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Aggregation and photophysical properties of water-soluble sapphyrins
Chemical Physics Letters 395 (2004) 82–86 www.elsevier.com/locate/cplett
Aggregation and photophysical properties of water-soluble sapphyrins P. Kuba´t a
a,*
, K. Lang b, Z. Zelinger a, V. Kra´l
c
J. Heyrovsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejsˇkova 3, 18223 Praha 8, Czech Republic, b Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, 25068 Rˇezˇ, Czech Republic c Institute of Chemical Technology, Technicka´ 5, 16628 Praha 6, Czech Republic Received 23 June 2004; in final form 15 July 2004 Available online 5 August 2004
Abstract Aggregation and photophysical properties of three sapphyrins were studied by UV/Vis, emission, resonance light scattering and laser kinetic spectroscopies. The relative abundance of various types of aggregates depends mainly on the structure of sapphyrin, solvent and temperature. The formation of H-dimers is related to negative entropy and enthalpy changes and it is controlled mainly by electrostatic interactions between the positively charged sapphyrin unit and negative substituents. Excitation of the H-dimers leads to the formation of the triplet states of the sapphyrin monomers. The quantum yields of the singlet oxygen in methanol vary between 0.30–0.33 and are independent of substitution. 2004 Elsevier B.V. All rights reserved.
1. Introduction Expanded porphyrins having more extensive p-electron conjugation pathways than porphyrins, are attractive as potential PDT agents because they combine many of the advantageous features of porphyrins (chemical stability, preferential localisation in tumours, etc.) with a shift of the absorption bands to longer wavelengths that allow deeper penetration through tissues [1,2]. In addition, some sapphyrins, the first expanded porphyrins reported in the literature [3], produce singlet oxygen O2(1Dg), which is the main cytotoxic species in PDT, in high quantum yields [4]. The behaviour of sapphyrins is complicated by aggregation in the polar solvent [5,6]. As with porphyrins [7], the tendency to form various types of aggregates should be influenced by peripheral substitution. So far three type of species have been identified spectroscopically: *
Corresponding author. Fax: +420 286 591 766. E-mail address: [email protected] (P. Kubat).
0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.07.040
(i) the sapphyrin monomer, characterised by a strong, sharp Soret-like absorbance around 450 nm; (ii) the dimer, which has a relatively broad absorbance around 420 nm with a lower extinction coefficient; and (iii) higher-order aggregates, which display a collective absorbance at ca. 410 nm [5]. In neutral aqueous media sapphyrins behave differently than porphyrins because sapphyrins are considerably more basic. While porphyrins become generally protonated at pH < 5, the pKas values of water-soluble sapphyrin monomers were estimated to be 4.8 and 8.8 [8]. Thus, monoprotonated sapphyrins and non-protonated porphyrin molecules are the dominant species at neutral pH. Increasing ionic strength causes aggregation of water-soluble porphyrins as a result of the compensation of the porphyrin charge. In contrast, protonated sapphyrin core binds some anions with binding constants up to 104 M1 for phosphate and 102 M1 for chloride [9]. In this Letter, we present the aggregation and photophysical behaviour of three sapphyrins S1, S2 and S3
P. Kuba´t et al. / Chemical Physics Letters 395 (2004) 82–86
83
R3 R2 R1
R1 N H
S1: R1 = R2 = H, R3 = CH2-COOH O
N
N
S2: R1 = R2 = H, R3 = CH2 C NH
SO3H
COOH NH
HN
O S3: R1 = C NH
, R2 = CH3, R3 = H
Fig. 1. Structure of studied sapphyrins S1, S2 and S3.
with a different peripheral substitution (Fig. 1) in aqueous and methanol solutions. The determination of the position and shape of the Soret bands in UV/Vis spectra allowed us to distinguish between monomeric sapphyrins and several types of aggregates. The extended aggregation of sapphyrins was evaluated by resonance light scattering experiments and photophysical properties by laser kinetic spectroscopy.
2. Experimental Sapphyrin derivatives were synthesised and characterised using procedures described in [10–12]. The stock solutions of S1–S3 (ca. 200 lM) were prepared in methanol (Lachema, Czech Republic, spectroscopic grade) and diluted with water, 10 mM PIPES buffer (pH 6.8), or methanol prior to use. UV/Vis absorption spectra were measured on a Perkin–Elmer Lambda 19 spectrophotometer. The concentrations of the sapphyrin monomer and dimer for calculation of the dimerisation constant were obtained from absorbances at selected wavelengths (420 and 443 nm) and corresponding absorption molar coefficients. Steady-state fluorescence emission spectra were recorded on a Perkin–Elmer LS 50B luminescence spectrophotometer. Resonance light-scattering (RLS) experiments were performed using simultaneous scans of the excitation and emission monochromators ranging from 300 to 600 nm. A Lambda Physik FL 3002 dye laser (kexc = 414440 nm, pulse length 28 ns, output 0.1–5 mJ/pulse) was used for production of the triplet states. The triplet–triplet spectra were measured using a laser kinetic spectrometer (Applied Photophysics, UK) equipped with a 250 W Xe lamp, pulse unit and R928 photomultiplier (Hamamatsu). Where appropriate, oxygen was removed from the solution by means of helium purging. Time-resolved near-infrared emission at 1270 nm of O2(1Dg) was monitored with a Ge diode (Judson J16-
8SP, USA) in conjunction with an interference filter. The quantum yields of singlet oxygen formation UD were estimated by a comparative method using 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) as a standard (UD = 0.69 in methanol) [13]. All measurements were performed using optically matched methanolic solutions (Aexc = 0.214 ± 0.002) at the excitation wavelength 414 nm for TPPS and 440 nm for S1–S3. Used excitation energy of 290 lJ is in the energy region where the intensity of a luminescence signal is proportional to incident energy.
