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Substituent effects in N-acetylated phenylazopyrazole photoswitches

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  4. 2 ORCID Logo and
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1Institute of Organic and Biomolecular Chemistry, Georg-August-University, Tammannstraße 2, 37077 Goettingen, Germany
2Department of Chemistry, Ångström laboratory, Uppsala University, Box 523, 751 20 Uppsala, Sweden
3Cluster of Excellence “Multiscale Bioimaging: from Molecular Machines to Networks of Excitable Cells” (MBExC), University of Göttingen, 37075 Göttingen, Germany
  1. Corresponding author email
  2. ‡ Equal contributors
This article is part of the thematic issue "Harnessing light energy with molecules".
Guest Editor: H. A. Wegner
Beilstein J. Org. Chem. 2025, 21, 830–838. https://doi.org/10.3762/bjoc.21.66
Received 06 Feb 2025, Accepted 10 Apr 2025, Published 25 Apr 2025
A non-peer-reviewed version of this article has been posted as a preprint https://doi.org/10.3762/bxiv.2025.7.v1
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Abstract

Phenylazopyrazole photoswitches proved to be valuable structural motifs for various applications ranging from materials science to medicine. Despite their potential, their structural diversity is still limited and a larger pool of substitution patterns remains to be systematically investigated. This is paramount as electronic effects play a crucial role in the behavior of photoswitches and a deeper understanding enables their straightforward development for specific applications. In this work, we synthesized novel N-acylpyrazole-based photoswitches and conducted a comparative study with 33 phenylazopyrazoles, comparing their photoswitching properties and the impact of electronic effects. Using UV–vis and NMR spectroscopy, we discovered that simple acylation of the pyrazole moiety leads to increased quantum yields of isomerization, long Z-isomer life-times, good spectral separation, and high photostability.

Keywords: azobenzenes; azopyrazoles; photochromism; photoswitches; substituent effects

Introduction

Organic photoswitches are molecules reversibly changing their optical and chemical properties upon irradiation. These features offer easy, precise, and reversible control over the system they are embedded in and make them attractive modulators for diverse applications. In the last decades, many classes of photoswitches have been described and extensively studied . Among these, azobenzenes belong to the most common ones . They were firstly explored almost 200 years ago and initially used mainly as dyes or pigments . Relatively recently, azobenzenes started to become an important part of state-of-the-art technologies ranging from energy-storage materials to pharmacology , materials chemistry , control of peptides structure or proteins , as antibacterial agents , smart coating , or multivalent photoresponsive systems , to name only a few examples.

Azobenzene and its derivatives show two characteristic absorption bands, namely a π→π* transition around 330 nm and an n→π* one around 450 nm, respectively . The molecule can populate the thermodynamically metastable Z isomer by addressing these transitions in the thermally stable E form. The relative position of the absorption bands in the azobenzene derivatives depends on the substitution pattern on the aromatic rings, which can act as a handle to affect the absorption properties of the compound class . For instance, push–pull systems or the introduction of tetra-ortho substituents were reported to either bathochromically shift the UV–vis absorption spectrum or lead to a better separation of the n→π* bands of the two photoisomers and allow for visible-light-responsive switches .

In recent years, heteroaryl azobenzene derivatives have revealed superior properties to classical azobenzenes. Heterocyclic rings offer, for example, enhanced polarity, electron pairs for metal coordination , better water-solubility, and variable pKa . Special attention has been given to 5-membered N-heterocyclic azobenzenes, which not only maintain the azobenzenes properties but often show higher quantum yields and increased thermal half-life of the metastable state. For the half-life, the choice of the heterocycle is crucial, as revealed through density functional theory (DFT) calculations, which showed that a 5-membered ring promotes the stability of the Z isomer . Within these, azo-photoswitches based on a 1,3,5-trimethylpyrazole ring (phenylazopyrazole; PAP) became particularly popular, showing almost quantitative back and forward photoswitching and high thermal stability . Moreover, 1H-pyrazole derivatives and arylazopyrazolium compounds were investigated in detail and allowed to correlate thermal relaxation rates and steric or electronic effects as well as mechanistic peculiarities, which has been in the spotlight recently for different classes of azobenzenes .

