# Excited-State Relaxation Pathways of 4‑Aminobiphenyl-2-Pyrimidine Derivatives: An Ultrafast Perspective

## TL;DR

This study explores how solvent and molecular structure affect the light-emitting behavior of push–pull molecules, offering insights for designing better optoelectronic materials.

## Contribution

The study systematically reveals how solvent and donor-group substituents influence excited-state dynamics in push–pull systems.

## Key findings

- D1 exhibits a TICT state in polar solvents, affecting its fluorescence.
- D2 lacks a TICT state due to conformational constraints.
- Solvent polarity and viscosity strongly influence excited-state relaxation pathways.

## Abstract

Push–pull systems are key molecular architectures
widely
studied for their unique photophysical properties and tunable excited-state
dynamics. Here, we present a systematic investigation of the influence
of (i) solvent environment and (ii) donor-group substituents on the
excited-state relaxation pathways and dynamics of two previously reported
push–pull systems, namely 4-[4-(4-N,N-dimethylaminophenyl)­phenyl]-2,6-diphenylpyrimidine and
4-[4-(4-N,N-diphenylaminophenyl)­phenyl]-2,6-diphenylpyrimidine
for D1 and D2, respectively. Previous findings
demonstrated the presence of a twisted intramolecular charge transfer
(TICT) state, which determined the fluorescent properties of D1 in highly polar solvents. However, this state was absent
in D2 due to conformational constraints. By integrating
steady-state emission mapping, time-correlated single photon counting
(TCSPC), and femtosecond/nanosecond transient absorption spectroscopy
(fs/ns-TAS), we are able to fully disentangle the complex landscape
behind the excited state relaxation pathways in these push–pull
systems both in nonpolar and polar solvents. Additionally, the correlation
between multiple excited-state components and factors such as viscosity
and solvent polarity is thoroughly rationalized. Altogether, these
findings shed more light on the complex interplay between molecular
conformation and solvent polarity in push–pull systems, which
provides valuable insights into the rational design of advanced optoelectronic
and photonic materials.