J . Phys. Chem. 1994, 98, 1454-1458
1454
Photoisomerization of Cyanines. A Comparative Study of Oxygen- and Sulfur-Containing Species M. M. Awad,+P. K. McCarthy, and G. J. Blanchard’ Department of Chemistry, Michigan State University, East Lansing, Michigan 48824 Received: September 30, 1993; In Final Form: November 29, 1993”
W e have compared the photoisomerization properties of two cyanine analogs, 3,3’-diethyloxadicarbocyanine iodide (DODCI) and 3,3’-diethylthiadicarbocyanineiodide (DTDCI). These two molecules are structurally similar, differing only in the presence of oxygen or sulfur at two heteroatom sites. Measurement of the radiative and nonradiative population relaxation kinetics of these molecules reveals a difference in their equilibrium geometries despite their outward similarities. We relate this difference to the steric constraints imposed by the oxygen and sulfur heteroatoms and to the occurrence of an excited state barrier predicted by semiempirical calculations of the ground- and excited-state isomerization surfaces for these molecules.
Introduction The cyanines are a class of molecules used for a wide range of applications related to their ability to change shape upon absorption of 1ight.l Cyanines have been used for many years for the photosensitization of silver halide particles in the photoimaging industry2 and have more recently found application as modelocking dyes in passively mode-locked lasers. Each of these applications depends to some extent on the details of the photoisomerization for the particular cyanine used. Understanding the chemical and structural dependence of cyanine isomerization surfaces is a necessary step in gaining predictive control over these processes. Cyanine photoisomerization has been studied extensi~ely,~-Il but remains understood to only a limited extent. The questions central to understanding photoisomerization of the cyanines are the identities of their ground-state equilibrium geometries in solution and the identification of the isomerization coordinate. We will examine the structure and isomerization coordinate of two outwardly similar cyanines through measurement of their radiative and nonradiative population relaxation kinetics. Since these photolabile molecules have found use primarily in polar and/or viscous environments, we have chosen to study their behavior in the liquid phase, using 1-propanol and ethylene glycol as solvents. Both of these liquids exhibit extensive hydrogen bonding but differ significantly in their bulk viscosities (-2 vs -20 cP). The viscosity difference between these two liquids is important in elucidating which energy relaxation pathways involve intramolecular motion and which step in a multistep relaxation process is rate limiting. We have also performed semiempirical molecular orbital calculations to understand the ground- and excited-state energy surfaces along which these molecules isomerize. Our calculated results are in excellent agreement with Rulliere’s model for cyanine is~merization.~For the rotation of one polar cyanine end group moiety, these calculations predict a sharp excited-state barrier. We believe the origin of this barrier is a “sudden polarization” effect,12-15 and the presence of this unexpected excited-state barrier provides an explanation for the anomalous relaxation kinetics of 3,3‘-diethylthiadicarbocyanine iodide (DTDCI), the sulfur-containing analog of DODCI.
Experimental Section We have studied the population relaxation kinetics of 3,3’diethyloxadicarbocyanine iodide (DODCI, Figure 1a) and 3,3’diethylthiadicarbocyanine iodide (DTDCI, Figure 1b) in l-pro+
Summer intern. Present address Brown University, Providence, RI.
* Author to whom correspondence should be addressed. 0
Abstract published in Aduance ACS Abstracts, January 1, 1994.
Figure 1. (a) Structureof 3,3’-diethyloxadicarbocyanineiodide (DODCI). (b) Structure of 3,3’-diethylthiadecarbocyanine iodide (DTDCI). For
each molecule the iodide counterion has been omitted and only one resonance structure is shown.
panol and ethylene glycol, and have modeled the ground- and excited-state isomerization surfaces of these molecules using semiempirical computational methods. The population relaxation dynamics of excited-state (SI) processes were studied using stimulated emission. While the information content of the stimulated emission response is much higher than that of the corresponding spontaneous emission response,16J7the fundamental electronic relaxation processes that determine the stimulated emission response are the same as those for spontaneous emission. Recovery of the ground-state population of DODCI and DTDCI was monitored through absorption band depletion recovery. Both of these techniques were performed with a time resolution of several picoseconds. The laser spectrometer used to acquire these data has been described before,I8 and we provide only the essential details here. The output of a mode-locked Ar+ laser operating at 514.4 nm (100-ps pulses, 82 MHz repetition rate) is used to excite synchronously two dye lasers. Each dye laser produces pulses of 5 ps duration, and the cross-correlation between output from the dye lasers is typically 10 ps fwhm. The pulses of one dye laser excite (pump) the sample and the pulses from the other dye laser interrogate (probe) the status of the excitation. The relative time registration between the two pulse trains is controlled mechanically. The wavelength of the pump laser is set to coincide with the absorption band of the particular cyanine under examination (Xpump = 595 nm for DODCI and hpump = 650 nm for DTDCI). The wavelength of the probe laser determines the phenomenon under investigation. For stimulated emission measurements, the probe laser is set to lie within the spontaneous emission envelope at a wavelength not overlapped with the absorption band (Apro& = 660 nm for DODCI and Xprok = 690 nm for DTDCI). These wavelengths were chosen to also miminize overlap with cyanine vibrational resonances, but vibrational relaxation effects were observed at short times (
