Effect of Diffusion on Photo-Induced Excited-State Energy Transfer between Fluorescent Semiconducting Molecules: Tris-(8-hydroxyquinoline) Aluminum and 6,13-Bis (Triisopropylsilylethynyl) Pentacene

Abstract

In the present work, the effect of diffusion on photo-induced excitation energy transfer between fluorescent organic semiconducting molecules tris-(8hydroxyquinoline) aluminum (AlQ 3 , n-type donor) and 6,13-bis (triisopropylsilylethynyl) pentacene (TIPS-P, p-type acceptor) at a concentration range of 10 −4 to 10 −6 M in chloroform solution was studied by steady-state and time-domain fluorescence measurements. The donor−donor interaction strength is significantly weaker than the donor−acceptor strength (the ratio of donor−donor to donor−acceptor interaction strengths is 1.55 × 10 3 ) in chloroform solution. Considerable overlap between donor emission and acceptor absorption and high interaction parameters favors direct Forster energy transfer and exciplex (D*A) formation. Excitation energy migration does not take place between donor molecules. However, material diffusion appears to influence the donor decay dynamics by forming charge-transfer exciplex complexes and modulating the excitation energy transfer rate from the metal-to-ligand chargetransfer (MLCT) state of the donor to vibronic states of the acceptor at a low acceptor concentration. At high acceptor concentrations, energy transfer from the excited donor to acceptor occurs through the exchange mechanism along with Forster long-range dipole−dipole interactions, and the corresponding critical transfer distance is found to be ∼42 Å. A blue-shift in AlQ 3 emission, a red-shift in the 0−0 vibronic peak of TIPS-P, and quenching of exciplex decay following Stern−Volmer quenching are observed, with the increase in acceptor concentration. No evidence of excimer formation is observed in acceptor molecules.

Förster Resonance Energy Transfer between Fluorescent Organic Semiconductors: Poly(9,9-dioctylfluorene-alt-benzothiadiazole) and 6,13-Bis(triisopropylsilylethynyl)pentacene

“…In our previous study, we observed a long decay time in the case of acceptor decay dynamics, which is not observed in the present case. The acceptor for that case was the same as in the present case (TIPS-P).…”

“…Finally, based on the above experimental analysis, a schematic mechanism for excitation energy transfer between the excited state of F8BT and the ground state of TIPS-P in chloroform solution is shown in Scheme through the energy-level diagram. Unlike AlQ3/TIPS-P, EET took place in the present system via both the FRET and RET mechanisms. Briefly, the mechanistic aspect of EET revealed in the present study would be important in designing efficient photovoltaic systems. − …”

“…The decrease in the intensity of the 0−0 transition of emission peak indicates the TIPS-P molecule can be attributed to concentration quenching due to reabsorption. The red-shift in the emission spectra indicates radiative migration of excitation energy among TIPS-P molecules, 10 which enhanced the intensity of the 0−1 emission band.…”

“…However, for higher acceptor concentration, EET follows the Forster mechanism of energy transfer. 9 In a recent work, 10 we reported the effect of diffusion on FRET between an n-type donor (AlQ 3 ) and a p-type acceptor 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-P) in a chloroform solution. This pair followed energy transfer by donor diffusion and an exciplex formation at low acceptor concentration.…”

“…The process of excitation energy transfer [EET] in donor–acceptor heterojunctions significantly implies an increase in the efficiency of organic fluorescent semiconductors. − In general, fluorescent semiconducting molecules transfer their excitation energy via long-range [Förster type] or short-range [Dexter type] interactions. In the Förster mechanism, the excitation energy of the donor is transferred nonradiatively to the acceptor via resonance coupling between excited donor dipole and unexcited acceptor dipole and is known as Förster resonance energy transfer (FRET). − In contrast, electronic energy transfer in the Dexter mechanism is due to dipole–quadrupole coupling between the donor–acceptor systems. − The essential requirement for EET is that there must be considerable overlap between the donor emission and acceptor absorption so that the acceptor is coupled effectively with the donor.…”