Computer aided chemical design: using quantum chemical calculations to predict properties of a series of halochromic guaiazulene derivatives

With the scientific community becoming increasingly aware of the need for greener products and methodologies, the optimization of synthetic design is of greater importance. Building on experimental data collected from a synthesized guaiazulene derivative, a series of analogous structures were investigated with time-dependent density functional theory (TD-DFT) methods in an effort to identify a compound with desirable photophysical properties. This in silico analysis may eliminate the need to synthesize numerous materials that, when investigated, do not possess viable characteristics. The synthesis of several computationally investigated structures revealed discrepancies in the calculation results. Further refined computational study of the molecules yielded results closer to those observed experimentally and helps set the stage for computationally guided design of organic photonic materials. Three novel derivatives were synthesized from guaiazulene, a naturally occurring chromophore, exhibiting distinct halochromic behaviour, which may have potential in a switchable optoelectronic system or combined with a photoacid generator for data storage. The protonated forms were readily excitable via two-photon absorption.

With the scientific community becoming increasingly aware of the need for greener products and methodologies, the optimization of synthetic design is of greater importance. Building on experimental data collected from a synthesized guaiazulene derivative, a series of analogous structures were investigated with time-dependent density functional theory (TD-DFT) methods in an effort to identify a compound with desirable photophysical properties. This in silico analysis may eliminate the need to synthesize numerous materials that, when investigated, do not possess viable characteristics. The synthesis of several computationally investigated structures revealed discrepancies in the calculation results. Further refined computational study of the molecules yielded results closer to those observed experimentally and helps set the stage for computationally guided design of organic photonic materials. Three novel derivatives were synthesized from guaiazulene, a naturally occurring chromophore, exhibiting distinct halochromic behaviour, which may have potential in a switchable optoelectronic system or combined with a photoacid generator for data storage. The protonated forms were readily excitable via two-photon absorption.
2016 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited. 1

. Introduction
Building on conventional chemistry, effective solutions for greener chemical practices are increasingly being implemented to tackle concerns of chemical hazards, resource scarcity and climate impact on the population and the planet. In his 1998 book, Paul Anastas features 12 principles for green chemistry, the first of which states: 'It is better to prevent waste than to treat or clean waste after it is formed' [1]. To this end, the use of quantum chemical calculations to provide a prediction of a compound's properties prior to generating it in the laboratory could help to minimize waste generated by synthesis of impractical derivatives. The use of in silico techniques is commonly used to aid in the explanation of experimental results [2,3], though more recently studies have been directed towards identifying sustainable solvents [4], as well as designing solar cell components [5] and complex metal oxides [6].
Another of Anastas's principles, and area of keen development, is the use of chemicals from renewable sources, or otherwise coming from nature [1]. Extractable from fungi and coral [7,8], 1,4-dimethyl-7isopropylazulene, or guaiazulene, is a natural derivative of azulene. Although azulene is an isomer of the colourless naphthalene, it exhibits a blue colour that has enchanted man since the late medieval period [9]. This is attributed to azulene's peculiar emission from the second excited state (S 2 ), an exception to Kasha's Rule, as a result of its unusually low-lying first excited state (S 1 ). Introducing electronic perturbing substituents on the seven-member ring and/or on the five-member ring of azulene was shown to change the electronic properties of azulene derivatives, accompanied by significant changes in their fluorescence behaviour [9,10]. Although the effect of resonantly electron withdrawing or donating groups on the HOMO, LUMO and LUMO +1 energies of azulene was reported, these theoretical calculations were initially limited to derivatives with mildly electronically perturbing (e.g. formyl or fluorine) substituents [11].
