Exciplex Formation in Non-linked and Peptoid- conjugated Systems with Experimental and Computational Approaches Suhyun Park College of Natural Sciences

Abstract

This thesis presents a comprehensive investigation of exciplex photophysics, progressing from fundamental studies in non-linked systems to complex dynamics in peptoid-conjugated system, ultimately establishing foundations for quantum information science applications. The research begins by revisiting Weller's classical framework for exciplex formation through systematic studies of anthracene (Ant) as electron acceptor with N,N-dimethylaniline (DMA) derivatives as donors in cyclohexane. By integrating steady-state and time-resolved spectroscopy with long-range corrected density functional theory (LC-DFT), this work reveals that the C–N rotational angle (φ) of the dimethylamine group critically modulates both the HOMO energy and its spatial distribution, thereby controlling photoinduced electron transfer (PET) rates. Notably, while electrochemical redox potentials follow the Rehm-Weller plot on logarithmic scales, they fail to explain specific cases on linear scales. LC-DFT calculations successfully rationalize these discrepancies, demonstrating that ortho-methyl substituted donors NN2 and NN26 exhibit restricted PET due to steric hindrance forcing twisted conformations, with only ~5% of NN2 molecules achieving the quasi-planar geometry required for PET while NN26 shows complete PET suppression. Furthermore, NN4 exhibits the fastest PET rate despite lower HOMO energy than julolidine due to extended HOMO distribution upon rotation. Vibrationally resolved emission spectra simulations using TD-DFT accurately reproduce experimental exciplex emissions, revealing that emission intensity depends on the donor's contribution to the exciplex HOMO. Building upon these fundamental insights, the study explores peptoid-conjugated exciplex systems featuring Ant and DMA in designed off-facial (i,i+2)-Ac and co-facial (i,i+3)-Ac arrangements on 9-mer peptoid scaffolds. Despite DFT-optimized ground-state structures indicating more favorable geometry for (i,i+3)-Ac with ~5 Å D-A distance versus ~14 Å for (i,i+2)-Ac, femtosecond transient absorption (fsTA) and fluorescence up- conversion (FLUP) unexpectedly reveal faster PET kinetics for the (i,i+2)-Ac system. This counterintuitive behavior persists even in high-viscosity solvents, ruling out simple scaffold reorganization mechanisms. In this context, low-temperature (77 K) experiments provide direct evidence for peptoid conformational heterogeneity, which is the other hypothesis for faster PET in (i,i+2)-Ac system. At low- temperature, (i,i+2)-Ac exhibits enhanced exciplex emission and efficient PET in frozen states, while (i,i+3)-Ac shows virtually no PET. TD-DFT calculations reveal that exciplex formation in (i,i+2)-Ac requires significant deviation in phi (φ) and psi (ψ) angles of the Ant-bearing residue from ground state of (i,i+2)-Ac calculated by DFT, not cis/trans isomerization. Molecular dynamics simulations using CGenFF-based peptoid force fields confirm a heterogeneous φ/ψ distribution in (i,i+2)-Ac ground states, with a significant population pre-organized for exciplex formation, contrasting with the more symmetric energy landscape of (i,i+3)-Ac. Extending beyond electron transfer systems, the research explores peptoid-conjugated chromophore- radical architectures featuring pyrene and BDPA (1,3-Bis(diphenylene)-2-phenylallyl). These systems exhibit red- shifted emission, ground-state complexation, and weak but detectable magnetic field effects (~0.15%), suggesting radical-exciplex formation potentially involving quartet states. To enable detailed spin dynamics investigations, a continuous-wave electron paramagnetic resonance (cwEPR) spectrometer was successfully constructed, featuring an X-band IF bridge, loop-gap resonator, high-resolution teslameter with analog feedback control, and custom cryostat for future time-resolved EPR studies. In summary, this work presents a thorough investigation of exciplex behavior, employing both experimental and computational methodologies. Starting with a detailed re-evaluation of photoinduced electron transfer in fundamental non-linked donor-acceptor systems which clarified the critical role of molecular conformation over classical electrochemical interpretations, the research progressed to uncover how peptoid structural heterogeneity dictates linked exciplex photophysics. The foundational knowledge and instrumental advances achieved, including the custom-built continuous-wave electron paramagnetic resonance spectrometer, open avenues for developing peptoid scaffolds for advanced photochemical applications and investigating their potential in quantum information science. List of Contents