Photophysical , Photochemical , and BQ Quenching Properties of Zinc Phthalocyanines with Fused or Interrupted Extended Conjugation

The effects of substituents and solvents on the photophysical and photochemical parameters of zinc(II) phthalocyanines containing four Schiff ’s base substituents attached directly and through phenyleneoxy-bridges on peripheral positions are reported. The group effects on peripheral position and the continual and intermittent conjugation of the phthalocyanine molecules on the photophysical and photochemical properties are also investigated. General trends are described for photodegradation, singlet oxygen, and fluorescence quantum yields of these compounds in dimethylsulfoxide (DMSO), dimethylformamide (DMF), and tetrahydrofurane (THF). Among the different substituents, phthalocyanines with cinnamaldimine moieties (1c and 2c) have the highest singlet oxygen quantum yields (Φ Δ ) and those with nitro groups (1a and 2a) have the highest fluorescence quantum yields in all the solvents used. The fluorescence of the substituted zinc(II) phthalocyanine complexes is effectively quenched by 1,4-benzoquinone (BQ) in these solvents.


Introduction
Phthalocyanines (Pcs) are remarkable macrocyclic compounds having magnificent physical and chemical properties [1].Metallophthalocyanines (MPc) have been investigated in detail for many years due to their high chemical and thermal stability, high degree of aromaticity, and synthetic flexibility [2,3].They have also found different applications in many fields ranging from industrial [4], technological [5,6] to medical [7,8].Metallophthalocyanine (MPc) derivatives are photoactive and may be employed in photosensitization when the central metal ion is diamagnetic or a nontransition metal element [9,10].In this case, it is worth emphasizing the Pcs' application as photosensitizers in the photodynamic therapy (PDT) of tumours.MPcs could lead them to be more efficient sensitizers owing to their high triplet state quantum yields and long triplet lifetimes.The photophysical properties of the Pc dyes are based on the presence and nature of the central metal ion.MPcs with paramagnetic metal centers produce low cytotoxic singlet oxygen.Closed shell and diamagnetic ions, like Zn 2+ , Ga 3+ and Si 4+ , play important role in Pc complexes and bring excellent properties such as high singlet oxygen generation which is very important for PDT efficiency of photosensitizes [11][12][13].ZnPcs have been widely studied because of closed shell d 10 configuration of the central Zn 2+ ion; they cause optical spectra that are not complicated by additional bands, as in partially filled transition-metal Pc complexes.Having intensive absorption in the low energy side (red) of visible region, high triplet yields, and efficient singlet oxygen generation makes ZnPcs valuable photosensitizers for PDT applications [14][15][16][17].The general synthesis of organic solvent soluble substituted phthalocyanines is now well established.Pc complexes containing S, N, or O containing substituents are known [18][19][20].However the study of the photochemical and photophysical properties of such complexes is still very limited.

Equipment.
Absorption spectra in the UV-visible region were recorded with a Shimadzu 2001 UV spectrophotometer.Fluorescence excitation and emission spectra were recorded on a Varian Eclipse spectrofluorometer using 1 cm path length cuvettes at room temperature.Photoirradiations were done using a General Electric quartz line lamp (300 W).A 600 nm glass cut-off filter (Schott) and a water filter were used to filter off ultraviolet and infrared radiations, respectively.An interference filter (Intor, 670 nm with a band width of 40 nm) was additionally placed in the light path before the sample.Light intensities were measured with a POWER MAX5100 (Molectron detector incorporated) power meter.

Photophysical Parameters
2.3.1.Fluorescence Quantum Yields.Fluorescence quantum yields (Φ  ) were determined by the comparative method (1) [29,30], as follows: where  and  Std are the areas under the fluorescence emission curves of the samples (1a-1d and 2a-2d) and the standard, respectively. and  Std are the respective absorbances of the samples and standard at the excitation wavelengths, respectively. and  Std are the refractive indices of solvents used for the sample and standard, respectively.Unsubstituted ZnPc (in DMSO) (Φ  = 0.20) [28] was employed as the standard.The absorbance of the solutions at the excitation wavelength ranged between 0.04 and 0.05.

