Excited-State Dynamics of a Substituted Fluorene Derivative. The Central Role of Hydrogen Bonding Interactions with the Solvent

Abstract

Substituted fluorene structures have demonstrated unusual photochemical properties. Previous reports on the substituted fluorene Schiff base FR0-SB demonstrated super photobase behavior with a ΔpK b of ∼14 upon photoexcitation. In an effort to understand the basis for this unusual behavior, we have examined the electronic structure and relaxation dynamics of the structural precursor of FR0-SB, the aldehyde FR0, in protic and aprotic solvents using time-resolved fluorescence spectroscopy and quantum chemical calculations. The calculations show three excited singlet states in relatively close energetic proximity. The spectroscopic data are consistent with relaxation dynamics from these electronic states that depend on the presence and concentration of solvent hydroxyl functionality. These results underscore the central role of solvent hydrogen bonding to the FR0 aldehyde oxygen in mediating the relaxation dynamics within this molecule.

Intramolecular Relaxation Dynamics Mediated by Solvent–Solute Interactions of Substituted Fluorene Derivatives. Solute Structural Dependence

“…18−20 In an effort to understand more fully the photoinduced change in reactivity of FR0-SB from a structural perspective, we have studied the excited-state relaxation dynamics of the aldehyde precursor, FR0 (Figure 1b). 24 Using steady-state and time-resolved emission spectroscopy in conjunction with ab initio quantum chemistry calculations, multiple unresolved bands were found to contribute to the broad and relatively featureless steadystate emission spectrum of FR0 in both protic and aprotic solvents. The individual bands were associated with three excited singlet electronic states in relatively close energetic proximity.…”

“…Steady-State Emission and Time-Resolved Fluorescence Spectroscopy. Collection of steady-state and timeresolved emission spectra was performed using the same instrumentation and parameters for analysis of FR0, 24 and the time-correlated single-photon counting (TCSPC) instrument has been described in detail elsewhere. 26 The time resolution of this instrument, determined by its instrument response function, is ca.…”

“…The Mulliken charges for the aldehyde functionality, specifically the carbonyl group, in the S 2 state of NCy-FR0 (Figure 2) are almost identical to those of FR0 in the S 2 excited state. 24 The carbonyl oxygen exhibits a more positive charge in the S 2 state, with an effective charge of ca. −0.1, than in the S 1 and S 3 states, where the carbonyl oxygen carries ca.…”

“…Quantum chemical computations can be of assistance in resolving the electronic structure of organic chromophores. 24,50,51 When multiple electronic states are shown to exist in close energetic proximity, intramolecular relaxation is expected to contribute to the observed spectral dynamics, although our quantum chemical computations cannot accurately predict the extent to which the intramolecular processes operate because such computations cannot account for intermolecular (solvent−solute) interactions at a level sufficient to model environmental mediation of the intramolecular relaxation processes.…”

“…Consistent with the analysis of FR0, the highest-energy emission band was assigned to the S 3 → S 0 emission, the intermediate band was assigned to S 2 → S 0 , and the lowestenergy band was assigned to the S 1 → S 0 emission. Taking the resulting time-dependent band intensities, we applied a kinetic model 24 to determine the fitted rate constants associated with the relaxations in each state (i.e., the relaxation of each fitted emission band), which are presented in Table 2 and Figure 5. We present this model in the Supporting Information (Figure S3 and eqs S.1−S.6 in the Supporting Information).…”