Synthesis and Photophysical Characterization of New Azaphthalocyanines and Azanaphthalocyanines for Semiconductor Interface Design
The aim of this work, which was carried out within the SFB1083, was the synthesis and characterisation of phthalocyanines (Pc), pyrazinoporphyrazines (Ppz), their hybrid compounds, the so called azaphthalocyanines (Pz, Nx-[Pc*M] with M = central metal or 2 H) and larger homologues, the naphthalocyan...
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|Summary:||The aim of this work, which was carried out within the SFB1083, was the synthesis and characterisation of phthalocyanines (Pc), pyrazinoporphyrazines (Ppz), their hybrid compounds, the so called azaphthalocyanines (Pz, Nx-[Pc*M] with M = central metal or 2 H) and larger homologues, the naphthalocyanines (Nc). The focus was, besides their optoelectronic application in dye sensitized solar cells (DSSCs), the systematic investigation of selected compounds, frontier MOs using electronic absorption spectroscopy and in cyclic voltammetry. In addition, photophysical properties of these compounds were investigated, in particular their ability to form singlet oxygen; their fluorescence quantum yields and the fluorescence lifetimes were also determined.
Organic precursors. Firstly, the literature known compounds PDN* 1, PyzDN* 2 and NDN* 3 were reproduced; the series was then completed by the synthesis of compounds NpzDN* 4, NqnDN* 5 and NppzDN* 6. Besides their synthesis and characterisations discussed in section 4.1, compound 3-6 are a novel series of C2v-symmetrical naphthalonitriles bearing alkyl groups in the peripheral position, while [-CH=] building blocks are systematically exchanged by [-N=] units.
In comparison to the commercially available PDNtBu, the advantages of these
2,2,5,5-tetramethylcyclohexane annulated compounds are the increased solubility in common organic solvents due to the reduced aggregation of the Pc caused by sterically demanding alkyl groups. Furthermore, in cyclotetramerisation of C2v-symmetrical phthalonitriles no mixture of 4 regioisomers is obtained, in contrast to [PctBuM]. Owing to the symmetry of the A4-type molecules resulting from a cyclisation of 1-6, the analysis in NMR spectroscopy is simplified, crystallisation of these compounds is favoured, and, therefore, an X-ray analysis is possible.
The key-step in the synthesis of PDN* 1 is the oxidation of the tetraline system to a dicarboxylic acid. The synthesis of PDN* 1 was firstly reported by MIKHALENKO. Up to now, the overall yield of ~30% over seven steps was limited due to the chosen conditions in the oxidation step. As oxidant potassium permanganate was used in excess in aq. pyridine. Following famous industrial processes, such as the aerobic oxidation of p-xylene to terephthalic acid, the side chain oxidation of tetraline was optimized in a catalytic procedure, increasing the yield
up to 97%.
Azaphthalocyanines (of Nx-[Pc*M] type with M = 2 H, Zn) were synthesised in a cocyclisation of two dinitriles PDN* 1 and PyzDN* 2, followed by metalation of chromatographically separated Nx-Pc*H2 using [Zn(hmds)2] or [Zn(OAc)2], respectively. Azaphthalocyanines
Nx-[Pc*M] with x = 0, 2, 4, 6, or 8 are discussed and described in section 4.2. All Nx-[Pc*M] were characterized by using NMR, UV-Vis, FS spectroscopy, cyclic voltammetry, mass spectrometry and photophysical experiments. First synthesised ABAB N4-[Pc*M] with
M = 2 H, Zn completed the up-to-now unfulfilled series. Both structural isomers ABAB and A2B2 could be clearly identified by using 1H NMR spectroscopy and UV-Vis spectroscopy. Besides the cocyclisation, A2B2 N4-Pc*H2 and
AB3 N6-Pc*H2 could be synthesised in a KOBAYASHI ring expansion of an AB2 N4-[Spc*BCl] or [Sppz*BCl], respectively, using the isoindoline of PDN* 1.
The values obtained by UV-Vis spectroscopy and CV were compared to TD-DFT calculations in cooperation with the TONNER group (Department of Chemistry of the Philipps-Universität Marburg). In both experimental data and theoretical calculations, a non-perfect linear trend of the hypsochromic shift of the Q-band was found when the HOMO-LUMO gap is increased with the increasing number of substitution of [-CH=] by [-N=] in the non-peripheral position. The reason for the HOMO-LUMO gap increase is the stronger decrease of the HOMO level relative to the one of the LUMO with increasing number of [-N=] units in Nx-[Pc*M].
In addition, photophysical properties were investigated. Singlet oxygen quantum yields , fluorescence quantum yields and fluorescence lifetimes were determined for both series of Nx-[Spc*BCl] and Nx-[Pc*M] in cooperation with ZIMČÍK and NOVÁKOVÁ (Department of Pharmacy of the Charles-University in Prague). The highest value for singlet oxygen quantum yield was observed for ABAB N4-[Pc*Zn]. The sum of values is for both Nx-[Pc*Zn] and Nx-[Spc*BCl] ~0.9. In comparison, a trend, not yet described in literature, was found for Nx-Pc*H2, which shows a significant decrease of both values with increasing number of [-N=] units.
