Phototunable Absorption and Nonlinear Optical Properties of Thermally Stable Dihydroazulene-Vinylheptafulvene Photochrome Pair.

The UV-vis absorption characteristics and nonlinear optical properties of a series of substituted dihydroazulene (DHA)/vinylheptafulvene (VHF) photoswitches are investigated by applying quantum calculations. Introduction of substituents at the seven-membered ring resulted in significant changes in their absorption properties depending on the nature and position of the substituent. Electron-donating groups at positions 5, 6, 7, and 8 generally exhibited red shifts with respect to the parent compound. However, the steric effect at positions 8a and 4 is responsible for the loss of planarity and conjugation, which generally leads to blue shifts. In contrast, any electron-withdrawing group, particularly at positions 8a and 4, would cause a blue shift. The presence of bulky groups at position 8a results in a loss of planarity and, as a result, a decrease in electronic conjugation within the molecule, resulting in a blue shift in the maximum absorption. When it comes to halogens, the red shift is directly correlated to the nucleophilicity; the higher the nucleophilicity, the larger the red shift. Regarding hyperpolarizability, the charge separation induces higher hyperpolarizabilities for all substituted VHFs compared to the corresponding DHAs, resulting in a much higher NLO response. In addition, for all DHA and VHF, the highest values of hyperpolarizabilities are calculated for 6-substituted systems. Finally, the objective of this detailed theoretical investigation is to continue exploring the photophysical properties of DHA-VHF through structural modifications.

Introduction

Molecular photoswitches[1] have received increasing attention from the scientific community because of their widespread applications in memory devices,[2,3] nonlinear optics (NLO),[4−6] electromagnetic switches,[7−9] synthetic ion channels,[10,11] liquid crystals,[12−22] biological processes,[23] and photopharmacology.[24] In addition to this, a large number of complex devices such as logic gates,[25] half adder,[26−34] and encoder–decoder[35] principally rely on the photoswitches as well. They possess a unique property related to isomeric interconversion occurring at different wavelengths. In general, there are several decisive factors, which enhance the efficiency of the photoswitches including fatigue resistance, thermal stability, nondestructive readout, and photochemical quantum yield. In order to prevent the occurrence of photostationary state (an isomeric coexistence at equilibrium), photoswitches require minimal overlap of absorption spectra of isomeric states for the applications in NLO and memory devices. Additionally, the polarizability difference among isomeric states should be considerably significant for an optimum performance.

Molecular photoswitches have several distinctive classes. Fulgides[36] and dithienylethenes[10] are extensively explored due to their thermal stability. Dihydroazulene (DHA) and vinylheptafulvene (VHF) comprise another important class (Figure ), which displays a number of practical applications and require further investigations. DHA is a colorless compound which is thermodynamically more stable than its colored counterpart VHF.[37−44] These isomers can easily undergo interconversion upon exposure to light and/or heat. Vinylheptafulvene consists of two conformers, i.e., s-cis and s-trans, which are at equilibrium and surprisingly, photochemically generated s-cis conformer is comparatively more stable than the s-trans conformer.[37−44] Thermal instability (which leads conversion of VHF into DHA (T-type photoswitch)[45] and associated synthetic difficulties render significant challenges toward further development of this class for industrial applications.

Figure 1

(A) DHA–VHF isomeric interconversion and (B) simplified optimized structures of (a) DHA and (b) VHF photoswitches. The arrows indicate the two possible pathways I or II for electron delocalization. Φ1 is the dihedral angle C2–C3–C3a–C8a and Φ2 the dihedral angle C2–C3–C3a–C8.

In order to enhance the efficiency of DHA–VHF photoswitches and gain further insight, considerable efforts have been made to design multimode photoswitches by combining dithienylethenes and other photochromic compounds with DHA–VHF photoswitches.[46] Moreover, the relationship between the physical properties of photoswitches and the substitution effect has been thoroughly investigated. Structural modifications by introducing a suitable functional atom/group have been successfully achieved for both five- and seven-membered rings of DHA,[47−49] though functionalization at the seven-membered ring is thwarted by serious synthetic challenges. However, successful functionalization at the seven-membered ring imparts significant value to DHA–VHF photoswitches as reported for the protonation of the amino group on the phenylethynyl DHA X.[48] This led to deactivation of VHF X into the corresponding DHA X under thermal conditions.

To gain further insight into the effect of substituents on the reactivity pattern, we have reported computational studies to evaluate the properties of DHA–VHF photoswitches and activation barriers for electrocyclization.[50] By performing thermal return reactions for several substituted VHFs to DHFs, it was revealed that the position and nature of the substituents at the seven-membered rings had a substantial effect on the electronic properties of the DHA–VHF photoswitches. In addition to this, a noticeable difference (up to 7 kcal mol–1) was observed for the electrocyclization by altering the position/nature of substituents. For example, 5-OH-VHF requires 23.7 kcal mol–1 for thermal cyclization into DHA, whereas 4-OH-VHF needs a 30.5 kcal mol–1 energy barrier for the same transformation. Contrary to this, the electron-withdrawing group imparts quite the opposite trend. The activation barrier for cyclization of 5-CHO-VHF requires 29.5 kcal mol–1 compared to 23.7 kcal mol–1 for 5-OH-VHF. Moreover, the presence of other groups at position 7 of VHF provides similar trend.[48]

In continuation to our ongoing interest in this field,[51] we present here a comprehensive study to explore the effect of substituents located at the seven-membered ring of DHA. The objective of this detailed theoretical investigation is to continue exploring the photophysical properties of DHA–VHF through structural modifications. Furthermore, the calculation of hyperpolarizabilities is also included in the study since the difference of hyperpolarizability of both isomers should be significant for the practical application in nonlinear optics devices.

