electron density distribution of a protease inhibitor
4.2 Prediction of electron density features in BCPs of phenyl-CH-bonds via vibrational spectroscopic and
4.3.2 The influence of substituents at the 4-phenyl position on the py-ph bond
Apart from complexation, shown to increase the π-character in the py-ph bond in the last section, a frequently used strategy to modify or extend π-systems is introducing sub-stituents in a given structure. With the aim of highly ordered supramolecular assemblies, the ph moiety was substituted in the para-position to achieve roughly linear structures.
The resulting derivatives were already introduced in table 4.8 of section 4.3.1.
4.3.2.1 Structural Characterization and Electron Density
According to the preceding section, the π-character of the py-ph bond was quantified by calculating the ellipticity ε within the py-ph BCP according QTAIM. Variations of ε should be accompanied with changes in the py-ph bond distance r(py-ph) and the dihe-dral angle between py and ph α(py-ph) which have been obtained from DFT-geometry-optimizations. The theoretically derived py-ph-characteristics are verified by experimental Raman spectra.
The subsequent introduction of further aromatic rings and lateral substituents (i.e. sol-ubilizing alkoxy side chains) into the terpyridine-ligands results in a disproportional rise in computational effort. Consequently, the analysis of the electron density according to QTAIM has not been performed for the complexes containing ligands L5-L9. Addition-ally, a vibrational spectroscopic analysis becomes increasingly complex, as the nuclear motions of the additional but structurally related groups are expected to mix with vibra-tional modes of the basic py-ph structure being the objects of investigation. Therefore, the investigations and discussions are focussed on the Zn(II) complexes with smaller ligands (L1-L4).
The highest value for ε(py-ph-BCP) and also the highest value for ρ(py-ph-BCP) within the series features [Zn(L3)2]2+, followed by [Zn(L4)2]2+, [Zn(L2)2]2+, [Zn(L1)2]2+ (table 4.10), whereas the differences in ρ(py-ph-BCP) are small. However, the introduction of unsaturated substituents in the para-position of the phenyl moiety leads to a significant increase of the π-character of the py-ph bond, while the effect of vinyl slightly exceeds the one of ethynyl. This order is mirrored by the py-ph bond length r(py-ph) and the dihedral angle α(py-ph) between pyridine and phenyl, both decreasing with higher ε(py-ph-BCP), as shown in table 4.10 and figure 4.22. Remarkably, the differences in r(py-ph) and α(py-ph), only arising from variation of the lateral substituents, are approximately one order of magnitude lower compared to the corresponding geometrical parameters previously obtained for L4 and [Zn(L4)2]2+. This indicates a minor change in π-character of the py-ph bond due to the nature of the lateral substituent compared to the change induced upon complexation of the free ligand.
4.3 The π-conjugation in tpy-ph derivatives and their metal complexes
Table 4.10: Selected parameters indicating variations in the π-electron delocalization due to complexation
α◦a rb ν˜c Raman intensity ratiod BCP(py-ph)e
Comp. [◦] [˚A] [cm−1] ν(ph-py(trig)) δ(tpy(trig))
ν(ph-py(W8a))
δ(tpy(trig)) ρ [e/˚A3] ε
Zn(L1)2+2 31.5 1.472 1367 1.138 0.647 0.253 0.073
Zn(L2)2+2 30.9 1.469 1368 1.418 0.477 0.254 0.078
Zn(L3)2+2 28.7 1.466 1368 1.160 0.672 0.256 0.083
Zn(L4)2+2 29.6 1.468 1369 1.371 0.585 0.255 0.080
a,b DFT-calculated dihedral angle and bond length between the central pyridine ring and the phenyl substituent (py-ph). c experimentally determined wavenumber value of ν(ph-py(trig)) (see text). d experimentally determined. e based on DFT calculations.
The bonding situation in the phenyl-pyridine bond has been visualized by the recently introduced electron-density-difference (∆ρ) plots (figure 4.22).221 For this purpose, com-plex [Zn(L1)2]2+ has been chosen as a reference system and ρ(r) in the plane of the central pyridine of the complexes [Zn(L2)2]2+, [Zn(L3)2]2+ and [Zn(L4)2]2+ has been subtracted from the one corresponding to [Zn(L1)2]2+, respectively. In general, the cal-culated changes between the different Zn(II) complexes with ligands L1, L2, L3 and L4, illustrated by the valleys in the middle of the graphs in figure 4.22, are small compared to the changes induced by complexation (valley-depth roughly one magnitude deeper) presented in section 4.3.1. Figure 4.22 illustrates that any para-substituent on the phenyl moiety (Br, vinyl, ethynyl) affects a higher π-electron density in the phenyl-pyridine bond compared to [Zn(L1)2]2+. The highest electron density is observed for [Zn(L3)2]2+ (figure 4.22, middle) as visualized by the magnitude of negative electron densities in the region of the phenyl-pyridine bond.
