Vibrational State Dependence of Interfacial Electron Transfer: Hot Electron Injection from the S₁ State of Azulene into TiO₂ Nanoparticles

Abstract

Synthesis of 6-methyl-azulene-2-carboxylic acid.Anhydrous cyclohexane (Acros Organics) was freshly distilled off sodium/benzophenone prior to use. Toluene was purchased anhydrous (Acros Organics) and used as received. Acetonitrile, methanol and ethanol were purchased HPLC grade (Acros Organics) and used without further purification. The 6-methyl-azulene (tech.) was purchased from Maybridge Ltd. Bis(pinacolato)diboron (B 2 Pin 2 , 98%), chloro(1,5-cyclooctadiene)iridium(I) dimer, 2,2'bipyridine (2,2'-bpy, 99%), potassium hydrogen fluoride (KHF 2 , 99%), ethyl chloroformate (99%), tetrakis(triphenylphosphine)palladium(0) (Pd(PPh 3 ) 4 , 99%), copper(I) oxide (Cu 2 O, 97%), potassium phosphate tribasic monohydrate (K 3 PO 4 *H 2 O, 96%), and sodium hydroxide were all purchased from Acros Organics. Anhydrous chlorotrimethylsilane (TMS-Cl) was purchased from Fluka and used as received. Purification by column chromatography was performed using silica gel (Acros Organics, 0.200 -0.500 mm, 60 Å). Aluminum backed silica gel plates (Sorbent Technologies) were used for thin layer chromatography (TLC). All synthetic manipulations were performed under nitrogen using standard Schlenk techniques. 6-methyl-azulene-2-pinacoloboronate (2) (6-MeAz-2-BPin). 6-methyl-azulene (1) (2.0 mmol, 284 mg), bis(pinacolato)diboron (2.2 mmol, 610 mg), 2,2'-bypyridine (0.14 mmol, 22 mg), and chloro(1,5-cyclooctadiene)iridium(I) dimer (0.05 mmol, 35 mg) were placed in a 25 mL Schlenck flask in the glove box. After that, 6.0 mL of dry cyclohexane were added with a syringe under nitrogen. The reaction mixture was then heated to a vigorous reflux under nitrogen for 26 hours. The reaction completion was confirmed by GC-MS and TLC. It is important to note that the reaction should be stopped before all 6-methyl-azulene has reacted (~90% conversion according to GC-MS) to ensure the functionalization occurred only at the 2-position. Unlike in the case of unsubstituted azulene, the excess of B 2 Pin 2 (1.1 eq.) and extended reaction time were

Phonon-Assisted Hot Carrier Generation in Plasmonic Semiconductor Systems

“…Carrier−carrier and carrier−phonon interactions regulate the relaxation of high-energy "hot" carriers, 18,19 while the energy level alignment and lifetime of the excited state controls charge injection from sensitizers to the semiconductor. 20 Temperature modulates the semiconductor energy levels due to excitation of phonons, which affects electron−phonon coupling and thermal conductance at a metal−semiconductor interface. Recent, ab initio calculations on TiSi 2 (metal)−Si (semiconductor) interface revealed that the strength of the coupling between electrons and interfacial joint phonon modes is similar to the strength of electron−phonon coupling in bulk metals and that the thermal conductance can be comparable to the phonon−phonon conductance across the interface.…”

“…Efficient systems are commonly characterized by an ultrafast injection (<1 ps) followed by a slow exponential decay(s) due to charge recombination. Carrier–carrier and carrier–phonon interactions regulate the relaxation of high-energy “hot” carriers, , while the energy level alignment and lifetime of the excited state controls charge injection from sensitizers to the semiconductor …”

Ultrafast Electron Transfer from Upper Excited State of Encapsulated Azulenes to Acceptors across an Organic Molecular Wall

“…The observation that Az S 2 can participate in eT led us to bind Az 2 @OA 2 and GAz@OA 2 capsuleplexes to TiO 2 colloidal aqueous suspensions and films cast on glass to study TiO 2 as the electron acceptor. Indeed, it has been demonstrated that the S 2 excited state of azulene derivatives lies above the conduction band of this metal oxide, as photoinduced electron transfer from the S 2 state of azulene to the conduction band of TiO 2 takes place efficiently, following selective excitation of the azulene chromophore. , As carboxylic acids are known to bind covalently to metal oxide semiconductors, some of the COOH groups present on OA’s exterior can be expected to bind to the TiO 2 nanoparticle surface, whether these are diluted colloidal solutions or mesoporous films. − …”

“…Indeed, it has been demonstrated that the S 2 excited state of azulene derivatives lies above the conduction band of this metal oxide, as photoinduced electron transfer from the S 2 state of azulene to the conduction band of TiO 2 takes place efficiently, following selective excitation of the azulene chromophore. 20,64 As carboxylic acids are known to bind covalently to metal oxide semiconductors, some of the COOH groups present on OA's exterior can be expected to bind to the TiO 2 nanoparticle surface, whether these are diluted colloidal solutions or mesoporous films. 65−67 An important prerequisite is that the Az 2 @OA 2 and GAz@ OA 2 capsuleplexes are stable in the experimental conditions used for binding.…”

Heterogeneous Electron-Transfer Dynamics through Dipole-Bridge Groups

“…37–39 Therefore, we conclude that the S 1 excited state is located below the conduction band edge at the TiO 2 interface and the slow decay is likely governed by injection into defect trap states. 40 Several TA measurements on similar systems found fast HET by evaluating the rise time of the probe signal around 600 nm. 38, 39 This assumption appears to be unreasonable in our case because the rise time resembles our instrument response function of 28 fs (Figure 7b).…”