The solvatochromic response of benzo[a]fluorenone in aprotic solvents compared with benzo[b]fluorenone and 9‐fluorenone

Abstract

The solvatochromism and possible excited state deactivation pathways of benzo[a]fluorenone were investigated and compared with the related aromatic compounds 9‐fluorenone and benzo[b]fluorenone. These molecules can serve as molecular building blocks with application in light emitting diodes, catalysts, and dye‐sensitized solar cells. A study of the effect changes in aromatic structure might have on their relative photophysical responses is therefore of interest. Steady‐state absorption and emission spectra, Lippert‐Mataga plots, fluorescence lifetimes, and absolute quantum yields were used in tandem with time dependent density functional theory (TD‐DFT) calculations to investigate solvatochromism, excited state decay, and the relative energies of singlet and triplet excited states. It was found that changes in aromaticity affect the energetic order and character of the excited singlet and triplet states differently among these fluorenones as solvent polarity is varied. The observed fluorescence emission from all these molecules can be understood in terms of their relative ability to undergo intersystem crossing. Additional calculations on the related benzo[c]fluorenone predict that its solvatochromic behavior should resemble that of benzo[a]fluorenone.

Investigation of excited‐state deactivation processes in benzofluorenones using time‐resolved transient absorption spectroscopy

“…This lifetime matches the fluorescence lifetime of 3.1 ns measured by TCSPC, and the emission wavelength is consistent with the 569 nm observed emission maximum in steady-state measurements. [10] For BFC, the stimulated emission is centered at 550 nm and has a lifetime of 5.74 ± 0.04 ns (Figure S26) that matches well with the 3.933 ± 0.003 ns fluorescence lifetime measured by TCSPC.…”

“…This would decrease the facility of ISC according to the El-Sayed selection rules. [10] These theoretical predictions are consistent with gas-phase experimental work that determined that the lowest triplet state lies 0.565 eV below S 1 . [11] The interesting solvatochromic response of 9-FL described above has motivated further investigation of the effects of structural changes to the basic 9-FL chromophore.…”

“…[13a] Using the slope obtained from the Lippert-Mataga plot of 3315 cm À1 , and an Onsager radius of 3.99 Â 10 À8 cm, a difference in excited-state dipole moment between the ground state and excited state of 4.57 D for BFC is obtained. This lies between the values obtained for BFA (5.20 D) [10] and BFB (4.10 D) [23] but is larger than that F I G U R E 2 Absorption and emission spectra of BFC in cyclohexane (red, solid line), ethyl acetate (green, dash-dot line), and acetonitrile (blue, dotted line)…”

“…[7b] More recent theory supports the claims of earlier calculations in that ISC to the lower lying triplet state is predicted to be El-Sayed forbidden in 9-FL in polar solvents and an allowed transition in nonpolar solvents. [10] T A B L E 1 Fluorescence lifetimes and fluorescence quantum yields of 9-FL, BFA, BFB, and BFC in cyclohexane, ethyl acetate, and acetonitrile…”

“…However, the nsTA of 9-FL in acetonitrile shows stimulated emission at 527 nm which recovers to the ground state in 16.12 ± 0.06 ns [8a] which matches previously obtained TCSPC fluorescence lifetime data. [10] From theory, we expect the S 1 excited-state character of 9-FL to switch from nπ* to ππ* in solvents with polarity between that of cyclohexane and acetonitrile, and ISC should switch from an allowed transition to a forbidden transition. In the fsTA of 9-FL in ethyl acetate, we observe a transient absorption band with a maximum of 500 nm which may have stimulated emission superimposed in the traces (Figure S7).…”