Despite these studies, the variety of substitution patterns in PAPs is limited compared to classical azobenzenes and remains to be better understood. Interestingly, Leistner et al. recently reported that introducing a formyl group in the para position of tetra-ortho-fluoro or -chloro-substituted azobenzenes leads to the decrease of the HOMO–LUMO gap resulting in a significant 50 nm bathochromic shift of the n→π* absorption band further moving their UV–vis absorption spectrum towards the therapeutic window . Moreover, a similar modification in stilbene-based photoswitches and molecular motors showed an increase in performance and photoisomerization quantum yield .

Consequently, we were interested to study the photoswitching properties of eleven novel N-acetylated analogous NAc-PAPs and the influence of substituent effects. Furthermore, we compared their photochromism with a set of 22 known N-methylated (NMe-) and unfunctionalized (NH-) PAPs in moderate to good yields (66–85%).

Results and Discussion

Synthesis

The PAPs in this study were obtained in a straightforward, three-step metal-free synthesis from commercially available aniline derivatives adapting known procedures . An overview of the compounds used in this study and their synthesis is displayed in Scheme 1. First, a diazotization of a given aniline 1 and reaction with 2,4-pentanedione gave intermediate 2, with yields which strongly depended on the residue in the para-position. Specifically, the residues bearing an electron-donating group (EDG) such as -OMe or -OH showed low yields because of the poor reactivity of the diazonium salt. When strong electron-withdrawing groups (EWGs) were introduced, the yield was also reduced, likely due to the low nucleophilicity of the aniline derivative and the ineffective formation of a diazonium salt. An annulation reaction of compound 2 was performed with either hydrazine or methylhydrazine to yield NH-PAPs or NMe-PAPs, respectively, in high to quantitative yields. The NAc-PAPs were synthesized via an acetylation reaction of NH-PAPs with acyl chloride, forming the novel NAc-PAPs. While the molecules could be extracted and purified by column chromatography without further issue, we observed some instability at acidic pH during analysis. Thus, we tested the stability of NAc-PAP-H (1 mM, MeOH, ambient conditions, 2 h) at pH 2, 12, and in the presence of DBU (10−2 M) and found that the acetyl group was lost (cf. Supporting Information File 1, section 2.4).

[1860-5397-21-66-i1]

Scheme 1: Reaction pathway for synthesizing NH-substituted, methylated-, and acetylated arylazopyrazoles. Conditions: A) NaNO2, AcOH + HCl at 0 °C, then, 2,4-pentanedione, NaOAc in EtOH + H2O, reflux; B) MeNHNH2, EtOH, reflux; C) NH2NH2, EtOH, reflux; D) AcCl, NaOAc in DCM, 0 °C to rt.

UV–vis absorption spectroscopy

With a set of photoswitches at hand, we studied the obtained PAPs through UV–vis absorption spectroscopy. The photochemical properties are summarized in Table 1. All compounds show absorption maxima in the range of 324–368 nm in CH3CN, which corresponds to the strongly allowed π→π* transition, while the weaker n→π* band shows absorption maxima in the range of 410–451 nm in CH3CN.

Table 1: Photophysical properties of synthesized PAP derivatives in CH3CN.

