Vertical gas phase absorption spectra
The ADC(2) and CAM-B3LYP vertical gas phase spectra of the pNA, pNA–NO2, and pNA–NH2 molecules employing the def2-TZVPD basis set are depicted in Fig. 1 along with the gas-phase experimental spectra [32].
By comparing our results with the gas phase experimental spectra [32], we found that the first ten states describe the two measured bands (Fig. 1). Concerning the first, the calculated spectra of pNA, pNA–NH2, and pNA–NO2 molecules overestimate the experimental absorption energies (i.e., blueshift) respectively by about 0.2 eV, 0.3 eV, and 0.2 eV for ADC(2)//def2-TZVPD and about by 0.2 eV, 0.3 eV, and 0.3 eV for CAM-B3LYP//def2-TZVPD. Overall, ADC(2) is slightly closer to the experiment for the first band. For the second, ADC(2)//def2-TZVPD overestimates the transition energy by about 0.4 eV, 0.1 eV, and 0.4 eV for the pNA, pNA–NH2, and pNA–NO2 molecules, whereas CAM-B3LYP//def2-TZVPD underestimates the maximum energy for pNA and pNA–NO2 by about − 0.2 eV and − 0.1 eV, and overestimates pNA–NH2 by about 0.1 eV. Although the CAM-B3LYP maximum energy for the second band is slightly closer to the experiment for pNA and pNA–NO2, it underestimates the relative intensity of the band in the spectrum.
For the other systems with no available experimental spectra, from now on we employ only the more accurate ADC(2)//def2-TZVPD method. The computed gas phase spectra of pNA with the –OCH3, –OH, –C6H5, –CCH, –CH3, –Br, –Cl, –F, –COOH, –CN, –SO2H, –CF3, and –CCl3 substituents are shown in Figures S1 to S5 of the SI.
Table 2 presents the properties of the unsubstituted pNA’s vertical electronic spectra. Figure 2 shows the NTOs of the pNA brightest transitions and Fig. 3 the charge transfer contributions. The pNA–NH2 and pNA–NO2 molecules are discussed in The substituent effect in the absorption spectra section. For the unsubstituted pNA, the brighter states (oscillator strengths \(f\) between parentheses) are S2 (0.440) in the first band and S7 (0.308) in the second. The other states have very small \(f\) values (between 0.000 and 0.010), thus giving very small contributions to the bands. The lowest bright excitation of unsubstituted pNA (S2 state) is dominated by a HOMO-LUMO π → π* transition – see Table 2 and Fig. 2. This first transition of unsubstituted pNA has a \(q\left(CT\right)\) value of 0.631\(e\) with a distinct CT character from the donor (–NH2) to the acceptor group (–NO2), in agreement with other theoretical calculations which found the same character [30, 47, 49, 51, 105–110]. The charge decomposition (Fig. 3) indicates that the S2 transition is dominated by a small CT to the nitro group, with \(q\left(NCT\right)=0.386e\). We also found for S2 a local excitation in the benzene ring, with \(q\left(RLE\right)=0.246e\). The NTOs corresponding to the bright S7 state of the unsubstituted pNA indicate some electron delocalization dominated by a HOMO–2\(\to\)LUMO transition with a small contribution by other orbitals (HOMO–1\(\to\)LUMO), as indicated by the weights of the corresponding NTOs shown in Table 2 and Fig. 2. For S7, the main contribution is mainly a excitation localized on the nitro group, with \(q\left(NLE\right)=0.346e\) and on the benzene ring (\(q\left(RLE\right)=0.177e\)), corresponding to a total CT character \(q\left(CT\right)=0.414e\). A charge transfer to the nitro group and to the benzene ring, with values \(q\left(NCT\right)=0.154e\) and \(q\left(RCT\right)=0.178e\), also contribute to the transition.
Table 2
Unsubstituted pNA ADC(2)//def2-TZVPD transition energies (), optical oscillator strengths (), and the transition assignments.