For the first time, an asymmetrical 2,2,5,5-tetramethylcyclohexane substituted aza-phthalocyanine N2-Pc*H2 was analysed by X-ray diffraction. In contrast to the herringbone pattern of unsubstituted Pcs, N2-Pc*H2 units are twisted almost 90° in their lattice structure. Similar to other obtained structures, such as A2B2 N4-[Pc*Zn], substituted [-CH=] and [-N=] units cannot be differentiated by X-ray analysis. In both structures, a characteristic statistical disorder is observed, in which the [-CH=] units are partly occupied by [-N=], to 25%, or 50%, respectively. Within the bachelor thesis of LANGE, the isoindoline units of the azaphthalocyanine series Nx-Pc*H2 were annulated with a benzene unit. These Nx-Npz*H2 are discussed in section 4.2.6. The series was also completed with the ABAB N4-Npz*H2 and differentiated from its A2B2 isomer by using 1H NMR and UV-Vis spectroscopy.
Azanaphthalocyanines (Nx,y-[Nc*M(NR)Cl] with M = Mo, W and R = tBu, Mes) were synthesised using 2,2,5,5-tetramethylcyclohexane annulated dinitriles 3-6 and group 6 metal precursors ([M(NR)2Cl2∙solv] with M = Mo, W, R = tBu, Mes and solv = py or dme).
This is the first described series of azanaphthalocyanines (section 4.2.9) bearing alkyl groups in the peripheral position. The compounds were analysed using at least UV-Vis and IR spectroscopy, elemental analysis and mass spectrometric methods, such as MALDI-ToF or LIFDI. The Q-bands of the compounds N0,0-[Nc*Mo(NtBu)Cl], N0,8-[Nc*Mo(NtBu)Cl],
N8,0-[Nc*Mo(NtBu)Cl] and N8,8-[Nc*Mo(NtBu)Cl] show a hypsochromic shift from 900 nm to 700 nm with increasing substitution of [-CH=] units by [-N=] and proximity to the central
[-MN4-] unit. The Q-band is uncommonly strongly shifted by a substitution of the axial ligand of N0,0-[Nc*Mo(NR)Cl] with R = tBu or Mes: for N0,0-[Nc*Mo(NtBu)Cl] and
N0,0-[Nc*Mo(NMes)Cl] a shift of 27 nm was observed. Both compounds,
N0,0-[Nc*Mo(NtBu)Cl] and N0,0-[Nc*Mo(NMes)Cl], were analysed by EPR spectroscopy.
In EPR measurements, the structural differences between the orthorhombic
N0,0-[Nc*Mo(NMes)Cl], contrary to the axially symmetric N0,0-[Nc*Mo(NtBu)Cl], is clearly visible. Calculations of orbital interaction with our cooperation partner BERGER (Department of Chemistry of the Philipps-Universität Marburg) are in progress.
Besides the synthesis of group 6 d1-metal naphthalocyanine complexes, the access to metal-free Nx,y-Nc*H2 ligands was attempted in a final stage of this work. N0,0-Nc*H2 and N0,8-Nc*H2 as well as their Zn-complexes, N0,0-[Nc*Zn] and N0,8-[Nc*Zn], were synthesised according to literature known methods (section 4.2.10). N8,0-[Nc*Zn] was obtained in 29% yield from compound 5 and [ZnCl2] in quinoline after 12 h at 230 °C. In first attempts to synthesise
N8,0-Nc*H2 and N8,8-Nc*H2, similar to the synthesis of Ppz*H2, a higher tendency to form
meso-Cnoctyl-substituted complexes Ppz*H2noctyl or N8,0-Nc*H2noctyl was observed. The mechanism, as well as the isolation of the pure N8,0-Nc*H2noctyl, has to be carried out in future studies. N8,0-Nc*H2 could be isolated by preparative TLC and was analysed by using mass spectrometry and UV-Vis spectroscopy. In the synthesis of N8,8-[Nc*Zn], no pure product could be isolated, but a cyclotetramerisation was proven by mass-spectrometry and UV-Vis spectroscopy.
Subphthalocyanines (Spc) and Subpyrazinporphyrazines (Sppz) bearing the annulated 2,2,5,5-tetramethylcyclohexane ring could be reproduced and axially substituted with halogen atoms. A series of these smaller, ring contracted, "umbrella-like", distorted Pc homologues [Spc*BR] and the first series of [Sppz*BR] with R = F, Cl, Br and OH were synthesised and characterized. Spcs (N0-[Spc*BCl]) absorb light in the higher energetic area at about 580 nm and electron deficient Sppzs (N6-[Spc*BCl]) at 534 nm. On the right side, cuvettes of a new
Nx-[Spc*BCl] series with X = 0, 2, 4 and 6 are shown (FS, illuminated at λ = 365 nm). Visible to the unaided eye is the change of the absorbance and emission behaviour, by exchanging [-CH=] by [-N=] building blocks. The structure of [Spc*BCl] is one example for the rare, substituted Spcs that was analysed by X-ray diffraction. In addition, first attempts were carried out to synthesise surrounded metal-complexes of [Me2M(-OBSpc)]2 type with M = Al, Ga using [MMe3] as metal precursor and [SpcBOH]. The synthesis of these compounds was monitored by 1H NMR spectroscopy and the resulting [Me2M(-OBSpc)]2 characterised by using UV-Vis, FS and IR spectroscopy. According to mass spectrometric results from MALDI-ToF, a dinuclear structure is proposed. Compared to axial substitution of phthalocyanines, the UV-Vis spectra of subphthalocyanine complexes [Me2M(-OBSpc)]2 with M = Al, Ga show a slight shift of the Q-band of about 1 nm in comparison to [SpcBOH].
