Indolizine-derived unsymmetric dimethine cyanines with electronic symmetry, strong π-delocalization, and nearinfrared solid-state emission
Abstract
Cyanine dyes are one of the most important classes of polymethine dyes because of their unique and highly tunable optical properties that can be adjusted through structural modifications. They play a central role as high-value-added functional dyes across numerous cutting-edge technologies. Unsymmetric cyanines exhibit optical properties that differ markedly from their symmetric counterparts. Most unsymmetric dimethine cyanines presented in the literature are indole-based, displaying strong fluorescence and often used as biological markers. However, only a few examples of dimethine cyanine dyes deviating from this common indole-based model are reported. In this work, a new family of unsymmetric dimethine cyanines incorporating an indolizine donor unit and different cationic heterocycles are presented (1a-1d). A comprehensive understanding of the structure and electronic properties of these new cyanine derivatives was achieved through the combined use of synthesis, NMR and UV-Vis spectroscopies, X-ray diffraction, and (TD)DFT calculations. These dyes absorb in the 588-705 nm range and are weakly fluorescent in solution but emit up to 914 nm with decent quantum yields in the solid-state. Despite their formal lack of symmetry, compounds 1b-1d exhibit a cyanine-like behaviour, including small bond-length alternation, narrow and vibronically structured absorption bands, small Stokes shifts, and negligible solvatochromism. In contrast, the quinolinium derivative 1a displays a markedly different behavior, with a decreased cyanine-like character, partial ground-state symmetry breaking, and enhanced solvatochromic absorption effect. These results demonstrate that indolizine constitutes an effective building block for the design of unconventional unsymmetric cyanines that retain a remarkably symmetric electronic structure and cyanine-like photophysical properties despite their formally unsymmetrical constitution.
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BibTeX
@article{lucas2026indolizinederived,
title = {{Indolizine-derived unsymmetric dimethine cyanines with electronic symmetry, strong π-delocalization, and nearinfrared solid-state emission}},
author = {Marie Lucas and Erwann Jeanneau and Josene Toldo and Yann Bretonnière},
journal = {Dyes and Pigments},
year = {2026},
month = aug,
doi = {10.1016/j.dyepig.2026.114075},
url = {https://hal.science/hal-05761515v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Indolizine-derived unsymmetric dimethine cyanines with electronic symmetry, strong π-delocalization, and nearinfrared solid-state emission
AU - Marie Lucas
AU - Erwann Jeanneau
AU - Josene Toldo
AU - Yann Bretonnière
PY - 2026
DA - 2026/08/02
JO - Dyes and Pigments
DO - 10.1016/j.dyepig.2026.114075
UR - https://hal.science/hal-05761515v1
AB - Cyanine dyes are one of the most important classes of polymethine dyes because of their unique and highly tunable optical properties that can be adjusted through structural modifications. They play a central role as high-value-added functional dyes across numerous cutting-edge technologies. Unsymmetric cyanines exhibit optical properties that differ markedly from their symmetric counterparts. Most unsymmetric dimethine cyanines presented in the literature are indole-based, displaying strong fluorescence and often used as biological markers. However, only a few examples of dimethine cyanine dyes deviating from this common indole-based model are reported. In this work, a new family of unsymmetric dimethine cyanines incorporating an indolizine donor unit and different cationic heterocycles are presented (1a-1d). A comprehensive understanding of the structure and electronic properties of these new cyanine derivatives was achieved through the combined use of synthesis, NMR and UV-Vis spectroscopies, X-ray diffraction, and (TD)DFT calculations. These dyes absorb in the 588-705 nm range and are weakly fluorescent in solution but emit up to 914 nm with decent quantum yields in the solid-state. Despite their formal lack of symmetry, compounds 1b-1d exhibit a cyanine-like behaviour, including small bond-length alternation, narrow and vibronically structured absorption bands, small Stokes shifts, and negligible solvatochromism. In contrast, the quinolinium derivative 1a displays a markedly different behavior, with a decreased cyanine-like character, partial ground-state symmetry breaking, and enhanced solvatochromic absorption effect. These results demonstrate that indolizine constitutes an effective building block for the design of unconventional unsymmetric cyanines that retain a remarkably symmetric electronic structure and cyanine-like photophysical properties despite their formally unsymmetrical constitution.
SN - 0143-7208
ER -