Recently, a number of studies have reported interesting optoelectronic properties of azulene derivatives having extended π-conjugated substituents that can be manipulated by protonation with strong acids. The formation of a resonance-stabilized 6-π-electron tropylium cation in protonated azulenes [12] resulted in a bathochromic shift in the S 0 → S 1 band of the absorption, as well as an increase in the luminescence intensity as a result of the now-dominant S 1 → S 0 decay pathway [13,14]. The unique electronic properties of azulene-based structures were instrumental in the development of derivatives for charge-transport, optoelectronic and sensor applications. The design of such structures is often based on theoretical calculations of the dipole moment of azulene derivatives; variation in calculated dipole moments is thought to be a result of various substituents on the azulene framework that acts as an electron bridge in a donor-acceptor-donor arrangement [12,15,16]. While these calculations predict a large dipole moment and large hyperpolarizability for 2,6-connected azulene systems, experimental investigation shows that a 4,7-connectivity often results in longer absorption and emission wavelengths, a desirable property for the development of various near-IR applications [10]. In order to avoid such discrepancies and minimize waste generated by preparing potentially impractical azulene derivatives, we exploited the naturally occurring guaiazulene to initially prepare a structurally simplified derivative tailored to guide the theoretical calculations to predict critical optical properties of more heavily conjugated systems (figure 1). Such in silico analysis could reveal any disadvantageous nature of certain derivatives, thus eliminating the wasteful necessity of producing derivatives with undesirable properties.
In its protonated state, azulene's tropylium cation acts as an electron acceptor when conjugated through a π-bridge to an electron-rich system. With that in mind, and using our established method [17], guaiazulene 1 was condensed with 4-hexyloxybenzaldehyde 2a in the presence of potassium tertbutoxide to afford 3a (scheme 1). Thus, upon treatment with trifluoroacetic acid (TFA), the ethylene moiety acts as a π-spacer between the tropylium cation and the electron-rich benzene ring (scheme 2). This is illustrated in the absorption and emission spectra of 3a and its protonated form 3aH+ (figures 2 and 3). In addition to the weak S 0 → S 1 transition (depicted as a broad peak centred at 630 nm) common to the azulene family, 3a shows an absorption peak with λ max = 365 nm, hence giving the solution of 3a a typical azure blue colour. On the other hand, an intense red colour is immediately observed when the solution is treated with TFA. This is reflected in the absorption spectrum of 3aH+ where an intense peak appears at λ max = 512 nm, while that of the neutral species 3a (λ abs = 630 nm) is no longer observed. Concomitantly, treatment of 3a with TFA resulted in a marked switch-on of fluorescence; in fact, a quantum yield of 0.12 was calculated for 3aH+ using the Lorentz-Lorenz equation (1.1) and (1.2), where OD is optical density, I is emission intensity, n x is the refractive index of solvent x, and ϕ x is the proportion of solvent x in the mixture [18,19]. As seen in figure 3, a prominent emission peak is seen at λ max = 620 nm, which was chosen in order to measure the excitation anisotropy for the protonated rsos.royalsocietypublishing.org R. Soc. open   TFA TEA Scheme 2. Structures for guaiazulene derivative 3 and its conjugate acid formed upon exposure to TFA, 3aH+.
species 3aH+ in a viscous medium (figure 3). The plateau of this trace through the main absorption band indicates that this is a single electronic transition (figure 3). Additionally, it was noted that acid treatment also results in 3a having significant two-photon absorptivity, with a two-photon absorption cross-section of approximately 170 GM at 1020 nm (figure 3). Given the unsymmetrical design of the molecule, the two-photon absorption (2PA) spectrum agrees well with that of the linear absorption.
The quantum yields of photodecomposition measured for 3a and 3aH+ show significant disparity, separated by three orders of magnitude (table 1). Though a value is not explicitly stated, the photostability of guaiazulene has been investigated previously; the sample was seen to persist for a longer period of time than 1 at an equivalent or greater irradiance but no quantitative value was reported [20]. It is possible that change of the solvent from methanol to dichloromethane (DCM) is responsible for the discrepancy: DCM can promote photooxidation and C-Cl bonds can be broken using UV light, leading to reactive species and additional damage to the compound in solution [21]. In the case of 3aH+, longer wavelength light was used, thus avoiding the degradation of the solvent, and obtaining a value similar to those of other fluorescent dyes that have been previously reported [22].
