Sumit Kumar Panja‡
,
Nidhi Dwivedi‡ and
Satyen Saha‡*
Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi-221005, India. E-mail: satyen.saha@gmail.com
First published on 20th October 2016
The intramolecular charge transfer (ICT) process in donor–acceptor systems has tremendous importance in various physical and biological systems. Three nitrophenolate salts were synthesized and studied here. The ICT and π → π* transition processes were identified in these derivatives using UV-Vis spectroscopy and theoretical calculations. It was observed that by simple substitution with nitro groups, one can generate and control the ICT process by regulating the charge distribution over the molecule. While for a monosubstitute nitro derivative, only one ICT band was observed, additional ICT processes can be generated at will by introducing a second nitro group. The intensity of this second ICT channel can be regulated with introduction of a third nitro group. Further, the association constants and solvation processes for these potassium nitrophenolate derivatives were found to be drastically dependent on the number of ICT channels present in the molecule. Theoretical studies (MEP analysis) support the experimental observations presented here. The results show that by simply introducing additional acceptor groups to the system, one can tune the ICT band efficiently in a conjugate system.
In this regard, it is advantageous to know the intrinsic absorption of the isolated molecule or ion to shed light on the electronic perturbation by a microenvironment. The degree of CT character in terms of an electronic transition differs significantly depending upon donor–acceptor groups and is modulated by the nearby environment. Ionophores are not easily dissolved in nonpolar solvents and even then there are field effects from counter ions. In polar solvents, the ground and excited states are stabilized to different extents, which results in solvatochromic shifts. When electron-withdrawing groups which can stabilize the phenoxide ion are present, substantial changes can occur in acidity of the OH group, which is highly sensitive towards its environment.4 Moreover, the hydroxyl group is a weak electron donating group, whereas the deprotonated hydroxyl group, that is O−, is a much stronger donating group with a negative charge that can be delocalized along a conjugated path involving the benzene ring. As a consequence, the phenol and the corresponding phenolate ion display very different absorption optical properties.
Phenolic compounds, specifically, nitrophenolic derivatives, are extensively used as model compounds for structural studies, extraction and transport through membranes. The polarizable and soft nitrophenolate ion is often used to facilitate and increase extraction and transport of alkaline metal cations through solvents of low polarity.5 Interesting nonlinear optical effects are observed extended to optical parametric amplifiers, optical parametric oscillators, Q-switched intracavity second harmonic devices, high optical damage threshold and other electro-optical applications.6 It has been reported that nitrophenolic derivatives show significant changes in spectral pattern and maxima of absorption spectra when a complex is formed with different amines in solution.7 So far, only few experimental and theoretical studies reported results with nitrophenolate derivatives.8 El-Dossoki measured the ion-pair association of sodium and potassium picrate in 2-butanone by conductometric and spectroscopic studies.9 Ahmed et al. reported that nitrophenolic derivatives are highly efficient for charge transfer complex formation.10 Kirkrterp et al. reported the effect of electronic coupling between two orbitals conveyed by a spacer group on ICT processes of nitro phenolate ions in vacuum and in solution.11
In the present study, the effect of nitro groups on these two different ICT processes was studied by experimental and theoretical methods. It is shown that it is possible to create two distinct ICT pathways by simple incorporation of nitro groups at different positions on the phenyl ring. Solvent effect on these two ICT bands was monitored and explained on the basis of standard solvent polarity model.
As can be seen from Fig. 1 (red line), incorporation of a second nitro group results in two ICT bands at 421 nm and 371 nm (for KDNP in ACN). The higher wavelength band is attributed to channel I and the lower wavelength band to channel II (schematically depicted in Fig. 2). As a result of longer conjugation and presence of channel 1, the absorption band has lower energy, that is higher wavelength.
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Fig. 2 Schematic representation of ICT channels in nitrophenolate derivatives according to nitro groups. |
Further, when a third nitro group is incorporated, a new ICT channel (channel III) is generated (resulting in a one donor-three acceptor system), but this new channel overlaps with channel II (at 375 nm) because of the similarity of their positions (both are at the ortho position in KTNP, Fig. 1, blue line).
The highly feasible and intense ICT bands in KNP are caused by delocalized charge over molecules. Similarly, two highly feasible ICT bands (one ICT at higher wavelength attributed to channel I and another at lower wavelength attributed to channel II) in KDNP (one donor-two acceptor system) result from the ICT process in the two different channels. Interestingly, in KTNP the band at lower wavelength becomes much stronger because of the combined contributions of channels II and III. The intensity and feasibility of these two ICT processes can be regulated, therefore, by varying the number of nitro groups.