Computational Methods

The calculations were performed with Gaussian 09.[52] Geometries of all structures were optimized without any symmetry constraints at PBE1PBE/6-311+G(d). The PBE1PBE is a well-performing, cost-effective method known to accurately predict the structural properties of organic dyes including DHA–VHF.[50] We have also shown in our recent report that the energies and geometries of DHA/VHF in the PBE0 method correlate nicely with the experimental results.[50] Frequency calculations have also been performed at PBE1PBE/6-311+G(d) in order to confirm the structures as true minima (no imaginary frequency). The UV–vis spectra are modeled for the TD-DFT method at the CAM-B3LYP/6-311+G(d) level of theory on PBE1PBE/6-311+G(d)-optimized structures. The reported absorption maxima are absorptions at maximum wavelengths with oscillator strengths greater than 0.2. The hyperpolarizability values are also calculated at the CAM-B3LYP/6-311+G(d) level of theory, which provides good agreement between accuracy and computational cost.

Results and Discussion

Organic photochromes find applications in many disciplines, but their practical applications primarily depend upon their thermal stability. This switching process should only be photocontrolled, and the two isomeric species should not have any spectral overlap in the UV–vis region. A spectral overlap causes an incomplete interconversion of both isomers leading to the formation of a photostationary state. The relative concentration of both components in the photostationary state depends on the irradiation time, irradiation wavelength, and rate of thermal return.[53] The purpose of the study is to explore the substituents which when installed on the DHA–VHF scaffold lead to spectral nonoverlap.

Since DHA and VHF absorb in the UV and visible region, respectively, spectral nonoverlap is better achieved if the UV–vis spectra of DHA and VHF are blue- and red-shifted, respectively. The UV–vis spectra of DHA and VHF are extensively studied experimentally and theoretically. Jacquemin and co-workers have studied the UV–vis spectra of DHA and VHF through a benchmark approach by applying CAM-B3LYP, ωB97XD, and PBE0 methods with different basis sets.[54] The experimental shift of the absorption maximum on the transition from phenyl DHA (1) (354 nm) to VHF (2) (459 nm) is 105 nm; however, the calculated differences at CAM-B3LYP, PBE0, and ωB97XD are 87, 87, and 90 nm, respectively. Based on their calculations, they have shown that the CAM-B3LYP method is the best for simulating the UV–visible spectra of the DHA–VHF pair that justifies the choice of the same method in our current study. Obviously, VHF has a planar seven-membered ring where in DHA the planarity is lost due to the presence of an sp3-hybridized carbon atom at the point of fusion. Based on the geometric parameter HOMA[55] (harmonic oscillator model of aromaticity), the aromatic behavior of substituted heptafulvenes is well-documented in the literature, which leads to the planarity of these structural motifs.[56] A detailed computational investigation involving electronic and geometric measures is underway to fully elaborate the planarity/aromaticity of the 7-membered ring, which will be documented in due course. The simplified unsubstituted DHA and VHF are presented in Figure .

Effect of Substitution on the UV–vis Spectra of VHF

A number of functional groups are evaluated for their effect on the UV–vis spectra of the DHA–VHF pair. The details of the functional groups studied are given in Tables –3. These groups are quite diverse and encompass mesomerically and inductively electron donor and acceptor groups. It is not simple to segregate these groups based on their nature; however, the discussion in the subsequent section is divided based on electronic nature of these groups.

Table 1

λhigh, λmax, and Oscillator Strength (f) for DHAs and VHFs (Electron Donating Groups: *∠C2–C3–C3a–C8 of DHAs)

Table 3

λhigh, λmax, and Oscillator Strength (f) for DHAs and VHFs (Electron-Withdrawing Groups: *∠C2–C3–C3a–C8 of DHAs)

Mesomerically Electron-Donating Groups

Mesomerically electron-donating functional groups on the VHF scaffold, generally, cause a red shift in the absorption spectrum where the magnitude of the effect is dependent on the position of the functionalization (Table ). The general shape of the calculated UV–vis absorption spectra consists of two peaks, and we are focusing on the near visible one, which is assigned to the third excited state mainly described by the HOMO (51)–LUMO (52) electronic transition. The study of other excited states reveals that at positions 8, 6, and 5 another transition is possible, characterized by maximum high wavelengths (λhigh). Based on the λmax values, we noticed that the NH2 group induces the bathochromic shift at all positions. The highest effect was at position 8a where the absorption of the maximum wavelength is observed at 471 nm, compared to 413 nm for the parent (H) VHF, giving a red shift of 58 nm. The next highest effect is observed in 4-NH2-VHF where the absorption maximum is shifted to 459 nm, quite comparable to that of 7-NH2 (450 nm). 8-NH2 and 6-NH2-VHF have similar absorption maxima (436 and 432 nm, respectively), while 5-NH2-VHF showed a very small red shift of 5 nm (418 nm). This can be clearly seen from the calculated UV–vis spectra (Figure ). More interestingly, a close look at λhigh revealed interesting differences that are not so obvious from these spectra especially because of their low oscillator strength and which can justify the structural modifications at those positions. Although we had taken the absorption with oscillator strength to be greater than 0.1, a close analysis reveals that, for 8-NH2-VHF, a peak is observed at 482 nm with an oscillator strength of 0.08 (Table ). This peak at 482 nm is of the same origin as the 471 nm peak of 8a-NH2-VHF. Thus, these results based on (λhigh) illustrate that the higher red shift for NH2-VHF is found for position 8, which is justified from its geometric properties (Figure S1).

