IR active intense out-of-plane porphyrin ring vibrations
An intense IR absorption starts to grow at 700 cm
−1 and gives the first very strong band at 785 cm
−1 in the H
2P molecule [
16,
20,
54,
55] by excitation of the vibrational mode ν
43 (following our throughout numeration of
Table 3) of
b1u symmetry. This is an out-of-plane wagging vibration of the N-H and C-H bonds in the protonated pyrrole rings with weak involvement of the C
m-H bonds (C
m are the methylene-bridge carbons). Because of the substitution in the TPPH
2 and HO-TPPH
2 molecules this mode is slightly shifted being mixed with the phenyl C-H bending. For the tetraphenyl derivatives in the same region there are also four close-lying intense lines determined by pure out-of-plane symmetric C-H bending in phenyl rings (δ
CHPh). By overlap with the porphyrin mode ν
43 they give one of the most intense lines at 790 cm
−1 in the TPPH
2 and HO-TPPH
2 molecules (
Figure 3). In the ZnP molecule this vibration corresponds to the ν
38 mode of
a2u symmetry (
Table 4). It consists of C-H out-of-plane wagging for C
β-H bonds and includes also the C
m-H wagging vibrations (out-of-phase to the former). Since the nature of this mode is rather different in the H
2P and ZnP molecules (no N-H bond in the latter) the frequency of the ν
38 mode is shifted in ZnP to 765 cm
−1 and the corresponding intensity decreased (
Table 4). The experimental frequency shift for this mode, n
H2P-n
ZnP = 20 cm
−1, can be compared with the calculated one (29 cm
−1). In TPPZn [
37] and HO-TPPZn (
Figure 3) this line is also overlapped by four intense δ
CHPh bands with frequencies 795 and 797 cm
−1, respectively; resulting in a larger shift compared to ZnP (in comparison with free-base variants) because of the stronger involvement of the C
m-H wagging vibrations. It is also more strongly mixed with the C-H bending vibrations of the phenyl rings. In the HO-prop variants (
Figure 3b) the intensity ratio for the free-base and Zn porphyrins is reversed in agreement with DFT/3-21G calculations. In dendrimers this IR band is more shifted to 802 cm
−1 and a new close lying intense band 829 cm
−1 occurs (
Figure 4). From the 3-21G calculation of the acetonide-G1-TPPZn dendrimer (
Figure 1) the latter band is connected with a few CH
2 modes of acetonide groups. The band 802 cm
−1 is the former ν
43 b1u mode of free-base porphyrin ring (
Table 3) mixed with the δ
CHPh vibrations and with the C-O-C bending in the acetonide groups. Its intensity is diminished for the dendrimer model in agreement with calculation; there are also a number of close lying new (acetonide) lines. A similar behaviour of this IR band was observed for Zn-containing dendrimers (
Figure 4).
The other intense line in the IR spectrum of the H
2P molecule [
38,
55] at 852 cm
−1 also belongs to the out-of-plane vibration of the
b1u symmetry (ν
48 in
Table 3, mostly the C
m-H bending) that slightly involves the C
β-H bond vibration. Since this mode is strongly affected by meso-tetrathenyl substitution of the porphyrin ring, it is less intense and shifted to 842 cm
−1 in TPPH
2 and HO-TPPH
2 molecules (
Figure 3). The shift and intensity reduction are supported by our calculations. In ZnP this mode appears similar. It corresponds to the ν
49 vibration of
a2u symmetry (
Table 4). Its measured frequency (849 cm
−1) is almost the same as for H
2P. The calculated frequencies and intensities are also very similar (
Table 3 and
Table 4).
The
b1u out-of-plane vibrations of H
2P correlate with the
a2u and
b2u symmetry of the ZnP molecule of the
D4h point group (
Table 2) and only the former vibrations are IR active. Although the optimized TPPZn and HO-TPPZn molecular structures are nonplanar, we can use correlation with the
D4h symmetry since some intense vibrations are determined by characteristic modes of the porphyrin ring. Since the ν
48 is one of the dominating out-of-plane vibrations of the C
m-H bonds, it is quite natural that the corresponding intensity is strongly reduced upon tetraphenyl substitution (
Figure 3). This mode is transformed in TPPZn in such a way that it includes out-of-plane vibrations of phenyl rings. In dendrimer substituted TPPs this vibration is quenched; the corresponding out-of-plane vibrations is shifted to the low-frequency region (the vibrations of light H atoms are transformed into out-of-plane movement of the massive and bulky substituent). It should be noted that the out-of-plane vibrations were not considered in empirical force-field calculations [
20,
54] and we need to use our throughout numeration of all modes, as presented in
Tables 3 and
4. The presented B3LYP/6-31G** calculations are in good agreement with the scaled results of Pulay
et al. [
7,
38,
55] with respect to intensity and polarization of IR and Raman spectra (
Tables 3–
6). Correlation with the AM1 results is strightforfard and obvious.
