A decade of cresyl violet – Turner Research Group

Cresyl violet perchlorate is a low-cost laser dye molecule that is easily solvated in methanol and other alcohol solvents and absorbs most of the visible spectrum, peaking at about 600 nm. This wavelength happens to coincide with the easiest wavelength range to produce from OPAs and NOPAs using BBO crystals pumped by femtosecond Ti:sapphire lasers.

Like other members of the oxazine family of dyes, cresyl violet has little practical value. Nevertheless, it has become popular for femtosecond laser spectroscopists because its vibrational quantum beats are extremely strong.

The quantum beats of cresyl violet are so strong that we once turned up the pump laser power and watching the quantum beats by eye! (We monitored the probe beam using a business card and observed the spot change from bright to dark, repeatedly, as we scanned the delay stage.)

Quantum beats of cresyl violet near peak signal at about 600 nm last for at least 5 picoseconds. From Ref. [25]

Cresyl violet therefore serves as a excellent baseline test of new femtosecond spectrometer designs: If one can observe the strong vibrational quantum beats at about 18 THz (590 cm^–1), then one’s spectrometer has at least a minimal level of functionality.

Vibrational quantum beat frequencies of cresyl violet in methanol as measured using ~7 fs laser pulses in transient-absorption spectroscopy.
From Ref. [22].

Here are some brief reflections after a decade of measuring this molecule. As a postdoc at U. Toronto, we wanted to study an important light-harvesting protein known as PC645, which, as you might anticipate, absorbs light at 645 nm. Based on sensible advice, we decided to test the newly constructed femtosecond 2D electronic spectrometer using a laser dye. We searched the classic dye catalog, Lambdachrome, for a dye with a similar spectral profile to PC645. A good match was cresyl violet perchlorate, and the saga began [10].

A few years later, while building my group’s spectroscopy lab at NYU, we again used cresyl violet as a minimal test of our femtosecond laser system. Because of enhancements and upgrades compared to the U. Toronto spectrometer, we were pleasantly surprised to find about a dozen other vibrational modes visible in our TA and 2D spectra [22, 23]. We then used cresyl violet in a test of a 2Q 2D ES setup [26]. We introduced a new style of wavelet transform analysis and postulated a role for nonadiabatic decay in cresyl violet [25]. Then an inquisitive graduate student, Will Carbery, was bold and thought about the actual photochemistry of the molecule. He purchased and synthesized some other oxazines and measured them. It became clear that the number of protons on the nitrogen atoms affected the molecule’s optical properties strongly. We then used cresyl as a reporter molecule in a collaboration with the Buccella group at NYU [40, 45]. Finally, cresyl violet holds a prominent place in my article describing how to quantify the specifications of a 2D spectrometer [39].

Representative 2D ES of cresyl violet in methanol using ~7 fs laser pulses, from Ref. [39]. The pattern of vibronic peaks in the ground-state bleach region is particularly impressive compared to other dye molecules.

Throughout the last decade, other research groups—including Anna, Brixner, Harel, Meech, Levis, and Scholes —have performed femtosecond TA and 2D ES measurements of cresyl violet, and an even larger number of researchers have used other oxazines to demonstrate new femtosecond methods or to evaluate the functionality of their time-resolved spectrometers.

Cresyl violet has had an amazing run, and it will be exciting see what it holds for the next decade!

Reference numbers refer to values on Publications page.