The McCamant lab studies the fast dynamics of photochemistry and photobiology. We want to learn how molecular geometry changes in electronic excited states on the nanosecond (billionth of a second) to femtosecond (millionth of a billionth of a second) time scale.

Many chemists are dedicated to understanding the relationship between molecular structure and chemical reactivity.  However, almost all of this work is dedicated to understanding the chemical structure of ground electronic states.  During a photochemical reaction, absorption of light excites the molecular electronic configuration and induces structural changes that allow new chemistry to proceed.  Because of the difficulties involved in performing spectroscopy on these photoexcited states, the structures of excited states are still only poorly understood.  Our research focuses on ultrafast nuclear dynamics in photochemistry and photobiology, illuminating how excited state structure is related to photochemical function. 
In the last decade, it has become possible to follow structural dynamics during photochemical reactions by combining ultrafast transient electronic absorption and ultrafast vibrational techniques, such as femtosecond stimulated Raman spectroscopy (FSRS) or femtosecond infrared absorption.(1)  Traditional femtosecond transient electronic absorption can map out the essential kinetics that occur during a photochemical reaction, but this technique is relatively insensitive to structural changes.  In contrast, vibrational spectra can be directly related to molecular structure.  Hence, time-resolved vibrational techniques such as FSRS, shown diagrammatically below, will allow us to unravel the structures of the photochemical intermediates and transition states.(2)

Two of our initial projects in this area are: excited state geometry changes in UV-excited DNA, and structural changes driving the ultrafast charge-transfer reorganization in “push-pull” chromophores.

Figure 1.  (a) Typical spectra of the laser pulses used in FSRS, with (b) the timing diagram, and (c) the energy level diagram for time-resolved FSRS.  The energy level diagram shown in (c) is specific to the situation when the actinic pump produces electronic excitation in the sample and the vibrational spectrum of the photoexcited electronic state is then obtained with the Raman pump and probe.

  The essential characteristics of FSRS are outlined in Figure 1, though a more detailed description of the technique and its application is given in recent reviews.(2,3)  A stimulated Raman spectrum of the sample is collected by observing the amplification of a continuum probe pulse that occurs in the presence of a narrow-bandwidth “Raman pump” pulse.  The amplification occurs at Raman resonances -- wavelengths that are red-shifted from the Raman pump by an amount equal to the frequency of a Raman-active molecular vibration in the sample (see Figure 1a).  The Raman pump is necessarily a > 1 ps duration pulse, to ensure that it has a narrow spectrum; however, the probe can be compressed to pulse durations of --20 fs because of its extremely broad spectrum, thereby enabling time-resolution better than 100 fs.  Time resolution is implemented by the addition of the actinic pump pulse, that initiates the photochemistry of interest (see Figure 1b and c) or initiates ground-state vibrational coherence (see Figure 4). 

Figure 2.  Electronic states involved in the photophysical relaxation of nucleic acids.  Also shown are the ground-state thymidine dinucleotide, T2, and the two primary photoproducts: the T< >T thymidine cyclobutane dimer, and the T[6-4]T cross-linked dimer.  Conical intersections on the potential energy surfaces are indicated by “CI”.

There has been an explosion of research in the last decade into the ultrafast photochemistry of nucleic acids, driven primarily by interest in understanding the damaging effects of ultraviolet (UV) light on DNA.(4)  It is known that the primary mechanism of UV damage is crosslinking of nucleotides to form cyclobutane pyrimidine dimers, e.g. the thymidine dimer “T<>T” in Figure 2, and the 6-4 pyrimidine adducts, shown as “T[6-4]T” in the figure.(5,6)  Unfortunately, although this damage is manifested as a structural change in the nucleic acids, there is still relatively little known about the nuclear dynamics during the electronic relaxation.  Recent femtosecond transient absorption studies have established the excited state lifetime following UV excitation as 0.3-0.7 ps for the four different nucleic acids.(7,8)  Theoretical studies have provided some information about the structural locations of conical intersections that accelerate the internal conversion from the p-p* and n-p* electronic states to the ground state, but there has been little experimental data with which to compare this.(4,9)  Femtosecond stimulated Raman can establish the structure of the excited electronic states and thereby help to establish the critical branching points at which a wrong turn on the potential energy surface can produce terminal damage to the molecule.