IR active in-plane porphyrin ring vibrations
There are no
b1u (
a2u) out-of-plane vibrations in the H
2P (ZnP) molecules with frequencies larger than 854 cm
−1 (
Tables 3 and
4). The same was found in calculations of the porphyrin ring out-of-plane vibrations in the tetraphenyl derivatives. Thus, the remainder of the porphyrin IR spectrum is caused by in-plane vibrations, for which assignment and numeration of Li and Zhierski [
20] are available. (Here, this numeration is denoted by a prime symbol ν′
i in order to avoid confusion with our complete numeration including also out-of-plane vibrations.) Analyses of the in-plane vibrations were also reported in a large number of experimental studies [
32–
34,
37,
54,
57]. We introduce the discussion of the vibrational structure with the ZnP analysis since it is more convenient to consider first the degenerate
eu vibrations, and then to discuss the corresponding vibrations of lower symmetry that occur in e.g., dendrimer capped porphyrins.
The most prominent features in the IR spectrum of the ZnP molecule [
20] occur at 993 and 1,052 cm
−1 and originate from the
eu vibrations (ν
54,55 and ν
62,63, respectively,
Table 4). The former mode includes the out-of-phase C
α-C
β stretching vibrations in the opposite-lying pyrrole rings with the corresponding Zn-N asymmetric stretches. This is the most intense (doubly degenerate) vibrational transition of the IR spectrum of ZnP (
Table 4) in agreement with experiment [
7]. In the notation of Ref. [
20] this is the ν′
47 mode, described as a pyrrole breathing mode. The results of our DFT calculations are not in complete agreement with the interpretation of Ref. [
20]. Our results also contradict the assignment of the 993 cm
−1 mode suggested in Ref. [
57]. The weaker line at 1,052 cm
−1 originates from C
β-H deformations being out-of-phase in the opposite-lying pyrrole rings. Both these modes (993 and 1,052 cm
−1) are mixed with the phenyl vibrations in TPPZn. Because of the specific character of the ν
54,55 vibrations, the C
m-Ph stretches are silent in TPPZn and the frequency 993 cm
−1 is hardly shifted in the tetraphenyl derivatives, TPPZn [
37], HO-TPPZn and HO-prop-TPPZn (
Figure 3). The mode at 993 cm
−1 of ZnP is split in the TPPZn molecule into two close lying frequencies (992 and 994 cm
−1 in our scaled DFT calculation), both having relatively high intensity (61 and 65 km/mole). They also include C-C vibrations of the phenyl rings. In the dendrimers, intensity of these very characteristic vibrations of the porphyrin ring are significantly changed. In the acetonide-G2-prop-TPPZn dendrimer the line at 993 cm
−1 is not the most intense and slightly shifted to 996.8 cm
−1. Its intensity is reduced by 25%, since the bulky acetonide groups withdraw electron density from the porphyrin ring and quench the dipole moment derivative along this C
α-C
β stretching vibration. In the acetonide-G5-prop-TPPZn dendrimer, the intensity of this line is reduced much more (by 67%) for similar reasons; the band is broadened because of mixing with acetonide vibrations and the maximum is shifted to 1002 cm
−1. At the same time the weaker line 1,052 cm
−1 of ZnP changes considerably upon tetraphenyl substitution, since these modes ν
62,63, directly involve C
m-H bending modes. In the the TPPZn and HO-TPPZn molecules this frequency shifts to 1,069 and 1,067 cm
−1, respectively. In HO-prop-TPPZn it is overlapped by an intense band at 1,055 cm
−1 (
Figure 3) originating from O-(CH
2)
3 vibrations. This intense IR absorption in dendrimers is further increased and shifted to 1,080 cm
−1 (
Figure 4). The line at 1,052 cm
−1 of ZnP is shifted approximately to 1,041 cm
−1 in the dendrimers. Since this C
β-H bending also involves some C
m-Ph bending character, its frequency is sensitive to substituents in the phenyl rings of TPPZn and is reduced in the more flexible bis-MPA dendrimers. The other
eu vibrations of low intensity in the IR spectrum of ZnP are not so informative as the intense lines mentioned above and we omit their discussion.