Figure 2

Calculated UV–vis spectra of different positions for NH2-VHF photoswitches.

The structure of VHF has zwitterionic character where the seven-membered ring has cationic character, whereas the open dicyano bearing chain has negative charge. Based on both electronic and structural analyses, one would expect that any electron-donating substituent at the 8a position would stabilize the positive charge and would result in the maximum red shift of the absorption spectrum (Table ). For all positions except 8a and 4, the donating NH2 group is in conjugation with the cyano substituent by electron delocalization by pathway I or II (Figure ). For position 4, the delocalization is possible only by pathway I and only by II for 8a (Figure S1). This discontinuance of conjugation mainly due to steric effects is accompanied by a loss of planarity that is clearly obvious from the dihedral angle C2–C3–C3a–C8a (Φ1) (Figure S1). Despite its lower effect on λmax, the behavior of 5-NH2-VHF is similar to that of 8a-NH2-VHF where a transition is observed at higher wavelength (473 nm) but with relatively low oscillator strength (0.035, Table ). The pronounced red shift of 5-NH2-VHF is consistent with the charge analysis presented above. The absorption of the higher wavelength for 7-NH2-, 6-NH2-, and 4-NH2-VHF is 450, 463, and 459 nm, respectively (Figure ).

The behavior of hydroxyl-substituted VHFs is very comparable to that of NH2-VHFs where the maximum red shift in the UV–vis spectrum is observed for 8a-OH-VHF (454 nm) followed by position 4 (447 nm), showing red shifts of 41 and 34 nm, respectively (Figure ). The UV–vis spectra of 7-OH- and 8-OH-VHFs show absorption of maximum wavelengths at 430 and 427 nm, respectively (Table ). We have observed similar behavior as for NH2 on positions 4-OH-VHF and 8a-OH-VHF where the electron delocalization was possible only by pathway I for 4 and II for 8a hydroxyl-substituted VHFs. Again, the higher wavelength (462 nm) is found at position 8.

Figure 3

Calculated UV–vis spectra of different positions for OH-VHF photoswitches.

From comparison of the UV–vis spectra of NH2- and OH-VHFs, it is obvious that the NH2-substituted VHFs show a more pronounced red shift compared to OH-substituted VHFs, which is attributed to the stronger mesomeric donating effect of NH2, compared to OH (Table ). Interestingly, from the structural comparison of substituted VHFs, we revealed that whereas the lone pair in nitrogen is included in the π system for all positions, the one in oxygen is not at positions 5, 6, and 7, which could explain their respective low red shifts. Generally, the differences in the UV–vis spectra of OH-VHFs are less compared to NH2-VHFs.

Following the same strategy, it can be expected that the SH groups (showing maximum mesomeric effect) at the VHF would have more pronounced red shifts in their UV–vis spectra if we only consider the mesomeric effect (Figure A). This is true for positions 5, 6, and 8. However, the results at 4, and 8a reveal quite the contrary situation. The red shifts in the absorption spectra of SH-VHFs are even less than those of OH-VHFs at these positions (Table ). This is clearly justified by the nonparticipation of the lone electron pair of sulfur in the conjugated system for all positions (Figure A). Nevertheless, for positions 5, 6, and 7 where the lone electrons are not participating even for the hydroxyl group, the thiol group would have stronger red shift (for position 7, the difference between OH and SH groups is not significant). Conversely, at 8a, 8, and 4 the OH-substituted VHFs would show a greater red shift because of the participation of the lone pair of electrons in the conjugation, but this is not the case for position 8 where the hydrogen of the hydroxyl group is involved in an intramolecular hydrogen bonding causing thus shorter wavelength compared to its thiol analogue (Figure B).

Figure 4

(A) Optimized structures of 8-OH and SH-VHF photoswitches indicating Φ1 and Φ2 and (B) the calculated UV–vis spectra of different positions for SH-VHF photoswitches.