3. Results and discussion Sapphyrins S1 (Fig. 2a) and S2 in a methanolic solution have the Soret band at 443 nm with the molar absorption coefficients 3.4 · 105 and 3.0 · 105 l mol1 cm1, respectively. At low concentrations the bands obey the Lambert–Beer law as investigated up to 2 · 105 M and the fluorescence excitation spectra recorded throughout the emission bands (670–820 nm) copy the absorption spectra. These facts indicate that both S1 and S2 at micromolar concentrations are monomeric. As reported earlier the pyrrole rings of monomeric sapphyrins are protonated in polar solvents such as methanol to form the protonated species, which is prone to dimerise [4,14]. The broadened and unsymmetrical Soret band of S3 with maximum at 449 nm (bandwidth of 18 vs. 12 nm of S1 and S2) (Fig. 2d), and a discrepancy between the Soret band and fluorescence excitation spectra indicate the presence of different protonated species. The addition of PIPES to S1 in methanol causes substantial changes in absorption and fluorescence spectra. A new absorption peak at 418 nm with e418 = 2.4 · 105 l mol1 cm1 (Fig. 2b), which appears in a methanol– PIPES mixture (2:1) has been attributed to sapphyrin dimers [9]. The dimer is non-emissive since any specific component in fluorescence emission spectra does not appear and fluorescence excitation spectra show only the
P. Kuba´t et al. / Chemical Physics Letters 395 (2004) 82–86
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(a) (d)
(b)
(c)
Fig. 2. Absorption spectra of 4.2 lM S1 in methanol (a), methanol:PIPES = 2:1 (b), and in PIPES buffer (c) and 4.0 lM S3 in methanol (d, dotted line).
presence of residual monomer species. Similar behaviour is observed for S2 and S3. The formation of dimers of S1 in a methanol–PIPES mixture (2:1) is characterised by the dimerisation constant KD = (4.1 ± 0.9) · 105 M1 at 25 C. The thermodynamic functions of the dimerisation process were determined using the VanÕt Hoff equation. The calculated enthalpy of the dimer formation (DH = 52.3 ± 0.7 kJ mol1) and the corresponding entropy change (DS = 71.1 ± 2.9 J mol1 K1) are negative and comparable to those typical for the dimerisation of porphyrins [15,16]. Large and negative DH suggests that the dimerisation is driven by the enthalpy term (high exothermicity). Thus, a simple consideration predicts enthalpy-driven ion-pairing on the basis of electrostatic interactions between the positively charged sapphyrin core and negative peripheral substituent(s). For comparison, the dimerisation of similar macrocycles–porphyrins, with highly polarisable and non-protonated ring in neutral aqueous solutions, is controlled mainly by (induced) dipole–(induced) dipole interactions and coulombic repulsion of the substituents [7,15]. The close proximity of two sapphyrin molecules in the dimer leads to the overlapping of their hydration shells resulting in the release of water molecules from the interacting surfaces to the bulk solution, which is expected to result in a positive entropy contribution that is, however, not
observed in our experiment. The negative entropy suggests a high ordering effect that favours a well-structured dimer. S2 is less stable at increased temperatures and S3 exhibits more complex spectral features including the formation of higher aggregates. The spectral features of the sapphyrin dimers or higher aggregates in general are readily explained on the bases of the point-dipole model proposed by Kasha and co-workers [17]. The transition dipole moments can adopt a range of orientations relative to the molecular axis of the dimer. The behaviour of the visible band of sapphyrins upon the addition of a buffer is typical of the formation of stacked dimers and the resulting exciton states. The simplest case is the parallel, so-called H-type dimer, in which the excited state levels of the monomer split in two upon dimerisation leading to a blue-shifted absorption peak. According to the simple model, the interaction energy DE between the neighbouring transition dipoles depends on the value of the transition dipole moment of the monomer M, the intermolecular distance R, and the mutual geometries in the following way: 1 1 DE ¼ hc kD kM ¼
hM 2 i ðcos h 3 cos /1 cos /2 Þ; 4pe0 R3
ð1Þ
where h is the Planck constant, c is the speed of light in vacuum, kD and kM are the band maxima of dimer and monomer, e0 is the permitivity of vacuum, ÆM2æ is the mean square of M, R is the centre-to-centre distance between neighbouring moieties, h is the angle between the transition dipoles of neighbour units and /1, /2 are the angles between the transition dipoles and the line connecting the chromophore centres. The transition dipole moment M can be obtained by integration of the corresponding absorption band expressed in wavenumbers [18,19] Z eð~mÞ hM 2 i ¼ 9:184 1039 d~m: ð2Þ ~m The interaction energy (DE = 1350 cm1) was obtained as the difference between the 0–0 band energies of S1 in the monomer state (443 nm) and in the dimerised form (418 nm). As an approximation, we can assume that the dimer preserves the local symmetry of the monomer and that the sapphyrin units are stacked vertically like plates (/1 = /2 = 90) (Fig. 3a). Then we ˚ between neighattain an approximate distance of 7 A bouring molecules in the dimer. This is considerably larger than the typical van der Waals separation of about ˚ observed in the p stacking of neutral aromatic 3.5 A compounds. In addition, sapphyrin carboxylate forms a dimer in the solid state as documented by single-crystal X-ray structure [20]. Here, a pair of sapphyrin units
P. Kuba´t et al. / Chemical Physics Letters 395 (2004) 82–86
α < 54.7 o
(a)
(b)
(c)
Fig. 3. Schematic arrangement of the sapphyrin units with positively charged pyrrole nitrogen atoms and a negatively charged substituent in H-dimer (a), X-ray single-crystal structure [17] (b) and in J-aggregate (c) (h = 0, /1 = a, /2 = a).