In free-base porphyrin and its derivatives the corresponding
eu vibrations of ZnP are split in the
D2h point group of the H
2P molecule into
b2u and
b3u modes [
7,
20]. The ZnP mode at 993 cm
−1 (ν
54,55,
Table 4) is split into 951 cm
−1 b2u and 971 cm
−1 b3u modes in H
2P (ν
53 and ν
55, in
Table 3). The nature of these modes is the same as in ZnP (asymmetric breathing of the opposite-lying pyrrole rings), but the absence of the Zn-ion and Zn-N stretches releases the force constants and leads to low-frequency shifts. The
b2u mode shifts more since it corresponds to unprotonated pyrrole rings. These two lines are not so intense in H
2P, like the double-degenerate line at 993 cm
−1 of the ZnP molecule (
Tables 3 and
4). Instead of the intense ZnP peak in this region, there is a gap (weaker absorption) in the IR spectrum of the H
2P molecule and this is the main difference between the two spectra. This trend is also well observed for IR spectra of the HO-TPPZn and HO-TPPH
2 molecules (
Figure 3), but not in the dendrimers (
Figure 4), since the line at 993 cm
−1 is not the most intense in acetonide-Zn-prop-TPPZn derivatives, as discussed above. The line corresponding to 993 cm
−1 of HO-TPPZn splits in the HO-TPPH
2 molecule into 983 and 966 cm
−1 lines (analogous the the case of
b3u and
b2u modes, respectively;
Figure 3). As far as the ZnP infrared line 1,052 cm
−1 is concerned, the behavior of free-base porphyrin variants is very peculiar; it splits into the 1,043 cm
−1 and 1,054 cm
−1 bands. They correspond to our ν
61 (
b3u) and ν
62 (
b2u) modes, respectively (
Table 3). In the H
2P molecule they become C
β- C
β -H bending vibrations of the out-of-phase type with respect to the opposite pyrrole rings. The ν
61 (
b3u) vibration involves the protonated rings, and the ν
62 (
b2u) mode involves the unprotonated rings (
Table 3). Only the ν
61 (
b3u) vibration is mixed with the C
m-Ph bendings and only this mode is seen in IR absorption of dendrimers. The striking difference of ZnP and H
2P vibrations of the C
β-H type has not been stressed before, as will be discussed more below, it is important for our further analysis of the dendrimers.
The separated strong line at 1,731–1,733 cm
−1 of all dendrimer samples is definitely connected with the carbonyl groups stretching. A very strong and narrow IR band at 1,080 cm
−1 in the region of intense porphyrin absorption was also present in all dendrimer samples. This originated from acetonide vibrations mixed with porphyrin modes. Even though the band is narrow, it consists of few close lying intense lines of similar nature. They include wagging vibrations of CH
2 and CH
3 groups, deformation of the Ph-O-CH chain and ν
61 - ν
63 modes of the porphyrin ring (
Table 3). The band 1,172 cm
−1 of HO-TPPX and HO-prop-TPPX molecules corresponds to the single bond C-O stretching of the terminal COH groups. It disappears in dendrimers because there are no such groups in the acetonide moiety. The C-O-H bending vibrations are assigned to the strong line at about 1220-1240 cm
−1 in the IR spectra of the HO-TTPX and HO-prop-TPPX molecules (X= Zn, H
2P). The less intense, closely lying bands at about 1,260–1,280 cm
−1 correspond to C-C-H bending vibrations of the phenyl rings; in the dendrimers they are shifted to lower frequency and overlapped by absorption of acetonide groups. The line at 1,349 cm
−1 of HO-TTPH
2 corresponds to the C
m-Phenyl stretching vibrations; it is shifted to 1,369 cm
−1 in dendrimers because of mixing with acetonide vibrations. The bands near 1,600 cm
−1 at the edge of the HO-TPPX infrared spectra belong to the phenyl C=C vibrations; these are sensitive to substituents and are strongly reduced in the dendrimers because of admixture of the ether-group stretching. In HO-TPPX in the region 1,500-1,420 cm
−1, there are a few intense IR bands of C-C stretching and C-C-H bending vibrations of the phenyl rings; some of them are mixed with C
m-C
α asymmetric modes of the porphyrin core. Introduction of the acetylide moieties leads to significant distortion of these bands. The broad strong IR absorption bands at 3,244 cm
−1 (HO-TTPZn) and 3,219 cm
−1 (HO-TTPH
2), data not shown, are attributed to O-H stretching vibrations in agreement with the calculated results. They further split into four modes of
ag and
bu symmetry; the two latter
bu modes correspond to paired combinations of O-H stretches from the opposite sides of the porphyrin ring. In the HO-prop-TPPX molecules these O-H stretching vibrational modes are shifted to higher frequency (about 3,320 cm
−1) because of the inductive effect of the alkyl groups.