R: NH-PAP NMe-PAP NAc-PAP
λmax [nm] ε (π→π*) 103 [L/(mol·cm)] λmax [nm] ε (π→π*) 103 [L/(mol·cm)] λmax [nm] ε (π→π*) 103 [L/(mol·cm)]
π→π* n→π* π→π* n→π* π→π* n→π*
OH 342 415* 16.8 ± 0.5 345 a 18.0 ± 0.3 344 443 13.0 ± 0.1
OMe 344 410 18.9 ± 0.8 345 a 19.4 ± 1.1 344 447 28.3 ± 0.4
Me 336 413* 19.1 ± 0.9 338 434 21.8 ± 1.3 333 425 24.1 ± 0.4
H 330 440 18.9 ± 0.1 337 433 19.5 ± 1.1 327 427 24.5 ± 0.4
F 332 437 17.2 ± 0.7 335 435 17.3 ± 0.5 327 426 20.2 ± 0.1
Cl 336 441 20.9 ± 0.6 341 447 22.1 ± 1.1 333 414 26.9 ± 0.3
Br 341 450 17.8 ± 0.5 343 451 27.1 ± 0.1 334 429 25.2 ± 0.2
I 343 451 24.2 ± 0.1 347 445 22.1 ± 1.3 340 422 29.1 ± 0.6
CF3 335 442 16.3 ± 0.1 341 444 23.3 ± 0.1 324 435 23.5 ± 0.5
CN 345 448 24.1 ± 1.4 352 451 13.8 ± 0.2 333 444 27.3 ± 0.5
NO2 361 449 21.3 ± 1.0 368 a 13.8 ± 0.1 345 448 26.4 ± 0.5

aTransition band appears as shoulder.

The introduction of methyl and acyl groups on one nitrogen of the pyrazole ring led to a hyperchromic effect and a minor bathochromic shift. Also, the introduction of an EDG or EWG on the R position led to minor hyperchromic effects and a bathochromic shift (exemplary the UV–vis absorption spectra of NAc-PAPs are displayed in Figure 1 and a full set is available in Supporting Information File 1, section 3.5). In the case of EWGs, we observed positive hyperchromic effects while for EDGs negative hyperchromic effects compared for NH- and NMe-PAP-H.

[1860-5397-21-66-1]

Figure 1: UV–vis absorption spectra of selected NAc-PAP derivatives in CH3CN. The strong π→π* can be observed in the region of 330–360 nm.

Photochemical isomerization

Upon irradiation with a 365 nm LED (exemplarily displayed for NMe-PAP-CN in Figure 2), we observed a decrease of the π→π* band for the E isomers. Simultaneously the π→π* bands around 253–369 nm and the n→π* bands around 426–457, respectively, of the Z isomers increased until reaching the photostationary state (PSS).

[1860-5397-21-66-2]

Figure 2: A) Time-resolved UV–vis absorption spectra of NAc-PAP-CN upon 365 nm irradiation (12.5 µM in CH3CN, at 25 °C). B) Absorbance of the same sample at 365 nm (Ar, 365nm) after reaching PSS365 or PSS455, respectively, to show the recyclability.

The UV–vis absorption spectra of the Z isomers show a more intense n→π* transition and less intense π→π* absorption band compared to the E isomers, because of the loss of the molecule’s planarity . This change in color can also be seen by naked eye with the solution changing from pale to dark yellow (cf. Supporting Information File 1, Figure S2). In Table 2, the absorption maxima of Z–PAP derivatives in CH3CN are summarized. For EWGs, such as CF3, CN, or NO2, the absorption maxima are redshifted, while for EDGs, such as OH or OMe, they are shifted towards the blue. Especially for NAc-PAPs, we observe a higher spectral separation of the π→π* and the n→π* transitions of the Z isomers compared to NMe-PAPs and NH-PAPs. For instance, the separation for the π→π* transition band and the n→π* is 176 nm for NAc-PAP-CN, while for NMe-PAP-CN 146 nm and for NH-PAP-CN 102 nm are found.

Table 2: UV–vis absorption maxima of Z-PAP derivatives in CH3CN (obtained through 365 nm irradiation to the corresponding PSS365).

