Photoinduced Intramolecular Charge Transfer in Donor-Acceptor Biaryls and Resulting Applicational As

<p>This study is focused on the effects of photoinduced intramolecular charge transfer (CT) in three differently twisted donor-acceptor (D-A) biphenyls. Taking into account a further pair of differently twisted D-A biaryls new universal insights into the photoinduced electronic and conformation dynamics of D-A biaryls are obtained. Furthermore possible applications in fields of solar energy conversion and fluorescence sensing of microenvironments are demonstrated.</p><p> </p><p>Experimental means of stationary and time-resolved (ps to s) luminescence transient absorption (sub-ps) polarization spectroscopy high pressure and low temperature techniques are employed in conjunction with quantum chemical calculations.</p><p>Twist angle and solvent dependent electron transfer (ET) interactions between the D and A aryl moieties are responsible for the low lying and solvatochromic intramolecular CT electron band which gains unusually high intensity through strong electronic coupling of the pure <sup>1</sup>ET with the ground (S<sub>0</sub>) and <sup>1</sup>L<sub>a</sub> state. As regards the class of biaryl compounds for the first time an excited state electron transfer from the D to the A could be monitored by dual spectrally separated stimulated fluorescence bands with precursor-successor relationship on a sub-ps timescale for the D-A biphenyls. It is concluded that in additon to the electronic interaction of <sup>1</sup>ET with S<sub>0</sub> and <sup>1</sup>L<sub>a</sub> the electronic interaction with a close lying <sup>1</sup>L<sub>b</sub> state plays a fundamental role in the ET dynamics and the <sup>1</sup>CT-S<sub>0</sub> transition probability in D-A biaryls.</p><p>The initial photoinduced conformational relaxation occurs towards planarity in all biaryls investigated. However various results evidence that the highly twisted D-A biphenyl additionally performs a slow "excited state intramolecular back twist rotation" leading to a solvent polarity dependent conformational equilibrium between a more planar (<strong>CT</strong>) and a more twisted (<strong>CTR</strong>) conformer in S<sub>1</sub>(<sup>1</sup>CT). Using global analysis of the biexponential fluorescence decays as a function of temperature and pressure in medium polar solvents the kinetics thermodynamics viscosity control and decomposed emission spectra associated with this adiabatic photoreaction are determined.</p><p>The twist angle dependent ability of the D-A biphenyls to serve as fluorescent probes of micropolarity changes of microviscosity or matrix order protic solvents and pH is investigated. In particular fluorescence sensing of pH seems to be promising.</p>