Characterization of a New Electron Donor-Acceptor Dyad in Conventional Solvents and Ionic Liquids

Article abstract of DOI:10.1021/acs.jpcb.9b07077

Ionic liquids are being tested as potential replacements for current electrolytes in energy-related applications. Electron transfer (ET) plays a central role in these applications, making it essential to understand how ET in ionic liquids differs from ET in conventional organic solvents and how these differences affect reaction kinetics. A new intramolecular electron donor-acceptor probe was synthesized by covalently linking the popular photoacceptor coumarin 152 with the donor dimethylaniline to create the dyad "C152-DMA" for potential use in probing dynamical solvent effects in ionic liquids. Molecular dynamics simulations of this dyad show the considerable conformational flexibility of the linker group but over a range of geometries in which the ET rate parameters vary little and should have minimal effect on reaction times >100 ps. Steady-state and time-resolved fluorescence methods show the spectra of C152-DMA to be highly responsive to solvent polarity, with ET rates varying over the range of 10⁸ to 10¹² s^-1 between nonpolar and high-polarity conventional solvents. The sensitivity to hydrolysis in the presence of acidic impurities limits the dyad's use to ionic liquids of high purity. The results in the few ionic liquids examined here suggest that in addition to solvent polarity, electron transfer in C152-DMA also depends on solvent fluidity or solvation times.

Full text of DOI:10.1021/acs.jpcb.9b07077

Subscriber access provided by Stockholm University Library
B: Liquids, Chemical and Dynamical Processes in Solution, Spectroscopy in Solution
Characterization of a New Electron Donor-Acceptor
Dyad in Conventional Solvents and Ionic Liquids
Marissa Saladin, Christopher A. Rumble, Durgesh Vinod Wagle, Gary A. Baker, and Mark Maroncelli
J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.9b07077 • Publication Date (Web): 09 Oct 2019
Downloaded from pubs.acs.org on October 12, 2019
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a service to the research community to expedite the dissemination
of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in
full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully
peer reviewed, but should not be considered the official version of record. They are citable by the
Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore,
the “Just Accepted” Web site may not include all articles that will be published in the journal. After
a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web
site and published as an ASAP article. Note that technical editing may introduce minor changes
to the manuscript text and/or graphics which could affect content, and all legal disclaimers and
ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or
consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W.,
Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 40
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
Characterization of a New Electron Donor-Acceptor
Dyad in Conventional Solvents and Ionic Liquids
1
1
2
Marissa Saladin, Christopher A. Rumble, Durgesh V. Wagle,
2
1*
Gary A. Baker, and Mark Maroncelli
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1Department of Chemistry, The Pennsylvania State University,
University Park, Pennsylvania 16802, United States
2
Department of Chemistry, University of Missouri – Columbia,
Columbia, Missouri 65211, United States
Abstract
Ionic liquids are being tested as potential replacements for current electrolytes in energy related
applications. Electron transfer (ET) plays a central role in these applications, making it essential
to understand how ET in ionic liquids differs from ET in conventional organic solvents and how
these differences affect reaction kinetics. A new intramolecular electron donor-acceptor probe
was synthesized by covalently linking the popular photo-acceptor coumarin 152 with the donor
dimethylaniline to create the dyad “C152-DMA” for potential use in probing dynamical solvent
effects in ionic liquids. Molecular dynamics simulations of this dyad show considerable
conformational flexibility of the linker group, but over a range of geometries in which the ET
rate to vary minimally for times >100 ps. Steady-state and time resolved fluorescence methods
show the spectra of C152-DMA to be highly responsive to solvent polarity, with ET-rates
8
12 -1
varying over the range 10 – 10 s between nonpolar and high-polarity conventional solvents.
Sensitivity to hydrolysis in the presence of acidic impurities limits the dyad’s use to ionic liquids
of high purity. Results in the few ionic liquids examined here suggest that, in addition to solvent
polarity, electron transfer in C152-DMA also depends upon solvent fluidity and/or solvation
times.
_________________________________
*
Corresponding author: maroncelli@psu.edu
1
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 2 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
I. Introduction
By virtue of its ubiquity in chemical and biological reactions, and its relevance in energy
capture and storage applications, electron transfer has been a perennial interest in many areas of
science and technology.1-2 Among the myriad ways researchers have sought to obtain a
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
fundamental understanding of electron transfer rates and how one might control them, studies of
covalently linked electron donor-acceptor (D-A),3-5 donor-bridge-acceptor molecules (D-B-A),6-
1
1
and more complex artificial photosynthetic systems12-14 have played a prominent role. The
present report describes the synthesis and photophysical characterization of new intramolecular
electron transfer probe intended for studies of the effects of solvation dynamics on electron
transfer in ionic liquid and conventional solvents.
This new probe (Scheme 1) is constructed by covalently linking the coumarin 152
(
C152) fluorophore, which acts as the photo-excited electron acceptor, to a dimethylaniline
DMA) donor via single methylene bridge. The C152 + DMA combination is one of the most
(
commonly used pairings in studies of bimolecular electron transfer, and it has already been
thoroughly characterized in conventional organic solvents,15-16 ionic liquids,17-20 micelles and
other complex environments,18, 21-22 and in neat liquid DMA.23-25 The C152-DMA construct
shown in Scheme 1 represents an intermediate case between strongly coupled D-A pairs linked
by a single bond that provides strong electronic coupling and ultrafast electron transfer, and D/A
pairs connected by more extended bridges, for which much weaker coupling leads to reaction in
the nanosecond and microsecond range. It was hoped that the simple linkage of the C152-DMA
pair used here would provide an example of electron transfer occurring on time scales
comparable to that of bimolecular C152/DMA pairs at first-solvation shell separations. While
this hope was largely fulfilled, we also found that the C152-DMA dyad has one serious flaw for
use in ionic liquids. The linkage between the 7-amino group of the coumarin ring and the DMA
readily dissociates in the presence of trace amounts of acidic impurities, impurities difficult to
completely remove from many ionic liquids. Nevertheless, this probe is suitable for use in a
wide range of conventional solvents and some ionic liquids, making it useful for many purposes,
if not for wide-ranging studies of ionic liquids. For this reason, the synthesis and photophysics
of the C152-DMA dyad are reported herein, after briefly describing some of the prior work on
bimolecular and intramolecular electron transfer in ionic liquid to which the present results will
be compared.
2
ACS Paragon Plus Environment

Page 3 of 40
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
Over the past decade, many groups have investigated basic the physical chemistry of
ionic liquids as reaction media, focusing on discerning how solvation in a purely ionic
environment changes reactive processes compared to that present in conventional high-fluidity
solvents, including studies of both bimolecular16-18, 20, 26-38 and intramolecular39-54 electron
transfer. Studies of bimolecular electron transfer in ionic liquids have thus far uncovered a
number of distinctive effects. The earliest of these studies compared reaction rates in ionic
liquids and conventional solvents against simple predictions for diffusion-controlled rates.17, 26-29
As anticipated, the high viscosities of ionic liquids lead to slower rates compared to conventional
solvents. However, in many cases reactions were found to be 10-100 times faster than expected
based on viscosity alone.30 This apparent rate acceleration was shown to be the result of two
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
55
compounding effects: enhanced rates of diffusion of neutral solutes in ionic liquids combined
with the fact that stationary reaction conditions are often not achieved in the fluorescence
quenching experiments typically used to measure rates.30-31 In more recent work, a number of
groups have measured the rates of electron transfer quenching of coumarin dyes by aromatic
amines in ionic liquids and observed an apparent Marcus turnover in the rate at smaller than
expected values of the driving force.18, 20, 32-33 Similar results were also reported in micelles and
other slowly relaxing media,21-22, 56-58 and attributed to reduction of the effective reorganization
energy due to the slow environmental response in these media. The validity of this interpretation
has been questioned16, 34 and is currently still being debated.20, 35-36 In very recent work, Wu et
al.37-38 employed a detailed model of the reaction-diffusion problem to analyze data a range of
fluorophore/quencher pairs and concluded that some new feature was leveling reaction rates in
ionic liquids in a way that is not apparent in conventional solvents. They proposed the slow
reorientation of contact pairs gates reaction in ionic liquids in a manner that is unimportant in
highly fluid solvents. Much has been learned from these studies of bimolecular electron transfer
reaction. Nevertheless, disentangling the diffusional portion of reaction from the “intrinsic”
electron transfer process in contact pairs remains subject to considerable uncertainty.
To focus on the solvent influence over the electron transfer event itself, a number of
groups have therefore turned to measurements of electron transfer rates between chemically
bonded donor/acceptor (D/A) pairs in ionic liquids.39-54 Most studies employed directly bonded
D-A pairs linked by a single bond, and thereby having significant through-bond coupling. In
such systems, electron transfer, or at least the achievement of the equilibrium extent of reaction,
3
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 4 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
is often observed to be much slower than in conventional solvents and to closely track solvation
times.40, 49, 53-54 Surprisingly few studies have so far employed dyads with the weaker coupling
produced by a more extended bridge between the donor and acceptor.47, 51-52 Of most relevance
to the present work are the studies of Castner, Wishart and coworkers.51-52 They synthesized two
dyads in which a coumarin dye was connected by one or two proline groups to the donor N,N-
dimethyl-p-phenylenediamine, and compared the kinetics observed in acetonitrile to that in
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
5
7
several ionic liquids. In the compound having a single proline bridge (“P1”), electron transfer
in acetonitrile was found to be dominated by a single component with a time constant near 0.5
ns. In contrast, reactions in the ionic liquids were markedly nonexponential, exhibiting a
distribution of time constants ranging between 0.1-10 ns. These researchers attributed the
distributed character of the kinetics to the fact that solvation spans many time scales in ionic
liquids.59 Because much of the solvation response occurs on time scales comparable to or slower
than reaction, substantial heterogeneity in electron transfer rates is produced.60-61 Curiously,
rather different behavior was observed in the case of the P2 dyad, linked by two proline units.58
The P2 dyad showed decays having three well separated components in both acetonitrile and in
the ionic liquid solvents, which the authors attributed to the presence of multiple conformations
adopted by the diproline linker. Average rate constants of the P2 dyad were also not markedly
different from those of the P1 dyad, presumably due to the ability of some P2 conformers to
achieve close D/A separations.
The remainder of this paper describes the synthesis and photophysics of the C152-DMA
dyad, hereafter referred to as simply “the dyad”, characterized using molecular dynamics
simulations and steady-state and time-resolved fluorescence spectroscopy in a variety of
solvents. Data on the solvent-dependent photophysics of two reference chromophores, C152 and
H152, shown in Scheme 1, are also reported.
II. Methods
A. Reagents and Materials
The C152-DMA dyad, 7-((4-(dimethylamino)benzyl)(methyl)amino)-4-(trifluoromethyl)-2H-
chromen-2-one, was synthesized using the route outlined in Scheme 2. The reagents 4-
dimethylaminobenzaldehyde (cat. no. 1030580100), 3-aminophenol (98%, 100242), ethyl 4,4,4-
®
trifluoroacetoacetate (99%, E50205), iodomethane (ReagentPlus , 99.5%, 289566), ethanol (200
4
ACS Paragon Plus Environment