R NH-PAP NMe-PAP NAc-PAP
λmax [nm] λmax [nm] λmax [nm]
π→π* n→π* π→π* n→π* π→π* n→π*
OH 304 436 307 435 309 445
OMe 307 426 307 411 307 445
Me 331 457 301 456 279 439
H 300 433 358 455 280 442
F 334 441 299 445 280 442
Cl 358 454 302 456 277 437
Br 366 452 302 457 275 435
I 366 455 366 456 279 445
CF3 369 453 304 442 253 440
CN 361 463 311 457 265 441
NO2 350 463 372 448 280 448

Subsequent illumination with a 445 nm LED led to the formation of the E isomer, which is accompanied by an increase of its π→π* band until reaching the PSS445. However, judging from the intensity of the E isomers’ π→π* band, the photostationary distribution (PSD) of the initial dark state could not be fully regenerated by photochemical means for all 33 PAPs (vide infra, ex-situ NMR measurements and thermal half-lifes).

The recyclability of NH-PAPs was previously studied by Rustler et al. by altered irradiation with 365 and 420 or 455 nm light, showing a great photostability of these compounds . We performed the same experiment on our NAc-PAPs (see Table 3 and section 3.2 in Supporting Information File 1) and did not observe any fatigue after 10–20 cycles of photoswitching, showing high photostability also of NAc-PAPs.

Table 3: PSS distribution of various NAc-PAP derivatives.a

R: 365 nm
E:Z UV–vis 		365 nm
E:Z NMR 		445 nm
E:Z UV–vis 		445 nm
E:Z NMR
OH 8:92 a 81:19 a
OMe 10:90 5:95 76:24 78:22
Me 4:96 4:96 77:23 77:23
H 22:78 22:78 81:19 81:19
F 8:92 8:92 79:21 79:21
Cl 16:84 4:96 73:27 75:25
Br 2:98 4:96 77:23 75:25
I 6:94 6:94 75:25 75:25
CF3 17:83 17:83 78:22 78:22
CN 15:85 5:95 78:22 78:22
NO2 14:86 27:73 84:16 88:12

aNot determined; relaxation towards the Z isomer is too fast.

To quantify the extent of photoisomerization, the isomer distributions of NAc-PAPs at the PSS365 and PSS445 were investigated using UV–vis and NMR spectroscopy. For the ex-situ 1H NMR irradiation experiments, we irradiated our samples in a cuvette until the PSS was reached and measured immediately afterwards the 1H NMR spectrum. We compared these results to simply estimating the isomer distribution at the PSS (cf. Supporting Information File 1, section 3.1) from the UV–vis spectra and observed good agreement, meaning the results from UV–vis spectroscopy can be used for rough PSD estimation, due to good spectral separation of the isomers. Since we encountered fast back isomerization only for NAc-PAP-OH (vide infra), the PSDs could only be determined from the UV–vis spectra.

NAc-PAPs showed high to quantitative formation of the Z-configurated isomers. For example, NAc-PAP-Br showed an isomerization distribution of 98% Z by UV–vis and 96% Z by 1H NMR. EWGs, such as NAc-PAP-CF3, on the other hand, showed a lower PSD for the Z isomer of 83% by both UV–vis and 1H NMR spectroscopy. For NMe- and NAc-PAPs, we observed increased Z content to almost quantitative photoisomerization after 365 nm illumination for most cases compared to NH-PAPs. For NMe-PAPs, West et al. attributed this to a pronounced spectral separation between the π→π* bands of E and Z-isomers, which we also found for NAc-PAPs .

For the back isomerization with 445 nm light, favoring the E isomer, we observed that the amount of E isomer present in the dark state could not be reached for any of the NAc-PAPs. For example, for NAc-PAP-Cl 73% of the E isomer could be reformed and for NAc-PAP-NO2 84%. Since we observed for NAc-PAPs a decrease of spectral separation of the n→π* bands for the E and Z isomers, a non-quantitative PSD upon irradiation with 445 nm can be explained by the presence of a competing E→Z isomerization at the same wavelength. In general, NAc-PAPs show only a minor substitution effect on the PSD upon irradiation with 365 or 445 nm LED light.

