Three biologically active conformers of retinal, all-trans, 9- and 13-cis, and their Schiff bases (SB), are studied using a combination of electroabsorption (Stark) spectroscopy and semiempirical calculations. All of the retinal isomers studied show both a large change in dipole moment between the ground and excited states (|Δμ| greater than 8 D) and a large change in polarizability between the ground and excited states ( greater than 300 Å3). The experimental |Δμ| values in the aldehydes are well predicted by semiempirical calculations. However, the calculated Δμ values for the SB are more than 2 times smaller than experimental values. This discrepancy suggests mixing of the 1Bu state with another state with a large dipole moment, most likely the 2Ag state, which is not probed by our calculational method. For both the aldehydes and SB, values are 2−8 times lower than experimental values. Possible reasons for the deviation between theory and experiment are discussed.

Using Stark effect (electroabsorption) spectroscopy to study the well-known solvatochromic probe molecule coumarin 153 (C153) in a variety of polymer matrices and organic glasses, we have found that the average change in polarizability ( ) that we measure depends critically on the rigidity of the matrix used. In rigid polymer and frozen organic glass matrices, the measured values are between 4 and 60 Å3. The smaller values in this range are similar to those obtained via semiempirical and ab initio calculations. In contrast, measurements made on polymer matrices that are above their glass-transition temperature or those containing trapped solvent are more than an order of magnitude higher. We postulate that large values of result from field-induced orientation of the C153 molecule and/or the dipoles of the surrounding matrix in matrices that are not fully rigid. The absolute value of the change in dipole moment between the ground and excited states (|Δμ|) of C153 measured here ranges from 4.4 to 7.0 D, depending on the polarity and the rigidity of the environment. In addition, an apparent local enhancement of the polarity of the cavity containing C153 is observed in both the solvent and polymer glass matrices, as inferred by the absorption maximum of C153 in these environments.

Mechanisms modulating the pituitary adenylate cyclase activating polypeptide (PACAP)-induced increase in excitability have been studied using dissociated guinea pig intrinsic cardiac neurons and intact ganglion preparations. Measurements of intracellular calcium (Ca2+) with the fluorescent Ca2+ indicator dye fluo-3 indicated that neither PACAP nor vasoactive intestinal polypeptide (VIP) at either 100 nM or 1 microM produced a discernible elevation of intracellular Ca2+ in dissociated intracardiac neurons. For neurons in ganglion whole mount preparations kept in control bath solution, local application of PACAP significantly increased excitability, as indicated by the number of action potentials generated by long depolarizing current pulses. However, in a Ca2+ -deficient solution in which external Ca2+ was replaced by Mg2+ or when cells were bathed in control solution containing 200 microM Cd2+, PACAP did not enhance action potential firing. In contrast, in a Ca2+ -deficient solution with Ca2+ replaced by strontium (Sr2+), PACAP increased excitability. PACAP increased excitability in cells treated with a combination of 20 microM ryanodine and 10 mM caffeine to interrupt release of Ca2+ from internal stores. Experiments using fluo-3 showed that ryanodine/caffeine pretreatment eliminated subsequent caffeine-induced Ca2+ release from intracellular stores, whereas exposure to the Ca2+ -deficient solution did not. In dissociated intracardiac neurons voltage clamped with the perforated patch recording technique, 100 nM PACAP decreased the voltage-dependent barium current (IBa). These results show that, in the guinea pig intracardiac neurons, the PACAP-induced increase in excitability apparently requires Ca2+ influx through Cd2+ -sensitive calcium permeable channels other than voltage-dependent Ca2+ channels, but not Ca2+ release from internal stores.

In this manuscript, a relatively simple and inexpensive INDO/SCI finite-field (FF) method for calculating polarizabilities (R) is demonstrated to give good agreement with results obtained by both the INDO/MRD/ SDCI sum-over-states routine and published results using the RPA method. The FF method is as effective as the other techniques in predicting both ground and excited state R's in all substituted and unsubstituted polyenes studied. We observe the correlation described by Marder and co-workers between bond-order-alternation (BOA) and dipolar properties, such as the change in R between the ground and excited states (∆R). In addition, qualitative, but not quantitative, agreement is seen between the calculated ∆R's of polar polyenes and those measured by Stark-effect spectroscopy.Recent interest in substituted polyenes which exhibit nonlinear optical (NLO) behavior has made accurate calculations of their properties an important predictive tool in materials development. For example, the hyperpolarizabilities ( ) of molecules are of interest because this parameter is directly related to efficiency in optical frequency doubling and to the Pockel effect. Experimentally, there are two common strategies for optimizing : [1][2][3][4][5][6][7] (1) changing the donor (D) and/or acceptor (A) strength of substituents on the polyene and (2) changing the solvent polarity and/or polarizability. Both alter the local field of the polyene chain, leading to a change in bond-order alternation (BOA) 8 and in NLO properties, including . In the two-state model, 9,10 is proportional to the change in dipole moment between the ground and excited states (∆µ), so an accurate calculation of solvated ∆µ is needed to correctly model the effects of solvation on . 3,6,[11][12][13][14][15][16] In dielectric cavity models, 17 the solvent-corrected ∆µ also depends on the change in polarizability between the ground and excited states (∆R). Therefore, quantitatively accurate calculations of both ground and excited state R are needed to correctly model the solvent-effects on and ∆µ, particularly in highly polarizable systems such as the substituted polyenes. This paper compares the two most common routines for calculating ground state R, the sum-over-states and finite-field methods, and applies them to the calculation of excited state R.While techniques for the calculations of first ( ) and second (γ) order hyperpolarizabilities and ground-state polarizabilities (R g ) of polyenes are well developed, 12,14,15,[18][19][20][21][22][23] we are aware of only one example of excited-state polarizability (R e ) calculations for such systems in the literature. 24 In this published work, the random phase approximation (RPA) was utilized to investigate a series of linear, unsubstituted polyenes. Marder and coworkers have investigated trends in ∆R 25 and R g 1 with respect to BOA, but to our knowledge, no quantitative calculations of R e or ∆R have been published on substituted systems similar to those being developed for NLO applications. Because linear unsub...