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Photophysics of an unsymmetrical Zn(II) phthalocyanine substituted with terminal alkynyl group
Author’s Accepted Manuscript Photophysics of an unsymmetrical Zn(II) phthalocyanine substituted with terminal alkynyl group Kamil Kędzierski, Bolesław Barszcz, Michał Kotkowiak, Bartosz Bursa, Jacek Goc, Hatice Dinçer, Danuta Wróbel www.elsevier.com/locate/jlumin
PII: DOI: Reference:
S0022-2313(16)30304-0 http://dx.doi.org/10.1016/j.jlumin.2016.08.010 LUMIN14171
To appear in: Journal of Luminescence Received date: 7 March 2016 Revised date: 29 July 2016 Accepted date: 2 August 2016 Cite this article as: Kamil Kędzierski, Bolesław Barszcz, Michał Kotkowiak, Bartosz Bursa, Jacek Goc, Hatice Dinçer and Danuta Wróbel, Photophysics of an unsymmetrical Zn(II) phthalocyanine substituted with terminal alkynyl group, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2016.08.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Photophysics of an unsymmetrical Zn(II) phthalocyanine substituted with terminal alkynyl group
Kamil Kędzierski1, Bolesław Barszcz1,2, Michał Kotkowiak1, Bartosz Bursa1, Jacek Goc1, Hatice Dinçer3, Danuta Wróbel1* 1
Faculty of Technical Physics, Institute of Physics, Poznan University of Technology, 60-965 Poznan, Poland 2 Institute of Molecular Physics, Polish Academy of Sciences, 60-179 Poznan, Poland 3 İstanbul Technical University, Faculty of Science and Letters, Department of Chemistry, 34469 Maslak, İstanbul, Turkey Corresponding author: Danuta Wróbel, email: [email protected], tel. +48 61 665 3179 Abstract This paper examines photophysical properties of an unsymmetrical zinc phthalocyanine substituted with a terminal alkynyl group. The studies were concentrated on absorption (in the UV-vis-IR range) and fluorescence (steady-state and time resolved) in chloroform of unsymmetrical 9(10),16(17),23(24)-tri-tert-butyl-2-(pent-4-yloxy)phthalocyaninato zinc(II) of different concentrations. Moreover, dye photodegradation studies were also done. Besides, a laser-induced optoacoustic spectroscopy was also used to determine thermal deactivation as well as singlet oxygen generation yield. The Langmuir layers of the dye were formed and the in-situ absorption technique was applied to follow ability of the dye to aggregate formation. The experimental data were supported by the quantum chemical calculations with the use of the time-dependent density functional theory (TD-DFT) to obtain information on the distribution of electron density and electronic transitions in the molecular systems. The spectroscopic results of the unsymmetrical zinc phthalocyanine were confronted with the data of the symmetric zinc phthalocyanine.
Keywords: UV-vis absorption, in-situ light absorption, fluorescence quantum yield, fluorescence quantum lifetime, singlet oxygen generation, quantum chemical calculations Abbreviations: ZnPc – zinc phthalocyanine, Pc – phthalocyanine, bromocresol purple – BCP, laser induced optoacoustic spectroscopy – LIOAS, time-dependent density functional theory – TD-DFT
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1. Introduction Unsymmetrical phthalocyanines (Pcs) belong to a huge family of porphyrins and they are highly versatile dyes that can find many practical applications in science and technology: in nanotechnology as optoelectronic materials, non-linear optical materials, sensors; in photodynamic diagnosis and therapy of cancer and other diseases; in biology in modeling photosynthesis, and many others [1–4]. Unsymmetrical Pcs are very unique species due to their high absorption coefficient, easily modified molecular structure and effective photoactivity, redox-active scaffolds for supramolecular design [5,6]. Moreover, because of Pc low production cost and easiness in construction these organic materials can replace the other ones as alternative to non-organic moieties [7]. Chemical synthesis procedure of the unsymmetrical Pcs under investigations was described and their basic studies were performed [8]. A variety of unsymmetrical Pcs of diverse molecular structures in different solvents [9] and in solid matrices have been investigated through physical/photophysical and chemical/photochemical experiments [10]. However, in this paper we would like to extend investigations previously done [8,11–13] and to examine an unsymmetrical Pc since there is still not very much known about its triplet states’ behavior and thermal deactivation. However, further studies seem to be necessary to get more information about their photophysical parameters important for photodynamic therapy of cancer (PDT). We put our attention on the photophysical properties of the unsymmetrical zinc Pc (ZnPc) the spectroscopic results obtained in this paper were compared to those of the symmetric zinc Pc. These studies included investigations of absorption (in the UV-vis range) and fluorescence (steady-state and time resolved) in non-polar solvent (chloroform (CHCl3). The dye concentrations were changed from 10-4 to 10-6 M. In our experiments CHCl3 was used because of its ability to very fast evaporation and low polarity (ε = 4.80). Moreover, examinations in CHCl3 are important for our future research of formation of Langmuir monolayers. However, we have to realize that the use of non-polar CHCl3 one can expect to have aggregated dye molecules at sufficient high concentration. Besides the UV-vis examinations, we also provided a laser-induced optoacoustic spectroscopy (LIOAS). The LIOAS technique was used for unsymmetrical Pcs for the first time and let to determine thermal deactivation as well as singlet oxygen generation. The experimental data were supported by the quantum chemical calculations with the use of the time-dependent density functional theory (TD-DFT) to obtain information on the distribution of electron density and electronic transitions in the molecular systems. The greatest advantage of unsymmetrical Pcs over symmetrical Pcs is lower ability to form aggregates. In our spectroscopic studies of the dyes in solutions aggregated forms of both symmetrical and unsymmetrical dyes have not revealed. Therefore we applied the in-situ absorption of a Langmuir layers experiment to expose the deference in aggregation ability. The distance between molecules in a Langmuir layer is much smaller than that in solution at 10-4 M concentration. The lower ability to form aggregates of unsymmetrical Pc, presented in the paper, was confirmed by the relatively small changes in the in-situ UV-Vis spectra with respect to the spectra collected for the dye in chloroform. 2
2. Materials and methods 2.1. Materials An unsymmetrical 9(10),16(17),23(24)-tri-tert-butyl-2-(pent-4-yloxy)phthalocyaninato zinc(II) (ZnPc_1) (concentration 10-4 – 10-6 M) was investigated and its properties were discussed versus those of a symmetric 2,9,16,23-tetra-tert-butyl-phthalocyaninato zinc(II) (ZnPc_2) that was used as a reference dye. The ZnPc_1 and the ZnPc_2 samples were obtained according to the procedure described previously [8] and purchased from SigmaAldrich, respectively. The molecular structures of the investigated Pcs are shown in Fig. 1. CHCl3 of spectroscopic grade was used as a solvent and it was purchased from POCH Poland S.A.