State
|
\(\varDelta \varvec{E}\)a
|
\(\varvec{f}\)b
|
Assignmentc
|
%d
|
\(q\left(CT\right)\)e
|
S1
|
3.768
|
0.000
|
HOMO–3\(\to\)LUMO
|
62.1
|
0.246
|
S2
|
4.415
|
0.440
|
HOMO\(\to\)LUMO
|
72.4
|
0.631
|
S3
|
4.443
|
0.001
|
HOMO–5\(\to\)LUMO
|
60.8
|
0.266
|
S4
|
4.697
|
0.010
|
HOMO\(\to\)LUMO+4
|
62.1
|
0.345
|
S5
|
5.595
|
0.001
|
HOMO\(\to\)LUMO+1
|
64.7
|
0.525
|
S6
|
5.651
|
0.002
|
HOMO–1\(\to\)LUMO
HOMO–2\(\to\)LUMO
HOMO\(\to\)LUMO+4
HOMO–1\(\to\)LUMO+1
|
35.6
25.6
11.3
10.0
|
0.434
|
S7
|
6.086
|
0.308
|
HOMO–2\(\to\)LUMO
HOMO–1\(\to\)LUMO
HOMO–2\(\to\)LUMO+1
|
41.2
19.9
12.3
|
0.414
|
S8
|
6.365
|
0.002
|
HOMO\(\to\)LUMO+12
HOMO\(\to\)LUMO+9
HOMO\(\to\)LUMO+8
|
34.3
15.8
10.3
|
0.533
|
S9
|
6.476
|
0.001
|
HOMO\(\to\)LUMO+2
|
82.0
|
0.522
|
S10
|
6.501
|
0.008
|
HOMO\(\to\)LUMO+3
|
66.8
|
0.466
|
a Transition energies (\(\varDelta E\)) in eV.
b Optical oscillator strengths (\(f\)).
c Main transition contributions.
d Percentage of the contributions (\(\%\)).
e Charge transfer \(q\left(CT\right)\) descriptor in units of elementary electrical charge \(e\).
The substituent effect in the absorption spectra
The individual spectral properties of all substituted pNA systems in the gas phase are presented in Tables S1–S15 of the SI. The charge transfer analysis is in Fig. S6 to Fig. S8 on SI. The NTOs of the brightest states of each band are shown in Fig. S9 to Fig. S13.
The EDGs –NH2, –OCH3, and –OH form extended conjugated systems in molecules and the electron-donating character of the substituents increases from –OH to –OCH3 and –NH2. As a result, more states have considerable optical oscillator strengths compared with unsubstituted pNA (see the red vertical lines in Fig. 1(b) and Fig. S1). Consequently, these molecules exhibit a broader absorption spectrum compared to those with smaller or non-conjugated systems [111]. This explains the wider bands in the experimental spectrum of pNA–NH2 compared with unsubstituted pNA (Fig. 1).
The energies of their first maximum band are redshifted – see Fig. 4(a) – compared with the unsubstituted pNA by approximately − 0.303 eV (pNA–NH2), − 0.369 eV (pNA–OH), and − 0.304 eV (pNA–OCH3). Similarly to unsubstituted pNA, their first band has state S2 as the major contributor to the intensity (HOMO\(\to\)LUMO for pNA–NH2; HOMO\(\to\)LUMO+1 for both pNA–OH and pNA–OCH3). The NTOs of S2 (Fig. S9) once again show CT from the remaining three regions (Scheme 1) to the nitro group. In the initial (ground) state, there is greater electron density over the substituent, while in the final state there is only a slight electron density over –OH and not any in –NH2 and –OCH3. These findings confirm that these substituents act as electron density donors, also indicated by the CT value (see Tables S1 to S3 of the SI), which increases from 0.631\(e\) for unsubstituted pNA to 0.651\(e\) for pNA–NH2, 0.653\(e\) for pNA–OH, and 0.670\(e\) for pNA–OCH3. Although previous works[72, 112] indicate a larger donor character of the –NH2 substituent (as discussed in subsection S2.1 of the SI), we found an increase of the CT to the nitro group for the first band in the pNA–NH2, pNA–OH, and pNA–OCH3 molecules, which is confirmed by the \(q\left(NCT\right)\)values of, respectively, 0.363e, 0.381e, and 0.396 e – see Fig. S7, panels (a), (b) and (c).
