Time-Resolved Absorption Studies on the Photochromic Process of 2H-Benzopyrans in the Picosecond to Submillisecond Time Domain

Abstract

Picosecond to submillisecond photochromic reactions of 2,4-diphenyl-2H-benzopyran and 2,2,4-triphenyl-2H-benzopyran have been investigated by time-resolved absorption spectroscopy. The C−O bond cleavage of the benzopyrans (closed forms) occurs via the first excited singlet state within 2 ps to produce vibrationally excited open forms in the ground electronic state. In the subnanosecond to submillisecond time domain, several decay components with almost the same spectral profiles are observed. These components are assigned to respective stereoisomers with respect to two double bonds and one single bond of the open enone forms. From the pump-laser power dependencies of the yields of the open forms, it is suggested that the photocleavage gives at first only the open forms revertible to the closed form by a single-bond rotation, and that the photoexcitation of the first generated open forms gives rise to other open forms which need a double-bond rotation for reversion to the closed form. The photochromic reactions of a series of 2H-benzopyrans bearing substituents on the pyran ring have also been studied using nanosecond time-resolved absorption spectroscopy. The size of a substituent in the 4-position fairly affects the rate constants of the thermal reversion of the open form to the closed form.

Photochemistry of Photochromic Benzopyrans Studied by Time-Resolved Absorption Spectroscopy

“…The yields of the major components with lifetimes of 48 s for Photochemistry of photochromic benzopyrans 149 2,4-Ph 2 BP and 12 s for 2,2,4-Ph 3 BP are linearly dependent on the nanosecond pump-laser power, while those of the minor components with longer lifetimes (>1 ms) are found to increase in the approximately second order of the nanosecond laser power 36 . This result indicates that the short-lived major components are produced through a one-photon process whereas the minor long-lived components are produced through a two-photon process within the duration of a pump-laser pulse (~ 20 ns).…”

“…We have therefore performed time-resolved absorption studies on the photoexcitation dynamics of 2,4-diphenyl-(2,4-Ph 2 BP) and 2,2,4-triphenyl-2H-benzopyran (2,2,4-Ph 3 BP) from picoseconds to submilliseconds 36 . However, very little is known about the time developments of 2H-benzopyrans immediately after the photoexcitation, although knowledge on the dynamics in all the time range involved is essential for a detailed understanding of these photochemical reactions.…”

“…Since both the constituent -electron systems do not interact significantly in the spiro form, the absorption spectrum of the spiropyran molecule is approximately reproduced by the superposition of the spectra of the two individual moieties 16,17 . It has been known that only 2H-benzopyrans (chromenes) and their analogues, which constitute the spiropyran molecules as the pyran moiety, also show photochromic behavior on UV irradiation ( Figure 2) 13,[18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] . Extensive conjugation of the -electron system in the open form leads to the appearance of the strong absorption in the visible region where the closed form does not absorb.…”

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

“…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).…”

Towards the understanding of the photochemical ring opening and closing mechanisms for spiropyrans using 2H-pyran and the maximum overlap method

“…The reversibility of the photochemical ring-opening was first reported by Bercovici et al [1] and since then there have been several experimental studies [2][3][4][5][6][7][8][9][10][11][12][13][14], including many on substituted spiropyrans and benzopyrans, which were considered as simplified models of spiropyrans. Recently, Buback et al [11] reported that the ring opening and closing processes of 6,8-dinitro-2 -3 -3 -trimethylspiro[2H-1-benzopyran-2-2 -indoline] (6,8-dinitro BIPS), as shown in Scheme 1, are reversible on a picosecond time scale, and this may lead to applications of spiropyrans as molecular switches.…”