Page 5 of 40
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
proof, E7023), hexane (anhydrous, 95%, 296090), N,N-dimethylformamide (ReagentPlus®,
≥99%, D158550), ethyl acetate (anhydrous, 99.8%, 270989), sodium borohydride (≥96% gas-
volumetric, 71320), potassium carbonate (anhydrous, free-flowing, Redi-Dri™, ACS reagent,
≥
99%, 791776), sodium sulfate (ACS reagent, ≥99.0%, anhydrous, granular, 239313), zinc
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
chloride (99.999% trace metals basis, 229997), and hydrogen bromide (ReagentPlus®, ≥99%,
295418) were used as received from MilliporeSigma (St. Louis, MO).
Preparation of 4-dimethylaminobenzyl alcohol (2): 1.0 g of 4-dimethylaminobenzaldehyde (1)
was dissolved in 10 mL of ethanol, followed by the slow addition of 0.279 g (1.1 equiv) of
NaBH over the course of 10 minutes. The reaction mixture was stirred for 2 h at room
4
temperature and monitored by TLC. The solvent was evaporated under reduced pressure
followed by addition of water and extraction with ethyl acetate. The organic phase was treated
with brine and dried over NaSO
-dimethylaminobenzyl alcohol, which was subsequently used for the next step without any
further purification.
4
. The ethyl acetate was removed on a rotary evaporator to afford
4
Preparation of 4-dimethylaminobenzylbromide hydrobromide (3): 3.5 mL of 48% HBr was
added to a pressure tube containing 0.5 g of 4-dimethylaminobenzyl alcohol (2). The tube was
tightly sealed and the reaction stirred while heating for 2 h at 120 °C. The reaction was allowed
to cool to room temperature and water then removed via rotary evaporation under reduced
pressure to afford crude product (brown oil) which was used without additional purification.
3
-Methylaminophenol (5) was synthesized from 3-aminophenol (4) following a literature
procedure (yield: 68%).62
Preparation of H152 (6): To generate the coumarin fluorophore by reaction of a phenol with a β-
keto ester (i.e., Pechmann condensation), 1.34 g of 3-methylaminophenol (5) was dissolved in 20
mL of ethanol, followed by the addition of 2.20 g (1.1 equiv) of ethyl 4,4,4-trifluoroacetoacetate
plus 1.78 g (1.2 equiv) of ZnCl . This mixture was refluxed for 12 h. The reaction mixture was
2
cooled to room temperature and poured into 104 mL of 0.1 N HCl solution. The crude product
was filtered, washed with water, and dried overnight. The crude product was further purified
using a silica gel column eluted with a 3:1 (v/v) hexane–ethyl acetate solvent system under
gravity (yield: 47%).
5
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 6 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1H NMR (500 MHz, DMSO) of 6: δ 7.832 (1H, d), 7.099 (1H, q), 6.678 (1H, dd), 6.483 (1H, d),
1
3
6.450 (1H, s), 2.775 (3H, d). C NMR (125 MHz) of 6: δ 159.42, 156.91, 154.07, 140.61,
1
2
40.36, 140.11, 139.11, 125.28, 123.08, 120.89, 118.70, 111.38, 107.28, 107.24, 101.59, 96.33,
9.11.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Synthesis of the C152-DMA Dyad (7): In a round bottom flask, 0.243 g of 3 was dissolved in 5
mL of acetone, neutralized with 0.125 g of K CO (1.1 equiv), and stirred for 5 min. This
reaction mixture was added to a 25 mL round bottomed flask equipped with a magnetic stirring
bar, followed by the addition of 0.200 g of 6 (1.0 equiv) plus another 0.125 g of K CO in 10 mL
2
3
2
3
of DMF. The reaction was stirred for 2 days at room temperature. The reaction was filtered and
the solvent removed via rotary evaporation (1 mmHg, 70 °C) over 8 h. The crude solid was
purified using flash column chromatography by eluting with trimethylamine–hexane–ethyl
acetate in a 2:29:69 (v/v/v) ratio (yield: 53%). The dyad was found to be highly susceptible to
cleavage of the amine linkage in the presence of trace acid.
1H NMR (500 MHz, DMSO) of 7: δ 7.426 (d, 2H), 7.046 (d, 2H), 6.886 (d, 1H), 6.707 (d, 1H),
13
6
.685 (d, 1H), 6.501 (s, 1H), 4.603 (s, 2H), 3.314 (s, 3H), 2.844 (s, 6H). C NMR (125 MHz) of
7
: 159.33, 156.25, 152.55, 149.64, 139.92, 127.54, 125.31, 124.52, 123.02, 120.83, 112.55,
110.27, 101.73, 98.14, 54.33.62
Coumarin 152 (7-dimethylamino)-4-(trifluoromethyl)coumarin, CAS# 53518-14-2) was
purchased from Exiton and used as received.
Solvents for spectroscopic measurements were HPLC grade acetonitrile and methanol
(
OmniSolv), spectroscopic grade ethylene glycol, dimethyl sulfoxide, propylene carbonate,
tetrahydrofuran, 1,4-dioxane, toluene, and cyclohexane (Sigma-Aldrich), and methyl acetate
Fluka). Series of two solvent mixtures, cyclohexane/tetrahydrofuran (CHEX/THF) and
(
cyclohexane/dibutyl ether (CHEX/DBE), were made volumetrically and compositions reported
as volume fraction, assuming incompressibility. All conventional solvents were tested for water
content using a Karl Fischer titrator. If the water content was less than 100 ppm, the solvent was
used as received. Otherwise, solvents were dried with molecular sieves overnight. Samples
were purged with nitrogen for three minutes before any experiments to prevent quenching by
dissolved O . Absorption and emission spectra were recorded for each solvent to confirm lack
2
absorbing impurities and minimal fluorescence in the visible region. Ionic liquid solvents 1-
6
ACS Paragon Plus Environment