is partially overlapped with one carboxylate oxygen from each sapphyrin within the hydrogen-bonding distance of the pyrrol NHs of the other sapphyrin and it ˚ (Fig. 3b). The foris separated by approximately 3.4 A mation of H-dimers should be accompanied by quenching of fluorescence emission because the optically allowed component reached by excitation of the dimer is located at a higher energy level and relaxes rapidly to the lower exciton component, for which the deactivation primarily occurs via intersystem crossing to generate the triplet states. As shown above, considerable fluorescence quenching was observed. The simple dimer model qualitatively explains the spectral features, however, this should be considered as tentative. Increasing the amount of buffer in a methanol–PIPES mixture increases the extent of aggregation as indicated by the appearance of additional broad peaks. In pure PIPES sapphyrins, S1–S3 form aggregates characterised with the blue-shifted Soret band centred at about 400 nm and with the red-shifted band located at 460–470 nm (Figs. 2c and 4a) [6]. The former band belongs to H-aggregates indicating that the dimers are converted to larger assemblies in the absence of methanol and the red-shifted absorption bands signify that the assembly consists of head-to-tail aggregates (J-aggregates). The contribution of respective aggregates is affected by sapphyrin functionalisation, temperature and ionic strength of a solution. Whereas S1 forms predominantly H-aggregates (Fig. 2c), both S2 and S3 yield a mixture of H- and J-aggregates (Fig. 4a).
(a)
(b)
Fig. 4. Absorption spectrum of S2 in PIPES buffer (left axis, (a)) is compared with corresponding resonance light scattering features (right axis, (b)).
85
The formation of J-aggregates is apparently driven by the stabilisation interaction between easily ionisable sulfonato or carboxy groups and the positively charged sapphyrin unit of the adjacent molecule (Fig. 3c). This binding motif has been previously reported for diprotonated 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin ðH2þ 4 TPPSÞ at high ionic strength where the J-aggregation is related to the appearance of a red-shifted and narrow absorption band [21,22]. The coupling of the tightly packed molecular transition dipoles in pure J-aggregates leads to the delocalisation of the exciton state through the coherent excitation of a number of monomers, which significantly contributes to the shift of the Soret band. In this case the width of the absorption band of the pure J-aggregate is narrower than that of the monomer by a factor of Neff1/2, where Neff is the average number of porphyrin units in direct communication [23]. In contradiction to H2þ 4 TPPS, the Soret bands of the J-aggregates of S1–S3 do not show a narrowing (Figs. 2c and 4a), which indicates a high disorder of the unit arrangement excluding delocalisation of the exciton states along the aggregate. The presence of extended electronically coupled aggregates can be detected using RLS experiments. This technique allows identification of extended aggregated species even at low concentrations since the amount of scattered light is directly proportional to the volume of particles and monomeric molecules and small oligomers show no enhanced scattering [24]. The appearance of the RLS features in Fig. 4b clearly shows that the size of aggregates is large enough to scatter light and that aggregates consist of multiple sapphyrin molecules. Two maxima in the light-scattering signal are associated with at least two different multiple arrangements, which is consistent with the presence of H- and J-aggregates. It should be pointed out that decreased scattering around 455 nm is not due to self-absorption of incident light because it corresponds to the absorption minimum between the absorption bands. The RLS profiles presented in Fig. 4 are similar for all studied sapphyrins. We also characterised the formation of the triplet states of S1–S3 and their quenching by oxygen leading to the formation of singlet oxygen 1O2. In oxygen-free methanol the decay kinetics of the triplet states is strictly monoexponetial and independent of the functionalisation giving the lifetimes of 118 ± 20 ls. The transient absorption spectrum recorded at short times after excitation shows a broad bleaching in the 400–475 nm region and increased absorption in the 330–390 nm and 475–520 nm (Fig. 5a). The triplet states of S1–S3 are effectively quenched by dissolved oxygen characterised by the bimolecular rate constant of oxygen quenching (3.3 ± 0.7) · 109 l mol1 cm1. The transient spectra in methanol–PIPES buffer (2:1) show bleaching of both peaks at 443 and 418 nm, respectively, suggesting that simultaneous excitation of
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Acknowledgement (c) (a) (d)
This research was supported by the Grant Agency of the Czech Republic (Grant Nos. 203/04/0426 and 203/ 02/0420).
