Digital Analysis of Electronic Absorption Spectre

Metzler, David E.; Harris, Carol M.; Reeves, Richard L.; Lawton, W. H.; Maggio, Mary S.

doi:10.1021/ac50019a717

Cited by 5 publications

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“…Since it is well established that NPE obeys the NEER principle38 (Scheme 1), the fluorescence decay functions used in KFA are given by 7 (A Am) = aa expHV) + ab exp(-V) (12) where ( )( «,0, «") is the fluorescence intensity at a specific «" from the conformer mixture (the bar over the symbol is used to identify quantities referring to the response of the conformer mixture) prepared by excitation at X«xc as a function of time following a excitation pulse. The biexponential decay parameters give conformer lifetimes t* = kA{ and tb = kB~\ and the preexponential factors give ratios of intensities of the fast and slow components.7•27 _ A_a _/a(\xe) 7**(Xem)^M "'V/AxAAnA (13) In eq 13 the //(X»c) represent fractions of conformers i = A or B molecules excited at each specific Xexc /AJ = f4H7=1"/fl(x««) (14) > where «(XeXC) are effective molar absorptivity coefficients of the conformer mixture, and the FfKm) represent fluorescence intensities of each conformer at Xem.…”

Section: Discussionmentioning

confidence: 99%

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Fluorescence, fluorescence-excitation, and ultraviolet absorption spectra of trans-1-(2-naphthyl)-2-phenylethene conformers

Saltiel¹,

Sears²,

Choi³

et al. 1994

J. Phys. Chem.

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Fluorescence spectra of trans-l-(2-naphthyl)-2-phenylethene (NPE) obtained under varying conditions of excitation wavelength and oxygen concentration in methylcyclohexane are resolved into two distinct components by application of principal component analysis with self-modeling. The key to obtaining unique spectral solutions is the constraint that Stern-Volmer quenching plots for the individual conformers be independent of excitation wavelength. Resolved conformer fluorescenwxcitation spectra are obtained by application of principal component analysis on a matrix of fluorescence-excitation spectra. Consistency between pure component fluorescence and fluorescen-xcitation spectra is established by use of a two-dimensional fluorescence excitationemission matrix. Fluorescence quantum yields and the pure component fluorescencbexcitation spectra are employed to resolve the UV absorption spectrum of N P E into pure component conformer absorption spectra.The results are compared with those from earlier studies based on the analysis of fluorescence decay data.Flexible molecules containing conjugated double bonds undergo facile rotation about essential single bonds and exist, in the ground state, as an equilibrium mixture of different conformations. The s-cis and s-trans conformers or rotamers of 1,3-butadiene provide the prototypical example.* Electronic excitation, in many such systems, leads to a reversal of single/double bond order, and as a consequence freely equilibrating ground-state conformers give noninterconverting excited molecules with different structures and properties. Since ground-state conformers have different absorption spectra, different excitation wavelengths, Lxc, usually lead to different populations of excited conformers and consequently to different photochemical and photophysical responses. Control of photochemical response through selective excitation of ground-state conformers was first postulated for trienes related to vitamin D by Havinga, who generalized this concept in his principle of nonequilibrating excited rotamers (NEER).3In achieving a quantitative elucidation of the photochemical behavior of such flexible molecules, the photochemist is faced with the challenge of determining the absorption and fluorescence spectra of each significant conformer. In our laboratory we have shown that matrices of spectra reflecting different mixtures of a set of conformers can be decomposed into pure component spectra by use of principal component analysis with self-modeling (EA-SM), a method first p r o p a d by Lawton and Sylvestre.4The first application of PCA-SM to a conformational problem involved a matrixoffluorescencespectraof tranr-l-(2-naphthyl)-2-phenylethene (NPE) obtained for different Lxc and oxygen concentrati~ns.~ The pure component spectra obtained were later shown to be at variance with spectra obtained by Bartocci, Mazzucato, and-workers6 through the treatment of fluorescence decay curves introduced by Birks et al., the KFA method.'In this paper we report a refinement of the PCA-SM appr...

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