a) Previously studied NBDs ( N5a–d , NN5e , and N4a ) and compounds made for this study ( N4b–d and NN4e ). b) Reaction protocol used to form N4b–d and NN4e . c) A representation of the structure of N4c obtained by X‐ray crystallography on crystals grown from dichloromethane/ n ‐heptane. The disorder of the NBD moiety (major/minor 0.59:0.41) is shown but hydrogen atoms are omitted. The conformation obtained by solving the crystal structure highlights that the C carbonyl ‐C2 NBD ‐C3 NBD ‐C aryl dihedral angle is 11.5° for the major form, indicating that the aryl and trifluoroacetyl groups are nearly coplanar.

With a target of MOST window laminates possessing a daily charging/discharging cycle in mind, we designed a series of NBD‐based molecules which should possess a good solar spectrum match and target half‐lives in the range of 4–8 h. Scheme 1 shows the structures of previously studied NBDs with the cyano acceptor N5a–d and NN5e and parent NBD with the trifluoroacetyl acceptor moiety N4a . Based on our previous studies, 11 we chose to investigate the properties of better aromatic donors used in conjunction with the strong trifluoroacetyl acceptor N4b–d and the double NBD analogs NN4e . These NBD derivatives were made using a Diels–Alder methodology between a selection of trifluoroacetylphenyl acetylenes with cyclopentadiene. The acetylenic precursors 3b–e were made following a literature procedure from their corresponding terminal alkynes. 16 Dienophiles 3b–e could be heated in the presence of cyclopentadiene to form N4b–d and NN4e in excellent yields (Scheme 1 ), primarily due to the strong trifluoroacetyl acceptor activating the alkyne toward the [4+2π] cycloaddition. The potential for extended conjugation of the C2–C3 substituents was confirmed through a single crystal X‐ray structure for N4c , which revealed near coplanarity of these substituents, although there is some disorder between the bridge C7 and the double bond of C5–C6 (Figure 3 ).

The quantum yields for the two processes in this photoconversion were examined at both 340 and 405 nm, where the former wavelength was close to the absorption maximum for NQ4e and the latter favored the absorbance of the NN4e . At both these wavelengths, comparable results were obtained, both showing a linear fit for the change in concentration versus irradiation time, and so the quantum yields for both processes were 77%. The light‐harvesting properties for NN4e were superior to NN5e , which had sequential quantum yields of 73% for the first photoconversion ( QQ5e→NQ5e ) and 51% for the second event ( NQ5e→NN5e ).

As observed for N4b–d the dimer NN4e also exhibited a red‐shifted onset for the absorbance relative to NN5e . Previously synthesized NN5e displayed stepwise photoconversions by irradiation at 405 nm, where an NQ5e intermediate could be identified. 11 In comparison, NN4e did not experience any sequential photoswitching events. However, stepwise kinetics did occur for the back‐conversion for QQ4e . It was found that the first conversion ( QQ4e→NQ4e ) was significantly faster than the second step ( NQ4e→NN4e ) (Figure 2 c), which is surprising since it is opposite to what we observed for previously explored double systems. 11 This could be clearly observed from the progressive change in the maxima for the absorbance profile during a kinetic experiment. As seen in Figure 2 b, the absorbance maximum initially remains unchanged, where the conversion of QQ4e to NQ4e had occurred relatively fast, and thereafter the maximum started to slowly red‐shift, a result of NQ4e to NN4e . Indeed, this progressive behavior was substantiated by following the back‐conversion by NMR (see the Supporting Information). Two different half‐lives were also observed for dimer NN5e , though in this case the conversion for NQ5e to NN5e was found to be faster than the conversion from QQ5e to NQ5e .

