Tuning the intramolecular charge transfer (ICT) process in push–pull systems: effect of nitro groups

Abstract

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.

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Introduction

Intramolecular charge transfer (ICT) is a process in conversion of solar energy into chemical energy allowing photosynthesis for crucial charge-separation processes.¹ Many protein biochromophores and bioluminophores have shown interesting photophysics. Hyperpolarizable organic molecules that display ICT are important building blocks for advance optical materials.² The degree of charge transfer (CT) character of an electronic transition differs significantly between chromophores and is modulated by the nearby environment. One such class of species is the nitrophenolates, which serve as donor–acceptor molecules, or so-called push–pull chromophores, as an electron is transferred from the negatively charged phenolate to the nitro group on photoexcitation, with a significant change in the charge distribution in the molecule. The ICT process depends on both donor and acceptor strength, that is the energy of the HOMO of the donor and LUMO of the acceptor, as well as the extent of electronic coupling between orbitals determined by a spacer. The spacer can act as an insulator or a conductor depending on the magnitude of coupling by the spacer between the two groups. Both the transition energy and oscillatory strength are linked to the coupling between the donor and acceptor group: the weaker the coupling, the smaller the excitation energy.³

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.

Experimental section

Chemicals and solvents

All starting materials were used as received from Sigma-Aldrich without further purification. Ultrapure water, acetonitrile, and dichloromethane, ethanol, DMSO, and others solvents are of analytical grade and found to be completely transparent in the range UV/Vis spectrophotometric studies.

Instrumentation

Electronic absorption spectra were measured on a Varian-UV-Visible spectrophotometer (Carry-100-Bio) in various solvents. The FT-IR spectra of the investigated compounds were recorded in the 4000–400 cm⁻¹ region with a Shimadzu FTIR-8900 spectrophotometer using KBr pellet.

Quantum chemical calculations

DFT calculations were performed with a hybrid functional B3LYP and CAM-B3LYP functional at 6-311G (d, p) basis set using the Berny method with the Gaussian 09W software package.¹² The electronic absorption spectra were calculated using the time-dependent density functional theory (TD-DFT) method at B3LYP/6-311G (d, p) and CAM-B3LYP/6-311G (d, p) using the polarizable continuum model (PCM) and ACN as solvent. The NBO and MEP were calculated using the same method (Chart 1).

Chart 1 Structural formula and abbreviations for investigated molecular systems.

Results and discussions

Tuning the ICT bands

The electronic properties of nitrophenolate derivatives depending on the number of nitro groups were investigated. First, the ICT process in various nitrophenolate derivatives was investigated. Fig. 1 shows that only one strong and highly feasible ICT process is observed at 420 nm in UV-Vis spectra for KNP in ACN. The number and strength of the ICT band can be tuned by increasing the number of nitro groups.

Fig. 1 Absorption spectra of KNP, KDNP and KTNP in ACN.

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.

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.

Solvent effect on ICT process

The solute–solvent interaction was investigated by variation of polarity and the nature of the surrounding solvent molecules. The spectral shift, intensity and shape are influenced by the environment and the polarity of solvents dependent on the number of nitro groups. KNP and KDNP show a larger spectral shift of ICT band at higher wavelength, and this was found to be highly sensitive to solvent polarity (Fig. 3 and 4). On the other hand, KTNP is comparatively less sensitive to solvent polarity and the nature of the solvents (Fig. 5). As shown in Fig. 3, in nonpolar solvents such as dioxane and THF, there is no ICT band, but the ICT band becomes prominent in polar solvent. Among polar solvents, DMSO shows the largest red shift. The low energy ICT band for KDNP is also regulated by the solvent polarity, and decreases with decreasing solvent polarity. But for KTNP, the lower energy ICT band is clearly separated from the higher energy ICT band with decreasing solvent polarity. In the main, a red shift of the lower energy ICT band is observed in polar aprotic solvents, and a blue shift in polar protic solvents (∼45 nm for KNP, ∼40 nm for KDNP and ∼38 nm for KTNP from methanol to DMSO solvent systems). KNP shows strong sensitivity of the ICT band for channel I in different solvents (Fig. 3), but this is absent in nonpolar solvent such as THF and dioxane. However, interestingly, for KTNP the ICT band for channel I is clearly observed even in THF and dioxane. Thus, from solvent-dependent studies (Fig. 3–5) of these nitrophenolate salts, it can be stated that differing sensitivity of ICT bands (lower energy ICT band compared with higher energy ICT band) is observed to solvent polarity.

Fig. 3 Normalized absorption spectra of KNP in various solvents.

Fig. 4 Normalized absorption spectra of KDNP in various solvents.

