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

Abstract

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.

---

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

The characteristic physical, chemical and biological properties 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 shifted Soret bands and the red^6,12,14–16 or blue shifted^11,14 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 and Q bands of porphyrins are due to the a_1u–e_g and a_2u–e_g transitions which are of E_u symmetry considering a D_4h symmetry for the porphyrin core. Therefore, the excited states resulted in by these transitions are of the same symmetry and consequently different degrees of configuration 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 configuration interaction depends to the difference between the energy of the E_u states involved in the interaction, so that a decreased difference between the energy of the interacting configurations causes an increased extent of configuration interaction.

The introduction of various substituents at the periphery of the free base porphyrins with a planar or nearly planar conformation influences 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 cm⁻¹ 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 cm⁻¹. It is noteworthy that there are ample evidence that the phenyl and porphyrin planes are significantly non-coplanar in such molecules^4,6,28,29 that limits the efficient resonance interactions between the π systems of the meso-aryl substituents and the porphyrin core.^2,6,10 Diprotonation of porphyrins which is accompanied with a significant decrease in the dihedral angle between the meso substituents and the porphyrin mean plane^2,9,10,12 influences the difference between the energy of the E_u 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 a_1u orbital in contrast to the stabilization of the a_2u and e_g 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 influenced by the electronic effects of meso substituents 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-deficient 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 find 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 substituents. 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 (ε) of the transitions.

Fig. 1 meso-Tetra(aryl)porphyrins used in this study.

Experimental

Synthesis and characterization

H₂TPP, H₂T(4-OMe)PP, H₂T(2-Me)PP, H₂T(4-Me)PP, H₂T(2-Cl)PP, H₂T(4-Cl)PP³⁴ were synthesized, characterized and purified according to the literature methods. The ¹H NMR, ¹³C NMR and UV-Vis spectral data of the porphyrins are presented in the ESI, S1a.†

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 dications^16,35 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.†

Extinction coefficient measurement

To calculate the ε values, 0.5 to 10 μM 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 fluorescence spectra were obtained using dilute solutions (∼10 μM).^10,36 As was previously reported,³⁷ the O₂ dissolved in air-saturated solvents was observed to reduce the fluorescence yield of porphyrins by nearly 15%. Accordingly, the dichloromethane 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 fluorescence quantum yields (Φ_f) were calculated relative to that of H₂TPP in CH₂Cl₂ as standard (Φ_f = 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 fluorescence cell on a Cary Eclipse fluorescence 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-tetra(aryl)porphyrins the aryl substituents are nearly perpendicular to the porphyrin mean plane and therefore there is no strong resonance type interactions between the π system of the meso groups and that of the porphyrin core.^6,9,40 Accordingly, there are no remarkable differences between the UV-Vis spectra of meso-tetra(aryl)porphyrins bearing various groups at the meso positions. However, the porphyrins with bulky meso substituents have an increased dihedral angle between the meso groups and the porphyrin moiety.²⁷ The data of ESI, S2† show that the position of the absorption bands are influenced by the electronic and steric effects of the meso substituents; the introduction of electron donating –OMe group at the para position of H₂TPP leads to a small shift of the Soret and Q bands to lower energies. Also, the presence of bulky methyl or chloro groups at the ortho positions with respect to the porphyrin ring causes a slight blue shift of the bands. However, the differences between the ε of the corresponding Soret and Q bands of different porphyrins are more obvious in comparison to those observed between the energy of the bands. In other words, the change of meso aryl substituents mainly affects the intensity of the Soret and Q bands. The intensity of the absorption bands may be interpreted on the basis of the ε values but the intensity of bands with different widths cannot be correctly compared using the ε of bands⁴¹ and therefore the f values should be used for the comparison of the intensities.²¹ The ε and f values of the Soret bands decrease in the order H₂T(2-Me)PP > H₂T(4-Me)PP > H₂T(4-Cl)PP ∼ H₂TPP > H₂T(2-Cl)PP > H₂T(4-OMe)PP and H₂T(4-Me)PP > H₂T(4-Cl)PP > H₂T(2-Me)PP > H₂TPP > H₂T(2-Cl)PP > H₂T(4-OMe)PP, respectively. Also, the ε and f values of the Q(0,0) bands decrease as H₂TPP > H₂T(2-Me)PP > H₂T(4-Me)PP > H₂T(4-Cl)PP ∼ H₂T(4-OMe)PP > H₂T(2-Cl)PP and H₂T(4-Me)PP > H₂T(4-Cl)PP > H₂T(2-Me)PP > H₂TPP ≥ H₂T(4-OMe)PP > H₂T(2-Cl)PP, respectively. As was mentioned above, the f value gives a better criterion for the comparison of the intensity of the absorption bands than the ε one. Also, the observed order of the ε and f values does not correlate with the electron-donating strength of the meso groups. The ratio of f_Soret/f_Q(0,0) has be used as a criterion for the comparison of the degree of CI between the excited states.^6,22 As may be seen from Table 1, the ratio decreases in the order of H₂T(2-Cl)PP ≫ H₂T(2-Me)PP > H₂T(4-Cl)PP ∼ H₂TPP > H₂T(4-OMe)P > H₂T(4-Me)PP. It should be noted that the increase of the f_Soret/f_Q(0,0) ratio is due to the decrease of the f_Q(0,0) resulted in by the CI between the e_u excited states.⁶ In other words, the above order of the ratio observed for the porphyrins suggests the decrease of degree of CI in the series from H₂T(2-Cl)PP to H₂T(4-Me)PP. Interestingly, H₂T(4-OMe)PP showed a greater degree of CI than H₂T(4-Me)PP which does not correlate with the relative electron donating ability of the meso substituents. Meot-Ner et al. evaluated a much greater basicity for H₂T(4-OMe)PP that was rationalized on the basis of better electron donor ability of 4-methoxyphenyl compared to that of 4-methylphenyl group. Also, they have found a greater f_Soret/f_Q(0,0) ratio for H₂T(4-Me)PP than H₂TPP, suggesting a greater degree of CI for the former. According to the four orbital model of porphyrins, for the a_2u orbital the compared electron densities are largest on the methene carbons and pyrrole nitrogens and therefore the substitution of meso positions with electron donating groups would expected to increase the energy of this orbital (Fig. 2).⁶ Also, the removal of the accidental degeneracy of the a_2u and a_lu orbitals will lead to increase in the oscillator strength of the Q bands which will be much more significant in the case of the forbidden Q(0,0) bands compared to the allowed Q(0,1) ones.^6,22,42,43 However, the order of f_Soret/f_Q(0,0) for H₂T(4-OMe)PP and H₂T(4-Me)PP can not be explained according to the relative electron donating ability of their meso groups. In other words, there is no close correlation between the f_Soret/f_Q(0,0) ratios and the electronic properties of the meso substituents. It is noteworthy that in spite of the much greater basicity of H₂T(4-OMe)PP (pK_a = 2.31) than H₂T(4-Me)PP (pK_a = 1.85) values of 107.9 and 73.2 were observed for the f_Soret/f_Q(0,0) ratio of the porphyrins, respectively.⁶ It should be noticed that, the Q bands of porphyrins are generally less intense and much broader than the Soret bands and therefore there is some errors in estimation of the f values for the Q bands. However, the observed difference between the f_Soret/f_Q(0,0) values for the two porphyrins is too much to be attributed to the experimental error in the measurement of f_Soret/f_Q(0,0) values. Accordingly, the order of porphyrin basicity for a series of free base porphyrins does not seem to be a good criterion for the comparison of the f_Soret/f_Q(0,0) ratio of the compounds. On the basis of the molecular orbital theory, the strength of a covalent bond depends inversely to the energy difference and directly to the overlap between the involved orbitals. While the introduction of good electron donating groups at the para position of the meso substituents increases the bond strength between the π systems of the aryl group and the porphyrin core, the stabilization of the aryl HOMO orbitals caused by the electron donating groups will weaken the π bond (Fig. 2). Furthermore, the dihedral angle between the meso aryl substituents and the porphyrin mean plane may be different for the diprotonated species of various meso-tetra(aryl)porphyrins, especially in the case of the porphyrins bearing bulky substituents at the ortho position of the aryl groups.² X-ray crystallographic studies show that the diprotonation of meso-tetra(aryl)porphyrins is accompanied with significant decreases in the dihedral angle between the meso aryl groups and the porphyrin mean plane.¹² Also, the presence of bulky groups such as methyl- and chlorine atom at the ortho positions leads to a decreased flexibility of the porphyrin core towards the out of plane deformation which in turn restricts the dihedral angles to greater ones.² According to the above discussion, the porphyrin basicity can not be explained only on the basis of the donating ability of the groups attached to the phenyl group of ortho, para or meta substituted meso-tetra(phenyl)porphyrins and the final dihedral angle between the meso aryl substituents and the porphyrin mean plane should be also considered. According to the ESI, S2,† the highest extent of CI was observed in the case of H₂T(2-Cl)PP with bulky Cl substituents at the ortho position. This observation seems to be due to the inefficient π resonance interactions between the meso groups and the porphyrin π systems. On the other hand, the methyl group is as bulky as the chlorine atom⁴⁴ and therefore H₂T(2-Me)PP was expected to behave as H₂T(2-Cl)PP and show a high degree of CI. However, the f_Soret/f_Q(0,0) ratio for H₂T(2-Me)PP is much smaller than that of H₂T(2-Cl)PP.

