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Статья Paper
Transformations in the Fluorescence Line Narrowing Spectra of Porphine upon the Formation of Diprotonated Species
Alexander S. Starukhin,a@ Mikalai M. Kruk,a Ol'ga L. Gladkova,b and Wouter Maescd
aB.I. Stepanov Institute of Physics of National Academy of Sciences, 220072 Мinsk, Republic of Belarus
bBelarusian State University of Informatics and Radioelectronics, 220013 Minsk, Republic of Belarus
cInstitute for Materials Research (IMO), Research Group Organic and (Bio)Polymer Chemistry, Hasselt University, B-3590
Diepenbeek, Belgium
Molecular Design and Synthesis, Department of Chemistry, Katholieke Universiteit Leuven, B-3001 Leuven, Belgium Corresponding author E-mail:astar@imaph.bas-net.by
Fluorescence line narrowing spectra of the diprotonated forms of the porphine have been obtained in solid acid solutions at liquid helium temperature. Quantum-chemical calculations offrequencies and forms of in-plane and out-of-plane vibrations for the diprotonatedforms were also performed. The manifestation of intense vibronic transitions in the fine-line fluorescence spectra of diprotonated forms of H2P with participation of out-of-plane modes have been estimated on the basis of mutual analysis of the experimental data and the results of quantum chemical calculations. The reason for the spectral activation of out-of-plane modes consists in saddle distortion of the porphine molecule upon formation of the diprotonated form.
Keywords: Porphine, fluorescence line narrowing spectra, diprotonated form, saddle conformation.
Introduction
The high importance of tetrapyrrolic compounds for current science and technology has led to intensive studies of these compounds by different spectral methods. It is well known that vibronic spectra of porphyrins in solid solutions have been unresolved even at liquid helium temperature. Upon narrow band laser excitation in the range of the S0^S1 absorption spectrum at liquid helium temperatures, the inhomogeneous broadening in fluorescence spectra is removed. This method was named the fluorescence line narrowing (FLN) procedure[1] and was used successfully for the study of tetrapyrrolic compounds in the different kinds of solid amorphous glasses. It is of particular interest to apply FLN method for the detection of fine-line fluorescence spectra for the protonated forms of porphyrins in acid solid solutions.
Free base porphyrins when reacted with acids are transformed. The spectral data for the diprotonated forms of octaethylporphyrin were detected in unresolved fluorescence spectra at 77 K.[2] Later, the occurrence of the monodeprotonated form of octaethylporphyrin has been detected in fluorescence and absorption spectra,1[3] also. The structure of the diprotonated forms of porphyrins has been studied on the basis of the X-ray data,[4,5] where saddle type of distortion of these forms was estimated.
At low temperatures only the diprotonated forms of tetrapyrrolic compounds are realized. First, the FLN spectra of free base porphine (H2P) in solid solution of trifluoroacetic acid matrix at 4.2 K were detected,[6] but the data have not interpreted. Several years ago the fine-line spectrum of diprotonated form of H2P in inorganic tetraethoxysilane matrix at 4.2 K was detected,[7] but these results have
essential differences if compared with the previous data[6] and the interpretation of the spectral data is also absent. Later single site fluorescence and absorption spectra of free base octaethylporphyrin, octaethylchlorin and their respective diacids in «-octane were reported also at 298 K and 7 K.[8]
In this work, we have measured high-resolution FLN spectra of the diprotonated forms of H2P. We have performed quantum-chemical calculations of the frequencies and forms of in-plane and out-of-plane vibrations of these compounds. Based on the experimental data and calculation results the manifestation of out-of-plane vibrations in the vibronic fluorescence spectra of diprotonated forms of H2P have been analyzed and the relationship between the activity of out-of-plane modes and their forms, was estimated.
Experimental
The H2P was synthesized according to the well known method.[9] For purification and identification of the structure standard procedures have been used.
Diprotonated species were prepared under dissolving of the H2P in mixture of acetonitrile/10"3 M HClO4 (H4P2+) and in mixture of2 acetonitrile/10"3 M D2SO4 (D4P2+). The D4P2+ is chemically similar to H4P2+ form but the central hydrogen atoms were exchanged on the deuterium atoms. The structural formulae of the H2P, H4P2+ and the D4P2+ are shown in Figure 1.
Dilute solutions of the porphyrins in acetonitrile-acids mixtures (Fluka, spectroscopic grade) were cooled in liquid helium (4.2 K). To minimize the concentration and aggregation effects the samples of the diprotonated forms of porphine with concentrations ~ 10"6 M have been used.