Next, we studied the photoisomerization quantum yields (QYs) ΦE→Z and ΦZ→E and the impact of substitution effects on the para position, using two types of LEDs (365 nm and 445 nm, details see Supporting Information File 1, section 3.5). The values determined are provided in Table 4.

Table 4: Quantum yield values for the E→Z and Z→E photoisomerization of PAP derivatives with the corresponding 365 nm and 445 nm LED in CH3CN.a

R: X = H X = Me X = Ac X = Ac
π→π* [%] n→π* [%]
OH 33 44 56 a
OMe 29 38 65 28
Me 18 45 44 31
H 11 22 14 30
F 29 56 72 61
Cl 26 36 59 42
Br 21 32 55 39
I 29 40 45 32
CF3 25 27 69 30
CN 25 26 26 10
NO2 21 18 16 20

aNot determined.

For ΦE→Z, we could observe for nearly all NAc-PAP derivatives, higher values compared to NMe-PAPs, followed by NH-PAPs in a descending order. For example, for NH-PAP-CF3 we found a QY of 25%, which went towards 27% for the NMe-PAP-CF3 and finally to 69% for the NAc-PAP-CF3. Especially for EDGs and weak EWGs, the NAc-PAPs showed higher ΦE→Z compared to NMe-PAPs and NH-PAPs, while strong EWGs show a higher to equal ΦE→Z. Interestingly, for R = Me and H, we recorded higher QYs in NMe-PAPs than in NAc-PAPs (22% and 45% for R = H and R = Me in NMe-PAP vs 14% and 44% for the acetylated ones).

In contrast to the excitation of the π→π* state in NAc-PAPs, the isomerization process varies in efficiency when the n→π* transition is selectively addressed, with similar or markedly reduced quantum yields. A deviation from Kasha's rule with the opposite outcome (viz. the quantum yield of isomerization proceeding from the excitation of the π→π* state is lower than the one from the n→π*) is reported in various studies on azobenzene . Another interesting aspect, that could point to a more complex picture in the excited state landscape of these switches, is that the QYs of NAc-PAPs with R = NO2 and R = H are lower for the π→π* than for the n→π* transition, while for the other substituents the opposite was found. We thus suspect that the substituents play a crucial role in in the population of the respective exited state and we can at this state not rule out a contribution also from the triplet state.

Moreover, we could not find a quantitative correlation between the R-substituents and ΦE→Z, however, some trends can be observed; EWG and EDG lead to higher ΦE→Z for the π→π* transition, while the opposite can be seen for the n→π* transition. We also observed increased numbers for the ΦE→Z of π→π* for NMe-PAPs compared to NH-PAPs. However, the highest values were observed for our newly synthesized NAc-PAPs. For example, for NH-PAP-CF3 we observed that the ΦE→Z has a value of 25%, for NMe-PAP-CF3 27%, while for NAc-PAP-CF3 we observed an increase of ΦE→Z to 69%.

Thermal half-lifes

The metastable Z isomers can be converted back to the thermodynamically favored E form by thermal means. Four mechanisms were predicted using quantum chemistry to describe the Z→E isomerization in azobenzenes, namely: rotation, inversion, inversion-assisted rotation, and concerted inversion depending on the structure of the azobenzene . For PAPs, Calbo et al. showed by DFT calculations that the inversion mechanism is one of the fastest relaxation mechanism for heterocyclic azobenzenes (typically for most of the azo dyes ). However, the nature of the mechanism is still a matter of current debate, and additional factors, such as the presence of tautomerizable groups , and the involvement of the triplet state , appear to play a role.

