Compartmentalizing Donor-Acceptor Stenhouse Adducts for Structure-Property Relationship Analysis

Abstract

The development of photoswitches that absorb low energy light is of notable interest due to the growing demand for smart materials and therapeutics necessitating benign stimuli. Donor-acceptor Stenhouse adducts (DASAs) are molecular photoswitches that respond to light in the visible to near-infrared spectrum. As a result of their modular assembly, DASAs can be modified at the donor, acceptor, triene, and backbone heteroatom molecular compartments for the tuning of optical and photoswitching properties. This Perspective focuses on the electronic and steric contributions at each compartment and how they influence photophysical properties through the adjustment of the isomerization energetic landscape. An emphasis on current synthetic strategies and their limitations highlights opportunities for DASA architecture, and thus photophysical property expansion.

Scheme 1. Donor–Acceptor Stenhouse Adduct Photoswitch

Scheme 2. DASA (Photo)isomerization Mechanism and Exemplary Isomerization Potential Energy Surface

Scheme 3. DASA Synthesis and Interrupted Aza-Piancatelli Rearrangement Mechanism

Figure 1

DASA molecular compartments and starting materials.

Figure 2

(a) DASA push–pull strength as characterized by solvatochromic slope from absorbance by Dimroth–Reichardt parameters.^, (b) DASA charge delocalization as characterized by bond-length alternations.^,

Figure 3

Amine donor scope in the general order of donating strength based on calculated pK_b values.

Figure 4

DASA absorbance tuned by donor compartment.^† (a) λ_max comparison of dialkyl and aryl amine donors, and the effect of arene planarity. λ_max comparison of para-substituents on (b) indoline and (c) N-methylaniline donors. λ_max values are in CH₂Cl₂ for (a) and (b) and in CDCl₃ for (c) with DASAs bearing Meldrum’s acid. ^†Absorbance bands illustrate the spectral characteristics but do not constitute experimental data. Graphics were created using data from refs (38) and (52).

Figure 5

(a) DASA isomerization energy surface with dialkyl and aryl amine donors in chloroform (M06-2X/def2-QZVP). (b) X-ray structures comparing the closed isomer charge character of DASAs with Meldrum’s acid acceptor and p-OMe-N-methylaniline (CCDC 1500305) or p-F-N-methylaniline donors (CCDC 1500307). Graphics were created using data from refs (41) and (52).

Figure 6

Cyclic β-carbonyl carbon acid acceptor scope in the general order of accepting strength based on calculated pK_a values.

Figure 7

DASA absorbance tuned by acceptor compartment.^† λ_max comparison of acceptors with varying electron withdrawing strength. λ_max values are in CDCl₃ for DASAs with 2-methylindoline donor. ^†Absorbance bands illustrate the spectral characteristics but do not constitute experimental data. Graphics were created using data from ref (53).

Figure 8

(a) DASA isomerization energy surface with strong and weak acceptors in chloroform (M06-2X/def2-QZVP). (b) Acceptor steric effect on DASA isomerization energy surface in toluene (ωB97x-D3/def2-TZVP(-f)). Graphics were created using data from refs (41) and (82).

Figure 9

(a) DASA absorbance tuned by triene compartment.^† Spectral shift of DASAs with C5–Me, C3–Ph, and C3–Br substitutions in comparison to unsubstituted counterparts. Spectral shifts were calculated from λ_max in CH₂Cl₂ for DASAs with indoline and Meldrum’s acid (C5–Me) or diheptylamine and N,N-dimethylbarbituric acid (C3–Ph and C3–Br). (b) Triene electronic effect on excitation energy. Hammett parameter correlation to calculated C3- and C4-substituted DASA excitation energies in toluene (B3LYP/6-31+G(d)).^†Absorbance bands illustrate the spectral characteristics but do not constitute experimental data. Graphics were created using data from refs (54) and (55).

Figure 10

(a) Triene electronic effect on photoexcitation charge transfer calculated in toluene (B3LYP/6-31+G(d)). (b) DASA isomerization energy surface with C3 substituents in toluene (M062X). (c) Calculated steric effect of C5 substituent on DASA closed isomer in toluene (M06-2X/6-31+G(d,p)). Graphics were created using data from refs (54) and (55).

Scheme 4. Heterocycle Ring-Opening Viability Based on ¹³C(7) NMR Spectral Shift in CDCl₃

Graphics were created using data from ref (57).

Figure 11

DASA absorbance tuned by backbone heteroatom compartment.^† λ_max comparison of hydroxy DASAs to (a) nonhydroxy and (b) sulfonyl amino DASAs. λ_max values are in CH₂Cl₂ for DASAs with (a) diethyl amine and Meldrum’s acid or (b) indoline and N,N-dimethylbarbituric acid. ^†Absorbance bands illustrate the spectral characteristics but do not constitute experimental data. Graphics were created using data from refs (57) and (84).

Figure 12

Proposed (photo)isomerization of (a) nonhydroxy DASA and (b) sulfonyl amino DASA. Graphics were created with data from refs (57) and (84).

Scheme 5. Structural Tuning of Tethered DASA To Improve Photoswitching in Polar Protic Media