Also, we noticed different trend for SH-VHF UV–vis spectrum compared to OH and NH2. The maximum red shift is observed in 8-SH-VHF where the absorption of the highest wavelength is seen at 459 nm, compared to 462 nm for 8-OH-VHF. However, the maximum wavelengths are 446 and 427 nm for 8-SH-VHF and 8-OH-VHF, respectively, which are in agreement with their respective electron-donating power. The next highest red shifts of the absorption maximum are seen at 6-SH-VHF (430 nm), 7-SH-VHF (429 nm), and 4-SH-VHF (429 nm). and the lowest is found for position 5-SH-VHF (419), while the absorption of the higher wavelengths in 5-SH-VHF, 6-SH-VHF, and 7-SH-VHF are quite comparable and appear at 448, 448, and 442 nm, respectively. The least shift in the UV–vis spectrum is observed for 4-SH-VHF (429 nm). The lower red shifts for SH-VHFs (compared to NH2 and OH), despite the strong mesomeric effect, are attributed to poor overlap of the SH with the VHF ring due to size differences between overlapping atoms (Figure B).

In summary, it can be clearly seen from Figure that NH2 has the greatest red shift, especially at position 8a followed by OH and SH with similar behaviors.

Figure 5

Calculated UV–vis spectra for selected 8a-EDG-substituted VHF photoswitches.

Methyl and Silyl Functional Groups

The effect of methyl substituent on the UV–vis spectrum is quite negligible (Table ). Nonsubstituted VHF has the main absorption maximum at 413 nm along with another peak at 438 nm, for which the oscillator strength is quite negligible (0.01). Introduction of a methyl substituent at all positions actually causes a slight (almost negligible) red shift in the UV–vis spectrum, except at position 4 where a 3 nm blue shift is found. The blue shift is mainly attributed to steric crowding which causes a break in conjugation, as discussed previously. The slight red shifts at other positions are in accordance with the positive charge at positions 5, 6, 7, and 8, which are stabilized by hyperconjugation.

The introduction of the SiH3 substituent induces a very similar red shift for all positions except 4, 7, and 8a, for which slight blue shifts were observed. The maximum wavelengths at positions 4, 7, and 8a are 9, 2, and 5 nm blue-shifted, respectively. Conversely, the introduction of this substituent into positions 5, 6, and 8 induces red shifts of 3, 1, and 7 nm, respectively (Table ).

The comparison between methyl and silyl groups reveals that substitution at positions 5 and 7 does not impart any significant impact. However, the effect is more pronounced at positions 6 and 8, where absorption maxima are red-shifted compared to parent VHF. For positions 8a and 4 the effect of these two substituents is opposite due to steric effects, which are responsible for the loss in planarity and conjugation as explained earlier (Figure S2).

Halogen-Substituted VHFs

Analysis of the results in Table reveals that halogens, in general, cause a blue shift of the absorption spectrum for positions 4 and 5; however, a slight red shift is observed for all studied halogens at position 7 as well as positions 8 and 6 for Cl- and Br-VHFs. Moreover, being the most electronegative, fluorine induces the blue shift at all positions except 7. Also, for the same reason fluorine substitution induces the highest effect on λhigh that reaches 446 nm at 5-F, whereas it is the least for bromine substitution (except for position 8a, vide infra). Thus, the electronegativity of halogens may be correlated to their effect on both λmax and λhigh. At all positions, halogens’ nucleophilicity and red shift are directly correlated; the higher the nucleophilicity, the larger the red shift. For example, the transitions of maximum wavelength for 5-F-, 5-Cl-, and 5-Br-VHFs are 405, 412, and 413 nm, respectively. Position 5 bears a positive charge when we see the charge-separated structure of VHF; therefore, it is expected that an electron-withdrawing halogen might cause destabilization of the positive charge and therefore would cause a blue shift of the absorption spectrum. A similar effect is also expected for positions 7 and 8a; however, the results here and above illustrate that positions 7 and 6 generally have higher red shifts than other positions which might be attributed to the quinone like resonance structure. A nonmonotonic behavior is seen for λhigh. For example, for 4-substituted VHFs, the absorption spectra show a maximum wavelength at 408 nm for all (4-F, 4-Cl, and 4-Br) VHFs along with peaks corresponding to λhigh at 429, 436, and 434 nm for 4-F, 4-Cl, and 4-Br, respectively, which are comparable to the 438 nm peak of the parent VHF (with very low oscillator strength). Among these, the 4-Cl-VHF shows the longest λhigh wavelength (436 nm), despite having the best planarization of the scaffold due to interaction between halogen atom and the neighboring proton for fluorine (Figure ). Moreover, the maximum electronic conjugation through both pathways is found for F, while a disconnection at pathway II was detected for both Cl and Br. As attempt to explain this situation, a comparison of these high wavelength oscillator strength has been carried out, which clearly demonstrates a higher oscillator strength for F (0.14) compared to Cl (0.08) and Br (0.09).

Table 2

λhigh, λmax, and Oscillator Strength (f) for DHAs and VHFs (Halogens: *∠C2–C3–C3a–C8 of DHAs)

Figure 6

Interaction between (a) fluorine, (b) chlorine, and (c) bromine atom with neighboring proton for 4-VHF.

A similar situation is observed for 8a halogen VHF where a similar transition has considerably higher oscillator strength and appears at 422, 413, and 415 nm for 8a-F-, 8a-Cl-, and 8a-Br-VHFs, respectively. The position 8a has the highest charge density; therefore, it is expected that large halogens would stabilize the positive charge because their inductive effect is dominated by their mesomeric effect. However, large halogen causes distortion in planarity of the seven-membered ring due to steric effect, which significantly induces the blue shift of the λhigh by 12, 26, and 23 nm for 8a-F-, 8a-Cl-, and 8a-Br-VHFs, respectively.

