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Viewpoint on the 2019 International Conference on Photochemistry

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Ffrom the scarcity of brightly red colored 1999 keywords. In
contrast, the 2019 chart highlights several emerging areas ofresearch interest, which have little or no mention 20 years
prior. Hybrid, perovskite, ef f icient, and solar all appear
predominantly in 2019, related to recent interest in emerging
photovoltaic and electro-optical materials, particularly inhybrid perovskites. We highlight other emerging andestablished topics in the context of some speciﬁc contributions
below.
Ultrafast and dynamics were some of the most frequently
occurring keywords within the 2019 ICP meeting, mentionedin ∼11% of all talk and poster titles. Ultrafast time-resolved
methods, made possible by developments in laser technologies
beginning in the 1960s, have been signiﬁcant in this ﬁeld forsome time. However, researchers have an increasing ability to
use these techniques to look at a larger variety of systems ingreater detail than ever before. This was well-reﬂected in the
topics presented in many of the plenary lectures, thesemethods being leveraged to investigate a wide variety of
biological, chemical, and physical systems.
Stefan Haacke (The University of Strasbourg and CNRS,IPCMS) gave a keynote presentation discussing the recent
developments in understanding the ultrafast processes that
occur in photosensory proteins relying on photoisomerizationof their chromophore. In retinal proteins, such as Rhodopsin,
the pigment of vision, this subpicosecond process allows theconversion of light into free energy via complex intermolecular
coupling. This triggers vision or ion pumping on femtosecond
time scales and with eﬃcient yields. It is known that theprotein environment has a large impact on the dynamics andquantum yield of this ultrafast reaction. In the framework of
the French−German project “Femto-ASR”, using a combination of quantum chemistry and ultrafast spectroscopy, theHaacke group and their partners show that electrostatic
properties of protein environment modulate these reaction
dynamics by changes in the electronic character along the ﬁrst
singlet excited state potential energy surface [M. D. Marin etal., J. Am. Chem. Soc., 2019, 141, 262−271], in addition tosteric eﬀects [P. P. Roy et al., J. Phys. Chem. Lett., 2019, 10,1012−1017]. Recent work in collaboration with Massimo
Olivucci (University of Siena and Bowling Green) has also
determined that isomerization can operate similarly to
Rhodopsin in biomimetic molecular switches [J. Briand etal., Phys. Chem. Chem. Phys., 2010, 12, 3178−3187]. Here, theobserved coherent nuclear motion is critically dependent on
minor chemical modiﬁcations, since these induce speciﬁc
geometric and thus electronic eﬀects [M. Gueye et al., Nat.
Commun., 2018, 9 (1), 1−8]. These recent ﬁndings identify
biomimetic switches as excellent model systems for experimentally addressing long-standing theoretical predictions ofwavepacket dynamics and branching at conical intersections.
Helen H. Fielding (University College, London) focused her
plenary session on the application of gas-phase and liquid-rom July 21, 2019 to July 25, 2019, a wide variety ofscientists converged upon the 40th Parallel North in
Boulder, CO in the United States, to discuss the latest
developments within the extensive ﬁeld of photochemistry.
The International Conference on Photochemistry (ICP) is theoldest and most established event on the topic in the worldand has provided a forum to report the eﬀects of light on
chemical, biological, and physical systems since 1962.
Returning to the United States for the ﬁrst time in morethan 20 years, the 2019 meeting was the 29th ICP, chaired by
Prof. Gary Rumbles (National Renewable Energy Laboratory,
University of Colorado, Boulder) and Prof. Gregory Scholes(Princeton University). Over time, the event and topics have
evolved from the pre-laser era to today’s emerging andexpanding methods to understand the photochemistry andphotophysics of a huge variety of systems and materials (seeFigure 1, with further details discussed below). From ultrafast
spectroscopy to spectroscopy at the nanoscale, from nextgeneration chemical reactions to device-inspired photophysics,
photochemists from around the world found value and insightin conference discussions and proceedings. This meetingfeatured over 240 talks and 150 posters in six concurrent
sessions, with a complete list available at www.colorado.edu/
conference/icp2019/technical-program. Participants traveledfrom their institutions in over 30 countries around the world,with the global distribution of attendees illustrated in Figure 2.
To gain some insight into the broad range of topics covered
within this conference, and how this has changed over time, we
analyzed keywords contained within the meeting’s talk andposter titles. In Figure 1, we show the most commonlyoccurring keywords across all titles in the 1999 ICP that washeld in Durham, NC, USA (left), and the 2019 meeting in
Boulder, CO, USA (right). The size of each circle indicates therelative occurrence of keywords within all conferences,normalized to the size of each meeting (in 1999, n = 236
contributions; in 2019, n = 397 contributions). For example, in
2019, the word ultrafast occurred in 43 separate talk and poster
titles, i.e., in 11% of all contributions. The color of each circle
indicates the similarity of a keyword’s occurrence between the1999 and 2019 conferences, where blue indicates a very similar
proportional level of occurrence, while red indicates a wordthat occurs much more commonly in one meeting than theother. Common word stems were used to compile statistics(e.g., coher* for coherence and coherent) and these stems
were completed in Figure 1 for clarity.
From this visualization, it is straightforward to identify major
topics that have continued as active and major areas ofresearch over the 20 year period. The study of excited states andelectron or charge transfer, and methods in f luorescence andspectroscopy all remain central to this research over time, having
also dominated the ﬁeld of photochemistry since its inception.
Further insights are given with regard to the research areasthat have changed and emerged over time. In the 1999
meeting, there are very few major topics that have notcontinued to be represented and studied in 2019, observable© 2019 American Chemical Society

