Ultrafast Relaxation Dynamics of the Excited States of 1‐Amino‐ and 1‐(N,N‐Dimethylamino)‐fluoren‐9‐ones

Abstract

The dynamics of the excited states of 1-aminofluoren-9-one (1AF) and 1-(N,N-dimethylamino)-fluoren-9-one (1DMAF) are investigated by using steady-state absorption and fluorescence as well as subpicosecond time-resolved absorption spectroscopic techniques. Following photoexcitation of 1AF, which exists in the intramolecular hydrogen-bonded form in aprotic solvents, the excited-state intramolecular proton-transfer reaction is the only relaxation process observed in the excited singlet (S(1)) state. However, in protic solvents, the intramolecular hydrogen bond is disrupted in the excited state and an intermolecular hydrogen bond is formed with the solvent leading to reorganization of the hydrogen-bond network structure of the solvent. The latter takes place in the timescale of the process of solvation dynamics. In the case of 1DMAF, the main relaxation pathway for the locally excited singlet, S(1)(LE), or S(1)(ICT) state is the configurational relaxation, via nearly barrierless twisting of the dimethylamino group to form the twisted intramolecular charge-transfer, S(1)(TICT), state. A crossing between the excited-state and ground-state potential energy curves is responsible for the fast, radiationless deactivation and nonemissive character of the S(1)(TICT) state in polar solvents, both aprotic and protic. However, in viscous but strong hydrogen-bond-donating solvents, such as ethylene glycol and glycerol, crossing between the potential energy surfaces for the ground electronic state and the hydrogen-bonded complex formed between the S(1)(TICT) state and the solvent is possibly avoided and the hydrogen-bonded complex is weakly emissive.

Ultrafast Dynamics of Hydrogen Bond Breaking and Making in the Excited State of Fluoren-9-one: Time-Resolved Visible Pump–IR Probe Spectroscopic Study

“…Solute–solvent interactions play important roles in nonequilibrium molecular processes in liquid-phase chemical reactions. , In particular, intermolecular hydrogen bonding, a site specific interaction, between the solute and solvent molecules significantly affects the chemical reactivity of the solute in many chemical and biological systems. − For example, the intermolecular photoinduced electron transfer dynamics between a donor and acceptor molecule may be strongly affected by hydrogen bond-mediated electronic coupling in the excited state. − Similarly, the photophysical properties and excited-state dynamics of molecules are also sensitive to the hydrogen bond-donating ability of the protic environment. , Thus, the dynamics of hydrogen bond breaking and making is of utmost importance for understanding these properties in a condensed phase. Both experimental and theoretical reserach on the dynamics of hydrogen bond reorganization are in progress, aiming at a molecular-level understanding of this ubiquitous process. − Dielectric relaxation or time-resolved fluorescence measurement techniques have extensively been used to probe the solvation dynamics of protic solvents, which indirectly provides the dynamics of the intermolecular hydrogen bond-reorganization process. − Molecular dynamics (MD) simulations have been successfully used to predict the hydrogen bond-making, -breaking, and -reorganization times. − However, measuring the vibrational frequencies in real time at the hydrogen-bonding site can probe the hydrogen bond-reorganization event in terms of hydrogen bond-breaking and -making processes.…”

Revisiting Fluorenone Photophysics via Dipolar Fluorenone Derivatives

“…The structure–mechanism relationship under which the IC takes place is still unclear . Fermi’s golden rule establishes that k IC is directly proportional to the excited state electronic coupling matrix element ( H AD ) .…”

“…The structureÀmechanism relationship under which the IC takes place is still unclear. 17 is directly proportional to the excited state electronic coupling matrix element (H AD ). 18 Previous observations imply that dipolar 2-FODs should have better electronic coupling than 3-FODs in the excited state, since their IC rates were always larger regardless of solvent polarity.…”

Excited State Dynamics of Brightly Fluorescent Second Generation Epicocconone Analogues

“…Polar protic solvents introduce additional solute charge reorganization and solvent reorientation mechanisms that can perturb the excited state. 23,24 The role of hydrogen bonding networks in ESIPT processes has also been established in N,N-dimethylanilino-1,3diketone and curcumin; both contain a β-keto−enol group similar to 1−6 and a propensity for charge transfer mediated ESIPT, that is on the 4−5 ps time scale. 25,26 From these studies, and comparing with our data, it appears that the shorter components (τ 1 ) in the 2−3 ps regime for 3 in acetonitrile and 20 mM SDS (Table 3) can be associated with its internal charge transfer, while in water this ubiquitous short component in 2 and 3 (0.6/2 ps) is most likely due to intermolecular proton transfer with solvent.…”