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Статья Paper
Excitation Energy Deactivation in Monodeprotonated Porphyrin
Mikalai M. Kruk@ and Aleksander S. Starukhin
B.I. Stepanov Institute of Physics of National Academy of Sciences, Minsk, 220072, Belarus Corresponding author E-mail: kruk@imaph.bas-net.by
Spectroscopic properties of monodeprotonated form of cationic 5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin (H2TMPyP) in solution are studied. Experimental results are interpreted according to Gouterman four-orbital model. Monodeprotonated form HTMPyP- can be assigned to the D4h symmetry point group, and has spectral and photophysical characteristics similar to that of the fluorescent metal complexes of H2TMPyP with the relatively low electronegativity of chelated metal ion. The intersystem crossing with the quantum yield @ISC=0.57 is the main pathway for the excitation energy deactivation of monodeprotonated form HTMPyP-.
Keywords: Porphyrin, acid-base equilibria, configuration interaction, symmetry, excited states, fluorescence.
Introduction
Porphyrin molecules are amphoteric compounds having properties of both bases and acids.[1] Basic properties are exhibited in protonation of the nitrogen atoms of the pyrrolenine rings when reacting with acids, and formation of monoprotonated and diprotonated forms. Acidic properties result in dissociation of the pyrrole protons during formation of metal complexes. The complete scheme for possible acid-base equilibria of porphyrin macrocycle is shown in Figure 1. While the monoprotonated and especially the doubly protonated forms of tetrapyrrolic compounds have been rather intensively studied, the monodeprotonated and doubly deprotonated molecules were out of focus of researchers. It was assumed that these forms are extremely unstable, and it is difficult to detect them experimentally.[1,2] At the moment when we have started these studies, it was known only two experimental papers devoted to the monodeprotonated forms of two compounds: porphine (H2P) and 5,10,15,20-tetrakis-(4-N-methylpyridyl)porphyrin (H2TMPyP),[3,4] and one theoretical paper where a quantum chemical study was carried out for the monodeprotonated form of porphine (HP). [5]
Fluorescence spectrum has been measured for the monodeprotonated form of porphine HP,[3] but the problem of excitation energy deactivation in such proton deficient porphyrin systems has not been studied. In our recent
publication we have reported on the absorption and fluorescence spectra and have discussed molecular symmetry of monodeprotonated form H2TMPyP (HTMPyP-) in the solution at room temperatures.[6] Here we report on the intersystem crossing quantum yield @1SC and discuss the excitation energy deactivation pathways in the monodeprotonated form HTMPyP-.
Experimental
Materials and Methods
5,10,15,20-Tetrakis(4-N-methylpyridyl)porphyrin tetratosy-late salt (Aldrich) was used without additional purification.
Twice-distilled water or 0.2 M solution of NaOH (p#=13.5) were used to prepare the solutions. The concentration of porphyrin in solutions was determined spectrophotometrically with known extinction coefficients [7] (s = 226000 M"1-cm"1 at 422 nm) and was ~2.410-6 M. The measurements were carried out in the standard quartz rectangular cells (1x1 cm, Hellma) in the air equilibrated solutions at 288 ± 2 K.
Deoxygenated solutions were used for the measurements of the fluorescence quantum yield &fl. Deoxygenation of the solutions was performed with argon bubbling during 20 min just before measurements. The fluorescence spectra and fluorescence excitation spectra were measured with the use of spectrofluoro-meter SFL-1211 (Solar, Belarus). The fluorescence decay kinetics
Ps- HP' H3P H3P+ HP*
Figure 1. Acid-base equilibria of porphyrin macrocycle; P2 is doubly deprotonated porphyrin, HP is monodeprotonated porphyrin, H2P is free base porphyrin, H3P+ is monoprotonated porphyrin and H.P2+ is doubly protonated porphyrin.
equimolar concentrations are shown in Figure 3. In going from the free base to the monodeprotonated form a long-wavelength shift of the Soret band from 422 nm to 453 nm is observed (the shoulder on the short-wavelength side of the Soret band is due to trace absorption of no more than 5% of the free base species). The transformation of the shape of the spectrum in the visible region was found too: instead of the four-band spectrum of the phyllo-type with maxima at 638 nm, 584 nm, 554 nm, and 518 nm typical to the free base, we observed a two-band spectrum with maxima at 627 nm and 581 nm, the shape of which is similar to the absorption spectra of the metal complexes.