Fig. 5 Normalized absorption spectra of KTNP in various solvents.

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.

Table 1 Absorption maxima (λ_max) of nitrophenolate derivative

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

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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, E_T (30). The E_T (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 E_T (30) dye) dissolved in the solvent or solvent mixture of interest.¹³ The E_T (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):

E_T (30)/(kcal mol⁻¹) = 28 591/λ_max (1)

where λ_max is the wavelength of the maximum of the long wavelength intramolecular CT absorption band of the betaine 30.

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 E_T values of nitrophenolate derivatives in THF are much higher than the E_T (30) value, ∼31.6 kcal mol⁻¹. Similarly, the difference between the E_T values of nitrophenolate derivatives and the E_T (30) value in H₂O is ∼7.0 kcal mol⁻¹. From the experimental results, it is observed that the difference between the E_T values of nitrophenolate derivatives and appropriate E_T (30) values decreases gradually with increasing E_T (30) value. This indicates that the ICT bands of nitrophenolate derivatives are not largely influenced solvents of lower polarity (E_T value in THF = 68.0 kcal mol⁻¹, E_T (30) value in THF = 37.4 kcal mol⁻¹), rather higher polarity solvents have more effect because of comparable E_T values of nitrophenolate derivatives (E_T value in H₂O = ∼70.0 kcal mol⁻¹, E_T (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 E_T (30) values with respect to solvents (Table 2).

Fig. 6 Solvatochromism of nitrophenolate derivatives. The absorption spectra were measured in H₂O (E_T (30): 63.1 kcal mol⁻¹), MeOH (55.4), EtOH (51.9), ACN (45.6), DMSO (45.1), THF (37.4). E_T (30) values of different solvents are taken from the literature.¹⁴

Table 2 Solvatochromic parameter E_T value of nitrophenolate derivatives

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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Equilibrium constant (K_eq) from Benesi Hildebrand spectroscopic method

To further investigate ICT processes in nitrophenolic derivatives, strong donor–acceptor systems were created from weak donor–acceptors using strong alkaline bases, such as KOH, as activators to investigate the combined effect of donor and acceptor. Initially, HNP (4-nitrophenol) showed only two characteristic bands attributed to the presence of a strong ICT band at 308 nm and a π → π* transition at 225 nm in ACN (Fig. 7). However, conversion of HNP to KNP by addition of KOH caused a new strong ICT band to appear at 420 nm in ACN, while the original ICT band disappeared (Fig. 7). The figure also clearly indicates that it is the ICT band at 308 nm which has shifted to 420 nm. A relatively insignificant blue shift of the π → π* transition band (234 nm to 225 nm) was also observed. In the case of HDNP (2,4-dinitrophenol), two absorption bands attributed to ICT and π → π* transition were observed at 331 nm and 258 nm, respectively, in ACN (Fig. 8). Conversion of KDNP to HDNP by addition of KOH, caused a new but similar characteristic strong ICT absorption band to appear at 421 nm along with a 371 nm band (shifted from 331 nm) (Fig. 8). Several isosbestic points were consequently observed (at 315, 232, 264 and 351 nm). The π → π* transition band shifted to 219 nm from 258 nm on KDNP formation with two clear isosbestic points at 240 nm and 330 nm. In the case of HTNP (2,4,6-trinitrophenol), an ICT band at 435 nm along with stronger ICT band at 338 nm were observed. At 235 nm another band is observed, which is results from π → π* transition. However, on formation of KTNP from HTNP, the ICT absorption band shifted to 375 nm from 338 nm (Fig. 9). The π → π* transition band was also blue shifted. Thus, increasing the number of nitro groups on phenolate moieties appears to significantly reduce low energy ICT bands, whereas the high energy bands are increased.

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).

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).

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 (K_eq) 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 (K_eq) was determined by monitoring the ICT bands, and calculated according to eqn (2) for a 1 : 1 system

where A₀ = absorbance without KOH, A = absorbance on addition of KOH, A_max = absorbance at the end of titration (final KOH addition), K_eq = equilibrium constant.

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 (K_eq), intercept and slope were obtained from linear fit curve (data presented in Table 3). The K_eq of KNP is 5.95 × 10⁵ cm³ mol⁻¹, 100 times higher than those of KDNP (K_eq = 7.55 × 10³ cm³ mol⁻¹) and KTNP (K_eq = 2.4 × 10³ cm³ mol⁻¹). The equilibrium constant values of KDNP and KTNP are comparable with previously reported values.⁹

Table 3 Equilibrium constant (K_eq) of nitrophenolate derivatives in ACN. χ² values indicate goodness-of-fit

	Intercept (1/[A − A0])	Slope (1/KA[A − A0])	χ^2	KA (cm^3 mol^−1)
KNP	0.10	1.68 × 10^−7	0.99	5.95 × 10^5 (@420 nm)
KDNP	0.43	5.69 × 10^−5	0.99	7.55 × 10^3 (@421 nm)
KTNP	0.17	6.90 × 10^−5	0.99	2.46 × 10^3 (@435 nm)

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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).

