Dark Photoswitching Induces Coulomb Blockade Diamond Collapse

Abstract

A derivative of the photochromic molecule dihydroazulene (DHA) undergoes bias-induced switching into its vinylheptafulvene (VHF) conformation when inserted into a silver junction. This dark switching mechanism, which induces a collapse of the Coulomb blockade diamonds, is explained by quantum calculations on the molecular transport junction. Analysis of the nonequilibrium populations of molecular redox states explains the observed bias threshold. Predictions are made that another DHA derivative will not, in future experiments, exhibit bias-induced switching. Thus, the dark switching mechanism depends on the structure of the photoswitch. The methodology applies to any molecular junction and offers a versatile tool for answering mechanistic questions.T he vision of a single-molecule device, the molecular rectifier, was proposed in 1974. 1 Today molecular electronics is a vibrant field studying functionalities of singlemolecule junctions including molecular switches, molecular transistors, and molecular memories. Also, molecular photoswitches comprise a growing field promising applications in harvesting of solar energy, biosensors, and fast and high density data storage. Recently, photoswitches have been placed in molecular transport junctions. 2−10 The complex interplay of the light-induced switching of, and the current passing through, the molecule poses challenging scientific questions and suggest new device functionalities. The most studied photoswitches in molecular junctions are derivatives of dithienylethene, 2−6 which undergo light-induced ring-opening, and azobenzene, 7,8 which experiences cis−trans isomerization upon photoexcitation.The switching behavior of photochromic molecules exposed to light is altered and often quenched in the vicinity of a metal surface. 2 Thus, to obtain reversible and robust photoswitching, the quenching pathway introduced by the strong electronic coupling to the metal surface must be avoided, for example, by inserting a linker group into the molecule. 5,9,10 Feringa and coworkers have presented reversible switching of a diarylethene with a phenyl group spacer in meta position 3,4 and further found that the switching also depends on the tilting of the molecule toward the metal surface. Similar results exist for other derivatives of diarylethene. In all cases, the closed form is found to be the more conducting formthe ON-state. Likewise, the ring-closed form of dimethyldihydropyrene photoswitch is found to be the more conducting form. 11 In addition, Scheer and coworkers showed that the ON−OFF ratio also depends on the side chains of the molecule. 6 Functionalized azobenzenes exhibit strong correlation between reversible switching and electronic coupling to the metal surface. 7 Theoretical investigations predict the trans-isomer to be more conducting than the cis-isomer for most bias voltages. 8 Transport experiments on the E-Z photoisomers of 4,4′-(ethene-1,2-diyl)dibenzoic acid demonstrate a significantly higher conductance for the Z isomer than for the E isomer. 12 Nielsen and...

A Study of Electrocyclic Reactions in a Molecular Junction: Mechanistic and Energetic Requirements for Switching in the Coulomb Blockade Regime

“…In a stability plot, the single‐molecule conductance is represented by a specific color code as a function of gate ( x ‐axis) and bias ( y ‐axis) potentials. In the Coulomb blockade regime, diamond‐shaped islands of no conductance are observed, corresponding to a specific redox state of the molecule . These Coulomb blockade diamonds may be influenced by a dark photoswitching event as the diamond of the switching state may collapse (i.e., resulting in a decrease in diamond size) or explode (i.e., resulting in an increase in diamond size).…”

“…However, recent experiments have shown that the switching mechanism of some photoswitches in a junction also can be promoted by a voltage stimulus, thus without a light stimulus . The process was named dark photoswitching due to the paradoxical behaviour of a photoswitch switching without light.…”

A Study of Electrocyclic Reactions in a Molecular Junction: Mechanistic and Energetic Requirements for Switching in the Coulomb Blockade Regime

“…In the Coulombb lockade regime, diamond-shaped islands of no conductance are observed, corresponding to as pecific redox state of the molecule. [11,17] These Coulombb lockade diamonds may be influenced by ad ark photoswitching event as the diamond of the switching state may collapse (i.e.,r esulting in ad ecrease in diamond size) or explode (i.e.,r esulting in an increasei nd iamond size). The physics behind ac ollapse and an explosion of ad iamond is dependent on the electron affinities (EA) and ionization potentials (IP) of two isomers,s ince the heighto fadiamond can be expressed as the sum of EA and IP times ac onstant( see the Computational section).…”

“…However,r ecent experiments have shown that the switching mechanism of some photoswitches in aj unction also can be promoted by av oltage stimulus, thus withoutal ight stimulus. [10,11] The process was named dark photoswitching due to the paradoxical behaviour of ap hotoswitch switching without light. Dark photoswitching was observed for ad ihydroazulene derivative( DHA) operating in the Coulomb blockader egime, [10] where the switching from DHA into its photo-isomer vinylheptafulvene (VHF) was observed by the change of the conductance properties.…”

Analytical gradients for MP2, double hybrid functionals, and TD‐DFT with polarizable embedding described by fluctuating charges

“…The recent extension of the QM/FQ model to analytical first and second derivatives for Hartree-Fock (HF) and Kohn-Sham (KS) methods [37] and to the corresponding response equations for various internal and external perturbations [38,39] makes this model particularly suitable for the evaluation of molecular properties and spectroscopic parameters. Use of polarizable QM/MM models (hereafter referred to as polarizable embedding [PE]) becomes even more significant for excited electronic states and has been considered by several groups [39][40][41][42][43] since the pioneering work by Luzhkow and Warshel exploiting the Langevin dipole solvent model. [44] However, to the best of our knowledge, those studies have been limited to excitation energies or excitation energy transfers, whereas excited state structures have never been addressed so far, not to speak about vibronic contributions.…”