Published: October 24, 2019
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Figure 1. Keyword occurrence and diﬀerence analysis between contributed talks and posters at the International Conference on Photochemistry
1999 (left) and 2019 (right). The circle size indicates the frequency of a keyword’s occurrence within all conference titles. The color of each circle
indicates the similarity in occurrence between the 1999 and 2019 meetings, where blue indicates keywords that appear often in both conferences’
titles, and red indicates keywords that occur much more often in one meeting than the other.

Figure 2. Countries of attendance for the 2019 International Conference in Photochemistry.

Phys. Chem. Lett., 2018, 9, 678−682], the aqueous condition
also aﬀorded the ultrafast generation of solvated electrons [A.
Henley et al., Faraday Discuss., DOI: 10.1039/c9fd00079h].
Fielding then moved from a discussion of phenol to thebiochromophore responsible for ﬂuorescence emission fromthe green ﬂuorescent protein (GFP). Again, using ultraviolet
PES, they conﬁrmed that, following photoexcitation, thechromophore shows signatures not only of internal conversion

microjet photoelectron spectroscopy to isolate and study
anionic forms of biomolecules, with the goal of using a bottomup approach to improve our understanding of their photophysical properties in their natural environment. By comparing
the relaxation dynamics of phenol using multiphoton ultraviolet photoelectron spectroscopy (PES) both in the gas phaseand in a liquid microjet, they found that, although the
dynamics in the two cases were very similar [J. Riley et al., J.
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at low temperature. Continuing with his theme of quantum
eﬀects, Wasielewski and co-workers also showcased their mostrecent advances in demonstrating high-ﬁdelity quantum
teleportation. Using a molecular donor−acceptor-radical
system, the organic entangled radical’s spin information wastransported to the donor moiety and was detected by timeresolved EPR, demonstrating prototypical quantum teleportation in an organic system [B. K. Rugg et al., Nat.
Chem., 2019, DOI: 10.1038/s41557-019-0332-8].
Graham R. Fleming (University of California, Berkeley) gave
his keynote talk centering on the application of twodimensional electronic vibrational spectroscopy, or 2DEV, totrack the photophysical dynamics in photosynthetic systems.
As opposed to two-dimensional electron spectroscopy (2DES),
2DEV is a technique that can directly correlate electronictransitions of a chromophore with its vibrational motion byusing a combination of visible and mid-IR pulses. Utilizing theinﬂuence of local protein structure on the energies of various
chlorophyll high-frequency modes in light-harvesting complex
II (LHCII), Fleming and co-workers demonstrated the ability
to track energy transfer and elucidate relaxation pathways inthis photosynthetic protein on the picosecond time scale [N.
H. C. Lewis et al., J. Phys. Chem. Lett., 2016, 7, 4197−4206].
Utilizing data from ultrafast spectroscopy, Fleming went on todescribe a model for energy ﬂow in the thylakoid membrane
and the regulation of photosynthetic light harvesting [D. I. G.
Bennett et al., Proc. Natl. Acad. Sci., 2018, 115,
E9523−E9531]. He further demonstrated his group’s dedication to elucidating the photophysical processes in photosynthetic organisms by performing ultrafast transient absorption spectroscopy on live cells of Nannochloropsis oceanica, in
an eﬀort to identify the presence and physiological triggeringfactors of nonphotochemical quenching in wild-type andmutant systems [S. Park et al., Proc. Nat. Acad. Sci., 2019, 116,
3385−3390].
While major themes emerged from our keyword analysis, it
was also evident that the huge diversity of topics covered
within the umbrella of photochemistry are not adequately
represented in Figure 1. An excellent example of this is thework presented in the lecture of Christy Landes (Rice
University). She presented strategies to leverage superresolution microscopic techniques in order to quantify protein
adsorption/desorption events and transport. She explained
that, while chemical separation uses as much as 15% of U.S.
domestic energy consumption, the science of this separation at
nanostructured interfaces has remained largely empirical.
Adsorption and desorption events can be modeled within theframework of statistical thermodynamics, providing a physicalconnection to the underlying chemistry that has been lacking
in previous methods. Fluorescence superlocalization coupledwith kinetic analysis makes it possible to distinguish between
speciﬁc, repeat protein adsorption at ligand sites. In addition,
Landes’ group combined these super-resolved techniques withimprovements in temporal resolution facilitated by the use ofrotating phase masks in the optical detection path during dataacquisition. This method of supertemporal-resolved microscopy was applied to the quantiﬁcation of adsorption−
desorption dynamics, protein unfolding, and nanoconﬁned
transport, revealing previously unresolvable dynamics in thisreinvigorated ﬁeld of research [N. A. Moringo et al., Annu. Rev.
Phys. Chem., 2018, 69 (1), 353−375].
Surface plasmons are a topic with a rich scientiﬁc history, yet
this topic was completely absent in our 1999 ICP title keyword