(b)
References
Fig. 5. Difference absorption spectra of 4.2 lM S1 recorded 42 ns after excitation by a 420 nm laser pulse in air-saturated methanol (monomeric S1) (a) and in methanol–PIPES buffer = 2:1 (mixture of the H-dimer and monomer) (b). Inset: Transient traces recorded at 390 nm in air-saturated methanol (c) and in methanol–PIPES = 2:1 (d).
the monomer and H-dimer (Fig. 5b). While in methanol the recovery curves and the transient absorption decay curves are monoexponetial (Fig. 5c) indicating decay of the produced triplet states (oxygen-free solution) or their quenching by oxygen, the kinetics in the methanol–PIPES mixture have a fast component with an estimated lifetime of less than 5 ns, which is evident at shorter wavelengths 340–390 nm (Fig. 5d). This fast kinetics is missing at longer wavelengths (470–550 nm). A full recovery is achieved with the lifetime of 130 ± 25 ls in an oxygen-free solution, which is close to the corresponding lifetime measured in pure methanol. In the presence of oxygen the slow recovery kinetics is accelerated because the triplet states are efficiently quenched by oxygen while the contribution of the fast kinetics is not influenced. These results indicate possible formation of the triplet states of the sapphyrin monomer by exciting the dimers, which is further corroborated by the fact that the rate of recovery of dimer bleaching is equivalent to triplet decay both in the presence and absence of dissolved oxygen. The presence of two different triplet states as recently found to be a consequence of aggregation [25] is less probable. Sapphyrins S1, S2 and S3 in methanol produce 1O2 with the quantum yields UD of 0.32 ± 0.06, 0.33 ± 0.07, 0.30 ± 0.06, respectively. These values are slightly greater than those of another sapphyrin derivatives in ethanol (UD = 0.160.18) [26] deuterated methanol, acetonitrile and dichloromethane (UD = 0.130.28) [4]. The measured lifetime of 1O2 is of sD = 8.6 ± 0.8 ls, the value that is in accordance with the behaviour of 1O2 in this solvent [27]. The production of 1O2 strongly decreases after the addition of the aqueous PIPES buffer due to the increasing contribution of dimers and larger aggregates. In general, with the increasing contribution of aggregates the value of UD dramatically decreases, e.g. UD 6 0.05, in a 1:1 methanol/PIPES mixture.
[1] J.L. Sessler, S.J. Weghorn, Expanded, Contracted and Isomeric Porphyrins, Elsevier, Oxford, 1997. [2] V. Kra´l, J. Davis, A. Andrievsky, J. Kra´lova´, A. Synytsya, P. Pouckova´, J.L. Sessler, J. Med. Chem. 45 (2002) 1073. [3] V.J. Bauer, D.L.J. Clive, D. Dolphin, J.B. Paine III, F.L. Harris, M.M. King, J. Loder, S.W.C. Wang, R.B. Woodward, J. Am. Chem. Soc. 105 (1983) 6429. [4] B.G. Maiya, M. Cyr, A. Harriman, J.L. Sessler, J. Phys. Chem. 94 (1990) 3597. [5] B.L. Iverson, K. Shreder, V. Kral, P. Sansom, V. Lynch, J.L. Sessler, J. Am. Chem. Soc. 118 (1996) 1608. [6] J.L. Sessler, J.M. Davis, V. Kra´l, T. Kimbrough, V. Lynch, Org. Biomol. Chem. 1 (2003) 4113. [7] P. Kuba´t, K. Lang, K. Procha´zkova´, P. Anzenbacher Jr., Langmuir 19 (2003) 422. [8] V. Kra´l, H. Furuta, K. Shreder, V. Lynch, J.L. Sessler, J. Am. Chem. Soc. 118 (1996) 1595. [9] J.L. Sessler, J.M. Davis, Acc. Chem. Res. 34 (2001) 989. [10] V. Kra´l, J.L. Sessler, Tetrahedron 51 (1995) 539. [11] J.L. Sessler, V. Kra´l, Preparation of water-soluble sapphyrins for photodynamic therapy, US, 1996, 21 pp Cont.-in-part of US 5,457,195. [12] J.L. Sessler, B.L. Iverson, V. Kra´l, K. Shreder and H. Furuta, Sapphyrin derivatives and conjugates, US, 1995, 52 pp Cont.-inpart of US 5,159,065. [13] C. Tanielian, C. Wolff, J. Phys. Chem. 99 (1995) 9825. [14] A. Regev, S. Michaeli, H. Levanon, M. Cyr, J.L. Sessler, J. Phys. Chem. 95 (1991) 9121. [15] K. Kano, K. Fukuda, H. Wakami, R. Nishiyabu, R.F. Pasternack, J. Am. Chem. Soc. 122 (2000) 7494. [16] D.J. Satterlee, J.A. Shelnutt, J. Phys. Chem. 88 (1984) 5487. [17] M. Kasha, H.R. Rawls, M. Ashraf El-Bayoumi, Pure. Appl. Chem. 11 (1965) 371. [18] F. Zsila, Z. Bikadi, Z. Keresztes, J. Deli, M. Simonyi, J. Phys. Chem. B 105 (2001) 9413. [19] N. Harada, Y. Takuma, H. Uda, J. Am. Chem. Soc. 100 (1978) 4029. [20] J.L. Sessler, A. Andrievsky, P.A. Gale, V. Lynch, Angew. Chem., Int. Ed. Engl. 35 (1996) 2782. [21] N. Maiti, S. Mazumdar, N. Periasamy, J. Phys. Chem. B 102 (1998) 1528. [22] D.L. Akins, H. Zhu, Ch. Guo, J. Phys. Chem. 100 (1996) 5420. [23] E.W. Knapp, Chem. Phys. 85 (1984) 73. [24] P.J. Collings, E.J. Gibbs, T.E. Starr, O. Vafek, C. Yee, L.A. Pomerance, R.F. Pasternack, J. Phys. Chem. B 103 (1999) 8474. [25] H. Garcia-Ortega, J.L. Bourdelande, J. Crusats, Z. El-Hachemi, J.M. Ribo, J. Phys. Chem. B 108 (2004) 4631. [26] L. Roithman, B. Ehrenberg, Y. Nitzan, V. Kra´l, J.L. Sessler, Photochem. Photobiol. 