In addition, for KNP, the π–π* transition is clearly visible in EtOH and MeOH, but completely absent in water. The ICT band (for channel I) is quite sensitive to polarity, as evident from the ∼45 nm shift of peak maxima from DMSO to methanol. Table 1 shows the variation of wavelength maxima of ICT band with solvent polarity.
Solvent | λmax (nm) | ||
---|---|---|---|
KNP | KDNP | KTNP | |
DMSO | 435 | 431 | 435 |
ACN | 420 | 421 | 430 |
H2O | 398 | 400 | 416 |
PrOH | 410 | 403 | 420 |
EtOH | 402 | 398 | 415 |
MeOH | 390 | 395 | 410 |
THF | 310 | 416 | 421 |
Dioxane | 302 | 417 | 421 |
After detailed analysis of UV-Vis spectra for K-salts in different solvents, it is suggested that ‘ionic couple’ solvation occurs in polar aprotic solvents such as ACN, DMSO, whereas ‘ion solvation’ occurs in polar protic solvents such as H2O, EtOH and MeOH. In the latter case, the ‘ion couples’ are completely disrupted by the protic polar solvents, thus the ions are solvated separately resulting in the ‘ion solvation’.
The solvatochromic behavior of nitrophenolate derivatives was investigated using the empirical solvent polarity parameter, ET (30). The ET (30) scale has widespread applications in study of chemical processes in solvents. This parameter is usually measured by UV-Vis spectrophotometric measurements of the longest wavelength ICT absorption band of Reichardt's pyridinium-N-phenolate betaine dye (known as ET (30) dye) dissolved in the solvent or solvent mixture of interest.13 The ET (30) values, empirically derived from solvatochromic measurements, are simply defined as the molar transition energies (in kcal mol−1) of the standard betaine dye 30, measured in solvents of different polarity at room temperature (25 °C) and normal pressure (1 bar), according to eqn (1):
ET (30)/(kcal mol−1) = 28![]() | (1) |
Using eqn (1), the molar transition energies (in kcal mol−1) of nitrophenolate derivatives in different solvents were calculated (Fig. 6 and Table 2). The ET values of nitrophenolate derivatives in THF are much higher than the ET (30) value, ∼31.6 kcal mol−1. Similarly, the difference between the ET values of nitrophenolate derivatives and the ET (30) value in H2O is ∼7.0 kcal mol−1. From the experimental results, it is observed that the difference between the ET values of nitrophenolate derivatives and appropriate ET (30) values decreases gradually with increasing ET (30) value. This indicates that the ICT bands of nitrophenolate derivatives are not largely influenced solvents of lower polarity (ET value in THF = 68.0 kcal mol−1, ET (30) value in THF = 37.4 kcal mol−1), rather higher polarity solvents have more effect because of comparable ET values of nitrophenolate derivatives (ET value in H2O = ∼70.0 kcal mol−1, ET (30) value in H2O = 63.1 kcal mol−1). Further, the transition energy is not changed drastically depending on the solvents, and is relatively higher than the ET (30) values with respect to solvents (Table 2).
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Fig. 6 Solvatochromism of nitrophenolate derivatives. The absorption spectra were measured in H2O (ET (30): 63.1 kcal mol−1), MeOH (55.4), EtOH (51.9), ACN (45.6), DMSO (45.1), THF (37.4). ET (30) values of different solvents are taken from the literature.14 |
Solvent | ET value (λmax (nm)) | ||
---|---|---|---|
KNP | KDNP | KTNP | |
DMSO | 65.7 (435 nm) | 66.3 (431 nm) | 65.7 (435 nm) |
ACN | 68.0 (420 nm) | 67.9 (421 nm) | 66.4 (430 nm) |
H2O | 71.8 (398 nm) | 71.4 (400 nm) | 68.7 (416 nm) |
PrOH | 69.7 (410 nm) | 70.9 (403 nm) | 68.0 (420 nm) |
EtOH | 71.1 (402 nm) | 71.8 (398 nm) | 68.8 (415 nm) |
MeOH | 73.3 (390 nm) | 72.3 (395 nm) | 69.7 (410 nm) |
THF | — | 68.7 (416 nm) | 67.9 (421 nm) |
Dioxane | — | 68.5 (417 nm) | 67.9 (421 nm) |
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Fig. 7 UV-Vis titration for formation of KNP. KOH is added successively (0–2.28 × 10−4 M) in HNP (3.35 × 10−5 M in ACN). |
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Fig. 8 UV-Vis titration for formation of KDNP. KOH is added successively (0–6.14 × 10−5 M) in HDNP (5.06 × 10−5 M in ACN). |
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Fig. 9 UV-Vis titration for formation of KTNP. KOH is added successively (0–7.60 × 10−5 M) in HTNP (4.65 × 10−5 M) ACN. |
It is clear the ICT process is modified by the presence of strong base such as KOH in ACN (aq. KOH solution), in that phenolic O–H (weaker donor) is converted to phenolic O− (strong donor), allowing generation of a strong donor–acceptor system and a highly feasible ICT process. The experimental results suggest that the relative charge density on the oxygen atom of the C–O group is altered according to the number of nitro groups. The charge density on the oxygen atom is higher and acts as an electron donor. This charge density easily can be delocalized to the benzo-nitro moiety causing a strong ICT process. The presence of two nitro groups (in KDP) means that the charge density on the oxygen atom (C–O) is redistributed over the molecule and delocalized primarily to two nitro groups, resulting in two strong ICT processes. However, the presence of three nitro groups (KTP) causes suppression of the ICT process at higher wavelengths because of the doubly favourable ICT processes at lower wavelengths. The above observations are supported by DFT calculations as discussed in the following section.