Table 1 The UV-Vis spectral data of the porphyrin dications with CF₃COOH in CH₂Cl₂^a

Dications		Bands			ΔνQ(0,0),Soret (cm^−1)
		Soret	II	I
a Oscillator strength; f = 4.32 × 10^−9∫ευdν.
H4TPP(CF3COO)2	λ (nm)	437	600	652	7546
	Δνfree base,dication (cm^−1)	−1097	−282	−118
	log ε	5.83	4.46	4.93
	f	2.042	0.053	0.288
	fSoret/fQ(0,0)	7.1
H4T(4-OMe)PP(CF3COO)2	λ (nm)	449		686	7694
	Δνfree base,dication (cm^−1)	−1481		−784
	log ε	5.77		5.07
	f	1.780		0.340
	fSoret/fQ(0,0)	5.2
H4T(2-Me)PP(CF3COO)2	λ (nm)	432	583	633	7350
	Δνfree base,dication (cm^−1)	−890	175	294
	log ε	5.99	4.64	4.89
	f	2.400	0.100	0.190
	fSoret/fQ(0,0)	12.6
H4T(4-Me)PP(CF3COO)2	λ (nm)	442		666	7609
	Δνfree base,dication (cm^−1)	−1299		−441
	log ε	5.85		5.01
	f	3.070		0.730
	fSoret/fQ(0,0)	4.2
H4T(2-Cl)PP(CF3COO)2	λ (nm)	431	580	632	7379
	Δνfree base,dication (cm^−1)	−837	206	271
	log ε	5.76	4.69	4.75
	f	1.520	0.044	0.058
	fSoret/fQ(0,0)	26.2
H4T(4-Cl)PP(CF3COO)2	λ (nm)	439		656	7535
	Δνfree base,dication (cm^−1)	−1144		−212
	log ε	6.06		5.16
	f	2.760		0.500
	fSoret/fQ(0,0)	5.5

---

Fig. 2 Schematic presentation of the effect of meso aryl groups on the energy level of the a_2u orbital. The e_1g symmetry of the HOMOs of each aryl group refers to a local symmetry of D_6h for the group.

The porphyrin dications with CF₃COOH. Diprotonation of meso-tetra(aryl)porphyrins with CF₃COOH led to the red shift of the Soret band and the red or blue shift of the Q bands (Table 1). Also, the Q bands reduce to an intense band at longer wavelength, i.e. Q(0,0) band, and a weak band at shorter wavelength. 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 (ε or f) of the Q bands. The red shifted 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 the porphyrin mean plane.² The ab initio calculations showed that the saddling of porphyrin core is accompanied with the red shift 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 shift of this band to higher wavelengths.⁶ The substitution of ortho positions with bulky substituents such as methyl group restricts the rotation around the C_meso–C_aryl band and prevents the efficient resonance interaction between the aryl group and the porphyrin core π systems.² The observed blue shift 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 shift of the Soret band is smaller than that of the porphyrins with no substituents at the ortho position (Table 1). The ε and f values of the Soret bands of the dications follow the following order:

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 > H₄T(4-OMe)PP(CF₃COO)₂ > H₄T(2-Cl)PP(CF₃COO)₂, respectively. Also, the ε and f values of the Q(0,0) bands decrease as H₄T(4-Cl)PP(CF₃COO)₂ > H₄T(4-OMe)PP(CF₃COO)₂ > H₄T(4-Me)PP(CF₃COO)₂ > H₄TPP(CF₃COO)₂ > H₄T(2-Me)PP(CF₃COO)₂ > H₄T(2-Cl)PP(CF₃COO)₂ and H₄T(4-Me)PP(CF₃COO)₂ > H₄T(4-Cl)PP(CF₃COO)₂ > H₄T(4-OMe)PP(CF₃COO)₂ > H₄TPP(CF₃COO)₂ > H₄T(2-Me)PP(CF₃COO)₂ > H₄T(2-Cl)PP(CF₃COO)₂, respectively. The order of f_Soret/f_Q(0,0) ratio for the dications is as H₄T(2-Cl)PP(CF₃COO)₂ > H₄T(2-Me)PP(CF₃COO)₂ > H₄TPP(CF₃COO)₂ > H₄T(4-Cl)PP(CF₃COO)₂ > H₄T(4-OMe)PP(CF₃COO)₂ > H₄T(4-Me)PP(CF₃COO)₂. With the exception of the position of H₄T(4-Cl)PP(CF₃COO)₂ relative to H₄TPP(CF₃COO)₂ and that of H₄T(4-Me)PP(CF₃COO)₂ compared to H₄T(4-OMe)PP(CF₃COO)₂ the order of f_Soret/f_Q(0,0) ratio for the dications correlates with the electron donating ability of the meso aryl groups. It should be noted that the saddle (or nearly saddle¹¹) shaped porphyrin core of the dications, facilitates the coplanarity of the aryl substituents and the porphyrin core. This in turn leads to more efficient π–π interactions between the aryl groups and the porphyrin core. Therefore, the difference between the electron donor ability of the aryl groups is more obvious in the case of the porphyrin dications compared to the corresponding free base porphyrins. Also, the electron deficiency on the porphyrin core caused by the shift of electron densities from the pyrrolenine nitrogen atoms to the CF₃COOH molecules seems to lead to dominance of the π-resonance interactions over the inductive sigma electron withdrawing effects of the chlorine atoms of the 4-Cl-phenyl groups in comparison with that of the phenyl group. Herein, the stronger electron donating aryl substituents resulted in a decreased degree of CI. Also, in the case of H₄T(4-Me)PP(CF₃COO)₂ and H₄T(4-OMe)PP(CF₃COO)_2, the unusual observation may be due to the more efficient overlap between the π systems of diprotonated porphyrin core and the meso substituents of the former than the latter (Fig. 3). It should be noted that due to the higher degree of saddling of porphyrin core of [H₄T(4-OMe)PP]²⁺ compared to that of [H₄T(4-Me)PP]²⁺, the a_2u orbital of the two porphyrin dications cannot be of the same energy. In other words, the a_2u orbital of the former seems to be more destabilized by the out-of-plane deformation of the porphyrin core. On the other hand, the energy gap between the a¹_1ue¹_g–a¹_2ue¹_g states is influenced by the change of the energy of the higher and lower states. In other word, the better electron donating ability of the 4-OMe group would lead to an increase in the energy of the a¹_1ue¹_g state of [H₄T(4-OMe)PP]²⁺ compared to that of [H₄T(4-Me)PP]²⁺(ΔE₁). Also, the higher degree of saddling of [H₄T(4-OMe)PP]²⁺ would resulted in a more destabilized a¹_1ue¹_g state for [H₄T(4-OMe)PP]²⁺ than [H₄T(4-Me)PP]²⁺ (ΔE₂). The observed higher CI for [H₄T(4-OMe)PP]²⁺ may be explained by higher values of ΔE₂ than ΔE₁ (Fig. 3).