Examination of the optimized structures for 3a and 3aH+ reveals that the neutral form does not exhibit a planar conformation (figure 4a), but, instead, a distinct dihedral angle, φ, between the guaiazulene moiety and the vinyl bond (φ ≈ 38°) is noted. When guaiazulene is protonated, the       optimized structure indicates that planarity is restored (φ ≈ 1°, figure 4b), hence amplifying electron donation from the adjacent anethole group. The ground state stationary dipole for the two forms is consistent in magnitude (1.90 D and 2.20 D for 3a and 3aH+, respectively) despite the presence of an electron deficient tropylium cation in 3aH+. However, the dipole of 3aH+ is polarized toward the cyclopentene ring rather than the anethole moiety.
The absorption spectra generated through time-dependent density functional theory (TD-DFT) calculations show that the weak intensity band of the S 0 → S 1 transition is present, though the bands   at shorter wavelength are resolved better than seen experimentally (figure 5a; electronic supplementary material, table S1). In the case of 3aH+ (figure 5b; electronic supplementary material, table S2), the absorption band centred at approximately 600 nm is anticipated by the calculation; however, the short wavelength bands are less well aligned. The consistency found between the experimental and calculated photophysical properties of 3a could, therefore, allow for prediction of the electronic properties of a series of structures similar to 3a and having various π-spacers (figure 6). Among potential spacers, phenyl rings offer a greater degree of rigidity over polyene groups; compared with these spacers, the fluorene moiety can be a better alternative as it confers higher rigidity given the bridging carbon in the 9-position. Alternatively, the use of thiophene, an isostere of benzene, is shown to result in a bathochromic shift of the linear absorption spectrum [21,22]. The length of conjugation is known to shift the absorption spectrum, but it can also lead to increased two-photon absorption cross-sections [23,24]. Eleven structures were studied in their neutral and protonated forms. Of this π-extended series, two structures were synthesized and their experimentally determined properties were compared with calculated properties.
Analysis of the results from TD-DFT calculations (table 2) reveals that the varying length of the chromophore system has little effect on the ground state dipole of the neutral form for this series. However, the protonated forms of the longer systems (vi-xiv) show a marked increase relative to their neutral counterparts, ranging from approximately threefold to eightfold increases. There is also a notable trend in the dihedral angles observed for the optimized structures: the structures with a thiophene ring adjacent to the guaiazulene system (i, iv, v, x and xiii) do not exhibit a change in dihedral angle between these systems upon protonation, instead both forms are planar. Conversely, structures with either a phenyl ring or fluorene moiety adjacent to the guaiazulene undergo approximately 25°rotation to relax the dihedral angle upon protonation. A possible explanation is that thiophene withdraws electron density from the vinyl hydrogen closest to the guaiazulene, thereby facilitating a planar conformation of the neutral form. Structure xiv, however, appears to be an exception to this trend, behaving similarly to the phenyl-substituted guaiazulenes.
These results also include the first transition wavelength and the peak of the calculated absorption spectrum. Although there is some fine variation, the first transition of the neutral forms, corresponding to the S 0 → S 1 transition, remains in the region of 650-680 nm, consistent with experimental results for 3a and other studies of azulene derivatives [9,12,14]. Results generally indicate that the absolute λ max steadily increased as the length of the chromophore increased, as expected (figure 7a). This implies that the energy gap between S 0 and S 2 is decreasing while that between S 0 and S 1 remains constant.
