The absorption and fluorescence emission spectra of meso-tetra(aryl)porphyrin dications with weak and strong carboxylic acids: A comparative study

Article abstract of DOI:10.1039/c5ra20445c

The stereoelectronic effects of meso substituents on the absorption and emission spectra of a series of para- or ortho-substituted meso-tetra(phenyl)porphyrins (H₂TPP, H₂T(2-Cl)PP, H₂T(2-Me)PP, H₂T(4-Cl)PP, H₂T(4-OMe)PP, H₂T(4-Me)PP) and their dications with CF₃COOH and HCOOH have been investigated. The f_Soret/f_Q(0,0) ratio was used as a criterion for the comparison of the degree of configuration interaction (CI) between the excited states. Reverse mirror image relationships were observed between the absorption and emission spectra of the porphyrins and their dication species. The much larger stokes shifts observed in the emission spectra of the dications compared to the free base porphyrins reveal major geometrical differences between the ground and excited states of the dications. With the exception of H₄T(4-OMe)PP, the fluorescence quantum yields (Φ_f) of the dications were much larger than those of the free base porphyrins. The natural radiative lifetimes (τ₁, τ_2,τ₃) of the dications estimated by the Forster, Strickler and Berg equations were generally much shorter than those of the free base porphyrins. Also, the radiative rate constant (k_r) and the non-radiative one (k_nr) of the dications were significantly larger than those of the free base porphyrins. The splitting of the bands in the absorption and emission spectra of the free base porphyrins was clearly observed, but little or no splitting was observed in the case of the spectra of the dications.

Full text of DOI:10.1039/c5ra20445c

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PAPER
The absorption and fluorescence emission spectra
of meso-tetra(aryl)porphyrin dications with weak
and strong carboxylic acids: a comparative study†
Cite this: RSC Adv., 2015, 5, 106774
*
Saeed Zakavi and Saiedeh Hoseini
The stereoelectronic effects of meso substituents on the absorption and emission spectra of a series of
para- or ortho-substituted meso-tetra(phenyl)porphyrins (H2TPP, H2T(2-Cl)PP, H2T(2-Me)PP, H2T(4-Cl)
PP, H2T(4-OMe)PP, H2T(4-Me)PP) and their dications with CF3COOH and HCOOH have been
investigated. The fSoret/fQ(0,0) ratio was used as a criterion for the comparison of the degree of
configuration interaction (CI) between the excited states. Reverse mirror image relationships were
observed between the absorption and emission spectra of the porphyrins and their dication species. The
much larger stokes shifts observed in the emission spectra of the dications compared to the free base
porphyrins reveal major geometrical differences between the ground and excited states of the dications.
With the exception of H4T(4-OMe)PP, the fluorescence quantum yields (Ff) of the dications were much
larger than those of the free base porphyrins. The natural radiative lifetimes (s1, s2, s3) of the dications
estimated by the Forster, Strickler and Berg equations were generally much shorter than those of the
free base porphyrins. Also, the radiative rate constant (kr) and the non-radiative one (knr) of the dications
were significantly larger than those of the free base porphyrins. The splitting of the bands in the
absorption and emission spectra of the free base porphyrins was clearly observed, but little or no
splitting was observed in the case of the spectra of the dications.
Received 2nd October 2015
Accepted 6th December 2015
DOI: 10.1039/c5ra20445c
www.rsc.org/advances
and the red6,12,14–16 or blue shied11,14 Q(0,0) bands for meso-
tetra(aryl)- and meso-tetra(alkyl)porphyrins, respectively. On the
basis of the four orbital model of porphyrin spectra,20 the Soret
Introduction
The structural, electronic, electrochemical and spectral changes
induced by the interaction of porphyrins with weak and strong
acids have been the subject of many studies over the past
decades.1–17
and Q bands of porphyrins are due to the a1u–eg and a2u–eg
transitions which are of Eu symmetry considering a D4h
symmetry for the porphyrin core. Therefore, the excited states
resulted in by these transitions are of the same symmetry and
consequently different degrees of conguration interaction are
considered for the interpretation of the UV-vis spectra of
porphyrins.21–23 The interaction was shown to affect both the
position and intensity of the absorption bands.21 The extent of
conguration interaction depends to the difference between the
energy of the Eu states involved in the interaction, so that
a decreased difference between the energy of the interacting
congurations causes an increased extent of conguration
interaction.
The introduction of various substituents at the periphery of
the free base porphyrins with a planar or nearly planar
conformation inuences the wavelengths of the Soret and Q
bands;6,24–27 the Soret band of most of meso-tetra(alkyl)- and
meso-tetra(aryl)porphyrins appears in the range of ca. 416–422
nm, which corresponds to an energy difference of ca. 342 cmꢀ1
between the bands. Also, the Q(0,0) band of the para- or meta-
substituted meso-tetra(phenyl)porphyrin with a planar porphyrin
core and that of meso-tetra(methyl)porphyrin appears at 648–661
nm, corresponding to an energy difference of ca. 303 cmꢀ1. It is
The characteristic physical, chemical and biological prop-
erties of the porphyrin are determined by the aromaticity of the
aromatic macrocycle.18 On the other hand, achievement of
planarity is an essential requirement for aromaticity of this
class of compounds. The nonplanar distortion of porphyrin
core resulted in by the molecular complexation of free base
porphyrins with different acceptors is expected to decrease the
porphyrin ring current. However, a decrease of only 5% was
determined for some highly nonplanar complexes of Co(III)
complexes of meso-tetra(tert-butyl)porphyrin.19 Accordingly, the
wavelength and intensity of the absorption bands of the
porphyrin diacids are expected to be resemble to those of the
free base porphyrins.
The protonation of porphyrins bearing different meso
substituents is accompanied with the red shied Soret bands
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS),
Zanjan 45137-66731, Iran. E-mail: zakavi@iasbs.ac.ir
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra20445c
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Porphyrin dictions were prepared by adding excess amounts
of CF3COOH (more than the 2 : 1 molar ratio of the acid to
porphyrin) to the solution of porphyrin in dichloromethane
followed by slow evaporation of the solvent and excess acid at
room temperature.11,14 In the case of HCOOH, the evaporation
of dichloromethane led to the decomposition of the dica-
tions16,35 and consequently the solution of dications were used
for the spectrophotometric studies. As the previous studies
showed, the UV-vis spectra of the porphyrin dications in the
presence of excess amounts of acid led to no detectable changes
in the position and intensity of the absorption bands.15 The 1H
NMR, 13C NMR and UV-Vis spectral data of the dications are
presented in the ESI, S1b.†
Fig. 1 meso-Tetra(aryl)porphyrins used in this study.
Extinction coefficient measurement
noteworthy that there are ample evidence that the phenyl and
porphyrin planes are signicantly non-coplanar in such mole-
cules4,6,28,29 that limits the efficient resonance interactions
between the p systems of the meso-aryl substituents and the
porphyrin core.2,6,10 Diprotonation of porphyrins which is
accompanied with a signicant decrease in the dihedral angle
between the meso substituents and the porphyrin mean
plane2,9,10,12 inuences the difference between the energy of the Eu
excited states and leads to remarkable changes in the UV-vis
spectra of the porphyrin diacids compared to those of the free
base ones.
We have previously reported the UV-vis spectral changes
induced by the interaction of different meso-tetra(alkyl)- and
meso-tetra(aryl)porphyrins with weak and strong acids.14
Furthermore, cyclic voltammetry and UV-vis spectroscopy have
provided evidence for the destabilization of the a1u orbital in
contrast to the stabilization of the a2u and eg orbitals as a results
of N-protonation of meso-tetra(aryl)porphyrins with carboxylic
acids.16 Also, the emission spectrum of porphyrin diacids was
shown to be inuenced by the electronic effects of meso substit-
uents and the type of counter anion.10,30–33 In the present work,
the effects of core protonation of a series of meso-tetra(aryl)
porphyrins (Fig. 1) with electron-rich and electron-decient meso
substituents on their absorption and emission spectra upon the
interaction of porphyrins with weak and strong acids have been
studied. Furthermore, an attempt was made to nd correlations
between the intensity and wavelength of the bands, natural
radiative lifetimes, radiative rate constant and the non-radiative
one and the stereoelectronic properties of the meso substitu-
ents. The intensity of different bands of the free base porphyrin
and corresponding dications with the acids was determined and
compared on the basis of both the oscillator strength (f) and the
extinction coefficient (3) of the transitions.
To calculate the 3 values, 0.5 to 10 mM solutions of the respected
porphyrin and the dications in CH2Cl2 were prepared and the
absorbances were measured. The extinction coefficients were
evaluated from the slope of a linear plot of absorbance versus
concentration.
