Electron transfer in silicon-bridged adjacent chromophores: the source for blue-green emission

Abstract

Si-Bridged chromophores have been proposed as sources for blue-green emission in several technological applications. The origin of this dual emission is to be found in an internal charge transfer reaction. The current work is an attempt to describe the details of these processes in these kinds of substances, and to design a molecular architecture to improve their performance. Nuclear motions essential for intramolecular charge transfer (ICT) can involve processes from twisted internal moieties to dielectric relaxation of the solvent. To address these issues, we studied ICT between adjacent chromophores in a molecular compound containing N-isopropylcarbazole (CBL) and 1,4-divinylbenzene (DVB) linked by a dimethylsilylene bridge. In nonpolar solvents emission arises from the local excited state (LE) of carbazole whereas in solvents of higher polarity dual emission was detected (LE + ICT). The CT character of the additional emission band was concluded from the linear dependence of the fluorescence maxima on solvent polarity. Electron transfer from CBL to DVB resulted in a large excited-state dipole moment (37.3 D) as determined from a solvatochromic plot and DFT calculations. Steady-state and picosecond time-resolved fluorescence experiments in butyronitrile (293-173 K) showed that the ICT excited state arises from the LE state of carbazole. These results were analyzed and found to be in accordance with an adiabatic version of Marcus theory including solvent relaxation.

Early Events in the Photoinduced Electron Transfer between Carbazole and Divinylbenzene in a Silylene-Bridged Donor–Acceptor Compound

“…This is a follow-up paper of our previously reported work , on photoinduced intramolecular electron transfer that occurs through a dimethylsilyl bridge between N -isopropylcarbazole (CBL, donor) and divinylbenzene (DVB, acceptor) moieties. The general idea of using this type of compound is visualized in Figure and can be realized within silicon-based polymers as well as within bichromophoric systems . A deeper insight into the mechanism of the excited-state processes of a simple bichromophoric model allows for an understanding of the photophysics of the respective light-emitting polymeric materials.…”

“…In the previous work we reported that the ICT state can be formed from the local excited state of carbazole. The mechanism was proposed on the basis of fluorescence experiments performed with ≈10 ps time resolution.…”

“…Carbazole and carbazole-based compounds are the subject of numerous papers, known for strong absorption and efficient fluorescence but mostly for their high potential in electroluminescence , due to their great hole mobility and their utility in optoelectronics (e.g., as emitters and hosts in thermally activated delayed fluorescence (TADF) materials for organic light-emitting diodes (OLEDs)). − Divinylbenzene is commonly associated as a base of a famous poly­( p -phenylenevinylene) polymer ( p -PPV), the first polymer employed in light-emitting diodes , which inspired many papers on its derivatives − including those containing silicon. − The Δ G el of compound 1 was determined in our previous work to be −0.11 eV (for more, see the Supporting Information).…”

Enzyme-Regulated DNA-Based Logic Device

“…These observations suggest that the enhancement in the fluorescence intensity along with the blue shift in the λ max em of the Cz is a consequence of the changing microenvironments of Cz upon binding with quadruplexes and nucleases. The dual emissive nature of Cz may be due to the locally excited state and intramolecular charge transfer, which is promoted by the extended conjugation along with electron donating amino and acceptor carbonyl groups. − …”

White Light from Dual Intramolecular Charge-Transfer Emission in a Silylene-Bridged Styrylcarbazole and Pyrene Dyad

“…We observe that if pyrene was not excited, the overall kinetics of CT2 would reduce the concentration profile for CT2 to two components and thus two amplitudes, which are equal but opposite in sign, that is, a growth of CT2 and a decay of CT1, respectively. Such a kinetic model resembles that of an excimer or an exciplex , in which k fast is the formation rate of the equilibrium between CT1 and CT2 and k slow is the rate of CT1 and CT2 decaying together via emission once equilibrium is achieved.…”