Synthesis of Carbazole‐Type D‐π‐A Fluorescent Dyes Possessing Solid‐State Red Fluorescence Properties

Abstract

Carbazole‐type donor‐π‐acceptor (D‐π‐A) fluorescent dyes (SO1 and SO2), each containing a diphenylamino system as an electron‐donating group and a nitro moiety as an electron‐accepting group, have been designed and synthesized, and their photophysical properties in solution and in the solid state have been investigated. The absorption and fluorescence properties of SO1 and SO2 in solution are similar, and both dyes exhibited moderate fluorescence quantum yields. In the solid state, however, the dye SO2, with a butyl substituent on the carbazole ring, exhibited red fluorescence at around 620 nm, whereas the dye SO1, with no substituent on the carbazole ring, did not exhibit these solid‐state fluorescence properties. Furthermore, the dye SO2 exhibited weak mechanofluorochromic (MFC) properties: grinding of as‐recrystallized dyes induces slight bathochromic shifts of the fluorescence excitation and emission maxima. To elucidate the effects of molecular and crystal structures on the solid‐state fluorescence properties, we performed semiempirical molecular orbital calculations (PM3 and INDO/S) and single‐crystal X‐ray structural analysis. The X‐ray crystal structures of SO1 and SO2 demonstrated that a continuous intermolecular π‐stacking between the fluorophores was observable in the crystal structure of SO1, but not in that of SO2. The MO calculations revealed that SO1 and SO2 have similarly large dipole moments in the ground state (μg ≈ 8 D). The relationship between the observed solid‐state photophysical properties and the molecular and crystal structures of the carbazole‐type D‐π‐A fluorescent dyes are discussed on the basis of experimental results and MO calculations. It is found that the formation of a continuous intermolecular π‐stacking between the fluorophores causes a drastic fluorescence quenching in the solid state and that D‐π‐A fluorescent dyes with very large dipole moments can reduce the MFC properties, due to strong dipole–dipole interactions between the fluorophores in the solid state.

Mechanofluorochromism of (D–π–)2A-type azine-based fluorescent dyes

“…e l abs, ex, or -solid max (heating) À l abs, ex, or -solid max (grinding). with the case of D-p-A-type uorescent dyes, [11][12][13][14][15] the MFC of the (D-p-) 2 A-type azine-based uorescent dyes was not just a matter of events originating from a reversible change between crystalline and amorphous states by grinding and heating, because OUY-2 as well as OTK-2 exhibited non-obvious MFC. More interestingly, the densities of the solids measured by the Archimedian method were found to increase by grinding from 1.40, 1.26, 1.25, and 1.13 g cm À3 to 1.45, 1.37, 1.39, and 1.31 g cm À3 for OTK-2, OUY-2, OUK-2, and OUJ-2, respectively, indicating that the dye molecules in the amorphous state aer grinding were more densely packed.…”

“…8 debye) show weak MFC due to the strong dipole–dipole interactions between the dye molecules, changes in which may be inhibited in the crystal-to-amorphous phase transition. 14 Moreover, it was suggested that negligible MFC observed in D–π–A-type fluorescent dyes possessing small dipole moments ( μ g = ca. 1–2 debye) is ascribable to small changes in the dipole–dipole interactions between the crystalline and amorphous states.…”

“…The D-p-A-type uorescent dyes are well known to undergo signicant uorescence quenching (i.e., ACQ: aggregation-caused quenching) and bathochromic shis of the photoabsoprtion and uorescence maximum wavelengths (l abs max and l  max ) from the solution to the solid state due to the delocalization of excitons or excimers by the formation of intermolecular p-p interactions between the dye molecules in the solid state, which can be controlled by changing the steric hindrance of substituents on the D and A moieties and/or pconjugated bridge. [11][12][13][14][15][16][71][72][73] Therefore, the D-p-A-type uorescent dyes occasionally show the intra-MFC based on the twisting and distortion between the p-conjugated bridge and D or A moiety or the inter-MFC based on changes in the intermolecular dipole-dipole and p-p interactions between the dye molecules. Most D-p-A-type mechanouorochromic dyes developed so far exhibit bathochromic shi-type MFC (b-MFC): grinding of the recrystallized dyes induces red shis of the uorescent colors, that is, bathochromic shis of l  max .…”

BODIPY dye possessing solid-state red fluorescence and green metallic luster properties in both crystalline and amorphous states

“…13 The absorption band at around 320 nm is attributed to the local p / p* transition. 20 The uorescence maximum appears at 535 nm for YHO-2 and 536 nm for YHO-3, respectively, which occurs from the BODIPY locally excited (LE) state. The uorescence quantum yields (F) of YHO-2 and YHO-3 are 0.62 and 0.70, respectively.…”

Tuning the solid-state emission of small push-pull dipolar dyes to the far-red through variation of the electron-acceptor group

“…On the other hand, recent works highlighted the interest of simple and small molecular weight D-π-A push-pull dipolar fluorophores, in which an electron donating group D is connected to an electron-withdrawing group A via a π-conjugated bridge, for the design of efficient solid-state emitters. [22][23][24][25][26][27] , [28][29][30][31] , [32][33][34][35][36][37][38][39] , [40] , [41] The permanent dipole moment associated with the D-π-A structure gives rise to strong dipole-dipole interactions that can induce specific organization and orientation of molecules in the solid-state and favors the formation of emissive aggregates. Moreover, since the fluorescence of such dipolar fluorophores is usually characterized by a large Stokes shift, red and even far-red emission wavelengths over 700 nm are accessible, providing appropriate combinations of electron-donor and acceptor groups.…”