Fig. 3 The comparison of H₄T(4-Me)PP²⁺ and H₄T(4-OMe)PP²⁺ excited states.

The porphyrin dications with HCOOH. The absorptions bands due to the porphyrin dications with HCOOH are demonstrated in Table 2. As was observed in the case of the dications with CF₃COOH, the interaction of porphyrins with HCOOH shifts the absorption bands to higher wavelengths. The comparison of Tables 1 and 2 shows larger red shift of the Soret and Q(0,0) bands for the dications of porphyrins with the latter that is more obvious in the case of the Q(0,0) band. HCOOH (pK_a = 3.75 (ref. 14)) is a much weaker acid than CF₃COOH.¹⁴ We have previously shown that the shift of electron density from the porphyrin core to the central acid molecules by itself causes a ca. 800 cm⁻¹ blue shift of the Q(0,0).¹⁴ The difference between the position of the Q(0,0) bands of the dications of a given porphyrin with CF₃COOH and HCOOH may be due to the larger shift of electron density from the porphyrin core to the acid molecules in the case of CF₃COOH which in turn leads to larger blue shift of the band for the dications of porphyrins with CF₃COOH. It should be noted that the final red shift of the band is probably due to the decreased HOMO-LUMO gap caused by the out-of-plane deformation of porphyrin core as well as the decreased dihedral angle between the meso aryl groups and the porphyrin core; the ab initio calculations on porphine revealed that the saddling of porphyrin core is accompanied with the red shift of the Q(0,0) band.^2,4,11,12 Also, the enhanced coplanarity of the meso aryl substituents with the porphyrin mean plane induces the red shift of the Q(0,0) band of the meso-tetra(aryl)porphyrin dication. The ε and f values of the Soret bands of the dications with HCOOH decrease in the order H₄T(4-Me)PP(HCOO)₂ > H₄T(2-Me)PP(HCOO)₂ > H₄TPP(HCOO)₂ > H₄T(2-Cl)PP(HCOO)₂ ∼ H₄T(4-Cl)PP(HCOO)₂ > H₄T(4-OMe)PP(HCOO)₂ and H₄T(4-Me)PP(HCOO)₂ > H₄T(2-Me)PP(HCOO)₂ ≥ H₄T(4-Cl)PP(HCOO)₂ > H₄TPP(HCOO)₂ > H₄T(4-OMe)PP(HCOO)₂ > H₄T(2-Cl)PP(HCOO)₂, respectively. Also, ε and f values of the Q(0,0) bands decrease in the order H₄T(4-Me)PP(HCOO)₂ > H₄T(4-Cl)PP(HCOO)₂ > H₄TPP(HCOO)₂ > H₄T(4-OMe)PP(HCOO)₂ > H₄T(2-Me)P(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 > H₄TPP(HCOO)₂ > H₄T(2-Me)P(HCOO)₂ > H₄T(2-Cl)PP(HCOO)₂, respectively.

Table 2 The UV-Vis spectral data of porphyrin dications with HCOOH in CH₂Cl₂^a

Dications		Bands			ΔνQ(0,0),Soret (cm^−1)
		Soret	II	I
a See the footnotes of Table 1.
H4TPP(HCOO)2	λ (nm)	439	604	657	7558
	Δνfree base,dication (cm^−1)	−1202	−393	−235
	log ε	5.75	4.42	4.89
	f	2.180	0.052	0.290
	fSoret/fQ(0,0)	7.5
H4T(4-OMe)PP(HCOO)2	λ (nm)	452		695	7735
	Δνfree base,dication (cm^−1)	−1629		−972
	log ε	5.36		4.73
	f	1.770		0.350
	fSoret/fQ(0,0)	5.1
H4T(2-Me)PP(HCOO)2	λ (nm)	433	582	635	7347
	Δνfree base,dication (cm^−1)	−944	204	244
	log ε	5.79	4.49	4.69
	f	2.830	0.100	0.170
	fSoret/fQ(0,0)	16.6
H4T(4-Me)PP(HCOO)2	λ (nm)	443		672	7692
	Δνfree base,dication (cm^−1)	−1350		−575
	log ε	5.88		5.09
	f	3.070		0.730
	fSoret/fQ(0,0)	4.2
H4T(2-Cl)PP(HCOO)2	λ (nm)	432	580	631	7300
	Δνfree base,dication (cm^−1)	−890	206	296
	log ε	5.65	4.41	4.52
	f	1.580	0.044	0.055
	fSoret/fQ(0,0)	28.3
H4T(4-Cl)PP(HCOO)2	λ (nm)	442		662	7519
	Δνfree base,dication (cm^−1)	−1299		−350
	log ε	5.65		4.91
	f	2.820		0.600
	fSoret/fQ(0,0)	4.7

---

In the case of the Soret band, the pattern is to some extent different from that observed for the corresponding dications with CF₃COOH. However, relative intensities of the Q(0,0) bands are the same as those observed for the dications with CF₃COOH.