Fluorescence Quenching by Benzoquinone (BQ).
Fluorescence quenching experiments on the substituted zinc phthalocyanine derivatives (1a-1d and 2a-2d) were carried out by the addition of different concentrations of BQ to a fixed concentration of the complexes, and the concentrations of BQ in the resulting mixtures were 0, 0.008, 0.016, 0.024, 0.032, and 0.040 mol dm −3 .The fluorescence spectra of substituted zinc phthalocyanine derivatives (1a-1d and 2a-2d) at each BQ concentration were recorded, and the changes in fluorescence intensity related to BQ concentration by the Stern-Volmer (S-V) equation [31] were shown in (2): where  0 and  are the fluorescence intensities of fluorophore in the absence and presence of quencher, respectively.The ratios of  0 / were calculated and plotted against [BQ] according to (2), and  SV is determined from the slope.

Photochemical Parameters
2.4.1.Singlet Oxygen Quantum Yields.Singlet oxygen quantum yield (Φ Δ ) determinations were carried out using the experimental setup described in the literature [32][33][34].Typically, a 3 mL portion of the phthalocyanine derivatives, (absorbance ∼1 at the irradiation wavelength) containing the singlet oxygen quencher, was irradiated in the Q band region with the photo-irradiation setup described in references [32][33][34].Singlet oxygen quantum yields (Φ Δ ) were determined in air using the relative method with ZnPc (in DMSO, DMF, and THF) as a reference.DPBF was used as chemical quencher for singlet oxygen in DMSO, DMF, and THF.Equation (3) was employed for the following calculations: where Φ Std Δ is the singlet oxygen quantum yields for the standard unsubstituted ZnPc (Φ Std Δ = 0.67 in DMSO [35], Φ Std Δ = 0.56 in DMF [36], and Φ Std Δ = 0.53 in THF [27]). and  Std are the DPBF photobleaching rates in the presence of the respective samples (1a-1d and 2a-2d) and standard, respectively. abs and  Std abs are the rates of light absorption by the samples (1a-1d and 2a-2d) and standard, respectively.To avoid chain reactions induced by DPBF in the presence of singlet oxygen [36], the concentration of quencher (DPBF) was lowered to ∼3.10 −5 M. Solutions of sensitizer ( = 1 × 10 −5 M) containing DPBF were prepared in the dark and irradiated in the Q band region using the photo-irradiation setup.DPBF degradation at 417 nm was monitored.The light intensity 6.75 × 10 15 photons s −1 ⋅cm −2 was used for Φ Δ determinations.

Photodegradation Quantum Yields.
Photodegradation quantum yield (Φ  ) determinations were carried out using the experimental setup described in the literature [32][33][34].Photodegradation quantum yields were determined using (4), as follows: where  0 and   are the samples (1a-1d and 2a-2d) concentrations before and after irradiation, respectively,  is the reaction volume,   is the Avogadro's constant,  is the irradiated cell area,  is the irradiation time, and  abs is the overlap integral of the radiation source light intensity and the absorption of the samples (1a-1d and 2a-2d).A light intensity of 2.24 × 10 16 photons s −1 cm −2 was employed for Φ  determinations.