Figure 3.  Electronic states of dimethylaminobenzonitrile (DMABN) in solution.  Ultraviolet excitation from the ground state (S0) to the locally excited (LE* or Lb) state is followed by formation of the charge-transfer (CT) state in about 6 picoseconds in acetonitrile (ACN).  The structure of the CT state is hypothesized to be either a twisted intramolecular charge transfer (TICT) state or a planar intramolecular charge transfer (PICT) state.

The modern fields of organic electron transfer and molecular electronics are built on foundational studies of smaller molecular charge-transfer (CT) systems, such as 4-dimethylamino-benzonitrile (DMABN, Figure 3).10,11  These CT compounds are of interest to material chemists because of their important optoelectronic properties, produced via the large excited-state dipole moment formed after absorption of light.  The excited state structure of these molecules has eluded description for over 40 years, with much debate centered on whether the amino group either twists out of the phenylene plane to form the “twisted intramolecular charge transfer” (TICT) state or becomes coplanar with the ring, forming the “planar intramolecular charge transfer” (PICT) state (see Figure 4).(11,12)  It is known that the locally excited state relaxes to the charge-transfer state in tens of picoseconds and recent infrared and resonance Raman spectra have supported the twisted intramolecular charge-transfer (TICT) model.(1,13,14)  However, with FSRS, we can obtain complete Raman spectra of the excited state species from 200-3000 cm-1 that will show not only the high-frequency vibrational modes that are sensitive to conjugation changes, but also the low-frequency modes that are sensitive probes of twist angles.

In addition to the effects attributed to changes in the excited state equilibrium geometry, the dynamics that occur while the molecule is moving between the two structures can have a dramatic effect on photochemistry.  The Franck-Condon principle states that the nuclear geometry remains fixed while a photon is being absorbed.  The result is that immediately after absorbing a photon, a molecule finds itself in the geometry of the ground electronic state and needing to reorganize itself to find the new excited state equilibrium structure.   The forces due to the new electronic configuration drive the molecule away from this Franck-Condon state and towards the relaxed excited state geometry along a small number of displaced internal coordinates—the particular bond lengths and angles that vary between the two electronic states—and as a result these internal coordinates pick up any excess energy deposited by the photon.  If this excess energy is deposited into modes that couple strongly to a photochemical reaction coordinate, for instance bond stretches that might lead to dissociation or molecular torsions that might lead to isomerization, then the excess energy will drive the molecular photochemistry. Because of this, to gain insight into photochemical reaction mechanisms we need to understand not just the excited state equilibrium geometry but also the dynamics that move energy into and out of the reactive vibrational modes.  This intramolecular vibrational reorganization (a.k.a. IVR) is controlled by vibrational anharmonicity which serves to couple vibrational coordinates to each other and allow energy to move between them.  Femtosecond stimulated Raman spectroscopy can reveal the anharmonic coupling that occurs between vibrational modes, thereby establishing both excited state structure and structural dynamics that are key elements in ultrafast photochemistry.

Figure 4.  Anharmonic coupling probed by impulsive Raman excitation of low-frequency modes in diphenyloctatetraene (DPO) and subsequent FSRS probing of the modulated high-frequency potential.  The phase of the low frequency wavepacket at the time the FSRS is initiated is visible in time dependent changes in intensity and frequency of the high-frequency mode.

Coupling between vibrational modes plays an important role in photochemistry by determining the rate at which energy can move into and out of reactive vibrational coordinates.(15)  As a gateway into this field, we will quantify the anharmonic couplings that determine intramolecular vibrational relaxation rates in ground state diphenyloctatetraene (DPO, Figure 4), a model chromophore for both retinal photochemistry and petroleum olefins and aromatics.  As shown in Figure 4, we will probe vibrational energy flow in DPO by driving low-frequency modes into coherence by impulsive Raman excitation(16) and subsequently collecting the high-frequency Raman spectrum at various time delays afterwards.  Coherent low-frequency motion, such as backbone torsions, will modulate both the polarizability and frequency of the high-frequency modes, such as C-C and C=C stretches.  This modulation is apparent in phase-shifted side bands that appear in the high frequency spectrum.(17)  This technique will visualize vibrational coupling at a range of frequencies (200-2000 cm‑1) that is not easily accessed by femtosecond IR spectroscopy and will be able to quantify the magnitude of this coupling that determines the rate of energy flow between these two sets of modes.  Our impulsively driven ground-state studies will be followed up with excited state studies that can reveal the magnitude of anharmonicity in the excited state potential.(18)