The kinetic decay for all QCs was measured at three different temperatures and from exponential fits, the rate constants and life‐times were obtained. From Arrhenius plots (see the Supporting Information), the room temperature life‐times were determined in toluene by extrapolation. Not surprisingly, the larger the dipole moment through the C2–C3 olefin for the NBD, the lower the life‐time for the corresponding QC forms. Metastable forms Q4b and Q4c exhibited markedly shorter half‐lives than Q4a , in the order of a few days for previously reported Q4a to around 7 h for Q4b and less than an 1 h for Q4c . In fact, parallel trends for the half‐lives of Q5a–d with a cyano group as the acceptor were noticed. Exchanging the cyano for trifluoroacetyl unit proportionately afforded an ≈20‐fold increase in the back‐conversion rate to the corresponding NBD ( Figure 2 a). Importantly, the effect of having a substituent at the ortho position was evident in the trifluoroacetyl acceptor series, stabilizing the QC and showing a similar trend in previously reported for Q5d and other 2‐cyano‐3‐arylNBDs. 11

To determine whether the newly synthesized NBD–QC photoswitches had the correct physical properties for polymer incorporation and daily cycle laminate applications, all NBDs were studied by UV–vis spectroscopy in toluene ( Table 1 ). This also included measuring the quantum yield of photoconversion and kinetic stabilities all QCs. Comparing the previously made NBDs N5b–d bearing a cyano acceptor group to analogs N4b–d with trifluoroacetyl acceptor group, the latter experienced red‐shifting for the NBD absorbance spectra of 70–100 nm. This gave NBD products with an improved overlap with the most intense region of the solar spectrum, especially N4c , which had an onset of absorbance at 529 nm. Meanwhile, there appeared to be no observable trend between the quantum yields for photoconversion between series N4b–d and N5b–d when measured using the high concentration regime (Abs >2). 17 Using the molecular absorptivity, quantum yield of photoisomerization and energy storage density (Δ H storage ) (see below) of the isomers as input parameters, we can estimate the fraction of the solar spectrum that can be captured and stored by these compounds. The estimates reveal energy capture efficiencies of 2.9%, 1.5%, and 3.8% for N4b , N4d , and NN4e respectively, (see the Supporting Information for details) rendering these the series of compounds with best solar capture efficiencies reported from our group so far.

2.3 Incorporation of NBDs into Polymers

With the new NBD–QC photoswitches and analysis of the photochemistry in hand, the study focused on the incorporation of the identified NBDs into a polymer matrix to assess possible applications in climate control for window laminating. Ideally, laminated films for temperature regulation require the NBD to have a half‐life in the order of 4–8 h. To investigate this, N4b was selected as it has a good solar spectrum overlap (λ onset = 457 nm) and a good quantum yield (Φ = 68%), but most importantly a half‐life of just under 7 h in toluene. This photoswitch was incorporated into four different polymers with the aim of investigating the effect of the matrix polymer on the switching properties of the incorporated NBD. This included the use of atactic polystyrene (PS), which was identified as a comparable host medium to solution based studies in toluene. Also, included in this study was atactic PMMA as it has been reported previously that the incorporation of NBD derivatives into PMMA resulted in composites with good cyclability. To compliment this investigation, more polar polymer matrices such as polycarbonate (PC) and polyvinylidene chloride (PVDC) were also tested. PVDC was chosen as it provides a good barrier for molecular oxygen, and it was evident from former work that molecular oxygen can affect the stability of NBD undergoing several cycles.11 Each polymer film was prepared by allowing a dichloromethane solution containing 0.1 wt% N4b. All composites of N4b@polymer gave free standing films of yellow‐colored plastic, and when subjected to irradiation these materials went colorless, through the formation of Q4b.

The absorbance profile for N4b within the polymer matrices were slightly redshifted compared to solution characterization in toluene (order of 3 nm). Figure 3a shows the UV–vis absorption profile found for N4b@PS before and after irradiation. In addition, cycling studies were performed on these films, and the results for the activity of N4b in the different hosts can be seen in Figure 3b. Each cycle consisted of measuring absorbance followed by irradiation of the sample until the color of the film had faded and the peak at 379 nm in the UV–vis absorption spectrum had disappeared. The sample was then left in the dark, and the UV–vis absorption spectrum was periodically measured until no change in the spectra was observed.

