Transition from Charge‐Transfer to Largely Locally Excited Exciplexes, from Structureless to Vibrationally Structured Emissions

Abstract

Exciplexes of 9,10-dicyanoanthracene (DCA) with alkylbenzene donors in cyclohexane show structureless emission spectra, typical of exciplexes with predominantly charge-transfer (CT) character, when the donor has a relatively low oxidation potential (Eox ), e.g. hexamethylbenzene (HMB). With increasing Eox and stronger mixing with a locally excited (LE) state, vibrational structure begins to appear with 1,2,3,5-tetramethylbenzene and becomes prominent with p-xylene (p-Xy). A simple theoretical model reproduces the spectra and the radiative rate constants, and it reveals several surprises: Even in this nonpolar solvent, the fractional CT character of a highly mixed exciplex varies widely in response to fluctuations in the microscopic environment. Environments that favor the LE (or CT) state contribute more to the blue (or red) side of the overall spectrum. It is known that sparsely substituted benzene radical cations, e.g., p-Xy(•+) , are stabilized more in acetonitrile than the heavily substituted HMB(•+) . Remarkably, ion pairing with DCA(•-) in cyclohexane leads to even larger differences in the stabilization of these radical cations. The spectra of the low-Eox donors are almost identical except for displacements that approximately equal the differences in Eox , even though the exciplexes have varying degrees of CT character. These similarities result from compensation among several nonobvious, but quantified factors.

Photochemistry and Photophysics of the 3‐Styrylidenebenz[e]indanes

“…When charge transfer is favorable, the CT state plays a major role and structureless emission is observed. There is strong evidence that when electron transfer is unfavorable, the LE state contribution in the exciplex is larger and vibronic structure is retained in exciplex fluorescence . It is puzzling and intriguing, therefore, that the t ‐BPE/TBA exciplex gives structureless fluorescence that appears so close to the energy of t ‐BPE fluorescence, Fig.…”

Recent progress on exciplex-emitting OLEDs

“…The CT in exciplexes can be considered as the partial CT, and the exciplex wavefunction can be expressed as the mixed state of pure locally ground, locally excited (LE) and CT states as a result of the configuration interaction which is the superposition of the configurations. In this form of wavefunction, the fraction of CT in the exciplex is defined as the square of the [9]); (c) PL spectrum and extinction coefficient of the m-MTDATA:PO-T2T blended film and extinction coefficients of m-MTDATA and PO-T2T films (ref. [11]); (d) Radiative decay rate constants (k f ), nonradiative decay rate constants (k nr ), and ISC rate constants (k isc) of the singlet exciplex states in which TCA and DCA are acceptors and various alkylbenzenes are donors in various solvents as a function of the average emission energy of the exciplexes (ref.…”

“…[12]). expansion coefficient for the pure CT state [8,9]. Exciplexes in solution and films were reported to exhibit various fractions of CT dependent on the microscopic environment, librational displacement, and geometries of exciplex-forming pairs [9,10].…”

“…As the fraction of CT in the exciplex increase, the PL spectra of exciplexes are redshifted from the PL spectra of the constituent molecules, broad, and featureless without the vibrational progression [9,13]. One example is shown in Figure 1b, where exciplexes are formed between 9,10-dicyanoanthracene (DCA) and various alkylbenzenes such as p-xylene, 1,2,4trimethylbenzene, 1,2,3,5-tetramethylbenzene, durene, pentamethylbenzene, and hexamethylbenzene in cyclohexane [9]. DCA and the alkylbenzens are the electron acceptor and electron donors, respectively.…”

“…The transition rate constants in exciplex-forming systems are also influenced by the degree of CT of exciplexes, f CT [9,12,13,[23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39]. The fluorescence rate constants (k f ), nonradiative rate constants (k nr ), and intersystem crossing (ISC) rate constants (k isc ) of the singlet exciplex states between 2,6,9,10-tetracyanoanthracene (TCA) and DCA as electron acceptors and various alkylbenzenes as electron donors in various solvents are plotted as a function of the average emission energy of singlet exciplex states in Figure 1(d) [12].…”