Ground State Electronic Absorption and Fluorescence Spectra.
The UV-Vis spectra of the phthalocyanine complexes exhibits characteristic Q-and B-bands.Two-principle - * transitions are seen for phthalocyanines: a Q-band (∼700 nm, a - * transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the complexes) and a B-band (∼300-350 nm, a deeper - * transition from the HOMO's (a 1u and a 2u ) to the LUMO (e g )) [37,38].The Q-band absorptions in the UV-Vis absorption spectra of the phthalocyanines (1a-1d and 2b-2d) were observed as a single high intensity band due to a - * transition at around 677-686 and 685-720 nm in DMSO, 674-682 and 685-714 nm in DMF, and 672-679 [22] and 683-711 nm in THF, respectively, Table 1 and Figure 1.The other bands (B) in the UV region at 348-363 and 349-363 nm in DMSO, 350-378 and 338-368 nm in DMF and 348-383 [22] and 349-357 nm in THF were observed due to the transitions from the deeper  levels to the LUMO [39,40].Aggregation behavior of Pc is depicted as a coplanar association of rings progressing from monomer to dimer and higher order complexes and it is dependent on concentration, nature of solvent and substituents, metal ions, and temperature [41].Aggregation is not desired in MPc complexes since aggregates are generally photoinactive.Aggregation is more enhanced for peripherally substituted MPc complexes when compared to nonperipherally substituted ones.When aggregation occurs, a band in the region of 630-645 nm is observed in the electronic absorption spectrum of MPcs as a result of the intramolecular interactions between the Pc units.Thus, the band at around 640 nm in DMSO for 2a can be attributed to aggregation.Normally, DMSO is known to prevent aggregation since it is strong coordinating solvent.However, it has been observed before that aggregation occurs in DMSO for some MPc derivatives [42].In the case of 2a, where strong electron withdrawing nitro groups are placed as substituents, aggregation occurs in polar solvents.Depending on the polarity of the solvent, aggregated species are more dominant in DMSO, at appreciable level in DMF and hardly present in THF (Figure 2).In DMF, DMSO, and THF, the Q band positions of 1b are red-shifted relative to those of 1a with the same red-shifting being observed in all the solvents.The red shift as a consequence of the presence of amino groups on MPc spectra is well known [22,23].
The flourescence behavior of the phthalocyanine complexes (1a-1d and 2a-2d) was studied in different solvents.Figure 2 shows fluorescence emission, absorption, and excitation spectra of complexes 1a and 2a in DMSO, DMF, and THF as examples of the studied zinc Pc complexes.Fluorescence emission and excitation peaks are listed in Table 1.The excitation spectra were similar to absorption spectra and both were mirror images of the fluorescent spectra for complexes 1a-1d.The shapes of the excitation spectra of studied compounds were similar to their absorption spectra.This proximity of the wavelength of the Q band absorption to the Q band maxima of the excitation spectrum for phthalocyanine compounds suggests that the nuclear configurations of the ground and excited states are similar and not affected by excitation in Figure 2 even though the absorption and fluorescence excitation spectra are not completely similar for 2a-2d due to conjugation effect of e Reference [27].
these groups and aggregation.It is evident from Figure 3 that the emission peaks of the two species (1a-1d and 2a-2d) are shifted to longer wavelength as a result of conversion of -NO 2 groups (1a and 2a) into -NH 2 on phenoxy groups (1b and 2b) in all solvents used.Spectral changes appearing as a result of enhanced conjugation have been clearly shown when the emission spectra of Pcs (2a-2d) are compared with those of analogous molecules (1a-1d) where identical substituents are attached not directly the Pc core but through phenylene-oxy bridges.As expected, emission bands of Pcs with conjugated cinnamaldimine groups show bathochromic shifts (Table 1).The observed Stokes shifts were within the region (∼5-35 nm) observed for zinc Pc complexes (1a-1d and 2a-2d).