Fig. 1. Molecular structures of investigated ZnPc_1 and ZnPc_2. 2.2. UV-vis absorption in solvent and in Langmuir monolayer The ground state absorption spectra were monitored with an UV-vis Varian Cary 4000 spectrophotometer in a quartz cuvette over the range of 250-900 nm. Langmuir monolayers of the samples were created with a KSV 2000 minitrough (KSV Instruments Ltd.). The Langmuir trough area was 2325 mm2 (length 310 mm and width 75 mm). Ultrapure water (electrical resistivity 18.2 MΩ·cm) as a subphase was obtained with a Mili-Q water purification system (Millipore Corp.). The sample dissolved in CHCl3 (10-4 M) was spread carefully onto the subphase and CHCl3 evaporated in 15 min. The floating film of the Langmuir layer was compressed symmetrically from both sides with motion barrier speed of 5mm/min. The in-situ method was used to follow electronic absorption spectra of the Langmuir layers in the UV-vis range (250-900 nm) with an Ocean Optics spectrometer QE65000. 2.3. Fluorescence studies The fluorescence spectra and fluorescence excitation spectra were measured with a Hitachi F4500 fluorometer (excitation wavelength λexc= 320 nm and observation wavelength λobs=400 nm or 720 nm). The fluorescence quantum yields (ΦF) were determined using the classical formula [14]: 3
(1) where A is the absorbance at the excitation wavelength, F is the area under the fluorescence and n is the refraction index. The subscripts r and s refer to the reference and to the sample of unknown quantum yield, respectively. As a fluorescence reference ZnPc dissolved in dimethylformamide was used (Φr=0.17) [15]. Fluorescence lifetime signals were obtained using EasyLife™ V lifetime fluorimeter with a 1.5 ns pulse of LED as an excitation source (635 nm) and the sample fluorescence responses were collected over the range of 650 - 750 nm. Fluorescence lifetimes (τ) were estimated by fitting the decay curves using a deconvolution procedure implemented into DecayFit 1.4 – Fluorescence Decay Analysis software. All measurements were done at room temperature. 2.4. Quantum chemical calculations To improve the interpretation of the experimental UV-vis spectra we performed calculations of the transition energies via the time-dependent density functional theory (TD-DFT). The calculations were performed using the B3LYP hybrid functional (Becke 3-parameter exchange functional combined with Lee-Yang-Parr correlation functional) and the standard 631G(d,p) basis set. Prior to TD-DFT calculations the geometries of the molecules were optimized using DFT method with the same functional and basis set. For the equilibrium geometries the normal mode frequencies were calculated (no imaginary frequencies were found proving that we reach the energetic minimum) and compared with the experimental IR data. TD-DFT calculations were performed for isolated molecules and including the influence of CHCl3 using the Polarized Continuum Model (PCM) as implemented in the Gaussian 03 program package [16]. The first 200/100 (ZnPc_1/ZnPc_2) optical transitions were calculated. To convolute the resulting transition energies and oscillator strengths in the absorption spectra the GaussSum program was used [17]. The spectra were generated assuming FWHM (Full Width at Half Maximum) parameters at 3000 cm-1 for all transitions. 2.5. Infrared absorption studies The infrared absorption spectra (middle infrared region) of the investigated ZnPcs were recorded using a JASCO FT-IR 6200 Fourier transform spectrometer. The samples were dissolved in CHCl3 and deposited on a potassium bromide (KBr) plate. After solvent evaporation the spectra of a thin film on KBr were recorded in the transmittance mode using the clear KBr plate as a reference. 2.6. Laser induced optoacoustic studies To get information on dye photothermal parameters, the time-resolved photothermal signals were recorded by means of the LIOAS method. Technical specifications and more details of a LIOAS apparatus were described elsewhere [18,19]. For the LIOAS experiments bromocresol purple (BCP) Sigma-Aldrich was chosen as a calorimetric reference due to its suitable spectral properties. All measurements were carried out at ambient temperature and in air
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atmosphere. The excitation wavelength was 337 nm; the absorbance of the dye was equal to 0.15 at the excitation wavelength. The analysis of the LIOAS waveform was carried out according to Marti et al. method [20]. In this approach, the first maximum (Hmax) of the LIOAS signal is expressed as: (2) where k is a proportionality factor that regards geometry and electric impedance of a device, αair means a part of energy changed into heat in air atmosphere (in time shorter than a time resolution of the apparatus), A is absorbance of a sample, and Ehν is the molar energy of incident photons 354.9 kJ·mol-1 at 337 nm. A part of the excitation energy exchanged into heat promptly (α parameter) was calculated by direct comparison of the slopes of the plots in the linear region obtained for the dye and for the reference dye in air [21]. The efficiency of singlet oxygen generation yield (ФΔ) can be evaluated from the following equation: (3) where ES is the molar energy of the singlet state and EΔ is the energy of the oxygen singlet state (EΔ = 94 kJ·mol-1) [21]. 3. Results and discussion 3.1. Electronic absorption experiments in solution The ground state normalized electronic absorption spectra of ZnPc_1 and ZnPc_2 in CHCl3 (concentration 10-4 – 10-6 M) are shown in Fig. 2; the spectra are normalized to unity at the highest Q band (679 nm). The absorption spectra are dominated by the π-π* transitions with an intense and narrow Q band and a much lower intensive Soret band. The main Soret bands with the left-side band are observed at 350 nm and 288 nm. The main Q band (679 nm) with two shorter-wavelength and tiny peaks (at 612 nm and about 650 nm) are seen. Such feature is characteristic for the family of Pc dyes.
Fig. 2. UV-vis absorption spectra of unsymmetrical ZnPc_1 and its analogue ZnPc_2; 5
(ZnPc_2 spectrum on the basis of [22]). From the absorption results one can see that sample concentration only slightly affected the absorption spectra. The spectra shapes were almost overlapped one by each other. Moreover, the band intensity ratios and their locations were not changed upon dyes increasing concentration. However, on the basis of behavior of the natural pigments and of synthetic porphin-like molecules [23] one can suppose the presence of poorly defined molecular aggregates. The highly concentrated ZnPc_1 in non-polar CHCl3 did not show notable changes when compared to the ZnPc_2 sample. The only differences in the absorption spectra can be found in the values of the relative intensities at 679 nm versus 350 nm. The selected absorption parameters of the dyes are collected in Table 1. Usually, a solvent condition may cause π-π* dyes associated into dimers (J, I, H, oblique coplanar dimers or other kinds [24]) which can be formed via direct π-π* interaction or indirect interaction between metal in the dye core of one molecule and an electron donating group of another molecule at adequately high concentration. Thus, on the basis of the results of other synthetic porphyrin-like molecules [25–27] one can suppose the presence of molecular aggregates. No particular changes in the absorption spectra observed at different concentrations could point out the dominance of the dye monomeric forms up to 10-4 M. However, a lack of changes in an absorption band shift does not exclude creation of an aggregate at very high dye concentration in non-polar solvent; the changes in intensity ratios in the spectral region 679 nm versus 350 nm can indicate the presence of aggregates (Table 1). Thus, on the basis of our absorption observation we cannot neglect the presence of a sort of aggregates in a highly concentrated sample. Table 1 Selected absorption parameters of ZnPc_1 and ZnPc_2 (a in CHCl3, b in Langmuir monolayer at 40 mN/m) Absorption Absorption Absorption Absorption Dye maxima band intensity maxima band intensity a a b [nm] ratio [nm] ratiob 679 690 0.16 (612/679) 0.57 (625-645/690) 612 625-645 ZnPc_1 0.39 (350/679) 1.17 (340/690) 350 340 679 682 0.16 (612/679) 0.92 (625-645/682) 612 625-645 ZnPc_2 0.34 (350/679) 1.41 (333/682) 350 333 To clearly decide on ability of the dyes to aggregate formation we applied an in-situ light absorption method to investigate the dyes in a Langmuir monolayer. The results are shown in Fig. 3 A and B for both ZnPc_1 and ZnPc_2 (the last one as a reference sample). It is worth to underline that the in-situ experiments at different surface pressure were done for the unsymmetrical Pcs for the first time. The first observation was the very different in-situ behavior of ZnPc_1 and ZnPc_2 in the Langmuir monolayer (see Table 1). The different shapes of the spectra of ZnPc_1 and 6
ZnPc_2 and relation in the bands’ intensities (682, 690 nm versus 625-645 nm) can origin from dissimilarity between the peripheral groups that affect the different configuration of the dipole transitions in ZnPc_1 and ZnPc_2. The presence of the intense 625-645 nm band confirms the aggregate formation [28]. Dimerization of metallic Pcs was also studied by Raman spectroscopy [29] and by atomic force microscopy [30]. In the Q band region two very intense bands were well seen: the first one at 690 nm (versus the band at 679 nm in CHCl3) and it can be assigned to a dye dimeric form. The second confirmation of the presence of aggregated species is the appearance of the intensive broad bands at about 625-645 nm. The location of these bands is in accordance with the results obtained for symmetric Pc dye and presented in one of our papers [22]. These bands originate from the formation of the oblique coplanar aggregate and/or of the I type aggregate [24]. The aggregates coexist with the monomeric dyes. In general, in the Langmuir layer the distance between dye molecules is much shorter than that in 10-4 M solution and thanks to high package of dye molecules in the layer the molecules are able to create aggregates. Thus, from our in-situ results we have every reason to believe in creation of aggregates. Moreover, the insitu spectra of ZnPc_1 are similar to the spectra monitored for the dye in solution. Otherwise, the in-situ spectra of ZnPc_2 are different when compare to those in solution. It is noticeable that the absorption band intensity ratio (Table 1) – the values of the ratio calculated from the in-situ and in solution spectra of ZnPc_1 are more similar than those of ZnPc_2. This clearly indicates lower ability to aggregate formation of unsymmetrical ZnPc_1.