Upon applying the FLN procedure the inhomogeneous spectral broadening is removed and the fluorescence spectra are transformed from broad bands in a set of narrow zero-phonon lines.[1] The difference in frequencies between lines of fluorescence and the
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a b c
Figure 1. The structure of the ^P (a), H4P2+ (b) and D4P2+ (c).
excitation laser line is equal to the frequency of the corresponding vibration in the ground state [1]. For molecules with relatively high symmetry the same set of vibrations will be manifested in FLN spectra as well as in the resonance Raman spectra (RRS).
The luminescence spectra of the compounds of interest have been recorded using a highly sensitive home-build spectrometer based on double monochromator of DFS-24 spectrometer. The samples were excited by radiation from a pulsed dye laser (wavelength range is 550 -700 nm, line width less than 0,1 nm). It was pumped by the second harmonic of Nd-YAG laser (wavelength is 532 nm, pulse width ~ 12 ns, repetition rate 20 Hz). The luminescent signals were detected with a photomultiplier tube (Hamamatsu, type R-928) and a two-channel boxcar integrator RIS-2002. More detailed the experimental setup was described elsewhere.[9]
Quantum-Chemical Calculations of Vibrational States
Quantum-chemical calculations of the frequencies and symmetry of the normal modes as well as the maximum amplitude change for natural coordinates have been done by a DFT method using the exchange-correlative functional BPE and software package "Priroda".[10] The quantum-chemical calculations for the equilibrium configuration of isolated molecules of the H4P2+ and D4P2+ were carried out. The calculated frequencies were not scaled.
Figure 2. Optimized structure of H4P2+. 86
The optimization of H4P2+ geometry for shows that this compound has an out-of-plane structure (D2d symmetry group) in contrary to the H2P molecule with a planar structure. In the optimized structure of H4P2+ (see Figure 2), the two of opposite protons turn relatively to the CC-bonds of the methine bridges so that its nitrogen atoms are spaced above the plane of methine bridges, and the two remaining protons are under the plane. It is a typical saddle form of a porphyrin macrocycle. This structure of H4P2+ corresponds very well to the X-ray data for diprotonated forms of porphyrins.[4,5] The tilting dihedral CbCaCmCa and CaCmCaN angles have a value of 9°. Under these conditions the corresponding NH band has a tilt above and under the plane of the pyrrolic rings (dihedral CbCaNH angle is
equal to 32°). b a
Results and Discussion
Highly resolved FLN spectra of H2P, the H4P2+ and D4P2+ have been detected in solid solution. Figure 3 shows the fine-structure fluorescence spectra of aforementioned compounds upon selective laser excitation at 4.2 K. The data on the frequencies of the vibronic lines in the FLN spectra of H4P2+ and D4P2+ are summarized in Table 1. The frequencies and symmetry of the normal modes have been established on the basis of quantum-chemical calculations for H4P2+ and D4P2+ and are presented in the Table 1, also.
The molecule of H4P2+ belongs to the D2d symmetry group and H2P - to the D2h symmetry group. The symmetrical metalloporphine with position of the metal atom in the plane of macrocycle belongs to the D4h symmetry group. As a result of the existing axes of fourth order for the D2d and D4h there is a possibility to compare vibrational types of symmetry for these groups. In total, the symmetric modes of A1 (D2d) correspond to the in-plane A vibrations and out-of-plane B2u modes; Bx modes (DJ - B2g §and Alu (DJ; B2 (DJ - Blg and
A2u (D4h); A (D2d) - A2g and Blu (D4h); E modes - wkh E and Eg (D4h) types of symmetry.
Combined discussion of the experimental data and the results of quantum-chemical calculations of the normal modes for our compounds starts from the comparison of the data for H2P and H4P2+. The frequencies of many modes in the FLN spectrum of H4P2+, in particular, in the range up to 1000 cm-1, have values similar to that in the spectrum of H2P. In the cases where one line may be compared with several calculated modes we keep the assignments for the corresponding interpretation of H2P.
The lines about 1600 cm-1 in the FLN spectra of H4P2+ belong to the modes with participation of the CaCm bonds. In contrast to the situation in the spectrum of H2P these vibrations are sensitive to deuterated atoms at central positions of H4P2+ (see Table 1). The changing value of the line with frequency 1618 cm-1 upon deuteration arises from the motion of nitrogen atoms, because the deformation shift of CaCm atoms for B2 mode is forbidden in symmetry. As a result the nitrogen atoms will shift hydrogen atoms along axes passing through the nitrogen atoms of opposite pyrrole rings. Calculated modes are not shifted upon deuteration and it means that the contribution of nitrogen atom will be more than the theoretical calculation predicts.
Similarly, the lines with frequencies 310, 723, 953, 1182, 1318 and 1388 cm-1 in the spectrum of H2P do not change their values upon deuteration of the center of H2P (D2P). At the same time upon deuteration the corresponding lines of H. P2+ are shifted on several cm-1.
