Ultrafast Relaxation Processes of Conjugated Polymer Nanoparticles in the Presence of Au Nanoparticles

Abstract

Considering the importance of conjugated polymer nanoparticles, major emphasis has been given for designing and understandingt he energy transfer and charge transfer processes of organic-inorganic hybridsf or light harvesting applications.I nt he present study, we have designed an aqueous solution-based light harvesting system using conjugatedp olymer nanoparticles (poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene],M EH-PPV) and Au nanoparticles. The change in photo-induced processes in the presence of metal nanoparticles are studied by steady-state absorption,t ime-resolved emission, time-resolved fluorescence up-conversion,u ltrafasta nisotropy and femtosecond transient absorption spectroscopy.G lobal and target analysis of transienta bsorption data validate the creation of ac ollective delocalized state in polymer nanoparticles, and the time scale for excitation energy funnelling from S 1 state to low lying collective delocalized state (CL s )i s1 8ps. Then, the electron transfer from the CL s state to Au NP occurs with a time constant of 150 ps. The 815 ps long lived charget ransfer (CT) state signifies the charget ransfer from the CL s state of polymer nanoparticles to Au NP.S uch basic understanding of relaxationp rocesses in hybrid systems is very important for designing inorganic-organic hybrid light-harvesting systems.

Revealing Complex Relaxation Processes of Collapsed Conjugated Polymer Nanoparticles in the Presence of Different Shapes of Gold Nanoparticles Using Global and Target Analysis

“…Here, the GSB peak at 525 nm is slightly blue-shifted from the plasmon band position of the Au NP. Therefore, the bleach at 525 nm in SADS3 does not arise due to energy transfer from F8BT PNP to Au NP; rather, SADS 3 is a signature of a charge-transfer state arising due to thermodynamically driven electron transfer from S 1 state of the F8BT PNP to Au NP. , The significant reduction of S 1 state from 45 to 12.5 ps as compared to the pure F8BT PNP confirm the electron transfer process. Therefore, we ascribe SADS 3 for charge transfer state S 1 δ+ –Au NP δ‑ and the time scale associated with the electron transfer from the S 1 state of F8BT PNP to Au NP is 6.8 ps (Figure C) (detailed calculation of time scale is provided in the Supporting Information).…”

“…This is because excitons are predominantly generated upon photoexcitation, and the PL decay represents the depopulation of charge carrier. Favorable interchain interactions between different chromophoric subunits with different energy levels along polymeric backbone leads to exciton diffusion (energy migration in form of exciton) from blue absorbing subunits to low lying red subunits after excitation. ,, It is evident that these energy funneling occurs in a collapsed state through a delocalized collective state which is the lowest energy sites of the collapsed form. − ,,,,, Interestingly, the diffusion length is found in the range of ∼2 nm for all of the systems, and the size of the pristine polymer nanoparticle is around 40 nm. Thus, the generated excitons after photoexcitation migrate through the chromophoric subunits and those excitons residing at the periphery of the nanoparticles can transfer from the polymer nanoparticle to the Au nanostructure.…”

“…Recently, polymer assemblies or aggregates and organic–inorganic heterostructures composed of polymers are used as an artificial light-harvesting antenna because of their reasonably high molar extinction coefficient and high photostability. , Fluorene-based conjugated polymers are attractive as active material in polymer light-emitting diodes (PLEDs) and photovoltaic devices. − With an alternation of poly­(fluorene-2,7-diyl) (F8) and benzothiadiazole (BT) units, poly­(9,9-dioctylfluorene- alt -benzothiadiazole) (F8BT) has been intensively utilized for PLED applications exhibiting high quantum and power efficiencies as a green emitter. , It is already evident that the photophysical processes of the collapsed form of polymer are significantly different from their extended form due to interchain interactions ,,, that eventually control the charge-transfer process. − In the case of polymer nanoparticles, polymeric chains are collapsed during the formation of nanoparticles in the presence of bad solvent where chromophoric units have interacted through intra- and inter-chromophoric interactions that modify the photophysical properties. ,, Favorable interchain interactions between different chromophoric subunits with different energy levels along the polymeric backbone lead to energy migration from blue absorbing subunits to low lying red subunits after excitation. , Our previous study revealed that different states with different free energy minima arise during collapsing of polymeric chains into nanoparticles which causes intra-/intrachain energy transfer and energy migration from higher energy states to the lower energy ones; this energy is funneled ultimately into a delocalized collective state which is the lowest energy sites of the collapsed form. ,,,, After photon absorption, the excited state carrier population of polymer nanoparticles relax from the vibrational hot S 1 state to relaxed S 1 state and then energy funneling occurs to low lying energy state, i.e., delocalized collective state (CL s ). These complex photophysical processes cannot be efficiently extracted using single wavelength analysis.…”

Deciphering the Relaxation Mechanism of Red-Emitting Carbon Dots Using Ultrafast Spectroscopy and Global Target Analysis

“…Reisner and co-workers have realized as many as 398 μmol of photocatalytic H 2 production per gram of C-dots per hour in the presence of a nickel co-catalyst . Significant efforts have been devoted to the synthesis and applications of C-dots; however, the study of the excited-state relaxation of C-dots using ultrafast spectroscopy and global and target analysis is less explored, which is essential for the development of efficient light-harvesting systems. − Wang et al have proposed an intrinsic dark state and a surface-related emissive state, which relaxes independently in the case of a carbon nanodot . On the contrary, previous works report the excitation transfer from the core to surface with various time scales ranging from 400 fs to 10 ps. , Yang and co-workers have demonstrated a detailed interplay between the core and surface in the excited state. , Wen et al have identified the time scale for various processes for the excited state of C-dots such as surface trapping (400 fs), optical phonon scattering (<5 ps), acoustic phonon scattering (50 ps), and excitonic recombination (1.5 ns) .…”

The Impact of Aggregation of Quaterthiophenes on the Excited State Dynamics

“…Understanding the essential structure–property correlation and the relaxation dynamics of multichromophoric organic aggregated materials is very important for designing potential materials for organic electronics, sensing devices, photocatalysis, and light-energy conversion devices. − The temperature, solvent effect, concentration, and solubility are the decisive factors for molecular aggregation. − Intermolecular interactions like π–π stacking, hydrogen bonding, hydrophobic or electrostatic interactions, van der Waals forces, etc., alter with the degree of molecules’ aggregation. All of those mentioned above eventually control the photophysical behavior. − Therefore, a systematic study is required to understand the excited state dynamics with varying aggregation degrees, which will benefit artificial light-harvesting systems. − …”