Fig. 3. In-situ light absorption spectra of ZnPc_1 (A) and ZnPc_2 (B) in Langmuir monolayer at different surface pressure. Fig. 4A shows the π-A isotherms of the dyes in monolayers. The dyes’ isotherms depend on the dye molecular structures – ZnPc_1 has one pent-4-ynoloxy and three tert-butyl groups 7
whereas ZnPc_2 has four tert-butyl groups. The presence of the different groups and asymmetry in the structure lead to a different molecular arrangement on the aqueous subphase. The main molecular core of the dyes under studies is the porphyrazine cycle-ring. On the basis of the π-A isotherm result we evaluated thermodynamic and molecular arrangement parameters of the dyes Langmuir layers. The values are collected in Table 2. The average area per molecule was evaluated with an extrapolation of the π-A isotherms. As well seen from Table 2 the values of parameters differ markedly between both dyes. On the basis of the literature data, the Pc geometrical size values are 1.2-1.9 nm2 (depending on dyes and substituted groups) and about 1 nm2 of the Pc skeleton [31]. The area of the dye skeletons was estimated taking advantage of the length of bonds calculated by the DFT method with the assumption that the external groups are stiffly united to the Pc skeleton. The results of our πA isotherm experiments let us to estimate a tilt angle between the molecular skeleton and the normal to water subphase. The tilt angles of the ZnPc_1 and ZnPc_2 skeleton are: 14 deg. and 24 deg., respectively. The difference between the tilt values explicitly indicates unlike molecular arrangements of the two dyes in the monolayer - ZnPc_1 is nearly oriented perpendicularly and ZnPc_2 less perpendicularly with respect to water subphase. The different angle values result from the presence of the hydrophobic group in ZnPc_1. Moreover, creation of the different types of aggregates and their different monolayer compressibility also affect the dye distribution.
Fig. 4. The π-A isotherms of ZnPc_1 (black lines) and ZnPc_2 (red lines) monolayers at room temperature (A) and compressibility (B). The angle values obtained for our samples are insignificantly different from those evaluated for similar Pcs [31]. However, taking into consideration the possibility of the external group free bending relative to the Pc ring plane and assuming 1 nm2 molecule skeleton surfaces, the tilt angles of the ZnPc_1 or ZnPc_2 skeleton are 40 deg. and 54 deg., respectively - this 8
result is similar to that obtained for similar Pcs [31]. It is obvious that the external groups have an effect on the arrangement of molecules in the layer [32,33], therefore the real tilt angles between the molecular skeleton and the normal to the water subphase has possible values between 14-40 deg. for ZnPc_1 and 24-54 deg. for ZnPc_2. We have also done investigations of Langmuir layer compressibility (Fig. 4 B); the 1 1 compressibility modules C S A(d / dA) [34,35] are collected in Table 2. The C S values higher than 200 mN/m indicate creation of the 2D solid. However, it is not the case for our samples. The compressibility parameter estimated for ZnPc_1 is 66 mN/m and indicates creation of the monolayer liquid state, whereas for ZnPc_2 it is 97 mN/m and confirms creation of the monolayer of properties between liquid and condense liquid phase. The compressibility results acknowledged the various orientation of the molecular plane with regard to the water surface. The differentiation in the module values shows various interactions of the Pcs. Moreover, the results confirmed the distinct thermodynamic properties of the dyes due to the presence of the different groups linked to the main molecular cores and asymmetry of the molecular structure. Table 2 Thermodynamic and molecular arrangement parameters of ZnPc_1 and ZnPc_2 (Aavr – average area per molecule; Ac – area at the collapse point; πc – collapse point; At – theoretical area of molecule; δa, δb – tilt angle under the assumption stiffly united external group and under the assumption of 1 nm2 surfaces of the molecule skeleton, respectively) Dye
Aavr [nm2]
Ac [nm2]
πc [mN/m]
At [nm2]
δa [deg.]
δb [deg.]
Cs-1 [mN/m]
ZnPc_1
0.64
0.27
39
2.48
14
40
66 (at π = 18 mN m-1)
ZnPc_2
0.81
0.59
30
1.89
24
54
97 (at π = 21 mN m-1)
3.2. Fluorescence examinations To follow the dye singlet state of ZnPc_1 and ZnPc_2 the emission and excitation spectra of the dyes (5×10-6 M) in CHCl3 were done and the results are shown in Figs 5 and 6. Using ZnPc_2 as a reference dye allowed us to observe the effect of the presence of the pent-4ynoloxy group on fluorescence. The fluorescence spectra of ZnPc_1 and ZnPc_2 differ markedly in their intensity and location of the maxima (fluorescence measurements were done at the same experimental conditions). The main band of the spectra located at 684 nm showed strong fluorescence and similar behavior were also observed for other Pcs [36–39]. The dominant fluorescence peak can be assigned to emission of the dye monomer. The much less intense shorter wavelength band was also observed at 397 nm but only for ZnPc_1, the dye with the pent-4-ynoloxy group. The origin of the ‘violet band’ in symmetric Pcs was widely discussed by Kaneko et al. [40] and by Chahraoui et al. [41] - the origin of the short wavelength emission was indicated as resulting from the intrinsic lowest singlet excited state that was formed by the red photons [40]. However, in our experiments we used the 320 nm light as excitation and we have also observed the 397 nm band in ZnPc_1 but not in the 9
symmetric ZnPc_2. Thus we have every reason to believe in a great influence of the pent-4ynoloxy group on the dye fluorescence behavior.
Fig. 5. Fluorescence spectra of ZnPc_1 (black line) and ZnPc_2 (red line) in CHCl3; insert – magnification of the spectra in the range 330-550 nm; λexc = 320 nm.