We have recently shown[6] that the shape of the absorption spectrum for the monodeprotonated form (HTMPyP) is due to the high symmetry of the molecule: it should be assigned to the D4h symmetry point group rather than to C2v, which could be in the case of fixed proton. We have hypothesized that such a situation was realized due to the highly effective tautomerism in the monodeprotonated porphyrin macrocycle. The proton delocalization over the porphyrin core leads to the formation of absorption spectrum like in the case of metalloporphyrins. The maximum of 0(0,0) band undergoes a monotonic blue shift: 627 > 602 > 587 > 566 > 545 nm in the series HTMPyP- > ZnTMPyP > CuTMPyP > PdTMPyP > PtTMPyP. Indeed, the monopro-tonated HTMPyP- species are in line with metal complexes for both 0(0,0) band maximum as well as for the absor-bance intensity ratio AQ(0,0)/AQ(1,0), indicating that in the case of the monodeprotonated form HTMPyP- 1E(alu, eg) < Electronic absorption spectra of the free base 1E(alueg).[6] Thus, the monodeprotonated HTMPyP- species H2TMPyP and the monodeprotonated form HTMPyP- in can be considered as a quasimetallocomplex of H2TMPyP,
Figure 2. Structure of 5,10,15,20-tetrakis(4-jV-methylpyridyl) porphyrin.
were measured with the use of photon counting system FLA-900 (Edinburgh Instruments, UK) which allowed to measure the decay kinetics with lifetimes down to 0.1 ns. The fluorescence quantum yield 0fl was determined using standard sample method.[8] The free base 5,10,15,20-tetrakis(4-iV-methylpyridyl)porphyrin was used as a standard sample (£>°fl = 0.044 [79]).
Results and Discussion
Electronic Absorption Spectra and Symmetry of Monodeprotonated Species
I I I I I I I I I J I I I I I I I I I J I I I I I I I I I g I I I I I I I I I J I I I I I I I I I
600 650 700 750 800
Wavelength, nm
Wavelength, nm
Figure 3. Absorption spectra for the monodeprotonated form Figure 4. Fluorescence spectra for the monodeprotonated form
HTMPyP- (solid line) and the free base H2TMPyP (dotted line). HTMPyP- (solid line) and the free base H2TMPyP (dotted line);
Porphyrin concentration of each form is 2.4T0"6 M. \xc = 580 nm. Porphyrin concentration of each form is 2.4T0"6 M.
M.M. Kruk and A.S. Starukhin
Table 1. Photophysical properties of studied compounds (the methods of the deactivation rates, kfl, knr, kISC and kSS determination were described earlier[10]).
Compound max Afl : nm % 0 fl kfP k , nr' 0 ISC kISC,
ß(0,0) ß(0,1) ns 107 s-1 108 s-1 108 s-1 108 s-1
H2TMPyP[7A13] 657 706 5.16 0.044 0.85 1.85 0.80 1.65 0.20
HTMPyP- 655 701 1.05 0.038 3.62 9.16 0.57 5.43 3.73
ZnTMPyP[1U2] 626 666 1.30 0.025 1.92 7.50 0.90 6.92 0.58
in which the proton delocalized over the porphyrin cavity plays the role of chelated metal ion.
Photophysical Properties of Monodeprotonated Species
Monodeprotonated form HTMPyP- fluoresces. The fluorescence spectrum consists of large band with maximum centered at 701 nm (Figure 4). Analysis of the fluorescence band shape shows that it is satisfactorily fitted by two Gaus-sians with maxima at 655 and 701 nm, corresponding to pure electronic 0(0,0) and vibronic 0(0,1) bands. The excitation spectrum for this luminescence, recorded in the region of Soret band, matches the absorption spectrum of HTMPyP-, which confirms our assignment of the luminescence to fluorescence of the monodeprotonated form. The fluorescence quantum yield of HTMPyP- is @fl = 0.038, and is of the same order of magnitude as for the free base and the Zn complex (Table 1). The fluorescence decay kinetics (Xexc = 460 nm, Xobs = 710 nm) are monoexponential with decay time Tfl = L05 ± 0.05 ns.