The significant contributions for the second band of the pNA–NH2, pNA–OCH3 and pNA–OH molecules are the S6, S7, and S8 states, respectively. Interestingly, the state position increases with the increase of the corresponding Hammett’s constant and the increase of the donor character of the substituents. Similarly to the unsubstituted pNA, they have two dominant NTOs (Fig. S9). The contributions of the secondary NTOs increase as the value of the Hammett’s constant decreases from pNA–OH to pNA–NH2. The overall effect of these substituents on the corresponding bright state of the second band is increasing the delocalization of NTOs, which is more pronounced in pNA–NH2, as seen by its NTO (Fig. S9), with the pNA–OCH3 and pNA–OH orbitals with higher delocalization making a more minor contribution (lower weight \({\lambda }_{2}\)). The delocalization caused by the substituents on the pNA–NH2, pNA–OCH3 and pNA–OH NTOs can be associated with the extended conjugation and resonance effects due to the substituent’s mesomeric lone pair.
The EDGs –C6H5, –CH3, and –CCH pNA spectra are presented in Fig. 4(b). The CH3 is an EDG by induction and σ conjugation, whereas –C6H5 and –CCH are EDGs by π conjugation (see the discussion in subsection S2.2 of the SI). The shifts of absorption bands were larger for pNA–C6H5 and pNA–CCH, of about − 0.144 and − 0.124 eV for the first band and − 0.777 and − 0183 eV for the second, and smaller for pNA–CH3, of about − 0.051 and − 0.044 eV for the first and second bands (see Fig. 4(b)). This can be attributed to the fact that the σ conjugation and inductive effects have a much smaller influence compared with the π conjugation in conjugated hydrocarbons[72, 113] due to the extended electron delocalization in the latter. The increase in the extension of the π-electron system is a well-known factor red shifting the absorption spectra [113]. The pNA–C6H5, pNA–CH3, and pNA–CCH brighter state of the first band is S2. The brightest state of the second band is S7 for pNA–CH3 and pNA–CCH, and S6 for pNA–C6H5. Fig. S10 shows that these substituents produce a significant electron delocalization in the NTOs for the brightest states of both bands. This suggests that these substituents are effectively conjugated with the aromatic system by resonance for the –C6H5 and –CCH substituents, and through hyperconjugation with the –CH3 substituent.
The halogens –F, –Cl, and –Br substituents can act both as electron donors through conjugation and electron withdrawers by induction, but the latter effect is more pronounced (see subsection S2.3 in the SI). These substituents resulted in a very weak redshift, which decreased with the electronegativity increase of the halogen in the ordering pNA–Br, pNA–Cl, and pNA–F, respectively by − 0.095 eV, − 0.079 eV, and − 0.038 eV for the first band, and − 0.093 eV, − 0.068 eV, and − 0.018 eV for the second (see Tables S7–S9 in the SI). These results are expected, given that the presence of halogens as EWGs is known to have little effect on the absorption spectra of conjugated compounds [113]. For pNA–Cl and pNA–F, the brightest state in the first band is the S2, while in the second band is the S7, and for pNA–Br is the S8. Although the position of the peak in the first band and the character of the NTOs remain almost unaffected by these halogenic substituents, the NTOs are slightly more delocalized, which is confirmed by the small decrease in the \(q\left(CT\right)\) values from pNA–F (0.630\(e\)) to pNA–Cl (0.621\(e\)) and pNA–Br (0.620\(e\)), compared with the S2 state value for the unsubstituted pNA (0.631\(e\)), which is mainly due to the a smaller CT from the remaining three regions (Scheme 1) to the nitro group: \(q\left(NCT\right)\) have 0.362\(e\), 0.341\(e\), and 0.339\(e\) values, respectively (see Fig. S6 in the SI). A smaller \(q\left(CT\right)\) also indicates less charge transfer for the S7 states (second band) for pNA–F (0.411\(e\)) and pNA–Cl (0.383\(e\)), and for the S8 state in pNA–Br (0.375\(e\)). These \(q\left(CT\right)\) values decreases are due to the withdrawal effect of halogens, which hinders CT from the remaining three regions (Scheme 1) to the nitro group. Therefore, the effect of the substituent on the magnitude of the energy shift, CT character, and NTO contributions of the brightest states of the first and second bands increases in the ordering –F, –Cl, and –Br. The brightest state of the second band has two major contributions from NTOs, whose weights of the main transition decrease in the same ordering, but increasing in the second transition, which has a more delocalized NTO. Intriguingly, these findings do not follow the increase in electronegativity from Br to F of the halogen substituents. Therefore, in this case, the higher electronegativity does not lead to a stronger electron attraction and reduced charge transfer, which suggests that other electronic effects play a major role in this phenomenon, reflecting on the magnitude and kinds of spectral shifts as well as in the pattern of CT and NTOs.
