Skip to main content

Main menu

  • Home
  • Articles
    • Current
    • Special Feature Articles - Most Recent
    • Special Features
    • Colloquia
    • Collected Articles
    • PNAS Classics
    • List of Issues
  • Front Matter
    • Front Matter Portal
    • Journal Club
  • News
    • For the Press
    • This Week In PNAS
    • PNAS in the News
  • Podcasts
  • Authors
    • Information for Authors
    • Editorial and Journal Policies
    • Submission Procedures
    • Fees and Licenses
  • Submit
  • Submit
  • About
    • Editorial Board
    • PNAS Staff
    • FAQ
    • Accessibility Statement
    • Rights and Permissions
    • Site Map
  • Contact
  • Journal Club
  • Subscribe
    • Subscription Rates
    • Subscriptions FAQ
    • Open Access
    • Recommend PNAS to Your Librarian

User menu

  • Log in
  • My Cart

Search

  • Advanced search
Home
Home
  • Log in
  • My Cart

Advanced Search

  • Home
  • Articles
    • Current
    • Special Feature Articles - Most Recent
    • Special Features
    • Colloquia
    • Collected Articles
    • PNAS Classics
    • List of Issues
  • Front Matter
    • Front Matter Portal
    • Journal Club
  • News
    • For the Press
    • This Week In PNAS
    • PNAS in the News
  • Podcasts
  • Authors
    • Information for Authors
    • Editorial and Journal Policies
    • Submission Procedures
    • Fees and Licenses
  • Submit
Commentary

Extreme cross-peak 2D spectroscopy

Gregory D. Scholes
  1. Department of Chemistry, University of Toronto, Toronto, ON, Canada M5S 3H6; and Department of Chemistry, Princeton University, Princeton, NJ 08544

See allHide authors and affiliations

PNAS July 15, 2014 111 (28) 10031-10032; first published July 7, 2014; https://doi.org/10.1073/pnas.1410105111
Gregory D. Scholes
Department of Chemistry, University of Toronto, Toronto, ON, Canada M5S 3H6; and Department of Chemistry, Princeton University, Princeton, NJ 08544
  • Find this author on Google Scholar
  • Find this author on PubMed
  • Search for this author on this site
  • For correspondence: gscholes@princeton.edu
  • Article
  • Figures & SI
  • Info & Metrics
  • PDF
Loading

Researchers have long recognized the value of ultrasfast time-resolved spectroscopy for revealing the mechanism of photophysical and light-induced biophysical processes (1⇓–3). Over recent years, we witnessed leaps in technology that have enabled new and ingenious femtosecond laser experiments to be demonstrated. In PNAS, Oliver et al. report a 2D spectroscopy that correlates electronic transition frequencies in a photo-excitation event with infrared transitions detected by a probe (4). They call this 2D electronic-vibrational (2DEV) spectroscopy.

2D spectroscopies including 2D electronic spectroscopy (2DES) and 2D infrared spectroscopy (2DIR) are femtosecond pump-probe techniques where both pump and probe frequencies are resolved. 2D spectroscopy thus correlates the absorption spectrum (UV, visible, or infrared), enabling line-broadening time scales to be distinguished by their different 2D line shapes, and states with a common origin can be identified by cross-peaks (5⇓⇓–8). 2DES and 2DIR are similar to transient absorption spectroscopy; however, two excitation pulses are used cooperatively to excite the sample, followed by a third “probe-pulse,” which interacts with the sample after the pump-probe time delay, causing a four-wave mixing signal to radiate. By Fourier transforming the signal amplitude with respect to the delay between the two excitation pulses, the excitation frequency axis is obtained for a given time delay. The probe axis is frequency resolved by dispersing signal in the detector, like in normal pump-probe methods.

In 2DEV (Fig. 1), time-resolved infrared spectroscopy is rendered multidimensional. This enables a vibrational spectroscopy probe—sensitive to chemical structure—to uncover how electronic excitation conditions promote and interplay with structural changes accompanying photophysical or photochemical transformations on a femtosecond time scale. Just some of the potential for this intriguing new method is illustrated by considering some thoughts on the work reported by Oliver et al. that complement their interpretations.

Fig. 1.
  • Download figure
  • Open in new tab
  • Download powerpoint
Fig. 1.

(A) Depiction of the free energies of the LE and ICT states relative to the ground state and an indication of how a 2DEV experiment probes the ICT reaction. (B) Regions of the 2D spectroscopic map relating 2DES and 2DIR, nominally diagonal experiments, to 2DEV, which is an off-diagonal experiment.