Page 7 of 40
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide ([Pr41][Tf
2
N]),
N]), and
trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide ([P14,666][Tf N]) were
purchased from IoLiTec having stated purities of 98% but selected for high optical quality.
methyltrioctylammonium bis(trifluoromethylsulfonyl)imide ([N8881][Tf
2
2
6
3
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
These ionic liquids were purified using the methods described in previous work in order to
minimize impurity emission overlapping with the S
0
-S absorption band of the solutes.
1
B. Spectroscopic Methods
Steady State Absorption and Emission: All steady state absorption measurements were made in
Parafilm-sealed 1-cm path length quartz cuvettes on a Hitachi UV-3010 UV-Vis
spectrophotometer. Solvent spectra were measured and subtracted from all sample spectra.
Fluorescence spectra were collected at 90° and 2 nm resolution using a SPEX Fluorolog 212
fluorimeter, calibrated with a set of emission standards using previously described methods.64
Emission spectra were excited at 400 nm and collected over the range 410 - 700 nm.
Fluorescence samples were also made in sealed 1 cm quartz cuvettes. Solute concentrations
were chosen to provide optical densities at 395 nm of ~1.0 for absorption and ~0.15 for emission
measurements.
Time Correlated Single Photon Counting (TCSPC): Time-correlated single photon counting
(TCSPC) was used to measure lifetimes in the ns-ps range. Instrumental details can be found in
previous work.65 For the present experiments, excitation pulses were provided by the doubled
output of a cavity-dumped Ti:Sapphire oscillator (Coherent Mira 900+PulseSwitch) tuned to 800
nm. All TCSPC samples were measured in sealed 1 cm quartz cuvettes. Emission decays at
selected wavelengths (4 nm resolution) were collected in a 90° geometry at magic angle
polarization. Stray excitation light was suppressed using a Kopp 3-73 filter. Use of a scattering
sample showed the instrument response function (IRF) to be approximately 25 ps FWHM in all
experiments.
Kerr-Gated Emission (KGE) Spectroscopy: For samples with decay times faster than 50 ps,
time-resolved fluorescence spectra were collected using a Kerr-gated emission instrument, which
was described in earlier work.66-67 Each sample was flowed through a 0.2 mm sample cell where
it was excited with 388 nm pulses from the frequency doubled output of an amplified
7
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 8 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Ti:Sapphire system (Coherent Verdi G18/Mira 900/RegA 9050). The emission spectrum
between 408 to 699 nm was collected at each time step (-5 to 5 ps in steps of 100 fs, from 5 to 50
ps in steps of 200 fs, and from 50 ps to 300 ps in 2 ps steps) using an Acton SpectraPro-300i
grating spectrograph and a liquid-nitrogen cooled CCD (1340 pixels, Princeton instruments).
The FWHM of the fitted instrument response was approximately 400 fs. These experiments
were performed in triplicate and the kinetic parameters averaged in producing the results
reported. Deconvolution of the instrument response function from the spectra was accomplished
using a global convolute-and-compare algorithm.66-67 Fits to log-normal functions were made
after deconvolution to quantify the spectral dynamics.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Temperature Control: Most of the experiments were performed at room temperature, 293 ± 1 K.
Steady state fluorescence and TCSPC measurements in ionic liquids were also performed over a
range of temperatures between 213 - 353 K. For temperatures above 273 K, regulation was
achieved using water from a recirculating water bath flowing through a brass cuvette holder. For
lower temperatures, an Oxford variable temperature liquid nitrogen cryostat (model DN1754)
was used. All samples were equilibrated at each temperature for at least thirty minutes prior to
data collection.
C. Computational Methods:
Molecular dynamics simulations of the dyad in vacuum and acetonitrile (ACN) solution were
6
8
69
performed using the GROMACS 2018.1 package and the GAFF force field. Atomic charges
70
were obtained from a CHELP fit of the electrostatic potential generated from an optimized
structure at the B3LYP/6-311G++ level using the Gaussian09 package.71 Charges on equivalent
atoms were symmetrized following the CHELP fit. Topology files were generated using
7
2
73
AmberTools17 and the acpype program. Simulations in ACN consisted of one dyad
molecule and 2450 ACN molecules in a ~6 nm cubic box with cubic periodic boundary
conditions. Integration was carried out using the Verlet leapfrog algorithm with a step size of 2
7
4
fs. The particle-mesh Ewald method was used to handle the long-range electrostatics and non-
bonded interactions were calculated using a Verlet neighbor list with a 1.4 nm cutoff. The P-
LINCS algorithm was used to constrain all hydrogen-containing bonds.75 The same methods
were applied to simulate behavior in the gas phase simply by omitting the ACN molecules.
Following a short energy minimization procedure, the simulation box was equilibrated
8
ACS Paragon Plus Environment

Page 9 of 40
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
using three consecutive 500 ps simulations in NVT, NPT, and NVT ensembles. The pressure of
the NPT simulation was set to 1 bar and regulated using a Berendsen barostat with a relaxation
time of 5 ps, whereas the temperature was 293.15 K and regulated using a modified Berendsen
thermostat with a relaxation time of 0.5 ps.76 Production NVT simulations were then carried out
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
5
for 50 ns with dyad coordinates saved every 0.1 ps, giving a total of 5×10 structures.
III. Results and Discussion
A. Molecular Dynamics Simulations
The methylene linkage chosen to connect the donor and acceptor groups might be
expected to lead to significant intramolecular flexibility in the C152-DMA dyad. To assess the
conformations present in the ground state, two approaches were used. First, attempts were made
7
7
with the conformational search algorithms available in MacroModel 11, followed by DFT
calculations of selected conformers. However, it became clear from such calculations that the
dyad structure near room temperature is not adequately described by a small number of well-
defined conformers. For this reason, classical MD simulations were performed both in
acetonitrile and the gas phase to provide more appropriate sampling of conformational space.
Three dihedral angles, defined in Figure 1(a), were used to coarsely specify the
geometries of the dyad observed in these simulations. Several representations of the
distributions observed for these three angles are shown in Figures 1(b,c) and Figure 2. There
are eight heavily populated regions in this conformational space, defined by population minima
approximately located by planes at D1 = ±90°, D2 = 0, and D3 = 0. For simplicity, if one uses
the 2-fold rotational symmetry about D1 to fold populations into the range -90°≤D1≤90°, the
four occupied regions of conformational space depicted in Figure 2 result. As shown by the
representative structures in Figure 2, all four heavily populated regions involve structures in
which the donor and acceptor aromatic planes are nearly orthogonal to one another. Comparison
of the structures observed in the gas- and solution-phase simulations (Figure 1(b) and Figure S3)
show them to be quite similar. The primary difference is that conformer distributions are slightly
more localized in solution, particularly with respect to the D2 angle.
The time evolution of the three dihedral angles (Figure S4) shows that the dyad samples
the conformational space within one of these four regions on a picosecond time scale and makes
9
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 10 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
transitions among regions on a much longer time scale. Specifically, in acetonitrile solution, fast
fluctuations within each region are such that D1-D3 vary by ±10-15° (1 σ) when viewed in 10 ps
windows. Transitions among conformer regions take place via ~180° jumps, which occur on
average once every 0.7 ns. Interconversion among conformers is faster in the gas-phase, as
might be expected given the absence of intermolecular friction. The remainder of this discussion
will consider only the more relevant solution-phase results.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
The presence of a distribution of conformers implies electron transfer will be initiated
from a distribution of ground-state geometries. This geometric variability should result in
distributions over the key parameters controlling electron transfer rates, namely the driving force,
reorganization energy, and most importantly, the electronic coupling. Rather than attempt
detailed quantum mechanical calculation of these quantities, two simple surrogates, the donor-
acceptor (center-of-mass) distance RDA, and the angle θ
donor and acceptor were monitored. In Figure 3(a, b) the distributions of these quantities are
plotted as the blue curves. Average values are RDA = 0.80 ± .08 nm and θ = 90 ± 23° (1σ).
These averages, and the way that RDA and θ are distributed, are remarkably similar in all four of
π
between the aromatic planes of the
π
π
the conformer regions, as shown in Figure S5. Thus, it seems reasonable to conclude that the
existence of these four slowly interconverting populations of conformers will have little effect on
π
the electron transfer kinetics observed. However, the variations of RDA and θ within each region
are not small and may be expected to give rise to distributed electron transfer kinetics.
To examine this possibility further, the main effect on electron transfer kinetics is
assumed to result from geometric variations modulating the electronic coupling, modeled as
2
functions of θ
π
and RDA using the model function k ∝ cos θ ⋅exp(−βR ). The electron
ET
π
DA
-1
coupling decay contant β is assumed to have a value of 15 nm , a value typical of condensed
media.30, 78 The dashed red curves in Figure 3 show how these model dependences samples the
distributions of conformer geometries. In both cases, the largest values of kET come from regions
π
well removed from the most probable conformations. Taking the RDA and θ variations
2
separately and correlating fluctuations in exp{−βR (t)} and cos θ (t) over the course of the
DA
π
simulation leads to the normalized time correlation functions shown in panels (c) and (d). These
functions both decay rapidly, with correlation times of ~5 ps. These times suggest that the
distribution of conformers present in C152-DMA should not lead to distributed kinetics for
10
ACS Paragon Plus Environment