From the spectral shifts in solvent-dependent studies, it appears that different solvated states exist depending on the nature and polarity of the solvents. The ion-pair solvation increases with variation of solvent polarity and feasible ICT process (red shift of ICT band with increasing solvent polarity with respect to dielectric constant). The results were interpreted to support the existence of a strong ionic O⋯K structure. Experimentally, a red shift of the ICT band is observed in aprotic polar solvents {(MeOH (32.7)) to DMSO (46.7)}, but a blue shift of the ICT band (∼40 nm) is observed in protic polar solvents.
The Benesi-Hildebrand method was employed to determine the equilibrium constant (Keq) by UV-Vis spectroscopy. This is an important parameter for understanding of the nature of ion-pairs in solution. On the basis of the Benesi-Hildebrand (BA) spectroscopic method, the equilibrium constant (Keq) was determined by monitoring the ICT bands, and calculated according to eqn (2) for a 1:
1 system
![]() | (2) |
Spectrophotometric titrations allowed determination of the ion equilibrium constant using the above equation. Fig. 6–8 depict the B–H plot obtained using eqn (1) from the UV-Vis titration of all. The B–H plot was carried out at particular wavelength (420 nm for KNP formation, 421 nm for KDNP and 435 nm for KTNP). In all cases, B–H plots were fitted linearly with eqn (1) (vide ESI-Fig. 1a–c†). The linearity of the B–H plots indicates that experimental results are acceptable for the 1:
1 system. Equilibrium constant (Keq), intercept and slope were obtained from linear fit curve (data presented in Table 3). The Keq of KNP is 5.95 × 105 cm3 mol−1, 100 times higher than those of KDNP (Keq = 7.55 × 103 cm3 mol−1) and KTNP (Keq = 2.4 × 103 cm3 mol−1). The equilibrium constant values of KDNP and KTNP are comparable with previously reported values.9
Intercept (1/[A − A0]) | Slope (1/KA[A − A0]) | χ2 | KA (cm3 mol−1) | |
---|---|---|---|---|
KNP | 0.10 | 1.68 × 10−7 | 0.99 | 5.95 × 105 (@420 nm) |
KDNP | 0.43 | 5.69 × 10−5 | 0.99 | 7.55 × 103 (@421 nm) |
KTNP | 0.17 | 6.90 × 10−5 | 0.99 | 2.46 × 103 (@435 nm) |
UV-Vis studies of these nitrophenolate derivatives allow prediction of a quantitative electronic energy level diagram to understand alternation of energy levels depending upon number of nitro groups. The ICT-1 state is for channel I and ICT-2 is for both channel II and channel III. As expected, the ratio of spectral areas of two ICT bands {area (channel I)/area (channel II/channel III)} decreases with increasing number of nitro groups, indicating strong interaction between benzene ring electrons and electron acceptor groups (Table 4). The ratio of A421 (channel I)/A369 (channel II) is 0.98 for KDNP and the ratio of A421 (channel I)/A369 (channel II and channel III) is 0.20 for KTNP. The above result indicates that incorporation of channel III (presence of a third nitro group) induces a drastic change in spectral area of the UV-Vis spectra of KTNP derivatives, implying the ICT-1 state is less probable compared with the ICT-2 state (Fig. 2).
