Sensitivity of Isomerization Kinetics of 1,3,5-Triphenylformazan on Cosolvents Added to Toluene

Abstract

Formazan molecules exhibit photochromism because isomerization processes following excitation may occur in both the azo group and the hydrazone group; thus, each formazan may be present in various forms with different colors. The ratio of these forms depends on the illumination conditions and the environment of the formazan with a most incisive sensibility of the thermal anti-syn relaxation of the CN toward slight traces of impurities in toluene solutions, as reported most prominently for 1,3,5-triphenylformazan. Here, we study the latter compound with transient absorption spectroscopy to investigate the role of these traces by adding small amounts of both protic and aprotic cosolvents. Whereas the activation barrier decreases if the binary solvent mixture has a higher polarity, the role of hydrogen bonding can have a reverse impact on the thermal isomerization rate. Both the addition of an aprotic cosolvent and the addition of a protic cosolvent can slow the reaction due to their hydrogen-bond accepting and hydrogen-bond donating properties, respectively. In the case of methanol as a cosolvent, this effect outweighed that of the polarity increase for small concentrations, which was not observed for the fluorinated alcohol hexafluoroisopropanol. The results are explained in the context of a competition between solute−cosolvent and cosolvent−cosolvent hydrogen bonding.

Monitoring the photochemistry of a formazan over 15 orders of magnitude in time

“…Comparison to the DADS from the ns-ms TA measurements (pink curve in Figures 5A,D) confirms that the same dynamics that were monitored up to 1 ms can now be followed completely. This absorption band was already identified in toluene solution (Wortmann et al, 2022) and assigned to the red II form of TPF, in accordance with the isomerization scheme of Figure 1. The DADS furthermore comprises negative contributions around 500 nm and around 405 nm.…”

“…TPF exhibits a unique photochromism which depends on the excitation conditions and the solvent environment. We investigated in an earlier study to which extent the thermal back relaxation around the C=N double bond is sensitive to the polarity and the hydrogen-bond donating ability of the solvent ( Wortmann et al, 2022 ). While polar solvents with a higher polarity result in a decrease of the activation barrier of the anti-syn isomerization around the C=N bond, hydrogen bonding has an oppositely directed effect and can stabilize the trans-anti isomer under certain conditions.…”

“…The photochemistry of the precursor TTC has already been explored in detail with various techniques ( Pechmann and Runge, 1894 ; Hausser et al, 1949b ; Nineham, 1955 ; Neugebauer and Russell, 1968 ; Umemoto, 1985 ; Gonzalez and San Roman, 1989 ; Kovács et al, 1996 ; Kanal et al, 2015 ; Bolze et al, 2018 ). Despite several studies with a focus on quantum-chemical calculations ( Buemi et al, 1998 ; Buemi and Zuccarello, 2002 ; King and Murrin, 2004 ; Tezcan and Tokay, 2010 ; Sherif, 2015 ), luminescence ( Turkoglu et al, 2015 ), solvatochromism ( Sherif, 1997 ; Kumar et al, 2014 ; Wortmann et al, 2022 ), Raman and infrared studies ( Schiele, 1965 ; Otting and Neugebauer, 1968 ; Otting and Neugebauer, 1969 ; Lewis and Sandorfy, 1983 ; Hiura and Takahashi, 1989a ; Hiura and Takahashi, 1989b ; Tezcan and Ozkan, 2003 ), or electrochemical properties ( Gökçe et al, 2005 ; Sherif, 2015 ; Turkoglu et al, 2015 ) of formazans, also investigated under temperature ( Kuhn and Weitz, 1953 ; Langbein, 1979 ; Grummt and Langbein, 1981 ; Sueishi and Nishimura, 1983 ) or pressure variation ( Sueishi and Nishimura, 1983 ), the interplay of light-induced processes which set in on an ultrafast time scale and extend to minutes has not been comprehensively studied for TPF. With regard to the involved intermediate species of TPF, several photoisomerization mechanisms of TPF are discussed in literature, with the one proposed by Grummt and Langbein which was derived from laser flash photolysis experiments ( Grummt and Langbein, 1981 ) being in closest accordance to the one inferred in our recent study ( Wortmann et al, 2022 ).…”

“…Despite several studies with a focus on quantum-chemical calculations ( Buemi et al, 1998 ; Buemi and Zuccarello, 2002 ; King and Murrin, 2004 ; Tezcan and Tokay, 2010 ; Sherif, 2015 ), luminescence ( Turkoglu et al, 2015 ), solvatochromism ( Sherif, 1997 ; Kumar et al, 2014 ; Wortmann et al, 2022 ), Raman and infrared studies ( Schiele, 1965 ; Otting and Neugebauer, 1968 ; Otting and Neugebauer, 1969 ; Lewis and Sandorfy, 1983 ; Hiura and Takahashi, 1989a ; Hiura and Takahashi, 1989b ; Tezcan and Ozkan, 2003 ), or electrochemical properties ( Gökçe et al, 2005 ; Sherif, 2015 ; Turkoglu et al, 2015 ) of formazans, also investigated under temperature ( Kuhn and Weitz, 1953 ; Langbein, 1979 ; Grummt and Langbein, 1981 ; Sueishi and Nishimura, 1983 ) or pressure variation ( Sueishi and Nishimura, 1983 ), the interplay of light-induced processes which set in on an ultrafast time scale and extend to minutes has not been comprehensively studied for TPF. With regard to the involved intermediate species of TPF, several photoisomerization mechanisms of TPF are discussed in literature, with the one proposed by Grummt and Langbein which was derived from laser flash photolysis experiments ( Grummt and Langbein, 1981 ) being in closest accordance to the one inferred in our recent study ( Wortmann et al, 2022 ). Note that also the orientation of the single bonds adjacent to the double bonds are sometimes drawn differently ( Lewis and Sandorfy, 1983 ; Veas-Arancibia, 1986 ) and lead to further stable isomers, as we will discuss in detail in Section 4 .…”

Excited‐State Proton Transfer Dynamics of a Super‐Photoacid in Acetone‐Water Mixtures

“…On the other hand, recording the emission in a variable environment also allows to obtain information on the latter. These versatile properties could be further manipulated by employing solvent mixtures with perturbed hydrogen-bond dynamics, [68][69][70][71][72][73][74] applying high pressures [26,[75][76][77] or spatially encapsulating the compound. The latter approach was demonstrated for other photoacids confined in small solvent droplets, [16] as e. g. also possible in (reverse) micelles, [24,[78][79][80][81][82][83][84][85][86] or small container molecules, [87][88][89] which further alters the ESPT characteristics.…”