal conversion. The reason
for the relatively low lifetimes of 1e and 2e is unclear at present.
Fluorescence quantum yields and lifetimes for both series of
porphyrins are largely independent of the nature of the sub-
stituent. Early work showed that H2TPP photophysical param-
eters are not particularly sensitive to substituents at the para
position of the phenyl ring, presumably owing to the nonplanar
orientation of the phenyl rings with the porphyrin macrocycle.46
Addition of ethynyl linkages does not appear to enhance
substituent effects on quantum yields and lifetimes, in contrast
to the enhancement in substituent effects described above for
porphyrin orbital energies.
Addition of TFA results in further red shifting of the emission
spectra for both 1aꢀd and 2aꢀd, accompanied by broadening of
the line shape (Figure 6). Moreover, Stokes shifts increase
significantly for the diacid derivatives, with slightly larger Stokes
shifts observed for 2aꢀd relative to 1aꢀd. Protonation of 1aꢀd
is accompanied by a substantial decrease in the fluorescence
quantum yield, while the quantum yields for the nonarylethynyl
spaced porphyrins 2aꢀd remain relatively constant, consistent
with previously reported data for H4TPP2+.25 For both 1aꢀd
and 2aꢀd, the fluorescence lifetime is observed to decrease upon
protonation, albeit more so for 2aꢀd than for 1aꢀd. The
resulting values for kr and knr, calculated according to eqs 2
and 3, respectively, show that both radiative and non-radiative
rates increase upon protonation. No fluorescence was detected
from the diacids of 1e and 2e bearing dimethylamine substitu-
ents, consistent with previous work.32
Porphyrins 1aꢀc and 2aꢀc, which bear weakly electron-
withdrawing or -donating substituents, exhibit increases in knr
upon protonation that are largely independent of the substituent,
and the observed increases are more substantial for 2aꢀc
(average increase is 5.00 ( 0.13 fold) than for 1aꢀc (4.38 (
0.19 fold). On the basis of the arguments outlined above, we
postulate that the yield of internal conversion (and therefore knr)
increases for both 1aꢀc and 2aꢀc upon protonation but that this
increase is attenuated in 1aꢀc owing to a smaller change in
structural distortion upon protonation. While dications H41d2+
and H42d2+ exhibit small but significant increases in knr relative
to the dications of 1aꢀc and 2aꢀc, the relative increase in the
rate of nonradiative decay upon protonation is smaller for 1d
than for 2d, further supporting the observed trend. However,
direct measurements of triplet yields are needed to substantiate
the contribution of both intersystem crossing and internal
conversion pathways to total nonradiative decay and allow for
the impact of protonation on porphyrin photophysics to be
correlated with structure. The absence of observable fluores-
cence for H41e2+ and H42e2+ indicates that excited state decay is
dominated by nonradiative pathways for these species.
The relative increase in nonradiative decay for the porphyrins
bearing highly electron-donating substituents (OMe and NMe2)
supports the notion outlined in an earlier section that the
dication porphyrin excited state exhibits an increasing degree
of charge-transfer character upon substitution with electron-
donating groups. Increasing charge-transfer character is com-
monly associated with substantial increases in internal conver-
sion rates owing to large differences in ground and excited state
geometries. Future studies are aimed at exploring porphyrin
substituent effects on the charge-transfer character, or “hyper-
porphyrin” character, of dication excited states.
Φfl
kr ¼
ð2Þ
τfl
1 ꢀ Φfl
knr
¼
ð3Þ
τfl
Chirvony et al.25 have shown that the porphyrin diacids
H4TPP2+ and H4OEP2+ exhibit larger Stokes shifts, reduced
fluorescence lifetimes, and increased rates of nonradiative decay
relative to the neutral derivatives owing to more accessible
internal conversion pathways upon protonation. Moreover, the
degree to which these properties are impacted was shown to
correlate with the degree of structural distortion exhibited by the
diacid. Measurement of fluorescence and triplet quantum yields
enabled calculation of the internal conversion yield for H2TPP
’ CONCLUSION
The impact of protonation on the photophysical properties of
H2TPP and its derivatives is proposed to originate primarily from
induced nonplanar structural distortions. To substantiate this
10458
dx.doi.org/10.1021/jp205309f |J. Phys. Chem. A 2011, 115, 10452–10460

The Journal of Physical Chemistry A
ARTICLE
notion, we present a comparative study of the photophysical
properties of free-base arylethynyl porphyrins 1aꢀe and the
trans-disubstituted H2TPP derivatives 2aꢀe in both the neutral
and the diacid form. We show that introduction of arylethynyl
functionalities can greatly enhance the light-harvesting efficiency
of porphyrins in the red region of the spectrum relative to H2TPP
derivatives. For neutral derivatives, porphyrin substituent effects
are enhanced in arylethynyl porphyrins owing to an increase in
conjugation of the appended phenyl group. Upon protonation,
both series of porphyrins exhibit substantial red-shifted spectra
and enhanced oscillator strengths. In the diacid form, the
structural distortion produced upon protonation allows for
porphyrin/phenyl group coplanarity for both 1aꢀe and 2aꢀe,
and the resulting substituent effects are similar for both series.