to the ground electronic state but also of resonant autodetachment of the extra electron [C. R. S. Mooney et al., J. Phys.
Chem. A, 2012, 116, 7943; J. Riley et al., manuscript in
preparation].
Laser light was also discussed by Hiroshi Masuhara(National Chiao Tung University), with regard to its ability
to physically manipulate particles. In optical manipulation,mechanical force is exerted on small objects by a large quantityof focused laser photons. This technique has a broad array of
applications, including photopolymerization, microstructuring,
photochromism, the crystallization of molecules and perovskites, and amyloid ﬁbril formation. Masuhara discussed recent
discoveries in an optically evolved assembly mechanism inwhich nanoparticle subjects of optical manipulation are notpassively moved by the trapping laser but play a dynamic rolein the system [T. Kudo et al., Nano Lett., 2018, 18, 5846].
Nanoparticles scatter, propagate, and interfere with photons
from the trapping laser, which modiﬁes the optical potential,
illuminating new phenomena and fabricating new assemblies.
Given the established expertise in renewable energy and theenvironment at the University of Colorado, Boulder, thismeeting particularly highlighted the latest work in solar fuels
and atmospheric photochemistry. Barbara J. Finlayson-Pitts(University of California−Irvine) presented ongoing investigations of photochemically driven reactions involving
pollutants, including at air/water or air/solid interfaces.
Photochemical reactions between primary pollutants in theenvironment produce secondary pollutants, many having
increased toxicity. For example, the photochemical decomposition of some neonicotinoid (NN) pesticides in air
generates products that have been shown by others to be
more toxic than the parent NN. In addition, the greenhousegas N2O is formed [K. Z. Aregahegn et al., Environ. Sci.
Technol., 2017, 51, 2660]. In collaboration with B. Gerber, D.
Tobias, P. Jungwirth, and M. Roeselova, Finlayson-Pitts andher group proposed that Cl− anions are readily available forreaction at the surface of seawater, and can be involved in O3,
OH, and particle formation [E. M. Knipping et al., Science,
2000, 288, 301]. Another example is humic acid acting as aphotosensitizer in and at the surface of seawater, facilitating
photochemical generation of a variety of products in the gas
and aqueous phases [M. Shrestha et al., Chem. Sci., 2018, 9,5716; P. R. Erickson, Environ. Sci. Technol., 2018, 52, 9170].
Photochemical reactions of pollutants in the environment atinterfaces and in the bulk, and the identity and role ofchromophores, remain important questions for increased
understanding of these processes.
The keywords quantum and coherent comprise an existing
theme in photochemical research observed more at ICP 2019
than 20 years prior. Michael R. Wasielewski (Northwestern
University) in his plenary session ﬁrst spoke on recent
developments in observing quantum interference eﬀects in aphotoinduced electron transfer system with identical electrontransfer acceptors and a single electron donor moiety. When
comparing the electron transfer rate constant between
molecules with only one acceptor as compared to two,
Wasielewski and co-workers discovered that the rate enhancement associated with having multiple acceptors is greater than
the sum of the two electron transfer rates [B. T. Phelan et al., J.
Am. Chem. Soc., 2019, 141, 12236−12239]. Importantly, thisphenomenon is only apparent at cryogenic temperatures,
suggesting the possibility of quantum coherence eﬀects as aresult of correlated system-bath ﬂuctuations in frozen solutions
