Communications analysis (PCA)t component syste n.which sts of the three pureis 002 that th A and B conform of -DTB and c-DTB have y -DTB 002002 at 77 k >300 nm) s可obt ed on irradiat of c-DTEP and 4) 400 5060 hree p in eigenvector combination DTB C-DTB→DTB tion coefficient points of th (Figure 2).Begi TB corner of the tetrah C-PTB-DTB edge and 400454 s(300 nm,77K in the fluc pectra bration ing a range of Figure 2)move to the c DTB -DTB.relaxation and the pres ence of residual cr of cl the same tetra dron. tion in the in Ep -DIB ormation by pathway in P(Figure).The structuredTB www.angewandte.org These are not the final page numbersanalysis (PCA) treatment[10, 18] of a matrix of spectra obtained on irradiation of both cis DTB isomers reveals a four￾component system, which consists of the three pure isomer spectra (insensitivity to changes in the excitation wavelength lexc shows that tt-DTB exists as the tt-DTBA conformer and that the A and B conformers of ct-DTB and cc-DTB have very similar spectra) and a fourth spectrum, obtained by self￾modeling,[18] corresponding to the higher-energy tt-DTBB conformer (Figure 1). The pure spectra define a tetrahedron in eigenvector combination coefficient space. The cc￾DTB!ct-DTB!tt-DTB sequence is revealed by the time evolution of the combina￾tion coefficient points of the spectra starting from cc-DTB (Figure 2). Beginning at the cc￾DTB corner of the tetrahe￾dron, the spectra move on the cc-DTB/ct-DTB edge and eventually proceed to a mix￾ture containing the two con￾formers, tt-DTBA and tt-DTBB. The broad features of the final fluorescence spectra narrow significantly on warming and recooling. Points for spectra obtained after thermal equili￾bration using a range of lexc (^ in Figure 2) move to the ct￾DTB/tt-DTBA edge of the tetrahedron, consistent with tt￾DTBB!tt-DTBA relaxation and the presence of residual ct￾DTB. Points for spectra starting from ct-DTB, omitted for the sake of clarity, lie on the ct-DTB/tt-DTBA/tt-DTBB plane of the same tetrahedron. Results in IP are strikingly different. Comparison of the initial spectral evolution of cc-DTB photoisomerization in the two media allows visual confirmation of tt-DTB formation by a sequential two-step pathway in EPA and by a direct pathway in IP (Figure 3). The structured tt-DTB fluorescence is clearly evident after 20 s of 300-nm irradiation in IP whereas the early spectra in EPA resemble the fluorescence spectrum of ct-DTB (see also Figures 1 and 4). These results mirror the behavior of cc-DPB in the two media.[7, 10] Analogous PCA treatment of the combined matrix of spectra starting from both cc- and ct-DTB in IP reveals a three-component system that is accounted for exactly by the three pure isomer spectra (Figure 4). Beginning from cc-DTB, spectral points in the combination coefficient plot (Figure 5) deviate from the cc-DTB/ct-DTB edge of the triangle from Figure 1. Normalized fluorescence spectra of tt-DTBB (ttB), tt-DTBA (ttA), ct-DTB (ct), and cc-DTB (cc) in EPA at 77 K. Figure 2. Combination coefficients (a, b, g) for the spectral matrix (lexc=300 *, 315 ~, 345 *, before +, and after thawing ^, 330– 350 nm, pure components &) obtained on irradiation of cc-DTB in EPA at 77 K. (lrad>300 nm). Figure 3. Evolution of fluorescence spectra (normalized) during the first 90 min of irradiation under similar conditions (300 nm, 77 K in the fluorimeter) in EPA (a) and in IP (b). Figure 4. Fluorescence spectra of pure tt-, ct-, and cc-DTB isomers in IP at 77 K. Communications 2 www.angewandte.org  2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2008, 47, 1–5

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