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S. Starukhin et al.
Figure 3. FLN spectra of H4P2+ in the mixture of acetonitryle/10-3M HClO4 (a) and D4P2+ in the mixture of acetonitryle/10-3M D2SO4 (b) upon selective excitation (Xexc=585 nm) at 4.2 K.
Deuteration of the center of H4P2+ leads to the following transformations in the spectra: the line with frequency 1386 cm-1 changes in form of vibration and the corresponding line with frequency 1374 cm-1 has no change of CaN bond and participation of deformation 5(NH) bond vanishes. The line at 1318 cm-1 is transformed into the line with frequency 1302 cm-1 in the spectrum of D4P2+. The shift is due to the changing form of the mode with frequency 1302 cm-1: the length of CaCb bonds are decreased and the shift of the imines hydrogens is absent.
Based on the results on the geometry optimization of the molecule, the length of the CaCb bonds for H4P2+ is smaller
Table 1. Vibrational assignments of the FLN spectra of H2P, D2P, H4P2+
than for H2P. As a result, the line in the spectrum of H2P with frequency 953 cm-1 (with participation of CaCb bonds) will be increased to 994 cm-1 in the spectrum of H4P2+.
Double degenerated deformation skeletal mode with frequencies about 720 cm-1 in the FLN spectrum of H2P are splitted into two modes in the FLN spectrum of H4P2+ which decrease the frequencies on 12 and 28 cm-1.[12]
The length of the bonds Ca'Cb' of the pyrrolic rings of H2P are more short than for the pyrroleine rings (about 0.02 A), and as a result, the frequency 953 cm-1 (A ) belongs to modes with participation of Ca"Cb" bonds, but a line with frequency 988 cm-1 (A1g) corresponds to Ca'Cb'. For the diprotonated forms all rings have the same pyrrolic ring type and frequencies are totally symmetric and have the same value of 994 cm-1 (986 and 972 cm-1 in the calculation results). The same value of 994 cm-1 has a mode of B1 symmetry type which is sensitive to deuteration (981 cm-1 in the spectrum of D4P2+).
In comparison with the spectrum of H2P, the values of the experimental and calculated frequencies for deformation modes 5(CbH) and 5(CmH) are increased in the spectrum of H4P2+ (see Table 1). It seems, that above mentioned effect depends on increasing force constants for angles CCH, because the lengths of CbCb bonds taking part in the forming of this mode are elevated and force constants are decreased.
The formation of the diprotonated form with four hydrogen atoms in the center of the porphyrin macrocycle leads to the stronger changes in vibrational frequencies. In the spectral range below 900 cm1 in the FLN spectra of H4P2+ (see Figure 3), a set of new lines with frequencies 337, 479, 675, 882 cm-1 is revealed. The corresponding lines are absent in the FLN spectrum and in the theoretical calculations for the in-plane modes of H2P. In the FLN spectrum of H2P the lines with frequencies in the range of430-720 cm1 are absent, but at the same time, one of the most intensive lines (479 and 675 cm1) appears in the FLN spectra of H4P2+ and D4P2+.
D4P2+ and theoretical data for H4P2+ and D4P2+.
H2P D2P H4P2+ D P2+ 4 Maximum amplitude change for
v exp v exp v exp v theor v exp Vtheor natural coordinates for H. P2+ 4
310 310 307 A1 298 304 295 CaCmCa
337 B2 315 334 314 p(Cm), p(N)
479 A2 462 475 463 P(CbX P(Cm)
675 B2 685 672 684 Fs, p(N)
723 723 695 A1 703 690 705 CCC, C CbN a m a a b
723 723 713 B2 705 711 702 CNCa,
790 781 800 B1 794 787 770 CaCbCb? CbCN
882 B2 867 900 858 p(CmH), p(CbH)
994 A1 974 952 940 CaN CbCb CaNC,
976 868 994 B1 981 876 871 C N, C CbCb, C C N a ' a b b' m a
994 B2 986 975 967 CaCb CaNCa
1066 1066 1072 A1 1073 1072 1073 8(CbH)CbCb>
1182 1182 1201 B2 1191 1192 1182 8(CmH), CaCb, CaNCa,
1318 1318 1318 A2 1346 1302 1335 CaN, CaCb,mS(CbH), S(CH)a S(NH)
1343 B1 1356 1349 CaN, CaCb, 8(CbH), 5(NH),
1388 1388 1385 A2 1363 1374 1356 S(qH), 8(CmH), S(NH), CaN
1385 A1 1370 1360 1357 CaN, CaCb a ' a b
1498 1498 1483 B2 1466 1483 1466 CbC^ 8(CbH)
1536 A1 1518 1535 1518 CaCm, CbCb, CaNCa
1602 1594 1597 A2 1572 1584 1564 CaCm,mS(CH), 8 (Nil)
1616 1615 1618 B2 1590 1602 1590 C C , 8(C H), C NC _a_m_1_m__a_a_
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In according to the results presented in Table 1 the line at 479 cm1 belongs to the practically pure out-of-plane displacement of the p-carbon atoms of the pyrrolic rings and the carbon atoms of the methine bridges. Intensity of this line is very high, what is due to maximum out-of-plane displacement of the p-carbon atoms of the pyrrolic rings (quantum-chemical calculation). The line at 675 cm1 corresponds to the mode with out-of-plane displacements of the nitrogen atoms of the pyrrolic rings. The line with frequency 337 cm-1 corresponds to the out-of-plane displacements of the meso-carbon atoms and the nitrogen atoms of pyrrolic rings. Out-of-plane vibration with frequency 882 cm1 is due to out-of-plane motion of the hydrogen atoms at meso-positions and the hydrogen atoms of the pyrrolic rings together and the corresponding line is absent in the FLN spectrum of H2P with planar structure.