The spectra of the EWGs –CN, –SO2H, and –COOH groups are depicted in Fig. 5(a). The pNA–CN and pNA–COOH molecules have Cs symmetry, with two types of irreducible representations of the molecular point group, the \(A{\prime }\) (symmetric) and \(A”\) (antisymmetric), the latter corresponding to states with the lower energy, while all bright states have \(A’\) symmetry. The brightest states of the first and second bands are, respectively, the S2 and S6 states for pNA–CN and pNA–COOH, and S2 and S4 for pNA–SO2H. These groups blue shifted the spectra, the exception being pNA–COOH for its first band, which was redshifted by about − 0.061 eV. The pNA–COOH and pNA–CN S2 states are similar to the pNA molecule, all having \(q\left(CT\right)\) values of about 0.63\(e\) and NTOs indicating a charge transfer character from the r three regions (Scheme 1) to the nitro group, as indicated by the \(\text{q}\left(NCT\right)\) values of 0.355\(e\) and 0.345\(e\), respectively. In contrast, the pNA–SO2H molecule has two major NTO transitions, the one with considerably greater weight occurring as a local excitation on the benzene ring, corresponding to a decrease in the \(q\left(CT\right)\)value from 0.631\(e\) in pNA to 0.441\(e\) in pNA–SO2H. As the –SO2H electron-withdrawing substituent counteracts the electron-withdrawing property of the nitro group, the electronic density becomes more concentrated in the benzene ring between them. This is confirmed by the smaller \(q\left(NCT\right)\) of 0.022 \(e\) and the considerably larger local excitation on the benzene ring and CT from the three regions (\(q\left(RCT\right)\) of 0.246\(e\) and \(q\left(RLE\right)\) of 0.489 \(e\) – see Fig. S8(d) in the SI). The pNA–COOH and pNA–CN S6 bright states are mainly a local excitation on the benzene ring (\(q\left(RLE\right)\)), which equals 0.436\(e\) and 0.319\(e\), respectively – see Fig. S8 (a) and (b) in the SI), whereas the S4 bright state of pNA–SO2H has a delocalized NTO spread over the benzene ring and the amino and nitro groups. The electron-withdrawing effect of these substituents is more pronounced compared with the halogenic substituents, hence the corresponding NTOs of these states are more delocalized over a larger portion of the molecule because the substituents compete with the withdrawing character of the nitro group already present in the unsubstituted pNA molecule. Due to this greater withdrawing nature, as confirmed by the corresponding larger Hammett’s constants, the brighter states of the second band show a small CT from the three regions to the substituent of pNA–CN (\(q\left(SCT\right)=0.075\)) and pNA–SO2H (\(q\left(SCT\right)=0.008e\)), and a local excitation for pNA–COOH (\(q\left(SLE\right)=0.023\)) and pNA–CN (\(q\left(SLE\right)=0.013e\)) – see Fig. S8 in the SI.
