Synthesis of Bis(arylethynyl)pyrrolo[3,2‐b]pyrroles and Effect of Intramolecular Charge Transfer on Their Photophysical Behavior

Abstract

3,6-Bis(arylethynyl)pyrrolo [3,2-b]pyrroles were synthesizedt hroughatwo-step procedure involving double direct alkynylation of the electron-rich core followed by Sonogashira coupling. In comparison with the parentt etraarylo-pyrrolo[3,2-b]pyrroles and benzo-fusedp yrrolopyrroles, these new dyes showed moderately redshifted absorption. Almosta ll derivatives showedp ositivef luorescence solvatochromism and, for the first time, red-emitting pyrrolopyrroles were obtained.C omputationals tudies revealedt hat, in most cases, there is negligible change in the geometry between grounda nd excited states. Interestingly,t here was af undamentald ifference between pyrrolopyrroles possessingelectron-withdrawings ubstituents at positions 2a nd 5a nd their analogslacking these substituents. The formerd yes behaved like dipolarc hromophores with the lowest excited state both one-photon and two-photon allowed, whichc orresponds to intramolecular charget ransfer occurringa long the branches perpendicular to the pyrrolopyrrole long axis. In compoundsl acking electron-withdrawing substituents at positions 2a nd 5, intramolecular charge transfer took place along the long axis of pyrrolopyrrole and consequently the one-photon transitions are not two-photon allowed.D espite displaying quadrupolar core-to-peripheral intramolecular charget ransfer,t hese derivatives showed two-photon absorptioncrosssections in the NIR1 region comparable to tetraaryl-pyrrolo[3,2-b]pyrroles lacking p-expansion (up to about5 00 GM).[a] Dr.Scheme1.Synthesis of dyes 7-12.Scheme2.Synthetic route towards bis(arylethynyl)pyrrolo[3,2-b]pyrroles.

Change of Quadrupole Moment upon Excitation and Symmetry Breaking in Multibranched Donor‐Acceptor Dyes

“…Comparison of the one-photon absorption (OPA) and TPA spectra in conjunction with quantum-chemical calculations reported in Ref. [44] points to mutually exclusive selection rules, namely large OPA and weak TPA S 1 ! S 0 cross-sections and weak OPA and large TPA S 2 !…”

“…The synthesis of dyes 1-4 was described in reference [44]. Tetrachloroethylene (TCE, purum, > 99.0 %) and p-xylene (PXY, puriss.…”

“…The basic photophysics, solvatochromism and two-photon absorption (TPA) properties of these dyes were already discussed in Ref. [44].…”

“…Dyes 1-3 are A-π-D-π-A systems consisting of a pyrrolo [3,2-b]pyrrole D core substituted in 3 and 6 positions with two cyanophenylethynyl π-A branches (Figure 1). [43,44] The magnitude of Q and ΔQ is tuned by appending groups with different electron-accepting strength, A', at 2 and 5 positions of the D core. This accepting strength increases by varying A' from tolyl (1), to cyanophenyl (2), and to nitrophenyl (3).…”

Tris(pentafluorophenyl)borane‐pyrrolo[3,2‐b]pyrrole Hybrids: Solid‐State Structure and Crystallization‐Induced Enhanced Emission

“…Their use as scaffolds for the synthesis of heterocyclic analogues of large curved polycyclic aromatic hydrocarbons has also been investigated . Although it still remains a challenge to bathochromically shift the fluorescence in this group of dyes while maintaining strong emission intensity, we have recently reported the photoluminescent color‐tuning of pyrrolo[3,2‐ b ]pyrrole derivatives from cyan to yellow emission by co‐crystallization methods. This was achieved through co‐crystallization of a pyrrolo[3,2‐ b ]pyrrole possessing two pyridine moieties with various Brønsted acids, such as phenols, carboxylic acids, and sulfonic acid, and we found that the degree of protonation affects the photoluminescent properties .…”