Systems | Peak position | Area (a.u.) | FWHM (nm) | Height (a.u.) | Ratio |
---|---|---|---|---|---|
KNP | 420 nm | 14.68 | 52.95 | 0.26 | — |
KDNP | 421 nm | 9.82 | 50.62 | 0.18 | A421/A369 = 0.98 |
369 nm | 10.42 | 51.33 | 0.19 | ||
KTNP | 435 nm | 27.12 | 35.37 | 0.60 | A435/A375 = 0.20 |
375 nm | 132.29 | 61.30 | 1.72 |
TD-DFT calculation at B3LYP/6-311G (d, p) and CAM-B3LYP/6-311G (d, p) using the polarizable continuum model (PCM) and gas phase was performed to better understand the electronic properties of these nitrophenolate derivatives. TD-DFT results at CAM-B3LYP/6-311G (d, p) using PCM model and ACN as solvent are reported in Table 5 and detailed in ESI-Table 1.†
This shows existence of a ICT and one π → π transition in KNP. The ICT transition in KNP appears at 354 nm attributed to HOMO to LUMO transition. The π → π transition in KNP is calculated at 268 nm (experimentally observed at 309 nm) attributed to HOMO to LUMO (+1) transition orbitals. Similarly, the two ICT transitions in KDNP appeared at 365 nm (experimentally 421 nm) and 330 nm (experimentally 371 nm) attributed to HOMO to LUMO and HOMO to LUMO (+1) transition orbitals, respectively. Further, the two ICT transitions in KTNP were calculated at 388 nm (experimentally at 435 nm) and 357 nm (experimentally at 375 nm) attributed to HOMO to LUMO (+1) and HOMO to LUMO (+1) transition orbitals, respectively. Hence, these TD-DFT calculations imply that the charge of anion is distributed to two different channels from orbital contribution during electronic transition for DNP and TNP anions from HOMO to LUMO. For the DNP anion, the first electronic transition (HOMO to LUMO; f = 0.259; ICT channel I) results from the participation orbital of the para nitro group. The second transition (HOMO to LUMO (+1); f = 0.3052; ICT channel II) results from higher contribution of the orbital of one ortho nitro group. For the TNP anion, the first electronic transition (HOMO to LUMO; f = 0.0154; ICT-channel I) results from involvement of the orbital of the para nitro group for the charge distribution process, giving a very weak ICT process for ICT channel I. The second electronic transition (HOMO to LUMO (+1) transition; f = 0.69258; ICT channel II) results mainly from the participation orbital of the two ortho nitro groups. The energy difference between the first and second excited state energy levels increases gradually with increase in nitro groups. From orbital contribution analysis, it was observed that it is mainly the π → π* transition that is involved in electronic transition. The electron density of the π* orbitals is altered depending on position and number of nitrogroups during electronic transition (Table 6).
CAM-B3LYP/6-311G (d, p) | B3LYP/6-311G (d, p) | |||
---|---|---|---|---|
GAS phase | ACN | GAS phase | ACN | |
a f = oscillator strength; nm = nanometre. | ||||
KNP | 468 nm (f = 0.0005) | 375 (f = 0.5156) | 363 (f = 0.0008) | 354 nm (f = 0.5597) |
KDNP | 402 (f = 0.1154) | 418 nm (f = 0.1987) | 352 nm (f = 0.1650) | 365 nm (f = 0.2593) |
KTNP | 406 nm (f = 0.1222) | 415 nm (f = 0.1606) | 348 nm (f = 0.0994) | 388 nm (f = 0.0154) |
The CAM-B3LYP functional provides better correlation compared with the B3LYP functional. From the involvement of orbitals in the ICT transition process, it can be stated that involvement of benzene π electrons with nitro groups is important in the ICT process. In the case of KNP, benzene π electrons strongly interact with orbitals of nitro groups. This interaction decreases with increase of nitro groups at ortho positions.
Distributions and energy levels of HOMO and LUMO orbitals computed with CAM-B3LYP/6-311++G (d, p) level are given in Table 6. From experimental and theoretical results, a schematic energy level diagram for the investigated nitrophenolate derivatives is given in Fig. 10.