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analysis. In contrast, surface plasmon appeared in ∼6% of 2019
ICP contributions, including the plenary lecture of Professor
Paul Mulvaney (The University of Melbourne). He spoke
about his group’s recent advances on the surface plasmon
spectroscopy of gold nanoparticles. These metal nanostructures oﬀer a unique sensitivity to changes in their electron
density in the form of their surface plasmon resonance. Theseeﬀects can be exploited by examining plasmon spectra in highpressure environments, where they can report on both thechange in solvent refractive index and the mechanicalproperties of the nanostructure itself [C. Martín-Sánchez etal., J. Phys. Chem. Lett., 2019, 10, 1587−1593; C. MartínSánchez et al., ACS Nano, 2019, 13, 498−504]. Similarly, thesurface plasmon of a particle can be modiﬁed by adding evenmodest numbers of electrons. Through the integration ofnanostructures into ion gel devices, rapid charging anddetection of as few as 110 electrons was possible using asingle gold rod as a sensor. Quite unlike a macroscopic
conductive object, here, a color change will indicate that a
particle is charged [S. S. E. Collins et al., Nano Lett., 2016, 16,6863−6869]. Another emerging technique developed by thegroup allows for the directed assembly of evenly spaced gold
particles into ordered arrays, opening up new avenues in single
nanocrystal spectroscopic sensing.
Another topic with increasing popularity at this year’s ICP
was photoredox catalysis. David W. C. MacMillan (Princeton
University), as a pioneer in this ﬁeld, presented several
examples of photoredox catalysis helping access traditionallychallenging organic transformations. Merging photocatalysis
with transition-metal catalysis, new nickel and copper catalyzed
cross-coupling reactions to form C−C, C−N, C−O, and C−Sbonds can be accomplished under much milder conditions
with various photocatalysts and blue light-emitting diode(LED) light. This approach takes advantage of the enhanced
redox properties of photocatalysts to generate high-valence
metal states to promote otherwise thermodynamicallyprohibitive elementary steps, such as reductive eliminationand oxidative addition. The enhanced redox properties can also
be exploited to promote facile generation of organic radicals toachieve C−H bond activation and selective isotopic substitution. For example, after photo-oxidation by an iridium
photocatalyst, the α-H becomes extremely acidic and can
exchange with deuterated and tritiated protic solvents [Y. Y.
Loh et al., Science, 2017, 358, 1182−1187]. These newreactions altogether contribute to a more complete toolkit forlarge molecule synthesis for pharmaceutical as well as energyrelated applications.
Poster prizes were awarded for 10 excellent contributions
across a broad variety of session topics. Singlet ﬁssion, ﬁrst
reported in 1965, has enjoyed a recent research renaissance(see Figure 1), due to new materials, methods, andapplications in the area of photovoltaics. The research ofMichelle Chen (Northwestern University) reﬂected this, withher poster, State Mixing Determines the Fate of the Multiexciton
State in Singlet Fission. Science with applications to lightharvesting by solar cells was further highlighted by Andressa
Vidal Müller (Universidade Federal do ABC and University ofNorth Carolina at Chapel Hill) in the poster, Tuning theElectron Transfer Rates in DSSCs with Ru(II) Dyes Having NHeterocyclic Aromatic Substituents. Muazzam Idris (Universityof Southern California) presented work entitled Phenanthro[9,10-d]imidazole as Hosts and Fluorescent Emitters for Hybrid