Fluorescence quantum yield
The uorescence spectra were obtained using dilute solutions
(ꢁ10 mM).10,36 As was previously reported,37 the O2 dissolved in
air-saturated solvents was observed to reduce the uorescence
yield of porphyrins by nearly 15%. Accordingly, the dichloro-
methane solutions of the porphyrins and dications were purged
with N2 before determination of the quantum yields. The
porphyrin and dication solutions were excited at the wavelength
of the corresponding Soret band and the emission spectra were
recorded in the range of 500 to 800 nm. Also, the uorescence
quantum yields (Ff) were calculated relative to that of H2TPP in
CH2Cl2 as standard (Ff ¼ 0.13)38 on the basis of the method
described by Parker and Rees.39
Instrumental
UV-Vis absorption spectra were measured on an Ultraspec 3100
Pro spectrophotometer. All optical investigations were carried
out in CH2Cl2. Fluorescence spectra and quantum yields were
obtained from dichloromethane solutions of the porphyrins
and dications in a high precision quartz uorescence cell on
a Cary Eclipse uorescence spectrophotometer. 1H and 13C
NMR spectra were obtained on a Bruker Avance DPX-400 MHz
spectrometer.
Results and discussion
The absorption spectra
The free base porphyrins. The absorption bands due to the
free base porphyrins are summarized in the ESI, S2.† X-ray
crystallographic studies revealed that in free base meso-tet-
Experimental
Synthesis and characterization
H2TPP, H2T(4-OMe)PP, H2T(2-Me)PP, H2T(4-Me)PP, H2T(2-Cl) ra(aryl)porphyrins the aryl substituents are nearly perpendic-
PP, H2T(4-Cl)PP34 were synthesized, characterized and puried ular to the porphyrin mean plane and therefore there is no
according to the literature methods. The 1H NMR, 13C NMR and strong resonance type interactions between the p system of the
UV-Vis spectral data of the porphyrins are presented in the ESI, meso groups and that of the porphyrin core.6,9,40 Accordingly,
S1a.†
there are no remarkable differences between the UV-Vis spectra
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of meso-tetra(aryl)porphyrins bearing various groups at the meso compared to the allowed Q(0,1) ones.6,22,42,43 However, the order
positions. However, the porphyrins with bulky meso substitu- of fSoret/fQ(0,0) for H2T(4-OMe)PP and H2T(4-Me)PP can not be
ents have an increased dihedral angle between the meso groups explained according to the relative electron donating ability of
and the porphyrin moiety.27 The data of ESI, S2† show that the their meso groups. In other words, there is no close correlation
position of the absorption bands are inuenced by the elec- between the fSoret/fQ(0,0) ratios and the electronic properties of
tronic and steric effects of the meso substituents; the intro- the meso substituents. It is noteworthy that in spite of the much
duction of electron donating –OMe group at the para position of greater basicity of H2T(4-OMe)PP (pKa ¼ 2.31) than H2T(4-Me)
H2TPP leads to a small shi of the Soret and Q bands to lower PP (pKa ¼ 1.85) values of 107.9 and 73.2 were observed for the
energies. Also, the presence of bulky methyl or chloro groups at fSoret/fQ(0,0) ratio of the porphyrins, respectively.6 It should be
the ortho positions with respect to the porphyrin ring causes noticed that, the Q bands of porphyrins are generally less
a slight blue shi of the bands. However, the differences intense and much broader than the Soret bands and therefore
between the 3 of the corresponding Soret and Q bands of there is some errors in estimation of the f values for the Q
different porphyrins are more obvious in comparison to those bands. However, the observed difference between the fSoret
/
observed between the energy of the bands. In other words, the fQ(0,0) values for the two porphyrins is too much to be attributed
change of meso aryl substituents mainly affects the intensity of to the experimental error in the measurement of fSoret/fQ(0,0)
the Soret and Q bands. The intensity of the absorption bands values. Accordingly, the order of porphyrin basicity for a series
may be interpreted on the basis of the 3 values but the intensity of free base porphyrins does not seem to be a good criterion for
of bands with different widths cannot be correctly compared the comparison of the fSoret/fQ(0,0) ratio of the compounds. On
using the 3 of bands41 and therefore the f values should be used the basis of the molecular orbital theory, the strength of
for the comparison of the intensities.21 The 3 and f values of the a covalent bond depends inversely to the energy difference and
Soret bands decrease in the order H2T(2-Me)PP > H2T(4-Me)PP > directly to the overlap between the involved orbitals. While the
H2T(4-Cl)PP ꢁ H2TPP > H2T(2-Cl)PP > H2T(4-OMe)PP and introduction of good electron donating groups at the para
H2T(4-Me)PP > H2T(4-Cl)PP > H2T(2-Me)PP > H2TPP > H2T(2-Cl) position of the meso substituents increases the bond strength
PP > H2T(4-OMe)PP, respectively. Also, the 3 and f values of the between the p systems of the aryl group and the porphyrin core,
Q(0,0) bands decrease as H2TPP > H2T(2-Me)PP > H2T(4-Me)PP > the stabilization of the aryl HOMO orbitals caused by the elec-
H2T(4-Cl)PP ꢁ H2T(4-OMe)PP > H2T(2-Cl)PP and H2T(4-Me)PP > tron donating groups will weaken the p bond (Fig. 2). Further-
H2T(4-Cl)PP > H2T(2-Me)PP > H2TPP $ H2T(4-OMe)PP > H2T(2- more, the dihedral angle between the meso aryl substituents
Cl)PP, respectively. As was mentioned above, the f value gives and the porphyrin mean plane may be different for the dipro-
a better criterion for the comparison of the intensity of the tonated species of various meso-tetra(aryl)porphyrins, especially
absorption bands than the 3 one. Also, the observed order of the in the case of the porphyrins bearing bulky substituents at the
3 and f values does not correlate with the electron-donating ortho position of the aryl groups.2 X-ray crystallographic studies
strength of the meso groups. The ratio of fSoret/fQ(0,0) has be show that the diprotonation of meso-tetra(aryl)porphyrins is
used as a criterion for the comparison of the degree of CI accompanied with signicant decreases in the dihedral angle
between the excited states.6,22 As may be seen from Table 1, the between the meso aryl groups and the porphyrin mean plane.12
ratio decreases in the order of H2T(2-Cl)PP [ H2T(2-Me)PP > Also, the presence of bulky groups such as methyl- and chlorine
H2T(4-Cl)PP ꢁ H2TPP > H2T(4-OMe)P > H2T(4-Me)PP. It should atom at the ortho positions leads to a decreased exibility of the
be noted that the increase of the fSoret/fQ(0,0) ratio is due to the porphyrin core towards the out of plane deformation which in
decrease of the fQ(0,0) resulted in by the CI between the eu excited turn restricts the dihedral angles to greater ones.2 According to
states.6 In other words, the above order of the ratio observed for the above discussion, the porphyrin basicity can not be
the porphyrins suggests the decrease of degree of CI in the explained only on the basis of the donating ability of the groups
series from H2T(2-Cl)PP to H2T(4-Me)PP. Interestingly, H2T(4- attached to the phenyl group of ortho, para or meta substituted
OMe)PP showed a greater degree of CI than H2T(4-Me)PP meso-tetra(phenyl)porphyrins and the nal dihedral angle
which does not correlate with the relative electron donating between the meso aryl substituents and the porphyrin mean
ability of the meso substituents. Meot-Ner et al. evaluated plane should be also considered. According to the ESI, S2,† the
a much greater basicity for H2T(4-OMe)PP that was rationalized highest extent of CI was observed in the case of H2T(2-Cl)PP
on the basis of better electron donor ability of 4-methoxyphenyl with bulky Cl substituents at the ortho position. This observa-
compared to that of 4-methylphenyl group. Also, they have tion seems to be due to the inefficient p resonance interactions
found a greater fSoret/fQ(0,0) ratio for H2T(4-Me)PP than H2TPP, between the meso groups and the porphyrin p systems. On the
suggesting a greater degree of CI for the former. According to other hand, the methyl group is as bulky as the chlorine atom44
the four orbital model of porphyrins, for the a2u orbital the and therefore H2T(2-Me)PP was expected to behave as H2T(2-Cl)
compared electron densities are largest on the methene carbons PP and show a high degree of CI. However, the fSoret/fQ(0,0) ratio
and pyrrole nitrogens and therefore the substitution of meso for H2T(2-Me)PP is much smaller than that of H2T(2-Cl)PP.
positions with electron donating groups would expected to
The porphyrin dications with CF3COOH. Diprotonation of
increase the energy of this orbital (Fig. 2).6 Also, the removal of meso-tetra(aryl)porphyrins with CF3COOH led to the red shi of
the accidental degeneracy of the a2u and alu orbitals will lead to the Soret band and the red or blue shi of the Q bands (Table 1).
increase in the oscillator strength of the Q bands which will be Also, the Q bands reduce to an intense band at longer wave-
much more signicant in the case of the forbidden Q(0,0) bands length, i.e. Q(0,0) band, and a weak band at shorter wavelength.