The f_Soret/f_Q(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 f_Soret/f_Q(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 f_Soret/f_Q(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 ability of the meso substituents. It should be noted that the f_Soret/f_Q(0,0) values of H₄T(2-Cl)PP(HCOO)₂ and H₄T(2-Me)PP(HCOO)₂ are significantly larger than those of the other dications. Also, there is no large difference between the f_Soret/f_Q(0,0) values of the latter. It may be concluded that the f_Soret/f_Q(0,0) value is mainly influenced by the steric hindrance of the bulky groups (ortho methyl or chloro substituents) which decreases the effective π–π interactions between the meso aryl groups and the porphyrin π system. Also, the difference between the electron donor ability of the groups attached to the para position has little effects on this ratio. Except in the case of H₄T(4-Cl)PP(HCOO)₂ and H₄T(4-OMe)PP(HCOO)₂, the f_Soret/f_Q(0,0) ratios for the dications of with HCOOH are larger than those of the corresponding dications with CF₃COOH (Table 1). In other words, the degree of CI for the dication of a given porphyrin with a weak carboxylic acid may be either greater or smaller than that of the corresponding dication with a strong one.

The f_Soret/f_Q(0,0) ratios for the dications compared to that of the free base porphyrins. As may be seen from Tables 1 and 2, the formation of porphyrin dications with HCOOH and CF₃COOH is associated with significant decreases in the f_Soret/f_Q(0,0) ratio indicating the decrease of CI upon the diprotonation of porphyrins with either weak or strong carboxylic acids. Accordingly, the energy gap between the two excited states involved in the a_1u to e_g and the a_2u to e_g transitions in the case of the free base porphyrins should be lower than that in the case of the porphyrin dications. 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 by different factors such as the out-of-plane deformation of porphyrin core, enhanced resonance interactions between the porphyrin and meso aryl group π systems, shift of electron density from the pyrrolenine nitrogen atoms to the central acid molecules and the configuration interaction between the two excited e_u states. Therefore, the values cannot be used as a criterion for the comparison of the CI for the free base porphyrins and the dications and the f_Soret/f_Q(0,0) ratios should be considered in this comparison. As was mentioned above, H₄T(2-Cl)PP(HCOO)₂ and H₄T(2-Me)PP(HCOO)₂ show much larger f_Soret/f_Q(0,0) ratios than the other dications. Also, the para-substituted meso-tetra(phenyl)porphyrins demonstrate a nearly similar low f_Soret/f_Q(0,0) value. With respect to the remarkably enhanced coplanarity of the meso substituents and the porphyrin mean plane of the dications compared to the free base porphyrins, the difference between the electron donating ability of the meso substituents is expected to be more dominant in the case of the dications. On the other hand, only small changes in the f_Soret/f_Q(0,0) ratio were observed on going from 4-Cl-phenyl to 4-OMe-phenyl substituent. Apparently, in the absence of high dihedral angles between the meso aryl substituents and the porphyrin mean plane which prevents the resonance interaction between the two π systems, the degree of CI seems to be nearly independent from the nature of groups attached to the meso phenyl substituents.

We have previously reported the remarkable influence 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-deficient 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 deficient and electron-rich porphyrins of the series, respectively. H₂T(thien-2-yl)P has a five membered ring at the meso position. The smaller size of the five 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 shifted 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-deficient porphyrin of the series appear at smaller wavelengths 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 shifted 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 shifts upon diprotonation 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 shifted) 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 confirm the positive effect due to the presence of more electron-donating meso substituents on the red shift of the Soret and Q(0,0) band of the corresponding dications to higher wavelengths. In this regard, the largest red shift 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 shift 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 dications of H₂T(2,6-Cl)PP shows the largest blue shift 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).

The emission spectra

The free base porphyrins. The emission spectrum of the free base porphyrins consists of a nearly broad band, Q^*_x(0,0) at ca. 650 and a broader one Q^*_x(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 influenced 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 π 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 for the relative intensity of the Q^*_x(0,0) or Q^*_x(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 influenced 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 (k_r). 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 influencing 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 porphyrins bearing electron-deficient ones. Herein, the smaller intensity 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 Q_x(0,0)/Q_x(1,0) ratio in the absorption spectrum and a Q^*_x(0,0)/Q^*_x(0,1) ratio of greater than 1 in the emission spectrum confirm this displacement.⁴⁹ Also, the substitution of heavy atoms such as chlorine on the meso aryl groups decreases the k_r value, due to the increased non-radiative intersystem crossing one (k_Isc)^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 Q^*_x(0,0) bands of the porphyrins with respect to those between the Q^*_x(0,1) ones. The fluorescence emission of a compound is the result of competition of the radiative and non-radiative transitions between electronic states. However, the efficiency of the radiative and non-radiative transitions depends on the lifetime of S₁ excited states.⁴⁹ Also, the extents of the out-of-plane distortion of porphyrin core of the ground and excited states of porphyrins are different which influences the dihedral angle between the porphyrin mean plane and the meso substituents. This in turn leads to an increase in the aryl-porphyrin π–π resonance interactions.⁴⁹ The stokes shift of porphyrins is defined as the energy gap between the Q^*_x(0,0) and Q_x(0,0) bands which provide information on the excited states; small stokes shifts give evidence of minor differences between the geometry of porphyrin macrocycle in the ground and excited states. On the other hand, large stokes shifts are expected in the case of the reverse mirror image spectra.⁴⁹ The data of the ESI, S4† show stokes shifts in the range of 118 to 262 cm⁻¹ which are in agreement with the reverse mirror image spectra of the porphyrins. Furthermore, the different conformations of the ground and excited states would lead to different dihedral angles between the meso substituents and the porphyrin mean plane and consequently different resonance interactions between the porphyrin and meso aryl group π systems. Therefore, the order of stokes shifts found for the free base porphyrins i.e. H₂T(2-Cl)PP > H₂T(4-OMe)PP > H₂T(4-Me)PP > H₂T(2-Me)PP > H₂TPP > H₂T(4-Cl)PP may be attributed to either the geometrical difference between the porphyrin cores or the varied dihedral angles of the ground and excited states porphyrin moieties. In the absorption spectra, the magnitude of splitting between the Q_x(0,0) and Q_x(1,0) bands may be used to evaluate the energy of the first vibrational state of the S₁ one. It should be noted that in the absorption spectrum of porphyrins, the involvement of vibrational states in the electron transitions is due to the vibrational states of the S₁ excited electronic state (Fig. 5).²⁰ For the free base porphyrins, the splitting of the absorption bands (Δν₁) decreases as H₂T(4-OMe)PP > H₂T(4-Cl)PP ∼ H₂TPP ∼ H₂T(4-Me)PP > H₂T(2-Cl)PP > H₂T(2-Me)PP. However, the differences between the splitting in the absorption bands are small and within 9–28 cm⁻¹ which are much less than those observed in the emission spectra (ESI, S4†). Accordingly, the energy of vibrational states of the free base porphyrin in the first electronic excited states is approximately the same for the used porphyrins, although there are large differences between the steric and electronic properties of the meso substituents of these compounds. The splitting in the emission spectra (Δν₂) follows the following order: H₂T(4-Cl)PP > H₂T(4-Me)PP > H₂T(2-Me)PP > H₂TPP > H₂T(4-OMe)PP > H₂T(2-Cl)PP which is different from that observed in the case of the splitting in the absorption spectra. Herein, a difference of ca. 164 cm⁻¹ is observed between the first and last members of the series which is much higher than that observed in the case of the splitting in the absorption spectra. Accordingly, there are large differences between the energy of the first vibrational states of the free base porphyrins in the S₀ ground state (Fig. 5). Furthermore, the differences between Δν₁ and Δν₂ for the free base porphyrins assigned as Δν₃ are in the range of 247–83 cm⁻¹ and decreases as H₂T(4-OMe)PP > H₂T(2-Cl)PP > H₂TPP > H₂T(2-Me)PP > H₂T(4-Me)PP > H₂T(4-Cl)PP. Thus, H₂T(4-OMe)PP demonstrates the larger difference between the energy of the first vibrational states of the S₀ and S₁ electronic states. The fluorescence quantum yields Φ_f for the free porphyrins are summarized in the ESI, S5.† The Φ_f 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 Φ_f values for a series of porphyrins depends on factors including the electronic 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 interactions between the porphyrin and meso aryl π systems. The latter would increase charge transfer emission and therefore decreases Φ_f. Indeed, fluorescence quantum yield is determined by the relative rate of the radiative and non-radiative transitions from the S₁ state. The presence of heavy atoms at the meso substituents, especially at the ortho position decreases the Φ_f due to the induced increase in intersystem crossing from S₁ to T₁ transition.^37,51 The rate of non-radiative deactivation of the S₁ excited state was shown to correlate with the Hammett constant of the substituent for both electron withdrawing substituents.⁴⁷ However, in the case of electron donating groups such as para-OMe and para-Me a very similar high quantum yield was observed.^46,47 Herein, H₂T(4-OMe)PP with the best electron donating meso substituents among the series shows the highest Φ_f and the porphyrins with heavy chlorine atom on the meso aryl group, H₂T(4-Cl)PP and H₂T(2-Cl)PP have the lowest quantum yields. The latter may be explained by the heavy metal atom effect. It should be noted that the distortion of porphyrin core in the excited state seems to be higher than that in the ground state.⁴⁹ Also, although the distortion of porphyrins with more bulky meso substituents is less than that of those with small meso groups, porphyrins with small meso substituents such as methyl may show high degrees of out-of-plane distortion which in turn would decrease their fluorescence quantum yields.⁴⁹ Accordingly, the anomalous behaviour of porphyrins such as H₂T(4-Me)PP in the series may be due to the increased distortion of H₂T(4-Me)PP relative to H₂TPP.