It is known that in going from the free base porphyrins to metallocomplexes, the fluorescence quantum yield @fl decreases by a factor of 2-5, and the probability of fluorescence kfl increases by a factor of about 2.[10] Fluorescence quantum yield of the free base H2TMPyP is initially low (0fl = 0.044[9]); the formation of Zn complex leads to almost two fold decrease in the @fl value (<Pfl = 0.025 [1112]), while the probability of fluorescence increases. Considering the monodeprotonated form HTMPyP- as quasimetallocomplex, we should stress that the probability of HTMPyP- fluorescence also follows the indicated pattern for the metallocomplexes (see Table 1). The magnitude of @fl decreases to 0.038, while kfl value increases up to 3.62-107 s-1. Obviously, the decrease in the fluorescence quantum yield @fl for such a substantial increase in kfl should be accompanied by a significant increase in the probabilities of nonradiative deactiva-tion channels (kISC for S1^T1 intersystem crossing and kSS for S1 ^S0 internal conversion). In fact, the overall probability of these processes knr = kST + kSS increases by a factor of five compared with that of the free base, and reaches 9.16 -108 s-1. For ZnTMPyP molecule, the knr value also increases up to 7.50-108 s-1. Thus, deactivation of the lower excited S1 state of HTMPyP- follows the patterns for deactivation for the Zn complex of H2TMPyP, i.e. the monodeprotonated form HTMPyP- also behaves itself like the metallocomplex.
The lowest triplet T1 state lifetime of the monodeprotonated form HTMPyP- was found to be sufficiently long to provide the efficient quenching by molecular oxygen. The triplet state lifetime in the air-equilibrated solution tt was found to be 2.8 |is, which was close to that of 3.5 |is for
ZnTMPyP.[14] Thus, we were able to use the singlet molecular oxygen photosensitization to determine the value of the intersystem crossing quantum yield 0ISC.[1516] The ^ISC value was found to be as high as 0.57 ± 0.05. This figure is somewhat lower that those for the free base and Zn complex and results from the competition between two nonradiative deactivation channels. It must be pointed out that fraction of the S1^T1 intersystem crossing in nonradiative deactivation of S1 state was found to be about 90% for both the free base and for its Zn complex,[7911] indicating the dominating role of the intersystem crossing rate kISC (Table 1). In the case of monodeprotonated form (HTMPyP) nonradiative deactivation of the lowest excited S1 state via the S1 ^S0 internal conversion (kSS = 3.73-108 s-1) seems to be successfully competing with the intersystem S1^T1 crossing (kISC = 5.43-108 s-1). As a result, the quantum yield of the internal conversion 0SS = 1 - 0fl - 0ISC is as high as ~0.4. Nevertheless, the intersystem crossing leading to population of the lowest triplet T1 state remains to be the main route of the excitation energy deactivation of monodeprotonated form HTMPyP- like it takes place in other metallocomplexes of H2TMPyP porphyrin.
Conclusions
The presented results show that monodeprotonated form HTMPyP- of cationic 5,10,15,20-tetrakis(4-N-methyl-pyridyl)porphyrin can be assigned to the D4h symmetry point group. The electronic absorption spectrum of the monodeprotonated form can be interpreted in the framework of Gouterman's four-orbital model. It was established that monodeprotonated form HTMPyP- has photophysical characteristics close to those known for the fluorescent metal complexes of H2TMPyP with relatively low electronegativity of the chelated metal ion. The triplet T1 state population is the main pathway for the excitation energy deactivation of the monodeprotonated form HTMPyP-.
Acknowledgments. This work was supported in part by the Foundation for Fundamental Research of the Republic of Belarus (project Ch08R-033).
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Received 19.05.2009 Accepted 11.08.2009