Similar to H2TPP and its derivatives, the dications of arylethy-
nylporphyrins exhibit reduced excited state lifetimes and in-
creased rates of nonradiative decay relative to the neutral
derivatives, although the impact of protonation on these param-
eters is attenuated in the presence of the arylethynyl groups
and correlates with the lower degree of structural distortion
expected in the dication form. Nonlinear decreases in orbital
energy and increased rates of nonradiative decay are indicative of
excited states with increasing charge-transfer character as the
substituent becomes more electron donating. Taken together,
this work represents the first photophysical study of metal-free
arylethynyl porphyrins and diacids.
(9) LeCours, S. M.; DiMagno, S. G.; Therien, M. J. J. Am. Chem. Soc.
1996, 118, 11854.
(10) LeCours, S. M.; Guan, H. W.; DiMagno, S. G.; Wang, C. H.;
Therien, M. J. J. Am. Chem. Soc. 1996, 118, 1497.
(11) Lin, V. S. Y.; Dimagno, S. G.; Therien, M. J. Science 1994,
264, 1105.
(12) Priyadarshy, S.; Therien, M. J.; Beratan, D. N. J. Am. Chem. Soc.
1996, 118, 1504.
(13) Ray, P. C.; Sainudeen, Z. J. Phys. Chem. A 2006, 110, 12342.
(14) Reeve, J. E.; Collins, H. A.; De Mey, K.; Kohl, M. M.; Thorley,
K. J.; Paulsen, O.; Clays, K.; Anderson, H. L. J. Am. Chem. Soc. 2009,
131, 2758.
(15) Tomizaki, K.; Lysenko, A. B.; Taniguchi, M.; Lindsey, J. S.
Tetrahedron 2004, 60, 2011.
(16) LeCours, S. M.; Philips, C. M.; de Paula, J. C.; Therien, M. J.
J. Am. Chem. Soc. 1997, 119, 12578.
(17) Lin, C. Y.; Chuang, L. C.; Lee, G. H.; Peng, S. M. J. Organomet.
Chem. 2005, 690, 244.
(18) Lin, C. Y.; Chuang, L. C.; Lin, Y. C.; Lin, C. L. Dalton Trans.
2004, 4006.
(19) Berlicka, A.; Latos-Grazynski, L.; Lis, T. Inorg. Chem. 2005,
44, 4522.
(20) Anderson, H. L.; Wylie, A. P.; Prout, K. J. Chem. Soc., Perkin
Trans. 1 1998, 1607.
(21) Milgrom, L. R.; Yahioglu, G. Tetrahedron Lett. 1995, 36,
9061.
(22) Milgrom, L. R.; Yahioglu, G. Tetrahedron Lett. 1996, 37,
4069.
(23) Nowak-Krol, A.; Koszarna, B.; Yoo, S. Y.; Chrominski,
J.; Weclawski, M. K.; Lee, C. H.; Gryko, D. T. J. Org. Chem. 2011,
76, 2627.
’ ASSOCIATED CONTENT
(24) Harriman, A. J. Chem. Soc., Faraday Trans. 2 1981, 77, 1281.
(25) Chirvony, V. S.; van Hoek, A.; Galievsky, V. A.; Sazanovich,
I. V.; Schaafsma, T. J.; Holten, D. J. Phys. Chem. B 2000, 104, 9909.
(26) Rosa, A.; Ricciardi, G.; Baerends, E. J.; Romeo, A.; Scolaro,
L. M. J. Phys. Chem. A 2003, 107, 11468.
S
Supporting Information. Details of the synthesis and
b
characterization of 1aꢀe and 2bꢀe. This material is available free
of charge via the Internet at http://pubs.acs.org.
(27) Cheng, B. S.; Munro, O. Q.; Marques, H. M.; Scheidt, W. R.
J. Am. Chem. Soc. 1997, 119, 10732.
’ AUTHOR INFORMATION
(28) Stone, A.; Fleischer, E. B. J. Am. Chem. Soc. 1968, 90, 2735.