As described in section 4.3.1,221 the properties of the py-ph bond are inter alia reflected by a particular normal mode involving a strong py-ph bond stretching vibration due to trigonal ring breathing vibrations on the tpy and lower on ph, denominated ν(ph-py(trig)).
This normal mode is well suited to compare the changes in py-ph by forming a homolep-tic complex, since the normal modes differ marginally between complex and free ligand.
Following this approach one has to stress that comparing ν(ph py(trig)) of differently substituted complexes is significantly more challenging than comparing the Raman spec-tra of L4 and the corresponding complex [Zn(L4)2](PF6)2. This additional complication arises from slight plane vibrations of ph, which contribute to ν(ph-py(trig)) and are in-fluenced somehow differently by different substituents. The well separated band at about 1368 cm−1, in the Raman spectra of [Zn(L1)2](PF6)2 to [Zn(L4)2](PF6)2, as depicted in figure 4.23, is assigned to ν(ph py(trig)). The increase of ε(py-ph-BCP) goes along with
4 Characterizing molecules in the electronic ground state [Zn(L1)2]2+. Each plot visualizes the change in ρ(r) in the py-ph bond (within the py-plane) which is located in the middle of each graph paral-lel to the x-axis (C(py): x=0 a.u., y=0 a.u.; C(ph): x≈3 a.u., y=0 a.u.).
4.3 The π-conjugation in tpy-ph derivatives and their metal complexes
1600 1500 1400 1300 1200 1100 1000
[Zn(L4) 2
](PF 6
) 2
Wavenumber / cm -1
[Zn(L1) 2
](PF 6
) 2 [Zn(L2)
2 ](PF
6 )
2 [Zn(L3)
2 ](PF
6 )
2
NormalizedRamanintensity
Figure 4.23: Experimental Raman spectra of Zn(L1)2+2 -Zn(L4)2+2 .
a stronger py-ph bond, what is assumed to affect the tentative trend to higher wave-number values of ν(ph-py(trig)) with increasing ε(py-ph-BCP) (figure 4.24). However, the observed wavenumber shifts are very small and are close to the error margins, even if the Raman spectra are corrected relatively to the Raman spectrum of TiO2, used as a standard. Nevertheless, the experimentally determined wavenumbers of ν(ph-py(trig)) corroborate the theoretically derived result of highest π-character of the py-ph bond in [Zn(L3)2](PF6)2 and [Zn(L4)2](PF6)2.
Apart from wavenumber shifts the Raman intensity of ν(ph-py(trig)) was shown to be a valuable quantity to conclude on changes in the electron density distribution (see section 4.3.1). However, as discussed above, ν(ph-py(trig)) involves slight in-plane phenyl vibrations. Since even aromatic in-plane vibrations contribute significantly to the Raman intensity of ν(ph-py(trig)), the latter is notably sensitive to a variation of para-phenyl substituents. Therefore, all modes involving significant py-ph-stretching fractions, thus leading to ph-vibrations, inherently differ somehow by varying the substituents on phenyl.
Hence, in the various complexes [Zn(L)2](PF6)2 the Raman intensities are also dependent
4 Characterizing molecules in the electronic ground state
0,072 0,075 0,078 0,081 0,084 1,466
0,072 0,074 0,076 0,078 0,080 0,082 0,084 1367
RI{ (ph-py(W 8a))}/RI{ (tpy(trig))}
RI{ (ph-py(trig))}/RI{ (tpy(trig))}
Exp.rel.Ramanintensity
Figure 4.24: Correlation between ε(ph-py), geometrical and Raman characteristics.
on the π-delocalization of the ph-R moiety and are not necessarily correlated with ε(py-ph-BCP). For example, both ν(ph-py(trig)) (1370 cm−1) and a mode assigned to Wilson No. 8a (on ph and py, ν(py-ph(W8a)), 1603 cm−1),235 involve strong py-ph-stretchings and their Raman intensities show opposite correlations with ε(py-ph-BCP) (see figures 4.23 and 4.24).