Specifically, the rotation mechanism does not explain the low activation entropy observed in azobenzene systems sparking new discussions on the possibility of alternative isomerization pathways . Recently, Reimann et al. computationally showed that the involvement of a triplet state mechanism, which crosses the transition state for the Z→E relaxation, could explain the low values of the activation entropy. The same authors also showed experimental evidence for this proposal by an external heavy atom effect on Z→E isomerization.

To understand the thermal Z→E isomerization in our newly synthesized NAc-PAPs, we recorded the process by time-resolved UV–vis absorption spectroscopy and calculated the thermal half-lifes of back isomerization (Table 5 and Supporting Information File 1, section 3.6). EDGs and weakly EWGs, such as halogen substituents, exhibited thermal half-lifes for thermal back isomerization in the range of days. For instance, NAc-PAP-Cl or -Br converts back to the form with half-lifes of roughly 1.5 days. In contrast, NAc-PAP-H or -Me demonstrated significantly longer thermal half-lifes, ranging from 9 days to 21.5 days. Notably, NAc-PAP-OMe, with a thermal half-life of 4.38 days, extends the thermal half-life significantly compared to the 19.7 minutes observed for NH-PAP-OMe.

Table 5: Overview over thermal half-lifes for NAc-PAPs in CH3CN at 30 °C.

R: τ1/2 [d]
OMe 4.38
Me 8.97
H 21.5
F 13.3
Cl 1.51
Br 1.57
I 3.26
R τ1/2 [s]
OH 19.0
CF3 1992
CN 696
NO2 608

We subsequently analyzed the electronic effects on the thermal relaxation rates in our NAc-PAPs using Hammett parameters for the substituents in the para-position and found a trending behavior (cf. Figure 3). Specifically, the Hammett plot shows linear trends for both EWGs and EDGs, with a minimum for electron-neutral R = H and R = OH as an outlier, likely due to a contribution of tautomerization . Both linear fits show high slope values, indicating a great dependency on the nature of the substituent. The observed trend behavior indicates an apparent change of mechanism for thermal relaxation to the E isomer. This was previously observed for NH-PAPs N-PEG-PAPs , and for azopyrazolium salts .

[1860-5397-21-66-3]

Figure 3: Hammett plot of NAc-PAP derivatives.

To obtain a deeper understanding of the transition states thermodynamic properties, we decided to perform an Eyring analysis of two representative PAPs. We chose NAc-PAP-CN and NAc-PAP-OMe, which lie on two different ends of the Hammett plot and hence should reveal the difference in relaxation mechanism. We measured the temperature-dependency of their relaxation rates in toluene to access higher temperatures (Table 6 and Supporting Information File 1, section 3.7). The Eyring plot for NAc-PAP-CN and NAc-PAP-OMe are depicted in Figure 4.

Table 6: Eyring analysis of NAc-PAP-CN and NAc-PAP-OMe determined in toluene (for details see Supporting Information File 1, section 3.7).

  NAc-PAP-CN NAc-PAP-OMe
ΔG‡ / kJ/mol 99.1 ± 0.07a 104.3 ± 0.1a
ΔH‡ / kJ/mol 90.0 ± 0.7 93.0 ± 1.0
ΔS‡ / J/(mol·K) −30.0 ± 2.0 −39.0 ± 4.0

aAt 298 K.

[1860-5397-21-66-4]

Figure 4: Eyring plots for NAc-PAP-CN and NAc-PAP-OMe.

Counterintuitively, the calculated thermodynamic data of the transition states show, within the error margin, similar values of activation enthalpy ΔH‡ and ΔS‡. In particular, NAc-PAP-CN showed a negative ΔS‡ = −30.0 ± 2.0 J/(mol·K), while NAc-PAP-OMe showed a relatively similar value (−39.0 ± 4.0 J/(mol·K)), hinting towards the same mechanism of relaxation operating for both compounds. Comparing these values to the calculations for azobenzene, we hypothesize that both compounds undergo isomerization via triplet intermediacy .