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a
Table 1 The UV-Vis spectral data of the porphyrin dications with CF3COOH in CH2Cl2
Bands
Dications
Soret
II
I
DnQ(0,0),Soret (cmꢀ1
)
H4TPP(CF3COO)2
l (nm)
437
600
652
7546
Dnfree base,dication (cmꢀ1
)
)
)
)
)
)
ꢀ1097
5.83
2.042
7.1
ꢀ282
ꢀ118
log 3
f
fSoret/fQ(0,0)
4.46
4.93
0.053
0.288
H4T(4-OMe)PP(CF3COO)2
H4T(2-Me)PP(CF3COO)2
H4T(4-Me)PP(CF3COO)2
H4T(2-Cl)PP(CF3COO)2
H4T(4-Cl)PP(CF3COO)2
l (nm)
449
686
7694
7350
7609
7379
7535
Dnfree base,dication (cmꢀ1
ꢀ1481
5.77
1.780
5.2
ꢀ784
log 3
f
fSoret/fQ(0,0)
5.07
0.340
l (nm)
432
583
175
4.64
0.100
633
294
4.89
0.190
Dnfree base,dication (cmꢀ1
ꢀ890
5.99
2.400
12.6
442
ꢀ1299
5.85
3.070
4.2
log 3
f
fSoret/fQ(0,0)
l (nm)
666
Dnfree base,dication (cmꢀ1
ꢀ441
log 3
f
fSoret/fQ(0,0)
5.01
0.730
l (nm)
431
580
206
4.69
0.044
632
271
4.75
0.058
Dnfree base,dication (cmꢀ1
ꢀ837
5.76
1.520
26.2
439
ꢀ1144
6.06
2.760
5.5
log 3
f
fSoret/fQ(0,0)
l (nm)
656
Dnfree base,dication (cmꢀ1
ꢀ212
log 3
f
fSoret/fQ(0,0)
5.16
0.500
Ð
a
Oscillator strength; f ¼ 4.32 ꢂ 10ꢀ9 3ydn.
the porphyrin mean plane.2 The ab initio calculations showed
that the saddling of porphyrin core is accompanied with the red
shi of both the Soret and Q(0,0) bands.45 Also, the decreased
dihedral angle between the aryl substituents and the porphyrin
mean plane caused the shi of this band to higher wavelengths.6
The substitution of ortho positions with bulky substituents such
as methyl group restricts the rotation around the Cmeso–Caryl band
and prevents the efficient resonance interaction between the aryl
group and the porphyrin core p systems.2 The observed blue shi
of the Q(0,0) band of H2T(2-Me)PP and H2T(2-Cl)PP may be
rationalized on the basis of the strong restrictions on the rotation
of the meso aryl groups caused by the presence of methyl and
chloro groups at the ortho positions. Furthermore, the degree of
saddling of the porphyrin core also decreases in the case of the
dications of H4T(2-Me)PP2+ and H4T(2-Cl)PP2+ and consequently
the red shi of the Soret band is smaller than that of the
porphyrins with no substituents at the ortho position (Table 1).
The 3 and f values of the Soret bands of the dications follow the
following order:
Fig. 2 Schematic presentation of the effect of meso aryl groups on
the energy level of the a2u orbital. The e1g symmetry of the HOMOs of
each aryl group refers to a local symmetry of D6h for the group.
The latter may be observed as a separated band or as a shoulder
on the lower wavelength side of the Q(0,0) band. The dication
formation is associated with an increase in the intensity (3 or f) of
the Q bands. The red shied Soret and Q(0,0) bands have been
attributed to the out-of-plane deformation of the porphyrin core
and the increased coplanarity of the meso aryl substituents with
H4T(4-Cl)PP(CF3COO)2 > H4T(2-Me)PP(CF3COO)2 > H4T(4-
Me)PP(CF3COO)2 > H4TPP(CF3COO)2 > H4T(4-OMe)P(CF3COO)2
> H4T(2-Cl)PP(CF3COO)2 and H4T(4-Me)PP(CF3COO)2 > H4T(4-
Cl)PP(CF3COO)2 > H4T(2-Me)PP(CF3COO)2 > H4TPP(CF3COO)2 >
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H4T(4-OMe)PP(CF3COO)2
>
H4T(2-Cl)PP(CF3COO)2, respec- saddling of [H4T(4-OMe)PP]2+ would resulted in a more desta-
tively. Also, the 3 and f values of the Q(0,0) bands decrease as bilized a11ueg1 state for [H4T(4-OMe)PP]2+ than [H4T(4-Me)PP]2+
H4T(4-Cl)PP(CF3COO)2 > H4T(4-OMe)PP(CF3COO)2 > H4T(4-Me) (DE2). The observed higher CI for [H4T(4-OMe)PP]2+ may be
PP(CF3COO)2 > H4TPP(CF3COO)2 > H4T(2-Me)PP(CF3COO)2
>
explained by higher values of DE2 than DE1 (Fig. 3).
H4T(2-Cl)PP(CF3COO)2 and H4T(4-Me)PP(CF3COO)2 > H4T(4-Cl)
The porphyrin dications with HCOOH. The absorptions
PP(CF3COO)2 > H4T(4-OMe)PP(CF3COO)2 > H4TPP(CF3COO)2 > bands due to the porphyrin dications with HCOOH are demon-
H4T(2-Me)PP(CF3COO)2 > H4T(2-Cl)PP(CF3COO)2, respectively. strated in Table 2. As was observed in the case of the dications
The order of fSoret/fQ(0,0) ratio for the dications is as H4T(2-Cl) with CF3COOH, the interaction of porphyrins with HCOOH shis
PP(CF3COO)2 > H4T(2-Me)PP(CF3COO)2 > H4TPP(CF3COO)2
>
the absorption bands to higher wavelengths. The comparison of
H4T(4-Cl)PP(CF3COO)2 > H4T(4-OMe)PP(CF3COO)2 > H4T(4-Me) Tables 1 and 2 shows larger red shi of the Soret and Q(0,0)
PP(CF3COO)2. With the exception of the position of H4T(4-Cl) bands for the dications of porphyrins with the latter that is more
PP(CF3COO)2 relative to H4TPP(CF3COO)2 and that of H4T(4- obvious in the case of the Q(0,0) band. HCOOH (pKa ¼ 3.75 (ref.