The vibrational assignments are performed in accordance to the one for Zn(L4)2+2 pre-sented in section 4.3.1.221 The Raman intensities shown in figure 4.24 have been de-termined via pseudo-Voigt fits and the Raman intensities of ν(ph-py(trig)) as well as ν(py-ph(W8a)) have been normalized to the Raman band at about 1030 cm−1. The lat-ter is assigned to a trigonal ring-breathing vibration of the pyridine rings δ(tpy(trig)) and was suggested as an appropriate reference mode in cause of negligibly involving the substituted phenyl ring and which is hence less dependent on the lateral substituent com-pared to ν(ph-py(trig)) and ν(py ph(W8a)).221 This band cannot be clearly assigned in the RS of [Zn(L1)2](PF6)2 and therefore, the data points associated to [Zn(L1)2](PF6)2
in figure 4.24 do not correspond to the trend of the points related to [Zn(L2)2](PF6)2
until [Zn(L4)2](PF6)2. However, the Raman bands between 1590 and 1640 cm−1, as-signed to in-plane vibrations involving phenyl, are most pronounced in [Zn(L3)2](PF6)2
and [Zn(L4)2](PF6)2, what is predominately a consequence of the π-conjugation between the phenyl moiety and the unsaturated substituents. Therefore, this finding does not al-low for conclusions on the π-conjugation between pyridine and phenyl. Further extension of the π-system by additional phenyl-based substituents on the vinyl- or the ethynyl-group (complexes [Zn(L5)2](PF6)2 - [Zn(L9)2](PF6)2) leads to an increase of the Raman intensity of the bands between 1590 and 1640 cm−1 in comparison to δ(tpy(trig)) (at
4.3 The π-conjugation in tpy-ph derivatives and their metal complexes
1030 cm−1) as shown in figure 4.25. In contrast, the ratios between the Raman intensi-ties of ν(ph-py(trig)) (1370 cm−1) and δ(tpy(trig)) are comparable for [Zn(L4)2](PF6)2, [Zn(L6)2](PF6)2 and [Zn(L9)2](PF6)2 (figure 4.25) as well as between [Zn(L3)2](PF6)2, [Zn(L5)2](PF6)2 and [Zn(L7)2](PF6)2 (figure 4.25, top). Therefore, one might conclude that the terminal phenyl-substituents do not influence the tpy-ph bond significantly. How-ever, the different substitution patterns on the terminal phenyl moiety influence the nor-mal modes differently, further complicating a comparison of single Raman bands. Hence, without a reliable normal coordinate analysis it appears not to be suggestive to conclude from the Raman spectra in figure 4.25 on ε(py-ph-BCP) of such complex systems based on ligands L5 - L9. Nevertheless, the para-ph substituents were found to influence the π-conjugation between the tpy- and the ph-moiety, whereas the vinyl-substituted deriva-tives offer the highest degree of tpy-ph π-conjugation according to our DFT-, AIM- and Raman-investigations. It is assumed, that the additional phenyl-substituents are less af-fecting the π-character of the py-ph bond, but nevertheless they significantly contribute to the electrochemical and photophysical properties of the respective zinc complexes.
The latter were determined by Winter and coworkers and are summed up in the follow-ing section.53 The focus of the discussion is to figure out if the relations of π-conjugations between tpy- and ph-moieties in [Zn(L)2](PF6)2 with L=L1, L2, L3, L4, determined by theoretical calculations and Raman spectra, can be verified by the results of the electro-chemical and photophysical studies.
4.3.2.2 Electrochemical and photophysical properties
To investigate electrochemical properties Winter and coworkers applied cyclic voltamme-try.53 In doing so, the Ered1/2 values in the range between -1.13 V and -1.22 V are assigned to the reduction of the tpy moiety. Applying Ferrocen as reference the LUMO energies were calculated and are summarized in table 4.11. To unravel at which molecular struc-tures HOMO and LUMO orbitals are predominantly located, these orbitals are plotted exemplarily for Zn(L3)2+2 in figure 4.26j. As can be seen, the HOMO is predominantly located on the ph-vinyl moiety while the LUMO is located at tpy. Even if the py-ph bond is influenced by the different ph-substituents (see section 4.3.1), the tpy-moiety and with it ELU M O is almost unaffected, hence leading to comparable ELU M O values. Oxidation processes were not observed for anodic scans up to 1.75 V and EHOM O values were not determined.
The absorption spectra of [Zn(L1)2](PF6)2 to [Zn(L4)2](PF6)2 determined by Winter and coworkers are characterized by a strong absorption band at 280 nm. The respective electronic transition is assigned to a π∗ ← π transition located at tpy which is roughly
jHOMO/LUMO-graphs are generated using the program gOpenMol and the results of the DFT calcu-lations. Contours are plotted at values of ±0.04.
4 Characterizing molecules in the electronic ground state
1700 1600 1500 1400 1300 1200 1100 1000
[ Zn(L7) 2
] (PF 6
) 2
[ Zn(L5) 2
] (PF 6
) 2
[ Zn(L3) 2
] (PF 6
) 2
Wavenumber / cm -1
Ramanintensity
2200 2100 1600 1500 1400 1300 1200 1100 1000
[ Zn(L9) 2
] (PF 6
) 2
[ Zn(L6) 2
] (PF 6
) 2
[ Zn(L4) 2
] (PF 6
) 2
Wavenumber / cm -1
Ramanintensity
Figure 4.25: Raman spectra of [Zn(L)2](PF6)2 containing vinyl (top: L = L3, L5, L7) or ethynyl (bottom: L = L4, L6, L9) groups in the para-ph position.