Based on the presented experimental data and the results of quantum-chemical calculations the main reason of activation of the out-of-plane modes in the fluorescence spectra of diprotonated forms is the distortion of the porphyrin macrocycle (saddle type of distortion). This kind of distortion should lead to realizing of out-of-plane oscillator with 3D character. In this case orientation of all electronic transition moments for H4P2+ would have three components (X, Y and Z) in contrary to the situation with in-plane X and Y components only of the electronic transition moments in the absorption and fluorescence spectra for the planar structure of H2P. The 3D oscillator of the electronic (vibronic) transitions may be presented as the superposition of the plane oscillator (X and Y) and out-of-plane Z linear oscillator with the orientation of transition moments being orthogonal to the plane (and X and Y) of the molecule. A similar 3D character would have all electronic transitions in absorption spectra of the diprotonated saddle distorted forms of H2P.
It is known that the vibrational structure of the luminescence spectra ofporphyrins is formed by the HerzbergTeller mechanism, i.e. by the mechanism by which vibronic transitions borrow the intensity from intense electronic transitions. Vibronic transitions with the participation of inplane vibrations are oriented in the plane of the porphyrin macrocycle and borrow the intensity from transitions in the absorption spectrum with the same orientation of transition moments. Correspondingly, out-of-plane vibrations can borrow intensity from transitions for which the orientation of the transition moments is orthogonal to the plane of the molecule (Z-components of electronic transitions).
Earlier it was discussed the possibility of the activation of out-of-plane Z component of the electronic transition for porphyrins.[13] This conclusion was done on the basis of the strong effects of the conformations in the porphyrin macrocycle at interaction with solvents and the influence on the photophysical properties of distorted porphyrins. However, unambiguous evidences of the spectral manifestation of the Z component have not been presented. In our opinion evidence of manifestation effect of the Z-component for the saddle distorted molecules possibly is in highly-resolved fluorescence spectra only in the frequency range up to 1000 cm1.
Recently essential out-of-plane Eg vibrations have been recorded in the fine-structure phosphorescence spectra of metallocomplexes of porphine with Pd11 and Pt11 ions,[14]
but the activity of these modes has another physical nature and increases with enhancing spin-orbital coupling upon changing to heavier chelated metal ions.
Conclusions
The combined theoretical and experimental investigation of the diprotonated forms of porphine allowed us to reveal a high activity of the vibronic transitions with the participation of out-of-plane vibrations in fluorescence spectra of saddle distorted above mentioned molecules. The absence of vibronic lines in the spectrum of H2P in 400-700 cm-1 range permits us to elucidate the manifestation of the out-of-plane modes in the FLN spectra of H4P2+ and D4P2+, because interference between in- and out-of-plane modes does not take place. High activity of the out-of-plane modes in the FLN spectra of the diprotonated forms was interpreted as a result of vibronic borrowing with participation of Z-component for 3D character electronic transitions for the distorted structure of the diprotonated forms.
Acknowledgements. This work was supported by the Foundation for Fundamental Research of Republic of Belarus (project Ch10R-001) and a FP-7 grant from the EC for Research, Technological Development and Demonstration Activities, "Dendrimers for Photonic Devices" IRSES-PE0PLE-2009-247260-DphotoD, under the "International Research Staff Exchange Scheme". W. Maes also thanks FWO (Fund for Scientific Research - Flanders), KU Leuven, Hasselt University and the Ministerie voor Wetenschapsbeleid for continuing financial support.
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Received 06.05.2011 Accepted 30.05.2011
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