Electron-Withdrawing Substituents

All of the studied electron-withdrawing groups (EWGs) behave very similar to one another, and some trends can easily be traced out (Table ). As stated above, the parent VHF has two transitions (413 and 438 nm, although the latter has very low oscillator strength). An electron-withdrawing substituent generally causes a blue shift of the peak at 413 nm, whereas a red shift is observed for the peak at 438 nm, except for positions 4 and 8a (Table , vide infra) where blue shifting is observed for both peaks, which is consistent with our previous inferences about these two particular positions. For example, for nitro VHFs, the absorption peaks for 4-NO2-VHF are 394 and 438 nm (the latter has very low oscillator strength and can be neglected, quite similar to the parent VHF). The absorptions peaks of high wavelength (λhigh) for 5-NO2-, 6-NO2-, 7-NO2-, and 8-NO2-VHFs are 471, 476, 480, and 460 nm, respectively, which are considerably red-shifted from the corresponding peak in the parent compound at 438 nm. 8a-NO2-VHF shows transitions at 374 and 432 nm, which show blue shifting when compared with the parent VHF. Geometries of all nitro VHFs are analyzed to rationalize the anomalous behavior of 4- and 8a-NO2-VHF. The optimized geometries reveal that NO2 in 4- and 8a-VHF are not communicating with the VHF scaffold; rather, they are oriented perpendicular to the plane of the seven-membered ring (Figure ). The lack of communication between the nitro and VHF ring is responsible for blue shift. In the absence of conjugation, inductive electronic effects are only operational, which illustrates that the electron-withdrawing nitro group should destabilize the positively charged seven-membered ring (and would cause blue shift). Further evidence of this notion is the pronounced blue shift for the 8a position where a substituent has direct destabilizing influence (Figure ). The conjugation at these two particular positions is broken at both pathways I and II.

Figure 7

Geometric properties for selected positions of EWG-substituted VHFs indicating Φ1 and Φ2.

To further support our hypothesis, we have calculated the UV–vis spectrum of 4-CN-VHF since the cyano group cannot orient perpendicular to the VHF scaffold (due to linear structure of this motif). Interestingly, the UV–vis spectrum of the cyano compounds showed very similar λmax but significant increasing of λhigh (467 nm for 4-CN-VHF compared to 438 nm for the parent VHF, Figure S3).

To extend this generalization to all EWGs, we also tested C(O)H and COOH (Table ). The maximum absorption wavelengths are blue-shifted with introduction of any EWG at all positions except at 8 where red shifts are found. However, when we consider λhigh, instead of λmax, we observed that, among all electron-withdrawing substituents, an aldehydic moiety at position 7 has the highest red shift with a value of 493 nm. When we compare the absorption spectra of all EWG VHFs (Figure S4), it is obvious that NO2 and COOH functional groups have the most pronounced blue shifts (39 nm) at the 8a position (374 nm for both groups at this position) whereas the CN (390 nm) has the least effect (Table ). Similarly, at position 4, the CHO (384 nm) is the substituent which has the highest hypsochromic effect. The effect at positions 5 and 6 is quite comparable for all withdrawing groups. Furthermore, structural analysis confirmed that the introduction of any EWG at positions 8a and 4 causes discontinuation of electronic conjugations at both pathways I and II, except for 4-CN in which the disconnection is only from pathway II.

Effect of Substitution on the UV–vis Spectra of DHA

The impact of electron-donating or electron-neutral moieties on the UV–vis spectra is dependent on the position of substitution (Table ). For example, for electroneutral (or very weakly donating) SiH3 and CH3, a slight red shift is observed at all positions, particularly for substitution at position 8a (234 and 223 nm for 8a-SiH3 and 8a-CH3 respectively). However, introducing two methyl groups in two positions at the seven-membered ring will induce opposite behavior as observed by Lubrin et al. when a dimethylated DHA was investigated.[57] Unsurprisingly, more nucleophilic silyl induced a slightly more pronounced red shift compared to the methyl group. Analysis of the geometries reveals that the presence of CH3 or SiH3 at position 8a causes planarization of the DHA skeleton in order to avoid steric interactions with the cyano substituents. The dihedral angle Φ2 (C2–C3–C3a–C8) is a measure of the planarity of the DHA skeleton. The Φ2 angle is −40.02° for the parent (H) DHA; however, it decreases to −24.31 for 8a-Me DHA, which indicates significant planarity of the skeleton. Similar is the case with SiH3-substituted DHA where the corresponding angle is −20.65. The more pronounced decrease in the angle for SiH3 is due to its bulky nature compared to CH3. The enhancement in the planarity of 8a-SiH3 DHA is reflected in a more pronounced red shift and larger λhigh (383 nm) compared to 336 nm for 8a-Me DHA. An inverse effect is seen at position 8 where presence of these substituents distorts the planarity of the ring. The dihedral angles for 8-Me and 8-SiH3 DHA are −50.11 and −48.99° respectively (Figure S5). The deviation from the planarity leads to a significant decrease of λhigh in the UV–vis spectra of DHAs. The absorptions of high wavelengths are observed at 316 and 318 nm for 8-Me- and 8-SiH3-DHAs, respectively. Similarly, a slight decrease of λhigh is observed when these substituents are present at all other positions.