60 (1994) 421. [27] F. Wilkinson, P. Helman, A.B. Ross, J. Phys. Chem. Ref. Data 24 (1995) 663.
Aggregation and photophysical properties of water-soluble sapphyrins P. Kuba´t a
a,*
, K. Lang b, Z. Zelinger a, V. Kra´l
c
J. Heyrovsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejsˇkova 3, 18223 Praha 8, Czech Republic, b Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, 25068 Rˇezˇ, Czech Republic c Institute of Chemical Technology, Technicka´ 5, 16628 Praha 6, Czech Republic Received 23 June 2004; in final form 15 July 2004 Available online 5 August 2004
Abstract Aggregation and photophysical properties of three sapphyrins were studied by UV/Vis, emission, resonance light scattering and laser kinetic spectroscopies. The relative abundance of various types of aggregates depends mainly on the structure of sapphyrin, solvent and temperature. The formation of H-dimers is related to negative entropy and enthalpy changes and it is controlled mainly by electrostatic interactions between the positively charged sapphyrin unit and negative substituents. Excitation of the H-dimers leads to the formation of the triplet states of the sapphyrin monomers. The quantum yields of the singlet oxygen in methanol vary between 0.30–0.33 and are independent of substitution. 2004 Elsevier B.V. All rights reserved.
1. Introduction Expanded porphyrins having more extensive p-electron conjugation pathways than porphyrins, are attractive as potential PDT agents because they combine many of the advantageous features of porphyrins (chemical stability, preferential localisation in tumours, etc.) with a shift of the absorption bands to longer wavelengths that allow deeper penetration through tissues [1,2]. In addition, some sapphyrins, the first expanded porphyrins reported in the literature [3], produce singlet oxygen O2(1Dg), which is the main cytotoxic species in PDT, in high quantum yields [4]. The behaviour of sapphyrins is complicated by aggregation in the polar solvent [5,6]. As with porphyrins [7], the tendency to form various types of aggregates should be influenced by peripheral substitution. So far three type of species have been identified spectroscopically: *
Corresponding author. Fax: +420 286 591 766. E-mail address: [email protected] (P. Kubat).
0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.07.040
(i) the sapphyrin monomer, characterised by a strong, sharp Soret-like absorbance around 450 nm; (ii) the dimer, which has a relatively broad absorbance around 420 nm with a lower extinction coefficient; and (iii) higher-order aggregates, which display a collective absorbance at ca. 410 nm [5]. In neutral aqueous media sapphyrins behave differently than porphyrins because sapphyrins are considerably more basic. While porphyrins become generally protonated at pH < 5, the pKas values of water-soluble sapphyrin monomers were estimated to be 4.8 and 8.8 [8]. Thus, monoprotonated sapphyrins and non-protonated porphyrin molecules are the dominant species at neutral pH. Increasing ionic strength causes aggregation of water-soluble porphyrins as a result of the compensation of the porphyrin charge. In contrast, protonated sapphyrin core binds some anions with binding constants up to 104 M1 for phosphate and 102 M1 for chloride [9]. In this Letter, we present the aggregation and photophysical behaviour of three sapphyrins S1, S2 and S3
P. Kuba´t et al. / Chemical Physics Letters 395 (2004) 82–86
83
R3 R2 R1
R1 N H
S1: R1 = R2 = H, R3 = CH2-COOH O
N
N
S2: R1 = R2 = H, R3 = CH2 C NH
SO3H
COOH NH
HN
O S3: R1 = C NH
, R2 = CH3, R3 = H
Fig. 1. Structure of studied sapphyrins S1, S2 and S3.