Fig. 4 Absorption (Q_X region) and emission spectra of all porphyrins in CH₂Cl₂ solutions normalized to each other at their respective Q_X00 maxima.

Fig. 5 The contribution from vibrational states in the absorption and emission bands.

The natural radiative lifetimes (τ₁, τ_2, τ₃) estimated by the formulas of Forster, Strickler and Berg,^52–54 decrease in the order H₂T(4-OMe)PP > H₂T(2-Cl)PP > H₂TPP > H₂T(2-Me)PP > H₂T(4-Cl)PP > H₂T(4-Me)PP (ESI, S5†). It is observed that there is no close correlation between the estimated lifetimes and the fluorescence quantum yields of the porphyrins. It should be noted that the higher fluorescence quantum yields are not necessarily accompanied by longer lifetimes.⁴⁹

The order of k_r for the porphyrins is as follows: H₂T(4-OMe)PP > H₂TPP > H₂T(2-Me)PP > H₂T(4-Me)PP > H₂T(4-Cl)PP > H₂T(2-Cl)PP. Also, the k_nr value decreases as H₂T(4-Me)PP > H₂T(4-Cl)PP > H₂T(2-Me)PP > H₂TPP > H₂T(2-Cl)PP > H₂T(4-OMe)PP. It is noteworthy that rate constants of charge transfer emission and non-radiative relaxations of S₁ state other than the internal conversion and intersystem crossing are not considered in the evaluation of k_r and k_nr constants. Therefore, a high k_r value does not necessitate a low k_nr one.

The porphyrin dications with CF₃COOH. The emission spectra of the porphyrin dications (Table 3, Fig. 6) consist of a broad band at the wavelengths between 652 to 734 nm. The wavelength and intensity of the band depends on the nature of groups substituted at the meso positions. The wavelength of the emission band of the dications decreases in the order H₄T(4-OMe)PP(CF₃COO)₂ > H₄T(4-Me)PP(CF₃COO)₂ > H₄T(4-Cl)PP(CF₃COO)₂ > H₄TPP(CF₃COO)₂ > H₄T(2-Me)PP(CF₃COO)₂ > H₄T(2-Cl)PP(CF₃COO)₂. With the exception of H₄T(4-Cl)PP(CF₃COO)₂, the pattern correlates with the electron donating ability of the meso substituents. The decrease in the dihedral angle between the porphyrin mean plane and the meso aryl substituents upon diprotonation of porphyrins leads to enhanced resonance interactions between these substituents and the porphyrin π systems in the ground state of porphyrin diacids. Therefore, the S₀ state of porphyrin dications with different meso substituents is expected to be of different energy. On the basis of the four orbital model of porphyrins,²⁰ in contrast to the a_2u orbital, the a_1u orbital has no electron density at the meso positions and therefore the main difference between the ground state energy of the porphyrin diacids is probably due to the difference between the π–π interaction between the former and the π system of the meso groups. Accordingly, the energy of S₀ state of the porphyrin dications bearing more electron donating groups should be higher than that of those with less electron donating or bulky substituents at the meso position (Fig. 7). On the other hand, the S₁ excited state of the dications is accompanied with the shift of electron density towards the porphyrin periphery which leads to a decrease in the electron donation from the meso substituents to the a_2u orbital. Accordingly, the effect of meso substituents on the raising of energy of the a_2u orbital is expected to be more pronounced in the S₀ state compared to the S₁ one (Fig. 7). As was seen above, the electron donation to the a_2u orbital through resonance interaction increases the energy level of this orbital. The red shifted emission band in the fluorescence spectra of the dications with electron rich meso substituents relative to those with electron deficient ones may be explained by greater destabilization of the S₀ state of the former compared to the latter. However, although the splitting of emission bands caused by the involvement of vibrational states has not observed in the spectra of the dications, the involvement of vibrational states (Fig. 7) in the observed difference between the energy of the emission bands can not be excluded. The intensity of the emission band of the porphyrin diacids decreases in the order H₄T(4-OMe)PP(CF₃COO)₂ > H₄TPP(CF₃COO)₂ > H₄T(2-Me)PP(CF₃COO)₂ > H₄T(4-Cl)PP(CF₃COO)₂ > H₄T(4-Me)PP(CF₃COO)₂ > H₄T(2-Cl)PP(CF₃COO)₂. Also, the lifetime of the S₁ excited state of the dications decreases as H₄T(2-Cl)PP(CF₃COO)₂ ≫ H₄T(2-Me)PP(CF₃COO)₂ > H₄TPP(CF₃COO)₂ > H₄T(4-OMe)PP(CF₃COO)₂ > H₄T(4-Cl)PP(CF₃COO)₂ > H₄T(4-Me)PP(CF₃COO)₂. It should be noted that there is a large difference between the lifetime of the S₁ state of H₄T(2-Cl)PP(CF₃COO)₂ and that of the other dications. The orders of k_r and k_nr for the diprotonated species are as H₄T(4-OMe)PP(CF₃COO)₂ > H₄TPP(CF₃COO)₂ > H₄T(4-Me)PP(CF₃COO)₂ > H₄T(2-Me)PP(CF₃COO)₂ > H₄T(4-Cl)PP(CF₃COO)₂ ≫ H₄T(2-Cl)PP(CF₃COO)₂ and H₄T(4-Me)PP(CF₃COO)₂ > H₄T(4-Cl)PP(CF₃COO)₂ ≫ H₄TPP(CF₃COO)₂ > H₄T(4-OMe)PP(CF₃COO)₂ > H₄T(2-Me)PP(CF₃COO)₂ > H₄T(2-Cl)PP(CF₃COO)₂, respectively. It is observed that with the exception of H₄T(4-Me)PP(CF₃COO)₂, the intensity of the emission bands correlates well with the k_r values. Also, the k_nr values of H₄T(4-Me)PP(CF₃COO)₂ and H₄T(4-Cl)PP(CF₃COO)₂ are significantly larger than those of the other dications. The splitting of the absorption bands of the porphyrin dications are summarized in Table 3. As may be seen from the ESI, S5† H₄TPP(CF₃COO)₂, H₄T(2-Me)PP(CF₃COO)₂ and H₄T(2-Cl)PP(CF₃COO)₂ show a distinct splitting of the Q(0,0) and Q(1,0) bands which decreases in the order H₄T(2-Cl)PP(CF₃COO)₂ > H₄T(2-Me)PP(CF₃COO)₂ > H₄TPP(CF₃COO)₂. In the case of H₄T(4-OMe)PP(CF₃COO)₂, H₄T(4-Me)PP(CF₃COO)₂ and H₄T(4-Cl)PP(CF₃COO)₂, the Q(1,0) band appears as a shoulder on the corresponding Q(0,0) one. On the other hand, in the emission spectra, generally the two emission bands of the free base porphyrins reduce to a broad single band. In other words, the energy spacing between vibrational levels decreases significantly in the case of the porphyrin dications and the overlap of the bands is observed. However, in the emission spectra of H₄T(2-Me)PP(CF₃COO)₂ and H₄T(2-Cl)PP(CF₃COO)₂ the Q*(0,1) band appears as a shoulder on the Q*(0,0) one. According to ESI, S6† a reverse mirror image relationships is observed between the absorption and emission spectra. Also, there are large Stokes shift between the emission and absorption spectra of the dications (vide infra). The Stokes shift of the dications decreases as H₄T(4-Me)PP(CF₃COO)₂ > H₄T(4-Cl)PP(CF₃COO)₂ > H₄T(4-OMe)PP(CF₃COO)₂ > H₄TPP(CF₃COO)₂ > H₄T(2-Me)PP(CF₃COO)₂ > H₄T(2-Cl)PP(CF₃COO)₂ (Table 3). The observed Stokes shifts are much larger than those of the corresponding free base porphyrins that consist with the observed reverse mirror image relationship between the absorption and emission spectra. As was mentioned above, major differences between the geometry of porphyrin macrocycle in the ground and excited states as well as the coplanarity of the meso aryl substituents are evident by the observation of large Stokes shift.⁴⁹ The fluorescence quantum yields of the dications decrease as follows:

Table 3 The fluorescence spectral data of the porphyrin dications with CF₃COOH in CH₂Cl₂^a

Dications with CF3COOH	H4TPP^2+	H4T(4-OMe)PP^2+	H4T(2-Me)PP^2+	H4T(4-Me)PP^2+	H4T(2-Cl)PP^2+	H4T(4-Cl)PP^2+
a Energy difference between the QX10 and QX00 bands.b Energy differences between the Q^*X00 and Q^*X01 bands.c Based on a reported value of 0.13 for Φf of TPP in CH2Cl2.^38d e f ^52–54g ∑k1 = τ1^−1.h ∑k2 = τ2^−1.i ∑k3 = τ3^−1.j kr1 = Φf × ∑k1.k kr2 = Φf × ∑k2.l kr3 = Φf × ∑k3.m knr1 = ∑k1 − kr1.n knr2 = ∑k2 − kr2.o knr3 = ∑k3 − kr3.^46
Absorption
Q10 (λ/nm)	600	686	583	666	580	656
Q00 (λ/nm)	652		633		632
Splitting (Δν1/cm^−1)^a	1329		1355		1418.6
Emission
Q^*00 (λ/nm)	694	734	660	714	652	700
Δν1/2 (cm^−1)	1316	1285	1195		1322
Q^*01 (λ/nm)			700		694
Splitting (Δν2/cm^−1)^b			866	1153	928	1323
Stokes shift (Δν/cm^−1)	928	953	646	1009	485	958
Φf^c	0.164	0.216	0.155	0.064	0.072	0.067
τ1 (ns)^d	22.68	21.15	31.83	9.23	104.60	13.44
τ2 (ns)^e	25.36	25.32	33.70	11.24	109.97	15.55
τ3 (ns)^f	27.40	26.00	37.80	11.80	123.30	16.67
∑k1 (×10^−6 s^−1)^g	44	47	31	108	10	74
∑k2 (×10^−6 s^−1)^h	39	40	30	89	9	64
∑k3 (×10^−6 s^−1)^i	37	39	26	85	8	60
Kr1 (×10^−6 s^−1)^j	7.2	10.2	4.9	6.9	0.7	5.0
Kr2 (×10^−6 s^−1)^k	6.5	8.5	4.6	5.7	0.7	4.3
Kr3 (×10^−6 s^−1)^l	6.0	8.3	4.1	5.4	0.6	4.0
Knr1 (×10^−6 s^−1)^m	36.9	37.1	26.5	101.4	8.9	69.4
Knr2 (×10^−6 s^−1)^n	33.0	31.0	25.1	83.3	8.4	60.0
Knr3 (×10^−6 s^−1)^o	30.5	30.2	22.4	79.3	7.5	56.0

---

Fig. 6 The emission spectra of the porphyrin dications with CF₃COOH in CH₂Cl₂.

Fig. 7 Schematic presentation of the effect of π–π interactions on the energy level of S₀ and S₁ of the free base porphyrins.

H₄T(4-OMe)PP(CF₃COO)₂ > H₄TPP(CF₃COO)₂ > H₄T(2-Me)PP(CF₃COO)₂ > H₄T(2-Cl)PP(CF₃COO)₂ > H₄T(4-Cl)PP(CF₃COO)₂ ≥ H₄T(4-Me)PP(CF₃COO)₂.

The pattern correlates approximately with the electron donor ability of the meso substituents. In the case of ortho or para chlorinated porphyrin dications, the heavy atom effect is involved in the observed order of Φ_f.