The SH moiety, when present on DHA, causes red shifts in the UV–vis spectrum at all positions. A significant increase of λhigh is also noticed at all positions, except at 8 (323 nm), where the UV–vis spectrum is comparable to the parent DHA, which is attributed to the distortion of the DHA skeleton (−50.62) (vide supra). The presence of an SH group at positions 4 (342 nm) and 8a (347 nm) has a much stronger influence on λhigh than on the other positions, while at positions 7 (289 nm) and 8 (234 nm) the effect on λmax was the greatest. The red shifting of UV–vis spectrum for 8a-SH DHA is not seen for its analogous 8a-OH DHA. In fact, the UV–vis spectrum of 8a-OH DHA shows a blue shift (214 nm) and lower λhigh (319 nm). The geometries of 8a-OH DHA and 8a-SH DHA are analyzed to rationalize the difference (Figure S6). The optimized geometries reveal a difference in the orientation of the SH and OH groups on the DHA skeleton. The proton of SH interacts with the cyano group while leaving the sulfur atom unaffected (H–C: 2.7 and S–C: 3.0 Å); however, for OH, the oxygen atom interacts with the cyano group (H–C and O–C: 2.7 Å). Interaction of the oxygen with the cyano group in the latter engages the lone pair of oxygen which is, in turn, not causing the red shift of the UV–vis spectrum. For the 8a position, if the lone pair of the electron-donating atom is engaged with the cyano group then a blue shift is observed. This is also could be applied for 8a-NH2 DHA (H–C: 2.7 and S–C: 2.6 Å) which has similar situation as OH. Due to its stronger electron-donating nature, introduction of NH2 at all positions induces more significant red shift, especially at position 4 for which the maximum wavelength is 241 nm. Moreover, the UV–vis spectra of NH2-substituted DHAs show an increase in the highest wavelength to 349, 347, 347, 346, and 352 nm for 8-NH2, 7-NH2, 6-NH2, 5-NH2, and 4-NH2, respectively. The presence of electron-donating substituents at the seven-membered ring and electron-withdrawing substituents at the five-membered ring generates extended conjugation due to the push–pull effect, which leads to red shift. The red shift is relatively less for OH-substituted DHAs, and it is mainly attributed to a weak mesomeric donating effect compared to the NH2 group.

Similarly, the UV–vis spectra of halogenated DHAs show a halogen-dependent effect. Unsurprisingly, the effect of chlorine and bromine substituents is very similar, which is consistent with their similar inductive and mesomeric properties. These two halogens cause slight to moderate bathochromic shifts when going from parent to Cl and Br, respectively. The maximum red shifting of λhigh for these two halogens is observed at position 8a (+12 and +27 nm for 8a-Cl and 8a-Br, respectively). However, the fluoro substituent (that has stronger inductive electron-withdrawing effect compared to Cl and Br) induces hypsochromic shift in the UV–vis spectrum, mostly at positions 8a and 8 (blue shift of about 9 and 11 nm, respectively). Conversely, the fluoro substitution at position 7 causes a small red shift in the UV–vis spectrum (Table ). As a general trend, halogens cause a red shift in the absorption spectrum for position 4 as well as position 8a for Cl and Br; however, a slight blue shift is observed at positions 8 and 8a for F and at positions 5 for F and Br DHAs. Furthermore, as the most electronegative element, fluorine causes the blue shift at all positions except 4. Also, bromine substitution induces the highest effect on λhigh that reaches 350 nm at 8a-Br, whereas it induces the least effect for fluorine at 8-F. Moreover, a large halogen causes planarity distortion in the seven-membered ring due to the steric effect, resulting in a significant blue shift of the λhigh by 7 and 9 nm for 8- and 8a-F, respectively. For example, the transitions of maximum wavelength for 8a-F-, 8a-Cl-, and 8a-Br-DHAs are 314, 335, and 350 nm, respectively, which can be explained as follows: the higher the nucleophilicity, the larger the red shift. A similar effect is also expected for position 4; therefore, the results here illustrate that this position generally has higher red shifts than other positions. Among these, the 4-Br-DHA shows the longest λhigh wavelength (241 nm); however, a large halogen causes distortion in the planarity of the seven-membered ring due to the steric effect, which significantly induces the blue shift of the λhigh.

Electron-withdrawing groups on the seven-membered ring of DHA have negligible effects on the UV–vis spectra. This is justified by the minor effect of these substituents on the geometries (Figure S7). For most of the substituents, the shifts (either blue or red) are mostly within the ±10 nm range of the spectrum of parent DHA; however, it is +29 nm in the case of 5-C(O)H-DHA and +26 nm for 4-NO2-DHA, compared to parent DHA (Table ). For example, the absorption maxima shifts for cyano-substituted DHAs are 325 (+2 nm), 334 (+11 nm), 320 (−3 nm), 334 (+11 nm), 328 (+5 nm), and 326 nm (+3 nm) for the 8a, 8, 7, 6, 5, and 4 positions, respectively. The maximum red shifting is observed for formyl-substituted DHAs where the absorptions of maximum wavelengths are calculated at 352 (+29 nm), 346 (+23 nm), and 344 nm (+21 nm) for 5-, 4-, and 8a-substituted DHAs. The nitro-substituted DHAs also showed strong red-shifting at positions 8 (+26 nm), 6 (+23 nm), and 5 (+20 nm). Carboxyl substituents, which show a strong red shift in VHF, show no considerable red shifting in the UV–vis spectra of DHA, for which the maximum red shifting is observed at position 6-CO2H (+15 nm).