Figure 3 Open in figure viewer PowerPoint a) Absorbance spectra of 0.1 wt% N4b@PS before and after irradiation. b) Cyclability of 0.1 wt% N4b in different polymers.

The stability of N4b was indeed dependent upon the host polymer. For instance, composite N4b@PVDC after one cycle had lost some 20% of its activity, whereas N4b@PS formed a much more robust film, experiencing very little degradation after four cycles.

The half‐lives of Q4b in PMMA, PC, and PS at room temperature were carried out and summarized in Table 2. It was found that the rate of back‐conversion for Q4b→N4b was significantly influenced by the polymer host. Composite Q4b@PMMA exhibited a fast rate of conversion with a half‐life of only 20 min, while for Q4b@PS and Q4b@PC, the photoswitch exhibited markedly longer half‐lives. In fact for Q4b@PS and Q4b@PC, studies of the switching kinetics revealed that the back‐conversion did not follow a first order kinetics, and fitting of the absorbance as a function of time instead agreed with a double exponential decay. For instance, Q4b@PS exhibited two half‐life times of 40 min and of 5.8 h for the second cycle. Similar results for other photoswitches in polymer films have been reported10 and could be explained by the inhomogeneous nature of local environment in the polymer composites. From these results, we can determine that polystyrene was the most suitable matrix polymer for the fabrication of N4b‐polymer composites on the account of its high durability and a high lifetime stability for the QC form. In addition, this aggregation does not appear to affect the stability of the photoswitch in PS. This makes N4b@PS a promising material for window tinting applications. However, this is complicated by the changing rates after successive cycles. Due to the low stability of this photoswitch in PVDC, no attempt was made to analyze the back‐conversion kinetics for this material.

Table 2. Results of kinetics measurements of polymer films, featuring the rate constants as 1 and 2 for the double exponential fit and corresponding half lives Cycles k1 [s−1] k2 [s−1] t ½ 1 [h] t ½ 2 [h] N4b@PMMA500 Cycle 1 5.70 · 10−4 0.34 N4b@PC Cycle 3 1.60 · 10−4 2.70 × 10−5 1.20 7.13 N4b@PC Cycle 6 9.46 · 10−5 2.02 × 10−5 2.03 9.55 N4b@PS Cycle 2 2.88 · 10−4 3.31 × 10−5 0.67 5.81 N4b@PS Cycle 7 5.73 · 10−5 1.01 × 10−5 3.36 19.1 N4b@PS Cycle 10 4.15 · 10−5 5.70 × 10−6 4.64 33.8

Given that PS was the optimal host for N4b and best preserved the properties of the photoswitch, longer cycling studies were necessary to ascertain the effects on NBD@PS, and most importantly on the kinetics for the back‐reaction. To quantify the effect on the rate of conversion over successive cycles we selected N4c, which has a faster rate of back‐conversion and therefore should allow for faster cycling. This investigation was also undertaken using different loadings of N4c (0.005–0.5 wt%) into PS to probe whether the concentration of the photoswitch also played a role. Composite N4c@PS was more colored than N4b@PS on the account of a greater degree of polarization in the π‐system and had a higher extinction coefficient. Compound Q4c has a shorter half‐life, which made it difficult to obtain a spectrum of fully converted Q4c@PS.

Photoswitch N4c was loaded into PS at four different concentrations, and the cyclability showed a decomposition per cycle of 0.02– 0.45% for the system (Figure 4b); however, this was not proportionate to the amount of N4c introduced to PS. Unlike for N4b@PS, it was not possible to obtain full conversion for the more concentrated samples, as the rate of back‐conversion is too fast. More surprising was the change in the conversion rate going from Q4c@PS→N4c@PS. As seen with N4b@PS, the back‐conversion rate slowed down over several cycles, and gradually reached a plateau (Figure 4c) and was independent of the relative concentration of N4c. This could be a result of progressive structural rearrangement of the NBD within the material as Q4c should occupy less space than N4c. After several cycles there are still two rate constants, which average out at t ½ 1:22 min and t ½ 2:178 min for the half‐lives for Q4c@PS at all measured concentrations. When examining the films with an optical microscope, it was found that small crystallites had formed within the composite films, suggestive of photoswitches aggregation, which could be related to the change in rate of back‐conversion. It should be noted that the measurements were performed at ambient temperature, well below the glass transition (T g ) of polystyrene (T g (PS) ≈ 100 °C).