The porphyrin dications with HCOOH. With the exception of H₄T(2-Cl)PP(HCOO)₂, the emission spectra of the dications of porphyrins with HCOOH consist of a broad band at 656 to 748 nm (ESI, S6†). The comparison of emission and absorption spectra of H₄T(2-Cl)PP(HCOO)₂ shows a reverse mirror image relationship, although the Q*(0,1) band of H₄T(2-Cl)PP(HCOO)₂ appears as a shoulder at the Q*(0,0) band (ESI, S8†). The position of the fluorescence emission bands of the dications with HCOOH shows a red shift relative to the corresponding band of the dication with CF₃COOH. However, as was observed in the case of the dications with CF₃COOH, the wavelength of the band decreases in the order of H₄T(4-OMe)PP(HCOO)₂ > H₄T(4-Me)PP(HCOO)₂ > H₄T(4-Cl)PP(HCOO)₂ > H₄TPP(HCOO)₂ > H₄T(2-Me)PP(H₃COO)₂ > H₄T(2-Cl)PP(HCOO)₂ (Table 4). The lower acid strength of HCOOH compared to that of CF₃COOH leads to smaller shift of electron density from the porphyrin core to the acid molecules. This effect on the other hand causes decreased coplanarity of the meso aryl substituents in the case of the dications with HCOOH and consequently less out-of-plane deformation of the porphyrin core. In other words, the dihedral angle between the porphyrin mean plane and the aryl groups is probably different in the case of the dications with HCOOH and CF₃COOH. The red shifted emission bands of the dications with the former compared to that of the latter seems to be due to different distortions of the porphyrin cores and the dihedral angles. The anion of trifluoroacetic acid is a weaker hydrogen bond acceptor than that of HCOOH. Indeed, the introduction of electron-withdrawing atoms at the carboxylate anion leads to a decrease in the hydrogen bond acceptor ability of the anion.⁵⁵ Furthermore, according to the literature,¹⁰ the necessity to optimize the hydrogen bond interactions between the counter anion and the porphyrin diprotonated species is among the factors determining the actual degree of saddling of porphyrin core. In this regard, the porphyrin dication with Br⁻ as the counter anion has a more distorted core with respect to the corresponding dication with F⁻. According to the above discussions, for a given porphyrin, the dication with CF₃COOH may be considered as a more distorted macrocycle. The intensity of the emission band of the diacids with HCOOH decreases in the order H₄T(2-Me)PP(HCOO)₂ > H₄T(4-OMe)PP(HCOO)₂ > H₄TPP(HCOO)₂ > H₄T(4-Cl)PP(HCOO)₂ > H₄T(2-Cl)PP(HCOO)₂ > H₄T(4-Me)PP(HCOO)₂ that is different from that observed in the case of the corresponding dications with CF₃COOH. In comparison with the porphyrin dications with CF₃COOH, the intensity of the emission bands of the dications with HCOOH is usually greater than that of those with CF₃COOH. However, the lifetime of the S₁ excited state of the dications decreases as H₄T(2-Cl)PP(HCOO)₂ ≫ H₄T(2-Me)PP(HCOO)₂ > H₄TPP(HCOO)₂ > H₄T(4-OMe)PP(HCOO)₂ > H₄T(4-Cl)PP(HCOO)₂ > H₄T(4-Me)PP(HCOO)₂ which is the same as that observed in the 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 k_r and k_nr for the diprotonated 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 > H₄TPP(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 shift 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 shift of dications was found to decrease in the order H₄T(4-Me)PP(HCOO)₂ > H₄T(4-OMe)PP(HCOO)₂ > H₄T(4-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 dications with CF₃COOH, the magnitudes of stokes shifts are much larger than those of the corresponding free base porphyrins. This finding 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 shifts 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 shifts 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 shift of H₄T(4-Cl)PP(HCOO)₂ and H₄T(4-Cl)PP(CF₃COO)₂, a difference of 50 cm⁻¹ was observed between the stokes shifts of H₄T(2-Me)PP(HCOO)₂ and H₄T(2-Me)PP(CF₃COO)₂.

Table 4 The fluorescence spectral data of the porphyrin dications with HCOOH in CH₂Cl₂^a

Dications with HCOOH	H4TPP^2+	H4T(4-OMe)PP^2+	H4T(2-Me)PP^2+	H4T(4-Me)PP^2+	H4T(2-Cl)PP^2+	H4T(4-Cl)PP^2+
a See the footnotes of Table 3.
Absorption
Q10 (λ/nm)	604	695	582	672	580	662
Q00 (λ/nm)	657		635		631
Splitting (Δν1/cm^−1)	1336		1434		1393
Emission
Q^*00 (λ/nm)	700	748	660	724	656	706
Δν1/2 (cm^−1)	1176				1446
Q^*01 (λ/nm)					698
Splitting (Δν2/cm^−1)		1308	1550	1128	917	1179
Stokes shift (Δν/cm^−1)	935	1019	596	1069	604	941
Φf	0.202	0.216	0.283	0.062	0.076	0.075
τ1 (ns)	22.50	21.13	35.42	9.23	111.25	11.30
τ2 (ns)	25.32	26.00	37.04	11.24	115.16	13.43
τ3 (ns)	27.42	26.35	41.40	11.80	131.58	14.20
∑k1 (×10^−6 s^−1)	44	47	28	108	9	88
∑k2 (×10^−6 s^−1)	39	38	27	89	9	74
∑k3 (×10^−6 s^−1)	36	38	24	85	8	70
Kr1 (×10^−6 s^−1)	9.0	10.2	8.0	6.7	0.7	6.6
Kr2 (×10^−6 s^−1)	8.0	8.3	7.6	5.5	0.7	5.6
Kr3 (×10^−6 s^−1)	7.4	8.2	6.8	5.3	0.6	5.2
Knr1 (×10^−6 s^−1)	35.4	37.1	20.2	101.6	8.3	81.9
Knr2 (×10^−6 s^−1)	31.5	30.2	19.4	83.5	8.0	68.9
Knr3 (×10^−6 s^−1)	29.1	29.7	17.3	79.7	7.0	65.1

---

The order of Φ_f 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)PP(HCOO)₂. Except for the position of H₄T(2-Me)PP(HCOO)₂ in the series, the order of Φ_f 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 Φ_f values of H₄TPP(HCOO)₂ and H₄T(2-Me)PP(HCOO)₂ are remarkably larger than those of the corresponding dications with CF₃COOH. In spite of the greater Φ_f value of the dications with HCOOH, there is no large difference between the Φ_f values of the two series of the dications in the case of the other porphyrins.

The orders of k_r and k_nr for the dications with HCOOH 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 > H₄TPP(HCOO)₂ > H₄T(2-Me)PP(HCOO)₂ > H₄T(2-Cl)PP(HCOO)₂, respectively. The patterns for the k_r and k_nr values are very similar to those observed for the dications with CF₃COOH.

Conclusion

A series of para- or ortho-substituted meso-tetra(phenyl)porphyrins and their dications with weak and strong carboxylic acids were investigated by fluorescence and absorption spectroscopy and found that: (i) the degree of configuration interaction (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 f_Soret/f_Q(0,0) values were much less than those observed for the free base porphyrins and larger differences were found between the f_Soret/f_Q(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 influenced 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 shifts 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 fluorescence quantum yields of the dications were much larger than those of the free base porphyrins. The natural radiative lifetimes (τ₁, τ_2, τ₃) of the dications were generally much shorter than those of the free base porphyrins. Furthermore, significantly 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.

Acknowledgements

Financial support of this work by the Institute for Advanced Studies in Basic Sciences (IASBS) is acknowledged.