Page 11 of 40
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
reactions taking place on time scales of 100 ps or more. For such reactions, homogeneous
kinetics of the geometrically averaged structures is expected. For faster reactions, some
distributed character and perhaps gating of reaction by conformational fluctuations are
anticipated.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
B. Steady-State Characterization
Steady-state absorption and emission spectra of the dyad and the reference chromophores
C152 and H152 were measured in a variety of solvents and solvent mixtures to characterize their
polarity dependence. Relevant data are summarized in Table S1. We note that whereas several
solvatochromic studies of C152 have already been reported,79-81 no such information is available
for H152.
Figure 4 compares absorption and emission spectra of these three solutes in the nonpolar
solvent cyclohexane and the highly polar solvent acetonitrile. In cyclohexane, the spectra of all
three molecules exhibit similar vibronic structure, and their absorption and emission spectra are
offset by comparable Stokes shifts. In acetonitrile, all spectra are broadened, and the Stokes
shifts are increased by solvation, but the similarities in spectral shape among the three are
maintained. These similarities suggest that the coumarin aromatic system is the absorbing
chromophore in all three cases, and that excitation of the dyad leads to a locally excited (LE)
state similar to those of other 7-amino substituted coumarins.82 Because the frequencies of the
and chromophore redox properties of the dyad are expected to be closer to those of C152, we
will use C152 as a reference for the photophysics expected in the absence of electron transfer.
Absorption and emission spectra of the dyad in solvents having a range of polarities are
shown in Figure 5. As solvent polarity increases, both the absorption and emission spectra red
shift and broaden. The emission shifts are considerably larger, due to the well-known increase in
dipole moment in S
1
compared to S , characteristic of this type of coumarin. Similar solvent
0
dependence is found for C152 and H152 (Table S1). One feature of the dyad emission of note is
the increased noise observed in higher polarity solvents (MeAC and ACN in Figure 5). This
noise reflects a roughly ~500-fold reduction in the dyad’s fluorescence quantum yield with
increasing solvent polarity. Such strong quenching is absent in C152 and H152 and is the first
indication that the anticipated electron transfer is occurring in the dyad.
In Figure 6, absorption and emission frequencies of the dyad are compared to those of
11
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 12 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
C152. The excellent correlations shown here underscore the similarity of the dyad LE state to
the S state of C152. However, whereas the transition energies of C152 and the dyad are linearly
1
related, the slopes in Figure 6 (0.71 and 0.64) are not close to unity. These slopes indicate the
dyad has only 2/3 of the solvent sensitivity of C152, which could be indicative of significantly
reduced charge separation in the LE state compared to C152. However, further analysis of the
solvatochromic data and electronic structure calculation indicate otherwise.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Estimates of the dipole differences between the absorbing (S
0
) and emitting (S
1
or LE)
states, ꢀꢁ = | ꢁ⃗ (ꢂ ) − ꢁ⃗ (ꢂ )|, were made using a Lippert-Mataga analysis of the Stokes shifts of
1
0
all three solutes. Modeling the system as a solute consisting of a polarizable point dipole
centered in a spherical cavity of radius a and the solvent as a dielectric continuum, such a model
predicts
(ꢅꢆ)ꢇ
2
ꢃabs − ꢃem = (ꢃabs − ꢃ )
em gas
+
ꢀꢉ.
4πε )ℎ ꢈ3
ꢄ
(1)
(
where ∆f is the solvent orientational polarizability,
ꢇ
ꢊꢋꢌ1
ꢍ ꢌ1
ꢎ
ꢀꢉ =
−
.
(2)
ꢇ
ꢊꢋ+2
ꢍ +2
ꢎ
with ε
r
the relative permittivity and n
D
the refractive index of the solvent. Because the solutes
examined here are not spherical in shape, there is no unambiguous way to assign an appropriate
8
3
value of the cavity radius a. Previous work on the similar chromophore coumarin 153 showed
that an effective cavity radius based on the van der Waals volume of the solute extended by 50%,
to account for the average size of solvent layer, provided estimates of ∆µ in line with electronic
structure calculations and more definitive experiments. Adopting this approach, the slopes of the
solvatochromic plots in Figure 7 yield the dipole difference estimates in Table 1. Insofar as this
choice of aeff is reasonable, these estimates show the solvatochromic data are consistent with the
charge shift between S
0
and the LE state of the dyad being similar to the S
1
-S difference in
0
C152. The value of ∆µ estimated for H152 is slightly smaller, as would be expected upon
replacing an electron donating alkyl group on the amine with a hydrogen atom.
For comparison, Table 1 also lists dipole moments calculated at the CAM-B3LYP/6-
3
11++G(d,p) level in the gas phase. Calculations of the dyad were performed on a number of
representative conformers in optimized S geometries, and the range of values for these
conformers are tabulated. These calculations confirm minimal mixing between the DMA and
coumarin portions of the molecule exists in either the S or (vertical) S /LE states, as well as the
0
0
1
12
ACS Paragon Plus Environment

Page 13 of 40
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
similarity of the charge shift in the S
the presence of a charge-transfer state of the dyad, as indicated by the large dipole moments of
the vertical S states of all conformers studied. The frontier molecular orbitals and some
0
→ S transitions of the dyad and C152. They also confirm
1
2
additional characteristics of the LE and CT states are provided in Figure S6.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
C. Time-Resolved Measurements
Representative fluorescence decays of the dyad are shown in Figure 8 and selected multi-
exponential fits are provided in Table 2. The decays in Figure 8 were all recorded using
TCSPC, which has an instrumental response time of ~25 ps. Five of the highest polarity
solvents, represented by acetonitrile (ACN) and methanol (MeOH) in Figure 8, have lifetimes
shorter than 25 ps, and these solvents were also measured using Kerr Gated Emission (KGE)
spectroscopy. Fits of the KGE data are compiled in Table 2B. Finally, emission decays of the
reference chromophores C152 and H152 were measured in all solvents, and their lifetimes are
also provided in Table 2.
84
With few exceptions the decays of C152 and H152 are well described by single
exponential functions. In contrast, in the case of the dyad, exponential or nearly exponential
decays are only observed in some of the least polar solvents, for example cyclohexane (CHEX)
and dibutyl ether (DBE) shown in Figure 8. In the majority of solvents, dyad emission is
significantly non-exponential, requiring 3-4 components for accurate representation. In many
solvents, the longest component has an amplitude of a few percent and a lifetime of several
nanoseconds, close to that of H152 in the same solvent. In the highest polarity solvents, such as
acetonitrile and methanol in Figure 8, this long component is readily distinguished, because it
decays on a much longer time constant than all other components. These nanosecond tails are
attributed to the presence of residual H152 impurity in the dyad samples. As discussed in the
Supporting Information, H152 is both a precursor in the dyad synthesis and is produced by acid-
catalyzed heterolysis of the dyad. In most solvents, the amplitude of this longest component is
<5 %. When such a small and well-separated component was obtained from unconstrained fits,
the TCSPC data were refit after constraining the lifetime of this component to be equal to the
H152 lifetime. These constrained fits are what are reported in Table 2A, and when calculating
average dyad lifetimes and rates, these putative impurity components are omitted.
Representative KGE data of the dyad in propylene carbonate are shown in Figure 9. With
13
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 14 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
the better time resolution of the KGE technique (~400 fs IRF), one observes the dyad emission to
undergo a dynamic Stokes shift. The peak frequencies of these spectra could be fit with bi-
exponential functions having times constants in the few picoseconds range. The average times
2
of these shifts are reasonably correlated (R =0.74, N=5) to the average times measured for the
nonreactive solute coumarin 153.83 As illustrated by the data in Figure 9(c), the time dependence
of the peak height and integrated intensity of the emission are quite similar. In all cases, the
intensity data could be represented by two primary components plus a slower component in the
tens of picoseconds range having an amplitude of ≤6% (Table 2B). Due to the lower signal-to-
noise and time range of the KGE experiment, the small (<3%) nanosecond components found in
the TCSPC data of these solvents are not observed.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
The non-exponential emission decays found for the dyad complicate description of the rate
of electron transfer and its solvent dependence. Rather than attempt any detailed characterization
of the distributed kinetics here, two limiting rate constants were extracted from the multi-
exponential fits of the decay data. Using the lifetime of C152 (τC152 ) to estimate the dyad
lifetime in the absence of electron transfer, the rate constants
(
ET
1)
ꢏ
= 1⁄⟨ꢐ⟩dyad − 1⁄ꢐꢑ152
⟨ꢐ⟩dyad = ∑ ꢒ ꢐ ⁄∑ ꢒ
ꢓ
(3)
(4)
ꢓ ꢓ
and
(
ET
2)
ꢏ
= ⟨ꢏ⟩dyad − 1⁄ꢐꢑ152
⟨ꢏ⟩dyad = ∑ ′(ꢒ ⁄ꢐ )⁄∑ ꢒ
ꢓ
ꢓ
ꢓ
were calculated. The symbol Σ′ in these equations indicates omission of small-amplitude (<6%)
(1)
(2)
impurity components. One can view ꢏ and ꢏET as providing approximate lower and upper
ET
bounds to the distribution of rates present.
Before discussing these rates, it should be mentioned that C152 undergoes a twisted
intramolecular charge-transfer (TICT) process in high polarity solvents.79-82, 85 It might be
thought that this process would render C152 unsuitable as a lifetime reference for the dyad.
This issue is discussed in detail in the Supporting Information (Section B) where it is shown that
the effect of the TICT process should have a negligible effect on the ET rates reported here.
Figure 10 plots the observed rate constants versus the steady-state emission frequency of
the dyad, <νem>, which is used here as a measure of effective solvent polarity. A similar
correlation is found when these rates are plotted versus the dielectric measure of total solvent
polarizability (ꢔ − 1 )⁄( ꢔ + 2), as shown in Figure S8. The filled symbols in Figure 10 are
ꢕ
ꢕ
14
ACS Paragon Plus Environment

