Reported here are the low-temperature photoluminescence (PL), energy-transfer mechanism, and exciton dynamics of Mn 2+ -doped two-dimensional (2D) perovskites that show interesting differences from their three-dimensionally doped counterpart. Dopant emission in 2D system shows increased PL intensity and shortened lifetime with increase of temperature and strong dopant emission even at low temperatures. Transient absorption (TA) spectroscopy reveals the dominant role of "hot" excitons in dictating the fast energy-transfer timescale. The operative dynamics of the generated hot excitons include filling up of existing trap states (shallow and deep) and energy-transfer channel from hot excitons to dopant states. Global analysis and target modeling of TA data provide an estimate of excitons (hot and band edge) to a dopant energy-transfer timescale of ∼330 ps, which is much faster than the band edge exciton lifetime (∼2 ns). Such fast energy-transfer timescale arises due to enhanced carrier exchange interaction resulting from higher exciton confinement, increased covalency, and involvement of hot excitons in the 2D perovskites. In stark contrast to three-dimensional systems, the high energy-transfer rate in 2D system results in high dopant emission intensity even at low temperatures. Increased intrinsic vibronic coupling at higher temperatures further supports efficient Mn 2+ sensitization that ultimately dictates the observed temperature dependence of the dopant emission (intensity, lifetime).

Understanding of exciton dynamics in semiconductor quantum dots (QDs) is of great importance due to their immense application potential on various photonic devices. In this work, we demonstrate hot electron transfer (HET) from higher excited states of cadmium telluride quantum dots (CdTe QDs) to tetrakis(4-carboxyphenyl)porphyrin (TCPP) and ultrafast electron transfer from photoexcited TCPP to CdTe QDs in newly prepared CdTe QD−tetrakis(4-carboxyphenyl)porphyrin nanocomposites (CdTe QD−TCPP NCs), where TCPP is noncovalently attached to CdTe QDs, by employing steady state, time-resolved emission, and femtosecond transient absorption spectroscopic techniques. The observation of efficient quenching of the photoluminescence (PL) of CdTe QDs with little/negligible change in photoluminescence (PL) decay profiles in nanosecond time regime (TCSPC method) of CdTe QD−TCPP NCs predominantly indicates static interaction between these two interacting species, CdTe QD and TCPP. Excitation wavelength and excitation intensity dependent femtosecond transient absorption studies of CdTe QDs and CdTe QD−TCPP NCs confirm the hot electron transfer (HET) from higher excited states (Σ − /1P(e)) to TCPP. Analysis of growth kinetics of band-edge or 1S bleach amplitude of band-edge bleach signal strength at the initial stage after excitation allows us to estimate 30−40% quantum efficiency (Φ HET ) of HET when CdTe QD−TCPP NCs are excited at higher excited state (∼390 nm). In contrast, no electron transfer was observed when CdTe QD−TCPP NCs are excited in the vicinity of the band-edge (1S) exciton. In another instance, femtosecond transient absorption studies upon 630 nm excitation at the Q-band of TCPP in CdTe QD−TCPP NCs suggest the occurrence of ultrafast electron transfer (<250 fs) from photoexcited TCPP to CdTe QDs in CdTe QD−TCPP NCs. The charge separation and charge recombination dynamics in CdTe QD−TCPP NCs are explored in detail. The fundamental understanding of this "to and fro" photoinduced electron transfer dynamics in CdTe QD−TCPP NCs opens up new possibilities to design an efficient light-harvesting system based on inorganic−organic hybrid systems.

Femtosecond time-resolved fluorescence and transient absorption studies are reported for three newly synthesized covalently linked N,N-bis(4'-tert-butylbiphenyl-4-yl)aniline (BBA) and pyrrolidinofullerenes (C60)-based donor-π conjugated bridge-acceptor dyads (D-B-A) as functions of the bridge length (7.1, 9.5 and 11.2 Å for Dyad-1, Dyad-2 and Dyad-3), dielectric constants of the medium and pump wavelengths. In polar solvent, ultrafast fluorescence quenching (kEET ≥ 2 × 1012 s-1) of the BBA moiety upon excitation of the BBA moiety (320 nm) is observed in the dyads and is assigned to a mechanism involving electron exchange energy transfer (EET) from 1BBA* to C60 followed by electron transfer from BBA to 1C60*. Cohesive rise and decay dynamics of conjugated BBA˙+-C60˙- anion pairs confirm the involvement of a distance independent adiabatic charge-separation (CS) process (kCS ≥ 2.2 × 1011 s-1) with near unity quantum efficiency (φCS ≥ 99.7%) and a distance-dependent non-adiabatic charge-recombination (CR) process [kCR ∼ (1010-108) s-1]. In contrast, excitation of the C60 moiety (λex = 430 to 700 nm) illustrates photoinduced electron transfer from BBA to 1C60*, involving non-adiabatic (diabatic) and distance-dependent CS (kCS in the range of 0.59-1.78 × 1011 s-1) with 98.86-99.6% (Dyad-3-Dyad-1) quantum efficiency and a CR process with kCR values [kCR ∼ (1010-108) s-1] up to three orders greater than kCS of the respective dyads. Both the processes, CS and CR, upon C60 excitation and the CR process upon BBA excitation show distance dependent rate constants with exponential factor β ≤ 0.5 Å-1, and electron transfer is concluded to occur through a covalently linked conjugated π bridge. Global and target analysis of fsTA data reveal the occurrence of two closely lying CS states, thermally hot (CShot) and thermally relaxed (CSeq) states, and two CR processes with two orders of different rate constants. Careful analysis of the kinetic and thermodynamic data allowed us to estimate the total reorganization energy and electronic coupling matrix (V), which decrease exponentially with distance. These novel features of the distance independent adiabatic CS process and the distance-dependent diabatic CR process upon donor excitation are due to extending the π-conjugation between BBA and C60. The demonstrated results may provide a benchmark in the design of light-harvesting molecular devices where ultrafast CS processes and long-lived CS states are essential requirements.