Fig. 6. Excitation emission spectra of ZnPc_1 (black line) and ZnPc_2 (red line) in CHCl3; main figure λobs = 720 nm, insert λobs = 400 nm. The Q bands in the fluorescence excitation spectra were consistent with the longwavelength UV-vis absorption spectra. The same locations of the 678 nm, 612 nm bands and of a hump at 640-650 nm were very well seen. The excitation spectra at 400 nm observation could confirm ‘violet fluorescence’ of ZnPc_1 which originate from absorption of 246 and 325 nm irradiation. On the other hand Mack et al. [42] have provided evidence that ‘violet emission’ may not be intrinsic to the molecule, but it can be the result of a photodegradation product. To validate these observations the absorption and emission spectra during ZnPc_1 irradiation have been presented in Fig. S1. In our experiment the ‘violet emission’ band was constant while irradiated. Furthermore, the intensities of the B and Q absorption bands and the S1 emission are changed significantly over the irradiation time. If the ‘violet emission’ (Fig. 5) is an intrinsic property of ZnPc_1 and hence it is a photophysical process with a fixed and reproducible quantum yield value, the observed ‘violet emission’ intensity can be expected to follow the same trend as observed in the S1 emission (panels B and D in Fig. S1). Moreover, the values of the B and Q absorption band intensities (shown in Fig. S1 A and C, respectively) 10
remain constant. Additional, changes in the shape of the spectra were not observed, as it was stated by Mack et al. [42]. The ΦF, and τ were also estimated. The τ values were evaluated taking in consideration the fluorescence kinetics presented in Fig. 7. The τ values are in accordance with those of Pc dyes. Evaluation of the τ values was done with two approaches: mono-exponential and double-exponential deconvolution. The best fitting results were obtained when monoexponential procedure was applied, and the estimated values are gathered in Table 3. Table 3 Parameters obtained on the basis of analysis of steady state and time resolved fluorescence of ZnPc_1 and ZnPc_2 in chloroform (5×10-5 M). Dye
τ [ns]
χ2
ZnPc_1 ZnPc_2
3.21 3.23
1.09 1.02
ΦF (±0.05) 0.15 0.16
Fig. 7. Fluorescence decay curves of ZnPc_1 (black curve) and ZnPc_2 (red curve) ; λexc = 635 nm. 3.3. Quantum chemical calculations In order to improve interpretation of the experimental results we have performed the DFT calculations of the investigated Pcs. The geometry of both molecules have been optimized and normal modes vibrations have been calculated. Additionally, two conformations of the ZnPc_1 have been optimized (Fig. S2, Supplementary Information). The lower total energy value (-4187.4890 versus -4187.4881 hartree) suggests that conformer 1 is preferred and it was used in the further investigations. The normal modes frequencies have been compared with the experimental results. The representative spectral region is displayed in Fig. S3. The contour plots of the frontier molecular orbitals of both Pcs are presented in Fig 10. The energy of LUMO is identical in both molecules (-2.58 eV) while the energy of HOMO is slightly different (-4.73 eV and -4.76 eV for ZnPc_1 and ZnPc_2, respectively) so the HOMO-LUMO energy gap is 2.15 eV and 2.18 eV for ZnPc_1 and ZnPc_2, respectively. The values for ZnPc_2 are comparable with those for symmetrical ZnPc obtained by Ueno and co-workers [43,44]. There is no change in the shape of the mentioned orbitals after including the influence of solvent. However, the energy of the HOMO and LUMO in the 11
solvent is slightly reduced (HOMO= -4.82 and -4.87 eV for ZnPc_1 and ZnPc_2, respectively, and LUMO= -2.69 eV for both molecules). The resulting HOMO-LUMO energy gap remains the same for ZnPc_2 (2.18 eV) but is slightly decreased for ZnPc_1 (2.12 eV).
Fig. 8. The contour plots of the frontier molecular orbitals of ZnPc_1 (left side) and ZnPc_2 (right side). The calculated UV-Vis absorption spectra of both dyes with and without the presence of solvent are gathered in Fig. 11. The differences between the results obtained for ZnPc_1 and ZnPc_2 are rather subtle – a small shift in the Soret and Q regions and some additional transition above 400 nm in the spectrum of ZnPc_1 is found. These differences are also seen in the spectra calculated including the influence of solvent. In general, including the solvent causes the bands in the calculated spectrum are in better agreement with the experimental results (Fig.2). The relation between the spectra is similar to the results obtained by Ueno et al. [43]. Additional absorption in the spectrum of ZnPc_1 mentioned above is related mainly to the transition HOMO-1→LUMO+1 so it involves the altered group in the ZnPc_1 (see Fig. S4). The details of the most intense calculated transitions are gathered in Table S1 (Supplementary Material).
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Fig. 9. The TD-DFT calculated UV-vis spectra of ZnPc_1 (black line without and grey line with the presence of solvent) and ZnPc_2 (red line and magenta line). 3.4. Triplet state studies Fig. 10 illustrates the relation between the Hmax of the sample ZnPc_1 and ZnPC_2 in CHCl3 as a function of excitation energy (Ehν).
Fig. 10. LIOAS signal Hmax of ZnPc_1 (red circles), ZnPc_2 (blue up triangles) and bromocresol purple (BCP) (black squares) in chloroform as a function of the laser energy Ehν; λexc = 337 nm; R2 ≥0.99. The αair values were 0.76±0.02 and 0.78±0.02 for ZnPc_1 and ZnPc_2, respectively (evaluated on the base of the relationships shown in Fig. 10). The presented results indicated that the αair values were not dependent on the ZnPcs substituents (within the experimental error). The ΦΔ values calculated (eq. 3) for ZnPc_1 and ZnPc_2 were 0.63±0.03 and 0.53±0.03, respectively. Slightly higher value was observed for unsymmetrically substituted ZnPc. The process that is responsible for singlet oxygen formation is the interaction between the dye excited triplet and oxygen. The obtained results indicated the modest efficient generation of singlet oxygen - ZnPc is limited due to high amount of energy change into heat (αair). The lower values of ΦΔ 0.47 were reported previously for unsymmetrical Pc bearing two 1-adamantylsulfanyl groups at adjacent peripheral positions in DMF and DMSO [45]. Novakova et al. reported ΦΔ lower than 0.47 for unsymmetrically substituted Mg2+ and Zn2+ Pcs in THF [46]. Dinçer et al. postulated potential application of polymer conjugated ZnPcs 13
as photosensitizer in PDT for which ΦΔ were 0.01 and 0.14 for symmetrical and asymmetrical dyes, respectively in aqueous solution [12]. 4. Conclusions The experimental spectroscopic results of the investigations of unsymmetrical phthalocyanine substituted with terminal alkynyl group in chloroform supported by the chemical calculations let us to draw very important conclusions on the properties of the singlet and triplet states of the dye and to confront the properties of terminal alkynyl-substituted phthalocyanine ZnPc_1 and ZnPc_2. The spectroscopic results presented in the paper can indicate rather weak influence of the asymmetry on the singlet state of ZnPc_1 as evidenced by the similar values of fluorescence quantum yields and of the life time values to those of the symmetric ZnPc_2 (τ 3.21 vs. 3.23; ΦF 0.15 vs. 0.16). However, a great diversity in fluorescence of ZnPc_1 and ZnPc_2 is found in the shortwavelength excited singlet state and it can originate from the intrinsic lowest singlet excited state as discussed in other papers. Moreover, we also showed weak ability of unsymmetrical Pc to formation of aggregates at sufficiently concentrated chloroform solution. However, the ability of the unsymmetrical dye to aggregate creation rises in the Langmuir nanolayers of high packaged molecules. It is possible due to much stronger dye dipole-dipole interaction of concentrated dyes and as a consequence oblique coplanar aggregates and/or of the I type aggregates can be formed. In general, in the Langmuir layer the distance between dye molecules is much shorter than that in 10-4 M solution and thanks to high package of dye molecules in the layer the molecules are able to