Page 15 of 40
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
(1)
(2)
ET
the logarithmic averages of ꢏET and ꢏ , and the limits of the error bars serve to denote their
individual values. Although there is considerable scatter and, in some cases a large difference
(
1)
(2)
between ꢏ and ꢏET in these data, a clear correlation exists between kET and <νem> in most
ET
solvents. A substantial dependence of the electron transfer rate on solvent polarity of this sort is
expected given the large difference in charge separation and thus dipole moments of the LE and
CT states of the dyad (~20 D based on the calculations in Table 1). It seems reasonable to
attribute the increasing rate with increasing solvent polarity as being primarily due changes in the
reactive driving force, assuming that the electron transfer lies within the normal regime.
In a number of solvents, labeled in Figure 10, values of kET appear to be smaller than
expected based on the relationship established by the majority of solvents. With the exception of
toluene, these solvents all have much higher viscosities than do the remaining solvents. (See
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table S1.) For example, the two ionic liquids near the bottom of the figure, [P14,666][Tf
2
N] and
-1
[
N
8881][Tf
2
N], have viscosities >500 mPa s near room temperature, and in these solvents
electron transfer is too slow to measure. (This situation persists even up to 350 K, as discussed
in SI Section C.) These observations suggest that, in addition to polarity, some other factor such
as solvent friction plays a role in determining the rate of electron transfer.
Support for friction being relevant to this reaction can be found in temperature-dependent
TCSPC measurements of the dyad in the ionic liquid [Pr41][Tf N], which are shown in Figure
2
11. Over the 25 K temperature range (278-303 K) the ionic liquid’s polarity (as judged by
<νem>) varies negligibly, whereas the viscosity decreases by nearly a factor of four and kET
doubles. An Arrhenius analysis of the data, shown in the inset to Figure 11, yields an activation
14 -1
energy of 21 kJ/mol and a prefactor of 1.3×10 s . This prefactor is too high to be interpreted in
terms of a reactive frequency in such a reaction. A more realistic description is a reaction which
−
p
is nearly activationless but has a frictional prefactor proportional to viscosity, η , with p~0.6.
This dependence on solvent viscosity could either reflect some large-amplitude solute motion
required for reaction, or a dependence on solvation time, which approximately tracks viscosity.
As discussed in Section D of the Supporting Information, the present data are unable to
distinguish between these two possibilities.
It is interesting to compare the electron transfer rates measured for C152-DMA dyad to
the similar intramolecular dyads studied by Castner, Wishart and coworkers,51-52 which were
15
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 16 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
constructed from an aromatic amine donor and a coumarin 343 photoacceptor linked by one (P1)
or two (P2) proline groups. Electron transfer in both the P1 and P2 molecules is much slower
than observed for the C152-DMA dyad. Focusing on the simpler P1 dyad,51-52 average rate
(1)
(2)
9
−1
constants calculated using Equations 3 and 4 provide values (k | k ) /10 s of (1.6 | 2.8) in
ET
ET
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
acetonitrile, (1.6 | 3.5) in methanol, and (0.73 | 1.4) in [Pr41][Tf
Table 2 shows electron transfer in the C152-DMA dyad are 100-1000 times faster in the two
conventional solvents and 10-40 times faster in the ionic liquid [Pr41][Tf N]. Differences in both
2
N] at 293 K. Comparison to
2
the electronic coupling and the redox properties of the donor and acceptor are likely responsible
for the different kinetics in these two dyads. For example, the free energy change in the C152 +
15
DMA bimolecular reaction in acetonitrile is estimated to be about -0.43 eV, and estimates for
the C152-DMA dyad are not expected to differ appreciably from this estimate. In contrast,
reaction free energies of the P1 and P2 dyads in acetonitrile were estimated to be -1.6 eV, 51-52
much more negative and the driving force much greater than in the C152-DMA dyad. The P1
and P2 dyads were thought to occur well within the Marcus inverted regime, whereas the C152-
DMA reaction is more likely to fall within the normal regime in most solvents.
Comparison can also be made between the present results and rates measured for the
equivalent bimolecular reaction between C152 and DMA. Most desirable for such a comparison
are effective unimolecular reaction rates between C152 and a single proximal DMA molecule.
The rate of this idealized process can be estimated from previously reported data only by making
some significant assumptions. Fewest assumptions are required for interpreting the quenching
dynamics of C152 in neat liquid DMA, which has been reported by two groups.24-25 Both groups
(
2)
reported quite similar bi-exponential fluorescence decays with initial rates (
k
, Eq. 4) of
ET
1
2 -1
(
2.0±.2)×10 s . Assuming reaction occurs between C152 and only one of the approximately
1
0 DMA molecules within its first solvation shell25, 37 acting independently, the rate constant for
(2)
11 -1
comparison to the dyad data is about 10-fold smaller, kET ~2×10 s . Measurements of
concentrated DMA solutions, where most reaction is expected to occur between preformed
ground-state C152/DMA pairs, also provide similar estimates. In solutions of >0.5 M DMA
2
5
25
86
20
mixed with toluene, chlorobenzene, cyclohexane, and two ionic liquids , one finds values
(
ET
2)
11 -1
of
k
in the range (1-5)×10 s . In both the neat and mixed solvent cases, these effective
unimolecular rate constants provide estimates of rates associated with reaction between a single
16
ACS Paragon Plus Environment

Page 17 of 40
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
C152 + DMA contact pair averaged over relative positions and orientations of first solvation
2
5
shell neighbors. The polarities of neat DMA (ε
r
=5.0 ) and these concentrated DMA mixtures
are expected to fall roughly in the middle of the range studied, where rates measured for the dyad
1
0 -1
are closer to 10 s . Thus, the C152-DMA dyad appears to provide an example of an
intramolecular electron transfer having electron transfer rates only slightly slower than average
rates of contact pairs in the intermolecular process it was intended to mimic.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
IV. Conclusions
This work documented the synthesis and photophysical properties of a new electron
donor-acceptor dyad, C152-DMA, made by covalently linking the photo-acceptor C152 and the
DMA donor via a single methylene bridge. Although the simple linker employed here confers
significant conformational flexibility to the dyad, molecular dynamics simulations suggest that
this flexibility only causes electron transfer rates to be distributed over a modest range.
Apart from the rapid fluorescence quenching caused by electron transfer, the absorption
and emission characteristics of the C152-DMA dyad are very similar to those of the reference
compounds C152 and H152, demonstrating weak substantial coupling between the S and LE
0
states of the dyad. The coumarin chromophore of the dyad and its references are all strongly
solvatochromic, as are most 7-aminocoumarins.
Time-resolved emission of the dyad was found to be multi-exponential indicating a
distributed character to the electron transfer reaction in most solvents. Conformational and
solvation dynamics are likely contributors to this distributed character, especially in high polarity
solvents where reaction occurs on the same time scales as both conformational relaxation and
solvation. Average rate constants in conventional solvents are strongly dependent on solvent
8
-1
12 -1
polarity. Rates are near 10 s in nonpolar solvents, increasing to ~10 s in the most polar
solvents. This variation is most likely the result of the increasing driving force for charge
separation with increasing solvent polarity. Absence of observable quenching in two high-
viscosity ionic liquids, together with the temperature dependence observed in a low-viscosity
ionic liquid, suggest that solvent friction also plays an important role in determining dyad
electron transfer rates. Further study is needed to determine whether this frictional effect is due
to a dependence on solvation times, as found in several other intramolecular cases40, 49, 53-54 or
17
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 18 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
perhaps due to the slowdown of conformational sampling with increasing viscosity.
The rates of electron transfer observed in the C152-DMA dyad are considerably faster
than rates previously reported for the related coumarin - aromatic amine dyads studied by
Castner, Wishart and coworkers.51-52 C152-DMA reaction rates appear to be closer to those
between contact DMA + C152 pairs, estimated from intermolecular quenching experiments. The
latter similarity suggests the C152-DMA dyad is an apt intramolecular analog of the bimolecular
process. Although its sensitivity to acid-catalyzed hydrolysis limits its use to only highly
purified ionic liquids, the C152-DMA dyad should nevertheless provide a useful new probe for
comparisons between electron transfer in ionic liquids and conventional solvents.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Acknowledgements: The authors thank Danny Sykes for help with attempts to quantify
impurities in dyad samples using HPLC. This work was supported by grants from the U.S.
Department of Energy, Office of Basic Sciences, Division of Chemical Sciences, Geosciences,
and Biosciences under contract nos. DE-SC0008640 and DE-SC0019200 (Penn State: MS and
CAR), and by the Research Corporation for Science Advancement (GAB).
Supporting Information: Mechanism and illustration of the acid-catalyzed hydrolysis of the
dyad; comparison of dyad and H152 lifetimes; dyad conformational distributions in the gas-
phase and acetonitrile; time dependence of the dyad dihedral angles D1-D3 in acetonitrile;
distributions of dyad interplanar angle and donor-acceptor distance from different conformer
regions; table of solvent properties and characteristics of the spectra of C152, H152, and the
C152-DMA dyad; frontier orbitals and some properties of the LE and CT states of C152-DMA;
lifetimes of C152, H152, and the dyad vs. dyad emission frequency; dyad electron transfer rate
constants vs. total solvent polarizability; steady-state emission spectra of C152 and the dyad in
[
[
P
14,666][Tf
2
N]; solvent viscosities, and emission frequencies of C152 and the dyad in
N]; table of fits to viscosity data in the ionic liquids; average
N]; Estimated frictional
N
8881][Tf N] and [P14,666][Tf
2
2
lifetimes of C152 and the dyad in [N8881][Tf
2
N] and [P14,666][Tf
2
component of kET versus viscosity and solvation time.
18
ACS Paragon Plus Environment