Photophysical Studies 3.2.1. Fluorescence Quantum Yields.
The fluorescence quantum yields (Φ  ) of zinc phthalocyanines (1a-1d and 2a-2d) were studied in DMSO, DMF, and THF.Fluorescence is strongly influenced by factors such as solvent polarity, viscosity, refractive index, temperature, and molecular structural features.Fluorescence quantum yields (Φ  ) were calculated in all solvents as shown in Table 2.
While fluorescence quantum yield (Φ  ) values for 1a-1d and 2a-2d in DMSO, DMF, and THF are lower than those for unsubstituted ZnPc (Φ  = 0.20 in DMSO [28], Φ  = 0.17 in DMF [25], and Φ  = 0.25 in THF [27], Table 2).This implies that the presence of the substituents (Scheme 1) caused low fluorescence of the parents ZnPc in DMSO, DMF, and THF.The continual conjugation of the substituents on the peripheral position of ZnPc increases electron density from Pc rings to substituents on peripheral position.
Higher Φ  values were observed for the phthalocyanines including intermittent conjugation compared to the phthalocyanines including continual conjugation in DMSO, DMF, and THF except 1d and 2d in DMF and THF.Quaternized  3).Singlet oxygen quantum yields were studied in DMSO, DMF, and THF (for 1a-1d and 2a-2d) using a chemical method (using DPBF as a singlet oxygen quencher).Figure 4 shows spectral changes observed during photolysis of complexes 1b and 2b in THF in the presence of DPBF as an example.The disappearance of DPBF was monitored using UV-Vis spectral changes.There were no changes in the Q band intensities during the Φ Δ determinations, confirming that complexes are not degraded during singlet oxygen studies [43].The complexes (1a-d and 2a-d) in this work were selected because they bore similar substituents; however they differed in their extended interrupted conjugation with phenoxy-bridges and continual conjugation bond linkages.We investigated the effect of this difference on the photophysicochemical properties.Phthalocyanines with fused extended conjugation (2a-2d) have lower singlet oxygen quantum yield when compared to phthalocyanines with interrupted extended conjugation (1a-1d) due to conjugation effect.Interrupted extended conjugation quenched singlet oxygen was generated.When the nature effect of groups on singlet oxygen quantum yields was investigated Schiff base groups (1c and 2c) showed the highest values and quaternized groups (1d and 2d) showed the lowest values in all solvents studied.The values of Φ Δ are higher for ZnPc complexes including intermittent conjugation when compared to ZnPc complexes including continual conjugation in all studied solvents.The Φ Δ values of complex 1c were higher than all the substituted complexes (1a-1d and 2a-2d) in DMSO, DMF and THF (Table 2).The values of Φ Δ for 1c (0.58 in DMF, and 0.85 in THF) and for 2c (0.62 in THF) are higher when compared to unsubstituted ZnPc in DMF (0.56 [26]) and THF (0.53 [44]), Table 2.The values of Φ Δ were the highest for 1a-1d in DMSO except 1c in THF.

Photodegradation Studies.
Photodegradation is a process where a phthalocyanine is degraded under light irradiation.It can be used to determinate MPcs stability and this is especially important for those molecules intended for use as photocatalysts.The photobleaching stabilities of studied zinc phthalocyanine complexes (1a-1d and 2a-2d) were determined in DMSO, DMF and THF by monitoring the decrease in the intensity of the Q band under irradiation with increasing time.The photodegradation quantum yield (Φ  ) values for the complexes are given in Table 2.All the complexes showed about the same stability with Φ  of the order of 10 −5 -10 −4 .These values show that the molecules are of moderate stability in all solvents used.Stable ZnPc molecules show values as low as 10 −6 and for unstable molecules, values of the order 10 −3 have been reported [45].The spectral changes observed for all the complexes (1a-1d and 2a-2d) during irradiation confirmed that photodegradation occurred without phototransformation (Figure 5).Φ  values of all complexes 1a-1d are higher in DMF than in DMSO and THF, corresponding to the low Φ Δ values in the former solvent, consistent with the notion that photodegradation is singlet oxygen mediated process.Complex 1a is more stable than complexes 1b-1d while complex 2a showed the highest stability in DMSO (Table 2).Photobleaching quantum yield values of complexes 2a-2d are lower than complexes 1a-1d in both DMSO, DMF, and THF and hence complexes 2a-2d are more stable than complexes 1a-1d in the same solvents.In general photobleaching quantum yield values of phthalocyanines complexes are lower in DMSO than DMF.This trend was observed our complexes 1a-1d and 2a, but other complexes 2b-2d.These complexes showed unexpected trend [46,47].