with a different peripheral substitution (Fig. 1) in aqueous and methanol solutions. The determination of the position and shape of the Soret bands in UV/Vis spectra allowed us to distinguish between monomeric sapphyrins and several types of aggregates. The extended aggregation of sapphyrins was evaluated by resonance light scattering experiments and photophysical properties by laser kinetic spectroscopy.
2. Experimental Sapphyrin derivatives were synthesised and characterised using procedures described in [10–12]. The stock solutions of S1–S3 (ca. 200 lM) were prepared in methanol (Lachema, Czech Republic, spectroscopic grade) and diluted with water, 10 mM PIPES buffer (pH 6.8), or methanol prior to use. UV/Vis absorption spectra were measured on a Perkin–Elmer Lambda 19 spectrophotometer. The concentrations of the sapphyrin monomer and dimer for calculation of the dimerisation constant were obtained from absorbances at selected wavelengths (420 and 443 nm) and corresponding absorption molar coefficients. Steady-state fluorescence emission spectra were recorded on a Perkin–Elmer LS 50B luminescence spectrophotometer. Resonance light-scattering (RLS) experiments were performed using simultaneous scans of the excitation and emission monochromators ranging from 300 to 600 nm. A Lambda Physik FL 3002 dye laser (kexc = 414440 nm, pulse length 28 ns, output 0.1–5 mJ/pulse) was used for production of the triplet states. The triplet–triplet spectra were measured using a laser kinetic spectrometer (Applied Photophysics, UK) equipped with a 250 W Xe lamp, pulse unit and R928 photomultiplier (Hamamatsu). Where appropriate, oxygen was removed from the solution by means of helium purging. Time-resolved near-infrared emission at 1270 nm of O2(1Dg) was monitored with a Ge diode (Judson J16-
8SP, USA) in conjunction with an interference filter. The quantum yields of singlet oxygen formation UD were estimated by a comparative method using 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) as a standard (UD = 0.69 in methanol) [13]. All measurements were performed using optically matched methanolic solutions (Aexc = 0.214 ± 0.002) at the excitation wavelength 414 nm for TPPS and 440 nm for S1–S3. Used excitation energy of 290 lJ is in the energy region where the intensity of a luminescence signal is proportional to incident energy.
3. Results and discussion Sapphyrins S1 (Fig. 2a) and S2 in a methanolic solution have the Soret band at 443 nm with the molar absorption coefficients 3.4 · 105 and 3.0 · 105 l mol1 cm1, respectively. At low concentrations the bands obey the Lambert–Beer law as investigated up to 2 · 105 M and the fluorescence excitation spectra recorded throughout the emission bands (670–820 nm) copy the absorption spectra. These facts indicate that both S1 and S2 at micromolar concentrations are monomeric. As reported earlier the pyrrole rings of monomeric sapphyrins are protonated in polar solvents such as methanol to form the protonated species, which is prone to dimerise [4,14]. The broadened and unsymmetrical Soret band of S3 with maximum at 449 nm (bandwidth of 18 vs. 12 nm of S1 and S2) (Fig. 2d), and a discrepancy between the Soret band and fluorescence excitation spectra indicate the presence of different protonated species. The addition of PIPES to S1 in methanol causes substantial changes in absorption and fluorescence spectra. A new absorption peak at 418 nm with e418 = 2.4 · 105 l mol1 cm1 (Fig. 2b), which appears in a methanol– PIPES mixture (2:1) has been attributed to sapphyrin dimers [9]. The dimer is non-emissive since any specific component in fluorescence emission spectra does not appear and fluorescence excitation spectra show only the
P. Kuba´t et al. / Chemical Physics Letters 395 (2004) 82–86
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(a) (d)
(b)
(c)
Fig. 2. Absorption spectra of 4.2 lM S1 in methanol (a), methanol:PIPES = 2:1 (b), and in PIPES buffer (c) and 4.0 lM S3 in methanol (d, dotted line).
