Picosecond−Microsecond Dynamics of Photoinduced Electron-Transfer Processes in Amorphous Solid Films of Dimeric Carbazolyl Compounds Doped with 1,2,4,5-Tetracyanobenzene

Abstract

Picosecond and nanosecond transient absorption spectroscopy and picosecond transient dichroism measurements were applied for elucidating photoinduced charge-separation, charge-recombination (CR), and hole-transfer (HT) processes in amorphous solid films of carbazolyl dimeric compounds (1,3-di(N-carbazolyl)propane and 1,2-trans-di(N-carbazolyl)cyclobutane) doped with 1,2,4,5-tetracyanobenzene (TCNB). Time-resolved fluorescence spectra were also measured for elucidating fluorescent states responsible for recombination emission. These results were compared with those for poly(N-vinylcarbazole) (PVCz), where the concentration of carbazolyl (Cz) chromophores is the same as the above dimeric compounds. The ground-state chargetransfer (CT) absorption and the steady-state CT fluorescence spectra were different between the present dimeric and the PVCz films. In both the films, the CR process was in competition with the HT process in the initial charge-separated state produced via the photoexcitation of the ground-state CT complex. The time constants of these processes in the amorphous dimeric compounds films were close to the values obtained for the PVCz ones. The holes (Cz + ) that escaped from the initial CR process and the immobile TCNB anion disappeared via the CR process with a t -0.5 time dependence. The CR process was interpreted as the hole diffusion-controlled geminate recombination process, as is the same as that of PVCz films. The behavior of time-resolved fluorescence attributable to the geminate CR process was different between the amorphous dimeric compounds and the PVCz films. These differences and similarities between the amorphous dimeric compounds and the PVCz films were discussed.

Direct Detection of the Hole Migration along the Polymer Chain by Means of Picosecond Transient Absorption and Dichroism Measurements: Poly(N-vinylbenzocarbazole) Systems in 1,2-Dichloroethane Solution

“…In the case where the dephasing process of the memory of the polarized excitation is regarded as an exponential process, the transmitted light intensity, I T ( t ), in the homodyned detection is given by − where A ( t ) is the time dependence of the population of the ionic species and τ DP is the time constant for the dephasing process of the memory of the polarized excitation. In the actual measurement, we employed the heterodyned detection method as performed in previous works. − In this optical configuration, the analyzer polarizer was uncrossed slightly (< a few degrees) and the electric field E LO is present in the absence of the pump pulse. I LO , which is in proportion with E LO 2 , is adjusted to make the heterodyne term ( E LO × E S ( t )) much greater than I S (∝ E S ( t ) 2 ).…”

“…In addition, the molecular structures of the benzocarbazolyl moiety attached to the main chain causes the difference in the overlap and the degree of interaction between neighboring aromatic groups in polymer chains. In the following, we will discuss the photoprimary processes in the present polymers as well as the corresponding monomer model system, by integrating the present results with those accumulated for the general transient ion pairs in solutions and with the primary electron-transfer processes in other aromatic vinyl polymer and related systems. − …”

Picosecond laser photolysis studies on photoinduced electron transfer processes in poly(1-vinylpyrene) in solutions

“…2,30,31 Actually, it was observed that the hole-migration process through pendant aromatic groups leading to the increase in the interionic distance resulted in the long-lived CS state formation in the nanosecond time region for aromatic vinyl polymers-electron acceptor systems in rather polar solutions. [9][10][11][12][13] In addition, for these systems, the time profile of the CS state was found to be reproduced by the model that the cation radical continuously migrates along pendant aromatic moieties in polymer chain with the charge recombination at the initial position of the charge separation, [6][7][8][9][10][11][12][13] as shown in Scheme 1. Hence, we employed this scheme for the analysis of the dynamic behaviors of PVPy-TCNB in pyridine solution.…”

“…Recently, we pointed out that the dichroism measurement of the transient absorption of ionic species directly probes the charge shift reactions in these polymer systems and applied this technique to the detection of the hole migration process in poly(N-vinylcarbazole) (PVCz) and its related systems in the solid amorphous phase [6][7][8][9] as well as PVCz and other aromatic vinyl polymers in the solution phase. [9][10][11][12][13] From these results, it was demonstrated that rapid hole migration with a time constant of 0.2-a few ns takes place in solutions as well as in solid films.…”

Recent progress on exciplex-emitting OLEDs

“…However, the existence of sub-band gap intermolecular CT absorption in exciplex-forming systems with very low intensity was reported employing elaborate measurements such as Fourier transform photocurrent spectroscopy (FTPS) and photothermal deflection spectroscopy (PDS). [11,[15][16][17][18][19][20][21][22]. These new absorption peaks originate from the direct optical excitation from the ground state to the exciplex state [21].…”