In following experiments, the synthesis of numerous borato coordinated metal complexes was attempted, whereby [(SpcBO)4Ti] as well as [(SpcBO)3B] were observed and characterised. These metal complexes show a weak broadening of the Q-band in UV-Vis spectroscopy, because the -systems are forced into physical proximity.
Application as Photosensitizer in Dye Sensitized Solar Cells (DSSCs) was carried out for equatorially functionalised compounds. Their synthesis is discussed in section 4.3. Unsubstituted Pc, [PcZn]FG, with different anchors were synthesised (FG = vinylphosphonic acid, phosphonic acid, vinylcarboxylic acid, or catechol) using a KOBAYASHI ring-expansion.
In addition, 2,2,5,5-tetramethylcyclohexane annulated Pcs ([Pc*M]OH with M = 2 H, Zn) with a catechol anchor were synthesised in yields of up to 10% in a cocyclisation.
All compounds were successfully bonded to ZnO semiconductor surfaces. Current-voltage characteristic (I-U curve) and IPCE measurements were carried out in collaboration with AG SCHLETTWEIN (Department of Physics of the Justus-Liebig-Universität Gießen). The most promising anchor moiety appears to be the phosphonic acid of [PcZn]vPA ( = 0.5%), followed by the carboxylic acid [PcZn]vCA ( = <0.2%) and the catechol [PcZn]OH ( ~ 0.001%), when correct solvent and conditions for the deposition are chosen. In comparison to the unsubstituted compound [PcZn]OH, the alkyl substituted [Pc*Zn]OH shows a higher efficiency in DSSCs by a factor 10, and with this, a better electron transfer to the semiconductor. This clearly shows how the electron donating and sterically demanding alkyl groups decrease aggregation and recombination processes. However, in accordance with the Nx-[Pc*M] series with M = 2 H, Zn, lower efficiencies were measured for Pc*H2OH compared to [Pc*Zn]OH, which is caused by increased "quenching" processes taking place when the central metal atom is removed.
Currently, photoluminescence measurements are being carried out for constructed cells. A fast electron transfer to the semiconductor was observed, but also a fast recombination process, which might explain the low electron transfer to the semiconductor and the low cell efficiencies. However, a favoured electron transfer from the dye to a semiconductor by using an equatorial anchor in comparison to previously described axial anchor moieties was proven. In future work, the described structure-property correlation obtained from previous sections can be used to design and modify equatorially functionalised Pcs. The readily accessible [Pc*Zn]OH can be converted into a vinylphosphonic acid substituted A3B type Pc, such as [Pc*Zn]vPA or [Pc*Zn]vCA. Furthermore, the complexes should be tested with additives such as cheno or axially coordinating moieties such as guanidine; meso-CFG Pc and derivatives could also be investigated.
Phthalocyanine Sandwich-Complexes of a [Pc*2MRE] and [Nc*2MRE] type were synthesised for MRE = Eu, Tb, Ce and Nd. The main focus was on the rare earth metals Eu3+ and Tb3+, because of their different spin of +/- ½ in comparison to the quite well investigated f7-Gd3+ phthalocyanine complexes, of a [Pc2Gd] type.
In this work, the easiest synthetic access to [Pc*2MRE] and [Nc*2MRE] was found to be the reaction of the free ligand Pc*H2 and Nc*H2 in presence of the respective [MRE(hmds)3]. For a complete formation to the [(Pc)- (Pc’)2- MRE, 3+]0 complex reaction times of up to 7 d at 110 °C in toluene were necessary. The obtained sandwich complexes show a split of the B-band in the UV-Vis spectrum at 323-331 nm and at 341-352 nm. In addition, both naphthalocyanine and phthalocyanine complexes show, with a decreasing radius of the central metal ion MRE, 3+, an increasing hypsochromic shift of the Q-band from 800 nm ([Nc*2Ce]) to 778 nm ([Nc*2Eu]) and from the metal free ligand at 711 nm (Pc*H2) over 694 nm ([Pc*2Eu]) to 692 nm ([Pc*2Tb]). Furthermore, typical v(C=N) valence vibrations in the IR spectrum of [Nc*2Ce] were observed at 1362 cm-1 and 1314 cm-1.
The presented work displayed multi-faceted chemistry of phthalocyanines, naphthalocyanines and their aza-analogue complexes. Within a set of selected examples, the prediction of optoelectronic properties using structure-property correlations has been demonstrated.|
|Physical Description:||410 Pages|