White Organic Light-emitting Diode Application, exploiting
singlet and triplet energy levels in new LED materials.
New spectroscopic techniques were applied in the awardwinning posters of Kathryn Ledbetter (Stanford University),
Time-Resolved X-ray Diff use Scattering as a Probe of Site-Specific
Solvation, and Daria Blach (Purdue University) contributing to
Ultrafast Imaging of Energy Transport in CdSe/CdS Lateral
Heterostructures. Nancy Awwad (North Carolina State
University) presented Photochemical Upconversion Induced
Polymerization in the “Photochemistry and Photoredox
session”, and Kylie Couch (University of Colorado, Boulder)won the session on “Atmospheric and Environmental
Chemistry” with her poster, Investigation of the Production of
Hydroxylating Species f rom Dissolved Organic Matter Photolysis.
Solar photochemistry remained a prevalent topic, withexcellent contributions from Yuchun Wang (Hokkaido
University) with In-Situ Surface Enhanced Raman Scattering
Observations of Water Oxidation Reactions Enhanced by the
Modal Strong Coupling; Maria Wächtler (Leibniz Institute of
Photonic Technology, Jena) with Charge-Separation in Nitipped CdSe@CdS Nanorods for Hydrogen Evolution; andAbdulaziz Alherz (University of Colorado, Boulder) with
Organic Hydrides for the Photoelectrochemical Catalytic Reduction of CO2.
The 2019 International Conference on Photochemistry has
continued to explore important and established topics in theﬁeld and has illuminated new avenues of fascinating and usefullight-induced phenomena across physics, biology, andchemistry. We would like to recognize the past and present
conference chairs, as well as the large number of student
volunteers, led in 2019 by NREL graduate student Justin
Earley, for their eﬀorts toward crafting an excellent meeting.
The 30th International Conference on Photochemistry wasrecently announced and will be held July 1823, 2021 in
Geneva, Switzerland, chaired by Prof. Eric Vauthey. This
meeting will undoubtably continue the tradition of bringing
together a wide variety of scientists to discuss and explore
many diﬀerent topics within the rich world of photochemical
research. Interested participants are encouraged to visit www.
icp2021.ch for further information.

Bryan KudischHannah J. Sayre
Lei Tian
Kyra N. Schwarz*■

Department of Chemistry, Princeton University, Princeton,
New Jersey 08544, United States

ASSOCIATED CONTENT

S Supporting Information*

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpca.9b09156.
Full list of conference talk and poster titles (PDF)■

AUTHOR INFORMATION

Corresponding Author*E-mail: kyras@princeton.edu.
Author Contributions

The manuscript was written through contributions of all
authors. All authors have given approval to the ﬁnal version of
the manuscript.
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Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
We would like to thank Prof. Cliﬀord Kubiak, Prof. David
Phillips, Prof. Ken Ghiggino, and Prof. Garry Rumbles for their
help in providing past conference data and information for thispublication. We would also like to thank the speakers
mentioned within this manuscript for feedback regarding
their presentation summaries.

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