Figure 4 Open in figure viewer PowerPoint a) 0.1 wt% N4c@PS before and after irradiation. b) Performance of N4c@PS with a weight percentage of 0.5% N4c subjected to photothermal cycling. c) Kinetics for 0.005–0.50 wt% Q4c→N4c@PS for multiple cycles showing the two reaction constants for the double exponential fit.

In order to demonstrate that these composites can not only absorb sunlight but also release the energy as heat, four different QC@PS films were prepared with a higher loading of the active molecule. Four NBDs were selected consisting of N4b–d and the symmetric dimer NN4e. Each sample was firstly irradiated under a solar simulator and immediately measured by differential scanning calorimetry (DSC). The polymer composite was recovered from the DSC pan to investigate the cyclability, then placed in a new DSC pan and subjected to two further irradiation‐heat release cycles. The heat release was measured using a gradient of 2 °C min−1 and the results are summarized in Table 3. The DSC analysis method was conducted at a markedly slower heating rate than what is normally used for measuring of heat release of neat QCs so as to avoid heating the sample above the temperatures where phase changes of the material could interfere with the anticipated exotherm. For N4b@PS, it was found that the heat release performance of corresponding QC remained unaffected after three cycles, when each DSC cycle heated the material to 90 °C, which is just below the glass transition temperature of the polystyrene matrix. Not surprisingly, it was not possible to obtain and analyze Q4c@PS, as the material back‐converted too rapidly. Material Q4d@PS was also measured in the DSC, though this sample could not be cycled as N4b@PS, as heating of the compound to 90 °C did not result in a full back‐conversion of N4b. Upon heating to 110 °C, we observed a full release in the DSC; however, the cyclability of the switching process diminished significantly. This could be related to softening of the polymer matrix at temperatures higher than the T g , which facilitates phase separation and the formation of NBD aggregates within the polymer matrix. Composite QQ4e@PS was also prepared, and the heat release of the material could be measured in a similar fashion to that of Q4d@PS. Collectively, these materials exhibited energy densities between 30 and 50 kJ kg−1 (see Table 3), though this was dependent on the amount of NBD in PS. For the sake of direct comparison, molar enthalpies for the heat release are also summarized. In all examples, the energy release per mole of QC was similar. Molecule QQ4e, possessing two photochromic units, leads to a higher energy density (0.48 MJ kg−1). We note that it was not possible to measure the heat release of the neat QCs due to their fast back‐conversion in their neat forms; however, the molar heat release could be calculated from the polymer samples. These values were found to be similar to those found for cyano analogs Q5b,d, and QQ5e (Table 3).

Table 3. Heat release for QCs in PS Loading of NBD [wt%] ΔH storage of composite [kJ kg−1] ΔH storage for NBD [MJ kg−1] ΔH storage for NBD [kJ mol−1] Q4b@PS 8.44 30.2 0.36 105 Q4d@PS 14.5 50.4 0.35 110 QQ4e@PS 10.8 51.8 0.48 216 Q5b 5 0.40 88.5 Q5d 11 0.50 118 QQ5e 11 0.77 238

To illustrate the potential of NBD–polystyrene composites for the lamination on windows, N4b@PS (0.8 wt% N4b) were cast onto glass substrates (see Figure 5) resulting in a film thickness of 70 µm (±5 µm). To prevent delamination of the N4b@PS coating we used either glass substrates which were subjected to a surface treatment with hexamethyldisilazane, or RadiSurf substrates, which comprise of a PS adhesion layer. These films converted within seconds upon exposure to sunlight, illustrating how the windows would work in real life (see Video SI in the Supporting Information).