Exciplex Formation in Blended Spin-Cast Films of Fluorene-Linked Dyes and Bisphthalimide Quenchers

Abstract

Spin-cast films of dyes (donor-π-donor, donor-π-acceptor, and acceptor-π-acceptor type, where the donor is Ph2N-, the acceptor is 2-benzothiazoyl, and the π-linker is 9,9-diethylfluorene) blended with nonconjugated bisphthalimides were prepared. Upon visible-light excitation of the dyes, quenching of the excited state occurs by exciplex formation between dye and bisphthalimide molecules or, in some cases, by excimer formation or aggregation-induced emission between two dye molecules. The extent of exciplex formation is dependent on the driving force, which can be calculated using the energy difference between the lowest unoccupied molecular orbitals (LUMOs) of the dyes and bisphthalimides. The results show that complete exciplex formation occurs when this driving force is greater than 0.57 eV whereas partial exciplex formation occurs when the driving force is between 0.28 and 0.57 eV. The exciplex emission energies can also be predicted by calculating the difference between the LUMO level of the bisphthalimide and the highest occupied molecular orbital (HOMO) of the dye. These calculated values, which were obtained from the electrochemically determined energy levels, showed good agreement with the observed emission energies. The exciplex lifetimes were found to be significantly longer than the lifetimes of the lone dyes. These exciplexes formed from nonlinked donors and acceptors in the solid state might have potential uses in nonlinear photonics.

Prediction and Design of Efficient Exciplex Emitters for High‐Efficiency, Thermally Activated Delayed‐Fluorescence Organic Light‐Emitting Diodes

“…It is interesting that the fastest decay component has a lifetime (0.9 ns) even shorter than those of the constituting molecules (2.6 ns for NPB and 5.0 ns for DPTPCz). This has been explained by Jankus et al and Stewart et al, who propose that the fastest decay component occurs due to quenching of the S 1 of the constituting molecules . On the other hand, the other two mixed films do not show any decay component faster than their constituting molecules.…”

“…Figure shows the exciplex formation process and related energy relationships. The driving force (–Δ G cs ) for exciplex formation in the solid state can be estimated based on a modified Rehm–Weller equation, which is given approximately by the difference in the exciton energy ( E A* or E D* ) of the constituting molecules with the exciplex energy ( E exciplex ) Equation . E exciplex are determined from the PL peak maxima (Figure ) of the TAPC:DPTPCz, the TCTA:DPTPCz and the NPB:DPTPCz films to be 2.47 (503 nm), 2.47 (502 nm), and 2.53 (491 nm) eV, respectively.…”

“… E exciplex are determined from the PL peak maxima (Figure ) of the TAPC:DPTPCz, the TCTA:DPTPCz and the NPB:DPTPCz films to be 2.47 (503 nm), 2.47 (502 nm), and 2.53 (491 nm) eV, respectively. Using the energy corresponding to the onset of DPTPCz's absorption spectrum in the deposited film as E exciton (3.10 eV), the values of –Δ G cs are calculated to be 0.63, 0.63, and 0.57 eV for TAPC:DPTPCz, TCTA:DPTPCz, and NPB:DPTPCz, respectively. These values are 0.97, 0.85, and 0.45 eV when calculated with E exciton of the three hole‐transporting materials.…”

Symmetry- and Solvent-Dependent Photophysics of Fluorenes Containing Donor and Acceptor Groups

“…These dyes have been studied previously in our group in spin-cast films blended with bisphthalimides for exciplex formation. 29,30 Solvent dependent studies by Belfield et al on similar dyes 31 and BT2F 32 revealed strong solvatochromism on the emission spectra of D-π-A derivatives, but little dependence in symmetrical molecules. They also showed that a solventstabilized state is formed in the asymmetrical fluorene, but showed no direct evidence of this process.…”

Bimodal Exciplex Formation in Bimolecular Photoinduced Electron Transfer Revealed by Ultrafast Time-Resolved Infrared Absorption

“…[2][3][4][5] They have indeed been invoked in intra- [6][7][8][9][10] and intermolecular electron transfer in solution, [11][12][13][14][15][16][17] in organic semiconductors for photovoltaics, [18][19][20][21][22][23] and organic light emitting diodes, [24][25][26][27][28] and in biological systems like DNA [29][30][31][32] and photosynthetic reaction centers, 33 some time under different names, such as charge-transfer excitons or excited chargetransfer complexes. All these monikers designate a species with an electronic structure between those of the reactants and products.…”