Table 4 Spectral band shape and area and full width half maxima (FWHM)

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

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Quantum chemical calculation

Quantum chemical calculations were performed for the nitrophenolate derivatives to obtain useful structural and electronic information from optimized geometry and TD-DFT calculation, respectively. Hybrid functional B3LYP and CAM-B3LYP (Becke's three parameter hybrid functional using the LYP correlation functional) at 6-311G (d, p) basis were used. Optimized geometry of KNP reveals that the potassium ion is directly and linearly attached to the oxygen atom of the hydroxyl group (ESI-Fig. 2a†). However, in KDNP the potassium ion acts as a bridging atom between the oxygen of the hydroxyl group and the oxygen of the nitro group at the ortho position. This allows an overlap with the oxygen atom of the neighbour nitro group having bond angle 66.9 Å, with 2.50 Å and 2.42 Å, respectively (ESI-Fig. 2b†). Prominent bridging activity of the K⁺ ion is observed in KTNP derivative, where the K⁺ ion makes a bridge with three oxygen atoms (one oxygen atom from the hydroxyl group and two oxygen atoms from adjacent nitro groups) with bonds 2.44 Å (hydroxyl group) and 2.91 Å (oxygen atoms from adjacent nitro groups), respectively, along with a bond angle of 61.4° (ESI-Fig. 2c†). Transformation from single bond to partial double bond character of the oxygen atom of hydroxyl group in nitrophenolate derivatives is observed with increasing nitro groups. The bond distance of the hydroxyl group (C–O) in KNP is found to be 1.29 Å, shorter than that of pure C–O bond (1.43 Å). The bond distance of the hydroxyl group (C–O) in KDNP and KTNP is 1.26 Å and 1.24 Å, respectively, comparable with that of pure C=O bond (1.20 Å). The above theoretical calculation suggests that the hydroxyl group from nitrophenolate derivatives moves to having partial double bond character because of charge transfer from the oxygen atom of the hydroxyl group to the benzene ring in the presence of the nitro group.

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.†

Table 5 HOMO–LUMO diagram obtained from TD-DFT calculation

KNP	KDNP	KTNP

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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).

Table 6 Electronic transition from TD-DFT calculation^a

	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)

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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.

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

where q_i is the donor orbital occupancy, ε_j and ε_i are diagonal elements and F(i,j) is the off diagonal NBO Fock matrix element. The larger the E⁽²⁾ value, the more intensive is the interaction between electron donor and acceptor, that is the more donation tendency from electron donors to electron acceptors and the greater the extent of conjugation of the whole system.

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.

Table 7 Second-order perturbation energies E⁽²⁾ (donor → acceptor)

Donor NBO (i) electron	Acceptor NBO (j) electron	E^(2)^a (kcal mol^−1)	ΔE^b = 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
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
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

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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.

Fig. 11 (a–c) Molecular electrostatic potential (MEP) of various molecular complexes studied.

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.

Conclusion

In conclusion, the intramolecular charge transfer (ICT) process was investigated in a series of synthesized nitrophenolate derivatives. It is found that depending on the number of nitro (acceptor) moieties present, the electron density redistributes via multiple ICT processes. In KNP, a highly feasible strong ICT process was observed at 420 nm (designated as channel I ICT process) in ACN (one donor (O⁻) and one acceptor (NO₃⁻) system). When an additional nitro group was introduced in the KDNP system (one donor-two acceptor system), a new additional ICT channel (designated as Channel II) opened up at 371 nm. Introduction of a third nitro group at the ortho position led to a considerable increased intensity of the 371 nm band, whereas the 421 nm band intensity diminished and no new band appeared. The ICT band (channel I) for KNP is highly influenced by solvent polarity, but this is much less evident for KTNP. The equilibrium constants (K_eq) for these complexes were also found to be dependent on number of nitro groups (∼10⁵ for KNP, ∼10³ for KDNP and KTNP in ACN).

Acknowledgements

We acknowledge CSIR, New Delhi (code: 01(2792)/14) for research grants and also Department of Chemistry, Banaras Hindu University, Varanasi-221005, India, for providing instrumentations and infrastructure facilities. Authors also acknowledge the CMSD facility at HCU, Hyderabad for computation.

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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.

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