create aggregates. Thermodynamic experiments done in the Langmuir layers showed that the value of the compressibility parameter indicates creation of the ZnPc_1 monolayer liquid state, and in contrast for ZnPc_2 the parameter confirms creation of the monolayer liquid-condense liquid phase. The compressibility results showed various interactions of ZnPc_1 and ZnPc_2 leading to diverse orientation of the molecular plane with regard to the water surface. Thus, the results confirmed the distinct thermodynamic properties of the dyes due to the presence of the different groups linked to the main molecular cores and asymmetry of the molecular structure. It was also shown that a large amount of energy is deactivated in a form of heat and it was not dependent on the ZnPcs substituents. However, the non-radiative process is able to affect oxygen generation. The slightly higher value of oxygen generation was observed for unsymmetrically substituted dye. Acknowledgements Presented work has been financed by the Ministry of Science & Higher Education in Poland in 2015 year under Project No 06/62/DSMK/0196 (KK, BB). MK, BaB, JG, DW thanks to Poznan University of Technology, grant DS PB 62/062/0216 and HD thanks to The Scientific & Technological Research Counsil of Turkey (TUBITAK) (project No 111T063) for financial support. 14
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PII: DOI: Reference:
S0022-2313(16)30304-0 http://dx.doi.org/10.1016/j.jlumin.2016.08.010 LUMIN14171
To appear in: Journal of Luminescence Received date: 7 March 2016 Revised date: 29 July 2016 Accepted date: 2 August 2016 Cite this article as: Kamil Kędzierski, Bolesław Barszcz, Michał Kotkowiak, Bartosz Bursa, Jacek Goc, Hatice Dinçer and Danuta Wróbel, Photophysics of an unsymmetrical Zn(II) phthalocyanine substituted with terminal alkynyl group, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2016.08.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Photophysics of an unsymmetrical Zn(II) phthalocyanine substituted with terminal alkynyl group
Kamil Kędzierski1, Bolesław Barszcz1,2, Michał Kotkowiak1, Bartosz Bursa1, Jacek Goc1, Hatice Dinçer3, Danuta Wróbel1* 1
Faculty of Technical Physics, Institute of Physics, Poznan University of Technology, 60-965 Poznan, Poland 2 Institute of Molecular Physics, Polish Academy of Sciences, 60-179 Poznan, Poland 3 İstanbul Technical University, Faculty of Science and Letters, Department of Chemistry, 34469 Maslak, İstanbul, Turkey Corresponding author: Danuta Wróbel, email: [email protected], tel. +48 61 665 3179 Abstract This paper examines photophysical properties of an unsymmetrical zinc phthalocyanine substituted with a terminal alkynyl group. The studies were concentrated on absorption (in the UV-vis-IR range) and fluorescence (steady-state and time resolved) in chloroform of unsymmetrical 9(10),16(17),23(24)-tri-tert-butyl-2-(pent-4-yloxy)phthalocyaninato zinc(II) of different concentrations. Moreover, dye photodegradation studies were also done. Besides, a laser-induced optoacoustic spectroscopy was also used to determine thermal deactivation as well as singlet oxygen generation yield. The Langmuir layers of the dye were formed and the in-situ absorption technique was applied to follow ability of the dye to aggregate formation. The experimental data were supported by the quantum chemical calculations with the use of the time-dependent density functional theory (TD-DFT) to obtain information on the distribution of electron density and electronic transitions in the molecular systems. The spectroscopic results of the unsymmetrical zinc phthalocyanine were confronted with the data of the symmetric zinc phthalocyanine.
Keywords: UV-vis absorption, in-situ light absorption, fluorescence quantum yield, fluorescence quantum lifetime, singlet oxygen generation, quantum chemical calculations Abbreviations: ZnPc – zinc phthalocyanine, Pc – phthalocyanine, bromocresol purple – BCP, laser induced optoacoustic spectroscopy – LIOAS, time-dependent density functional theory – TD-DFT
1
1. Introduction Unsymmetrical phthalocyanines (Pcs) belong to a huge family of porphyrins and they are highly versatile dyes that can find many practical applications in science and technology: in nanotechnology as optoelectronic materials, non-linear optical materials, sensors; in photodynamic diagnosis and therapy of cancer and other diseases; in biology in modeling photosynthesis, and many others [1–4]. Unsymmetrical Pcs are very unique species due to their high absorption coefficient, easily modified molecular structure and effective photoactivity, redox-active scaffolds for supramolecular design [5,6]. Moreover, because of Pc low production cost and easiness in construction these organic materials can replace the other ones as alternative to non-organic moieties [7]. Chemical synthesis procedure of the unsymmetrical Pcs under investigations was described and their basic studies were performed [8]. A variety of unsymmetrical Pcs of diverse molecular structures in different solvents [9] and in solid matrices have been investigated through physical/photophysical and chemical/photochemical experiments [10]. However, in this paper we would like to extend investigations previously done [8,11–13] and to examine an unsymmetrical Pc since there is still not very much known about its triplet states’ behavior and thermal deactivation. However, further studies seem to be necessary to get more information about their photophysical parameters important for photodynamic therapy of cancer (PDT). We put our attention on the photophysical properties of the unsymmetrical zinc Pc (ZnPc) the spectroscopic results obtained in this paper were compared to those of the symmetric zinc Pc. These studies included investigations of absorption (in the UV-vis range) and fluorescence (steady-state and time resolved) in non-polar solvent (chloroform (CHCl3). The dye concentrations were changed from 10-4 to 10-6 M. In our experiments CHCl3 was used because of its ability to very fast evaporation and low polarity (ε = 4.80). Moreover, examinations in CHCl3 are important for our future research of formation of Langmuir monolayers. However, we have to realize that the use of non-polar CHCl3 one can expect to have aggregated dye molecules at sufficient high concentration. Besides the UV-vis examinations, we also provided a laser-induced optoacoustic spectroscopy (LIOAS). The LIOAS technique was used for unsymmetrical Pcs for the first time and let to determine thermal deactivation as well as singlet oxygen generation. The experimental data were supported by the quantum chemical calculations with the use of the time-dependent density functional theory (TD-DFT) to obtain information on the distribution of electron density and electronic transitions in the molecular systems. The greatest advantage of unsymmetrical Pcs over symmetrical Pcs is lower ability to form aggregates. In our spectroscopic studies of the dyes in solutions aggregated forms of both symmetrical and unsymmetrical dyes have not revealed. Therefore we applied the in-situ absorption of a Langmuir layers experiment to expose the deference in aggregation ability. The distance between molecules in a Langmuir layer is much smaller than that in solution at 10-4 M concentration. The lower ability to form aggregates of unsymmetrical Pc, presented in the paper, was confirmed by the relatively small changes in the in-situ UV-Vis spectra with respect to the spectra collected for the dye in chloroform. 2
2. Materials and methods 2.1. Materials An unsymmetrical 9(10),16(17),23(24)-tri-tert-butyl-2-(pent-4-yloxy)phthalocyaninato zinc(II) (ZnPc_1) (concentration 10-4 – 10-6 M) was investigated and its properties were discussed versus those of a symmetric 2,9,16,23-tetra-tert-butyl-phthalocyaninato zinc(II) (ZnPc_2) that was used as a reference dye. The ZnPc_1 and the ZnPc_2 samples were obtained according to the procedure described previously [8] and purchased from SigmaAldrich, respectively. The molecular structures of the investigated Pcs are shown in Fig. 1. CHCl3 of spectroscopic grade was used as a solvent and it was purchased from POCH Poland S.A.