In summary, by exploring different substituents, it is clear that the spectral nonoverlap is better achieved generally when electron-donating groups have been inserted at positions other than 8a and 4, thus inducing significant red shift in the VHF UV–vis spectra, especially NH2 and OH. Also halogenated DHA–VHF provided interesting results especially at position 6.

Hyperpolarizability of Substituted DHA–VHF

The literature reveals a number of strategies to improve the nonlinear optical response of organic and inorganic systems. For organic compounds, an electron push–pull mechanism is a powerful approach to get a large nonlinear (NLO) response. Among DHA and VHF, the latter has zwitterionic character with significant charge separation, which can impart a much higher NLO response in a system. On the contrary, DHA has a closed structure with no charge separation, and it is expected that DHA will have a low NLO response. In VHF, electrons are withdrawn from the seven-membered ring by the cyanide group which results in charge separation. The effect of charge separation is very similar to the push–pull mechanism leading to higher hyperpolarizability. Moreover, the VHF form has continuous conjugation, whereas in DHA conjugation breaks to some extent (carbons 8a and 1 are sp3, Figure S2). Moreover, the DHA (5) is also bent away from planarity, which is also responsible for its low hyperpolarizability value. The calculated hyperpolarizabilities, which are measure of the NLO response, are consistent with the expectations. The calculated hyperpolarizabilities of parent nonphenyl DHA and VHF are 0.55 and 11.3 esu.

The effect of substituents on the NLO response in DHA is different from that of VHF. Any substituent on the DHA skeleton increases the hyperpolarizability of the system (Table –6). For example, the hyperpolarizabilities of 8a-Me, 8-Me, 7-Me, 6-Me, 5-Me, and 4-Me DHAs are 0.59, 1.25, 1.26, 2.94, 1.28, and 0.63 esu, respectively (Table ). On the other hand, for VHF, the hyperpolarizabilities first decrease and then increase and become highest for 6- or 7- (in some cases) substituted VHFs. The hyperpolarizabilities of 8a-Me-, 8-Me-, 7-Me-, 6-Me-, 5-Me-, and 4-Me-VHFs are 9.02, 9.15, 13.41, 14.03, 12.50, and 11.20 esu, respectively (Table ). It is interesting to note that, for all electron-donating substituents, a common trend is observed where the maximum values of hyperpolarizabilities are calculated for 6-substituted DHAs and VHFs (with very few exceptions). For amino-substituted DHAs and VHFs, the highest values are calculated for 6-NH2-DHA (10.32 esu) and 6-NH2-VHF (14.97 esu). The high hyperpolarizabilities of DHAs and VHFs at positions 6 may be attributed to the maximum separation between electron-withdrawing and electron-donating groups (and hence the maximum dipole moment). Moreover, the substituent at position 6 does not cause deformation of the DHA skeleton. The high hyperpolarizabilities at position 6 are also consistent with the UV–vis results where greater red shift is observed when electron-donating NH2 or OH groups are present at this position for both DHA and VHF (Table ).

Table 4

Hyperpolarizability (βtot) Values (esu) for DHAs and VHFs (Electron-Donating Groups)

No.	Substituents	DHA	VHF	No.	Substituents	DHA	VHF
1	parent (H)	0.55	11.3
2	8a-Me	0.59	9.02	20	8a-SiH3	1.63	12.23
3	8-Me	1.25	9.15	21	8-SiH3	0.48	10.87
4	7-Me	1.26	13.41	22	7-SiH3	0.51	12.33
5	6-Me	2.94	14.03	23	6-SiH3	1.54	12.63
6	5-Me	1.28	12.50	24	5-SiH3	1.71	12.07
7	4-Me	0.63	11.20	25	4-SiH3	1.49	13.11
8	8a-NH2	0.42	10.09	26	8a-SH	0.87	6.56
9	8-NH2	3.17	7.43	27	8-SH	1.44	7.10
10	7-NH2	2.42	14.96	28	7-SH	1.11	22.43
11	6-NH2	10.32	14.97	29	6-SH	6.51	26.30
12	5-NH2	3.87	11.13	30	5-SH	1.45	22.19
13	4-NH2	2.53	6.04	31	4-SH	1.49	15.80
14	8a-OH	0.42	4.37	17	6-OH	6.83	15.08
15	8-OH	2.61	1.85	18	5-OH	3.39	13.23
16	7-OH	2.26	14.45	19	4-OH	2.08	4.92

Table 6

Hyperpolarizability (βtot) Values (esu) for DHAs and VHFs (Electron-Withdrawing Groups)