Me)PP(CF3COO)2 compared to H4T(4-OMe)PP(CF3COO)2 the 14)) is a much weaker acid than CF3COOH.14 We have previously
order of fSoret/fQ(0,0) ratio for the dications correlates with the shown that the shi of electron density from the porphyrin core
electron donating ability of the meso aryl groups. It should be to the central acid molecules by itself causes a ca. 800 cmꢀ1 blue
noted that the saddle (or nearly saddle11) shaped porphyrin core shi of the Q(0,0).14 The difference between the position of the
of the dications, facilitates the coplanarity of the aryl substitu- Q(0,0) bands of the dications of a given porphyrin with CF3COOH
ents and the porphyrin core. This in turn leads to more efficient and HCOOH may be due to the larger shi of electron density
p–p interactions between the aryl groups and the porphyrin from the porphyrin core to the acid molecules in the case of
core. Therefore, the difference between the electron donor CF3COOH which in turn leads to larger blue shi of the band for
ability of the aryl groups is more obvious in the case of the the dications of porphyrins with CF3COOH. It should be noted
porphyrin dications compared to the corresponding free base that the nal red shi of the band is probably due to the
porphyrins. Also, the electron deciency on the porphyrin core decreased HOMO-LUMO gap caused by the out-of-plane defor-
caused by the shi of electron densities from the pyrrolenine mation of porphyrin core as well as the decreased dihedral angle
nitrogen atoms to the CF3COOH molecules seems to lead to between the meso aryl groups and the porphyrin core; the ab initio
dominance of the p-resonance interactions over the inductive calculations on porphine revealed that the saddling of porphyrin
sigma electron withdrawing effects of the chlorine atoms of the core is accompanied with the red shi of the Q(0,0) band.2,4,11,12
4-Cl-phenyl groups in comparison with that of the phenyl Also, the enhanced coplanarity of the meso aryl substituents with
group. Herein, the stronger electron donating aryl substituents the porphyrin mean plane induces the red shi of the Q(0,0)
resulted in a decreased degree of CI. Also, in the case of H4T(4- band of the meso-tetra(aryl)porphyrin dication. The 3 and f values
Me)PP(CF3COO)2 and H4T(4-OMe)PP(CF3COO)2, the unusual of the Soret bands of the dications with HCOOH decrease in the
observation may be due to the more efficient overlap between order H4T(4-Me)PP(HCOO)2
> H4T(2-Me)PP(HCOO)2 > H4-
the p systems of diprotonated porphyrin core and the meso TPP(HCOO)2 > H4T(2-Cl)PP(HCOO)2 ꢁ H4T(4-Cl)PP(HCOO)2
>
substituents of the former than the latter (Fig. 3). It should be H4T(4-OMe)PP(HCOO)2 and H4T(4-Me)PP(HCOO)2 > H4T(2-Me)
noted that due to the higher degree of saddling of porphyrin PP(HCOO)2 $ H4T(4-Cl)PP(HCOO)2 > H4TPP(HCOO)2 > H4T(4-
core of [H4T(4-OMe)PP]2+ compared to that of [H4T(4-Me)PP]2+
,
OMe)PP(HCOO)2 > H4T(2-Cl)PP(HCOO)2, respectively. Also, 3 and
the a2u orbital of the two porphyrin dications cannot be of the f values of the Q(0,0) bands decrease in the order H4T(4-Me)
same energy. In other words, the a2u orbital of the former seems PP(HCOO)2 > H4T(4-Cl)PP(HCOO)2 > H4TPP(HCOO)2 > H4T(4-
to be more destabilized by the out-of-plane deformation of the OMe)PP(HCOO)2 > H4T(2-Me)P(HCOO)2 > H4T(2-Cl)PP(HCOO)2
porphyrin core. On the other hand, the energy gap between the and H4T(4-Me)PP(HCOO)2 > H4T(4-Cl)PP(HCOO)2 > H4T(4-OMe)
a11ue1g–a12ue1g states is inuenced by the change of the energy of PP(HCOO)2 > H4TPP(HCOO)2 > H4T(2-Me)P(HCOO)2 > H4T(2-Cl)
the higher and lower states. In other word, the better electron PP(HCOO)2, respectively.
donating ability of the 4-OMe group would lead to an increase in
In the case of the Soret band, the pattern is to some extent
the energy of the a11ueg1 state of [H4T(4-OMe)PP]2+ compared to different from that observed for the corresponding dications
that of [H4T(4-Me)PP]2+(DE1). Also, the higher degree of with CF3COOH. However, relative intensities of the Q(0,0)
bands are the same as those observed for the dications with
CF3COOH.
The fSoret/fQ(0,0) ratio for the dications with HCOOH (Table 2)
decreases in the order H4T(2-Cl)PP(HCOO)2 > H4T(2-Me)
PP(HCOO)2 > H4TPP(HCOO)2 > H4T(4-OMe)PP(HCOO)2 > H4T(4-
Cl)P(HCOO)2 > H4T(4-Me)PP(HCOO)2.
In comparison with the fSoret/fQ(0,0) ratio of the CF3COOH
dications, H4T(4-OMe)PP(HCOO)2 shows a greater ratio than
H4T(4-Cl)P(HCOO)2.
The values of fSoret/fQ(0,0) are in the range of 28.3 to 4.2 which
with the exception of H4T(2-Cl)PP(HCOO)2 and H4T(2-Me)
PP(HCOO)2 do not correlate with the relative electron donating
Fig.
3
The comparison of H4T(4-Me)PP2+ and H4T(4-OMe)PP2+
excited states.
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a
Table 2 The UV-Vis spectral data of porphyrin dications with HCOOH in CH2Cl2
Bands
Dications
Soret
II
I
DnQ(0,0),Soret (cmꢀ1
)
H4TPP(HCOO)2
l (nm)
439
604
657
7558
Dnfree base,dication (cmꢀ1
)
)
)
)
)
)
ꢀ1202
5.75
2.180
7.5
ꢀ393
ꢀ235
log 3
f
fSoret/fQ(0,0)
4.42
4.89
0.052
0.290
H4T(4-OMe)PP(HCOO)2
H4T(2-Me)PP(HCOO)2
H4T(4-Me)PP(HCOO)2
H4T(2-Cl)PP(HCOO)2
H4T(4-Cl)PP(HCOO)2
l (nm)
452
695
7735
7347
7692
7300
7519
Dnfree base,dication (cmꢀ1
ꢀ1629
5.36
1.770
5.1
ꢀ972
log 3
f
fSoret/fQ(0,0)
4.73
0.350
l (nm)
433
582
204
4.49
0.100
635
244
4.69
0.170
Dnfree base,dication (cmꢀ1
ꢀ944
5.79
2.830
16.6
443
ꢀ1350
5.88
3.070
4.2
log 3
f
fSoret/fQ(0,0)
l (nm)
672
Dnfree base,dication (cmꢀ1
ꢀ575
log 3
f
fSoret/fQ(0,0)
5.09
0.730
l (nm)
432
580
206
4.41
0.044
631
296
4.52
0.055
Dnfree base,dication (cmꢀ1
ꢀ890
5.65
1.580
28.3
442
ꢀ1299
5.65
2.820
4.7
log 3
f
fSoret/fQ(0,0)
l (nm)
662
Dnfree base,dication (cmꢀ1
ꢀ350
log 3
f
fSoret/fQ(0,0)
4.91
0.600
a
See the footnotes of Table 1.
ability of the meso substituents. It should be noted that the energy gap between the two excited states involved in the a1u to eg
fSoret/fQ(0,0) values of H4T(2-Cl)PP(HCOO)2 and H4T(2-Me) and the a2u to eg transitions in the case of the free base porphyrins
PP(HCOO)2 are signicantly larger than those of the other should be lower than that in the case of the porphyrin dications.
dications. Also, there is no large difference between the fSoret
fQ(0,0) values of the latter. It may be concluded that the fSoret
/
/
It should be noted that the values of energy gap between the
Soret and Q(0,0) bands summarized in Tables 1 and 2 are affected
fQ(0,0) value is mainly inuenced by the steric hindrance of the by different factors such as the out-of-plane deformation of
bulky groups (ortho methyl or chloro substituents) which porphyrin core, enhanced resonance interactions between the
decreases the effective p–p interactions between the meso aryl porphyrin and meso aryl group p systems, shi of electron density
groups and the porphyrin p system. Also, the difference from the pyrrolenine nitrogen atoms to the central acid molecules
between the electron donor ability of the groups attached to the and the conguration interaction between the two excited eu
para position has little effects on this ratio. Except in the case of states. Therefore, the values cannot be used as a criterion for the
H4T(4-Cl)PP(HCOO)2 and H4T(4-OMe)PP(HCOO)2, the fSoret
/
comparison of the CI for the free base porphyrins and the dica-
fQ(0,0) ratios for the dications of with HCOOH are larger than tions and the fSoret/fQ(0,0) ratios should be considered in this
those of the corresponding dications with CF3COOH (Table 1). comparison. As was mentioned above, H4T(2-Cl)PP(HCOO)2 and
In other words, the degree of CI for the dication of a given H4T(2-Me)PP(HCOO)2 show much larger fSoret/fQ(0,0) ratios than
porphyrin with a weak carboxylic acid may be either greater or the other dications. Also, the para-substituted meso-tetra(phenyl)
smaller than that of the corresponding dication with a strong porphyrins demonstrate a nearly similar low fSoret/fQ(0,0) value.
one.
The f /f
With respect to the remarkably enhanced coplanarity of the meso
ratios for the dications compared to that of the substituents and the porphyrin mean plane of the dications
Soret Q(0,0)
free base porphyrins. As may be seen from Tables 1 and 2, the compared to the free base porphyrins, the difference between the
formation of porphyrin dications with HCOOH and CF3COOH is electron donating ability of the meso substituents is expected to be
associated with signicant decreases in the fSoret/fQ(0,0) ratio indi- more dominant in the case of the dications. On the other hand,
cating the decrease of CI upon the diprotonation of porphyrins only small changes in the fSoret/fQ(0,0) ratio were observed on going
with either weak or strong carboxylic acids. Accordingly, the from 4-Cl-phenyl to 4-OMe-phenyl substituent. Apparently, in the
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absence of high dihedral angles between the meso aryl substitu-
ents and the porphyrin mean plane which prevents the resonance
interaction between the two p systems, the degree of CI seems to
be nearly independent from the nature of groups attached to the
meso phenyl substituents.
The emission spectra
The free base porphyrins. The emission spectrum of the free
base porphyrins consists of a nearly broad band, Qx(0,0) at ca.