The model reaction studied by Oliver et al. (4) is closely related to the famous twisted intramolecular charge transfer (TICT) phenomenon (9, 10). TICT molecules show duel fluorescence: two fluorescence bands that depend strongly on solvent polarity. One fluorescence band has a smaller Stokes shift from the absorption and indicates the locally excited (LE) state. This LE state undergoes a photophysical transformation in moderately polar solvents to populate an intramolecular charge transfer (ICT) state that, in turn, is revealed by a red-shifted fluorescence that is strongly solvent dependent. This interpretation and the idea that the prototypical TICT molecule, dimethylaminobenzonitrile (DMABN), undergoes a marked twist to stabilize the charge-transfer state was proposed by Grabowski and coworkers (11).

For polar molecules in solution, the change in dipole moment between ground and excited electronic states is stabilized by reorganization of the solvent around the molecule to lower the free energy. Seen as a solvent-dependent (and time-dependent) Stokes shift, this process is called nonequilibrium solvation (12). An interesting aspect of the photophysics of TICT compounds is the interplay between solvent reorganization and structural degrees of freedom in the reaction (13). Therefore, in Fig. 1A, I depict the reaction in terms of free energy that accommodates both ensemble reorganization of the solvent around the LE and ICT states, as well as the potential energy surface for the TICT geometry reorganization.

The intramolecular aspects of the TICT reaction are notable because the degree of structural distortion is much greater than found in typical electron transfer reactions. The extent and precise nature of the structural changes accompanying charge separation in TICT molecules has long been controversial—for example, is it really a full twist of the dimethylamino group (9, 14, 15)? This kind of question lends itself to tools that can probe structure. Time-resolved infrared (16) and Raman (17, 18) experiments have therefore provided essential insights.

To understand, in part, what Oliver et al. are observing, consider the 2D spectroscopy map shown in Fig. 1B. 2DES correlates the UV-visible absorption spectrum: it tells us about pathways for interconversion of electronic states (7, 19). 2DEV resolves pathways along the reaction coordinate for producing a product or intermediate detected by its infrared absorption signature. In the present case, it appears that the infrared (IR) band at 1,480 cm−1 signals such a product, as indicated in Fig. 1A. What is interesting is that the pump frequency (at visible wavelengths) that produces this IR signature depends on the time that the 1,480 cm−1 band is detected. The red-most pump frequency (Fig. 1, red arrow) produces the signal quickly—the product is formed quickly and seen in the 2DEV spectrum as indicated by the red circle, which then decays. The blue excitation more slowly produces the intermediate indicated by the 1,480 cm−1 IR band, so the blue circle rises on a slower time scale in the 2DEV spectrum. This interpretation is rather speculative but instructive to convey principles of the experiment. It suggests that part of the reaction coordinate is sampled by each pump frequency. For example, the red excitation frequencies excite closer to the transition state and therefore produce the ICT product more quickly than the blue wavelengths (the rise time of the blue signal is indicated as T3, longer than T1, the rise of the red signal). Such inhomogeneity in reaction kinetics on ultrafast time scales was recently described for DMABN by Joo and coworkers recently (20).

This discussion illustrates some of the potential of future 2DEV experiments. More subtle and particularly interesting is the fact that 2DEV is a correlation spectroscopy and not simply a pump-probe technique. Therefore, the 2D line shapes carry information about correlations between the evolution of electronic and nuclear states—insights that are central for elucidating dynamics not described in the framework of the Born–Oppenheimer approximation. The demonstration and, above all, promise of 2DEV spectroscopy highlight the significance of this new advance in multidimensional nonlinear spectroscopy.

Footnotes

  • ↵1Email: gscholes{at}princeton.edu.
  • Author contributions: G.D.S. wrote the paper.

  • The authors declare no conflict of interest.

  • See companion article on page 10061.

