Photoswitches are molecules that are reversibly and chemically interconverted between two states with light. Azobenzene is the most widely studied photoswitch and has a well-documented synthesis via diazonium coupling reactions. The relatively small size of azobenzene, along with the significant chemical and structural changes resulting from isomerization, has enabled its use for the spatiotemporal control of proteins, lipids, neurotransmitters, neurons, and carbohydrates. Phenyl functionalization can improve photophysical properties for applications in chemical biology and materials science. Scheme 1 shows the photochemical E → Z conversion of azobenzene and the subsequent Z → E thermal reversion. Azobenzene requires ultraviolet light (320 nm) to promote the E → Z isomerization, limiting its utility in vivo due to undesired lightpromoted dimerizations, destruction of living tissue from UV-light, and low light penetrability in living tissue. Further, the meta-stable Z-isomer thermally reverts to the E isomer and has a half-life (t1/2) of 2 days at room temperature in acetonitrile. This relatively short half-life results in an unstable photostationary state (PSS), thus limiting photoswitch efficacy where bistability is neded. As such, an ideal photoswitch features a long-lived t1/2 (e.g., weeks to months), an E-isomer λ max in the visible range, and well-separated λ max values for the E and Z isomers to prevent bimodal isomerization. Woolley and Fuchter reported that λ max absorptions are redshifted into the visible range by replacing phenyl groups with heteroaryl rings (benzodioxanes , diazinines , and imidazoles ) and by functionalizing the aryl groups. These heteroazoarenes have relatively high E → Z photochemical reaction yields, ranging between 46-98, and Z-isomer half-lives ranging from 1 second up to 46 years. The range in E-isomer λ max for these heteroazoarenes is 310-720 nm, pushing them deep into the visible light range. There is an inverse relationship between λ max and t1/2 for these heteroazoarenes, those with the highest t1/2 have the smallest λ max . Scheme 2 shows heteroazoarenes with heteroaryl rings, and the heteroazoarenes of each type with the longest t1/2 values are shown in Figure . Thermal Z → E reactions can proceed through inversion or rotation mechanistic pathways. Past computational studies have found that the inversion mechanism is preferred for azoarene thermal Z → E isomerization reactions. Thiophene-containing heteroazoarenes have well-separated E and Z absorption bands and a high yield of Z-isomer for the E → Z photoisomerization reaction (94-98%). 2-hemi-azothiophene and its derivatives have been studied by Wegner, Dreuw, and Wachtveitl (Scheme 2b), and two (3,3) bis-azothiophenes (Figure ) have been studied by Perry. The unsubstituted 2-hemiazothiophene (Scheme 2b, left) has a Z-isomer t1/2 of 7 hours in acetonitrile. Wegner and Heindl showed that electron-donating groups (EDG) at the phenyl para position (Me and OMe groups) of 2-hemi-azothiophenes lower the t1/2 of the Z-isomer to 0.5 and 1.9 hours (methyl and methoxy substituent, respectively). Electron withdrawing groups (EWGs) increase the t1/2 to 14.3 and 17.8 hours (CF3 and CN, respectively). Substitution of the thiophene ring with an electron-donating group (OMe) lowered the t1/2 to 2.8 hours. Installing push-pull substituents (OMe and CN groups) on an azothiophene lowered the t1/2 to 9 minutes. The λ max is 365 nm for the unsubstituted hemiazothiophene and 405 nm for the push-pull hemi-azothiophene. All hemi-thiophene E-isomer λ max values are red-shifted towards the visible range, an improvement over azobenzene. Figure shows the heteroazoarenes photoswitches of each type with the longest measured half-lives. Although recent studies have increased the Z-isomer t1/2 or the E isomer λ max , experimental and theoretical studies on heteroazoarenes generally show an inverse relationship between t1/2 and λ max . The reason for this relationship has not been well defined; there is no clear connection between how the E-isomer λ max relates to the Z-isomer half-life. We have computed the thermal and photophysical properties of isomeric bis-heteroazoarenes photoswitches (two azobisfurans and two azobisthiophenes) using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The Z isomers of 1, 2, 3, and 4 are shown in Figure . Watchtveitl and co-workers initially reported derivatives of 3-Z. The (3,3)-azobisthiophene (4) and one derivative with ester groups substituted at the 2-position were synthesized, and UV/Vis spectra and Z-isomer half-lives were measured by Perry and co-workers. These molecules had t1/2 and λ max values of 38 days and 316 nm (unsubstituted azothiophene) and 14 days and 350 nm (estersubstituted azothiophene).