650 and a broader one Qx(0,1) at 720 nm (ESI, S4†). According to
*
*
Fig. 4, reverse mirror image relationships are observed between
the absorption and emission spectra. As was seen in the case of
the absorption spectra of these compounds, the position of the
emission bands is fairly inuenced by the stereoelectronic
effects of the meso substituents. Indeed, due to the almost
perpendicular orientation of the aryl substituents and the lack
of efficient resonance interactions between the p systems of the
porphyrin core and the aryl groups, there is no large difference
between the absorption and emission spectra of the free base
porphyrin. However, according to the data of ESI, S2 and S5,†
large differences are observed between the intensity and
broadness of the emission bands of the porphyrins. The total
intensity of the emission bands of the porphyrins decreases in
the order H2T(4-OMe)PP > H2TPP > H2T(2-Me)PP > H2T(4-Me)PP
> H2T(4-Cl)PP > H2T(2-Cl)PP. Also, the same pattern is observed
We have previously reported the remarkable inuence of
the electronic and steric effects of the meso substituents on
the position of the absorption bands of porphyrin dications
with different substituents including thien-2-yl, 4-thioanisole
and (2-NO2)phenyl at this position15 (ESI, S3†). Also, the data
concerning the dications of meso-tetra(2,6-dichlorophenyl)
porphyrins as an electron-decient porphyrin with bulky
substituents at the meso positions are summarized in ESI,
S3.† H2T(2,6-Cl)PP and H2T(thien-2-yl)P may be considered
as the most electron decient and electron-rich porphyrins of
the series, respectively. H2T(thien-2-yl)P has a ve membered
ring at the meso position. The smaller size of the ve
membered thienyl ring with respect to phenyl one of the
other porphyrins allows for increased coplanarity between
thienyl groups and porphyrin mean plane and consequently
leads to better electron donation to the porphyrin core.
Accordingly, the Soret and Q(0,0) bands of H2T(thien-2-yl)P
are red shied relative to the corresponding bands of the
other free base porphyrins of Tables ESI, S2 and S3.† Also, the
absorption bands of H2T(2,6-Cl)PP as the most electron-
decient porphyrin of the series appear at smaller wave-
lengths in comparison with those of the other free base
porphyrins. High-level ab initio calculations showed the
better electron-donating ability of 4-thioanisole to porphyrin
core relative to that of 4-anisole.15 In other words, H2T(4-
SCH3)PP is a more electron-rich porphyrin than H2T(4-OCH3)
PP. As may be seen from Tables S3-1 and S3-2,† the Soret and
Q(0,0) bands of the former are red shied relative to those of
the latter. Also, in the case of the porphyrin dications, the
same patterns are observed; again the Q(0,0) and Soret bands
of H2T(thien-2-yl)P show the largest red shis upon dipro-
tonation with CF3COOH and HCOOH. On the other hand, the
bands of the dications of H2T(2,6-Cl)PP are observed at
wavelengths that are remarkably smaller (blue shied) than
those of the other dications.
H2T(2,6-Cl)PP has the most bulky substituents at the meso
position compared to the other porphyrins. The results clearly
conrm the positive effect due to the presence of more electron-
donating meso substituents on the red shi of the Soret and
Q(0,0) band of the corresponding dications to higher wave-
lengths. In this regard, the largest red shi of the Soret and
Q(0,0) bands is observed for the dications of H2T(thien-2-yl)PP
and H2T(4-SCH3)PP that are more electron-rich than the
others. Also, the smallest red shi of the Soret band upon
diprotonation of porphyrins was observed in the case of the
dications of H2T(2,6-Cl)PP. Also, the Q(0,0) band of the dica-
tions of H2T(2,6-Cl)PP shows the largest blue shi of the band
among the used porphyrins. Interestingly, for the dications of
porphyrins with a bulky group (2-Cl, 2-NO2 and 2-Me) at the
ortho position of the phenyl ring the Soret and Q(0,0) bands
appear at a similar wavelength i.e. ca. 430 (the Soret band) and
ca. 630 (the Q(0,0) band).
*
*
for the relative intensity of the Qx(0,0) or Qx(0,1) bands. It should
be noted that the band area was used in this comparison,
although the comparison of intensity of the bands at the
maximum emission also leads to nearly the same order. The
emission intensity of free base and metalloporphyrin has been
shown to be inuenced by the steric and electronic effects of the
meso substituents.37,46–50 In the absence of the steric hindrance
at the ortho position of the meso aryl groups, the introduction of
electron-rich substituents at the meso position and the
contaminant increased resonance interactions between the
porphyrin core and the meso substituents would lead to an
increase in the radiative rate constant (kr). This is due to the
smaller dihedral angle between the aryl groups and the
porphyrin mean plane in the excited state of porphyrins (S1)
with respect to that in the ground state (S0). However, the
electronic effect of the peripheral substituents is not the only
factor inuencing the emission spectra of porphyrins and
distortion of the porphyrin core from planarity may be also
involved; the enhanced distortion of porphyrin core in the
excited state (S1) and increased displacement between the
ground and excited state potential curves have been used to
explain the unusual emission spectra of porphyrins with
electron-rich meso groups in comparison with that of porphy-
rins bearing electron-decient ones. Herein, the smaller inten-
sity of H2T(4-Me)PP than H2T(2-Me)PP and H2TPP may be
rationalized on the basis of the presence of different degrees of
porphyrin core distortion for these compounds in the S1 excited
state. It is noteworthy that, the observation of a value less than 1
for the Qx(0,0)/Qx(1,0) ratio in the absorption spectrum and
*
*
a Qx(0,0)/Qx(0,1) ratio of greater than 1 in the emission spec-
trum conrm this displacement.49 Also, the substitution of
heavy atoms such as chlorine on the meso aryl groups decreases
the kr value, due to the increased non-radiative intersystem
crossing one (kIsc)
37,51 which is much more obvious in the case of
H2T(2-Cl)PP. However, the spectral data summarized in the ESI,
S4† show larger differences between the intensity of the
*
Qx(0,0) bands of the porphyrins with respect to those between
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the Qx(0,1) ones. The uorescence emission of a compound is should be noted that in the absorption spectrum of porphyrins,
the result of competition of the radiative and non-radiative the involvement of vibrational states in the electron transitions
transitions between electronic states. However, the efficiency is due to the vibrational states of the S1 excited electronic state
of the radiative and non-radiative transitions depends on the (Fig. 5).20 For the free base porphyrins, the splitting of the
lifetime of S1 excited states.49 Also, the extents of the out-of- absorption bands (Dn1) decreases as H2T(4-OMe)PP > H2T(4-Cl)
plane distortion of porphyrin core of the ground and excited PP ꢁ H2TPP ꢁ H2T(4-Me)PP > H2T(2-Cl)PP > H2T(2-Me)PP.
states of porphyrins are different which inuences the dihedral However, the differences between the splitting in the absorp-
angle between the porphyrin mean plane and the meso tion bands are small and within 9–28 cmꢀ1 which are much less
substituents. This in turn leads to an increase in the aryl- than those observed in the emission spectra (ESI, S4†).
porphyrin p–p resonance interactions.49 The stokes shi of Accordingly, the energy of vibrational states of the free base
*
porphyrins is dened as the energy gap between the Qx(0,0) and porphyrin in the rst electronic excited states is approximately
Qx(0,0) bands which provide information on the excited states; the same for the used porphyrins, although there are large
small stokes shis give evidence of minor differences between differences between the steric and electronic properties of the
the geometry of porphyrin macrocycle in the ground and excited meso substituents of these compounds. The splitting in the
states. On the other hand, large stokes shis are expected in the emission spectra (Dn2) follows the following order: H2T(4-Cl)PP
case of the reverse mirror image spectra.49 The data of the ESI, > H2T(4-Me)PP > H2T(2-Me)PP > H2TPP > H2T(4-OMe)PP >
S4† show stokes shis in the range of 118 to 262 cmꢀ1 which are H2T(2-Cl)PP which is different from that observed in the case of
in agreement with the reverse mirror image spectra of the the splitting in the absorption spectra. Herein, a difference of
porphyrins. Furthermore, the different conformations of the ca. 164 cmꢀ1 is observed between the rst and last members of
ground and excited states would lead to different dihedral the series which is much higher than that observed in the case
angles between the meso substituents and the porphyrin mean of the splitting in the absorption spectra. Accordingly, there are
plane and consequently different resonance interactions large differences between the energy of the rst vibrational
between the porphyrin and meso aryl group p systems. There- states of the free base porphyrins in the S0 ground state (Fig. 5).