Page 19 of 40
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
19
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 20 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
References:
1
. Jortner, J.; Bixon, M., Electron Transfer- From Isolated Molecules to Biomolecules, Parts 1-
. [In: Adv. Chem. Phys., 1999; 106]. John Wiley & Sons: 1999.
2
2
. Balzani, V., Electron Transfer in Chemistry, Volumes 1-5. Wiley-VCH Verlag GmbH: 2001.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3. Tominaga, K.; Walker, G. C.; Jarzeba, W.; Barbara, P. F., Ultrafast charge separation in
ADMA: experiment, simulation, and theoretical issues. J. Phys. Chem. 1991, 95, 10475-
1
0485.
4. Heitele, H., Dynamic solvent effects on electron-transfer reactions. Angew. Chem. Int. Ed.
Engl. 1993, 32, 359-377.
5
6
7
8
. Kumpulainen, T.; Lang, B.; Rosspeintner, A.; Vauthey, E., Ultrafast Elementary
Photochemical Processes of Organic Molecules in Liquid Solution. Chemical Reviews
(Washington, DC, United States) 2017, 117, 10826-10939.
. Miller, J. R.; Calcaterra, L. T.; Closs, G. L., Intramolecular long-distance electron transfer in
radical anions. The effects of free energy and solvent on the reaction rates. J. Am. Chem. Soc.
1984, 106, 3047-3049.
. Penfield, K. W.; Miller, J. R.; Paddon-Row, M. N.; Cotsaris, E.; Oliver, A. M.; Hush, N. S.,
Optical and thermal electron transfer in rigid difunctional molecules of fixed distance and
orientation. J. Am. Chem. Soc. 1987, 109, 5061-5065.
. Napper, A. M.; Head, N. J.; Oliver, A. M.; Shephard, M. J.; Paddon-Row, M. N.; Read, I.;
Waldeck, D. H., Use of U-shaped Donor-Bridge-Acceptor Molecules To Study Electron
Tunneling through Nonbonded Contacts. J. Am. Chem. Soc. 2002, 124, 10171-10181.
9
. Zimmt, M. B.; Waldeck, D. H., Exposing Solvent's Roles in Electron Transfer Reactions:
Tunneling Pathway and Solvation. J. Phys. Chem. A 2003, 107, 3580-3597.
1
0. Natali, M.; Campagna, S.; Scandola, F., Photoinduced electron transfer across molecular
bridges: electron- and hole-transfer superexchange pathways. Chem. Soc. Rev. 2014, 43,
4
005-4018.
1
1. Gilbert, M.; Albinsson, B., Photoinduced charge and energy transfer in molecular wires.
Chem. Soc. Rev. 2015, 44, 845-862.
12. Wasielewski, M. R., Energy, Charge, and Spin Transport in Molecules and Self-Assembled
Nanostructures Inspired by Photosynthesis. J. Org. Chem. 2006, 71, 5051-5066.
13. Mora, S. J.; Odella, E.; Moore, G. F.; Gust, D.; Moore, T. A.; Moore, A. L., Proton-Coupled
Electron Transfer in Artificial Photosynthetic Systems. Acc. Chem. Res. 2018, 51, 445-453.
1
1
4. Whang, D. R.; Apaydin, D. H., Artificial Photosynthesis: Learning from Nature.
ChemPhotoChem 2018, 2, 148-160.
5. Nad, S.; Pal, H., Electron Transfer from Aromatic Amines to Excited Coumarin Dyes:ꢀ
Fluorescence Quenching and Picosecond Transient Absorption Studies. J. Phys. Chem. A
2
000, 104, 673-680.
20
ACS Paragon Plus Environment

Page 21 of 40
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
1
1
6. Rosspeintner, A.; Koch, M.; Angulo, G.; Vauthey, E., Spurious Observation of the Marcus
Inverted Region in Bimolecular Photoinduced Electron Transfer. J. Am. Chem. Soc. 2012,
1
34, 11396-11399.
7. Sarkar, S.; Pramanik, R.; Seth, D.; Setua, P.; Sarkar, N., Photoinduced electron transfer
PET) from N,N-dimethylaniline to 7-amino Coumarin dyes in a room temperature ionic
(
liquid (RTIL): Slowing down of electron transfer rate compared to conventional solvent.
Chem. Phys. Lett. 2009, 477, 102-108.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
1
8. Das, A. K.; Mondal, T.; Sen, M. S.; Bhattacharyya, K., Marcus-like Inversion in Electron
Transfer in Neat Ionic Liquid and Ionic Liquid-Mixed Micelles. J. Phys. Chem. B 2011, 115,
4
680-4688.
9. Yuan, S.; Zhang, Y.; Lu, R.; Yu, A., Photoinduced electron transfer between coumarin dyes
and N,N-dimethylaniline in imidazolium based room temperature ionic liquids: Effect of the
cation's alkyl chain length on the bimolecular photoinduced electron transfer process. J.
Photochem. Photobiol., A 2013, 260, 39-49.
2
2
0. Kumbhakar, M.; Manna, A.; Sayed, M.; Kumar, A.; Pal, H., Observation of the Marcus
Inverted Region for Bimolecular Photoinduced Electron-Transfer Reactions in Viscous
Media. 118 2014, 36.
1. Chakraborty, A.; Chakrabarty, D.; Hazra, P.; Seth, D.; Sarkar, N., Photoinduced
intermolecular electron transfer between Coumarin dyes and electron donating solvents in
cetyltrimethylammonium bromide (CTAB) micelles: evidence for Marcus inverted region.
Chem. Phys. Lett. 2003, 382, 508-517.
2
2. Chakraborty, A.; Chakrabarty, D.; Seth, D.; Hazra, P.; Sarkar, N., Photoinduced
intermolecular electron-transfer from electron donating solvents to Coumarin dyes in bile salt
aggregates: Role of diffusion in electron transfer reaction. Spectrochim. Acta, Part A 2006,
63A, 594-602.
2
2
3. Yoshihara, K.; Tominaga, K.; Nagasawa, Y., Effects of the solvent dynamics and vibrational
motions in electron transfer. Bull. Chem. Soc. Jpn. 1995, 68, 696-712.
4. Shirota, H.; Pal, H.; Tominaga, K.; Yoshihara, K., Substituent Effect and Deuterium Isotope
Effect of Ultrafast Intermolecular Electron Transfer: Coumarin in Electron-Donating
Solvent. Journal of Physical Chemistry A 1998, 102, 3089-3102.
2
2
5. Castner, E. W. J.; Kennedy, D.; Cave, R. J., Solvent as Electron Donor: Donor/Acceptor
Electronic Coupling Is a Dynamical Variable. J. Phys. Chem. A 2000, 104, 2869-2885.
6. Gordon, C. M.; McLean, A. J., Photoelectron transfer from excited-state ruthenium(II)
tris(bipyridyl) to methylviologen in an ionic liquid. Chemical Communications 2000, 1395-
1
396.
27. McLean, A. J.; Muldoon, M. J.; Gordon, C. M.; Dunkin, I. R., Bimolecular rate constants for
diffusion in ionic liquids. Chem. Commun. 2002, 0, 1880-1881.
28. Skrzypczak, A.; Neta, P., Diffusion-controlled electron-transfer reactions in ionic liquids. J.
Phys. Chem. A 2003, 107, 7800-7803.
21
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 22 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2
3
9. Vieira, R. C.; Falvey, D. E., Photoinduced Electron-Transfer Reactions in Two Room-
Temperature Ionic Liquids: 1-Butyl-3-methylimidazolium Hexafluorophosphate and 1-Octyl-
3
-methylimidazolium Hexafluorophosphate. J. Phys. Chem. B 2007, 111, 5023-5029.
0. Liang, M.; Kaintz, A.; Baker, G. A.; Maroncelli, M., Bimolecular Electron Transfer in Ionic
Liquids: Are Reaction Rates Anomalously High? J. Phys. Chem. B 2012, 116, 1370-1384.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
31. Koch, M.; Rosspeintner, A.; Angulo, G.; Vauthey, E., Bimolecular Photoinduced Electron
Transfer in Imidazolium-Based Room-Temperature Ionic Liquids Is Not Faster than in
Conventional Solvents. J. Am. Chem. Soc. 2012, 134, 3729-3736.
32. Sarkar, S.; Mandal, S.; Pramanik, R.; Ghatak, C.; Rao, V. G.; Sarkar, N., Photoinduced
Electron Transfer in a Room Temperature Ionic Liquid 1-Butyl-3-methylimidizolium Octyl
Sulfate Micelle: A Temperature Dependent Study. J. Chem. Phys. B 2011, 115, 6100-6110.
33. Sarkar, S.; Mandal, S.; Ghatak, C.; Rao, V. G.; Ghosh, S.; Sarkar, N., Photoinduced Electron
Transfer in an Imidazolium Ionic Liquid and in Its Binary Mixtures with Water, Methanol,
and 2-Propanol: Appearance of Marcus-Type of Inversion. J. Phys. Chem. B 2012, 116,
1
335-1344.
34. Angulo, G.; Rosspeintner, A.; Koch, M.; Vauthey, E., Comment on "Observation of the
Marcus Inverted Region for Bimolecular Photoinduced Electron-Transfer Reactions in
Viscous Media". J. Phys. Chem. B 2016, 120, 9800-9803.
35. Kumbhakar, M.; Pal, H., Reply to "Comment on 'Observation of the Marcus Inverted Region
for Bimolecular Photoinduced Electron-Transfer Reactions in Viscous Media'". J. Phys.
Chem. B 2016, 120, 9804-9809.
36. Mukherjee, P.; Das, A.; Sengupta, A.; Sen, P., Bimolecular Photoinduced Electron Transfer
in Static Quenching Regime: Illustration of Marcus Inversion in Micelle. J. Phys. Chem. B
2
017, 121, 1610-1622.
37. Wu, B.; Maroncelli, M.; Castner, E. W., Photoinduced Bimolecular Electron Transfer in
Ionic Liquids. J. Am. Chem. Soc. 2017, 139, 14568-14585.
3
8. Wu, B.; Liang, M.; Zmich, N.; Hatcher, J.; Lall-Ramnarine, S. I.; Wishart, J. F.; Maroncelli,
M.; Castner, E. W., Jr., Photoinduced Bimolecular Electron Transfer in Ionic Liquids:
Cationic Electron Donors. J. Phys. Chem. B 2018, 122, 2379-2388.
3
9. Castner, E. W., Jr.; Margulis, C. J.; Maroncelli, M.; Wishart James, F., Ionic Liquids:
Structure and Photochemical Reactivity. Annual Reviews of Physical Chemistry 2011, 62, 85-
105.
4
4
0. Nagasawa, Y.; Miyasaka, H., Ultrafast solvation dynamics and charge transfer reactions in
room temperature ionic liquids. Phys. Chem. Chem. Phys. 2014, 16, 13008-13026.
1. Lockard, J. V.; Wasielewski, M. R., Intramolecular Electron Transfer within a Covalent,
Fixed-Distance Donor-Acceptor Molecule in an Ionic Liquid. J. Phys. Chem. B 2007, 111,
1
1638-11641.
4
2. Banerji, N.; Angulo, G.; Barabanov, I.; Vauthey, E., Intramolecular Charge-Transfer
Dynamics in Covalently Linked Perylene-Dimethylaniline and Cyanoperylene-
Dimethylaniline. J. Phys. Chem. A 2008, 112, 9665-9674.
22
ACS Paragon Plus Environment