Fig. 1. Molecular structures of investigated ZnPc_1 and ZnPc_2. 2.2. UV-vis absorption in solvent and in Langmuir monolayer The ground state absorption spectra were monitored with an UV-vis Varian Cary 4000 spectrophotometer in a quartz cuvette over the range of 250-900 nm. Langmuir monolayers of the samples were created with a KSV 2000 minitrough (KSV Instruments Ltd.). The Langmuir trough area was 2325 mm2 (length 310 mm and width 75 mm). Ultrapure water (electrical resistivity 18.2 MΩ·cm) as a subphase was obtained with a Mili-Q water purification system (Millipore Corp.). The sample dissolved in CHCl3 (10-4 M) was spread carefully onto the subphase and CHCl3 evaporated in 15 min. The floating film of the Langmuir layer was compressed symmetrically from both sides with motion barrier speed of 5mm/min. The in-situ method was used to follow electronic absorption spectra of the Langmuir layers in the UV-vis range (250-900 nm) with an Ocean Optics spectrometer QE65000. 2.3. Fluorescence studies The fluorescence spectra and fluorescence excitation spectra were measured with a Hitachi F4500 fluorometer (excitation wavelength λexc= 320 nm and observation wavelength λobs=400 nm or 720 nm). The fluorescence quantum yields (ΦF) were determined using the classical formula [14]: 3
(1) where A is the absorbance at the excitation wavelength, F is the area under the fluorescence and n is the refraction index. The subscripts r and s refer to the reference and to the sample of unknown quantum yield, respectively. As a fluorescence reference ZnPc dissolved in dimethylformamide was used (Φr=0.17) [15]. Fluorescence lifetime signals were obtained using EasyLife™ V lifetime fluorimeter with a 1.5 ns pulse of LED as an excitation source (635 nm) and the sample fluorescence responses were collected over the range of 650 - 750 nm. Fluorescence lifetimes (τ) were estimated by fitting the decay curves using a deconvolution procedure implemented into DecayFit 1.4 – Fluorescence Decay Analysis software. All measurements were done at room temperature. 2.4. Quantum chemical calculations To improve the interpretation of the experimental UV-vis spectra we performed calculations of the transition energies via the time-dependent density functional theory (TD-DFT). The calculations were performed using the B3LYP hybrid functional (Becke 3-parameter exchange functional combined with Lee-Yang-Parr correlation functional) and the standard 631G(d,p) basis set. Prior to TD-DFT calculations the geometries of the molecules were optimized using DFT method with the same functional and basis set. For the equilibrium geometries the normal mode frequencies were calculated (no imaginary frequencies were found proving that we reach the energetic minimum) and compared with the experimental IR data. TD-DFT calculations were performed for isolated molecules and including the influence of CHCl3 using the Polarized Continuum Model (PCM) as implemented in the Gaussian 03 program package [16]. The first 200/100 (ZnPc_1/ZnPc_2) optical transitions were calculated. To convolute the resulting transition energies and oscillator strengths in the absorption spectra the GaussSum program was used [17]. The spectra were generated assuming FWHM (Full Width at Half Maximum) parameters at 3000 cm-1 for all transitions. 2.5. Infrared absorption studies The infrared absorption spectra (middle infrared region) of the investigated ZnPcs were recorded using a JASCO FT-IR 6200 Fourier transform spectrometer. The samples were dissolved in CHCl3 and deposited on a potassium bromide (KBr) plate. After solvent evaporation the spectra of a thin film on KBr were recorded in the transmittance mode using the clear KBr plate as a reference. 2.6. Laser induced optoacoustic studies To get information on dye photothermal parameters, the time-resolved photothermal signals were recorded by means of the LIOAS method. Technical specifications and more details of a LIOAS apparatus were described elsewhere [18,19]. For the LIOAS experiments bromocresol purple (BCP) Sigma-Aldrich was chosen as a calorimetric reference due to its suitable spectral properties. All measurements were carried out at ambient temperature and in air
4
atmosphere. The excitation wavelength was 337 nm; the absorbance of the dye was equal to 0.15 at the excitation wavelength. The analysis of the LIOAS waveform was carried out according to Marti et al. method [20]. In this approach, the first maximum (Hmax) of the LIOAS signal is expressed as: (2) where k is a proportionality factor that regards geometry and electric impedance of a device, αair means a part of energy changed into heat in air atmosphere (in time shorter than a time resolution of the apparatus), A is absorbance of a sample, and Ehν is the molar energy of incident photons 354.9 kJ·mol-1 at 337 nm. A part of the excitation energy exchanged into heat promptly (α parameter) was calculated by direct comparison of the slopes of the plots in the linear region obtained for the dye and for the reference dye in air [21]. The efficiency of singlet oxygen generation yield (ФΔ) can be evaluated from the following equation: (3) where ES is the molar energy of the singlet state and EΔ is the energy of the oxygen singlet state (EΔ = 94 kJ·mol-1) [21]. 3. Results and discussion 3.1. Electronic absorption experiments in solution The ground state normalized electronic absorption spectra of ZnPc_1 and ZnPc_2 in CHCl3 (concentration 10-4 – 10-6 M) are shown in Fig. 2; the spectra are normalized to unity at the highest Q band (679 nm). The absorption spectra are dominated by the π-π* transitions with an intense and narrow Q band and a much lower intensive Soret band. The main Soret bands with the left-side band are observed at 350 nm and 288 nm. The main Q band (679 nm) with two shorter-wavelength and tiny peaks (at 612 nm and about 650 nm) are seen. Such feature is characteristic for the family of Pc dyes.
Fig. 2. UV-vis absorption spectra of unsymmetrical ZnPc_1 and its analogue ZnPc_2; 5
(ZnPc_2 spectrum on the basis of [22]). From the absorption results one can see that sample concentration only slightly affected the absorption spectra. The spectra shapes were almost overlapped one by each other. Moreover, the band intensity ratios and their locations were not changed upon dyes increasing concentration. However, on the basis of behavior of the natural pigments and of synthetic porphin-like molecules [23] one can suppose the presence of poorly defined molecular aggregates. The highly concentrated ZnPc_1 in non-polar CHCl3 did not show notable changes when compared to the ZnPc_2 sample. The only differences in the absorption spectra can be found in the values of the relative intensities at 679 nm versus 350 nm. The selected absorption parameters of the dyes are collected in Table 1. Usually, a solvent condition may cause π-π* dyes associated into dimers (J, I, H, oblique coplanar dimers or other kinds [24]) which can be formed via direct π-π* interaction or indirect interaction between metal in the dye core of one molecule and an electron donating group of another molecule at adequately high concentration. Thus, on the basis of the results of other synthetic porphyrin-like molecules [25–27] one can suppose the presence of molecular aggregates. No particular changes in the absorption spectra observed at different concentrations could point out the dominance of the dye monomeric forms up to 10-4 M. However, a lack of changes in an absorption band shift does not exclude creation of an aggregate at very high dye concentration in non-polar solvent; the changes in intensity ratios in the spectral region 679 nm versus 350 nm can indicate the presence of aggregates (Table 1). Thus, on the basis of our absorption observation we cannot neglect the presence of a sort of aggregates in a highly concentrated sample. Table 1 Selected absorption parameters of ZnPc_1 and ZnPc_2 (a in CHCl3, b in Langmuir monolayer at 40 mN/m) Absorption Absorption Absorption Absorption Dye maxima band intensity maxima band intensity a a b [nm] ratio [nm] ratiob 679 690 0.16 (612/679) 0.57 (625-645/690) 612 625-645 ZnPc_1 0.39 (350/679) 1.17 (340/690) 350 340 679 682 0.16 (612/679) 0.92 (625-645/682) 612 625-645 ZnPc_2 0.34 (350/679) 1.41 (333/682) 350 333 To clearly decide on ability of the dyes to aggregate formation we applied an in-situ light absorption method to investigate the dyes in a Langmuir monolayer. The results are shown in Fig. 3 A and B for both ZnPc_1 and ZnPc_2 (the last one as a reference sample). It is worth to underline that the in-situ experiments at different surface pressure were done for the unsymmetrical Pcs for the first time. The first observation was the very different in-situ behavior of ZnPc_1 and ZnPc_2 in the Langmuir monolayer (see Table 1). The different shapes of the spectra of ZnPc_1 and 6
ZnPc_2 and relation in the bands’ intensities (682, 690 nm versus 625-645 nm) can origin from dissimilarity between the peripheral groups that affect the different configuration of the dipole transitions in ZnPc_1 and ZnPc_2. The presence of the intense 625-645 nm band confirms the aggregate formation [28]. Dimerization of metallic Pcs was also studied by Raman spectroscopy [29] and by atomic force microscopy [30]. In the Q band region two very intense bands were well seen: the first one at 690 nm (versus the band at 679 nm in CHCl3) and it can be assigned to a dye dimeric form. The second confirmation of the presence of aggregated species is the appearance of the intensive broad bands at about 625-645 nm. The location of these bands is in accordance with the results obtained for symmetric Pc dye and presented in one of our papers [22]. These bands originate from the formation of the oblique coplanar aggregate and/or of the I type aggregate [24]. The aggregates coexist with the monomeric dyes. In general, in the Langmuir layer the distance between dye molecules is much shorter than that in 10-4 M solution and thanks to high package of dye molecules in the layer the molecules are able to create aggregates. Thus, from our in-situ results we have every reason to believe in creation of aggregates. Moreover, the insitu spectra of ZnPc_1 are similar to the spectra monitored for the dye in solution. Otherwise, the in-situ spectra of ZnPc_2 are different when compare to those in solution. It is noticeable that the absorption band intensity ratio (Table 1) – the values of the ratio calculated from the in-situ and in solution spectra of ZnPc_1 are more similar than those of ZnPc_2. This clearly indicates lower ability to aggregate formation of unsymmetrical ZnPc_1.
