Practical Design of 3,6-Di-tert-butyldiphenyldibenzofulvene Derivatives with Enhanced Aggregation-Induced Emission

Abstract

Diphenyldibenzofulvene derivatives consisting of an aromatic tertbutyl-substituted fluorene stator and different rotors consisting of nonsubstituted phenyl groups (3,6-dtb-DPBF) and monomethyl-substituted (3,6-dtb-DPBFMe) and dimethyl-substituted [3,6-dtb-DPBF(Me) 2 ] forms have been synthesized and found to display aggregation-induced emission (AIE). The incremental number of substituents from 3,6-dtb-DPBF to the 3,6-dtb-DPBFMe and 3,6-dtb-DPBF-(Me) 2 derivatives promotes significant changes, from a good solvent (acetonitrile, MeCN), where it is very poorly emissive, to thin films or aggregates, in MeCN/ water mixtures, and a huge increment in fluorescence emission, which is found to be dependent on the water fraction, f w . The characteristics (size and distribution) of the aggregates were further corroborated with dynamic light scattering measurements. From time-resolved fluorescence experiments (TCSPC and FLIM), the increase in the contribution of the longer decay component is linked to the emission of the aggregate (AIE effect). To assist in the elucidation of the aggregation process at a molecular level, the data were complemented with computational studies [time-dependent density functional theory (TDDFT) and molecular dynamics (MD) simulations]. From MD, the octamer properly addresses the properties of the aggregate. As determined by the X-ray data, the crystal structure of a two-unit special disposition is identical to the geometry of the most stable structure obtained from MD and TDDFT calculations.

Tuning the AIE Properties of Di-tert-Butyl-diphenyldibenzofulvene Derivatives

“…The compounds show similar absorption and emission spectra, with the same spectral profile as that in acetonitrile (see Figure S10). This behavior is reciprocal to that of 3,6- dtb -DPBFs, where a blue shift of the solid compared to the solution behavior was observed . On the other hand, for 2,7- dtb -DPBFs and 3,6- dtb -DPBFs, in the powder, the emission wavelength maximum is very close to the value observed in acetonitrile (see Table S9).…”

“…The absorption spectra of 2,7- dtb -DPBF and 2,7- dtb -DPBFMe in MeCN and in MeCN/H 2 O mixtures, as a function of the water fraction ( f w ), Figure S10, displays a change in the absorption spectra with f w , indicating a change in the nature of the absorbing species. In previous studies with DPBF derivatives, this was shown to be associated with the formation of aggregates. , However, while in the case of 3,6- dtb -DBPF, a red shift of the longer-wavelength absorption band (associated with the aggregated species), when compared to the monomer, was observed, here, the case of the 2,7- dtb -DPBF’s, this shift is not present and a loss of vibronic resolution is detected with the increase in the water fraction in the mixture . The evidence that the spectra are devoid of vibronic resolution is evidence to the presence of aggregates of different conformational degreedepending on the position of the tert -butyl group on the fluorene moietyin 2,7- dtb -DPBF and 3,6- dtb -DPBF.…”

“…The photophysical studies of the mixtures were conducted immediately after sample preparation. Thin films from the compounds were prepared using a desktop precision spin-coating system, specifically the P6700 series from Speedline Technologies, as described elsewhere. , Thin films from the samples were obtained by depositing approximately 50 μL of a solution of the compounds onto a circular sapphire substrate (12 mm diameter). Subsequently, spin-coating was performed at 2500 rpm for 60 s in a nitrogen-saturated atmosphere (2 psi).…”

“…Upon aggregation, constraining intramolecular motion is expected to significantly affect the internal conversion from the first singlet (S 1 ) state to the conical intersection (CI) on the potential energy surface (PES). , In such cases, the more generic term “solid-state luminescence enhancement” could be applicable. , This simple concept opens new possibilities for the design of new luminescent materials in various areas, particularly in the field of organic light-emitting devices (OLED) and optical sensors. − Previous studies indicate that the diphenyldibenzofulvene (DPBF) structure is, in acetonitrile, weakly fluorescent. It is insoluble in water and forms aggregates in acetonitrile/water mixtures, with an increase in the fluorescence quantum yield. ,,, In recent studies, we have demonstrated that the tert -butyl group plays a significant role in shaping the AIE properties of 3,6- dtb -diphenyldibenzofulvene derivatives (3,6- dtb -DPBF’s) . In this work, we extend the rational design of DPBF compounds to two new derivatives with tert -butyl substituents in the 2 and 7 positions of the fluorene core stator, which are closer to the diphenyl moiety rotor than in the case of the 3,6- dtb -DPBF’s, and with or without methyl groups at the phenyl groups: the 2,7-di- tert -butyl-diphenyldibenzofulvene derivatives (2,7- dtb -DPBF’s), see Scheme .…”

“…It is insoluble in water and forms aggregates in acetonitrile/water mixtures, with an increase in the fluorescence quantum yield. ,,, In recent studies, we have demonstrated that the tert -butyl group plays a significant role in shaping the AIE properties of 3,6- dtb -diphenyldibenzofulvene derivatives (3,6- dtb -DPBF’s) . In this work, we extend the rational design of DPBF compounds to two new derivatives with tert -butyl substituents in the 2 and 7 positions of the fluorene core stator, which are closer to the diphenyl moiety rotor than in the case of the 3,6- dtb -DPBF’s, and with or without methyl groups at the phenyl groups: the 2,7-di- tert -butyl-diphenyldibenzofulvene derivatives (2,7- dtb -DPBF’s), see Scheme . In order to fully equate the luminescent properties of the compounds, and to shed light on the nature of the aggregates, a study, in solution and solid state, including photophysical (including steady-state and time-resolved data), dynamic light scattering (DLS) and TDDFT electronic quantum calculations was undertaken.…”