References

1. S. Aronoff, J. Phys. Chem., 1958, 62, 428.
2. B. Cheng, O. Q. Munro, H. M. Marques and W. R. Scheidt, J. Am. Chem. Soc., 1997, 119, 10732.
3. M. Y. Choi, J. A. Pollard, M. A. Webb and J. L. McHale, J. Am. Chem. Soc., 2003, 125, 810.
4. E. B. Fleischer, Acc. Chem. Res., 1970, 3, 105.
5. E. B. Fleischer and A. L. Stone, Chem. Commun., 1967, 332.
6. M. Meot-Ner and A. D. Adler, J. Am. Chem. Soc., 1975, 97, 5107.
7. E. C. Ojadi, H. Linschitz, M. Gouterman, R. I. Walter, J. S. Lindsey, R. W. Wagner, P. Droupadi and W. Wang, J. Phys. Chem., 1993, 97, 13192.
8. S. Rayati, S. Zakavi, A. Ghaemi and P. J. Carroll, Tetrahedron Lett., 2008, 49, 664.
9. A. Rosa, G. Ricciardi and E. J. Baerends, J. Phys. Chem. A, 2006, 110, 5180.
10. A. Rosa, G. Ricciardi, E. J. Baerends, A. Romeo and L. Monsù Scolaro, J. Phys. Chem. A, 2003, 107, 11468.
11. M. O. Senge, Z. Naturforsch., B: J. Chem. Sci., 2000, 55, 336.
12. A. Stone and E. B. Fleischer, J. Am. Chem. Soc., 1968, 90, 2735.
13. R. I. Walter, E. C. Ojadi and H. Linschitz, J. Phys. Chem., 1993, 97, 13308.
14. S. Zakavi and N. G. Gharab, Polyhedron, 2007, 26, 2425.
15. S. Zakavi, R. Omidyan, L. Ebrahimi and F. Heidarizadi, Inorg. Chem. Commun., 2011, 14, 1827.
16. S. Zakavi, M. N. Ragheb and M. Rafiee, Inorg. Chem. Commun., 2012, 22, 48.
17. S. Zakavi, H. Rahiminezhad and R. Alizadeh, Spectrochim. Acta, Part A, 2010, 77, 994.
18. K. H. Buchel, J. Falbe, H. Hagemann, M. Hanack, D. Klamann, R. Kreher, H. Kropf, M. Regitz and E. Schaumann, Huban-Wely Method of Organic Chemistry, Thieme, New York, 1997, vol. E 9d, p. 577.
19. C. Medforth, C. Muzzi, K. Shea, K. Smith, R. Abraham and J. Shelnutt, J. Chem. Soc., Perkin Trans. 2, 1997, 839.
20. M. Gouterman, J. Mol. Spectrosc., 1961, 6, 138.
21. M. Gouterman, J. Chem. Phys., 1959, 30, 1139.
22. C. Weiss Jr, H. Kobayashi and M. Gouterman, J. Mol. Spectrosc., 1965, 16, 415.
23. H. Longuet-Higgins, C. Rector and J. Platt, J. Chem. Phys., 1950, 18, 1174.
24. J. B. Kim, J. J. Leonard and F. R. Longo, J. Am. Chem. Soc., 1972, 94, 3986.
25. N. Z. Mamardashvili and O. A. Golubchikov, Russ. Chem. Rev., 2001, 70, 577.
26. C. J. Medforth, M. O. Senge, K. M. Smith, L. D. Sparks and J. A. Shelnutt, J. Am. Chem. Soc., 1992, 114, 9859.
27. R. E. Haddad, S. Gazeau, J. Pécaut, J.-C. Marchon, C. J. Medforth and J. A. Shelnutt, J. Am. Chem. Soc., 2003, 125, 1253.
28. J. Hoard, Science, 1971, 174, 1295.
29. P. W. Codding and A. Tulinsky, J. Am. Chem. Soc., 1972, 94, 4151.
30. E. Austin and M. Gouterman, Bioinorg. Chem., 1978, 9, 281.
31. V. Kuz'mitskii and K. Solov'ev, J. Appl. Spectrosc., 1999, 66, 627.
32. M. M. Kruk, A. S. Starukhin and W. Maes, Macroheterocycles, 2011, 4, 69.
33. J. R. Weinkauf, S. W. Cooper, A. Schweiger and C. C. Wamser, J. Phys. Chem. A, 2003, 107, 3486.
34. A. D. Adler, F. R. Longo, J. D. Finarelli, J. Goldmacher, J. Assour and L. Korsakoff, J. Org. Chem., 1967, 32, 476.
35. S. Zakavi and M. N. Ragheb, Inorg. Chem. Commun., 2013, 36, 113.
36. S. Gentemann, C. J. Medforth, T. P. Forsyth, D. J. Nurco, K. M. Smith, J. Fajer and D. Holten, J. Am. Chem. Soc., 1994, 116, 7363.
37. D. J. Quimby and F. R. Longo, J. Am. Chem. Soc., 1975, 97, 5111.
38. M. Bergkamp, J. Dalton and T. Netzel, J. Am. Chem. Soc., 1982, 104, 253.
39. C. A. Parker and W. Rees, Analyst, 1960, 85, 587.
40. A. Wolberg, J. Mol. Struct., 1974, 21, 61.
41. B. N. Figgis and M. A. Hitchman, Ligand Field Theory and its Applications, Wiley-VCH, 2000, p. 180.
42. M. H. Perrin, M. Gouterman and C. L. Perrin, J. Chem. Phys., 1969, 50, 4137.
43. M. Gouterman, G. H. Wagnière and L. C. Snyder, J. Mol. Spectrosc., 1963, 11, 108.
44. L. Pauling, Selected Scientific Papers, Volume 2 (World Scientific Series in 20^th centry Chemistry), ed. Barclay Kamp, World Scientific, New Jersey, 2001, ch. 14, vol. 10, p. 1100.
45. S. Zakavi, R. Omidyan and S. Talebzadeh, Polyhedron, 2013, 49, 36.
46. H. N. Fonda, J. V. Gilbert, R. A. Cormier, J. R. Sprague, K. Kamioka and J. S. Connolly, J. Phys. Chem., 1993, 97, 7024.
47. P. G. Seybold and M. Gouterman, J. Mol. Spectrosc., 1969, 31, 1.
48. A. Harriman and R. J. Hosie, J. Chem. Soc., Faraday Trans. 2, 1981, 77, 1695.
49. G.-Z. Wu, W.-X. Gan and H.-K. Leung, J. Chem. Soc., Faraday Trans., 1991, 87, 2933.
50. S. Gentemann, C. J. Medforth, T. Ema, N. Y. Nelson, K. M. Smith, J. Fajer and D. Holten, Chem. Phys. Lett., 1995, 245, 441.
51. A. C. Serra, M. Pineiro, A. M. d'A. Rocha Gonsalves, M. Abrantes, M. Laranjo, A. C. Santos and M. F. Botelho, J. Photochem. Photobiol., B, 2008, 92, 61.
52. S. Strickler and R. A. Berg, J. Chem. Phys., 1962, 37, 814.
53. G. N. Lewis and M. Kasha, J. Am. Chem. Soc., 1945, 67, 994.
54. P. G. Seybold, M. Gouterman and J. Callis, Photochem. Photobiol., 1969, 9, 229.
55. K. F. Purcell and J. C. Kotz, Inorganic Chemistry, Saunders, Philadelphia, 1977, p. 190.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra20445c

---