The spectra of the EWGs –CCl3, –NO2, and –CF3 are depicted in Fig. 5(b). The –CF3 and –CCl3 substituents are electrically neutral and bulky moieties, which restrict their interaction with the other functional groups thus limiting the shift of the absorption spectrum [72]. The pNA–NO2 molecule has Cs symmetry, with all bright states having \(A’\) symmetry. These EWGs blue shifted the spectra, the exception being the first band of pNA–CCl3 and the second band of pNA–NO2, which were red shifted by about − 0.0751 eV and − 0.131 eV. The pNA–CF3 molecule is unique in the set because the first bright state is S3 instead of S2, with CT from three regions to the pNA nitro group, but having also a local excitation on the benzene ring – \(q\left(NCT\right)\) and \(q\left(RLE\right)\)of about 0.35\(e\) and 0.26\(e\). For pNA–CCl3, on the other hand, there is a slightly larger CT from the three regions to the substituent CCl3, with \(q\left(SCT\right)\) 0.048\(e\), thus increasing the delocalization of the NTOs throughout the entire molecule. As a result of the electron-withdrawing substituents counteracting the effect of the electron-withdrawing nitro group, there is a smaller CT, with the \(q\left(CT\right)\) value decreasing from 0.631\(e\) in pNA to 0.620\(e\) in pNA–CF3, 0.618\(e\) in pNA–NO2, and 0.608\(e\) in pNA–CCl3, which indicated a slightly larger localized nature. This competition of withdrawing effects between the substituents and the strong electron-withdrawing original nitro group is more pronounced in the second band – see Fig. S13 in the SI. The NTOs of the pNA–CCl3 S9 bright state are delocalized all over the molecule. The pNA–CF3 S7 bright state has the same behavior but with a smaller delocalization degree. The nitro substituent in pNA–NO2 is strong enough to pull more density towards itself, so the S6 bright state is mostly a local excitation on the substituted nitro group with \(q\left(SLE\right)\) = 0.341\(e\) and with a small CT from the three regions to the substituent, with \(q\left(SCT\right)=\) 0.181\(e\). Therefore, the two bands of the experimental spectrum of pNA–NO2[32] shown in Fig. 1 has some charge transfer character.
The aforementioned substituent effects on the spectral properties of pNA can be rationalized by Hammett’s constants. The σm0 (Figure S14), σm (Figure S15), σR (Figure S16), and σI (Figure S17) constants were plotted against the following molecular properties: HOMO-LUMO gap (a), HOMO energy (b), LUMO energy (c), first band maximum energy (d), and second band maximum energy (e). We did not find a correlation between Hammett's constant and the \(q\left(CT\right)\) values of the bright state for both bands. The σI constant did not lead to a good correlation with such properties.
The σm0 constant, the modified substituent effect on the meta position, showed the best correlation with the HOMO and LUMO energies (R2 of 0.83 and 0.99, respectively) – see Figure S14 (b) and (c), showing that LUMO energies decrease with all the studied substituents compared with the unsubstituted pNA. In contrast, the HOMO energies increase for EDGs (i.e., become less negative) and drecrease for EWGs (become more negative). The best correlation with HOMO-LUMO energy gaps was obtained for the σR constants, which quantifies the substituent effect on the electron density of a molecule through resonance. However, it was not very good (R2 = 0.58). Despite that, a discernible trend was found, namely: smaller σR values (EDGs) correspond to smaller energy gaps, whereas larger σR values (EWGs) correspond to larger energy gaps. This trend can be explained by the fact that EDGs usually increase the HOMO energy levels, as discussed earlier, which consequently reduces the HOMO-LUMO energy gap. Conversely, EWGs tend to lower the HOMO energy levels, thereby causing an increase in the HOMO-LUMO energy gap. Even though the –NH2 substituent has the most negative σR value (–0.76), the smallest energy gap is for pNA–OH (9.919 eV), followed by pNA–OCH3 (9.949 eV), and pNA–NH2 (9.988 eV). These results point to potential applications in optoelectronic devices as low-gap materials of the pNA–OH, pNA–OCH3, and pNA–NH2 molecules. These properties make them also valuable for applications as functional dyes and information-storage materials [114].
The best correlation for the theoretical maximum energy computed this study of the first band was obtained for σR (R2 = 0.64). The most significant shifts were red shifts for the strong EDGs pNA–OCH3 (0.369 eV), pNA–OH (0.304 eV), and pNA–NH2 (0.303 eV). In contrast, the largest blueshifts were found for pNA–SO2H (0.108 eV), pNA–NO2 (0.084 eV), and pNA–CN (0.044 eV). These results agree with the discussion in the previous paragraph, considering that the HOMO-LUMO is the dominant transition for the observed spectral band. The computed peak of the second band obtained does not have a good correlation with the Hammett’s constants.