Fluorescence Quenching Studies by Benzoquinone.
The fluorescence quenching of zinc phthalocyanine complexes (1a-1d and 2a-2d) by benzoquinone (BQ) in DMSO, DMF, and THF was found to obey Stern-Volmer kinetics, which is consistent with diffusion-controlled bimolecular reactions.Figure 6 shows the quenching of complex 1d and 2d by BQ in DMSO as an example.The slope of the plots shown in Figure 7 gave  SV values of 1a-1d in THF as an example, listed in Table 3.The Stern-Volmer plots for studied complexes (1a-1d and 2a-2d) gave straight lines, depicting diffusioncontrolled quenching mechanisms (Figure 7).Quinones have high electron affinities and their involvement in electron transfer processes is well documented [48].The energy of the lowest excited state for quinones is greater than the energy of the excited singlet state of MPc complexes [49]; hence, energy transfer from the excited MPc to BQ is not likely Scheme 1: Synthesis of zinc complexes.1a-1d and 2a-2d ((i) Pd/C, hydrazine hydrate, dioxane, and reflux; (ii) 4-(dimethylamino) cinnamaldehyde, THF, and reflux; (iii) CH 3 I, CHCl 3 , and reflux) [22,23].
THF > DMF > DMSO except 2d in DMSO, respectively.In different solvents, the  SV values for BQ quenching of phthalocyanine complexes vary directly with the solvents' polarity.

Conclusion
This work has described the spectral and photophysicochemical properties of Zn(II) phthalocyanines with fused or interrupted extended conjugation.The photophysical and photochemical properties of the phthalocyanines (1a-1d and 2a-2d) were investigated in DMSO, DMF, and THF.The effect of the nature of substituent and solvents on the photophysical and photochemical parameters of the phthalocyanine complexes (1a-1d and 2a-2d) are also reported in this study.The fluorescence quantum yields (Φ  ) values of complex 1a were higher than all the substituted complexes (1b-1d and 2a-2d) in DMSO, DMF, and THF.The highest singlet oxygen quantum yield values were obtained for only 1c and 2c among the all complexes synthesized in DMF, DMSO, and THF.These properties give an indication of the potential of the complexes as photosensitizers in applications where singlet oxygen is required (type II mechanism), especially complexes 1c and 2c may be good candidate as a photosensitizer for PDT application.

Figure 5 :Figure 6 :
Figure 5: The photodegradation of compound 1d (a) in DMF showing the disappearance of the Q band at 5 min intervals.

Table 2 :
Photophysical and photochemical properties of 1a-1d and 2a-2d in DMSO, DMF, and THF. ) of quaternized phthalocyanines (1d and 2d) has the lowest values than the other complexes.Quaternized group quenched fluorescence quantum yield due to group effects.Comparing Φ  values of complexes among the three solvents, complex 1a has the highest fluorescence quantum yield Φ  values in DMSO, DMF, and THF.Complexes 1d and 2d in DMSO, 1d in DMF, and 1c and 2b in THF (Table2) have the lowest values, respectively.The trend was reversed in DMSO, DMF, and THF Φ  values were obtained for the Pc derivatives.
[27]ference[25].cReference[26].dReference[27].group 2d has higher Φ  values than 1d partners due to showing less aggregation tendency.The zinc phthalocyanine complexes bearing nitro group (1a and 2a) are more fluorescent than other studied phthalocyanine complexes in all solvents used due to the nature of the electron withdrawing nitro group.The zinc phthalocyanine complexes (1b and 2b) showed that lowest emission could be as a result of converted NO 2 group into NH 2 on the Pc ring.Complexes 1a and 2a have higher fluorescence quantum yield (Φ  ) values than the other complexes and in DMSO, DMF, and THF due to electron withdrawing group effect.Fluorescence quantum yield (Φ  ) values of phthalocyanines substituted Schiff base groups on peripheral positions (1c and 2c) lowed when compared to 1b and 2b in DMSO and DMF 2c except in THF.This effect can be explained by electrondonating feature of Schiff groups.Fluorescence quantum yield (Φ 3.3.Photochemical Studies 3.3.1.Singlet Oxygen Quantum Yields.Singlet oxygen quantum yield Φ Δ is a measure of singlet oxygen generation and the Φ Δ values were obtained using (

Table 3
complexes (1a-1d and 2a-2d), the substitution with Schiff base Zn(II) complex group including continual conjugation seems to increase the  SV values of the complexes in DMF and THF.The orders of  SV values for substituted complexes (1a-1d and 2a-2d) among the studied solvents are as follows: DMSO > DMF > THF except 1d in DMSO and 1c