Dynamics of the excited states of chromenes studied by fast and ultrafast spectroscopies

Abstract

In this paper, the photodynamics of three chromenes (2,2-spiro-adamantyl-7,8-benzo(2H)chromene, 2,2-diphenyl(2H)chromene and 2,2-diphenyl-5,6-benzo(2H)chromene) has been investigated by nano- and femtosecond time resolved techniques in hydrocarbon solutions at room temperature. Using pump-probe techniques, ultrafast steps of molecular dynamics characterizing the photoresponse of chromenes have been investigated: the breakage of the C-O bond was found to occur within a few hundred femtoseconds producing a short-lived transient that converts to the cisoid-cis open form in less than one picosecond and then to the metastable transoid-cis structure within a few picoseconds. The effect of different excitation wavelengths on the formation rate of the first shortest-lived transient of 5,6DPBC is in agreement with the model previously proposed to explain the wavelength dependence of the reaction and emission quantum yields in photostationary experiments. The results obtained in the nanosecond time domain show that the triplet marginally participates in the photoreaction for only one of the studied compounds (2,2-spiro-adamantyl-7,8-benzo(2H)chromene).

The photochemical ring opening reaction of chromene as seen by transient absorption and fluorescence spectroscopy

“…12,13 A large number of experimental and theoretical studies are currently available 13,14,16,[18][19][20][21] concentrating on the molecule DPBC (Scheme 1) or related compounds. These compounds were investigated in different solvents ( polar vs. non-polar) 13 and for different excitation conditions, (S 1 vs. S N excitation) 19 using continuous-wave spectroscopy 12,13,15,22,23 or timeresolved techniques, e.g., transient absorption, TA.…”

“…1a). 14,[18][19][20] In this model, it was hypothesized that photoexcitation to the Franck-Condon range leads to ultrafast bond-cleavage (Scheme 1, O 1 -C 2 ), i.e., the formation of an open-state precursor in the excited state (Fig. 1a, Model with fast excited state decay) on a time-scale faster than the actual time-resolution (∼150 fs) of the experiment.…”

“…It should be mentioned that up to now there has been no experimental evidence to support the hypothesis for ultrafast formation of a zwitter-ionic or biradicaloid open-state. 14,[18][19][20] The first directly observable process in the TA-data is a rapid decay (<0.5 ps, τ 1 ) of a broad and unstructured induced absorption, an observation that was interpreted as decay of the excited state and population of a "cisoid" ground-state conformation S 0 (OF cis ) of DPBC (Fig. 1a).…”

“…1a). 14,[18][19][20] Subsequently, a prominent build-up of absorption in the spectral region of OF (tc) and OF (tt) with time constant τ 2 of the order of picoseconds was assigned to singlebond rotation around C 3 -C 4 (Scheme 1) and hence formation of product isomers. Finally, smaller changes of the induced absorption on timescales >10 ps (τ 3 ) were assigned to thermal equilibration between product isomers OF (tc) and OF (tt) .…”

“…These compounds were investigated in different solvents ( polar vs. non-polar) 13 and for different excitation conditions, (S 1 vs. S N excitation) 19 using continuous-wave spectroscopy 12,13,15,22,23 or timeresolved techniques, e.g., transient absorption, TA. 14,[18][19][20]24,25 Based on the data from transient absorption measurements and the spectral dynamics observed after photoexcitation, at first a sequential model with fast excited state decay was suggested for the ring-opening reaction (Fig. 1a).…”