4.3 The π-conjugation in tpy-ph derivatives and their metal complexes
Table 4.11: LUMO energy (derived from electrochemical investigations) and photochemi-cal properties:53 absorption and luminescence maxima λabs, λP L, energy gaps Eoptgap (estimated from absorption spectra), Stokes shifts ∆S and photo lumi-nescence quantum yields ΦP L.
Substance ELUM O λabs Eoptgap λP L ∆S ΦP L
[eV] [nm] [eV] [nm] [cm−1]
[Zn(tpy)2](PF6)2 -3.22 232, 281, 320, 333 3.63 425 6.5 0.35 [Zn(L1)2](PF6)2 -3.23 252, 282, 323 3.59 425 11.93 0.37 [Zn(L2)2](PF6)2 -3.28 252, 276 3.57 427 12.81 0.14 [Zn(L3)2](PF6)2 -3.27 230, 285 3.54 438 12.26 0.59 [Zn(L4)2](PF6)2 -3.26 230, 284 3.58 427 11.79 0.34 [Zn(L5)2](PF6)2 -3.25 254, 282, 326 3.44 454 8.65 0.06 [Zn(L6)2](PF6)2 -3.28 233, 281, 326, 384 2.90 449 3.77 0.20 [Zn(L7)2](PF6)2 -3.23 234, 285, 323, 392 2.69 487 4.98 0.35 [Zn(L8)2](PF6)2 -3.25 234, 285, 323 3.27 447 8.59 0.64 [Zn(L9)2](PF6)2 -3.26 233, 286, 326, 398 2.70 477 4.16 0.53
independent from complexationk even if the latter affects an increase in the π-character of the py-ph bond.53
For complexes with ligands L5-L9 absorption bands at 325 and 400 nm which are also assigned to π∗ ← π transitions become more pronounced. Since they are supposed to be related to the π-system involving ph, these transitions are expected to be more af-fected by complexation than the tpy located transitions, what is confirmed by significant bathochrome shifts of the low-energetic absorption bands. In particular, for ligands L7 (red shift ≈ 3260 cm−1) and L9 (red shift ≈ 1190 cm−1) the strong red shift due to complexation can be explained by ILCT processes.
Since the study of the π-character of the py-ph bond reveals a maximum for [Zn(L3)2](PF6)2
in the series of complexes with ligands L1-L4, the lowest HOMO-LUMO energy gap is expected for [Zn(L3)2](PF6)2 which is in accordance to the estimations made from the absorption spectral. The respective Eoptgap-values are listed in table 4.11. In agreement to the low Eoptgap-value, [Zn(L3)2](PF6)2 also shows a significant red shifted maximum of the luminescence spectrum, compared to [Zn(L)2](PF6)2 with L=L1, L2, L4 (see table 4.11).
Furthermore, the photo luminescence quantum yield ΦP L of [Zn(L3)2](PF6)2 is roughly twice the one of [Zn(L4)2](PF6)2.
kλabs of ligand and respective Zn(II) complex are almost identical for L1-L4.
lEnergy at 10% of the intensity of the bathochrome absorption edge
4 Characterizing molecules in the electronic ground state
Figure 4.26: HOMO (bottom) and LUMO (top) of Zn(L3)2+2 .
4.3.2.3 Conclusion
The tuning of the opto-electronic properties has been realized by a subsequent exten-sion of the π-conjugated system on the coordinated terpyridine ligand. The investiga-tions of the ground state properties have been supported by Raman spectroscopy, DFT-calculations and analysis of the electron densities according to QTAIM. As visualized by ρ-difference plots, the presence of a vinyl group generally leads to a much higher degree of π-conjugation compared to an ethynyl moiety. Therefore, the tuning of the emission properties is enabled. Intense violet emission is observed for the smaller complexes and the photo luminescence maximum is strongly shifted resulting in bright cyan photolumi-nescence for the complexes based on terpyridines L7 and L9 with the highest degree of π-conjugation. Along with this, also the HOMO energy level is significantly influenced, whereas the energy of the LUMO appears independent from the electronic nature of the terpyridine ligand.
4.3 The π-conjugation in tpy-ph derivatives and their metal complexes
In continuation of the study the materials have been incorporated into PMMAm matri-ces resulting in homogeneous thin solid films. The bright emission also in the solid state indicates that the herein described Zn(II) bis-terpyridine complexes are highly promising candidates with respect to potential LED applications.