(29) Gouterman, M.; Snyder, L. C.; Wagniere, G. H. J. Mol. Spectrosc.
1963, 11, 108.
(30) De Luca, G.; Romeo, A.; Scolaro, L. M. J. Phys. Chem. B 2006,
110, 14135.
Corresponding Author
*Phone: 651-696-6109. Fax: 651-696-6432. E-mail: splank@
macalester.edu.
(31) Ojadi, E. C. A.; Linschitz, H.; Gouterman, M.; Walter, R. I.;
Lindsey, J. S.; Wagner, R. W.; Droupadi, P. R.; Wang, W. D. J. Phys.
Chem. 1993, 97, 13192.
(32) Weinkauf, J. R.; Cooper, S. W.; Schweiger, A.; Wamser, C. C.
J. Phys. Chem. A 2003, 107, 3486.
(33) Guo, H. W.; Jiang, J. G.; Shi, Y. Y.; Wang, Y. L.; Wang, Y. Z.;
Dong, S. J. J. Phys. Chem. B 2006, 110, 587.
(34) Wasbotten, I. H.; Conradie, J.; Ghosh, A. J. Phys. Chem. B 2003,
107, 3613.
’ ACKNOWLEDGMENT
This work was supported by the donors of the Petroleum
Research Fund, administered by the American Chemical Society
(ACS-PRF 50467-UNI4) and by Macalester College. We also
thank Prof. David Blank (University of Minnesota) for assistance
with lifetime measurements, helpful discussions, and critical
reading of the manuscript.
(35) Kalyanasundaram, K. Inorg. Chem. 1984, 23, 2453.
(36) Seybold, P. G.; Gouterman, M. J. Mol. Spectrosc. 1969, 31, 1.
(37) Laha, J. K.; Dhanalekshmi, S.; Taniguchi, M.; Ambroise, A.;
Lindsey, J. S. Org. Process Res. Dev. 2003, 7, 799.
(38) Taylor, P. N.; Anderson, H. L. J. Am. Chem. Soc. 1999, 121,
11538.
’ REFERENCES
(1) Martinez-Diaz, M. V.; de la Torrea, G.; Torres, T. Chem.
Commun. 2010, 46, 7090.
(2) Wasielewski, M. R. Acc. Chem. Res. 2009, 42, 1910.
(3) Aratani, N.; Kim, D.; Osuka, A. Acc. Chem. Res. 2009, 42, 1922.
(4) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001,
34, 40.
(39) Littler, B. J.; Ciringh, Y.; Lindsey, J. S. J. Org. Chem. 1999, 64,
2864.
(5) Chang, J. C.; Ma, C. J.; Lee, G. H.; Peng, S. M.; Yeh, C. Y. Dalton
Trans. 2005, 1504.
(6) Huang, T. H.; Chen, Y. J.; Lo, S. S.; Yen, W. N.; Mai, C. L.; Kuo,
M. C.; Yeh, C. Y. Dalton Trans. 2006, 2207.
(7) Krivokapic, A.; Anderson, H. L.; Bourhill, G.; Ives, R.; Clark, S.;
McEwan, K. J. Adv. Mater. 2001, 13, 652.
(40) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165.
(41) Exner, O. Correlation Analysis in Chemistry; Plenum Press:
New York, 1978.
(42) Meot-ner, M.; Adler, A. D. J. Am. Chem. Soc. 1972, 94, 4763.
(43) Ransdell, R. A.; Wamser, C. C. J. Phys. Chem. 1992, 96,
10572.
(8) Kuo, M. C.; Li, L. A.; Yen, W. N.; Lo, S. S.; Lee, C. W.; Yeh, C. Y.
Dalton Trans. 2007, 1433.
(44) Koerner, R.; Wright, J. L.; Ding, X. D.; Nesset, M. J. M.;
Aubrecht, K.; Watson, R. A.; Barber, R. A.; Mink, L. M.; Tipton, A. R.;
10459
dx.doi.org/10.1021/jp205309f |J. Phys. Chem. A 2011, 115, 10452–10460

The Journal of Physical Chemistry A
ARTICLE
Norvell, C. J.; Skidmore, K.; Simonis, U.; Walker, F. A. Inorg. Chem.
1998, 37, 733.
(45) Vitasovic, M.; Gouterman, M.; Linschitz, H. J. Porphyrins
Phthalocyanines 2001, 5, 191.
(46) Harriman, A.; Hosie, R. J. J. Chem. Soc., Faraday Trans. 2 1981,
77, 1696.
10460
dx.doi.org/10.1021/jp205309f |J. Phys. Chem. A 2011, 115, 10452–10460