Contributions made to an organization that hasn't yet received tax exemption aren't tax deductible yet -- but they will be, retroactively, if and when the organization receives its tax exemption.
When considering motivating donors, however, don't forget that many of them will never receive a tax deduction for their donations anyway, because they don't itemize their deductions on their personal income tax return. Instead, they use the standard deduction. Clearly, many people are motivated by more than just how the dollars shake out every April 15th. For tips on attracting donors to your organization, see Nolo's article Getting Charitable Donors For Your Nonprofit.