Protonation induces the same bathochromic shift seen for compound 3a in the calculated absorption data. Disappearance of the weak S 0 → S 1 band of the azulene at approximately 650 nm and emergence of a new band from the azulenium cation extending into the NIR are observed [23]. In the case of the Table 2. TD-DFT calculation data for the series of compounds shown in figure 6.               longer species, such as xiiiH, the first transition no longer represents the peak absorption as it has been superseded by a higher transition (figure 7b).
To confirm the validity of these results, as well as probe further into the photophysical properties of these extended guaiazulene derivatives, compounds similar to structures vii and xiii were synthesized and characterized. These compounds were selected as they represent the midpoint and the extreme of the series, as well as having two and three moieties, respectively, in addition to guaiazulene. Given the increased length of the conjugated systems, and the inclusion of a thiophene moiety (in the case of xiii), these structures should have greater 2PA properties over 3a.
The approach for the synthesis of 3b is depicted in scheme 3. In this synthetic route, a Wittig reaction was employed to prepare styrene 4 by condensing benzaldehyde 2 with methyltriphenylphosphonium bromide under basic conditions. Alternatively 6, which was prepared by a lithium-halogen exchange reaction employing intermediate 5 in the presence of n-BuLi, was coupled with 4 to furnish the aldehyde rsos.royalsocietypublishing.org R. Soc. open 7. Finally, a condensation of 7 with guaiazulene 1 afforded the target compound 3b. It is noteworthy that the formation of 3b can be preliminarily observed as change in colour of the spot corresponding to the desired product from green to dark blue when exposing the compound spotted on a thin layer chromatography (TLC) plate to TFA vapour.
The experimental absorption spectra of 3b and 3c appear hypsochromically shifted in relation to their computationally determined counterparts (electronic supplementary material, tables S3-S6), though they share a somewhat similar structure (figures 8a and 9a). With the intense S 0 → S 2 bands pushed to lower wavelength, the S 0 → S 1 bands are more visible, appearing at approximately 650-660 nm in both instances. Weak emission was observed through excitation of the S 0 → S 2 band, though no emission was apparent for the S 0 → S 1 transition. Upon protonation, the absorption spectrum was altered in a manner similar to 3a, though there was no evidence of the absorption bands in the NIR (figures 8b and 9b). These protonated forms also fluoresce upon excitation of the azulenium cation bands at 586 nm and 664 nm, respectively (table 3).
It was shown that lengthening the conjugated system of the chromophore increased the molar absorptivity of 3b and 3bH+ and the inclusion of a thiophene ring imposed a detrimental effect on 3c rsos.royalsocietypublishing.org R. Soc. open   and 3cH+ molar absorptivity (tables 1 and 3). These effects are unexpected as the addition of a thiophene ring has been noted to generally increase molar absorptivity [24,25]; while increasing the length of the conjugated system has been shown to have the opposite effect [26]. Across the series of compounds, trends in fluorescence quantum yield can be seen (tables 1 and 3). The decreased fluorescence quantum yield with increased conjugation length can be correlated to the higher degree of flexibility in the structure, allowing for more rotation and vibration, though the inversion of this trend in the neutral forms is equally counterintuitive.
With the increased conjugation length, an increase in photodecomposition quantum yield also occurs (table 3). Although a longer wavelength was utilized for the excitation (405 nm), the neutral forms exhibit lower photostability than their conjugate acids. Despite this, 10% TFA/DCM solutions of the compounds were noted to degrade between experiments, though this is more likely to be a case of chemical stability in the present of an acid.
The excitation anisotropy trace calculated for 3bH+ presents a gradient within the region of the long wavelength absorption band, inferring that this is an overlap of more than one electronic transition (figure 10a). The two-photon absorption (2PA) spectrum obtained in 10% TFA/DCM of 3bH+ seems erratic, which may be explained by a variety of possible transitions. Unlike 3bH+, the excitation anisotropy of 3cH+ plateaus within the long wavelength absorption band, in accordance with a sole transition. This plateau also suggests that the absorption and emission are collinear, given its anisotropy value of 0.4 [18]. The 2PA spectrum of 3cH+ initially shows reasonable agreement with the linear absorption spectrum, though appears erratic beyond 1300 nm (figure 10b).