Page 23 of 40
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
4
4
4
3. Khara, D. C.; Paul, A.; Santhosh, K.; Samanta, A., Excited state dynamics of 9,9'-bianthryl in
room temperature ionic liquids as revealed by picosecond time-resolved fluorescence study.
J, Chem. Sci. (Bangalore) 2009, 121, 309-315.
4. Nagasawa, Y.; Oishi, A.; Itoh, T.; Yasuda, M.; Muramatsu, M.; Ishibashi, Y.; Ito, S.;
Miyasaka, H., Dynamic Stokes Shift of 9,9'-Bianthryl in Ionic Liquids: A Temperature
Dependence Study. Journal of Physical Chemistry C 2009, 113, 11868-11876.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
5. Santhosh, K.; Banerjee, S.; Rangaraj, N.; Samanta, A., Fluorescence response of 4-(N,N'-
Dimethylamino)benzonitrile in room temperature ionic liquids: observation of
photobleaching under mild excitation condition and multiphoton confocal microscopic study
of the fluorescence recovery dynamics. J. Phys. Chem. B 2010, 114, 1967-1974.
4
4
6. Santhosh, K.; Samanta, A., Modulation of the Excited State Intramolecular Electron Transfer
Reaction and Dual Fluorescence of Crystal Violet Lactone in Room Temperature Ionic
Liquids. J. Phys. Chem. B 2010, 114, 9195-9200.
7. Wu, H.; Wang, H.; Xue, L.; Shi, Y.; Li, X., Hindered Intramolecular Electron Transfer in
Room-Temperature Ionic Liquid. J. Phys. Chem. B 2010, 114, 14420-14425.
48. Khurmi, C.; Berg, M. A., Dispersed Kinetics without Rate Heterogeneity in an Ionic Liquid
Measured with Multiple Population-Period Transient Spectroscopy. J. Phys. Chem. Lett.
2
010, 1, 161-164.
49. Li, X.; Liang, M.; Chakraborty, A.; Kondo, M.; Maroncelli, M., Solvent-Controlled
Intramolecular Electron Transfer in Ionic Liquids. J. Phys. Chem. B 2011, 115, 6592-6607.
5
0. Muramatsu, M.; Katayama, T.; Ito, S.; Nagasawa, Y.; Matsuo, D.; Suzuma, Y.; Peng, L.;
Orita, A.; Otera, J.; Miyasaka, H., Photoinduced charge-transfer dynamics of sequentially
aligned donor-acceptor systems in an ionic liquid. Photochem. Photobiol. Sci. 2013, 12,
1
885-1894.
51. Lee, H. Y.; Issa, J. B.; Isied, S. S.; Castner, E. W., Jr.; Pan, Y.; Hussey, C. L.; Lee, K. S.;
Wishart, J. F., A Comparison of Electron-Transfer Dynamics in Ionic Liquids and Neutral
Solvents. J. Phys. Chem. C 2012, 116, 5197-5208.
52. DeVine, J. A.; Labib, M.; Harries, M. E.; Rached, R. A. M.; Issa, J.; Wishart, J. F.; Castner,
E. W., Electron-Transfer Dynamics for a Donor-Bridge-Acceptor Complex in Ionic Liquids.
J. Phys. Chem. B 2015, 119, 11336-11345.
5
3. Takeuchi, E.; Muramatsu, M.; Katayama, T.; Yoneda, Y.; Ito, S.; Nagasawa, Y.; Miyasaka,
H., Sub-100 fs Charge Separation and Subsequent Diffusive Solvation Observed for
Asymmetric Bianthryl Derivative in Ionic Liquid. J. Phys. Chem. C 2016, 120, 14502-14512.
5
4. Rumble, C. A.; Maroncelli, M., Solvent controlled intramolecular electron transfer in
mixtures of 1-butyl-3-methylimidizolium tetrafluoroborate and acetonitrile. J. Chem. Phys.
2018, 148, 193801/193801-193801/193807.
5
5
5. Kaintz, A. Translational and Rotational Diffusion in Ionic Liquids. PhD, The Pennsylvania
State University, University Park, PA, 2013.
6. Kumbhakar, M.; Nath, S.; Mukherjee, T.; Pal, H., Intermolecular electron transfer between
coumarin dyes and aromatic amines in Triton-X-100 micellar solutions: Evidence for Marcus
inverted region. J. Chem. Phys. 2004, 120, 2824-2834.
23
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 24 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
5
7. Dutta Choudhury, S.; Kumbhakar, M.; Nath, S.; Sarkar, S. K.; Mukherjee, T.; Pal, H.,
Compartmentalization of reactants in different regions of sodium 1,4-bis(2-
ethylhexyl)sulfosuccinate/heptane/water reverse micelles and its influence on bimolecular
electron-transfer kinetics. J. Phys. Chem. B 2007, 111, 8842-8853.
5
8. Samanta, P.; Rane, S.; Bahadur, P.; Dutta Choudhury, S.; Pal, H., Tetronic Star Block
Copolymer Micelles: Photophysical Characterization of Microenvironments and
Applicability for Tuning Electron Transfer Reactions. J. Phys. Chem. B 2018, 122, 6079-
6093.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
5
6
6
9. Zhang, X.-X.; Liang, M.; Ernsting, N. P.; Maroncelli, M., Complete Solvation Response of
Coumarin 153 in Ionic Liquids. J. Phys. Chem. B 2013, 117, 4291-4304.
0. Jin, H.; Li, X.; Maroncelli, M., Heterogeneous Solute Dynamics in Room-Temperature Ionic
Liquids. J. Phys. Chem. B 2007, 111, 13473-13478
1. Sahu, K.; Kern, S. J.; Berg, M. A., Heterogeneous reaction rates in an ionic liquid:
quantitative results from two-dimensional multiple population-period transient spectroscopy.
J. Phys. Chem. A 2011, 115, 7984-7993.
62. Tsai, C.-S.; Liu, P.-Y.; Yen, H.-Y.; Hsu, T.-L.; Wong, C.-H., Development of trifunctional
probes for glycoproteomic analysis. Chem. Comm. 2010, 46, 5575-5577.
63. Jin, H.; O'Hare, B.; Dong, J.; Arzhantsev, S.; Baker, G. A.; Wishart, J. F.; Benesi, A.;
Maroncelli, M., Physical Properties of Ionic Liquids Consisting of the 1-Butyl-3-
Methylimidazolium Cation with Various Anions and the Bis(trifluoromethylsulfonyl)imide
Anion with Various Cations. J. Phys. Chem. B 2008, 112, 81-92.
64. Gardecki, J. A.; Maroncelli, M., A Set of Secondary Emission Standards for Calibration of
Spectral Responsivity in Emission Spectroscopy. Appl. Spectrosc. 1998, 52, 1179-1189.
6
6
6
6
5. Breffke, J.; Williams, B. W.; Maroncelli, M., The Photophysics of Three Naphthylmethylene
Malononitriles. J. Phys. Chem. B 2015, 119, 9254-9267.
6. Arzhantsev, S.; Maroncelli, M., Design and characterization of a femtosecond fluorescence
spectrometer based on optical Kerr gating. Appl. Spectrosc. 2005, 59, 206-220.
7. Rumble, C. A.; Breffke, J.; Maroncelli, M., Solvation Dynamics and Proton Transfer in
Diethylaminohydroxyflavone. J. Phys. Chem. B 2017, 121, 630-637.
8. Abraham, M. J.; Schultz, R.; Páll, S.; Smith, J. C.; Hess, B.; Lindahl, E., GROMACS: High
Performance Molecular Simulations through Multi-Level Parallelism from Laptops to
Supercomputers. Software X 2015, 1, 19-25.
6
9. Wang, J.; Wolf, R. M.; Caldwell, J. W.; Kollman, P. A.; Case, D. A., Development and
testing of a general Amber force field. J. Comput. Chem. 2004, 25, 1157-1174.
70. Chirlian, L. E.; Francl, M. M., Atomic charges derived from electrostatic potentials: a
detailed study. J. Comput. Chem. 1987, 8, 894-905.
7
1. Gaussian09, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H.; et al. (Gaussian,
Inc., Wallingford, CT, 2013).
24
ACS Paragon Plus Environment