Contrary to what was observed by Reimann et al. , however, we did not observe any heavy atom effect. This could be explained by the different ways in which the heavy atom is introduced: in our case as a substituent, in the case of the literature example by adding tetrabutylammonium iodide to the solution.

Conclusion

In this study, we synthesized and systematically investigated various PAP derivatives, including NAc-PAPs, NMe-PAPs, and NH-PAPs. As similar functional groups were reported as highly beneficial for the photochemical properties in other classes of photoswitches, our focus was on the novel NAc-PAPs, which exhibit an acetyl group on one of the pyrazole nitrogens. The functional group could be installed easily via acetylation from the corresponding NH-PAPs in high yields to result in a set of eleven novel compounds that we could compare to a set of 22 NMe-PAPs and NH-PAPs.

We then analyzed the molecules' photophysical and photochemical properties and studied the metastable isomers' thermal relaxation mechanism. In particular, photophysical studies highlighted the impact of structural modifications on the π→π* and n→π* transitions, showing that substitution of nitrogen with methyl or acetyl groups resulted in a small bathochromic shift and hyperchromic effects.

Anti-Kasha behavior was observed with distinct trends in the π→π* and n→π* transitions when studying the quantum yields (ΦE→Z and ΦZ→E ). Strong EWGs or EDGs enhanced the quantum yields for the π→π* transitions, whereas the n→π* transitions exhibited no clear correlation with substitution patterns. Notably, the acetylation of nitrogen significantly increased the ΦE→Z for π→π* transitions in almost all compounds studied (excluding NAc-PAP-H and -Me), even surpassing the effects of methylation.

Hammett analysis showed that the thermal population of the triplet state seem to be preferred as the thermal relaxation mechanisms of the back isomerization. EWGs and EDGs accelerated the relaxation dynamics compared to NAc-PAP-H. Acylation of the pyrazole moiety led to an enhanced metastable half-life compared to the NH-PAPs. For NAc-PAP-H, we observed increased half-lifes (21.5 days, 30 °C), compared to the reported NH-PAP-H (0.066 days; 25 °C ) or NMe-PAP-H (10 days; 25 °C , all in CH3CN). In the presence of OH as substituent, tautomerism can become feasible and result in particularly fast relaxation rates. These results highlight the complex interplay between electronic effects and thermal isomerization pathways in this class of compounds.

To summarize, this work provides a comprehensive understanding of how structural modifications affect the synthesis, photochemical, and thermal behavior of PAP derivatives introducing NAc-PAPs as novel compound set with enhanced photochemical performance. Our findings provide valuable guidance for designing functional PAPs with tailor-made photochemistry and photophysical properties, which may broaden their application in areas such as molecular switches, photodynamic materials, and optoelectronics.

Supporting Information

Supporting Information File 1: Materials and methods, analytical equipment, experimental procedures, compound characterization, UV–vis spectra at different concentrations, photochemical experiments, thermal isomerization analysis, and NMR spectra.
Format: PDF Size: 12.5 MB Download

Funding

We thank the Swedish Vetenskapsrådet for a Starting Grant (2021-05414 to SC and to 2023-04088 to NAS). This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) in the framework of RTG BENCh (389479699/GRK2455 to RT, PW, and NAS) and under Germany's Excellence Strategy (EXC 2067/1-390729940 to NAS).

Author Contributions

Radek Tovtik: conceptualization; data curation; formal analysis; investigation; methodology; visualization; writing – original draft; writing – review & editing. Dennis Marzin: conceptualization; formal analysis; investigation; methodology; visualization; writing – original draft; writing – review & editing. Pia Weigel: data curation; formal analysis; writing – original draft. Stefano Crespi: data curation; formal analysis; methodology; writing – review & editing. Nadja A. Simeth: conceptualization; funding acquisition; investigation; supervision; validation; writing – review & editing.

Data Availability Statement

All data that supports the findings of this study is available in the published article and/or the supporting information of this article.

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