Influence of Chromophore Length and Acceptor Groups on the Optical Properties of Rigidified Merocyanine Dyes

Abstract

Scheme 1. Framework of polyenes (above) with even number of p-centres and polymethines (below) with odd number of p-centres. A = acceptor, D = donor.Scheme 3. Synthesised merocyanine derivatives. For better comparison of experimental and theoretical data a common atom numbering system was used, which is different from IUPAC numbering. Hex = hexyl.

Rigidified Push–Pull Dyes: Using Chromophore Size, Donor, and Acceptor Units to Tune the Ground State between Neutral and the Cyanine Limit

“…As outlined in Scheme vinylogous acids keto‐n‐OH were chosen as the starting materials for the synthesis of mal‐n‐OC12 and barb‐n‐OC12 with a dodecyloxy donor. Trimethine keto‐1‐OH is commercially available, whereas the pentamethine keto‐2‐OH and heptamethine keto‐3‐OH can be accessed by literature‐known two‐ and five‐step syntheses . The acid‐catalyzed etherification of the vinylogous acids keto‐n‐OH with dodecanol provided the products keto‐2‐OC12 and keto‐3‐OC12 in 91 % and 74 % yield, respectively.…”

“…The synthetic route was also used for the preparation of compounds mal‐2‐NR 2 and barb‐2‐NR 2 with a piperidyl donor (Scheme ). The vinylogous acid of pentamethine keto‐2‐OH was reacted with piperidinium acetate under microwave conditions to give the piperidyl‐substituted ketone keto‐2‐NR 2 in 81 % yield . Ketone keto‐2‐NR 2 was converted to the nitrile NC‐2‐NR 2 in 63 % yield by a HWE reaction with diethyl cyanomethylphosphonate.…”

“…Recently, we reported on rigid merocyanines with a strong piperidyl donor and several acceptor moieties of differing strengths: ketone 2 < cyanoacetate 3 < malononitrile 4 (Scheme ) as well as their homologues with fewer rings . Derivatives 3 and 4 with stronger acceptors showed cyanine‐like absorption spectra with a dominant 0–0 vibronic transition.…”

“…As the synthesis of a systematic library of dyes often requires different routes, the aim of the current study was to develop a widely applicable and efficient synthetic access to a series of partially rigidified dyes carrying different acceptor groups. The acceptor should be attached by a flexible exocyclic vinylidene unit, whereas the donor is directly connected to the cyclic system (Scheme ).…”

“… Series of rigidified dyes with barbiturate and malononitrile acceptors, different chromophore sizes n =1–3 as well as dodecyloxy and piperidyl donors and the known merocyanine 4 for comparison.…”