1.         Nibbering, E. T. J.; Fidder, H.; Pines, E., Ultrafast chemistry: Using time-resolved vibrational spectroscopy for interrogation of structural dynamics. Annual Review of Physical Chemistry 2005, 56, 337-367.
2.         McCamant, D. W.; Kukura, P.; Yoon, S.; Mathies, R. A., Femtosecond broadband stimulated Raman spectroscopy: Apparatus and methods. Review of Scientific Instruments 2004, 75, (11), 4971-4980.
3.         Kukura, P.; McCamant, D. W.; Mathies, R. A., Femtosecond Stimulated Raman Spectroscopy. Annual Review of Physical Chemistry 2007, 58, (1), 461-488.
4.         Crespo-Hernandez, C. E.; Cohen, B.; Hare, P. M.; Kohler, B., Ultrafast excited-state dynamics in nucleic acids. Chemical Reviews 2004, 104, (4), 1977-2019.
5.         Sancar, A., Structure and function of DNA photolyase and cryptochrome blue-light photoreceptors. Chemical Reviews 2003, 103, (6), 2203-2237.
6.         Marguet, S.; Markovitsi, D., Time-resolved study of thymine dimer formation. Journal of the American Chemical Society 2005, 127, 5780-5781.
7.         Pecourt, J. M. L.; Peon, J.; Kohler, B., Ultrafast internal conversion of electronically excited RNA and DNA nucleosides in water (vol 122, pg 9348, 2000). Journal of the American Chemical Society 2001, 123, (21), 5166-5166.
8.         Pecourt, J. M. L.; Peon, J.; Kohler, B., Ultrafast internal conversion of electronically excited RNA and DNA nucleosides in water. Journal of the American Chemical Society 2000, 122, (38), 9348-9349.
9.         Ismail, N.; Blancafort, L.; Olivucci, M.; Kohler, B.; Robb, M. A., Ultrafast decay of electronically excited singlet cytosine via pi,pi* to n(o)pi* state switch. Journal of the American Chemical Society 2002, 124, (24), 6818-6819.
10.       Lippert, E.; Lüder, W.; Boos, H., In Advances in Molecular Spectroscopy, Mangini, A., Ed. Pergamon Press: Oxford, 1962; p 443.
11.       Grabowski, Z. R.; Rotkiewics, K.; Rettig, W., Structural changes accompanying intramolecular electron transfer: Focus on twisted intramolecular charge-transfer states and structures. Chemical Reviews 2003, 103, (10), 3899-4031.
12.       Dahl, K.; Biswas, R.; Ito, N.; Maroncelli, M., Solvent dependence of the spectra and kinetics of excited-state charge transfer in three (alkylamino)benzonitriles. Journal of Physical Chemistry B 2005, 109, 1563-1585.
13.       Kwok, W. M.; Ma, C.; Matousek, P.; Parker, A. W.; Phillips, D.; Towrie, M., Picosecond time-resolved study of 4-dimethylaminobenzonitrile in polar and nonpolar solvents. Journal of Physical Chemistry A 2000, 104, (18), 4188-4197.
14.       Okamoto, H., Picosecond transient infrared spectrum of 4-(Dimethylamino)benzonitrile in the fingerprint region. Journal of Physical Chemistry A 2000, 104, (18), 4182-4187.
15.       Wang, Z. H.; Pakoulev, A.; Dlott, D. D., Watching vibrational energy transfer in liquids with atomic spatial resolution. Science 2002, 296, (5576), 2201-2203.
16.       Pollard, W. T.; Dexheimer, S. L.; Wang, Q.; Peteanu, L. A.; Shank, C. V.; Mathies, R. A., Theory of Dynamic Absorption-Spectroscopy of Nonstationary States .4. Application to 12-Fs Resonant Impulsive Raman-Spectroscopy of Bacteriorhodopsin. Journal of Physical Chemistry 1992, 96, (15), 6147-6158.
17.       Kukura, P.; Frontiera, R.; Mathies, R. A., Direct observation of anharmonic coupling in the time domain with femtosecond stimulated Raman scattering. Physical Review Letters 2006, 96, (23), 238303.
18.       Fuss, W.; Haas, Y.; Zilberg, S., Twin states and conical intersections in linear polyenes. Chemical Physics 2000, 259, (2-3), 273-295.