Straightforward Synthesis and Properties of Highly Fluorescent [5]‐ and [7]‐Helical Dispiroindeno[2,1‐c]fluorenes

Abstract

This work presents a general approach for synthesis of substituted [5]‐helical dispiroindeno[2,1‐c]fluorenes based on Rh‐catalyzed intramolecular cyclotrimerization of triynes. This approach was further extended for the first synthesis of configurationally stable [7]‐helical dispiroindeno[2,1‐c]fluorenes. A series of variously substituted derivatives was prepared and their photophysical and electrochemical properties were evaluated. Their fluorescence emission maxima were in the region of 351–428 nm and quantum yields up to 88 % are the highest measured among the full‐carbon helical compounds.

Rhodium‐Catalyzed Enantioselective Synthesis of Highly Fluorescent and CPL‐Active Dispiroindeno[2,1‐c]fluorenes

“…Recently, we have introduced a fast, simple, and modular approach to various compounds possessing fluorene scaffold, [33–35] and subsequently, we have combined this structural motive with the element of helicity in the preparation of highly fluorescent [5]‐ and [7]‐helical indeno[2,1‐c]fluorenes [36] . The expected configurational stability and superior fluorescence quantum yields of the later ones drive us to attempt enantioselective synthesis [37–41] of these compounds and evaluate also their chiroptical properties (Scheme 1).…”

“…At the outset, we synthesized a set of various triynediols 1 according to our previously reported procedure (see Section I in the Supporting Information) [36] . Triynediol 1 a was chosen as a representative substrate to study catalytic enantioselective cyclotrimerization under different conditions by using combinations of Rh(cod) 2 BF 4 (5 mol%) and chiral phosphines (6 or 12 mol%) in CH 2 ClCH 2 Cl at 80 °C on 0.1 mmol scale (for details, see Tables S1–S3 in the Supporting Information).…”

A General Synthetic Approach and Photophysical Properties of Regioselectively Fluorinated [5]‐ and [6]‐Helical Bispiroindenofluorenes

“…This method allows synthesis of specifically and regioselectively substituted fluorenes [10][11][12] and [5]-and [7]-helical dihydroindenofluorenes (DSF-IFs). [13] Since the precursors for the cyclotrimerization step were formed by a rather simple transformation of halobenzaldehydes, we envisioned that the use of fluorinated halobenzaldehydes could serve our purpose and be used to synthesize the desired target compounds. For a retrosynthetic analysis see Scheme 1.…”

“…As a result, compounds 1 a-g (mono-to tetrafluorinated DSF-IFs, Figure 3) presented very similar absorption spectra and molar attenuation coefficients with the characteristic strong absorption band at 308-312 nm and the moderate intensity band at 319-329 nm typical for all of known [5]-helical DSF-IFs. [13,21,22] Although some differences can be noted in the absorption spectra, such as an additional band of lowest energy absorption (343 nm) for the tetrafluorinated DSF-IF (1 g) and a slightly red-shifted lowest energy absorption of 1 b and 1 e (both with at least one fluorine in position 2), we can conclude that fluorination of the one terminal benzene ring of the SBIFs helical core (neither the number nor position of fluorine atoms) does not have a significant effect on the absorption properties. The absorption spectrum of octafluorinated 1 h (both terminal rings of the helical core are tetrafluorinated) evinces much lower molar absorption, and additionally, abated fine vibrational structure (noticeable also for 1 g) suggesting increasing polar character of the ground state with increasing number of the fluorine atoms attached to the terminal rings of the dihydroindenofluorene core.…”

“…With regard to the fluorescence properties, all compounds presented unresolved emission spectra ( Figure 5) with maxima at 386-395 nm for 1 a-1 g, 413 nm for 1 h and 412 nm for 2 (Table 1 and Figure 6). Even if comparison with non-fluorinated DSF-IFs is intricate, [13,23] emission maxima of mono-to tetrafluorinated 1 a-1 g are rather slightly red-shifted up to~15 nm reached by the tetrafluorinated 1 g. The highest bathochromic shift (~33 nm) assignable to the presence of fluorine atoms was observed for octafluorinated 1 h. This compound also possesses higher absolute quantum yield (0.89 vs~0.50) and unexpectedly high Stokes shift of 7135 cm À 1 which is more than 2000 cm À 1 higher than any of the 1 a-1 g. In a similar manner, the red shifted emission maximum (412 nm) was recorded for [6]-helical tetrafluorinated DSF-BIF 2 as well as high absolute quantum yield (0.92). These results suggest that the extension of π-system from [5]-helical DSF-IF to [6]-helical DSF-BIF or the intramolecular charge transfer promoted by polyfluorination of terminal benzene rings in DSF-IFs are responsible for the consequent bathochromic shifts of emission showed by 1 h and 2.…”