fore, the order of stokes shis found for the free base porphy- Furthermore, the differences between Dn1 and Dn2 for the free
rins i.e. H2T(2-Cl)PP > H2T(4-OMe)PP > H2T(4-Me)PP > H2T(2- base porphyrins assigned as Dn3 are in the range of 247–83 cmꢀ1
Me)PP > H2TPP > H2T(4-Cl)PP may be attributed to either the and decreases as H2T(4-OMe)PP > H2T(2-Cl)PP > H2TPP > H2T(2-
geometrical difference between the porphyrin cores or the Me)PP > H2T(4-Me)PP > H2T(4-Cl)PP. Thus, H2T(4-OMe)PP
varied dihedral angles of the ground and excited states demonstrates the larger difference between the energy of the
porphyrin moieties. In the absorption spectra, the magnitude of rst vibrational states of the S0 and S1 electronic states. The
splitting between the Qx(0,0) and Qx(1,0) bands may be used to uorescence quantum yields Ff for the free porphyrins are
evaluate the energy of the rst vibrational state of the S1 one. It summarized in the ESI, S5.† The Ff of the porphyrins decreases
in the order H2T(4-OMe)PP > H2TPP > H2T(2-Me)PP > H2T(4-Me)
PP > H2T(4-Cl)PP > H2T(2-Cl)PP. The order of Ff values for
a series of porphyrins depends on factors including the elec-
tronic properties of the meso substituents, the heavy atom
effect, the lifetime of the relaxed excited singlet state, the
distortion of porphyrin in the ground state (S0), the extent of
distortion of the S1 state and the efficiency of resonance inter-
actions between the porphyrin and meso aryl p systems. The
latter would increase charge transfer emission and therefore
Fig. 4 Absorption (QX region) and emission spectra of all porphyrins in
CH2Cl2 solutions normalized to each other at their respective QX00 Fig. 5 The contribution from vibrational states in the absorption and
maxima.
emission bands.
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decreases Ff. Indeed, uorescence quantum yield is determined H2T(4-Me)PP in the series may be due to the increased distor-
by the relative rate of the radiative and non-radiative transitions tion of H2T(4-Me)PP relative to H2TPP.
from the S1 state. The presence of heavy atoms at the meso
The natural radiative lifetimes (s1, s2, s3) estimated by the
substituents, especially at the ortho position decreases the Ff formulas of Forster, Strickler and Berg,52–54 decrease in the order
due to the induced increase in intersystem crossing from S1 to H2T(4-OMe)PP > H2T(2-Cl)PP > H2TPP > H2T(2-Me)PP > H2T(4-
T1 transition.37,51 The rate of non-radiative deactivation of the S1 Cl)PP > H2T(4-Me)PP (ESI, S5†). It is observed that there is no
excited state was shown to correlate with the Hammett constant close correlation between the estimated lifetimes and the uo-
of the substituent for both electron withdrawing substituents.47 rescence quantum yields of the porphyrins. It should be noted
However, in the case of electron donating groups such as para- that the higher uorescence quantum yields are not necessarily
OMe and para-Me a very similar high quantum yield was accompanied by longer lifetimes.49
observed.46,47 Herein, H2T(4-OMe)PP with the best electron
The order of kr for the porphyrins is as follows: H2T(4-OMe)
donating meso substituents among the series shows the highest PP > H2TPP > H2T(2-Me)PP > H2T(4-Me)PP > H2T(4-Cl)PP >
Ff and the porphyrins with heavy chlorine atom on the meso aryl H2T(2-Cl)PP. Also, the knr value decreases as H2T(4-Me)PP >
group, H2T(4-Cl)PP and H2T(2-Cl)PP have the lowest quantum H2T(4-Cl)PP > H2T(2-Me)PP > H2TPP > H2T(2-Cl)PP > H2T(4-
yields. The latter may be explained by the heavy metal atom OMe)PP. It is noteworthy that rate constants of charge trans-
effect. It should be noted that the distortion of porphyrin core in fer emission and non-radiative relaxations of S1 state other than
the excited state seems to be higher than that in the ground the internal conversion and intersystem crossing are not
state.49 Also, although the distortion of porphyrins with more considered in the evaluation of kr and knr constants. Therefore,
bulky meso substituents is less than that of those with small a high kr value does not necessitate a low knr one.
meso groups, porphyrins with small meso substituents such as
The porphyrin dications with CF3COOH. The emission
methyl may show high degrees of out-of-plane distortion which spectra of the porphyrin dications (Table 3, Fig. 6) consist of
in turn would decrease their uorescence quantum yields.49 a broad band at the wavelengths between 652 to 734 nm. The
Accordingly, the anomalous behaviour of porphyrins such as wavelength and intensity of the band depends on the nature of
a
Table 3 The fluorescence spectral data of the porphyrin dications with CF3COOH in CH2Cl2
Dications with CF3COOH
H4TPP2+
H4T(4-OMe)PP2+
H4T(2-Me)PP2+
H4T(4-Me)PP2+
666
H4T(2-Cl)PP2+
H4T(4-Cl)PP2+
656
Absorption
Q10 (l/nm)
Q00 (l/nm)
600
652
1329
686
583
633
1355
580
632
1418.6
Splitting (Dn1/cmꢀ1 a
)
Emission
*
Q00 (l/nm)
694
1316
734
1285
660
1195
700
866
646
0.155
31.83
33.70
37.80
31
714
652
1322
694
928
485
0.072
104.60
109.97
123.30
10
9
8
0.7
0.7
0.6
8.9
8.4
7.5
700
Dn1/2 (cmꢀ1
)
*
Q01 (l/nm)
Splitting (Dn2/cmꢀ1 b
)
1153
1009
0.064
9.23
11.24
11.80
108
89
85
6.9
5.7
1323
958
0.067
13.44
15.55
16.67
74
64
60
5.0
4.3
Stokes shi (Dn/cmꢀ1
)
928
0.164
22.68
25.36
27.40
44
39
37
7.2
6.5
953
0.216
21.15
25.32
26.00
47
40
39
10.2
8.5
8.3
Ffc
s1 (ns)d
s2 (ns)e
s3 (ns)f
P
k1 (ꢂ10ꢀ6 sꢀ1 g
)
)
)
P
k2 (ꢂ10ꢀ6 sꢀ1 h
30
26
4.9
4.6
P
k3 (ꢂ10ꢀ6 sꢀ1 i
Kr1 (ꢂ10ꢀ6 sꢀ1 j
)
)
)
Kr2 (ꢂ10ꢀ6 sꢀ1 k
Kr3 (ꢂ10ꢀ6 sꢀ1 l
6.0
4.1
5.4
4.0
Knr1 (ꢂ10ꢀ6 sꢀ1 m
)
)
)
36.9
33.0
30.5
37.1
31.0
30.2
26.5
25.1
22.4
101.4
83.3
79.3
69.4
60.0
56.0
Knr2 (ꢂ10ꢀ6 sꢀ1 n
Knr3 (ꢂ10ꢀ6 sꢀ1 o
a
b
c
*
*
Energy difference between the QX10 and QX00 bands. Energy differences between the QX00 and QX01 bands. Based on a reported
ð
ð
3
ꢀ
ꢁ
gl
gl ð2n0 ꢀ nÞ
d
e
n2 3ðnÞdn:
s2 ¼ 2:880 ꢂ 10ꢀ9n2
3 ðnÞdn:
ꢀ1
ꢀ1
value of 0.13 for Ff of TPP in CH2Cl2.38
s1 ¼ 2:880 ꢂ10ꢀ9n2
gu
gu
n
ð
​
ð
FðnÞdn
ꢀ
ꢁꢀ1
ꢀ
ꢁꢀ1
P
¼
P
P
P
gl
3 ðnÞ
f
g
52–54
ꢀ1
h
ꢀ1
i
ꢀ1
j
ꢀ3
ꢀ3
ꢀ1
s3 ¼ 2:880 ꢂ 10ꢀ9n2
nf
dn and nf
¼
:
k1 ¼ s1
.
k2 ¼ s2
.
k3 ¼ s3
.
kr1 ¼ Ff
ꢂ
k1.