Fig. 3. In-situ light absorption spectra of ZnPc_1 (A) and ZnPc_2 (B) in Langmuir monolayer at different surface pressure. Fig. 4A shows the π-A isotherms of the dyes in monolayers. The dyes’ isotherms depend on the dye molecular structures – ZnPc_1 has one pent-4-ynoloxy and three tert-butyl groups 7
whereas ZnPc_2 has four tert-butyl groups. The presence of the different groups and asymmetry in the structure lead to a different molecular arrangement on the aqueous subphase. The main molecular core of the dyes under studies is the porphyrazine cycle-ring. On the basis of the π-A isotherm result we evaluated thermodynamic and molecular arrangement parameters of the dyes Langmuir layers. The values are collected in Table 2. The average area per molecule was evaluated with an extrapolation of the π-A isotherms. As well seen from Table 2 the values of parameters differ markedly between both dyes. On the basis of the literature data, the Pc geometrical size values are 1.2-1.9 nm2 (depending on dyes and substituted groups) and about 1 nm2 of the Pc skeleton [31]. The area of the dye skeletons was estimated taking advantage of the length of bonds calculated by the DFT method with the assumption that the external groups are stiffly united to the Pc skeleton. The results of our πA isotherm experiments let us to estimate a tilt angle between the molecular skeleton and the normal to water subphase. The tilt angles of the ZnPc_1 and ZnPc_2 skeleton are: 14 deg. and 24 deg., respectively. The difference between the tilt values explicitly indicates unlike molecular arrangements of the two dyes in the monolayer - ZnPc_1 is nearly oriented perpendicularly and ZnPc_2 less perpendicularly with respect to water subphase. The different angle values result from the presence of the hydrophobic group in ZnPc_1. Moreover, creation of the different types of aggregates and their different monolayer compressibility also affect the dye distribution.
Fig. 4. The π-A isotherms of ZnPc_1 (black lines) and ZnPc_2 (red lines) monolayers at room temperature (A) and compressibility (B). The angle values obtained for our samples are insignificantly different from those evaluated for similar Pcs [31]. However, taking into consideration the possibility of the external group free bending relative to the Pc ring plane and assuming 1 nm2 molecule skeleton surfaces, the tilt angles of the ZnPc_1 or ZnPc_2 skeleton are 40 deg. and 54 deg., respectively - this 8
result is similar to that obtained for similar Pcs [31]. It is obvious that the external groups have an effect on the arrangement of molecules in the layer [32,33], therefore the real tilt angles between the molecular skeleton and the normal to the water subphase has possible values between 14-40 deg. for ZnPc_1 and 24-54 deg. for ZnPc_2. We have also done investigations of Langmuir layer compressibility (Fig. 4 B); the 1 1 compressibility modules C S A(d / dA) [34,35] are collected in Table 2. The C S values higher than 200 mN/m indicate creation of the 2D solid. However, it is not the case for our samples. The compressibility parameter estimated for ZnPc_1 is 66 mN/m and indicates creation of the monolayer liquid state, whereas for ZnPc_2 it is 97 mN/m and confirms creation of the monolayer of properties between liquid and condense liquid phase. The compressibility results acknowledged the various orientation of the molecular plane with regard to the water surface. The differentiation in the module values shows various interactions of the Pcs. Moreover, the results confirmed the distinct thermodynamic properties of the dyes due to the presence of the different groups linked to the main molecular cores and asymmetry of the molecular structure. Table 2 Thermodynamic and molecular arrangement parameters of ZnPc_1 and ZnPc_2 (Aavr – average area per molecule; Ac – area at the collapse point; πc – collapse point; At – theoretical area of molecule; δa, δb – tilt angle under the assumption stiffly united external group and under the assumption of 1 nm2 surfaces of the molecule skeleton, respectively) Dye
Aavr [nm2]
Ac [nm2]
πc [mN/m]
At [nm2]
δa [deg.]
δb [deg.]
Cs-1 [mN/m]
ZnPc_1
0.64
0.27
39
2.48
14
40
66 (at π = 18 mN m-1)
ZnPc_2
0.81
0.59
30
1.89
24
54
97 (at π = 21 mN m-1)
3.2. Fluorescence examinations To follow the dye singlet state of ZnPc_1 and ZnPc_2 the emission and excitation spectra of the dyes (5×10-6 M) in CHCl3 were done and the results are shown in Figs 5 and 6. Using ZnPc_2 as a reference dye allowed us to observe the effect of the presence of the pent-4ynoloxy group on fluorescence. The fluorescence spectra of ZnPc_1 and ZnPc_2 differ markedly in their intensity and location of the maxima (fluorescence measurements were done at the same experimental conditions). The main band of the spectra located at 684 nm showed strong fluorescence and similar behavior were also observed for other Pcs [36–39]. The dominant fluorescence peak can be assigned to emission of the dye monomer. The much less intense shorter wavelength band was also observed at 397 nm but only for ZnPc_1, the dye with the pent-4-ynoloxy group. The origin of the ‘violet band’ in symmetric Pcs was widely discussed by Kaneko et al. [40] and by Chahraoui et al. [41] - the origin of the short wavelength emission was indicated as resulting from the intrinsic lowest singlet excited state that was formed by the red photons [40]. However, in our experiments we used the 320 nm light as excitation and we have also observed the 397 nm band in ZnPc_1 but not in the 9
symmetric ZnPc_2. Thus we have every reason to believe in a great influence of the pent-4ynoloxy group on the dye fluorescence behavior.
Fig. 5. Fluorescence spectra of ZnPc_1 (black line) and ZnPc_2 (red line) in CHCl3; insert – magnification of the spectra in the range 330-550 nm; λexc = 320 nm.