presence of residual monomer species. Similar behaviour is observed for S2 and S3. The formation of dimers of S1 in a methanol–PIPES mixture (2:1) is characterised by the dimerisation constant KD = (4.1 ± 0.9) · 105 M1 at 25 C. The thermodynamic functions of the dimerisation process were determined using the VanÕt Hoff equation. The calculated enthalpy of the dimer formation (DH = 52.3 ± 0.7 kJ mol1) and the corresponding entropy change (DS = 71.1 ± 2.9 J mol1 K1) are negative and comparable to those typical for the dimerisation of porphyrins [15,16]. Large and negative DH suggests that the dimerisation is driven by the enthalpy term (high exothermicity). Thus, a simple consideration predicts enthalpy-driven ion-pairing on the basis of electrostatic interactions between the positively charged sapphyrin core and negative peripheral substituent(s). For comparison, the dimerisation of similar macrocycles–porphyrins, with highly polarisable and non-protonated ring in neutral aqueous solutions, is controlled mainly by (induced) dipole–(induced) dipole interactions and coulombic repulsion of the substituents [7,15]. The close proximity of two sapphyrin molecules in the dimer leads to the overlapping of their hydration shells resulting in the release of water molecules from the interacting surfaces to the bulk solution, which is expected to result in a positive entropy contribution that is, however, not
observed in our experiment. The negative entropy suggests a high ordering effect that favours a well-structured dimer. S2 is less stable at increased temperatures and S3 exhibits more complex spectral features including the formation of higher aggregates. The spectral features of the sapphyrin dimers or higher aggregates in general are readily explained on the bases of the point-dipole model proposed by Kasha and co-workers [17]. The transition dipole moments can adopt a range of orientations relative to the molecular axis of the dimer. The behaviour of the visible band of sapphyrins upon the addition of a buffer is typical of the formation of stacked dimers and the resulting exciton states. The simplest case is the parallel, so-called H-type dimer, in which the excited state levels of the monomer split in two upon dimerisation leading to a blue-shifted absorption peak. According to the simple model, the interaction energy DE between the neighbouring transition dipoles depends on the value of the transition dipole moment of the monomer M, the intermolecular distance R, and the mutual geometries in the following way: 1 1 DE ¼ hc kD kM ¼
hM 2 i ðcos h 3 cos /1 cos /2 Þ; 4pe0 R3
ð1Þ
where h is the Planck constant, c is the speed of light in vacuum, kD and kM are the band maxima of dimer and monomer, e0 is the permitivity of vacuum, ÆM2æ is the mean square of M, R is the centre-to-centre distance between neighbouring moieties, h is the angle between the transition dipoles of neighbour units and /1, /2 are the angles between the transition dipoles and the line connecting the chromophore centres. The transition dipole moment M can be obtained by integration of the corresponding absorption band expressed in wavenumbers [18,19] Z eð~mÞ hM 2 i ¼ 9:184 1039 d~m: ð2Þ ~m The interaction energy (DE = 1350 cm1) was obtained as the difference between the 0–0 band energies of S1 in the monomer state (443 nm) and in the dimerised form (418 nm). As an approximation, we can assume that the dimer preserves the local symmetry of the monomer and that the sapphyrin units are stacked vertically like plates (/1 = /2 = 90) (Fig. 3a). Then we ˚ between neighattain an approximate distance of 7 A bouring molecules in the dimer. This is considerably larger than the typical van der Waals separation of about ˚ observed in the p stacking of neutral aromatic 3.5 A compounds. In addition, sapphyrin carboxylate forms a dimer in the solid state as documented by single-crystal X-ray structure [20]. Here, a pair of sapphyrin units
P. Kuba´t et al. / Chemical Physics Letters 395 (2004) 82–86
α < 54.7 o
(a)
(b)
(c)
Fig. 3. Schematic arrangement of the sapphyrin units with positively charged pyrrole nitrogen atoms and a negatively charged substituent in H-dimer (a), X-ray single-crystal structure [17] (b) and in J-aggregate (c) (h = 0, /1 = a, /2 = a).
is partially overlapped with one carboxylate oxygen from each sapphyrin within the hydrogen-bonding distance of the pyrrol NHs of the other sapphyrin and it ˚ (Fig. 3b). The foris separated by approximately 3.4 A mation of H-dimers should be accompanied by quenching of fluorescence emission because the optically allowed component reached by excitation of the dimer is located at a higher energy level and relaxes rapidly to the lower exciton component, for which the deactivation primarily occurs via intersystem crossing to generate the triplet states. As shown above, considerable fluorescence quenching was observed. The simple dimer model qualitatively explains the spectral features, however, this should be considered as tentative. Increasing the amount of buffer in a methanol–PIPES mixture increases the extent of aggregation as indicated by the appearance of additional broad peaks. In pure PIPES sapphyrins, S1–S3 form aggregates characterised with the blue-shifted Soret band centred at about 400 nm and with the red-shifted band located at 460–470 nm (Figs. 2c and 4a) [6]. The former band belongs to H-aggregates indicating that the dimers are converted to larger assemblies in the absence of methanol and the red-shifted absorption bands signify that the assembly consists of head-to-tail aggregates (J-aggregates). The contribution of respective aggregates is affected by sapphyrin functionalisation, temperature and ionic strength of a solution. Whereas S1 forms predominantly H-aggregates (Fig. 2c), both S2 and S3 yield a mixture of H- and J-aggregates (Fig. 4a).
(a)
(b)
Fig. 4. Absorption spectrum of S2 in PIPES buffer (left axis, (a)) is compared with corresponding resonance light scattering features (right axis, (b)).