ð
​
av
av
gu
v
FðnÞvꢀ3 dn
P
P
P
P
P
kr2 ¼ Ff ꢂ k2. l kr3 ¼ Ff ꢂ k3. m knr1
¼
k1 ꢀ kr1. n knr2
¼
k2 ꢀ kr2. o knr3
k3 ꢀ kr3.46
k
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groups substituted at the meso positions. The wavelength of the emission band of the porphyrin diacids decreases in the order
emission band of the dications decreases in the order H4T(4- H4T(4-OMe)PP(CF3COO)2 H4TPP(CF3COO)2 H4T(2-Me)
OMe)PP(CF3COO)2 H4T(4-Me)PP(CF3COO)2 H4T(4-Cl) PP(CF3COO)2 > H4T(4-Cl)PP(CF3COO)2 > H4T(4-Me)PP(CF3-
PP(CF3COO)2 > H4TPP(CF3COO)2 > H4T(2-Me)PP(CF3COO)2 COO)2 > H4T(2-Cl)PP(CF3COO)2. Also, the lifetime of the S1
>
>
>
>
>
H4T(2-Cl)PP(CF3COO)2. With the exception of H4T(4-Cl) excited state of the dications decreases as H4T(2-Cl)PP(CF3-
PP(CF3COO)2, the pattern correlates with the electron donating COO)2 [ H4T(2-Me)PP(CF3COO)2 > H4TPP(CF3COO)2 > H4T(4-
ability of the meso substituents. The decrease in the dihedral OMe)PP(CF3COO)2
> H4T(4-Cl)PP(CF3COO)2 > H4T(4-Me)
angle between the porphyrin mean plane and the meso aryl PP(CF3COO)2. It should be noted that there is a large difference
substituents upon diprotonation of porphyrins leads to between the lifetime of the S1 state of H4T(2-Cl)PP(CF3COO)2
enhanced resonance interactions between these substituents and that of the other dications. The orders of kr and knr for the
and the porphyrin p systems in the ground state of porphyrin diprotonated species are as H4T(4-OMe)PP(CF3COO)2 > H4-
diacids. Therefore, the S0 state of porphyrin dications with TPP(CF3COO)2 > H4T(4-Me)PP(CF3COO)2 > H4T(2-Me)PP(CF3-
different meso substituents is expected to be of different energy. COO)2 > H4T(4-Cl)PP(CF3COO)2 [ H4T(2-Cl)PP(CF3COO)2 and
On the basis of the four orbital model of porphyrins,20 in H4T(4-Me)PP(CF3COO)2
>
H4T(4-Cl)PP(CF3COO)2
[
H4-
contrast to the a2u orbital, the a1u orbital has no electron density TPP(CF3COO)2 > H4T(4-OMe)PP(CF3COO)2 > H4T(2-Me)PP(CF3-
at the meso positions and therefore the main difference between COO)2 > H4T(2-Cl)PP(CF3COO)2, respectively. It is observed that
the ground state energy of the porphyrin diacids is probably due with the exception of H4T(4-Me)PP(CF3COO)2, the intensity of
to the difference between the p–p interaction between the the emission bands correlates well with the kr values. Also, the
former and the p system of the meso groups. Accordingly, the knr values of H4T(4-Me)PP(CF3COO)2 and H4T(4-Cl)PP(CF3-
energy of S0 state of the porphyrin dications bearing more COO)2 are signicantly larger than those of the other dications.
electron donating groups should be higher than that of those The splitting of the absorption bands of the porphyrin dications
with less electron donating or bulky substituents at the meso are summarized in Table 3. As may be seen from the ESI, S5†
position (Fig. 7). On the other hand, the S1 excited state of the H4TPP(CF3COO)2, H4T(2-Me)PP(CF3COO)2 and H4T(2-Cl)
dications is accompanied with the shi of electron density PP(CF3COO)2 show a distinct splitting of the Q(0,0) and Q(1,0)
towards the porphyrin periphery which leads to a decrease in bands which decreases in the order H4T(2-Cl)PP(CF3COO)2
>
the electron donation from the meso substituents to the a2u H4T(2-Me)PP(CF3COO)2 > H4TPP(CF3COO)2. In the case of
orbital. Accordingly, the effect of meso substituents on the H4T(4-OMe)PP(CF3COO)2, H4T(4-Me)PP(CF3COO)2 and H4T(4-
raising of energy of the a2u orbital is expected to be more Cl)PP(CF3COO)2, the Q(1,0) band appears as a shoulder on the
pronounced in the S0 state compared to the S1 one (Fig. 7). As corresponding Q(0,0) one. On the other hand, in the emission
was seen above, the electron donation to the a2u orbital through spectra, generally the two emission bands of the free base
resonance interaction increases the energy level of this orbital. porphyrins reduce to a broad single band. In other words, the
The red shied emission band in the uorescence spectra of the energy spacing between vibrational levels decreases signi-
dications with electron rich meso substituents relative to those cantly in the case of the porphyrin dications and the overlap of
with electron decient ones may be explained by greater the bands is observed. However, in the emission spectra of
destabilization of the S0 state of the former compared to the H4T(2-Me)PP(CF3COO)2 and H4T(2-Cl)PP(CF3COO)2 the Q*(0,1)
latter. However, although the splitting of emission bands band appears as a shoulder on the Q*(0,0) one. According to
caused by the involvement of vibrational states has not observed ESI, S6† a reverse mirror image relationships is observed
in the spectra of the dications, the involvement of vibrational between the absorption and emission spectra. Also, there are
states (Fig. 7) in the observed difference between the energy of large Stokes shi between the emission and absorption spectra
the emission bands can not be excluded. The intensity of the of the dications (vide infra). The Stokes shi of the dications
decreases as H4T(4-Me)PP(CF3COO)2 > H4T(4-Cl)PP(CF3COO)2 >
Fig. 6 The emission spectra of the porphyrin dications with CF3COOH Fig. 7 Schematic presentation of the effect of p–p interactions on the
in CH2Cl2.
energy level of S0 and S1 of the free base porphyrins.
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H4T(4-OMe)PP(CF3COO)2
> H4TPP(CF3COO)2 > H4T(2-Me) Me)PP(HCOO)2 > H4T(4-Cl)PP(HCOO)2 > H4TPP(HCOO)2 >
PP(CF3COO)2 > H4T(2-Cl)PP(CF3COO)2 (Table 3). The observed H4T(2-Me)PP(H3COO)2 > H4T(2-Cl)PP(HCOO)2 (Table 4). The
Stokes shis are much larger than those of the corresponding lower acid strength of HCOOH compared to that of CF3COOH
free base porphyrins that consist with the observed reverse leads to smaller shi of electron density from the porphyrin
mirror image relationship between the absorption and emis- core to the acid molecules. This effect on the other hand causes
sion spectra. As was mentioned above, major differences decreased coplanarity of the meso aryl substituents in the case
between the geometry of porphyrin macrocycle in the ground of the dications with HCOOH and consequently less out-of-
and excited states as well as the coplanarity of the meso aryl plane deformation of the porphyrin core. In other words, the
substituents are evident by the observation of large Stokes dihedral angle between the porphyrin mean plane and the aryl
shi.49 The uorescence quantum yields of the dications groups is probably different in the case of the dications with
decrease as follows:
HCOOH and CF3COOH. The red shied emission bands of the
H4T(4-OMe)PP(CF3COO)2 > H4TPP(CF3COO)2 > H4T(2-Me) dications with the former compared to that of the latter seems
PP(CF3COO)2 > H4T(2-Cl)PP(CF3COO)2 > H4T(4-Cl)PP(CF3COO)2 to be due to different distortions of the porphyrin cores and the
$ H4T(4-Me)PP(CF3COO)2.
dihedral angles. The anion of triuoroacetic acid is a weaker
The pattern correlates approximately with the electron donor hydrogen bond acceptor than that of HCOOH. Indeed, the
ability of the meso substituents. In the case of ortho or para introduction of electron-withdrawing atoms at the carboxylate
chlorinated porphyrin dications, the heavy atom effect is anion leads to a decrease in the hydrogen bond acceptor ability
involved in the observed order of Ff.