References

  1. ↵
    1. Fleming GR,
    2. Morris JM,
    3. Robinson GW
    (1976) Direct observation of rotational diffusion by picosecond spectroscopy. Chem Phys 17(1):91–100.
    OpenUrlCrossRef
  2. ↵
    1. Porter G
    (1978) The Bakerian Lecture, 1977: In vitro models for photosynthesis. Proc R Soc Lond A Math Phys Sci 362(1710):281–303.
    OpenUrlCrossRef
  3. ↵
    1. Zewail AH
    (1996) Femtochemistry: Recent progress in studies of dynamics and control of reactions and their transition states. J Phys Chem 100(31):12701–12724.
    OpenUrlCrossRef
  4. ↵
    1. Oliver TAA,
    2. Lewis NHC,
    3. Fleming GR
    (2014) Correlating the motion of electrons and nuclei with two-dimensional electronic–vibrational spectroscopy. Proc Natl Acad Sci USA 111:10061–10066.
    OpenUrlAbstract/FREE Full Text
  5. ↵
    1. Hamm P,
    2. Lim MH,
    3. Hochstrasser RM
    (1998) Structure of the amide I band of peptides measured by femtosecond nonlinear-infrared spectroscopy. J Phys Chem B 102(31):6123–6138.
    OpenUrlCrossRef
  6. ↵
    1. Jonas DM
    (2003) Two-dimensional femtosecond spectroscopy. Annu Rev Phys Chem 54:425–463.
    OpenUrlCrossRefPubMed
  7. ↵
    1. Brixner T,
    2. et al.
    (2005) Two-dimensional spectroscopy of electronic couplings in photosynthesis. Nature 434(7033):625–628.
    OpenUrlCrossRefPubMed
  8. ↵
    1. Mukamel S
    (2000) Multidimensional femtosecond correlation spectroscopies of electronic and vibrational excitations. Annu Rev Phys Chem 51:691–729.
    OpenUrlCrossRefPubMed
  9. ↵
    1. Grabowski ZR,
    2. Rotkiewicz K,
    3. Rettig W
    (2003) Structural changes accompanying intramolecular electron transfer: Focus on twisted intramolecular charge-transfer states and structures. Chem Rev 103(10):3899–4032.
    OpenUrlCrossRefPubMed
  10. ↵
    1. Rettig W
    (1986) Charge separation in excited-states of decoupled systems: TICT compounds and implications regarding the development of new laser-dyes and the primary processes of vision and photosynthesis. Angew Chem Int Ed Engl 25(11):971–988.
    OpenUrlCrossRef
  11. ↵
    1. Rotkiewicz K,
    2. Grellman KH,
    3. Grabowski ZR
    (1973) Reinterpretation of anomalous fluorescence of para-N,N-dimethylamino-benzonitrile. Chem Phys Lett 19(3):315–318.
    OpenUrlCrossRef
  12. ↵
    1. van der Zwan G,
    2. Hynes JT
    (1985) Time-dependent fluorescence solvent shifts, dielectric friction, and noneqilibrium solvation in polar solvents. J Phys Chem 89(20):4181–4188.
    OpenUrlCrossRef
  13. ↵
    1. Scholes GD,
    2. Fournier T,
    3. Parker AW,
    4. Phillips D
    (1999) Solvation and intramolecular reorganization in 9,9′-bianthryl: Analysis of resonance Raman excitation profiles and ab initio molecular orbital calculations. J Chem Phys 111(13):5999–6010.
    OpenUrlCrossRef
  14. ↵
    1. Zachariasse KA,
    2. et al.
    (1996) Intramolecular charge transfer in the excited state. Kinetics and configurational changes. J Photochem Photobiol Chem 102(1):59–70.
    OpenUrlCrossRef
  15. ↵
    1. Sobolewski AL,
    2. Domcke W
    (1996) Charge transfer in aminobenzonitriles: Do they twist? Chem Phys Lett 250(3-4):428–436.
    OpenUrlCrossRef
  16. ↵
    1. Hashimoto M,
    2. Hamaguchi H
    (1995) Structure of the twisted-intramolecular-charge-transfer excited singlet and triplet-states of 4-(dimethylamino)benzonitrile as studied by nanosecond time-resolved infrared-spectroscopy. J Phys Chem 99(20):7875–7877.
    OpenUrlCrossRef
  17. ↵
    1. Kwok WM,
    2. et al.
    (2003) Further time-resolved spectroscopic investigations on the intramolecular charge transfer state of 4-dimethylaminobenzonitrile (DMABN) and its derivatives, 4-diethylaminobenzonitrile (DEABN) and 4-dimethylamino-3,5-dimethylbenzonitrile (TMABN) Phys Chem Chem Phys 5(6):1043–1050.
    OpenUrlCrossRef
  18. ↵
    1. Kwok WM,
    2. et al.
    (2001) A determination of the structure of the intramolecular charge transfer state of 4-dimethylaminobenzonitrile (DMABN) by time-resolved resonance Raman spectroscopy. J Phys Chem A 105(6):984–990.
    OpenUrlCrossRef
  19. ↵
    1. Ostroumov EE,
    2. Mulvaney RM,
    3. Cogdell RJ,
    4. Scholes GD
    (2013) Broadband 2D electronic spectroscopy reveals a carotenoid dark state in purple bacteria. Science 340(6128):52–56.
    OpenUrlAbstract/FREE Full Text
  20. ↵
    1. Park M,
    2. Kim CH,
    3. Joo T
    (2013) Multifaceted ultrafast intramolecular charge transfer dynamics of 4-(dimethylamino)benzonitrile (DMABN) J Phys Chem A 117(2):370–377.
    OpenUrlCrossRefPubMed
PreviousNext
Back to top
Article Alerts
Email Article

Thank you for your interest in spreading the word on PNAS.