No.	Substituents	DHA	VHF	No.	Substituents	DHA	VHF
1	8a-NO2	1.60	11.83	13	8a-C(O)H	1.63	10.43
2	8-NO2	1.98	13.41	14	8-C(O)H	2.49	12.58
3	7-NO2	0.36	10.41	15	7-C(O)H	0.75	12.67
4	6-NO2	8.05	4.06	16	6-C(O)H	5.79	8.45
5	5-NO2	3.01	6.22	17	5-C(O)H	2.38	7.02
6	4-NO2	2.21	14.13	18	4-C(O)H	2.38	12.63
7	8a-CN	0.72	10.31	19	8a-CO2H	0.96	13.12
8	8-CN	1.07	14.94	20	8-CO2H	0.71	13.52
9	7-CN	0.13	16.57	21	7-CO2H	0.46	12.46
10	6-CN	1.08	16.76	22	6-CO2H	4.19	9.30
11	5-CN	0.71	14.15	23	5-CO2H	1.96	9.16
12	4-CN	0.91	14.47	24	4-CO2H	1.82	14.70

In general, the hyperpolarizabilities of the substituted VHFs are higher than the hyperpolarizabilities of the corresponding DHAs, due to the reasons mentioned above. For DHA, the lowest hyperpolarizabilities are calculated at positions 8a, probably due to the perpendicular orientation of the substituent to the DHA skeleton where it shows minimum interaction with electron withdrawing cyano groups. For VHFs, the lowest values are generally calculated for 4 substituted VHFs probably because of unfavorable orientation of the dipole moment.

The hyperpolarizabilities of halogen-substituted DHAs and VHFs are very comparable to electron-donating substituents (Table ). For example, the hyperpolarizabilities of 8a-F-, 8a-Cl-, and 8a-Br-DHAs are 0.58, 0.40, and 2.06 esu. Among halogen DHAs and VHFs, the highest values of hyperpolarizabilities are calculated for 6-substituted systems, quite similar to studied electron-donating substituents. The hyperpolarizabilities of 6-F-, 6-Cl-, and 6-Br-DHAs (VHFs) are 4.02 (15.70), 4.76 (22.12), and 3.98 (22.16) esu, respectively.

Table 5

Hyperpolarizability (βtot) Values (esu) for DHAs and VHFs (Halogens)

No.	Substituents	DHA	VHF	No.	Substituents	DHA	VHF
1	8a-F	0.58	12.26	10	6-Cl	4.76	22.12
2	8-F	1.58	11.35	11	5-Cl	1.64	18.48
3	7-F	1.72	15.68	12	4-Cl	1.60	14.41
4	6-F	4.02	15.70	13	8a-Br	2.06	12.29
5	5-F	2.21	14.56	14	8-Br	1.42	13.26
6	4-F	1.47	13.46	15	7-Br	0.51	19.58
7	8a-Cl	0.40	12.40	16	6-Br	3.98	22.16
8	8-Cl	1.90	13.00	17	5-Br	0.89	19.30
9	7-Cl	1.52	19.71	18	4-Br	0.92	14.44

In addition, the hyperpolarizability of electron-withdrawing group substituents on DHA and VHF have been evaluated (Table ). For example, the hyperpolarizabilities of 8a-CN-, 8-CN-, 7-CN-, 6-CN-, 5-CN-, and 4-CN-DHAs are 0.72, 1.07, 0.13, 1.08, 0.71, and 0.91esu, respectively, while the hyperpolarizabilities of 8a-CN-, 8-CN-, 7-CN-, 6-CN-, 5-CN-, and 4-CN-VHFs are 10.31, 14.94, 16.57, 16.76, 14.15, and 14.47 esu. This illustrates that the maximum values of hyperpolarizabilities are observed for 6-substituted DHAs and-VHFs, whereas the lowest values are calculated for 7-substituted DHAs and VHFs, for all electron-withdrawing substituents (with very few exceptions).

In summary, the hyperpolarizabilities of all substituted VHFs are higher than those of the corresponding DHAs for the reasons mentioned above. In addition, the highest values of hyperpolarizabilities are calculated for 6-substituted systems for all DHA and VHF.

Conclusion

The effect of substitution at the seven-membered ring on the absorption properties of DHA–VHF is described, and it is noticed that any electron-donating group would stabilize the conjugation and will result in the maximum red shift of the VHF absorption spectra. However, at the 8a and 4 positions, the steric hindrance would exhibit a distortion of planarity that lowers the conjugation. Conversely, any electron-withdrawing group especially at the 8a and 4 positions would cause a blue shift. The presence of bulky groups at position 8a causes loss of planarity and consequently a decrease of electronic conjugation within the molecule leading to a blue shift on the maximum absorption. In the case of halogens, as the most electronegative element, fluorine causes a blue shift at all positions except 4-DHA and 7-VHF. At all positions, halogens’ nucleophilicity and red shift are directly correlated; the higher the nucleophilicity, the larger the red shift. Moreover, large halogens cause planarity distortion in the seven-membered ring due to steric effects, resulting in a significant blue shift of the λhigh. The hyperpolarizabilities of all substituted VHFs are higher than those of the corresponding DHAs due to charge separation, which imparts a much higher NLO response. In addition, the highest values of hyperpolarizabilities are calculated for 6-substituted systems for all DHA and VHF.