of the anion.55 Furthermore, according to the literature,10 the
The porphyrin dications with HCOOH. With the exception of necessity to optimize the hydrogen bond interactions between
H4T(2-Cl)PP(HCOO)2, the emission spectra of the dications of the counter anion and the porphyrin diprotonated species is
porphyrins with HCOOH consist of a broad band at 656 to 748 among the factors determining the actual degree of saddling of
nm (ESI, S6†). The comparison of emission and absorption porphyrin core. In this regard, the porphyrin dication with Brꢀ
spectra of H4T(2-Cl)PP(HCOO)2 shows a reverse mirror image as the counter anion has a more distorted core with respect to
relationship, although the Q*(0,1) band of H4T(2-Cl)PP(HCOO)2 the corresponding dication with Fꢀ. According to the above
appears as a shoulder at the Q*(0,0) band (ESI, S8†). The posi- discussions, for a given porphyrin, the dication with CF3COOH
tion of the uorescence emission bands of the dications with may be considered as a more distorted macrocycle. The inten-
HCOOH shows a red shi relative to the corresponding band of sity of the emission band of the diacids with HCOOH decreases
the dication with CF3COOH. However, as was observed in the in the order H4T(2-Me)PP(HCOO)2 > H4T(4-OMe)PP(HCOO)2 >
case of the dications with CF3COOH, the wavelength of the H4TPP(HCOO)2 > H4T(4-Cl)PP(HCOO)2 > H4T(2-Cl)PP(HCOO)2 >
band decreases in the order of H4T(4-OMe)PP(HCOO)2 > H4T(4- H4T(4-Me)PP(HCOO)2 that is different from that observed in the
a
Table 4 The fluorescence spectral data of the porphyrin dications with HCOOH in CH2Cl2
Dications with HCOOH
H4TPP2+
H4T(4-OMe)PP2+
H4T(2-Me)PP2+
H4T(4-Me)PP2+
672
H4T(2-Cl)PP2+
H4T(4-Cl)PP2+
662
Absorption
Q10 (l/nm)
Q00 (l/nm)
604
657
1336
695
582
635
1434
580
631
1393
Splitting (Dn1/cmꢀ1
)
Emission
*
Q00 (l/nm)
700
1176
748
660
724
656
1446
698
917
604
0.076
111.25
115.16
131.58
9
9
8
0.7
0.7
0.6
8.3
8.0
7.0
706
Dn1/2 (cmꢀ1
)
*
Q01 (l/nm)
Splitting (Dn2/cmꢀ1
)
1308
1019
0.216
21.13
26.00
26.35
47
38
38
10.2
8.3
8.2
1550
596
0.283
35.42
37.04
41.40
28
27
24
8.0
7.6
1128
1069
0.062
9.23
11.24
11.80
108
89
85
6.7
5.5
1179
941
0.075
11.30
13.43
14.20
88
74
70
6.6
5.6
Stokes shi (Dn/cmꢀ1
Ff
)
935
0.202
22.50
25.32
27.42
44
39
36
9.0
8.0
s1 (ns)
s2 (ns)
s3 (ns)
P
k1 (ꢂ10ꢀ6 sꢀ1
k2 (ꢂ10ꢀ6 sꢀ1
k3 (ꢂ10ꢀ6 sꢀ1
)
)
)
P
P
Kr1 (ꢂ10ꢀ6 sꢀ1
)
)
)
Kr2 (ꢂ10ꢀ6 sꢀ1
Kr3 (ꢂ10ꢀ6 sꢀ1
7.4
6.8
5.3
5.2
Knr1 (ꢂ10ꢀ6 sꢀ1
Knr2 (ꢂ10ꢀ6 sꢀ1
Knr3 (ꢂ10ꢀ6 sꢀ1
)
)
)
35.4
31.5
29.1
37.1
30.2
29.7
20.2
19.4
17.3
101.6
83.5
79.7
81.9
68.9
65.1
a
See the footnotes of Table 3.
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case of the corresponding dications with CF3COOH. In PP(HCOO)2 [ H4T(4-OMe)PP(HCOO)2 > H4TPP(HCOO)2
>
comparison with the porphyrin dications with CF3COOH, the H4T(2-Me)PP(HCOO)2 > H4T(2-Cl)PP(HCOO)2, respectively. The
intensity of the emission bands of the dications with HCOOH is patterns for the kr and knr values are very similar to those
usually greater than that of those with CF3COOH. However, the observed for the dications with CF3COOH.
lifetime of the S1 excited state of the dications decreases as
H4T(2-Cl)PP(HCOO)2
[
H4T(2-Me)PP(HCOO)2
>
H4-
TPP(HCOO)2 > H4T(4-OMe)PP(HCOO)2 > H4T(4-Cl)PP(HCOO)2 > Conclusion
H4T(4-Me)PP(HCOO)2 which is the same as that observed in the
A
series of para- or ortho-substituted meso-tetra(phenyl)
case of the dications with CF3COOH. As was observed in the
case of the dications with CF3COOH, again the lifetime of the S1
state of H4T(2-Cl)PP(HCOO)2 is much higher than that of the
other dications. The orders of kr and knr for the diprotonated
porphyrins and their dications with weak and strong carboxylic
acids were investigated by uorescence and absorption spec-
troscopy and found that: (i) the degree of conguration inter-
action (CI) between the excited states was estimated to decrease
as H2T(2-Cl)PP [ H2T(2-Me)PP > H2T(4-Cl)PP ꢁ H2TPP >
H2T(4-OMe)P > H2T(4-Me)PP that with the exception of H2T(4-
OMe)P correlates with the electron-donor ability of the aryl
groups; (ii) in the case of the dications with CF3COOH and
HCOOH, similar patterns were observed, but the fSoret/fQ(0,0)
values were much less than those observed for the free base
species are as H4T(4-OMe)PP(HCOO)2 > H4TPP(HCOO)2
>
H4T(2-Me)PP(HCOO)2 > H4T(4-Me)PP(HCOO)2 $ H4T(4-Cl)
PP(HCOO)2 [ H4T(2-Cl)PP(HCOO)2 and H4T(4-Me)PP(HCOO)2
>
H4T(4-Cl)PP(HCOO)2 [ H4T(4-OMe)PP(HCOO)2 > H4-
TPP(HCOO)2 > H4T(2-Me)PP(HCOO)2 > H4T(2-Cl)PP(HCOO)2,
respectively. The orders are very similar with those observed for
the dications with CF3COOH. The stokes shi of porphyrins
which are large in the case of the reverse image spectra, provide
information on the differences between the geometry of
porphyrin macrocycle in the ground and excited states.46 The
order of stokes shi of dications was found to decrease in the
order H4T(4-Me)PP(HCOO)2 > H4T(4-OMe)PP(HCOO)2 > H4T(4-
porphyrins and larger differences were found between the fSoret
/
fQ(0,0) values; (iii) reverse mirror image relationships were
observed between the absorption and emission spectra of the
porphyrins and their dication species; (iv) the position of the
emission bands was fairly inuenced by the stereoelectronic
effects of the meso substituents. However, the total intensity of
the emission bands of the porphyrins decreased in the order
H2T(4-OMe)PP > H2TPP > H2T(2-Me)PP > H2T(4-Me)PP > H2T(4-
Cl)PP > H2T(2-Cl)PP; (v) in the emission spectra of the dications
compared to the free base porphyrins, the much larger stokes
shis observed reveal major geometrical differences between
the ground and excited states of the dication; (vi) except in the
case of H4T(4-OMe)PP, the uorescence quantum yields of the
dications were much larger than those of the free base
porphyrins. The natural radiative lifetimes (s1, s2, s3) of the
dications were generally much shorter than those of the free
base porphyrins. Furthermore, signicantly larger radiative and
non-radiative rate constants were observed for the dications;
(vii) little or no splitting was observed in the absorption and
emission spectra of the dications.
Cl)PP(HCOO)2
> H4TPP(HCOO)2 > H4T(2-Cl)PP(HCOO)2 >
H4T(2-Me)PP(HCOO)2. As was observed in the case of the dica-
tions with CF3COOH, the magnitudes of stokes shis are much
larger than those of the corresponding free base porphyrins.
This nding reveals large differences between the geometry of
the porphyrin diacids in the S0 and S1 states. Again, the change
in the dihedral angles between the porphyrin core and the meso
aryl groups should be considered to be involved upon the
conformational changes. In comparison with the stokes shis
observed for the dications with CF3COOH, with the exception of
H4T(2-Me)PP(HCOO)2 and H4T(4-Cl)PP(HCOO)2, the stokes
shis of the other dications with HCOOH are greater than those
of the corresponding dications with CF3COOH. It should be
noted that while there is a small difference between the stokes
shi of H4T(4-Cl)PP(HCOO)2 and H4T(4-Cl)PP(CF3COO)2,
a difference of 50 cmꢀ1 was observed between the stokes shis
of H4T(2-Me)PP(HCOO)2 and H4T(2-Me)PP(CF3COO)2.
Acknowledgements
The order of Ff values of the dications decreases as H4T(2-
Me)PP(HCOO)2 > H4T(4-OMe)PP(HCOO)2 > H4TPP(HCOO)2
H4T(2-Cl)PP(HCOO)2 H4T(4-Cl)PP(HCOO)2 H4T(4-Me)
>
Financial support of this work by the Institute for Advanced
Studies in Basic Sciences (IASBS) is acknowledged.
$
>
PP(HCOO)2. Except for the position of H4T(2-Me)PP(HCOO)2 in
the series, the order of Ff for the other dications is the same as
that was observed in the case of the dications with CF3COOH. It
should be noted that the Ff values of H4TPP(HCOO)2 and H4T(2-
Me)PP(HCOO)2 are remarkably larger than those of the corre-
sponding dications with CF3COOH. In spite of the greater Ff
value of the dications with HCOOH, there is no large difference
between the Ff values of the two series of the dications in the
case of the other porphyrins.
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>
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>
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Paper
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