Page 25 of 40
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
7
2. AMBER, Case, D.A.; Cerutti, D. S.; Cheatham, I.; Darden, T. A.; Duke, R. E.; Giese, T. J.;
Gohlke, H.; Goetz, A. W.; Green, D.; Homeyer N.; et al. (University of California, San
Francisco, 2017).
7
3. Sousa de Silva, A. W.; Vranken, W. F., ACPYPE - AnteChamber PYthon Parser InterfacE.
BMC Res. Notse 2012, 5.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
74. Essmann, U.; Perera, L.; Berkowitz, M. L.; Darden, T.; Lee, H.; Pedersen, L. G., A smooth
particle mesh Ewald method. J. Chem. Phys. 1995, 103, 8577-8593.
75. Hess, B., P-LINCS: A Parallel Linear Constraint Solver for Molecular Simulation. J. Chem.
Theory Comput. 2008, 4, 116-122.
7
7
7
6. Bussi, G.; Donadio, D.; Parrinello, M., Canonical sampling through velocity rescaling. J.
Chem. Phys. 2007, 126, 014101/014101-014101/014107.
7. MacroModel 11, Schrödinger Release 2019-2: MacroModel, Schrödinger, LLC, New York,
NY, 2019.
8. Wenger, O. S., How Donor-Bridge-Acceptor Energetics Influence Electron Tunneling
Dynamics and Their Distance Dependences. Acc. Chem. Res. 2011, 44, 25-35.
79. Guo, C.; Feng, Y., Solvent and substituent effects on intramolecular charge transfer of
selected derivatives of 4-trifluoromethyl-7-aminocoumarin. J. Chem. Soc., Faraday Trans. 1
1
987, 83, 2533-2539.
80. Rechthaler, K.; Koehler, G., Excited state properties and deactivation pathways of 7-
aminocoumarins. Chem. Phys. 1994, 189, 99-116.
8
8
8
1. Nad, S.; Kumbhakar, M.; Pal, H., Photophysical properties of Coumarin-152 and Coumarin-
481 dyes: unusual behavior in nonpolar and in higher polarity solvents. J. Phys. Chem. A
2
003, 107, 4808-4816.
2. Jones, G., II; Jackson, W. R.; Choi, C. Y.; Bergmark, W. R., Solvent effects on emission
yield and lifetime for coumarin laser dyes. Requirements for a rotatory decay mechanism. J.
Phys. Chem. 1985, 89, 294-300.
3. Horng, M. L.; Gardecki, J. A.; Papazyan, A.; Maroncelli, M., Subpicosecond Measurements
of Polar Solvation Dynamics: Coumarin 153 Revisited. J. Phys. Chem. 1995, 99, 17311-
17337.
8
8
4. In some mixtures and ionic liquids these fluorophores are show slightly non-exponential
decays, presumably due to some effect of solvation dynamics.
5. Dahiya, P.; Kumbhakar, M.; Mukherjee, T.; Pal, H., Effect of protic solvents on twisted
intramolecular charge transfer state formation in coumarin-152 and coumarin-481 dyes.
Chem. Phys. Lett. 2005, 414, 148-154.
8
6. Letrun, R.; Vauthey, E., Excitation Wavelength Dependence of the Dynamics of Bimolecular
Photoinduced Electron Transfer Reactions. J. Phys. Chem. Lett. 2014, 5, 1685-1690.
25
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 26 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 1: S
Calculations
1
-S
0
Dipole Difference Estimates from Solvatochromic Analysis and DFT
A. Solvatochromic Analysis
B. DFT Calculations

∆
ν / ∆f
aeff
/ Å
µ(S0)
µ(S1)
/ D
µ(S2)
/ D
| ∆µ |
/
∆µ
/ D
10
solute
3
-1
/
10 cm
/ D
D
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
C152
H152
dyad
3.08
2.43
1.94
5.43
5.28
6.35
7.0±.5
6.0±.5
7.0±.9
7.79
7.45
8.6-11
13.23
12.48
13-17
11.21
10.93
30-39
5.53
5.10
5.6-6.6
A. ∆ν / ∆f is the slope of the fit to Eq. 1.
B. Calculated values are from gas-phase CAM-B3LYP/6-311++G(d,p) calculations.
Geometries were optimized for the ground state and excited-state values derived from TDDFT
calculations at these S geometries. For the dyad, a number of representative conformations
0
were selected, and the range of values obtained from these conformers is tabulated.
26
ACS Paragon Plus Environment

Page 27 of 40
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
Table 2: Multi-Exponential Representations of Dyad Emission Decays and ET Rate Constants
A. TCSPC Measurements
(
1)
(2)
τ
ps
1
τ
/ ps
2
τ
/ ps
3
τ
/ ns
4
τ
H152
τ
C152
<τ>dyad
k
ET
k
ET
Solvent
a
1
a
2
a
3
a
4
/ s-1
/ s-1
6.9×10
1.1×10
8.8×10
1.7×10
1.4×10
2.1×10
1.3×10
5.5×10
/
/ ns
3.7
3.9
4.1
3.9
4.5
4.7
4.8
4.4
5.1
4.8
5.2
/ ns
4.1
3.8
4.8
4.4
4.7
4.9
5.4
1.1
4.8
4.5
3.4
/ ps
7
7
Cyclohexane (CHEX)
50/50 vol% CHEX+DBE
Dibutyl Ether (DBE)
Toluene (TOL)
1.00
0.22
0.21
0.26
0.83
0.59
0.97
0.97
0.093
0.10
0.64
-
-
3183
21
-
3183
1531
1014
855
81
6.9×10
3.9×10
7.8×10
9.4×10
1.2×10
1.1×10
1.0×10
2.8×10
8
8
8
10
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
0.77
0.78
0.69
0.16
0.36
0.02
0.02
0.91
0.90
0.30
-
0.02
0.01
0.05
0.01
0.004
0.004
0.02
-
1956
1148
1091
170
-
(3.9)
(4.1)
(3.9)
(4.5)
(4.7)
(4.8)
(4.4)
-
8
-
529
216
63
-
9
-
-
10
10
5
1
0/50 vol% CHEX+THF
,4-Dioxane (DIOX)
-
-
10
10
11
10
0.05
32
135
477
92
1
1
1
0
Tetrahydrofuran (THF)
Ethylene Glycol (EG)
-
8
104
-
10
-
18
980
-
34
<107
<107
[N8881][Tf
2
N] (N8881
)
-
-
1123
1201
12
5001
4921
105
-
-
-
[
[
P
14,666][ Tf N] (P14,666
2
)
-
-
-
10
10
Pr41][Tf
2
N] (Pr41
)
0.02
0.04
2138
(5.2)
88
1.1×10
5.8×10
B. KGE Measurements
(1)
(2)
τ
/ ps
1
τ
/ ps
2
τ
/ ps
3
<τ>dyad
/ ps
τ
/ns
H152
τ
/ ns
C152
k
/ s
ET
k
/ s
ET
-1
Solvent
a
1
a
2
a
3
-1
1.5×1011
2.2×1011
Methyl Acetate (MEAC)
0.47
0.49
0.04
0.3
1.9
42
2.2
5.0
4.8
3.4×1011
1.9×10
1.6×10
4.2×10
6.4×10
12
Propylene Carbonate (PC)
Dimethylsulfoxide (DMSO)
Acetonitrile (ACN)
0.56
0.56
0.39
0.23
0.40
0.40
0.55
0.73
0.05
0.04
0.06
0.04
0.4
0.1
2.0
3.8
2.3
0.9
10
39
64
50
43
3.0
2.8
3.6
5.0
5.5
5.0
5.4
4.9
2.0
3.4×1011
12
0.97
2.1
11
12
3.5×10
2.3×1011
11
Methanol (MeOH)
10
0.96
Fits are to the function Σ a exp(−t /τ ) with Σ a =1. TCSPC decays were measured at the steady-state emission whereas the KGE
i
i
i
i i
fits show the average time dependence of the peak height and integrated intensity of the emission band. <τ>dyad is the average decay
'
'
time of the dyad Σ a exp(−t /τ ) / Σ a where the prime denotes exclusion of component 4, which is assumed to be due to emission
i
i
i
i i
(1)
(2)
from H152 impurity. k and k are estimated rates of electron transfer calculated according to Eqs. 3-4.
ET
ET
27
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 28 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Scheme 1
Scheme 2
28
ACS Paragon Plus Environment

Page 29 of 40
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 1: (a) Dihedral angle definitions. (b) One-dimensional distributions of angles D1-D3 in
acetonitrile (solid curves) and in the gas phase (dashed). (c) 2-dimensional distributions in
acetonitrile. In both the 1d and 2d distributions the values shown are the numbers of occurrences
5
out of 5×10 samples.
29
ACS Paragon Plus Environment