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Fig. 10 Comparison of theoretically calculated (CAM-B3LYP) energy levels with experimentally obtained energy levels. |
Natural Bond Orbital (NBO) analysis provides a possible natural Lewis structure with all orbitals mathematically chosen to include the highest possible percentage of the electron density. Information on interaction between both filled and virtual orbital spaces as correctly explained by NBO analysis would enhance the analysis of intra- and inter-molecular interactions. A second-order Fock matrix was carried out to evaluate donor (i)–acceptor (j), that is donor level bond to acceptor level bond interaction in NBO analysis. The result of interaction is a loss of occupancy from the concentration of electron NBO of the idealized Lewis structure into an empty non-Lewis orbital. For each donor (i) and acceptor (j), the stabilization energy E(2) associated with the delocalization i → j is estimated as
Delocalization of electron density between occupied Lewis type (bond or lone pair) NBO orbitals and formally unoccupied (anti bond or Rydberg) non-Lewis NBO orbitals corresponds to a stabilizing donor–acceptor interaction. One deprotonated hydroxyl group (electron donor) in the aromatic ring and lone pair on the oxygen atom interact with the π system of the aromatic ring and nitro group. The intramolecular interaction is formed by orbital overlap between LP(O) → BD*(1)C1–C6 and → BD*(1)C5–C6 bond orbital, causing intra-molecular charge transfer and system stabilization. One of the most important interactions in this molecule having lone pair LP(3)O14 with that of BD*(1)C5–C6 results in stabilization of 56.3 kcal mol−1 in KNP. Interaction between the lone pair of LP(2)O13 with BD*(1)C5–C6 results in a stabilization energy of 13.86 kcal mol−1. Further, interaction between the lone pair of LP(2)O12 with BD*(1)C5–C6 results in stabilization energy of 17.49 kcal mol−1 in KTNP. These results clearly state that delocalization of charge is highly effective for KNP but lowest for KTNP, as shown in Table 7.
Donor NBO (i) electron | Acceptor NBO (j) electron | E(2)a (kcal mol−1) | ΔEb = E(i) − E(j) (a.u.) | F(ij)c (a.u.) |
---|---|---|---|---|
a Second-order perturbation energies.b The stabilization energy.c Off diagonal NBO Fock matrix element. | ||||
KNP | ||||
LP(2)O14 | BD*(1)C1–C6 | 12.02 | 0.79 | 0.088 |
LP(2)O14 | BD*(1)C5–C6 | 12.02 | 0.79 | 0.088 |
LP(3)O14 | BD*(2)C5–C6 | 56.39 | 0.29 | 0.118 |
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KDNP | ||||
LP(1)O13 | BD*(1)C1–C6 | 1.91 | 1.17 | 0.043 |
LP(1)O13 | BD*(1)C5–C6 | 1.70 | 1.20 | 0.040 |
LP(2)O13 | BD*(1)C1–C6 | 16.75 | 0.73 | 0.100 |
LP(2)O13 | BD*(1)C5–C6 | 13.86 | 0.76 | 0.093 |
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KTNP | ||||
LP(1)O12 | BD*(1)C1–C6 | 1.91 | 1.17 | 0.043 |
LP(1)O12 | BD*(1)C5–C6 | 1.91 | 1.17 | 0.043 |
LP(2)O12 | BD*(1)C1–C6 | 17.49 | 0.73 | 0.102 |
LP(2)O12 | BD*(1)C5–C6 | 17.49 | 0.73 | 0.102 |
The molecular electrostatic potential (MEP) is related to the electronic density and is a very useful descriptor in understanding sites of electrophilic attack and nucleophilic reactions as well as hydrogen-bonding interactions. The electrostatic potential V(r) is also well suited for analysing processes based on ‘recognition’ of one molecule by another, as in drug–receptor, and enzyme–substrate interactions, because it is through their potentials that the two species first “see” each other.
Being a real physical property, V(r) can be determined experimentally by diffraction or by computational methods. To visualize consider the most probable sites of on the nitrophenolate derivatives for interaction with electrophilic and nucleophilic species, MEP was calculated at the DFT/B3LYP/6-311++G (d, p). MEP of the nitrophenolate derivatives in presented in Fig. 11.
While electrophilic reactivity is visualized by the yellow colour indicative of negative regions of the molecule, the nucleophilic reactivity is coloured by blue, indicating the positive regions of the molecule. The MEP of KNP shows a yellow colour of the nitro group at the para position whereas the potassium atom is blue. Similarly, it is shown that the nitro group at the para position is light yellow along with appearance of a very light yellow colour of nitro group at the ortho position and the potassium atom from MEP of KDNP. In KTNP, the colour of the nitro group at the para position is yellow along with appearance of a very light yellow for the nitro group at the ortho position (both) and the potassium atom is blue.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17521j |
‡ Present address of SKP is Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, India. |
This journal is © The Royal Society of Chemistry 2016 |