85
The formation of J-aggregates is apparently driven by the stabilisation interaction between easily ionisable sulfonato or carboxy groups and the positively charged sapphyrin unit of the adjacent molecule (Fig. 3c). This binding motif has been previously reported for diprotonated 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin ðH2þ 4 TPPSÞ at high ionic strength where the J-aggregation is related to the appearance of a red-shifted and narrow absorption band [21,22]. The coupling of the tightly packed molecular transition dipoles in pure J-aggregates leads to the delocalisation of the exciton state through the coherent excitation of a number of monomers, which significantly contributes to the shift of the Soret band. In this case the width of the absorption band of the pure J-aggregate is narrower than that of the monomer by a factor of Neff1/2, where Neff is the average number of porphyrin units in direct communication [23]. In contradiction to H2þ 4 TPPS, the Soret bands of the J-aggregates of S1–S3 do not show a narrowing (Figs. 2c and 4a), which indicates a high disorder of the unit arrangement excluding delocalisation of the exciton states along the aggregate. The presence of extended electronically coupled aggregates can be detected using RLS experiments. This technique allows identification of extended aggregated species even at low concentrations since the amount of scattered light is directly proportional to the volume of particles and monomeric molecules and small oligomers show no enhanced scattering [24]. The appearance of the RLS features in Fig. 4b clearly shows that the size of aggregates is large enough to scatter light and that aggregates consist of multiple sapphyrin molecules. Two maxima in the light-scattering signal are associated with at least two different multiple arrangements, which is consistent with the presence of H- and J-aggregates. It should be pointed out that decreased scattering around 455 nm is not due to self-absorption of incident light because it corresponds to the absorption minimum between the absorption bands. The RLS profiles presented in Fig. 4 are similar for all studied sapphyrins. We also characterised the formation of the triplet states of S1–S3 and their quenching by oxygen leading to the formation of singlet oxygen 1O2. In oxygen-free methanol the decay kinetics of the triplet states is strictly monoexponetial and independent of the functionalisation giving the lifetimes of 118 ± 20 ls. The transient absorption spectrum recorded at short times after excitation shows a broad bleaching in the 400–475 nm region and increased absorption in the 330–390 nm and 475–520 nm (Fig. 5a). The triplet states of S1–S3 are effectively quenched by dissolved oxygen characterised by the bimolecular rate constant of oxygen quenching (3.3 ± 0.7) · 109 l mol1 cm1. The transient spectra in methanol–PIPES buffer (2:1) show bleaching of both peaks at 443 and 418 nm, respectively, suggesting that simultaneous excitation of
P. Kuba´t et al. / Chemical Physics Letters 395 (2004) 82–86
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Acknowledgement (c) (a) (d)
This research was supported by the Grant Agency of the Czech Republic (Grant Nos. 203/04/0426 and 203/ 02/0420).
(b)
References
Fig. 5. Difference absorption spectra of 4.2 lM S1 recorded 42 ns after excitation by a 420 nm laser pulse in air-saturated methanol (monomeric S1) (a) and in methanol–PIPES buffer = 2:1 (mixture of the H-dimer and monomer) (b). Inset: Transient traces recorded at 390 nm in air-saturated methanol (c) and in methanol–PIPES = 2:1 (d).
the monomer and H-dimer (Fig. 5b). While in methanol the recovery curves and the transient absorption decay curves are monoexponetial (Fig. 5c) indicating decay of the produced triplet states (oxygen-free solution) or their quenching by oxygen, the kinetics in the methanol–PIPES mixture have a fast component with an estimated lifetime of less than 5 ns, which is evident at shorter wavelengths 340–390 nm (Fig. 5d). This fast kinetics is missing at longer wavelengths (470–550 nm). A full recovery is achieved with the lifetime of 130 ± 25 ls in an oxygen-free solution, which is close to the corresponding lifetime measured in pure methanol. In the presence of oxygen the slow recovery kinetics is accelerated because the triplet states are efficiently quenched by oxygen while the contribution of the fast kinetics is not influenced. These results indicate possible formation of the triplet states of the sapphyrin monomer by exciting the dimers, which is further corroborated by the fact that the rate of recovery of dimer bleaching is equivalent to triplet decay both in the presence and absence of dissolved oxygen. The presence of two different triplet states as recently found to be a consequence of aggregation [25] is less probable. Sapphyrins S1, S2 and S3 in methanol produce 1O2 with the quantum yields UD of 0.32 ± 0.06, 0.33 ± 0.07, 0.30 ± 0.06, respectively. These values are slightly greater than those of another sapphyrin derivatives in ethanol (UD = 0.160.18) [26] deuterated methanol, acetonitrile and dichloromethane (UD = 0.130.28) [4]. The measured lifetime of 1O2 is of sD = 8.6 ± 0.8 ls, the value that is in accordance with the behaviour of 1O2 in this solvent [27]. The production of 1O2 strongly decreases after the addition of the aqueous PIPES buffer due to the increasing contribution of dimers and larger aggregates. In general, with the increasing contribution of aggregates the value of UD dramatically decreases, e.g. UD 6 0.05, in a 1:1 methanol/PIPES mixture.
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