In an effort to resolve the difference observed between the experimental results and those of the B3LYP TD-DFT calculation (figures 7 and 8), a series of additional methods were implemented by varying the proportion of Hartree-Fock (HF) calculations. Ab initio methods, like HF, yield results that are less erratic than semiempirical methods, though at the cost of time [27]. Hybrid functions, including B3LYP (20% HF), have been developed to bridge this gap, where the ratios can be adjusted. With respect to spectroscopic data, increasing the ab initio portion decreases, or blue shifts, the wavelengths of the transitions.  When the TD-DFT calculation was repeated for 3bH+ using the M06-HF method (100% HF), the calculated spectrum was in closer semblance to the experimental spectrum than previous results (figure 11a). The first transition appeared hypsochromically shifted from the experimental main absorption band, as is the shorter wavelength transition. There is greater difference in the experimental and calculated 2PA spectrum; in the region of the main linear absorption band, accounting for the wavelength shift, they were similar, though at higher energy the data diverges.
Likewise, the results obtained for 3cH+ using the same M06-HF method show a hypsochromic shift compared with the experimental data, for both the one-and two-photon absorption spectra (figure 11b). Here, the 2PA data are more closely matched, with results becoming erratic towards the limit of the germanium detectors (1600 nm).
The use of a method with a higher proportion of HF provided greater similarity between the calculated and experimental results for these compounds because of the presence of the large dipole moments that are more akin to charge transfer, a phenomenon that traditional DFT methods struggle to depict [28,29].

General procedure for the preparation of guaiazulenes 3a-c and 10
A solution of t-BuOK (3.0 mmol) in t-amyl alcohol (10 ml) was heated at 105°C for 30 min. To this solution was added the appropriate aldehyde (3.0 mmol) followed by the dropwise addition of a solution of guaiazulene 1 (1.0 mmol) in t-amyl alchol (5 ml). The solution was heated for 3 h at 105°C, then cooled to room temperature and poured into dilute HCl (1 N). The mixture was extracted with DCM (2 × 25 ml) and the solvent was removed under reduced pressure. The resulting crude mixture was purified using Flash column chromatography using hexanes and EtOAc (0-3%).

Quantum chemical calculations
Taking the output from the GaussView5 program, structures were optimized to the B3LYP/D95* level with the GAUSSIAN09 software package [35]. TD-DFT calculations looking at the first 15 transitions utilized these optimized conformations, and, initially, the B3LYP method. Further calculations used M05, M06, M06-2X and M06-HF methods. The alkoxy and alkyl chains in the para-position of phenyl and 9-position of fluorene were shortened to methoxy and methyl groups, respectively, due to the minimal impact these groups have on the results of the calculations [36]. The calculated 2PA spectrum was determined from the permanent and state-to-state transition dipoles, which were obtained using a posteriori Tamm-Dancoff approximation (ATDA) in a locally modified version of GAUSSIAN09 [37].

Conclusion
Through the use of quantum chemical calculations, a library of guaiazulene derivatives based on an initial synthesized compound led to the discovery of molecules with high linear absorption dipoles. Measurements on two synthesized structures from the series illustrated issues in the calculations performed. As a result of the numerous terms that factor into nonlinear absorption, the larger dipole did not correspond to the great 2PA.
Despite the discrepancies between experimental and computational results, the use of quantum chemical calculations to consider the properties of designed, yet unsynthesized, compounds is promising. Conducting in silico analysis and prediction, combined with deriving materials from renewable resources, is a noteworthy course of action for chemists in this increasingly ecologically and sustainability-conscious world.
Data accessibility. Computational data and molecular orbital illustrations along with 1 H and 13 C NMR spectra are provided in the electronic supplementary material available from the publisher.