Fig. 6. Excitation emission spectra of ZnPc_1 (black line) and ZnPc_2 (red line) in CHCl3; main figure λobs = 720 nm, insert λobs = 400 nm. The Q bands in the fluorescence excitation spectra were consistent with the longwavelength UV-vis absorption spectra. The same locations of the 678 nm, 612 nm bands and of a hump at 640-650 nm were very well seen. The excitation spectra at 400 nm observation could confirm ‘violet fluorescence’ of ZnPc_1 which originate from absorption of 246 and 325 nm irradiation. On the other hand Mack et al. [42] have provided evidence that ‘violet emission’ may not be intrinsic to the molecule, but it can be the result of a photodegradation product. To validate these observations the absorption and emission spectra during ZnPc_1 irradiation have been presented in Fig. S1. In our experiment the ‘violet emission’ band was constant while irradiated. Furthermore, the intensities of the B and Q absorption bands and the S1 emission are changed significantly over the irradiation time. If the ‘violet emission’ (Fig. 5) is an intrinsic property of ZnPc_1 and hence it is a photophysical process with a fixed and reproducible quantum yield value, the observed ‘violet emission’ intensity can be expected to follow the same trend as observed in the S1 emission (panels B and D in Fig. S1). Moreover, the values of the B and Q absorption band intensities (shown in Fig. S1 A and C, respectively) 10
remain constant. Additional, changes in the shape of the spectra were not observed, as it was stated by Mack et al. [42]. The ΦF, and τ were also estimated. The τ values were evaluated taking in consideration the fluorescence kinetics presented in Fig. 7. The τ values are in accordance with those of Pc dyes. Evaluation of the τ values was done with two approaches: mono-exponential and double-exponential deconvolution. The best fitting results were obtained when monoexponential procedure was applied, and the estimated values are gathered in Table 3. Table 3 Parameters obtained on the basis of analysis of steady state and time resolved fluorescence of ZnPc_1 and ZnPc_2 in chloroform (5×10-5 M). Dye
τ [ns]
χ2
ZnPc_1 ZnPc_2
3.21 3.23
1.09 1.02
ΦF (±0.05) 0.15 0.16
Fig. 7. Fluorescence decay curves of ZnPc_1 (black curve) and ZnPc_2 (red curve) ; λexc = 635 nm. 3.3. Quantum chemical calculations In order to improve interpretation of the experimental results we have performed the DFT calculations of the investigated Pcs. The geometry of both molecules have been optimized and normal modes vibrations have been calculated. Additionally, two conformations of the ZnPc_1 have been optimized (Fig. S2, Supplementary Information). The lower total energy value (-4187.4890 versus -4187.4881 hartree) suggests that conformer 1 is preferred and it was used in the further investigations. The normal modes frequencies have been compared with the experimental results. The representative spectral region is displayed in Fig. S3. The contour plots of the frontier molecular orbitals of both Pcs are presented in Fig 10. The energy of LUMO is identical in both molecules (-2.58 eV) while the energy of HOMO is slightly different (-4.73 eV and -4.76 eV for ZnPc_1 and ZnPc_2, respectively) so the HOMO-LUMO energy gap is 2.15 eV and 2.18 eV for ZnPc_1 and ZnPc_2, respectively. The values for ZnPc_2 are comparable with those for symmetrical ZnPc obtained by Ueno and co-workers [43,44]. There is no change in the shape of the mentioned orbitals after including the influence of solvent. However, the energy of the HOMO and LUMO in the 11
solvent is slightly reduced (HOMO= -4.82 and -4.87 eV for ZnPc_1 and ZnPc_2, respectively, and LUMO= -2.69 eV for both molecules). The resulting HOMO-LUMO energy gap remains the same for ZnPc_2 (2.18 eV) but is slightly decreased for ZnPc_1 (2.12 eV).
Fig. 8. The contour plots of the frontier molecular orbitals of ZnPc_1 (left side) and ZnPc_2 (right side). The calculated UV-Vis absorption spectra of both dyes with and without the presence of solvent are gathered in Fig. 11. The differences between the results obtained for ZnPc_1 and ZnPc_2 are rather subtle – a small shift in the Soret and Q regions and some additional transition above 400 nm in the spectrum of ZnPc_1 is found. These differences are also seen in the spectra calculated including the influence of solvent. In general, including the solvent causes the bands in the calculated spectrum are in better agreement with the experimental results (Fig.2). The relation between the spectra is similar to the results obtained by Ueno et al. [43]. Additional absorption in the spectrum of ZnPc_1 mentioned above is related mainly to the transition HOMO-1→LUMO+1 so it involves the altered group in the ZnPc_1 (see Fig. S4). The details of the most intense calculated transitions are gathered in Table S1 (Supplementary Material).
12
Fig. 9. The TD-DFT calculated UV-vis spectra of ZnPc_1 (black line without and grey line with the presence of solvent) and ZnPc_2 (red line and magenta line). 3.4. Triplet state studies Fig. 10 illustrates the relation between the Hmax of the sample ZnPc_1 and ZnPC_2 in CHCl3 as a function of excitation energy (Ehν).
Fig. 10. LIOAS signal Hmax of ZnPc_1 (red circles), ZnPc_2 (blue up triangles) and bromocresol purple (BCP) (black squares) in chloroform as a function of the laser energy Ehν; λexc = 337 nm; R2 ≥0.99. The αair values were 0.76±0.02 and 0.78±0.02 for ZnPc_1 and ZnPc_2, respectively (evaluated on the base of the relationships shown in Fig. 10). The presented results indicated that the αair values were not dependent on the ZnPcs substituents (within the experimental error). The ΦΔ values calculated (eq. 3) for ZnPc_1 and ZnPc_2 were 0.63±0.03 and 0.53±0.03, respectively. Slightly higher value was observed for unsymmetrically substituted ZnPc. The process that is responsible for singlet oxygen formation is the interaction between the dye excited triplet and oxygen. The obtained results indicated the modest efficient generation of singlet oxygen - ZnPc is limited due to high amount of energy change into heat (αair). The lower values of ΦΔ 0.47 were reported previously for unsymmetrical Pc bearing two 1-adamantylsulfanyl groups at adjacent peripheral positions in DMF and DMSO [45]. Novakova et al. reported ΦΔ lower than 0.47 for unsymmetrically substituted Mg2+ and Zn2+ Pcs in THF [46]. Dinçer et al. postulated potential application of polymer conjugated ZnPcs 13
as photosensitizer in PDT for which ΦΔ were 0.01 and 0.14 for symmetrical and asymmetrical dyes, respectively in aqueous solution [12]. 4. Conclusions The experimental spectroscopic results of the investigations of unsymmetrical phthalocyanine substituted with terminal alkynyl group in chloroform supported by the chemical calculations let us to draw very important conclusions on the properties of the singlet and triplet states of the dye and to confront the properties of terminal alkynyl-substituted phthalocyanine ZnPc_1 and ZnPc_2. The spectroscopic results presented in the paper can indicate rather weak influence of the asymmetry on the singlet state of ZnPc_1 as evidenced by the similar values of fluorescence quantum yields and of the life time values to those of the symmetric ZnPc_2 (τ 3.21 vs. 3.23; ΦF 0.15 vs. 0.16). However, a great diversity in fluorescence of ZnPc_1 and ZnPc_2 is found in the shortwavelength excited singlet state and it can originate from the intrinsic lowest singlet excited state as discussed in other papers. Moreover, we also showed weak ability of unsymmetrical Pc to formation of aggregates at sufficiently concentrated chloroform solution. However, the ability of the unsymmetrical dye to aggregate creation rises in the Langmuir nanolayers of high packaged molecules. It is possible due to much stronger dye dipole-dipole interaction of concentrated dyes and as a consequence oblique coplanar aggregates and/or of the I type aggregates can be formed. In general, in the Langmuir layer the distance between dye molecules is much shorter than that in 10-4 M solution and thanks to high package of dye molecules in the layer the molecules are able to create aggregates. Thermodynamic experiments done in the Langmuir layers showed that the value of the compressibility parameter indicates creation of the ZnPc_1 monolayer liquid state, and in contrast for ZnPc_2 the parameter confirms creation of the monolayer liquid-condense liquid phase. The compressibility results showed various interactions of ZnPc_1 and ZnPc_2 leading to diverse orientation of the molecular plane with regard to the water surface. Thus, the results confirmed the distinct thermodynamic properties of the dyes due to the presence of the different groups linked to the main molecular cores and asymmetry of the molecular structure. It was also shown that a large amount of energy is deactivated in a form of heat and it was not dependent on the ZnPcs substituents. However, the non-radiative process is able to affect oxygen generation. The slightly higher value of oxygen generation was observed for unsymmetrically substituted dye. Acknowledgements Presented work has been financed by the Ministry of Science & Higher Education in Poland in 2015 year under Project No 06/62/DSMK/0196 (KK, BB). MK, BaB, JG, DW thanks to Poznan University of Technology, grant DS PB 62/062/0216 and HD thanks to The Scientific & Technological Research Counsil of Turkey (TUBITAK) (project No 111T063) for financial support. 14
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