NOTE: We only request your email address so that the person you are recommending the page to knows that you wanted them to see it, and that it is not junk mail. We do not capture any email address.

Enter multiple addresses on separate lines or separate them with commas.
Extreme cross-peak 2D spectroscopy
(Your Name) has sent you a message from PNAS
(Your Name) thought you would like to see the PNAS web site.
CAPTCHA
This question is for testing whether or not you are a human visitor and to prevent automated spam submissions.
Citation Tools
Extreme cross-peak 2D spectroscopy
Gregory D. Scholes
Proceedings of the National Academy of Sciences Jul 2014, 111 (28) 10031-10032; DOI: 10.1073/pnas.1410105111

Citation Manager Formats

  • BibTeX
  • Bookends
  • EasyBib
  • EndNote (tagged)
  • EndNote 8 (xml)
  • Medlars
  • Mendeley
  • Papers
  • RefWorks Tagged
  • Ref Manager
  • RIS
  • Zotero
Request Permissions
Share
Extreme cross-peak 2D spectroscopy
Gregory D. Scholes
Proceedings of the National Academy of Sciences Jul 2014, 111 (28) 10031-10032; DOI: 10.1073/pnas.1410105111
del.icio.us logo Digg logo Reddit logo Twitter logo CiteULike logo Facebook logo Google logo Mendeley logo
  • Tweet Widget
  • Facebook Like
  • Mendeley logo Mendeley

Article Classifications

  • Physical Sciences
  • Chemistry

See related content:

  • Two-dimensional electronic–vibrational spectroscopy
    - Jun 09, 2014
Proceedings of the National Academy of Sciences: 111 (28)
Table of Contents

Submit

Sign up for Article Alerts

Jump to section

  • Article
    • Footnotes
    • References
  • Figures & SI
  • Info & Metrics
  • PDF

You May Also be Interested in

Smoke emanates from Japan’s Fukushima nuclear power plant a few days after tsunami damage
Core Concept: Muography offers a new way to see inside a multitude of objects
Muons penetrate much further than X-rays, they do essentially zero damage, and they are provided for free by the cosmos.
Image credit: Science Source/Digital Globe.
Water from a faucet fills a glass.
News Feature: How “forever chemicals” might impair the immune system
Researchers are exploring whether these ubiquitous fluorinated molecules might worsen infections or hamper vaccine effectiveness.
Image credit: Shutterstock/Dmitry Naumov.
Venus flytrap captures a fly.
Journal Club: Venus flytrap mechanism could shed light on how plants sense touch
One protein seems to play a key role in touch sensitivity for flytraps and other meat-eating plants.
Image credit: Shutterstock/Kuttelvaserova Stuchelova.
Illustration of groups of people chatting
Exploring the length of human conversations
Adam Mastroianni and Daniel Gilbert explore why conversations almost never end when people want them to.
Listen
Past PodcastsSubscribe
Panda bear hanging in a tree
How horse manure helps giant pandas tolerate cold
A study finds that giant pandas roll in horse manure to increase their cold tolerance.
Image credit: Fuwen Wei.

Similar Articles

Site Logo
Powered by HighWire
  • Submit Manuscript
  • Twitter
  • Facebook
  • RSS Feeds
  • Email Alerts

Articles

  • Current Issue
  • Special Feature Articles – Most Recent
  • List of Issues

PNAS Portals

  • Anthropology
  • Chemistry
  • Classics
  • Front Matter
  • Physics
  • Sustainability Science
  • Teaching Resources

Information

  • Authors
  • Editorial Board
  • Reviewers
  • Subscribers
  • Librarians
  • Press
  • Cozzarelli Prize
  • Site Map
  • PNAS Updates
  • FAQs
  • Accessibility Statement
  • Rights & Permissions
  • About
  • Contact

Feedback    Privacy/Legal

Copyright © 2021 National Academy of Sciences. Online ISSN 1091-6490