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Ultrafast dynamics and computational studies on diaminodicyanoquinodimethanes (DADQs).

Szablewski, M. and Fox, M.A. and Dias, F.B. and Namih, H. and Snedden, E.W. and King, S.M. and Dai, D.C. and Pålsson, L.O. (2014) 'Ultrafast dynamics and computational studies on diaminodicyanoquinodimethanes (DADQs).', Journal of physical chemistry B., 118 (24). pp. 6815-6828.

Abstract

Three diaminodicyanoquinodimethanes, 4-(R1R2C)-1-[(NC)2C]-C6H4 (R1,R2 = H2N, 1; R1 = 3,5-Me2-4-OCH4H6N–, R2 = H2N, 2; R1 = 3,5-Me2-4-OCH4H6N–, R2 = 4-Me-C5H9N, 3), were investigated using carbon-13 NMR, steady-state, and ultrafast transient absorption and ultrafast fluorescence spectroscopies to unravel the unusual characteristics of this class of chromophores. Computed (GIAO)B3LYP/6-31G* data for the zwitterions 1–3 using necessary solvation (PCM) models were shown to be in excellent agreement with observed structural and carbon-13 NMR data. The ground-state geometries of 1–3 contain a cationic methine group R1R2C– twisted from the C6H4 ring and an anionic methine group (NC)2C– in plane with the C6H4 ring in solution and solid state. The 13C chemical shifts of the peak corresponding to the methine carbon at the (NC)2C– group of 1–3 are observed at 32.5–34.7 ppm, which are some 55 ppm upfield compared with the 13C chemical shift for the methine carbons in TCNQ, 1,4-[(NC)2C]2-C6H4. The decay of the excited state in diaminodicyanoquinodimethanes is fast and dominated by nonradiative processes on the picosecond time scale, which depends on the viscosity of the medium. The dynamics of the excited-state decay is therefore limited by conformational changes through an intramolecular twisting motion. This twisting motion is hindered by friction, which, in turn, also depends on the functional group size of the system. The dominant nonradiative pathways after excitation are due to twisted excited-state conformers according to TD-DFT computations.

Item Type:Article
Full text:(AM) Accepted Manuscript
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Status:Peer-reviewed
Publisher Web site:http://dx.doi.org/10.1021/jp411358d
Publisher statement:This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of Physical Chemistry B, copyright © 2014 American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://dx.doi.org/10.1021/jp411358d.
Date accepted:22 May 2014
Date deposited:18 November 2014
Date of first online publication:June 2014
Date first made open access:No date available

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